Download The North Sea — A shelf sea in the Anthropocene

Journal of Marine Systems 141 (2015) 18–33
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Journal of Marine Systems
journal homepage: www.elsevier.com/locate/jmarsys
The North Sea — A shelf sea in the Anthropocene
Kay-Christian Emeis a,b,⁎, Justus van Beusekom a,b, Ulrich Callies a, Ralf Ebinghaus a, Andreas Kannen a,
Gerd Kraus c, Ingrid Kröncke d, Hermann Lenhart e, Ina Lorkowski b, Volker Matthias a, Christian Möllmann b,
Johannes Pätsch b, Mirco Scharfe a, Helmuth Thomas f, Ralf Weisse a, Eduardo Zorita a
a
Helmholtz Zentrum Geesthacht, Institute of Coastal Research, Max-Planck-Str. 1, 21502 Geesthacht, Germany
CEN, University of Hamburg, Germany
Thuenen Institute of Sea Fisheries, Hamburg, Germany
d
Senckenberg Gesellschaft für Naturforschung, Wilhelmshaven, Germany
e
Research Group Scientific Computing, University Hamburg, Germany
f
Dalhousie University, Department of Oceanography, Halifax, Canada
b
c
a r t i c l e
i n f o
Article history:
Received 3 May 2013
Received in revised form 24 March 2014
Accepted 28 March 2014
Available online 5 April 2014
Keywords:
North Sea
Long-term development
Natural variability
Eutrophication
Pollution
Fisheries
Economic uses
Policies
a b s t r a c t
Global and regional change clearly affects the structure and functioning of ecosystems in shelf seas. However,
complex interactions within the shelf seas hinder the identification and unambiguous attribution of observed
changes to drivers. These include variability in the climate system, in ocean dynamics, in biogeochemistry, and
in shelf sea resource exploitation in the widest sense by societies. Observational time series are commonly too
short, and resolution, integration time, and complexity of models are often insufficient to unravel natural
variability from anthropogenic perturbation. The North Sea is a shelf sea of the North Atlantic and is impacted
by virtually all global and regional developments. Natural variability (from interannual to multidecadal time
scales) as response to forcing in the North Atlantic is overlain by global trends (sea level, temperature, acidification) and alternating phases of direct human impacts and attempts to remedy those. Human intervention started
some 1000 years ago (diking and associated loss of wetlands), expanded to near-coastal parts in the industrial
revolution of the mid-19th century (river management, waste disposal in rivers), and greatly accelerated in
the mid-1950s (eutrophication, pollution, fisheries). The North Sea is now a heavily regulated shelf sea, yet
societal goals (good environmental status versus increased uses), demands for benefits and policies diverge increasingly. Likely, the southern North Sea will be re-zoned as riparian countries dedicate increasing sea space
for offshore wind energy generation — with uncertain consequences for the system's environmental status.
We review available observational and model data (predominantly from the southeastern North Sea region) to
identify and describe effects of natural variability, of secular changes, and of human impacts on the North Sea ecosystem, and outline developments in the next decades in response to environmental legislation, and in response
to increased use of shelf sea space.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Coastal systems at the interface of land and sea are susceptible to
environmental effects emanating from the sea-saw of oceanic or land
influence, driven essentially by weather and climate patterns that either
promote inflow of oceanic waters from the ocean, or dominance of terrestrial runoff. The North Sea is a good example of such a system, where
an extended gradient stretches from oceanic conditions (in the deep
parts of the northern North Sea) to more brackish conditions near continental Europe. Salinities in the coastal zone of the North Sea are low,
because variable fresh water inflow from continental rivers creates an
⁎ Corresponding author.
E-mail address: [email protected] (K.-C. Emeis).
http://dx.doi.org/10.1016/j.jmarsys.2014.03.012
0924-7963/© 2014 Elsevier B.V. All rights reserved.
irregular gradient bracketing salinities of 0 (at river barrages and
weirs) over alternating salinities in tidal estuaries, coastal water masses
(in the North Sea bounded by the 33 isohaline) to the oceanic (S = 35)
water masses offshore in the central North Sea (Fig. 1). The extent and
character of the mixing zone is defined by alternating strengths of either
oceanic water masses, or terrestrial runoff. The catchment of continental rivers discharging into the southern area is 428,500 km2, and it is
populated by 140 Mio inhabitants (total North Sea catchment, including
UK and Norway, is 184 Mio) (OSPAR, 2010). The mixing zone also corresponds to environmental problem areas in the North Sea, because it
coincides with high nutrient concentrations imported from land, high
biomass production fuelled by these excess nutrients, and high concentrations of pollutants exported from industrialized and intensely farmed
catchments. In consequence, the entire German Bight has been identified as a problem area in the assessment of environmental status
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
0.9
19
1.5
0.5
0.02
0.1
Fig. 1. Map of the North Sea and generalized properties. Areas and features referred to in the text are marked. Arrows with numbers denote average water mass transports (Sv).
(OSPAR, 2008). This is where the human footprint is strongest, so that
we put one focus on the SE North Sea where the land influence is
distinctly felt and where human activities have early started to alter
the natural environment.
The North Sea is an exceptionally well-studied shelf sea, and few
shelf seas have comparable data and model coverage on current status
and history. Regular assessments of environmental status are published
by the OSPAR Commission (OSPAR, 2010) and national agencies
(Loewe, 2009). In spite of the well-developed knowledge base, robust
statements about any long-term change and/or variability in natural
state and external forcing, and internal variability require analysis of
consistent and homogeneous meteorological and oceanographic data
(Weisse et al., 2009). Usually, there are different sources and types of
data, all of which have their advantages and disadvantages. In-situ
observations are usually considered as relatively robust and reliable,
however, time-series are often short and spatial extrapolation is difficult. Remotely sensed data usually provide higher spatial coverage,
but cover even shorter time spans. More recently, coupled atmosphere–ocean models in combination with measurements were used
to derive consistent high-resolution reconstructions (hindcasts) for
the past decades. One of the most comprehensive efforts carried out
so far for the North Sea is the coastDat reconstruction (1948–2012)
(Weisse et al., 2009). Typically, and with the limitations inherent in
the approach, such reconstructions may nowadays be carried out for
some decades. To obtain a better picture of the full range of natural
variability, even longer time series are needed that ideally bracket the
periods of external forcing. Proxy data available for much longer time
spans represent an approach to address this issue (Alexandersson
et al., 1998; Krüger and von Storch, 2011; WASA, 1998). To analyze
time series at fixed stations, or to establish synoptic states on ship
expeditions has traditionally been the domain of institutions charged
with environmental monitoring or fish stock assessments, and data
are reported and are being curated by national or international data
banks. The Royal Netherlands Institute for Sea Research has in 2001 initiated a new time series dedicated to basin-wide biogeochemical observations in the North Sea and mounted expeditions in 2001/2002, 2005,
2008 and 2011 (Salt et al., 2013; Thomas et al., 2004; Thomas et al.,
2005, 2007). Furthermore, numerous time-series observations in the
20
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
coastal zones have helped substantially to unravel natural from anthropogenic processes affecting the marine environment in nearshore areas
(see below).
This paper attempts to trace three types of influences on the
environmental status of the North Sea, especially in its SE part. The
first influence is variability in the coupled ocean–atmosphere system
of the North Atlantic that distinctively drives ecosystem change on
multi-annual to multi-decadal time scales. The second influences are
global trends in sea level and CO2 concentrations that guide the corridor
of natural variability. Here we chose to include model outlooks on the
effects of warming temperatures for seasonal water-column stratification and resulting oxygen depletion in isolated deep water bodies. The
third set of influences is the spread of human influences on system
state that ranges from medieval diking over pollution and eutrophication to modern marine resource use.
1.1. Paleo- and long-term evolution
The largest temporal framework of reference with regard to recent
change in the North Sea region is the climatic transition from the last
glacial to the present interglacial, the Holocene, that started around
10,000 years ago. Major components of that climate transition are
changes in the geographical and seasonal patterns of solar insolation
and melting of inland ice, both causing the modern coastal zone to develop where it is now (the global sea level has risen by 120–130 m
since approximately 19,000–21,000 years ago). Considering that many
adjustment processes are slow (building of soils, filling of estuaries,
delta building, salt-marsh buildup), it is relevant that eustatic sea level
did not come to rest until approximately 6000 years ago, the birth of
the present-day coastal zone (Vink et al., 2007). That implies that
many coastal features (sand bodies, deltas and estuaries) may not
have reached equilibrium with regard to sea level (Eisma et al., 1981).
That equilibration process between subsidence, sea level rise, and material fluxes and accommodation was interrupted by the onset of human
activities in the amphibian landscape at around 1000 years AD.
On shorter time scales of several centuries, climate variability in the
North Sea region is mainly linked to ocean dynamics in the North Atlantic. At the temporal resolution of decades, atmospheric climate patterns
come into play. These are large-scale atmospheric patterns of variability,
and include such important features as the Northern Annular Mode,
associated North Atlantic Oscillation (NAO) and Atlantic Multidecadal
Oscillation, each with distinct footprints.
Fig. 2. Multidecadal variations in eastward wind averaged over the North Sea from two
global climate simulations over the past millennium. Black: ECHO-G (atmosphere–ocean
general circulation model; prescribed greenhouse gas concentrations, no vegetation
changes, large changes in past solar irradiance). Red: ECHAM5-OM (atmosphere–ocean
general circulation model; interactive vegetation and carbon cycle, small changes in past
solar irradiance).
winds in warmer periods and weaker winds during the Little Ice Age.
The multidecadal variations superimposed on the multi-centennial
trends are clearly observed in both simulations, but they are not closely
correlated in time and may be a result of internal random dynamics
generated in each simulation independently.
On shorter time scales, the strength of the westerly winds in this region is linked to the NAO, more closely in the winter season and more
loosely in other seasons. Both NAO and sea surface temperature (SST)
have undergone different long-term evolution in the last 120 years
(Fig. 3). Variations of the SST are mainly driven by atmosphere–ocean
2. Natural variability in physical forcing
12
4
2.1. Forcing from the North Atlantic Ocean
NOA index
2
11
SST (°C)
The most important large-scale hydro-meteorological drivers of the
North Sea (eco-)system are the sea-surface temperatures and the wind
(Edwards et al., 2002; Ottersen et al., 2001). Both may be influenced by
regional factors, but also by the large-scale climate dynamics emanating
from the North Atlantic and in general in the Northern Hemisphere.
Fig. 2 shows time series of eastward zonal wind (deviations from the
long term mean) simulated by two global climate simulations over the
past millennium with the climate models ECHO-G (von Storch et al.,
2004) and ECHAM5-OM (Jungclaus et al., 2010) and spatially averaged
over the North Sea. The time series have been low-pass filtered to highlight the variability at multidecadal and longer time periods. The global
and European temperatures over the past millennium are characterized
by relatively warmer values around 1000 AD and recently during the
20th century, and relatively colder temperatures during the intermediate centuries, loosely denoted as the Little Ice Age. This is generally
interpreted as a response to the variations in the external forcing comprising solar irradiance, volcanism and more recently anthropogenic
greenhouse gases. The wind time series at these multi-centennial time
scales also reflect this behavior, with slightly stronger zonal westerly
0
10
-2
-4
9
1860
1880
1900
1920
1940
1960
1980
2000
year
Fig. 3. Time series of annual mean North Sea (1 × 1°, geographical box: 0°–10°E; 50°–
60°N) sea-surface temperature derived from HadSST data set (Rayner et al., 2003) in
1870–2013 (thin red line), and time series of the North Atlantic Oscillation index calculated from the difference between the normalized monthly mean pressure in Azores and
Iceland, averaged over the months December to March (Jones et al., 1997) (thin blue
line) (retrieved from https://climatedataguide.ucar.edu/). The 11-year-sliding means of
the time series are also shown as thick lines.
