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REPORT NO. 2392
LAND-USE CHANGES IN THE TUKITUKI RIVER
CATCHMENT: ASSESSMENT OF ENVIRONMENTAL
EFFECTS ON COASTAL WATERS
CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
EXECUTIVE SUMMARY
Rivers have a strong influence on coastal processes and are important conduits for the
delivery of sediments and nutrients to estuaries and coastal waters. A range of land uses can
modify the input of sediments, nutrients and contaminants carried by rivers into the marine
environment, which in turn can lead to adverse effects on marine ecosystems. This report
assesses the nature and extent of effects on the coastal environment that may arise from
changes in nutrient loading in the Tukituki River as a consequence of the Ruataniwha Water
Storage Scheme (RWSS) and Plan Change 6. Plan Change 6 aims to address specific water
quality and allocation issues in the area that may lead to subsequent changes in irrigation
and land use (e.g. intensification of dairying) in the Tukituki River catchment. A particular
concern with regard to the RWSS and associated land-use change is the potential for an
increase in anthropogenic nutrient loading, which in turn could lead to adverse effects in the
marine environment.
It has been predicted that the RWSS and Plan Change 6 will result in an increase in river
nitrogen and phosphorus of 32% and 6%, respectively. Phosphate is the nutrient of primary
concern in the river with regard to controlling periphyton growth. Once near the river mouth
and in coastal waters, nitrogen is more likely to limit primary production; therefore, an
increase in nitrogen loading is the primary concern with regard to ecological effects on the
coastal receiving environment. Depending on the time of year and river flows, a 32%
increase in nitrogen concentrations (primarily in the form of nitrate) could equate to large,
periodic increases in the amount of nitrogen being transported within the Tukituki River out
welling plume into Hawke Bay.
Placed within the context of Hawke Bay (assuming no changes in the other rivers or outfalls),
the predicted increase in nitrogen loads from the Tukituki River represents a ~4% increase in
total annual inputs for all rivers and the East Clive and NCC wastewater outfalls combined,
and a ~9% increase for the southern region that includes inputs from the Tutaekuri /
Ngaruroro / Clive, Tukituki and Maraetotara Rivers and the East Clive and NCC wastewater
outfalls. A simple model was applied to estimate increases in downstream nitrate
concentrations in coastal waters influenced by the Tukituki River plume. Model outputs
highlighted that (1) the incoming river water will be rapidly diluted as it progressively mixes
with the much larger volume of seawater in Hawke Bay, and (2) the predicted change in
nitrogen loading will have the largest impact during winter months when nitrate
concentrations are highest and during flood flows when the majority of inputs occur.
Contrary to concerns in the river, levels of increased nitrogen loading will not result in nitrate
concentrations in coastal waters that are considered toxic to organisms. Due largely to
dilution, the range of concentrations in nearby coastal waters are roughly 10 times lower than
those in the river; hence increases in the order of 32% are highly unlikely to result in
concentrations considered toxic to marine organisms. The most likely ecological effects in
the marine environment to arise from increases in nitrate concentrations relate to symptoms
of eutrophication. One of the symptoms of eutrophication is an increased abundance of
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nuisance benthic macroalgae such as sea lettuce (Ulva spp.). Such effects are more likely to
occur in shallow estuaries with low levels of flushing. The physical environment and habitats
adjacent to the Tukituki River mouth are unlikely to be conducive to blooms of nuisance
macroalgae. A more likely effect to arise from the predicted increase in nutrient loading is
enhanced growth and abundance of phytoplankton in the water column. With an increase in
phytoplankton abundance, water colour and clarity may also change.
The magnitude of effects on primary producers such as phytoplankton will depend on a
number of factors, including the extent to which nutrient concentrations increase and the
amount of light available for photosynthesis (which relates to season as well as water clarity).
Increases in nitrogen loading under the RMSS and Plan Change 6 represent a small portion
of the cumulative loading of nitrogen from multiple rivers, outfalls and (likely much larger)
oceanic inputs in Hawke Bay. Consequently, added nutrients from the Tukituki River would
potentially enhance (rather than drive) levels of primary production observed in Hawke Bay.
Increased nitrate concentrations are most likely to influence important biological processes
(phytoplankton production and follow-on food web effects) when the river floods for a
prolonged period followed by a period of high light availability. This commonly occurs during
late winter to early spring months.
Increases in nitrate concentration are unlikely to result in immediate biological effects, such
as a measurable increase in phytoplankton biomass. Such responses will be lagged over a
period of days to weeks and significantly dampened as the river plume moves offshore and is
further diluted with seawater. Consequently, the effects on the wider marine environment
arising from incremental increases in nutrient loading from the Tukituki River will be difficult
to isolate from changes occurring in response to the cumulative loading of nutrients from
multiple rivers, outfalls and (likely much larger) oceanic inputs.
Due to the cumulative nature of land-use effects on marine ecosystems, it is particularly
important to maintain long-term datasets for establishing baseline conditions and enabling
the effects of cumulative stressors (including anthropogenic nutrient loading from multiple
sources) to be assessed against a backdrop of natural variability. It is recommended that
HBRC carry out routine water quality monitoring off the mouth of the Tukituki River that in
turn contributes to a wider coastal monitoring programme for assessing cumulative
environmental change in Hawke Bay. It is also recommended that HBRC implement the use
of satellite imagery as an additional tool for monitoring temporal and spatial trends in water
quality conditions in the vicinity of the Tukituki River mouth and wider Hawke Bay. The
Council should also consider developing a coastal hydrology model (perhaps building on the
model developed for the East Clive wastewater outfall) to better understand transport
processes in Hawke Bay and the behaviour of the Tukituki River outwelling plume within a
context that includes other river plumes and point source discharges (outfalls).
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TABLE OF CONTENTS
1. INTRODUCTION .............................................................................................................. 1 1.1. Scope and objectives ..................................................................................................................................... 1 2. BACKGROUND ................................................................................................................ 3 2.1. Rivers and the coastal environment ............................................................................................................... 3 2.2. Coastal issues associated with increased nutrients........................................................................................ 5 3. DESCRIPTION OF THE EXISTING ENVIRONMENT ...................................................... 9 3.1. Hawke Bay ..................................................................................................................................................... 9 3.1.1. Circulation ................................................................................................................................................. 9 3.1.2. Nutrient loading and water quality ........................................................................................................... 10 3.2. Tukituki coastal receiving environment ......................................................................................................... 14 3.2.1. Coastal water quality ............................................................................................................................... 15 4. PREDICTED CHANGES IN FLOWS AND NUTRIENT LOADING ................................. 19 5. POTENTIAL EFFECTS ON THE COASTAL RECEIVING ENVIRONMENT .................. 20 6. CONCLUSIONS AND RECOMMENDATIONS ............................................................... 25 7. ACKNOWLEDGEMENTS ............................................................................................... 26 8. REFERENCES ............................................................................................................... 27 v
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LIST OF FIGURES
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Mean river flows for major rivers around New Zealand and New Zealand’s coastal
zone as defined using satellite imagery and remote sensing data for a number of
parameters closely linked with terrestrial runoff.................................................................. 4 Schematic of the eutrophication process, whereby nutrient loading from multiple
sources combined with additional stressors and natural characteristics of receiving
waters leads to a range of early to late-stage symptoms once an ecosystem’s capacity
to assimilate nutrients is exceeded. ................................................................................... 6 Example of modelled depth-averaged currents over Hawke Bay. ................................... 10 Average river flows and annual nitrogen loading for rivers and streams discharging
more than 20 tonnes N per annum into Hawke Bay. ........................................................ 11 Chlorophyll-a concentrations in Hawke Bay on 15 September 2007 based on MODIS
Aqua data. ........................................................................................................................ 13 Hawke Bay following a dry summer period and a wet and wild winter period.. ................ 13 Example of modelled depth-averaged currents near the Tutaekuri / Ngaruroro and
Tukituki Rivers................................................................................................................... 15 Water quality data for routine monitoring carried out by Hawke’s Bay Regional Council
at the Awatoto coastal monitoring site located approximately 5 km north of the Tukituki
River mouth... .................................................................................................................... 18 Projected nitrate concentrations within the Tukituki River outwelling plume associated
with river inputs following a period of high flooding and at mean flow over a 24-hour
period. ............................................................................................................................... 22 LIST OF TABLES
Table 1. Table 2. vi
Typical water column characteristics for different trophic states in marine waters, as
summarised by Smith et al. (1999) and based on the review by Håkanson (1994). .......... 8 Statistics describing the Tukituki and Motueka Rivers and their respective coastal
receiving environments. .................................................................................................... 17 CAWTHRON INSTITUTE | REPORT NO. 2392
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1. INTRODUCTION
This report provides an assessment of the potential coastal marine effects of changes
in river flows and increased nutrient loading that may arise through the Ruataniwha
Water Storage Scheme (RWSS) and Plan Change 6. Plan Change 6 aims to address
specific water quality and allocation issues in the area that may lead to subsequent
changes in irrigation and land use (e.g. intensification of dairying) in the Tukituki River
catchment. Rivers play an important role in driving coastal processes via the influx of
freshwater and the delivery of nutrients and sediments to estuaries and coastal
waters. Land uses (in particular, urbanisation and agriculture), can increase the input
of a range of contaminants carried by rivers into the marine environment. A particular
concern with regard to the RWSS and associated land-use change is the potential for
an increase in anthropogenic nutrient loading, which in turn could lead to adverse
effects in the marine environment (e.g. symptoms of eutrophication). Although not
addressed within this report, changes in land use may also increase the amount of
sediment and other contaminants such as faecal bacteria entering coastal waters.
