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Journal of Marine Systems 141 (2015) 18–33 Contents lists available at ScienceDirect 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 24 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. 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