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
heat exchanges, i.e. local conditions that depend on the state of large
scale atmospheric circulation (Becker and Pauly, 1996; Meyer et al.,
2011). Observed annual mean sea-surface temperature averaged over
the North Sea display two different phases over the last 130 years.
Until about 1975, the SST remain remarkably stable with only a slight
upward trend of about 0.03 °C/decade and superposed decadal variations of an approximate amplitude of 0.5 °C. Around 1980, a stepwise
increase in the SST took place which did not reverse to the long-term
mean. At the end of the 20th century SST had increased about 1.2 °C
relative to the pre-1980 mean value with a trend of 0.4 °C/decade, a
ten-fold sudden increase. Model sensitivity experiments suggest that
the shift in observed SST — effected mainly by rising autumn temperatures — is best explained by high air temperature and local air–sea interactions (Meyer et al., 2011). The analysis of Meyer et al. (2011) ended
with the year 2007. Low temperatures from the very last years (2007–
2013), however, illustrate the general difficulty to identify a clear
trend based on observations with high interannual variability (Fig. 3).
Regarding the wind, a comprehensive indicator of the atmospheric
circulation in the North Atlantic is the NAO index (Fig. 3). Several almost
equivalent definitions of the NAO index have been used in the past,
based on different statistical analyses and seasons, but all basically
represent the meridional air pressure gradient in the North Atlantic
and thus the strength of the westerly winds in the North Atlantic–
European sector. The NAO has displayed a stationary behavior over
the last 130 years, with no long-term trend, but with marked
multidecadal variations. It is not clear from the time series displayed
in Fig. 3 whether the shift observed in the North Sea SSTs can be also detected in the NAO index. Extreme values of the index are regularly distributed along this period, with the exception of an extremely low
NAO index observed in the winter of 2010, which is clearly an outlier
against the backdrop long-term climatology. Some studies have related
extreme negative values of the NAO index to the melting of Arctic seaice and to the associated heat flux from the ocean, but this remains to
be confirmed.
On-going analysis with the suite of millennium climate simulations
included in the Fifth Assessment Report of the IPCC strongly suggests
that NAO variability is almost completely of random character, and is
likely independent of the external climate forcing. Previous studies
have found links between the NAO and Indian Ocean sea-surface
temperatures (Hoerling et al., 2001), sea-ice cover (Petoukhov and
Semenov, 2010), and Eurasian snow cover (Cohen et al., 2010). Thus,
as an internal mode of variability (that is also present in control simulations with no variations in the external forcing), the NAO may be influenced by a series of entangled remote climate patterns. Simulations of
future climate do show a response of the NAO to increasing concentrations of greenhouse gas forcing, but so far this response is not seen in
observations due to large interannual and interdecadal variability
(Stephenson et al., 2006).
2.2. Circulation and circulation anomalies
Atlantic waters can be very different from North Sea waters in terms
of their physical, chemical and also biological properties. Variability of
hydrodynamic conditions in the North Sea in combination with a
changing impact of Atlantic water masses (Leterme et al., 2008) has
clear effects on marine ecosystems (Stenseth et al., 2002). Faced with
a lack of long-term synoptic observations, model based long-term
reconstructions of hydrodynamic conditions are a central building
block within the analysis of system change. In particular, a detailed
description of hydrodynamic variability is needed for a proper estimate
of the extent to which observations at a given marine monitoring station are influenced by changing water masses in the station's vicinity.
We based an analysis of the spatio-temporal variation of North Sea
hydrodynamics on long-term simulations (Callies et al., 2011) with
the 3-dimensional, baroclinic model TRIM forced by the regional atmospheric model SN-REMO (Meinke et al., 2004) that is nested into NCEP/
21
NCAR re-analyses (Kistler et al., 2001). Monthly mean fields of vertically
integrated simulated volume transports in the entire North Sea were
subjected to Empirical Orthogonal Function (EOF) analysis. The leading
EOF (anomaly pattern with regard to the mean conditions 1962–2004)
shown in Fig. 4 represents 75.8% of total variance. Regions of particularly
high variability (reddish areas) include the inflow areas of Atlantic
waters via the northern boundary of the North Sea and the English
Channel, respectively.
The time coefficient (principal component, PC1) associated with
EOF1 proves the anomaly pattern to vary on different time scales
(Fig. 5). Positive values of PC1 (intensified inflow of Atlantic waters)
correspond with positive values of the NAO index (Fig. 5). The causal
mechanism behind this correlation is that positive (negative) NAO
values correspond with enhanced (weakened) westerly winds, which
in turn result in an intensified (weakened) cyclonic North Sea circulation (Schrum, 2001).
2.3. Nutrients and CO2
The main nutrient source of the North Sea is inflow from the Atlantic
Ocean. New potential production estimated from nutrient inventories
ranges from around 30 to 100 g C m−2 a−1 along the British east coast
and in the central area (Heath and Beare, 2008). Additional nutrient
input from rivers raises productivity to up to 430 g C m−2 a−1 in the
fertile continental coastal waters of the southern and southeastern
North Sea (German Bight) (Rick et al., 2006), to which river nutrient
inputs contribute an estimated 24% (Heath and Beare, 2008). Annual
production is scaled to winter nitrate concentrations (van Beusekom
and Diel-Christiansen, 1994), and locked to variability in nutrient influx
across the northern boundary (Heath and Beare, 2008). A simulation of
the North Sea nutrient inventory for high and low NAO years (1994/
1995 and 1995/1996) suggested that annual nitrate budgets in the German Bight reflect interannual differences in NAO forcing (Pätsch and
Kühn, 2008). Under oceanic dominance during high NAO winters,
large imports of reactive nitrogen across the northern boundary coupled
with larger river exports due to more precipitation on land increase the
N-inventory of the southern North Sea significantly over that of low
NAO winters.
The North Sea is a two-part system (Fig. 1), where the southern shallow water column is mixed and has high nutrient concentrations; this
sector is a seasonal carbon dioxide sink during spring blooms, and a
source to the atmosphere for the remaining seasons when CO2 is
produced from mineralization of organic matter. The central and northern North Sea — seasonally stratified and nutrient limited in the surface
mixed layer — is a CO2 sink all year round, because CO2 from organic
matter remineralization builds up in deep water layers below the
pycnocline and is ultimately advected to the intermediate depth
Atlantic Ocean. Once biological activity ceases in late summer and autumn, both parts of the North Sea system head toward CO2 equilibrium
with the atmosphere. In the southern part this means near neutral or
weak CO2 fluxes into the atmosphere, and in the northern part near
neutral to weak CO2 uptake, respectively (Bozec et al., 2005; Thomas
et al., 2004; Thomas et al., 2005). Observational studies reveal that
NAO-driven variability affects the carbon cycling and pH in the North
Sea via various processes, occurring locally within the North Sea and
also via variability in the water mass characteristics entering the
North Sea from the North Atlantic (Salt et al., 2013; Thomas et al.,
2007; Thomas et al., 2008). In the central part of the North Sea stratification strength and depth respond to the NAO driven changes in the circulation patterns (stronger inflow during years of positive NAO), as well
as local weather (i.e., temperature). As a consequence the mixed layer
depth (MLD) varies over more than 10 m in late summer between
different NAO stages, which corresponds to a range of 50%, relative to
the shallow MLD of 20 m, as observed during positive NAO stages. Besides control of carbon and nutrient budgets “on the supply side”,
such variability in stratification also exerts controls on bottom-water
22
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Fig. 4. Leading EOF (EOF1) of monthly mean vertically integrated volume transports obtained from 3D baroclinic simulations covering the period 1962–2004. The anomaly pattern (technically: loadings of the corresponding principal component) explains 75.8% of overall variability. Vectors indicate directions of transport anomalies, corresponding strengths are color
coded.
1.0
oxygenation (see below), and on CO2 fluxes by confining the depth
range in which primary productivity controls the CO2 uptake: At a
given productivity inventory, the CO2 concentration change is stronger
during years with shallow MLD. The CO2 concentration change in turn
governs the CO2 air–sea exchange (Salt et al., 2013).
2.0
0.5
1.0
0.0
0.0
-0.5
-1.0
1962
NAO index
PC 1
PC 1
NAO index
-1.0
1968
1974
1980
1986
1992
1998
-2.0
2004
Year
Fig. 5. Time coefficient (PC1) of EOF1 shown in Fig. 4 and NAO index (retrieved from
Hurrell and The National Center for Atmospheric Research Staff, 2013). Time series
show moving annual averages based on monthly values.
2.4. Biotic response
The effect of the physical forcing from the North Atlantic is visible
over all trophic levels of the North Sea ecosystem, especially during
the period of increasing and highly positive NAO anomalies from 1970
and culminating in 1990 (Fig. 3). A long-term increase in phytoplankton
biomass paralleled the temperature increase caused by the NAO since
the early 1960s (Edwards et al., 2001; Reid et al., 1998b). Combined
analyses of monitoring data derived by the Continuous Plankton
Recorder (CPR) Survey (Reid et al., 1998a) and satellite data (SEAWIFS)
(McQuatters-Gollop et al., 2007) revealed an increase in chlorophyll a
concentration during the 1980s by 13% in the open North Sea and 21%
in coastal waters. Phytoplankton biomass increased in coastal waters
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
despite decreasing nutrient levels effected by environmental legislation
to combat eutrophication (see below). Continuing temperature increase
and decreased turbidity are the most probable stimuli, the latter
allowing the normally light limited coastal phytoplankton to more
effectively utilize lower nutrient concentrations (McQuatters-Gollop
and Vermaat, 2011; Wiltshire et al., 2010). In contrast, phytoplankton
biomass in the Wadden Sea and the adjacent coastal zone did react
instantaneously to decreasing riverine nutrient loads (Cadée and
Hegeman, 2002; van Beusekom et al., 2008; van Beusekom et al.,
2009). Here we see evidence that river input, and especially riverine nitrogen loads, and turbidity both control the coastal pelagic phytoplankton production (Loebl et al., 2007).
Species composition of phytoplankton may also be affected by atmospheric forcing. A long-term increase in the ratio between diatoms and
dinoflagellates since 1990 reflects an interacting effect of increasing sea
surface temperature and increasingly windy conditions during summer
(Hinder et al., 2012). An important aspect of phytoplankton species
composition are the so-called Harmful Algae Blooms (HABs) that impact the entire ecosystem through massive biomass or toxin production.
CPR data suggest anomalously favorable conditions for HAB occurrence
in the central North Sea at the end of the 1980s (Edwards et al., 2006). A
general decline in HABs along the eastern UK coast and an increase
especially along the Norwegian coast since 1990 (Hinder et al., 2012)
are attributed to a combination of altered oceanic inflow and nutrientrich low-salinity coastal waters. Several HAB species benefit from
warming (Peperzak, 2003) and enhanced stratification (Edwards
et al., 2006; Peperzak, 2003). The coincidence of regional warming
and freshwater input with continental runoff and nutrient discharges
during positive NAO phases makes it difficult to discriminate climatic
effects from those of eutrophication in the occurrence and regional distribution of HABs (Edwards et al., 2006). Apart from changes in biomass
and primary production, changes in the timing (phenology) of algal
blooms may impact higher trophic levels (Edwards and Richardson,
2004). The latter authors showed that especially nowadays dinoflagellates reach their seasonal maximum earlier, whereas spring and autumn
diatom bloom showed little change. In general the timing of the spring
bloom in German Bight coastal waters remained remarkably stable
(Wiltshire et al., 2008).