1.1. Scope and objectives
This report focuses on the following objectives and describes the potential effects that
increased nutrient loading in the Tukituki River may have on the coastal receiving
environment.

Provide a brief background on the role of rivers in relation to coastal processes
and nutrient loading in coastal waters

Describe the existing environment and current conditions at a regional (Hawke
Bay) to local (Tukituki catchment and coastal receiving environment) scale

Briefly summarise forecasted changes in river discharges and nutrient loading that
are likely to occur as a consequence of the RWSS and Plan Change 6

Assess the nature, extent and likely effects of these changes on the coastal
receiving environment and place such effects within the context of wider,
cumulative environmental change

Provide recommendations for monitoring baseline conditions in Hawke Bay and
potential effects of increased nutrient loading on coastal water quality downstream
of the Tukituki River.
The above objectives were met using information gathered from published literature
and reports and water quality data provided by Hawke’s Bay Regional Council
(HBRC). The assessment is primarily based on forecasted changes in river flows and
nutrient losses derived from modelling (Rutherford 2013) and effects described in
Young et al. (2013). Of particular value in the assessment are data that have been
collected by HBRC since 2007 at a coastal monitoring site (located within the
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REPORT NO. 2392 | CAWTHRON INSTITUTE
influence of the Tutaekuri / Ngaruroro / Clive Rivers and ~ 5 km from the Tukituki
River mouth. The Tukituki River is one of several rivers that flow into Hawke Bay.
Time-series water quality data from the HAWQi coastal buoy located to the north,
coupled with satellite imagery, enable the effects to be placed within a regional
context that includes inputs from multiple rivers and larger-scale oceanic processes.
In general, information regarding the effects of out-welling river plumes on coastal
ecology in New Zealand is limited to a few case study areas. The outcomes from the
Motueka River Integrated Catchment Management Programme and related research
in Tasman Bay are compared with the Tukituki/Hawke Bay situation to provide an
indication of the likelihood and level of downstream effects that may arise from
changes in the Tukituki river catchment.
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2. BACKGROUND
2.1. Rivers and the coastal environment
The coastal marine environment is highly productive relative to most open ocean
environments. This is attributed largely to a greater availability of nutrients that
support high rates of primary production on both the seabed (e.g. sea grasses,
macroalgae and microphytobenthos) and by phytoplankton in the water column.
Nutrient inputs to coastal waters include those derived from the upwelling of deeper,
nutrient-rich waters into near shore waters, which is largely driven by physical
processes (interactions between currents and coastal morphology). Depending on
coastal hydrology and the volume of freshwater inflow, rivers can play an important
role in this process by driving estuarine-type circulation, whereby buoyant, out-welling
river plumes enhance the input of deep, nutrient-rich oceanic water. . Rivers are also
important conduits for terrestrial sediments and nutrients that support healthy, highly
productive estuaries and near shore waters. However, as explained further below,
changes in land cover through habitat loss (e.g. wetlands) and modification (shoreline
hardening), and land use, such as conversion of native bush to forestry and pastoral
farm land, can greatly modify the amounts of sediments, nutrients, and contaminants
entering coastal waters.
Collectively, rivers have a very large influence on New Zealand’s territorial seas
(12 nm) and even beyond (Figure 1). Typically, coastal zones are defined by distance
from shorelines and bathymetry, but a study conducted by Gibbs et al. (2006)
demonstrated that the coastal zone, if defined by waters influenced by land, actually
extends much farther than what is classically considered ‘coastal’. For example, levels
of turbidity (or water clarity), which are influenced by sediment inputs, as well as
production of phytoplankton, which is dependent on nutrients, reveal patterns
consistent with a ‘coastal footprint’ that varies considerably in time and space and can
extend over 100 kilometres offshore (see Figure 1). This illustrates how rivers, and the
water, sediments, and nutrients that they deliver, can significantly contribute to landsea interactions and larger-scale processes in the marine environment.
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Figure 1.
REPORT NO. 2392 | CAWTHRON INSTITUTE
Mean river flows for major rivers around New Zealand (left, from Gillespie et al. 2011) and
New Zealand’s coastal zone (right, from Gibbs et al. 2006) as defined using satellite
imagery and remote sensing data for a number of parameters (turbidity and chlorophyll-a)
closely linked with terrestrial runoff. The blue line is the maximum extent and the pink line
is the median for the period 1997-2002.
The influence of a river on the ecology of downstream coastal waters will largely
depend on the physical characteristics of the coastal receiving environment. For
instance, rivers flowing into large, bar-built estuaries or semi-enclosed harbours (e.g.,
Tauranga Harbour), where freshwaters are retained, mixed and processed for varying
periods, may have a larger influence on the ecosystem than rivers flowing directly into
the sea where they are rapidly diluted and mixed with offshore waters. The
importance of riverine inputs can also vary between coastal bays. For instance, both
the Firth of Thames and Tasman Bay receive significant river inputs; however, due to
differences in coastal hydrology and retention times, the Firth of Thames is a more
river-dominated system, whereas Tasman Bay is a more ocean-dominated system in
terms of the sources of nutrient inputs available for primary production (Zeldis et al.
2008).
Using a mass-balance budgeting approach, the contribution of rivers to inputs of
dissolved inorganic nitrogen (DIN) in Tasman Bay has been estimated at
approximately 9%, indicating that incoming ocean water is the primary source of
nitrogen in the Bay (Zeldis et al. 2008). This low percentage based on high-level
calculations should not downplay the potential importance of rivers in coastal systems.
For instance, studies have demonstrated that productivity (phytoplankton production)
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in the shallow (< 30 m), near shore regions of Tasman Bay can be affected by river
inflows that increase nutrient supply directly via nutrient loading (Mackenzie &
Adamson 2004, MacKenzie 2004). In addition, the inflowing freshwaters indirectly
influence nutrient loading from the ocean by enhancing estuarine circulation patterns
resulting in influx of deeper nutrient (and sometimes plankton) rich ocean waters into
the bay (Gibbs 2001; Mackenzie & Adamson 2004). Very similar processes and
patterns to those described for Tasman Bay have been observed in Hawke Bay
(Bradford et al. 1980). Incoming freshwater is also more turbid and less dense than
the seawater, and can lead to strong water column stratification which in turn affects
light penetration. As a result, biological responses such as increased phytoplankton
production can lag for periods of days to weeks following wet periods and increased
nutrient loading (see Gillespie et al. 2011). So, although the rivers may carry a high
level of nutrients, the extent to which nutrients directly influence biological processes
such as primary production will depend on a number of factors, including light
availability and water column stratification.