The response of zooplankton to environmental change is complex
due to a mixture of both direct (e.g., temperature, salinity, water column
stability, advection) and indirect (via changes in trophodynamics, via
species dominance, and/or phenological shifts in prey availability)
impacts. Direct impacts have been inferred from correlations between
changes in zooplankton community characteristics (e.g., species composition and diversity of the copepod community) and changes in
hydroclimate including a positive relationship between copepod diversity and the NAO index (Beaugrand and Reid, 2003). Consistent with the
prediction that continued warming will drive species ranges toward the
poles, strong biogeographical shifts in calanoid copepod assemblages in
the North Sea and adjacent waters have occurred with a northward extension of more than 10° latitude of warm-water species and a decrease
in the number of colder-water species (Beaugrand et al., 2002;
Beaugrand et al., 2009). Importantly, biogeographical changes have resulted in a shift in dominance between Calanus finmarchicus and Calanus
helgolandicus, congeners with markedly different thermal preferences
(Helaouet and Beaugrand, 2007; Helaouet et al., 2011).
Warming-related changes have been registered in the benthic communities of the sublittoral North Sea since the late 1980s. The number of
warm-temperate species in the in- and epifauna communities has
increased, while cold-temperate species have decreased (Kröncke
et al., 2013; Neumann et al., 2009). For example, the warm-temperate
angular crab (Goneplax rhomboides) usually occurring in the Eastern
Atlantic and the Mediterranean Sea, was first detected in the southern North Sea in 2000 with increasing abundances since then
(Neumann et al., 2013) (Fig. 6). The introduction of the angular
crab was due to larval dispersal into the North Sea and enhanced
23
larval survival due to increased water temperature. Similarly, infauna communities off the island of Norderney and Dogger Bank reacted
to the NAO-related temperature rise (Kröncke, 2011; Kröncke et al.,
1998). Since 2001 the correlation between infauna abundance,
biomass and species number and the NAO collapsed, potentially
indicating a major change in the response of the ecosystem to the climate and oceanic circulation system of the northern Atlantic
(Dippner et al., 2010; Junker et al., 2012).
Long-term changes of the fish communities are usually driven by a
combination of exploitation and climate effects (Rijnsdorp et al.,
2009). In the North Sea the most prominent case is the collapse of Atlantic cod (Gadus morhua) that has been explained by overfishing (Cook
et al., 1997) in combination with detrimental effects of warming on recruitment (Beaugrand et al., 2003; O'Brien et al., 2000). A further effect
of warming on cod is an invoked shift to a more northern distribution
(Rindorf and Lewy, 2005). Overall, both exploited and non-exploited
North Sea fishes strongly responded to recent warming with a northward shift (Perry et al., 2005). Fig. 7 shows an example of a significant increase in southern species from 1985 to 2010 (Sell et al.,
2010). Among these are, e.g., red mullet (Mullus surmuletus), red
gurnard (Trigla lucerna) and several non-commercial flatfish species,
e.g. Buglossidium luteum species (Ehrich et al., 2007). Using the example of red mullet, spatial analyses of the International Bottom
Trawl Survey data from ICES revealed that this species re-entered
the North Sea in the early 1990s through the English Channel and
spread northward along the UK coast (following preferred rocky
habitats), as well as to a limited degree along the sandy Dutch and
German coasts. Generally, warming has caused a reorganization of
fish communities on the European shelf and especially the North
Sea (Simpson et al., 2011).
Overall, adjustments to altered external forcing occurred over all trophic levels in the North Sea ecosystem and suggest that the system has
experienced a major regime shift during the late 1980s (Alheit et al.,
2005; Beaugrand, 2004; Kenny et al., 2009; Reid and Edwards, 2001;
Schlüter et al., 2008; Weijerman et al., 2005). Regime shifts are abrupt
transitions between quasi-stationary states (Scheffer et al., 2001) that
have been observed widely in the marine environment to include
major changes in ecosystem structure and function (Möllmann and
Diekmann, 2012). Regime-like ecosystem changes are of concern for
the management of ecosystems since they may appear without warning
and are theoretically difficult to reverse (Scheffer and Carpenter, 2003).
In the North Sea and other Northern Hemisphere ecosystems marine regime shifts have been related mainly to external, atmospheric forcing
during the late 1970s and 1980s with an emphasis on the effect of
the NAO (Möllmann and Diekmann, 2012). Recently, the importance
of multi-decadal climate variability as indicated by the Atlantic
Multidecadal Oscillation (AMO) has also been proposed to influence
North Sea ecosystem dynamics (Alheit et al., 2014; Edwards et al.,
2013), but that influence has not yet been fully unraveled.
2.5. Global trends and their impact on the North Sea
Consequences of global change for shelf sea systems are increase in
sea level, CO2 uptake and acidification, sea surface temperature
increases and more stable stratification. Linked to this is a possible
expansion of low oxygen conditions that is aggravated by river- and
atmosphere-borne eutrophication. Drivers for these developments are
globally acting and not regionally confined, and these are developments
that may affect every shelf system on the planet, although with more or
less dramatic consequences due to diverging sensitivities and
resiliences of individual shelf sea systems.
2.6. Sea level
The long-term trend in absolute mean sea level in the North Sea was
about 1.6 mm/year over the past 110 years (Wahl et al., 2013), an
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K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Fig. 6. Spreading of the non-native angular crab Goneplax rhomboides (inset; size approximately 5 cm) in the North Sea since 2000.
storm activity (Weisse and Günther, 2007; Weisse and Pluess, 2006) on
time scales of years and decades over the North Sea (Fig. 8). Storm activity was high around 1995 and at the beginning of the 20th century
(WASA, 1998), whereas relatively low storm activity was observed
around 1960 and over the most recent years (Krüger et al., 2013;
Rosenhagen and Schatzmann, 2011). Changes in tidal signals may further contribute to sea level variability. Such changes may reflect for example, local changes in bathymetry such as caused by construction
works, or effects of the nodal tidal cycle (Woodworth, 2012).
8
7.8
annual mean high water
and linear trend (m)
estimate broadly consistent with the global mean figure (Bindoff et al.,
2007). Locally, relative mean sea trends may deviate substantially
caused by the known vertical land movement patterns. For the same period, relative mean sea level changes range from about −0.1 mm/year
in northern Denmark to about 2.3 mm/year in Holland and the eastern
Wadden Sea (Wahl et al., 2013). Moreover, when inter-annual and decadal time scales are considered there is pronounced variability in North
Sea sea levels caused by corresponding variations in any of the driving
factors. Decadal trends show considerable variability with relatively
high values within the most recent decades (Albrecht et al., 2011;
Wahl et al., 2011; Wahl et al., 2013). However, when compared to earlier decades, no outstanding acceleration could be inferred (Fig. 8).
Inter-annual and decadal variations in storm surge and wave climate
are closely linked and reflect the corresponding changes in atmospheric
7.6
7.4
7.2
7
6.8
6.6
6.4
6.2
6
1860
1880
1900
1920
1940
1960
1980
2000
Year
Fig. 7. Increase in the number of southern fish species (Ehrich and Stransky, 2001) in the
southern North Sea over the last 20 years (Sell et al., 2010).
Fig. 8. Annual mean high water and linear trend (in m) for the time period 1843–2012 at
Cuxhaven, Germany (bottom) and corresponding difference between annual 99percentile and annual mean high water levels (top); In addition an 11-year running
mean is shown in the upper panel.
Updated from von Storch and Reichardt (1997).
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
2.7. Changes in the CO2-system in the North Sea
The exchange of CO2 between shelf seas and the atmosphere is
determined by a variety of factors in the CO2 system, including not
only atmospheric CO2 partial pressure, but also by temperature, alkalinity runoff from land, and biological processes (Artioli et al., 2012;
Gypens et al., 2011; Thomas et al., 2005). Atmospheric CO2 partial pressure has risen by 1.32 ppmv yr−1 before 1995, and at a rate of
2.0 ppmv yr−1 after 2005 (http://www.esrl.noaa.gov/gmd/ccgg/trends/
#mlo_full). At the same time sea surface temperature in the North
Sea — a crucial influence on the solubility of CO2 in seawater — has
shown a long-term upward trend of 0.027 °C yr−1 that steepened to
0.08 °C yr−1 over the last decade (Loewe, 2009) (Fig. 3). The average
pH of the North Sea has decreased from 8.08 to 8.01 (corresponding to
an increase in acidity of approximately 17%) over the time period
1970–2006 according to models (Lorkowski et al., 2012). Together
with the increase in sea surface temperatures of 1.8 °C over the last decades, falling pH raised the partial pressure of CO2 (pCO2) in surface
water, which in turn reduced the modeled annual CO2 uptake from the
atmosphere from 700 Gmol C (at 1.4 mol C m−2 in 1970) to 480 Gmol
C (at 0.94 mol C m−2 in 2006) (Lorkowski et al., 2012).
The effects of eutrophication (discussed below) are closely related to
the secular trend of ocean acidification, because both affect the concentrations of dissolved inorganic carbon (DIC) and the DIC/alkalinity ratio
in coastal waters. Increased nutrient loads may eventually lead to enhanced production and respiration of organic matter, which in turn releases DIC and thus lowers the pH. On shorter time scales, this process
overrides any effects of ocean acidification, which rather acts at centennial time scales (Borges and Gypens, 2010).
2.8. Stratification and oxygen status
Warming sea surfaces cause thermoclines to become either deeper
or more stable, with considerable effects on cross-thermocline nutrient
supply to the mixed layer, and oxygen status of below-thermocline waters (Queste et al., 2012). For large areas of the North Sea near bottom
oxygen concentrations are N125 μM, which is the tolerance level for
many marine animals (Diaz and Rosenberg, 2008). Permanent mixing
of the shallow water column is the reason for good oxygen status in
25
coastal waters, despite high organic matter production rates in surface
waters and concomitant high respiration rates near the sea floor. In
deeper areas, where seasonal stratification inhibits the oxygen supply
via mixing, organic matter rain rates are lower and limit oxygen consumption by remineralization. Winter mixing regularly replaces deep
waters, and the adjacent North Atlantic provides oxygen-rich water to
the northern and central North Sea throughout the year.
However, there are some areas prone to low oxygen concentrations
near the bottom, and all of them are located offshore the Danish coast
(see Fig. 1). The depth of this area is around 50 m and it receives a
moderately high input of organic material; depending on the weather
situation, stable stratification develops here in summer months. Following a previous study (Lorkowski et al., 2012) with the 3D ecosystem
model ECOHAM, we identified differences in stratification and simulated oxygen conditions between the 1970–1979 and 2000–2009 time periods. Fig. 9a depicts the areal distribution of the relative frequency of
stratification in the month of September for the time 1970–1979 (% of
time). The water column is almost permanently mixed near the British
and in continental coastal areas and well oxygenated. The deep waters
of the north eastern and north central North Sea are usually well stratified in September (90% of the time) (Fig. 9a). The area with almost permanently stratified water columns in September increased in the
decade 2000–2009 as compared to 1970–1979. This can be seen clearly
in Fig. 9b, where the differences 2000–2009 minus 1970–1979 are
shown. A band of prolonged stratification (positive values) developed
in the middle of the North Sea reaching from the northern areas to
about 55°N. Landward of this band of intensified stratification the
model results suggest slightly decreased stratification durations in
September for the time interval 2000–2009 as compared to the 1970–
1979 period.
The distribution of mean oxygen concentrations in the deepest layer
of the model grid for the 1970s exhibit values around 250 μM in near
coastal regions, while the central and northern deep areas are nearly
saturated (N260 μM) (Fig. 10a). Values below 240 μM prevail in the
offshore area in the south-eastern North Sea. Lowest values
(b 200 μM) occupy a south-east to north-west orientated corridor beginning in the inner German Bight. In the second model interval
(2000–2009) similar or slightly decreased oxygen concentrations
occur all over the North Sea, but significantly decreased concentrations
Fig. 9. a) Relative time of stratification in the months of September for the model period 1970–1979. 100% = stratification during all days of Septembers. b) Difference in relative time of
stratification in the months of September for the model period 2000–2009 minus 1970–1979. Positive numbers: Increase in time of stratification.
26
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Fig. 10. a) September oxygen concentrations (μM) in the deepest grid cell of the model period 1970–1979 outline a zone of depressed oxygen levels originating in the inner German Bight
and widening between 54°N and 57°N. b) Difference in bottom oxygen concentrations of Septembers for the model period 2000–2009 minus 1970–1979. Negative numbers: Decrease in
bottom oxygen concentrations (change in μM).
(up to 15 μM lower than in September of the period 1970–1979) occur
in the SE German Bight around 54°S (Fig. 10b). Part of this may be due to
the oxygen solubility decrease at rising temperatures. But the patterns
of change are too restricted in space and argue for other causes. The
most obvious changes are a decrease of modeled oxygen concentrations
offshore Denmark and an improvement of modeled oxygen conditions
in the deepest model layer in the southern German Bight.
An explanation for the spatially discrete changes in oxygen concentrations is a regional change in the oxygen consumption due to biological activities, in most cases remineralization. An indicator for the
latter effect is the apparent oxygen utilization (AOU) which is simply
the difference between the local oxygen saturation concentration and
the concentration in a given grid cell of the model. The area with a deepened September O2 minimum around 56°N/6°E exhibits larger AOU
values in the model period 2000–2009 as compared to the time interval
1970–1979. This region is a critical low oxygen area (see Fig. 10a), so
that enhanced stratification may aggravate the oxygen situation under
continued warming in summer months. Evidence for more organic matter production and thus higher oxygen demand was not seen in the
model results that used realistic data for nutrient inputs from the atmosphere and rivers (see below). Thus, the effect of warming on oxygen
levels in deep-water layers is not ubiquitous in the North Sea, but is
most pronounced in intermediate water depths where a trend to summer stratification developed over the period 1970–2009 due to a significant temperature increase of the sea surface.
This effect is also shown by observations comparing the summer
bottom temperature change between the periods 1900–1990 and
1990–2000 (Queste et al., 2012). Temperature decreased off the Danish
coast and between the Oyster Ground and the North Dogger site, while
temperature increased in almost all other parts of the North Sea. Further
intensification and prolongation of seasonal stratification is indicated by
model experiments with warming surface waters over the next
100 years (Gröger et al., 2013; Meire et al., 2013). The effect of warming
on stratification emerges as the most prominent physical driver for the
decrease of bottom oxygen concentrations, more important than decreased oxygen solubility in warmer waters and enhanced respiration
rates at higher temperature (Meire et al., 2013). More dramatic
effects of warming by 2 °C at the sea surface may ensue from enhanced
stratification at the shelf break to the North Atlantic (Gröger et al.,
2013). Such a restriction may reduce nutrient advection into the
North Sea by 50% and reduce primary production by 25%.
2.9. Direct anthropogenic impacts
Interactions of various modes of climate and oceanographic variability create complex patterns of spatial and temporal variability in the
North Sea system. Some of these are expressions of geophysical variability in the North Atlantic realm, others are globally acting trends emanating from human activities, such as temperature and associated sea-level
rise and stratification, or changes in the CO2 system that affect the entire
shelf sea. More direct human influences are most easily recognized in
near shore areas bordering discharge areas of rivers. The main threats
among the diverse activities at the land–sea interface are the introduction of non-indigenous species, industrial and agricultural pollution,
overfishing and trawling, dredging, human-induced eutrophication,
construction on coastal breeding and feeding grounds, sand and gravel
extraction, offshore construction, and heavy shipping traffic. Human
activities staggered in time overprinted and in some cases aggravated
undesired effects of natural variability (Fig. 11). Undesired environmental consequences of human activities, e.g., pollution, eutrophication and
de-oxygenation, harmful algal blooms, in several cases resulted in social
and political action, so that adverse activities have been regulated (see
below). Other pressures on the shelf sea environment and in particular
on coastal zones were wetland destruction by diking (Reise, 2005),
estuarine and riverine management that eliminated wetlands and
annihilated estuarine functions (Dähnke et al., 2008), and a substantial
increase in economic exploitation of shelf sea resources (see below),
including transport.
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Fig. 11. Time line (quasi-log-scale) of the relative intensity of human activities and resource exploitation in the coastal zone of the southern North Sea, as well as expected
trends over the next decades. The list of activities is far from being exhaustive, and the relative intensities reflect the assessment of the authors.
In the following paragraphs we describe some of the historical and
recent trends in direct influence of human activities, either on adjacent
land or in the coastal zone and shelf sea.
2.10. Development of the North Sea fisheries
For many centuries, the southern North Sea has been heavily fished.
Even before the documented post-medieval expansion of England's sea
fisheries there was a revolutionary expansion of marine fishing within a
few decades of AD 1000 with cod and herring in focus (Barrett et al.,
2004). From about 1850 onwards the strong fluctuations of North Sea
fish stocks were discussed by the surrounding countries as part of an
“overfishing problem” (Smed and Ramster, 2002), but right whales,
sturgeon, shad, rays, skates and salmon among other species were still
very common in the North Sea well into the 20th century. Today, over
230 species of fish live in the North Sea with cod, haddock, whiting,
saithe, plaice, sole, mackerel, herring, pouting, sprat, and sandeel
being the principal commercially fished species. Norway lobster, deepwater prawns, and brown shrimp are the most important commercial
crustacean species, but other species of lobster, shrimp, oyster, mussels
and clams are also common. Annual catches in the North Sea (ICES
27
Sub-area IV) grew until the late 1970s, when a historic maximum of
more than 3 million metric tons was reached (Fig. 12). Since the early
1980s, the landings fluctuated around 2.5 million tons until the rapid
decline to present days levels of 1.3 million tons occurred during the
1990s (Fig. 12) (ICES, 2012).
Strong declines in North Sea fish populations caused by overfishing
have only been recorded during the 19th and 20th century, following
several revolutionary developments in fishing technology (Kaiser
et al., 2006; Reiss et al., 2009). The first noticeable event was the invention of a modern, mainly Scottish, steam trawler fleet at the end 19th
century. During the 1920s Germany and the Netherlands established
the first diesel engine cutter trawler fleets operating mainly in the
southern and central North Sea. The invention of modern bottom
trawl fleets after the second World War led to strongly increasing landings of cod and other demersal white fish species peaking during the so
called “gadoid outburst” in the 1970s and early 1980s (Fig. 12). This was
not sustainable and led to the yet only partly resolved overfishing
problem of North Sea fish stocks.
Since the 1960s a large beam trawl fleet has been harvesting sole
and plaice using 4 and 12 m beam trawls with tickler chains that frequently plow or rake most of the sea floor in the German Bight area
(Fig. 13). Large amounts of unwanted by-catch are discarded and die.
The long-term effects include destruction of habitats and shifts in biodiversity, species composition or age structure of benthic invertebrate and
fish communities. Recent studies show clear differences between the
fished and non-fished areas. Bottom trawling reduces biomass, production, and species richness of the benthic fauna (Hiddink et al., 2006). The
impacts of trawling are greatest in areas with low levels of natural
disturbance, while the impact of trawling is small in areas with high
rates of natural disturbance. Large species disappear at a faster rate in
response to trawling than small species, as would be expected given
the less resilient life histories of larger animals (Hiddink et al., 2006).
At a critical level of disturbance, such communities may approach an
equilibrium disturbed state so that further increase in disturbance has
little additional impact. In a region of the German Bight (North Sea)
that has been intensively trawled for decades (see Fig. 13), the effects
of variation in fishing disturbance on the secondary production, species
diversity, abundance, biomass, and community structure of benthic infauna were studied (Reiss et al., 2010). Variation in fishing disturbance
across the study area was determined using automated position
registration (APR) and vessel monitoring through satellite (VMS; see
Fig. 13 for an example). Even in such a heavily fished area, linear regression analyses revealed that biomass, species richness, and productivity
decreased significantly with increasing fishing intensity. Although
Fig. 12. Total international fishery landings in the North Sea (ICES Subarea IV) from 1903 to 2010.
Data source: ICES, Bulletin Statistique des Pèches Maritimes/ICES Fisheries Statistics.
28
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Fig. 13. Fine-scale distribution of the total international beam trawl fishing effort for ships b221 kw and N221 kw. Resolution: 3 × 3 nautical miles (nm) in 2006 in the German EEZ and
territorial waters of the North Sea. Green and blue shaded areas indicate Natura 2000 conservation areas.
After Pedersen et al. (2009).
redundancy analyses (RDA) showed that sediment characteristics were
influential in determining the structure of the infauna community, partial RDA revealed that fishing continued to have an impact on community structure in terms of biomass of benthic invertebrates. These
results suggest that resource management will need to be aware that
further increases in fishing activity may cause additional damage to
benthic invertebrate communities even in areas of high chronic fishing
disturbance.
2.11. Pollution
The contamination of the global coastal and marine environment by
persistent pollutants is one of the key features of the “Chemical
Anthropocene”. For an assessment of risks related to sediment contamination the “Effect Range Low” (ERL) concept of the US National Oceanic
and Atmospheric Administration (NOAA, 1999) is a helpful tool for scientist and practitioners. Based on biologically oriented effect data, numerical quality criteria data have been derived and are summarized in
the “NOAA Sediment Quality Guidelines”. These threshold data refer
to concentration levels of pollutants that should not be exceeded to
avoid negative effects in biological systems. Negative effects known to
occur from the wide variety of pollutants and their synergies include endocrine disruption, direct toxic effects, and bio-magnification to harmful concentration levels through food chains, and the associated
aspects of food safety (EEA, 2011). The link between pollutant levels
and observed direct or indirect environmental effects is often tenuous
(Laane et al., 2013), but policy on national and international levels in
Europe concerning pollution and pollutants is firmly rooted in the
precautionary principle.
2.12. Heavy metals
Earliest indications of rising metal concentrations in dated sediment
cores suggest a close link to the industrial revolution in the late 18th
century (Hebbeln et al., 2003). Metals of highest relevance in the coastal
margin are cadmium (Cd), copper (Cu), mercury (Hg), nickel (Ni), lead
(Pb) and zinc (Zn) and have been routinely monitored in the fine grain
fraction (b20 μm) (Loewe, 2009). Besides Ni, all other metals exceed the
effect threshold value in certain regions of the German coastline, Zn
along the entire coast, and Hg between the Elbe estuary and the North
Frisian coast. In the case of Pb, the effect threshold value is exceeded
in the entire German EEZ. Concentrations for Cu, Zn and Pb as well as
for Hg have been decreasing in the time period 1998 to 2007 (Loewe,
2009). The effect range levels of Hg and Pb in the sediments continue
to pose a risk to the Wadden Sea ecosystem in the majority of sub-
areas, as well as in the vicinity of river discharge areas (Bakker et al.,
2009; OSPAR, 2010).