2.2. Coastal issues associated with increased nutrients
Nutrient inputs from terrestrial sources contribute to maintaining healthy and
productive coastal ecosystems. However, under certain conditions, excessive
amounts of nutrients may lead to a number of effects that impact the ecosystem and
in turn the services and values the ecosystem provides. Most of the adverse effects
relate to the symptoms of eutrophication (see Figure 2). Eutrophication is the process
whereby nutrient inputs to a water body accelerate primary production (phytoplankton
and/or macroalgal growth). In extreme cases this can lead to reduced water clarity,
physical smothering of biota and reductions in dissolved oxygen concentrations (DO)
because of microbial decay (Figure 2; Degobbis 1989; Cloern 2001; Paerl 2006).
Runoff from land-based agriculture has been associated with intense eutrophication of
coastal environments and an increasing number of hypoxic (low oxygen) zones (Diaz
et al. 2012). The clearest evidence for the wide spread issue of eutrophication is the
increasing number of ‘dead zones’, where advanced symptoms of eutrophication,
such as anoxic conditions, are exacerbated to the point where the ecosystem no
longer functions normally. Extremely developed catchments combined with modified
river deltas have led to some classic worst-case examples, such as the Mississippi
River plume in the Gulf of Mexico. In New Zealand, cases of severe eutrophication are
currently limited to lakes (e.g. Rotorua Lakes; see www.lernz.co.nz) and shallow,
poorly flushed estuaries (e.g. a number of estuaries in Southland); these systems are
more susceptible to eutrophication from increased nutrient loading than exposed
coastal waters.
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Figure 2.
REPORT NO. 2392 | CAWTHRON INSTITUTE
Schematic of the eutrophication process, whereby nutrient loading from multiple sources
combined with additional stressors and natural characteristics of receiving waters
(flushing, retention) leads to a range of early to late-stage symptoms once an
ecosystem’s capacity to assimilate nutrients is exceeded (i.e. measureable changes
occur beyond the envelope of natural variability). 1Ocean sources to coastal waters
include dissolved nutrients through breakdown of organic matter, nitrification, and
onwelling / upwelling of nutrient rich deeper waters. 2Atmospheric deposition of nutrients
from fossil fuel combustion, agricultural fertilizers and livestock operations can also
significantly contribute to nutrients in coastal waters (see Diaz et al. 2012) (HABs =
Harmful algal blooms).
Nitrogen is generally considered the key nutrient limiting growth of primary producers
in temperate coastal waters (e.g. Howarth & Marino 2006). Based on reported
seawater ratios of nitrogen, phosphorus and silicate (N:P:Si), this is also the case for
Hawke Bay (see Section 3) and other coastal regions in New Zealand (MacKenzie
2004). Thus the growth of primary producers such as phytoplankton and macroalgae
(seaweeds) is likely to be limited at certain times of year by the supply of dissolved
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CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
inorganic nitrogen (DIN; as nitrate-N and ammonium-N), rather than by the supply of
other nutrients such as phosphorus or silicate (Eppley et al. 1969).
Once nitrogen is introduced into the receiving environment, numerous processes
influence its form and concentration. Dissolved inorganic nitrogen is directly
assimilated by phytoplankton and macroalgae. Transformation occurs through
processes such as microbial activity (denitrification), oxidation or consumption.
Denitrification, particularly within coastal sediments, can represent a major loss of
biologically available nitrogen from an ecosystem.
Dissolved inorganic nitrogen (primarily NO3-N + NH4-N) associated with river nutrient
inputs is biologically available and therefore likely to influence important biological
processes in coastal waters. In order to avoid over-enrichment (and symptoms of
eutrophication), the inputs of these nutrients must not exceed the assimilative capacity
of the receiving environment at local and larger scales. A system’s assimilative
capacity is a complex function of its biotic and abiotic characteristics, including
flushing rate, light and temperature regime, several nutrient cycling processes (e.g.
microbial remineralisation and denitrification rates), grazing pressure (Tett & Edwards
2002) and native epibiota composition and biomass (e.g. macrophytes).
There are no widely accepted guidelines as to what constitutes an acceptable level of
nitrogen input to coastal systems. ANZECC (2000) contains guidelines for nutrients in
coastal waters, however, standards are based on South-East Australian coastal
values where nutrients are naturally lower than New Zealand’s temperate coastal
waters; hence they are considered here as having little relevance to most New
Zealand situations. One way of gauging potential downstream effects of increased
nutrient loading is to determine the existing state of the system and the degree of
potential modification from additional nutrient inputs. The degree of enrichment for a
body of water based on water column nutrient and phytoplankton concentrations is
commonly defined as its ‘trophic state’ or state of enrichment.
Waters receiving low inputs of nutrients with low concentrations of phytoplankton are
considered oligotrophic, whereas waters with high nutrient inputs and concentrations
of phytoplankton are considered eutrophic. Mesotrophic conditions lie along the
continuum between oligotrophic and eutrophic. Increases in trophic status occur when
additional nutrients contribute to elevated rates of primary production (the production
of phytoplankton and other autotrophs). At lower trophic states production rates are
balanced with efficient transfer through the food web. In a eutrophic state this
relationship can start to break down to where increasing nutrient inputs lead to
conditions not suitable for some species (e.g. reduced dissolved oxygen levels). At
extreme levels of nutrient inputs, anoxia and azoic conditions can occur periodically,
this trophic state is termed hypertrophic or dystrophic (Smith et al. 1999; Table 1).
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Table 1.
REPORT NO. 2392 | CAWTHRON INSTITUTE
Typical water column characteristics for different trophic states in marine waters, as
summarised by Smith et al. (1999) and based on the review by Håkanson (1994). TN=
total nitrogen, TP= total phosphorus, SD= Secchi disc depth (a measure of water clarity).
Trophic
state
TN
(mg/m3)
TP
(mg/m3)
Chl a
(mg/m3)
SD (m)
Oligotrophic
< 260
< 10
<1
>6
Mesotrophic
260–350
10–30
1–3
3–6
Eutrophic
350–400
30–40
3–5
1.5–3
> 400
> 40
>5
< 1.5
Hypertrophic
Describing the trophic state of a water body implies that a system stays within a given
band of values for water column characteristics, but in reality a coastal system such
as Hawke Bay will fluctuate between states over time in response to varying levels of
nutrient inputs and primary production, which in turn vary according to rainfall, coastal
processes, season, etc. It is noted that the bands in Table 1 are lower than those
used to classify estuaries, which generally exhibit higher levels of nutrients and
productivity than open coastal environments. For instance, eutrophic levels of
chlorophyll-a (chl-a) in Table 1 are considered as the low (oligotrophic) band for 138
estuaries throughout North America (Bricker et al. 2003). Similarly, nitrogen
concentrations indicative of eutrophication in estuaries is considered to be > 1000
mg/m3. These differences in values for classifying trophic status is expected when
comparing estuaries, which typically have high levels of nutrient cycling and
productivity, to coastal and offshore marine systems.
In reality, the trophic state of New Zealand’s coastal bays (e.g. Hawke Bay) will
fluctuate, primarily as a function of seasonal variation in nutrient availability, light
levels, and water column stratification. For instance, conditions during a spring
phytoplankton bloom may equate to eutrophic conditions, followed by a prolonged
period of low nutrients and phytoplankton consistent with oligotrophic conditions.
Hence, the values in Table 1 provide only a guide as to the bands of water quality
properties that are indicative of the various stages of nutrient enrichment. Ultimately,
long time-series data on nutrient concentrations and biological indicators
(phytoplankton concentrations and community composition, dissolved oxygen
concentrations) are required to gauge whether conditions are trending toward
increased symptoms of eutrophication (see Figure 2). There are numerous previously
published monitoring approaches that utilise multi-parameter indices for gauging
trophic conditions (e.g. Bricker et al. 2003; Ferreira et al. 2007).
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3. DESCRIPTION OF THE EXISTING ENVIRONMENT
3.1. Hawke Bay
Hawke Bay is a large coastal water body spanning an area of 2,950 km2 (at high tide)
and with a volume of 1.8 x 1011 m3 (at low tide; Heath 1976). To put the volume of
Hawke Bay into perspective, it would take approximately 130 years for the Tukituki
River to fill its basin. Hawke Bay has a long retention time of 47 tidal periods
(approximately 24 days) (Heath 1976), which is attributed to its horizontal circulation,
small tidal amplitude (1.2-1.3 m) and its large size and volume.