2.13. Classical organic contaminants
The contamination of sediments with organic substances decreases
substantially from the coastline towards the open sea. Only very few
contaminants, such as the hexachlorocyclohexane (HCH) isomers, behave conservatively in the marine environment (Loewe, 2009). High
quality data for this compound class in the water-phase are available
since 1975. Since 1986 a sharp decline has been detected which levels
out around 1999 and reflects variation of river-borne loads of the Elbe
River (Loewe, 2009). Most other organic contaminants are lipophilic
with high affinity to suspended matter and sediments. In the absence
of local sources, sediment characteristics determine the accumulation
(and concentrations) of these contaminants in sediments. Consequently, sediment concentrations are often normalized to TOC. None of the
lipophilic chlorinated compounds monitored had a significant trend in
the observation period of approx. 10 years, which is mostly likely due
to the high variability of measured concentration within this fairly
short time span (Theobald, 2011). The same applies to the group of
polyaromatic hydrocarbons (PAH) in the aqueous phase and in
sediments. The group of PAHs is the most homogeneously distributed
contaminant class in sediments of the German Bight (Loewe, 2009).
OSPAR sediment data for the Wadden Sea indicate that the 6 of the
Borneff PAHs are mainly above the ERL, caused mainly by benzo-ghiperylene and to a lesser extent indeno[123-cd]pyrene (Bakker et al.,
2009).
In river mouths, such as the Scheldt, concentrations of
polychlorinated biphenyls (PCBs and DDTs) in sediment cores were
found to have a fairly steady concentration with a slight decline in top
layers, suggesting a decrease in inputs in recent years. In contrast, concentrations of some brominated flame retardants, such polybrominated
diphenyl esters (PBDEs), have increased exponentially (Covaci et al.,
2005), although they have in part been banned. Concentrations of
PBDEs exceeded predicted effect levels according to Sediment Quality
Guidelines at some sampling stations and for every sample, at least,
one of the discussed threshold values was reached (Biselli et al.,
2005). Furthermore, elevated concentrations and toxicological effects
were not restricted to areas close to industrial sites and harbors, but
were also found in open sea areas.
Although disruption of hormone-regulated endocrine processes in
marine mammals has been identified to be caused by many other compounds besides the classical PCBs, the ecotoxicological effects of these
compounds are often not well known (Bakker et al., 2009). Hormonal
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
disruption of among fish and invertebrates has been detected in estuarine and coastal waters in the UK and the southern Baltic Sea, but there
is little evidence for such effects in the coastal areas of the German Bight
(Bakker et al., 2009).
New compounds are being developed continuously, some to replace
the ones that have been banned, mostly for new purposes and applications. A significant change in the contamination characteristics has been
observed in recent years: The pollution caused by classical hydrophobic
and lipophilic compounds is no longer dominating and has been partly
replaced by more polar and persistent substances, such as pesticides.
These compounds show fairly conservative mixing, consequently correlation to salinity enables source attribution (Theobald, 2011). Most of
the emerging contaminants are polar and non-volatile, so that commonly used analytical techniques are not applicable for detection and
quantification. Many of the “new” pollutants are not really new compounds, but their detection was not possible before. Remarkably,
many of the newly detected compounds exhibit far higher concentrations in the water phase than the old, “classical” pollutants (Theobald,
2011).
2.14. Nutrient loading
1000
50
800
40
600
30
400
20
200
10
0
1979 1983 1987 1991 1995 1999 2003 2007
0
dissolved inorganic phosphorus (t *1000)
dissolved inorganic nitrogen (t*1000)
There is indication in dated sediment cores that river runoff introduced reactive nitrogen and caused eutrophication as early as 1870
AD in the German Bight (Serna et al., 2010), but the onset of significant
eutrophication coincides with the 1950s expansion of intensive farming
and peaked in the early 1980s. Fig. 14 depicts the cumulative annual
load of dissolved inorganic nitrogen (DIN) and phosphate (DIP) of the
most important continental rivers entering the North Sea. The river
load data since 1977 for the German, Dutch and Belgium coast are
from Pätsch and Lenhart (2011), updated from a previous detailed analysis (Radach and Paetsch, 2007). Average winter concentrations of nitrate in waters of the German Bight in the salinity range 30–34.5 were
largely independent of changes in river discharges, whereas lowsalinity waters (range 18–30) affected by river plumes clearly track development of river loads since 1977 (U. Brockmann, pers. comm. 2013).
The increase in river discharge of DIN and DIP was the major reason
for the eutrophication in the coastal waters of the North Sea in the 1980s
(Jickells, 1998). In order to mitigate the apparent negative effects of
eutrophication, the ministers of environment of riparian countries in
1987 agreed to reduce the river nutrient loads of dissolved inorganic
phosphorus (DIP) and nitrogen (DIN) by 50% of discharges in 1985 by
the year 1995. This goal was generally reached for phosphorus as
reflected in decreasing annual loads after 1985 (Fig. 14), due to
Year
Fig. 14. River loads of dissolved inorganic nitrogen (gray bars) and phosphorus (black
bars) delivered by continental rivers (Elbe, Weser, Ems, Ijssel, Rhine, Scheldt, Meuse) to
the German Bight from 1977 to 2010 (Pätsch and Lenhart, 2011). Note that some missing
annual ammonium and phosphate loads of individual rivers have been interpolated.
29
improvements of municipal waste-water treatment plants and the
replacement of phosphates by tensides in detergents. Reactive nitrogen
inputs could not be as effectively reduced (Claussen et al., 2008), but
continue to decline since 1985. The unbalanced reduction of N and P increased the N/P-ratios in most of the continental rivers. In the Rhine, for
example, the average N/P-ratio rose from 23 to 62 between 1980 and
1992, and in the Elbe from 75 to 124, respectively. There is an ongoing
debate if elevated or unbalanced (N/P/Si ratios) nutrient loads will
cause the occurrence of toxic algae (Graneli et al., 2008).
Reactive N from a variety of emission sources (such as nitrogen oxides stemming from burning of fossil and non-fossil fuels for electricity,
heat generation, and transportation activities, or reduced nitrogen that
originates mainly from agricultural activities) is transported through
the atmosphere and deposited in the North Sea. A part of these reactive
compounds is transformed into other more long-lived substances like
ammonium and nitrate aerosols that can be transported over several
hundreds of kilometers. Their main deposition pathway is through
their incorporation into cloud and rain droplets and their subsequent
washout through precipitation. Thus, nitrogen compounds can reach
distant areas of the North Sea where riverine inputs of reactive nitrogen
are less important. It is difficult to measure the wet deposition flux of
reactive nitrogen into the North Sea in a representative way, because
precipitation can be very inhomogeneously distributed even in small
areas, and because measurements are difficult to perform on the open
sea. Therefore the atmospheric nitrogen input into the North Sea is
usually estimated from model results which have been compared and
quality controlled against the available observations at remote coastal
stations.
In 1985, when atmospheric concentrations peaked, deposition of reactive N from the atmosphere was 440,000 t yr−1 and of the same magnitude as input by all rivers (Bartnicki and Fagerli, 2008). Model
estimates for atmospheric deposition over the North Sea area in 1970
were 370,000 t yr− 1 (Pätsch and Radach, 1997), 190,000 t yr− 1 for
the 1960s, and 93,000 t yr−1 for the pre-industrial 1860s (Serna et al.,
2010). Since the peak of atmospheric deposition in 1985, deposition
rates in recent years have almost halved to 280,000 t yr−1 in 2006.
Central Europe and in particular the North Sea riparian states Belgium,
The Netherlands and Germany, but also Northern France continue to
be areas with the highest nitrogen emissions to the atmosphere in
Europe. Consequently, large parts of the emitted nitrogen compounds
are transported by the predominant south westerly winds over the
North Sea and are deposited there. On average about 2 mg N/(m2 d)
enter the North Sea via the atmosphere in winter and 1 mg N/(m2 d)
in summer (Matthias et al., 2008). A pronounced onshore–offshore gradient is caused by the fact that the main emission sources are on land
and that most of the reactive nitrogen emitted into the atmosphere is
deposited close to the sources.
How can the actual eutrophication status of the North Sea be described, and how can possible mitigation measures be evaluated? To answer these questions, a European Model Comparison was carried out
within the OSPAR framework (Lenhart et al., 2010). The aim was to
evaluate the effects of reductions in N and P according to international
reduction targets, compared to the standard model reference year
2002. The distribution of net primary production for the 2002 reference
run of the ECOHAM model is shown in Fig. 15a, based on realistic forcing
including river nutrient loads. The model output shows a pattern consistent with observations: high net primary production within the shallow
southern North Sea receiving continental river inputs, an area with net
primary production values above 250 g C m−2 yr−1 that stretches from
Belgium to the northern tip of Denmark. Near the inlets of the rivers
Rhine and Elbe net primary production rises up to 350 g C m−2 yr−1,
whereas net primary production is below 100 g C m−2 yr− 1 in the
northern North Sea.
Fig. 15b depicts differences between the 2002 standard run and
model results when implementing the OSPAR reduction scenarios
with a 50% reduction of DIN and DIP river loads of 1985. A 50% reduction
30
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Fig. 15. (a) Depth-integrated net primary production modeled for 2002 (g C m−2 a−1); b) percentage difference in net primary production between the standard run (year 2002, panel a)
minus the NPP modeled for a 50% OSPAR reduction scenario for river DIN and DIP load.
in loads reduces net PP in the model by as much as 25%. The area where
reduction is significant (N 10% of net PP) stretches from the northern
Danish coast towards the Humber and Thames estuaries in the UK. In
a model experiment with only 50% reduction in DIN (not shown),
there are no changes in the direct vicinity of the major inlets of Rhine
and Elbe. Compared to only nitrate reduction the simultaneous reduction of DIN and DIP in the model also decreased NPP in coastal zones
and the discharge areas of the Rhine and the Elbe Rivers. An explanation
for these regional differences is that coastal zone NPP is either light or
phosphorus-limited, and that DIN reduction has little effect here,
whereas the near-coastal areas react mainly to a reduction in DIP. As
indicated by the two different river load reduction scenarios in the
resulting distribution of the net primary production (Fig. 15), the
desired final reduction levels will depend on the definition of good
environmental status within the EU policy framework (see below).
Although river loads are decreasing, a major source for oxidized nitrogen at sea is dramatically increasing, namely from increasing ship
traffic. Model results suggest that 17% of the nitrogen deposition into
the North Sea stems from ships (Bartnicki and Fagerli, 2008), and nitrate
aerosol concentrations in air are enhanced by 10–50% due to ship traffic,
depending on region (Matthias et al., 2010; Matthias et al., 2012). Increased ship traffic resulted in an increase of the nitrate wet deposition
of 20% in summer and 10% in winter averaged over the entire North Sea
based on emission data for the year 2000. Projections for the year 2030
show that the nitrogen emissions from ships will double compared to
2008 if no emission control measures are implemented (Matthias
et al., 2012). Compared to 2008 the contribution by ships will increase
to more than 25% in large regions and could be even higher, given that
land based reactive nitrogen emissions will decrease by 50% until
2020 as projected in the National Emission Ceilings (NEC) for the EU
(Wagner et al., 2010).
2.15. Policy and the great acceleration of offshore uses
European seas are increasingly recognized as an economic space.
Established sea uses are becoming more intense, while technological
progress enables new concepts of sea use, thereby increasing the
range of marine uses available. Traditional offshore installations are primarily related to hydrocarbon extraction of predominantly gas in the
southern and predominantly oil in the northern sectors of the North
Sea. While this energy resource is nearing depletion, offshore wind
farming is massively ramping up demand for sea space and will be a
strong competitor for space in the coming decades — competing inter
alia with nature conservation and traditional sea uses such as fisheries
(Gee, 2010; Kannen et al., 2008). Balancing the different interests
poses a new challenge to marine and maritime policies.
2.16. Development of European policies
Since the establishment of the European Economic Community
(EEC) in the late 1950s development of marine and maritime economies
of member states became more and more influenced by European policies and related legislation. Whereas in the beginning EEC marine and
maritime policy was designed to create a free trade area for fish and
fish products, EU policy in recent times follows two major goals: to promote competitiveness and growth of European maritime industries, and
secondly, to achieve social and ecological sustainability and conservation targets. Environmental monitoring and assessment of environmental status of the western European catchments and coasts has since 1992
been the mandate of the OSPAR Commission, in which 15 national
states and the European Union cooperate (http://www.ospar.org/).
While the countries of Europe and the European Commission recognize
that seas and oceans are drivers for the European economy with great
potential for innovation and growth, EU environmental policy at the
same time promotes the implementation of the ecosystem approach
to management of natural resources and achievement of good environmental status (GES), for example in its Marine Strategy Framework
Directive (Kannen, 2012). In order to balance nature conservation aspects with an increasing amount of stakeholder interests in the sea, an
overarching EU Integrated Maritime Policy (IMP) was created. However,
conflicts between economic, social and ecological policy goals are
glaringly obvious and create tensions in the attempt to reconcile the
multitude of human activities in the sea with these diverging goals.
Most important European environmental regulations for the North
Sea are the Water Framework Directive (WFD) and the European Marine
Strategy Framework Directive (MSFD). The WFD is a European legal
framework in which all member states have to define the reductions
to achieve a healthy environment for their river systems from their
springs to about 1 mile off the discharge point at the coast. The MSFD
extends the WFD to cover the entire European seas area. Based on 11
descriptors of good environmental status and a set of related of indicators and criteria, MFSD seeks to achieve good environmental status of
the EU's marine waters by 2020. The aim here is to protect the resource
base, upon which marine-related economic and social activities depend.
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Embedded in WFD and MFSD, the Birds & Habitats Directive complements the major European legal frameworks for protecting the marine
environment.
While there is legally binding EU legislation in place for environmental protection and certain sea uses such as fisheries, there is no such legally binding EU legislation in place for blue growth. On the other hand,
the development of the offshore energy sector is strongly supported by
European Commission and national strategies, such as the Blue Growth
Strategy (http://ec.europa.eu/maritimeaffairs/policy/blue_growth/)
and the recent Blue Energy Action Plan (http://ec.europa.eu/ news/energy/140122_en.htm). Consequently, supported by those strategies, the
growing demand for use of ecosystem services strongly increases the
pressure on marine and maritime space.
2.17. Offshore wind energy
Offshore wind capacity in Europe grew by 33% in 2012, a faster rate
of growth than the onshore wind sector (EWEA, 2013). At the end of
2012 the industry produced approximately 0.5% of the EU's total electricity consumption with an installed capacity of nearly 5 gigawatts
(GW). In the first six months of 2013, 277 new offshore wind turbines
were connected to the electricity grid, totaling a further 1 GW (EWEA,
2014). By 2020 total installed capacity is projected to reach 43 GW, producing approximately 3% of the EU's total electricity consumption
(COM, 2014). The largest amount of installations is expected in the
North Sea, in particular in the UK and Germany (De Decker and
Kreutzkamp, 2011). Such large scale development requires turning a
significant amount of marine space into wind farm sites over the next
20 years. Competing uses are in particular shipping and fisheries. For
example, more than 50% of sole catches in the German North Sea
come from areas for which wind farms are planned (Berkenhagen
et al., 2009). Because most of the remaining areas for this particular
type of fishing are protected as Natura 2000 areas, substitution with
catches from other areas may be difficult or impossible (Berkenhagen
et al., 2009).
Marine (or maritime) spatial planning (MSP) is currently evolving as
one of the major tools for integration of different demands for marine
space and resources and is supported by the Commission's proposal for
a Directive on Maritime Spatial Planning (MSP) and Integrated Coastal
Management (ICM). A number of MSP activities currently exist throughout Europe, in stages ranging from early beginnings and pilot projects to
well-established statutory systems.
Mature spatial plans already exist for the German EEZ, triggered by
the rapid emergence of offshore wind farming and the need to support
the licensing process. The zoning concept of the spatial plan focuses particularly on the spatial conflict between shipping and offshore wind
farms. The spatial plan divides the EEZ into priority and reservation
areas for shipping, pipelines and cables, and wind energy development,
thereby prohibiting other uses in priority areas, unless they are compatible with the priority use. Future challenges to the German as well as
other EU countries' spatial plans are cross-border integration and the
need to recognize a range of sectorial and marine policy arenas,
discourses and actors — including their different values, beliefs and
interests. MSP also must deal with issues such as conflicts and compatibility among different sea uses, cumulative impacts from the existing or
developing use patterns, and transnational cooperation needs (Kannen,
2012).
3. Conclusions
The sensitivity of the North Sea environment to natural variability in
the coupled ocean–atmosphere of the North Atlantic emerges only now
from long observational time series and model experiments. A key driver of surprisingly many of the observed environmental deteriorations
observed in the North Sea since the 1970s was temperature, and that
is closely linked to hemispheric and regional weather and climate
31
oscillations. Natural variability (from interannual to multidecadal time
scales) as yet appears to override direct human impacts on pollutants,
eutrophying nutrients and food webs; most impacts of eutrophication
and pollution are confined to the immediate coastal strip. But with global temperatures expected to rise as a result of rising CO2 atmospheric
concentrations, several crucial assets of North Sea functioning will be
tested, such as the free water exchange of water and nutrients with
the North Atlantic, and mixing of the water column near discharge
areas of riverborne pollutants and nutrients. The North Sea (as other
seas in Europe) is a kingpin of political plans to push economic development in Europe, in particular through large-scale offshore wind energy
generation. There has to our knowledge not been any serious scientific
effort to gauge the possible system-wide and interlinking consequences
of altered sea-floor and sea use. But can likely/postulated/speculative
changes (e.g., in winds, currents, turbulence/mixing, suspended matter
transport, gas exchange, Wadden sea mass balances, sedimentation
patterns, effects on pollution patterns and their synergies) indeed be
unambiguously recognized in observations and models against a backdrop of a highly dynamic and variable natural system? Answering this
question requires the use of both observations and model data, some
of them involving a large degree of uncertainty. There clearly is a need
to step up efforts towards inter- and transdisciplinary North Sea science.
Acknowledgments
We thank the two anonymous reviewers for their detailed and insightful suggestions on how to improve the paper, and our colleagues
at IMBIZO-3 in Goa/India for helpful discussions.
References
Albrecht, F., Wahl, T., Jensen, J., Weisse, R., 2011. Determining sea level change in the
German Bight. Ocean Dyn. 61, 2037–2050.
Alexandersson, H., Smith, T., Iden, K., Tuomenvirta, H., 1998. Long-term trend variations of
storm climate over NW Europe. Global. Atmos. Ocean Syst. 6, 97–120.
Alheit, J., et al., 2005. Synchronous ecological regime shifts in the central Baltic and the
North Sea in the late 1980s. ICES J. Mar. Sci. 62, 1205–1215.
Alheit, J., et al., 2014. Atlantic Multidecadal Oscillation (AMO) modulates dynamics of
small pelagic fishes and ecosystem regime shifts in the eastern North and Central
Atlantic. J. Mar. Syst. 131, 21–35.
Artioli, Y., et al., 2012. The carbonate system in the North Sea: sensitivity and model
validation. J. Mar. Syst. 102–104, 1–13.
Bakker, J., Lüerssen, G., Marencic, H., Jung, K., 2009. Hazardous substances. Thematic report no. 5.1. In: Marencic, H., Vlas, J.d. (Eds.), Quality Status Report 2009 Common
Wadden Sea Secretariat. Trilateral Monitoring and Assessment Group, Wilhelmshaven,
pp. 1–56.
Barrett, J., Locker, A., Roberts, C., 2004. The origins of intensive marine fishing in Medieval
Europe: the English evidence. Proc. Biol. Sci. 27 (1556), 2417–2421.
Bartnicki, J., Fagerli, H., 2008. Airborne load of nitrogen to European Seas. Ecol. Chem. Eng.
15 (3), 297–314.
Beaugrand, G., 2004. The North Sea regime shift: evidence, causes, mechanisms and
consequences. Prog. Oceanogr. 60 (2–4), 245–262.
Beaugrand, G., Reid, D.G., 2003. Long-term changes in phytoplankton, zooplankton and
salmon related to climate. Global Change Biol. 9, 801–817.
Beaugrand, G., Reid, P.C., Ibanez, F., Lindely, J.A., Edwards, M., 2002. Reorganization of
North Atlantic marine copepod biodiversity and climate. Science 296, 1692–1694.
Beaugrand, G., Brander, K.M., Lindley, J.A., Souissi, S., Reid, P.C., 2003. Plankton effect on
cod recruitment in the North Sea. Nature 426, 661–664.
Beaugrand, G., Luczak, C., Edwards, M., 2009. Rapid biogeographical plankton shifts in the
North Atlantic Ocean. Glob. Chang. Biol. 15, 1790–1803.
Becker, G.A., Pauly, M., 1996. Sea surface temperature changes in the North Sea and their
causes. ICES J. Mar. Sci. 53 (887–898).
Berkenhagen, J., et al., 2009. Decision bias in marine spatial planning of offshore
windfarms: problems of singular versus cumulative assessments of economic
impacts on fisheries. Mar. Policy 34 (3), 733–736.
Bindoff, N.L., et al., 2007. Observations: oceanic climate change and sea level. In: Solomon, S.,
et al. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on
Climate Change. Cambridge University Press, Cambridge and New York, NY, USA,
pp. 385–432.
Biselli, S., et al., 2005. Bioassay-directed fractionation of organic extracts of marine surface
sediments from the North and Baltic Sea, part I: determination and identification of
organic pollutants. J. Soils Sediments 5 (1), 171–181.
Borges, A.V., Gypens, N., 2010. Carbonate chemistry in the coastal zone responds more
strongly to eutrophication than to ocean acidification. Limnol. Oceanogr. 55,
346–353.
32
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
Bozec, Y., Thomas, H., Elkalay, K., De Baar, H.J.W., 2005. The continental shelf pump for
CO2 in the North Sea — evidence from summer observation. Mar. Chem. 93, 131–147.
Cadée, G.C., Hegeman, J., 2002. Phytoplankton in the Marsdiep at the end of the 20th century; 30 years monitoring biomass, primary production, and Phaeocystis blooms. J.
Sea Res. 48 (2), 97–110.
Callies, U., Plüß, A., Kappenberg, J., Kapitza, H., 2011. Particle tracking in the vicinity of
Helgoland, North Sea: a model comparison. Ocean Dyn. 61 (12), 2121–2139.
Claussen, U., Zevenboom, W., Brockmann, U., Topcu, D., Bot, P., 2008. Assessment of the
eutrophication status of transitional, coastal and marine waters within OSPAR.
Hydrobiologia 629, 49–58.
Cohen, J., Foster, J., Barlow, M., Saito, K., Jones, J., 2010. Winter 2009–2010: a case study of
an extreme Arctic Oscillation event. Geophys. Res. Lett. 37 (L17707).
COM, 2014. Blue Energy Action needed to deliver on the potential of ocean energy in
European seas and oceans by 2020 and beyond. Communication from the Commission to the European Parliament, the Council, the European Economic and
Social Committee and the Committee of the RegionsEuropean Commission, Brussels pp. 1–11.
Cook, R.M., Sinclair, A., Stefansson, G., 1997. Potential collapse of North Sea cod stocks.
Nature 385, 521–522.
Covaci, A., et al., 2005. Polybrominated diphenyl ethers, polychlorinated biphenyls and organochlorine pesticides in sediment cores from the Western Scheldt river (Belgium):
analytical aspects and depth profiles. Environ. Int. 31, 367–375.