The shoreline of Hawke Bay is dominated by short beaches with soft coarse
sediments. The beaches are wave dominated with breakers typically between 0-1 m
(NIWA Coastal Explorer tool). In general, Hawke Bay beaches are gently sloping,
going to 10 m depth within about a 2 km distance, and 20 m depth within about 8 to
10 km of the shoreline. Beyond about 30 km, the slope drops off more steeply to a
depth of 100 m.
3.1.1. Circulation
Circulation in the Bay is driven mainly by horizontal flows that are heavily influenced
by prevailing winds and the adjacent Wairarapa Coastal Current (WCC). The WCC,
which in turn is influenced by the East Cape Current, plays a significant role in the
movement of coastal waters in Hawke Bay (Chiswell 2002). Measurements of
currents collected as part of the Open Ocean Aquaculture (OOA) research
programme conducted in Hawke Bay (Heasman 2009) were consistent with earlier
observations by Ridgeway & Stanton (1969), who described the circulation as a main
current entering the middle of the Bay and splitting into two currents, running north
and south parallel with the coast, respectively. Recent hydrodynamic modelling
conducted to assess the East Clive wastewater outfall agrees with this general pattern
(Figure 3).
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Figure 3.
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Example of modelled depth-averaged currents (m/s) over Hawke Bay. This figure was
produced from the model developed for assessing the East Clive wastewater outfall
(MetOcean and Cawthron 2010)
3.1.2. Nutrient loading and water quality
Rivers and streams in the Hawke Bay catchments collectively contribute an estimated
6,021 tonnes of nitrogen (N) per annum to the Bay (based on the NIWA Water
Resources Explorer for streams and rivers contributing more than 20 tonne per
annum; Figure 4). The majority of this input is provided by four rivers, including the
Wairoa and Mohaka rivers in the northern region and the Tutaekuri / Ngaruroro / Clive
and Tukituki rivers in the southern region. The East Clive and Napier City Council
(NCC) outfalls contribute an additional 1,010 and 1,271 tonnes of N per annum,
respectively (McWilliams 2012; NCC data provided by HBRC). There are also other
diffuse sources of nutrients that enter Hawke Bay from direct runoff and groundwater.
The magnitude of nitrogen loads discharged into the coastal zone from rivers can vary
considerably as a function of variability in rainfall over seasonal, annual and longer
time-scales, such as ocean-climate cycles (e.g. El Nino). For example, the Motueka
River discharges 731 tonnes of nitrogen on average into Tasman Bay each year, but
the annual nitrogen discharge actually ranged between 397 and 829 tonnes over a 5year period as a function of variability in rainfall and river flows (Gillespie et al. 2011).
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Taehaenui River
Wairoa River
Te Kiwi Stream
10 m
Nuhaka River
20 m
Mohaka River
Waitaha Stream
30 m
Aropaonui River
OOA research site
Pakuratahi Stream
HAWQi buoy
Esk River
50 m
Average flow (m³/sec)
1
10
100 m
50
TNC Rivers
Awatoto monitoring site
Tukituki River
Maraetotara River
100
¯
0
5
10
20 Km
Napier City outfall
TNC
Rivers
Tukituki
River
East
Clive
outfall
Maraetotara
River
Taehaenui River
10 m
Nuhaka River
Te Kiwi Stream Wairoa River
20 m
Mohaka River
Waitaha Stream
30 m
Aropaonui River
Pakuratahi Stream
50 m
Esk River
Total Nitrogen (tonnes/annum)
100 m
10
TNC Rivers
Tukituki River
Maraetotara River
50
100
500
1,000
0
Figure 4.
5
10
¯
20 Km
Average river flows (top panel) and annual nitrogen (N) loading (bottom panel) for rivers
and streams discharging more than 20 tonnes N per annum into Hawke Bay. Discharges
for the East Clive and Napier City Council outfalls are included in the inset figure. Depth
contours and locations where water quality was monitored as part of the Open Ocean
Aquaculture (OOA) research programme, the HAWQi buoy site, and the Awatoto coastal
monitoring site is also shown in the top panel. TNC refers to the combined Tutaekuri /
Ngaruroro / Clive Rivers. The larger orange circle around the Tukituki River represents
the estimated 32% increase in N loading.
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REPORT NO. 2392 | CAWTHRON INSTITUTE
Indicators of water quality (e.g. chl-a, nutrients) in Hawke Bay vary over time, showing
seasonal patterns similar to other coastal regions of New Zealand. Based on data
collected during the OOA programme in waters approximately 10 km offshore, water
quality in Hawke Bay primarily varies from oligotrophic (low nutrients and productivity)
to mesotrophic (moderately productive; see Table 1) conditions. On occasion, chl-a
concentrations are consistent with eutrophic conditions. Chlorophyll-a concentrations
measured at the OOA sampling sites between 2004 and 2007 averaged 0.8 mg chla/m3, peaking between 2 and 6 mg chl-a/m3 during late-winter (August–September)
months and at depths of 20 and 30 m (Heasman et al. 2009). Data is also collected
for chl-a in shallower waters (5 m depth) at the Council’s HAWQi water quality buoy.
Based on available data from five months in 2012, chl-a concentrations at the HAWQi
site averaged 0.48 mg chl-a/m3 and peaked at 2.3 mg chl-a/m3. The range of chl-a
concentrations observed in Tasman Bay (MacKenzie & Adamson 2004), which
receives less nutrient loading from rivers, are similar to those observed in Hawke Bay.
Phytoplankton blooms in late winter to spring months are common in many of New
Zealand’s coastal regions. In Tasman Bay they are a relatively consistent feature with
similar levels of chl-a as observed in Hawke Bay. Nutrients from rivers likely contribute
to elevated production in Hawke Bay during the more productive late winter and
spring months (see Figure 5 for example). The summer months are often associated
with higher chl-a concentrations in deeper waters (> 30 m) and very low
concentrations nearer the surface. These conditions are thought to reflect temperature
stratification (i.e. poor mixing) and very low nutrient concentrations near the surface
(Heasman et al. 2009). These observations are similar to those in Tasman Bay, where
strong water column stratification in summer months leads to low concentrations of
nutrients and phytoplankton in surface waters (MacKenzie 2004).
Rivers are also major conduits for land-derived sediments. During and following
stormy periods, when rivers flood and wave action causes re-suspension of near
shore sediments, highly turbid waters cover almost the entire surface of the Bay.
Conversely, during calm periods, the waters are very clear, and during summer
months, low nutrient concentrations typically support low abundances of
phytoplankton (Figure 6).
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CAWTHRON INSTITUTE | REPORT NO. 2392
Figure 5.
AUGUST 2013
Chlorophyll-a (chl-a) concentrations in Hawke Bay on 15 September 2007 based on
MODIS Aqua data (NASA OceanColor Web). Note that the algorithm has not been
validated, and a portion of the signal for chl-a is likely due to suspended solids and back
scatter in the water column, particularly closer to the shoreline.
Figure 6.
Hawke Bay following a dry summer period (left; 10 February 2013) and a wet and wild
winter period (right; 25 June 2013). Photographs are from the NASA MODIS Aqua
satellite.
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REPORT NO. 2392 | CAWTHRON INSTITUTE
3.2. Tukituki coastal receiving environment
Although the Tukituki River is the fourth largest river in the region in terms of average
flows, it is the region’s second largest in terms of annual nitrogen loads (see Figure 4).
This reflects the nature of land use in the Tukituki River catchment relative to other
catchments in the region. The river mouth at the coast is narrow (35 m) and consists
of a small area defined as estuarine (about 0.23 km2 at spring high water; Hume
2013). The estuary is well flushed and has a very short retention time of 1.2 tidal
cycles. As described by Hume (2013), the estuary is essentially an extension of the
river that is tidally influenced; its short retention means that the estuary is unlikely to
experience adverse effects due to increases in nutrient loading. Hence the river is
effectively a direct conduit for freshwater and associated sediments, nutrients and
other materials carried by the river into Hawke Bay.
River waters enter Hawke Bay across a gently sloping beach, as described in Section
3.1 and Figure 4. Based on a side scan survey of the area, the seabed directly off the
river mouth appears to be comprised of a small patch of sandy gravel surrounded by
relatively uniform sands (MetOcean Solutions Ltd. 2011). Within a 2 km radius of the
shoreline there appear to be no notable benthic features or habitats such as rocky
reefs, or stands of macroalgae (e.g. kelp). The near shore zone along the primary
current direction (to the southeast) becomes a mixture of sandy and rocky bottom
extending at least 2 km offshore.
A close-up of simulated currents in the southern region of Hawke Bay illustrates that
the river plume is likely to flow predominantly to the southeast along the shoreline and
then northward as the water mass is influenced by the northerly-flowing Wairarapa
Coastal Current (WCC; Figure 7). At times, the Tutaekuri / Ngaruroro / Clive river
plume likely mixes with the discharges from the NCC and East Clive outfalls, and in
turn the Tukituki River plume, propagating south-eastward along the shoreline and
then northward into the wider Hawke Bay and offshore. This circulation pattern is
consistent with the satellite image of regional chl-a distribution (see Figure 5).
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CAWTHRON INSTITUTE | REPORT NO. 2392
Figure 7.
AUGUST 2013
Example of modelled depth-averaged currents (m/s) near the Tutaekuri / Ngaruroro and
Tukituki Rivers. This figure was produced from the model developed for assessing the
East Clive wastewater outfall (MetOcean and Cawthron 2010).
3.2.1. Coastal water quality
Hawke’s Bay Regional Council has monitored surface water quality at the Awatoto
coastal monitoring site just north of the Tukituki River mouth and near the Tutaekuri /
Ngaruroro / Clive Rivers since 2006. Although ‘up-current’ of the Tukituki, the dataset
at this site provide a good time-series for gauging the influence of river plumes on
water quality in the region. Concentrations of nutrients (with the exception of
ammonium-N) are considerably lower for coastal waters than nutrient concentrations
in the river, which reflects a high level of dilution (Table 2). Nitrate concentrations
within the influence of the river plumes are higher than some regions such as Tasman
Bay (MacKenzie 2004) and lower than others (e.g. off Horizons east coast; Cornelisen
2010).
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REPORT NO. 2392 | CAWTHRON INSTITUTE
Time-series data from coastal water quality monitoring conducted by HBRC just north
of the Tukituki River mouth also indicates an N:P ratio based on dissolved inorganic
nitrogen and phosphorus typically below an optimal ratio of 16:1 for marine
phytoplankton growth (Redfield 1934), which is consistent with other coastal regions
including Tasman Bay. These concentrations suggest that nitrogen is most likely the
macronutrient limiting primary production. This means that increases in nitrogen
loading associated with river discharges could in turn enhance primary production
(assuming conditions are right) in coastal waters, but that additions of other nutrients
such as phosphorus are less likely to result in increased production. This is a
generality based on average concentrations of macronutrients, and at times it is
possible (and likely) that other nutrients (including micronutrients) may limit growth of
some phytoplankton species.
Time-series results are similar to those from other coastal regions that demonstrate a
seasonal variation in nutrient concentrations. For example, nitrate concentrations
peak during the winter months and are lowest (sometimes undetectable) during the
summer months (Figure 8). Peaks in nitrate concentrations also coincide with periods
of low conductivity, indicating the influence of waters from nearby rivers and outfalls.
Results from this monitoring are consistent with those described for a location further
offshore in Hawke Bay and monitored as part of the OOA programme (see Figure 4
for the location). As was the case for the OOA monitoring site, the data collected at
the HBRC monitoring site indicate that the waters vary between oligotrophic and
mesotrophic conditions, occasionally reaching nutrient and chl-a concentrations
characteristic of eutrophic conditions (see Table 1). However, the water quality data
are not indicative of a system exhibiting advanced symptoms of eutrophication, which
would be expected to include other signs such as the presence of nuisance algal
blooms, and periods of low dissolved oxygen (see Figure 2).
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CAWTHRON INSTITUTE | REPORT NO. 2392
Table 2.
AUGUST 2013
Statistics describing the Tukituki and Motueka Rivers and their respective coastal
receiving environments. Catchment area, river flows and nutrient loading values were
obtained from the NIWA Wrenz tool. Nutrient concentrations (mean (max)) were compiled
from a number of sources referenced in HBRC reports (Young et al. 2013; Rutherford
2013). Values for the Tukituki and Motueka rivers are based on data from downstream
Red Bridge and Woodman’s Bend locations, respectively. Data on nutrients in Hawke
Bay are based on HBRC data collected between 2007 and 2013 at the Awatoto coastal
monitoring site, and those for Tasman Bay are based on MacKenzie (2004).
Tukituki River
Hawke Bay
Motueka River
Tasman Bay
2,502
2,058
Mean
43
67
Median
21
36
1,407
887
1,330
731
667 (2415)
170 (660)
Coastal
38 (270)
~14 (70)
River
10 (71)
6 (114)
Coastal
17 (140)
~8 (21)
River
882 (2765)
314 (2150)
Coastal
155 (420)
NA
River
12 (91)
5 (57)
Coastal
6 (15)
~12 (15)
Catchment area (km2)
3
River flows (m /s)
Annual flood peak
Total N loading (tonnes/yr)
3
Nutrient concentrations (mg/m )
Nitrite-Nitrate-N
River
Ammonium-N
Total-N
Phosphorus (SRP)
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REPORT NO. 2392 | CAWTHRON INSTITUTE
6
5
4
Chl a
mg/m3
3
2
1
0
300Jul‐06
Jan‐07
Jul‐07
Jan‐08
Jul‐08
Jan‐09
Jul‐09
Jan‐10
Jul‐10
Jan‐11
Jul‐11
Jan‐12
Jul‐12
Jan‐13
Jan‐07
Jul‐07
Jan‐08
Jul‐08
Jan‐09
Jul‐09
Jan‐10
Jul‐10
Jan‐11
Jul‐11
Jan‐12
Jul‐12
Jan‐13
Jan‐07
Jul‐07
Jan‐08
Jul‐08
Jan‐09
Jul‐09
Jan‐10
Jul‐10
Jan‐11
Jul‐11
Jan‐12
Jul‐12
Jan‐13
250
Nitrate
mg/m3
200
150
100
50
0
60Jul‐06
50
Cond
mmho/cm 40
30
20
10
25Jul‐06
20
Water Temp
15
°C
10
5
Jul‐06 Jan‐07 Jul‐07 Jan‐08 Jul‐08 Jan‐09 Jul‐09 Jan‐10 Jul‐10 Jan‐11 Jul‐11 Jan‐12 Jul‐12 Jan‐13
Figure 8.
18
Water quality data for routine monitoring carried out by Hawke’s Bay Regional Council at
the Awatoto coastal monitoring site located approximately 5 km north of the Tukituki River
mouth. Data are for samples collected at the surface, and when the water column is
reasonably well mixed. The point of elevated chlorophyll-a (chl-a) indicated with an arrow
was on 18 Sept 2007, three days after the satellite image shown in Figure 5.
CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
4. PREDICTED CHANGES IN FLOWS AND NUTRIENT LOADING
The RWSS and Plan Change 6 are expected to lead to changes in irrigation as a
result of more efficient water storage, and, in turn, increased nutrient runoff from the
land as a function of changes in irrigation. Changes in river flows and nutrient loading
to the river and downstream coastal waters are therefore likely. River flows are
expected to be least affected downstream near the river mouth (Young et al. 2013).
Median monthly flows for modelled scenarios are forecasted to be relatively similar
from July to September, whereas at other times the effects of the proposed RWSS
may reduce monthly median flows by between 1 to 11% (Young et al. 2013).
Freshwater entering the coast influences hydrological processes, including the
stratification of the water column and estuarine circulation (outwelling plume, inwelling
marine water). The changes predicted for river flows are small (Waldron et al. 2012;
Young et al. 2013) and likely to have inconsequential effects on the hydrology of
the coastal receiving environment and are therefore not considered further.
The main issue to consider is the increase in nutrient runoff from the land, and the
subsequent increase in nutrient loading to the coastal receiving environment. With
regard to nutrients, it has been predicted that the RWSS and Plan Change 6 will result
in an increased intensity of agriculture and an associated increase in nitrogen and
phosphorus of 32% and 6%, respectively (Rutherford 2013). Phosphate is the nutrient
of primary concern in the river with regard to periphyton growth, whereas the main
concern with regard to nitrogen in the river relates to elevated levels of nitrate and
associated toxicity. Once near the river mouth and in coastal waters, nitrogen most
likely becomes limiting to primary production and algal growth. The level of increase in
nitrogen concentration reduces as you move downstream (Uytendaal & Ausseil 2013;
Young et al. 2013). This is likely in part due to assimilation of nitrate by periphyton and
possibly increased rates of denitrification.