Dähnke, K., Serna, A., Blanz, T., Emeis, K.-C., 2008. Sub-recent nitrogen-isotope trends in
sediments from Skagerrak (North Sea) and Kattegat: changes in N-budgets and
N-sources? Mar. Geol. 253, 92–98.
Offshore electricity grid infrastructure in Europe — a techno-economic assessment. In: De
Decker, J., Kreutzkamp, P. (Eds.), Offshore Grid Final Report (22).
Diaz, R.J., Rosenberg, R., 2008. Spreading dead zones and consequences for marine
ecosystems. Science 321 (5891), 926–929.
Dippner, J.W., Junker, K., Kröncke, I., 2010. Biological regime shifts and changes in predictability. Geophys. Res. Lett. 37 (L24701), 1–5.
Edwards, M., Richardson, A.J., 2004. Impact of climate change on marine pelagic
phenology and trophic mismatch. Nature 430 (7002), 881–884.
Edwards, M., Reid, P., Planque, B., 2001. Long-term and regional variability of phytoplankton biomass in the Northeast Atlantic (1960–1995). ICES J. Mar. Sci. 58, 39–49.
Edwards, M., Beaugrand, G., Reid, P.C., Rowden, A.A., Jones, M.B., 2002. Ocean climate
anomalies and the ecology of the North Sea. Mar. Ecol. Prog. Ser. 239, 1–10.
Edwards, M., Johns, D.G., Leterme, S.C., Svendsen, E., Richardson, A.J., 2006. Regional climate change and harmful algal blooms in the northeast Atlantic. Limnol. Oceanogr.
51 (2), 820–829.
Edwards, M., Beaugrand, G., Helaouet, P., Alheit, J., Coombs, S., 2013. Marine ecosystem
response to the Atlantic Multidecadal Oscillation. PLoS One 8 (2).
EEA, 2011. Hazardous Substances in Europe's Fresh and Marine Waters — An
OverviewEuropean Environmental Agency, Copenhagen.
Ehrich, S., Stransky, C., 2001. Spatial and temporal changes in the southern species
component of North Sea bottom fish assemblages. Senckenb. Marit 31 (2), 143–150.
Ehrich, S., et al., 2007. 20 years of the German Small-Scale Bottom Trawl Survey (GSBTS):
a review. Senckenb. Marit 37 (1), 13–82.
Eisma, D., Mook, W.G., Laban, C., 1981. An early Holocene tidal flat in the Southern Bight.
Spec. Publ. Intern. Ass. Sediment. 5, 229–237.
EWEA, 2013. The European Offshore Wind Industry — Key Trends and Statistics
2012European Wind Energy Association, Brussels.
EWEA, 2014. The European Offshore Wind Industry — Key Trends and Statistics
2013European Wind Energy Association, Brussels.
Gee, K., 2010. Sea use and offshore wind farming. In: Lange, M., et al. (Eds.), Analysing
Coastal and Marine Changes: Offshore Wind Farming as a Case Study. Zukunft
Küste — Coastal Futures Synthesis Report. GKSS Research Center, Geesthacht,
pp. 13–20.
Graneli, E., Weberg, M., Salomon, P.S., 2008. Harmful algal blooms of allelopathic
microalgal species — the role of eutrophication. Harmful Algae 8, 94–102.
Gröger, M., Maier-Reimer, E., Mikolajewicz, U., Moll, A., Sein, D., 2013. NW European shelf
under climate warming: implications for open ocean — shelf exchange, primary
production, and carbon absorption. Biogeosciences 10, 3767–3792.
Gypens, N., Lacroix, G., Lancelot, C., Borges, A.V., 2011. Seasonal and inter-annual variability of air–sea CO2 fluxes and seawater carbonate chemistry in the Southern North Sea.
Prog. Oceanogr. 88 (1–4), 59–77.
Heath, M.R., Beare, D.J., 2008. New primary production in northwest European shelf seas,
1960–2003. Mar. Ecol. Prog. Ser. 363, 183–203.
Hebbeln, D., Scherle, C., Lamy, F., 2003. Depositional history of the Helgoland mud area,
German Bight, North Sea. Geo-Mar. Lett. 23, 81–90.
Helaouet, P., Beaugrand, G., 2007. Macroecology of Calanus finmarchicus and C.
helgolandicus in the North Atlantic and adjacent seas. Mar. Ecol. Prog. Ser. 345,
147–165.
Helaouet, P., Beaugrand, G., Reid, J.B., 2011. Macrophysiology of Calanus finmarchicus in
the North Atlantic Ocean. Prog. Oceanogr. 91, 217–228.
Hiddink, J.G., et al., 2006. Cumulative impacts of seabed trawl disturbance on benthic biomass, production, and species richness in different habitats. Can. J. Fish. Aquat. Sci. 63,
721–736.
Hinder, S.L., et al., 2012. Changes in marine dinoflagellate and diatom abundance under
climate change. Nat. Clim. Chang. 2 (4), 271–275.
Hoerling, M.P., Hurrell, J.W., Xu, T., 2001. Tropical origins for recent North Atlantic climate
change. Science 292, 90–92.
Hurrell, J., The National Center for Atmospheric Research Staff, 2013. The Climate Data Guide:
Hurrel North Atlantic Oscillation (NAO) Index (Station Based). (Retrieved from) https://
climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-indexstation-based.
ICES, 2012. Report of the Working Group on the Assessment of Demersal Stocks in the
North Sea and Skagerrak (WGNSSK), 27 April–3 May 2012ICES Headquarters,
Copenhagen.
Jickells, T.D., 1998. Nutrient biochemistry of the coastal zone. Science 281, 217–222.
Jones, P.D., Jonsson, T., Wheeler, D., 1997. Extension to the North Atlantic Oscillation using
early instrumental pressure observations from Gibraltar and South-West Iceland. Int.
J. Climatol. 17, 1433–1450.
Jungclaus, J., et al., 2010. Ocean circulation and tropical variability in the coupled model
ECHAM5/MPI-OM. J. Clim. 19, 3952–3972.
Junker, K., Sovilj, D., Kröncke, I., Dippner, J.W., 2012. Climate induced changes in benthic
macrofauna — a non-linear model approach. J. Mar. Syst. 96–97, 90–94.
Kaiser, M.J., Clarke, K.R., Hinz, H., Austen, M.C.V., Somerfield, P.J., Karakassis, I., 2006.
Global analysis of response and recovery of benthic biota to fishing. Mar. Ecol. Prog.
Ser. 311, 1–14.
Kannen, A., 2012. Challenges for marine spatial planning in the context of multiple sea
uses, policy arenas and actors based on experiences from the German North Sea.
Reg. Environ. Chang. 1–12.
Kannen, A., Gee, K., Licht-Eggert, K., 2008. Managing changes in sea use across scales:
North Sea and North Sea coast of Schleswig–Holstein. In: Krishnamurthy, R.R., et
al. (Eds.), Integrated Coastal Zone Management. Research Publishing, Singapore,
pp. 93–108.
Kenny, A.J., Skjoldal, H.R., Engelhard, G.H., Kershaw, P.J., Reid, J.B., 2009. An integrated
approach for assessing the relative significance of human pressures and environmental forcing on the status of large marine ecosystems. Prog. Oceanogr. 81 (1–4),
132–148.
Kistler, R., et al., 2001. The NCEP-NCAR 50-year reanalysis: monthly means CD-ROM and
documentation. Bull. Am. Meteorol. Soc. 82, 247–267.
Kröncke, I., 2011. Changes in Dogger Bank macrofauna communities in the 20th century
caused by fishing and climate. Estuar. Coast. Shelf Sci. 94 (3), 234–245.
Kröncke, I., Dippner, J.W., Heyen, H., Zeiss, B., 1998. Long-term changes in the macrofauna
communities off Norderney (East Frisia, Germany) in relation to climate variability.
Mar. Ecol. Prog. Ser. 167, 25–36.
Kröncke, I., Reiss, H., Dippner, J.W., 2013. Effects of cold winters and regime shifts on
macrofauna communities in the southern North Sea. Estuar. Coast. Shelf Sci. 119,
79–90.
Krüger, O., von Storch, H., 2011. Evaluation of an air pressure based proxy for storm activity. J. Clim. 24, 2612–2619.
Krüger, O., Schenk, F., Feser, F., Weisse, R., 2013. Inconsistencies between long-term
trends in storminess derived from the 20CR reanalysis and observations. J. Clim. 26,
868–874.
Laane, R.W.P.M., et al., 2013. Chemical contaminants in the Wadden Sea: sources,
transport, fate and effects. J. Sea Res. 82, 10–53.
Lenhart, H.-J., et al., 2010. Predicting the consequences of nutrient reduction on the eutrophication status of the North Sea. J. Mar. Syst. 81 (1–2), 148–170.
Leterme, S.C., et al., 2008. Decadal fluctuations in North Atlantic water inflow in the North
Sea between 1958–2003: impacts on temperature and phytoplankton populations.
Oceanol. 50, 59–72.
Loebl, M., Dolch, T., van Beusekom, J.E.E., 2007. Annual dynamics of pelagic primary production and respiration in a shallow coastal basin. J. Sea Res. 58, 269–282.
Loewe, P. (Ed.), 2009. System Nordsee — Zustand 2005 im Kontext langzeitlicher
Entwicklungen. Berichte des BSH, 44. Bundesamt für Seeschifffahrt und
Hydrographie, Hamburg und Rostock, Hamburg. 270 pp.
Lorkowski, I., Paetsch, J., Moll, A., Kuehn, W., 2012. Interannual variability of carbon fluxes
in the North Sea from 1970 to 2006 — competing effects of abiotic and biotic drivers
on the gas exchange of CO2. Estuar. Coast. Shelf Sci. 100, 38–57.
Matthias, V., Aulinger, A., Quante, M., 2008. Adapting CMAQ to investigate air pollution in
North Sea coastal regions. Environ. Model. Softw. 23, 356–368.
Matthias, V., Bewersdorff, I., Aulinger, A., Quante, M.T., 2010. The contribution of ship
emissions to air pollution in the North Sea regions. Environ. Pollut. 158 (6),
2241–2250.
Matthias, V., Bewersdorff, I., Aulinger, A., Quante, M., 2012. Enhanced aerosol formation
and nutrient deposition by ship emissions in North Sea coastal regions. In: Steyn,
D.G., Trini Castelli, S. (Eds.), Air Pollution Modeling and Its Application, XXI. Springer,
Dordrecht, pp. 699–703.
McQuatters-Gollop, A., Vermaat, J.E., 2011. Covariance among North Sea ecosystem state
indicators during the past 50 years — contrasts between coastal and open waters. J.
Sea Res. 65 (2), 284–292.
McQuatters-Gollop, A., et al., 2007. A long-term chlorophyll data set reveals regime shift
in North Sea phytoplankton biomass unconnected to nutrient trends. Limnol.
Oceanogr. 52 (2), 635–648.
Meinke, I., von Storch, H., Feser, F., 2004. A validation of the cloud parameterization in the
regional model SN-REMO. J. Geophys. Res. 109 (D13205).
Meire, L., Soetaert, K.E.R., Meysman, F.J.R., 2013. Impact of global change on coastal
oxygen dynamics and risk of hypoxia. Biogeosciences 10, 2633–2653.
Meyer, E.M.I., Pohlmann, T., Weisse, R., 2011. Thermodynamic variability and change in
the North Sea (1948–2007) derived from a multidecadal hindcast. J. Mar. Syst. 86,
35–44.
Möllmann, C., Diekmann, R., 2012. Marine ecosystem regime shifts induced by climate
and overfishing: a review for the Northern Hemisphere. Adv. Ecol. Res. 47, 303–347.