Nitrate is clearly the dominant form of dissolved inorganic nitrogen in the river (Table
2), and a 32% increase in nitrogen loading means that a proportional increase in
nitrate concentrations are likely to follow. Based on observations of nitrate
concentrations near Red Bridge during winter (Figure 14A in Uytendaal & Ausseil
2013), a 32% increase in nitrogen may at times lead to nitrate concentrations of
around 2400 mg/m3 under this scenario. Depending on the time of year, rainfall
patterns, and river flows, a 32% increase in nitrate concentrations could equate to
large, periodic increases in the amount of nitrogen being transported within the
Tukituki River outwelling plume into Hawke Bay. The following section focuses on
assessing the nature and extent of environmental effects that could arise from such
increases in nitrogen loading.
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5. POTENTIAL EFFECTS ON THE COASTAL RECEIVING
ENVIRONMENT
The priority concern with regard to potential ecological effects on the coastal receiving
environment associated with the RMSS and Plan Change 6 is the loading of nutrients,
and in particular, dissolved inorganic nitrogen in the form of nitrate (NO3-N). Contrary
to concerns in the river, levels of nitrate loading will not result in concentrations in
coastal waters that are considered toxic to organisms. Due largely to dilution, the
range of concentrations in nearby coastal waters are roughly 10 times lower than
those in the river (Table 2); hence increases in the order of 32% are highly unlikely to
result in concentrations considered toxic to marine organisms.
Beyond toxicity effects, the most likely ecological effect to arise from increases in
nutrient concentrations relate to symptoms of eutrophication. One of the early
symptoms of eutrophication is an increased abundance of nuisance benthic
macroalgae such as sea lettuce (Ulva spp.). Such effects are more likely to occur in
shallow estuaries with low levels of flushing. The physical environment (wave action,
currents) and habitats adjacent to the Tukituki River mouth (sandy beaches and
sandy/gravely benthos) are unlikely to be conducive to blooms of nuisance
macroalgae. More likely to arise from increased nutrient loading is enhanced growth
and abundance of phytoplankton in the water column. With an increase in
phytoplankton abundance, water colour and clarity may also change. The magnitude
of effects on primary producers such as phytoplankton will depend on a number of
factors, including the extent to which nutrient concentrations increase and the amount
of light available for photosynthesis (which relates to season as well as water clarity).
An approximation of the likely extent of change in nutrient concentrations assists in
gauging the potential for biological effects. A 32% increase in nitrogen loading
equates to an increase of 426 tonnes on average per year, which will increase the
total input from 1,330 to 1,756 tonnes of nitrogen. Based on the area of the Tukituki
River catchment and average river flows, the current and projected nutrient loads
represent a significant input of nitrogen to Hawke Bay when compared to other
similarly sized catchments that are less developed. The predicted annual total N load
is more than double that of the Motueka River catchment (see Table 2) and
considerably more than the Tutaekuri / Ngaruroro / Clive Rivers (Figure 4).
Placed within the context of Hawke Bay (assuming no changes in the other rivers or
outfalls), the predicted increase in nitrogen loads from the Tukituki River represents a
~4% increase in total annual inputs for all rivers and the East Clive and NCC
wastewater outfalls combined, and a ~9% increase for the southern region that
includes inputs from the Tutaekuri / Ngaruroro / Clive, Tukituki and Maraetotara Rivers
and the East Clive and NCC wastewater outfalls (see inset in Figure 4). This latter
increase is the most relevant for the area down current of the Tukituki inflow since
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CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
these inputs have the potential to mix and cumulatively influence downstream coastal
areas and offshore waters.
While useful for comparing overall inputs between various rivers and point source
discharges (outfalls), annual loads provide no context with regard to the temporal
variability in nutrient inputs. As was demonstrated for the Motueka River, the actual
amount of additional nutrient loading from the Tukituki River will vary over time
depending on levels of rainfall and flooding which vary as a function of season and
among years (Gillespie et al. 2011). In lieu of using a sophisticated hydrodynamic
model for estimating transport and mixing processes, rough estimates can be made of
potential downstream nitrate concentrations in the coastal environment as a result of
increased river flows and nutrient loading. Assuming a 32% increase in nitrogen
loading, river nitrate concentrations at Red Bridge may increase from about 1,800
mg/m3 to 2,400 mg/m3 during the winter months (when nutrient concentrations are
highest), and from about 250 mg/m3 to 330 mg/m3 during summer months (Uytendaal
& Ausseil 2013). Using these concentrations and either mean river flow (43 m3/s) or a
high flood flow (average of 800 m3/s over 24 hours), estimates of downstream
concentrations can be made assuming the plume has covered an area of 100 km2
over a 24 hour period (Figure 9). In reality, the area and depth that is influenced by
the plume will vary in response to winds, currents and wave action and will increase
over time. The depth of mixing across the area will be a function of wave action and
currents and will also vary, perhaps between 2 and 10 m within the first 24 hours.
These are reasonable assumptions based on the behaviour of the Motueka River
plume, which is known to cover areas in excess of 100 km2 within 24 to 48 hours
following a moderate flood (see Cornelisen et al. 2011 for example).
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AUGUST 2013
REPORT NO. 2392 | CAWTHRON INSTITUTE
900
120
Winter
Summer
800
100
Solid = current
Dashed = change
700
80
600
500
60
Flood flow (800 m3/s)
Nitrate‐N 400
(mg/m3)
Flood flow (800 m3/s)
40
300
200
20
Mean flow (43 m3/s)
100
0
0
2
Figure 9.
Mean flow (43 m3/s)
4
6
Mixing depth (m)
8
10
2
4
6
Mixing depth (m)
8
10
Projected nitrate concentrations within the Tukituki River outwelling plume associated
with river inputs following a period of high flooding (average of 800 m3/s) and at mean
flow (43 m3/s) over a 24-hour period. Nitrate concentrations of river inputs were based on
the highest observed at Red Bridge in winter (1,800 mg/m3) and summer (250 mg/m3;
Uytendaal & Ausseil 2013) with the change based on a 32% increase in these
concentrations.
The simple model outputs summarised in Figure 9 are intended to illustrate two
important points:
1.
2.
That the incoming river water will be rapidly diluted as it progressively mixes with
the much larger volume of seawater in Hawke Bay.
That the predicted change in nitrogen loading will have the largest impact during
winter months when nitrate concentrations are highest and during flood flows
when the majority of inputs occur.
Assuming the river plume becomes reasonably well mixed (to 5 m depth) when
covering an area of 100 km2, the potential for increases in nitrate concentration range
from about 1 to 10 mg/m3 during summer months for dry and flood conditions,
respectively, and from about 5 to 100 mg/m3 during winter months for dry and flood
conditions, respectively.
There are no available data to directly ground-truth the accuracy of these estimates;
however, predicted nitrate concentrations following flood flows in winter and when the
water is well mixed to a depth of 8-10 meters (about 200 mg/m3 in Figure 9) fall within
the range of the existing peaks in NO3-N (range 150 to 280 mg/m3) observed in
waters off the Tutaekuri / Ngaruroro / Clive River mouth when the water column (in 810 m depth) is well mixed (Figure 9). This comparison suggests that the estimates
provided here are most likely conservative since they are only based on river inputs
from the Tukituki River which are lower than the combined inputs of the Tutaekuri /
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CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
Ngaruroro / Clive Rivers and nearby outfalls. The estimates also do not include nitrate
already present in seawater.
The potential for production of carbon by primary producers based on a direct
conversion of the molar mass of NO3-N to Carbon (C) (Redfield 1934) suggests that
increases in the order of 5 to 100 mg NO3-N/m3 could theoretically fuel phytoplankton
production of 28 to 570 mg C/m3. Based on an approximate 50:1 ratio of carbon to
chl-a in phytoplankton, the increase in phytoplankton equates to a potential increase
in chl-a concentrations of 0.5 to 11 mg chl-a/m3. These levels of increase in chl-a
concentrations based on a direct conversion of nitrate from the river to chl-a
concentrations in downstream coastal waters are unlikely to be realised due to
number of ecological processes (e.g., microbial uptake, denitrification, zooplankton
grazing, filter feeding, etc.) and other factors driving levels of primary production, such
as light availability. Indeed, an increase in chl-a concentrations of 11 mg chl-a/m3
solely due to the incremental increase in Nitrate loading would be very large when
compared to available water column data for the region. As an example, chl-a levels
rarely exceed 3 mg chl-a/m3 downstream of the Tutaekuri / Ngaruroro / Clive Rivers
and within proximity of the East Clive wastewater outfall (Figure 8), where combined
nutrient inputs are greater than current and projected inputs for the Tukituki River (see
Figure 4).
Based on data from the OOA programme (Heasman et al. 2009) and 2012 data from
the HAWQi buoy, chl-a concentrations in wider Hawke Bay also cover a similar range
as observed at the Awatoto monitoring site. Offshore concentrations of chl-a have
been observed to be greater at depths of 20 m than in near-surface waters (Heasman
et al. 2009). At these depths and levels of mixing, increases in nitrate loading from
rivers would likely represent a very small contribution to the total nitrogen pool in
comparison to oceanic inputs to Hawke Bay.
The above estimates of nutrient concentrations in downstream coastal waters are
based on a given area / volume of water. Hawke Bay is not a closed system, and the
extent to which coastal concentrations increase, the duration of the impact and the
area affected will ultimately depend on a number of factors, including the magnitude of
flooding, and physical (currents, mixing) and biological processes (including lag times
in phytoplankton production). The latter is dependent on factors such as light
availability, which in turn can be affected by the river plume as a function of turbidity.
When rivers are in flood, the turbidity of near shore waters increases, reducing the
level of light that can penetrate the water and in turn the extent to which primary
producers photosynthesise and utilise nutrients. Accordingly, while nutrient
concentrations may be greater with closer proximity to a river mouth, the ecological
consequences of such increases will depend on the ability of primary producers such
as phytoplankton to assimilate and use the excess nitrogen. This assimilative capacity
will in turn be minimised due to higher levels of turbidity and light attenuation. As river
plumes move offshore, nutrients are more rapidly assimilated by phytoplankton as
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REPORT NO. 2392 | CAWTHRON INSTITUTE
waters become clearer and light is more available (Devlin and Brodie 2005; Brodie et
al. 2010; Davies 2004; Dagg et al. 2004). However, nutrients from the river will also
become more and more diluted with distance due to mixing with seawater, which will
usually have much lower nitrate concentrations than the incoming river water.
A 32% increase in nitrogen loading predicted to occur in future under the RMSS and
Plan Change 6 represents a small portion of the cumulative loading of nitrogen from
multiple rivers, outfalls and (likely much larger) oceanic inputs in Hawke Bay.
Consequently, added nutrients from the Tukituki River would enhance (rather than
drive) levels of primary production observed in Hawke Bay. Increased nitrate
concentrations are most likely to influence important biological processes
(phytoplankton production and follow-on food web effects) when the river floods for a
prolonged period followed by a period of high light availability (see Figure 6 and
Gillespie et al. 2011). This commonly occurs during late winter, early spring months,
when nitrate concentrations in the river are high, and phytoplankton production in the
water column is increasing. However, recent July 2013 records from the HAWQi
chlorophyll sensor (5 m depth) demonstrate the potential for elevated chl-a
concentrations (~ 11 mg chl-a/m3) during winter months. It is during these times that
the increased nutrient loading may potentially result in a lagged increase in primary
production. At times, increases in nutrients may enhance blooms of certain
phytoplankton species such as the non-toxic dinoflagellate Akashiwo sanguinea
known to cause ‘red tides’ and potentially harmful (toxin producing) phytoplankton
species (e.g. Pseudo-nitzschia spp.).
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CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
6. CONCLUSIONS AND RECOMMENDATIONS
Increased nutrient concentrations in coastal waters associated with the RWSS and
Plan Change 6 will be highly variable and will decrease with distance from the river
mouth and with time as the river plume mixes and becomes more diluted with
seawater. Subsequently, the effects of increased nutrient loading on the coastal
receiving environment are likely to be far removed from the river mouth and be limited
to periodic enhancement of phytoplankton production in Hawke Bay. The ecological
implications of estimated increases in nitrogen loading from the Tukituki River are
difficult to predict, since increases in downstream nutrient concentrations are only one
of many variables that influence biological and biogeochemical processes in the
marine environment. Furthermore, increases in nitrate concentration are unlikely to
result in immediate biological effects, such as a measurable increase in phytoplankton
biomass. Such responses will be lagged over a period of days to weeks and
significantly dampened as the river plume moves offshore and is further diluted with
seawater. Consequently, the effects on the wider marine environment arising from
incremental increases in nutrient loading from the Tukituki River will be difficult to
isolate from changes occurring in response to the cumulative loading of nutrients from
multiple rivers, outfalls and (likely much larger) oceanic inputs.
Because of the cumulative nature of land-use effects on marine ecosystems, it is
particularly important to maintain long-term datasets for establishing baseline
conditions. This would facilitate assessment of the effects of cumulative stressors
(including anthropogenic nutrient loading from multiple sources) against a backdrop of
natural variability. The following recommendations are made in relation to monitoring
and understanding downstream effects of increased nutrient loading associated with
RWSS and Plan Change 6 and placing effects within the context of the wider Hawke
Bay ecosystem.


Conduct routine water quality monitoring off the mouth of the Tukituki River that
in turn contributes to a wider coastal monitoring programme for assessing
cumulative environmental change in Hawke Bay; consider increasing sampling
frequency when the potential for effects (e.g. phytoplankton blooms) are likely to
be greatest (late winter to early spring months). In addition, consider collecting
depth-integrated samples to minimise variability associated with changes in
water column stratification.
Chlorophyll-a is not always a good predictor of phytoplankton concentrations,
which is partly due to variations in the amount of chl-a contained within different
species and at different stages of phytoplankton development. Consider use of
integrated, multi-parameter indices (e.g. see Bricker et al. 2003; Ferreira et al.
2007) and possibly the addition of phytoplankton species composition to the mix
of variables used to assess abundance/frequency of potentially harmful
phytoplankton.
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AUGUST 2013


REPORT NO. 2392 | CAWTHRON INSTITUTE
Remote sensing methods and technologies continue to advance and enable
visualisation and assessment of large-scale coastal processes. It is
recommended that HBRC implement the use of satellite imagery as an
additional tool for monitoring temporal and spatial trends in water quality
conditions in the vicinity of the Tukituki River mouth and wider Hawke Bay.
Consider developing a coastal hydrology model (perhaps building on the model
developed for the East Clive wastewater outfall) to provide a better
understanding of transport processes in Hawke Bay and the behaviour of the
Tukituki River outwelling plume within a context that includes other river plumes
and point source discharges (outfalls).
Rivers are an important, natural source of nutrients to coastal waters; however, in the
case of the Tukituki catchment, the loading of nutrients to the coastal environment is
much higher than less developed catchments; these inputs are expected to increase
further under Plan Change 6. Increases in nitrate concentrations based on forecasted
levels are not expected to have a large affect over and beyond current conditions
downstream of the Tukituki River. However, the levels of nitrogen loading have the
potential to enhance important biological processes in Hawke Bay and efforts should
be made to better understand environmental implications of further increasing
nitrogen loading in the region within a whole-of-ecosystem context. This includes the
development and setting of appropriate limits for coastal receiving waters that take
into account cumulative inputs and the wider ecosystem processes.
The development of methods and tools for establishing limits lie outside the scope of
this report, and are best dealt with on a National level and through working with other
Regional Councils (and central government) to develop frameworks and standards for
coastal receiving waters. Nonetheless, implementation of the above recommendations
alongside wider efforts by HBRC to monitor coastal waters (e.g. additional sites along
the coast and time-series data collected by remote platforms such as HAWQi) will
enable the council to work toward a whole-of-ecosystem approach to monitoring and
managing its catchments and downstream coastal waters in Hawke Bay.
7. ACKNOWLEDGEMENTS
Assistance from Robyn Dunmore with the generation of Figure 4 and feedback from
Paul Gillespie was greatly appreciated. I also would like to thank Anna MadaraszSmith at HBRC for providing related reports and monitoring data.