Neumann, H., Ehrich, S., Kröncke, I., 2009. Variability of epifauna and temperature in the
northern North Sea. Mar. Biol. 156, 1817–1826.
Neumann, H., de Boois, I., Kröncke, I., Reiss, H., 2013. Climate change facilitated range
expansion of the non-native Angular crab Goneplax rhomboides into the North Sea.
Mar. Ecol. Prog. Ser. 484, 143–153.
NOAA, 1999. Sediment Quality Guidelines Developed for the National Status and Trends
Program. http://www.ccma.nos.noaa.gov/publications/sqg.pdf.
K.-C. Emeis et al. / Journal of Marine Systems 141 (2015) 18–33
O'Brien, C.M., Fox, C.J., Planque, B., Casey, J., 2000. Climate variability and North Sea cod.
Nature 404, 142.
OSPAR, 2008. Second OSPAR Integrated Report on the Eutrophication Status of the OSPAR
Maritime AreaOSPAR Commission.
OSPAR, 2010. Quality Status Report 2010OSPAR, London.
Ottersen, G., et al., 2001. Ecological effects of the North Atlantic oscillation. Oecologia 128,
1–14.
Pätsch, J., Kühn, W., 2008. Nitrogen and carbon cycling in the North Sea and exchange
with the North Atlantic — a model study, part I. Nitrogen budget and fluxes. Cont.
Shelf Res. 28, 767–787.
Pätsch, J., Lenhart, H.-J., 2011. Daily Nutrient Loads of Nutrients, Total Alkalinity, Dissolved
Inorganic Carbon and Dissolved Organic Carbon of the European Continental Rivers
for the Years 1977–2009Zentrum für Meeres- und Klimaforschung, Hamburg.
Pätsch, J., Radach, G., 1997. Long-term simulation of the eutrophication of the North Sea:
temporal development of nutrients, chlorophyll and primary production in comparison to observations. J. Sea Res. 38, 275–310.
Pedersen, S.A., et al., 2009. Natura 2000 sites and fisheries in German offshore waters.
ICES J. Mar. Sci 66, 1–15.
Peperzak, L., 2003. Climate change and harmful algal blooms in the North Sea. Acta Oecol.
24, S139–S144.
Perry, A.L., Low, P.J., Ellis, J.R., Reynolds, J.D., 2005. Climate change and distribution shifts
in marine fishes. Science 308, 1912–1915.
Petoukhov, V., Semenov, 2010. A link between reduced Barents‐Kara sea ice and cold
winter extremes over northern continents. JGR 115 (D21111).
Queste, B.Y., Fernand, L., Jickells, T.D., Heywood, K.J., 2012. Spatial extent and historical
context of North Sea oxygen depletion in August 2010. Biogeochemistry 1–16.
Radach, G., Paetsch, J., 2007. Variability of continental riverine freshwater and nutrient inputs into the North Sea for the years 1977–2000 and its consequences for the assessment of eutrophication. Estuar. Coasts 30 (1), 66–81.
Rayner, N.A., et al., 2003. Global analyses of sea surface temperature, sea ice, and night
marine air temperature since the late nineteenth century. JGR 108 (D14, 4407), 1–37.
Reid, P.C., Edwards, M., 2001. Long-term changes in the pelagos, benthos and fisheries of
the North Sea. Senckenberg. Marit. 32, 107–115.
Reid, P.C., Edwards, M., Hunt, H.G., Warner, A.J., 1998a. Phytoplankton change in the
North Atlantic. Nature 391 (6667), 546-546.
Reid, P.C., Planque, B., Edwards, M., 1998b. Is observed variability in the long-term results
of the Continuous Plankton Recorder survey a response to climate change? Fish.
Oceanogr. 7, 282–288.
Reise, K., 2005. Coast of change: habitat loss and transformations in the Wadden Sea.
Helgol. Mar. Res. 59, 9–21.
Reiss, H., Greenstreet, S.P.R., Sieben, K., Ehrich, S., 2009. Effects of fishing disturbance on
benthic communities and secondary production within an intensively fished area.
Mar. Ecol. Prog. Ser. 394, 201–213.
Reiss, H., et al., 2010. Unsuitability of TAC management within an ecosystem approach to
fisheries: an ecological perspective. J. Sea Res. 63, 85–92.
Rick, H.J., et al., 2006. Primary productivity in the German Bight (1994–1996). Estuar.
Coasts 29 (1), 4–23.
Rijnsdorp, A.D., Peck, M.A., Engelhard, G.H., Möllmann, C., Pinnegar, J.K., 2009. Resolving
the effect of climate change on fish populations. ICES J. Mar. Sci. 66 (7), 1570–1583.
Rindorf, A., Lewy, P., 2005. Warm, windy winters drive cod north and homing of spawners
keeps them there. J. Appl. Ecol. 43, 445–453.
Rosenhagen, G., Schatzmann, M., 2011. Das Klima der Metropolregion auf Grundlage
meteorologischer Messungen und Beobachtungen. In: von Storch, H., Claussen, M.
(Eds.), Klimabericht für die Metropolregion Hamburg. Springer, Berlin, Heidelberg,
New York, pp. 19–60.
Salt, L., et al., 2013. Variability of shelf sea pH and CO2 pumping in response to North
Atlantic Oscillation forcing. JGR Biogeosci. 118 (4), 1584–1592.
Scheffer, M., Carpenter, S., 2003. Catastrophic regime shifts in ecosystems: linking theory
to observation. Trends Ecol. Evol. 18, 648–656.
Scheffer, M., Carpenter, S., Foley, J., Folke, C., Walker, B., 2001. Catastrophic shifts in ecosystems. Nature 413, 591–596.
Schlüter, M.H., Merico, A., Wiltshire, K.H., Greve, W., von Storch, H., 2008. A statistical
analysis of climate variability and ecosystem response in the German Bight. Ocean
Dyn. 58, 169–186.
Schrum, C., 2001. Regionalization of climate change for the North Sea and the Baltic Sea.
Clim. Res. 18, 31–37.
Sell, A., Ehrich, S., Rieck, N., Stelzenmuller, V., Wegner, G., 2010. Climate change and
spatial scales: evaluating responses in bottom fish communities. Proceedings from
33
the 2010 AGU Ocean Sciences Meeting, Portland. American Geophysical Union,
Oregon.
Serna, A., et al., 2010. History of anthropogenic nitrogen input to the German Bight/SE
North Sea as reflected by nitrogen isotope composition of surface sediments,
sediment cores and hindcast models. Cont. Shelf Res. 30 (15), 1626–1638.
Simpson, S.D., et al., 2011. Continental shelf-wide response of a fish assemblage to rapid
warming of the Sea. Curr. Biol. 21, 1565–1570.
Smed, J., Ramster, J., 2002. Overfishing, science, and politics: the background in the 1890s
to the foundation of the International Council for the Exploration of the Sea. ICES Mar.
Sci. Symp. 215, 13–21.
Stenseth, N.C., et al., 2002. Ecological effects of climate fluctuations. Science 297,
1292–1296.
Stephenson, D.B., Pavan, V., Collins, M., Junge, M.M., Quadrelli, R., 2006. North Atlantic Oscillation response to transient greenhouse gas forcing and the impact on European
winter climate: a CMIP2 multi-model assessment. Clim. Dyn. 27 (4), 401–420.
Theobald, N., 2011. Emerging persistent organic pollutants in the marine environment. In:
Quante, M., Ebinghaus, R., Flöser, G. (Eds.), Persistent Pollution — Past, Present and
Future. Springer Verlag Heidelberg, Dordrecht, London, New York.
Thomas, H., Bozec, Y., Elkalay, K., de Baar, H.J.W., 2004. Enhanced open ocean storage of
CO2 from shelf sea pumping. Science 304, 1005–1008.
Thomas, H., et al., 2005. The carbon budget of the North Sea. Biogeosciences 2, 87–96.
Thomas, H., et al., 2007. Rapid decline of the CO2 buffering capacity in the North Sea and
implications for the North Atlantic Ocean. Glob. Biogeochem. Cycles 21 (GB4001).
Thomas, H., et al., 2008. Changes in the North Atlantic Oscillation influence CO2 uptake in
the North Atlantic over the past 2 decades. Global Biogeochem. Cycles 22. http://dx.
doi.org/10.1029/2007GB003167.
van Beusekom, J.E.E., Diel-Christiansen, S., 1994. A Synthesis of Phyto and Zooplankton
Dynamics in the North Sea EnvironmentWWF, Godalming, UK 148 pp.
van Beusekom, J.E.E., Weigelt-Krenz, S., Martens, P., 2008. Long-term variability of winter
nitrate concentrations in the Northern Wadden Sea driven by freshwater discharge,
decreasing riverine loads and denitrification. Helgol. Mar. Res. 62 (1), 49–57.
van Beusekom, J.E.E., Loebl, M., Martens, P., 2009. Distant riverine nutrient supply and
local temperature drive the long-term phytoplankton development in a temperate
coastal basin. J. Sea Res. 61 (1–2), 26–33.
Vink, A., Steffen, H., Reinhardt, I., Kaufmann, G., 2007. Holocene sea-level change, isostatic
subsidence and the radial viscosity structure of the mantle of northwest Europe
(Belgium, The Netherlands, Germany, southern North Sea). Quat. Sci. Rev. 26,
3249–3275.
von Storch, H., Reichardt, H., 1997. A scenario of storm surge statistics for the German
Bight at the expected time of doubled atmospheric carbon dioxide concentration. J.
Clim. 10, 2653–2662.
von Storch, H., et al., 2004. Reconstructing past climate from noisy data. Sciencexpress 10.
1126 (1096109), 1–8.
Wagner, F., et al., 2010. Baseline Emission Projections and Further Cost-effective Reductions of Air Pollution Impacts in Europe — A 2010 PerspectiveIIASA, Vienna.
Wahl, T., Jensen, J., Frank, T., Haigh, I., 2011. Improved estimates of mean sea level changes
in the German Bight over the last 166 years. Ocean Dyn. 61, 701–715.
Wahl, T., et al., 2013. Observed mean sea level changes around the North Sea coastline
from 1800 to present. Earth Sci. Rev. 124, 51–67.
WASA, 1998. Changing waves and storms in the Northeast Atlantic? Bull. Am. Meteorol.
Soc. 79, 741–760.
Weijerman, M., Lindeboom, H.J., Zuur, A., 2005. Regime shifts in marine ecosystems of the
North Sea and Wadden Sea. Mar. Ecol. Prog. Ser. 289, 21–39.
Weisse, R., Günther, H., 2007. Wave climate and long-term changes for the Southern
North Sea obtained from a high-resolution hindcast 1958–2002. Ocean Dyn. 57 (3),
161–172.
Weisse, R., Pluess, A., 2006. Storm-related sea level variations along the North Sea coast as
simulated by a high-resolution model 1958–2002. Ocean Dyn. 56 (1), 16–25.
Weisse, R., et al., 2009. Regional meteorological-marine reanalyses and climate change
projections: results for Northern Europe and potential for coastal and offshore applications. Bull. Am. Meteorol. Soc. 90 (6), 849–860.
Wiltshire, K.H., et al., 2008. Resilience of North Sea phytoplankton spring bloom dynamics:
an analysis of long-term data at Helgoland Roads. Limnol. Oceanogr. 53, 1294–1302.
Wiltshire, K.H., et al., 2010. Helgoland roads: 45 years of change. Estuar. Coasts 33,
295–310.
Woodworth, P., 2012. A note on the nodal tide in sea level records. J. Coastal Res. 28,
316–323.