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8. REFERENCES
Bradford JM, Ridgeway NM, Robertson DA, Stanton BR 1980. Hydrology, plankton
and nutrients in Hawke Bay, September 1976. New Zealand Oceanographic
Institute Field Report. No.15.
Bricker SB, Ferreira JG, Simas T 2003. An integrated methodology for assessment of
estuarine trophic status. Ecological Modelling. 169: 39-60.
Brodie JE, Schroeder T, Rohde K, Faithful JW, Masters B, Dekker A, Brando V,
Maughan M 2010. Dispersal of suspended sediments and nutrients in the
Great Barrier Reef lagoon during river discharge events: conclusions from
satellite remote sensing and concurrent flood plume sampling. Marine and
Freshwater Research. 61:651-664.
Chiswell S 2002. Wairarapa Coastal Current influence on sea surface temperature in
Hawkes Bay, New Zealand. New Zealand Journal of Marine and Freshwater
Research 36: 267–279.
Cloern JE 2001. Our evolving conceptual model of the coastal eutrophication problem.
Marine Ecology Progress Series 210: 223-253.
Cornelisen CD 2010. Recommendations for monitoring the coastal marine area in the
Manawatu-Wanganui region. Prepared for Horizons Regional Council.
REPORT No.1752. 23 p. plus appendices
Cornelisen CD, Gillespie PA, Kirs M, Young RG, Forrest RW, Barter PJ, Knight BR,
Harwood VJ 2011. Motueka River plume facilitates transport of ruminant
faecal contaminants into shellfish growing waters, Tasman Bay, New Zealand.
New Zealand Journal of Marine and Freshwater Research. 45: 477-495.
Dagg, M.J, Bianchi, T, McKee, B., Powell, R 2008. Fates of dissolved and particulate
materials from the Mississippi River immediately after discharge into the
northern Gulf of Mexico, USA, during a period of low wind stress. Continental
Shelf Research 28, 1443–1450.
Davies P 2004. Nutrient processes and chlorophyll in the estuaries and plume of the
Gulf of Papua. Continental Shelf Research 24, 2317–2341.
Degobbis D 1989. Increased eutrophication of the northern Adriatic Sea, second act.
Marine Pollution Bulletin 20: 452-457.
Devlin M, Brodie J 2005. Terrestrial discharge into the Great Barrier Reef lagoon:
nutrient behaviour in coastal waters. Marine Pollution Bulletin 51, 9–22.
Díaz R, Rabalais NN, Breitburg DL 2012. Agriculture’s Impact on Aquaculture:
Hypoxia and Eutrophication in Marine Waters. Report under Directorate for
Trade and Agriculture for the OECD (Organisation for Economic Co-operation
and Development). 45 pp.
27
AUGUST 2013
REPORT NO. 2392 | CAWTHRON INSTITUTE
Eppley RW, Rogers JN, McCarthy JJ 1969. Half-saturation constants for uptake of
nitrate and ammonium by marine phytoplankton. Limnology and Oceanography
14: 912-920.
Ferreira JG, Vale C, Soares CV, Salas F, Stacey PE, Bricker SB, Silva MC, Marques
JC 2007. Monitoring of coastal and transitional waters under the EU water
framework directive. Environmental Monitoring and Assessment 135 (1-3):
195-216.
Gibbs MM 2001. Sedimentation, suspension, and re-suspension in Tasman Bay and
Beatrix Bay, New Zealand, two contrasting coastal environments which
thermally stratify in summer. New Zealand Journal of Marine and Freshwater
Research. 35: 951-970.
Gibbs MT, Hobday AJ, Sanderson B, Hewitt CL 2006. Defining the seaward extent of
New Zealand’s coastal zone Estuarine, Coastal and Shelf Science. 66 : 240254.
Gillespie P, Forrest R, Knight B, Cornelisen C, Young R 2011. Variation in nutrient
loading from the Motueka River into Tasman Bay, 2008: Implications for the
river plume ecosystem. New Zealand Journal of Marine and Freshwater
Research 45.
Hakanson L 1994. A review of effect-dose-sensitivity models for aquatic ecosystems.
Internationale Revue der Gesamten Hydro-biologie 79, 621-667.
Heasman K, Keeley N, Roberts B, Batstone C, Knight B, Mussely H 2009. Feasibility
of Open Ocean Aquaculture in New Zealand. Prepared for the Foundation for
Research, Science & Technology and Offshore Technology Development
(OTD) Ltd. Cawthron Report No. 1481. 213 p.
Heath RA 1976. Broad classification of New Zealand inlets with emphasis on
residence times. New Zealand Journal of Marine and Freshwater Research 10
(3): 429–444.
Howarth RW, Marino R 2006. Nitrogen as the limiting nutrient for eutrophication in
coastal marine ecosystems: Evolving views over three decades. Limnology
and Oceanography 51 (1): 364-376.
Hume T, 2013. Key statistics and flushing time for Tukituki Estuary. Letter dated 6
May 2013 for Hawke’s Bay Regional Council. 2 p.
MacKenzie L 2004. River inputs, re-mineralisation and the spatial and temporal
distribution of inorganic nutrients in Tasman Bay, New Zealand. New Zealand
Journal of Marine and Freshwater Research 38: 681-704.
MacKenzie L, Adamson J 2004. Water column stratification and the spatial and
temporal distribution of phytoplankton biomass in Tasman Bay, New Zealand.
New Zealand Journal of Marine and Freshwater Research 38: 705-728.
28
CAWTHRON INSTITUTE | REPORT NO. 2392
AUGUST 2013
McWilliams R 2012. Discharge Consent Annual Report: Wastewater services annual
report 2011 to 2012. Hastings District Council.
MetOcean Solutions Ltd 2011. Hawke’s Bay October 2011 Sidescan Sonar Survey
Interpretation. Report prepared for Hawke’s Bay Regional Council. MetOcean
Solutions Ltd Report P0088-01. 8 p.
MetOcean Solutions Ltd, Cawthron Institute 2010. East Clive wastewater outfall:
Hydrodynamic modelling. Report prepared for Hastings District Council. 97 p.
Paerl H W 2006. Assessing and managing nutrient-enhanced eutrophication in
estuarine and coastal waters: Interactive effects of human and climatic
perturbations. Ecological Engineering 26: 40-54.
Redfield AC 1934. On the proportions of organic derivations in sea water and their
relation to the composition of plankton. In University Press of Liverpool. James
Johnstone Memorial Volume. (ed. R.J. Daniel): 177-192.
Ridgeway NM, Stanton BR 1969. Some hydrological features of Hawke Bay and
nearby shelf waters. New Zealand Journal of Marine and Freshwater Research
3: 545-559.
Rutherford K 2013. Effects of land use on streams – Phase 2 modelling studies in the
Tukituki River, Hawke’s Bay. Prepared for Hawke’s Bay Regional Investment
Company Limited. NIWA report No: HAM2013-026. 90 p.
Smith V, Tilman G, Nekola J 1999. Eutrophication: impacts of excess nutrient inputs
on freshwater, marine, and terrestrial ecosystems. Environmental pollution 100
(1-3): 179-196.
Tett P, Edwards V, 2002. Review of harmful algal blooms in Scottish coastal waters.
School of Life Sciences, Napier University, Edinburgh. 120 p.
Uytendaal A, Ausseil O 2013. Tukituki Catchment: Recommended Water Quality
Limits and Targets for the Tukituki Plan Change 6. Resource Management
Group Technical Report. EMT 13/04. HBRC Plan No. 4463. Hawke’s Bay
Regional Council.
Waldron R, Baalousha H 2012. Assessment of potential effects of the proposed
Ruataniwha Water Storage scheme on groundwater and surface water
resources in the Tukituki River Catchment. Hawke's Bay Regional Council. 60
p. plus appendices.
Young RG, Allen C, Shearer KA, Doehring K, Berkett N, Holmes R, Hay J 2013.
Ruataniwha Water Storage Scheme – Aquatic Ecology Assessment of Effects.
Prepared for Hawke’s Bay Regional Investment Company Limited. Cawthron
Report No. 2307. 194 p. plus appendices.
Zeldis J 2008. Origin and processing of nutrients in Golden and Tasman Bays.
Tasman District Council Envirolink Advice Grant and Client Report May 2008
CHC 2008 052 (Client Tasman District Council).
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