REPORT NO LAND-USE CHANGES IN THE TUKITUKI RIVER CATCHMENT: ASSESSMENT OF ENVIRONMENTAL EFFECTS ON COASTAL WATERS

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1 REPORT NO LAND-USE CHANGES IN THE TUKITUKI RIVER CATCHMENT: ASSESSMENT OF ENVIRONMENTAL EFFECTS ON COASTAL WATERS

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5 CAWTHRON INSTITUTE REPORT NO 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 iii

6 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE 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). iv

7 CAWTHRON INSTITUTE REPORT NO AUGUST 2013 TABLE OF CONTENTS 1. INTRODUCTION Scope and objectives BACKGROUND Rivers and the coastal environment Coastal issues associated with increased nutrients DESCRIPTION OF THE EXISTING ENVIRONMENT Hawke Bay Circulation Nutrient loading and water quality Tukituki coastal receiving environment Coastal water quality PREDICTED CHANGES IN FLOWS AND NUTRIENT LOADING POTENTIAL EFFECTS ON THE COASTAL RECEIVING ENVIRONMENT CONCLUSIONS AND RECOMMENDATIONS ACKNOWLEDGEMENTS REFERENCES v

8 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE LIST OF FIGURES Figure 1. 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 Figure 2. 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 Figure 3. Example of modelled depth-averaged currents over Hawke Bay Figure 4. Average river flows and annual nitrogen loading for rivers and streams discharging more than 20 tonnes N per annum into Hawke Bay Figure 5. Chlorophyll-a concentrations in Hawke Bay on 15 September 2007 based on MODIS Aqua data Figure 6. Hawke Bay following a dry summer period and a wet and wild winter period Figure 7. Example of modelled depth-averaged currents near the Tutaekuri / Ngaruroro and Tukituki Rivers Figure 8. 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 Figure 9. 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 LIST OF TABLES Table 1. Table 2. 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 vi

9 CAWTHRON INSTITUTE REPORT NO AUGUST 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 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 1

10 AUGUST 2013 REPORT NO 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. 2

11 CAWTHRON INSTITUTE REPORT NO AUGUST 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. 3

12 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE Figure 1. 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 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) 4

13 CAWTHRON INSTITUTE REPORT NO AUGUST 2013 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 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 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. 5

14 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE Figure 2. 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). 1 Ocean sources to coastal waters include dissolved nutrients through breakdown of organic matter, nitrification, and onwelling / upwelling of nutrient rich deeper waters. 2 Atmospheric 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 6

15 CAWTHRON INSTITUTE REPORT NO 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 NO 3 -N + NH 4 -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). 7

16 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE Table 1. 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/m 3 ) TP (mg/m 3 ) Chl a (mg/m 3 ) SD (m) Oligotrophic < 260 < 10 < 1 > 6 Mesotrophic Eutrophic Hypertrophic > 400 > 40 > 5 < 1.5 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/m 3. 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). 8

17 CAWTHRON INSTITUTE REPORT NO AUGUST DESCRIPTION OF THE EXISTING ENVIRONMENT 3.1. Hawke Bay Hawke Bay is a large coastal water body spanning an area of 2,950 km 2 (at high tide) and with a volume of 1.8 x m 3 (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 ( 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 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). 9

18 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE Figure 3. 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) 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 5- year period as a function of variability in rainfall and river flows (Gillespie et al. 2011). 10

19 CAWTHRON INSTITUTE REPORT NO AUGUST 2013 Wairoa River Te Kiwi Stream Mohaka River Waitaha Stream Taehaenui River 10 m Nuhaka River 20 m 30 m Aropaonui River OOA research site Pakuratahi Stream HAWQi buoy Esk River TNC Rivers Awatoto monitoring site Tukituki River Maraetotara River Average flow (m³/sec) m 100 m Km Napier City outfall TNC Rivers East Clive outfall Tukituki River Maraetotara River Te Kiwi Stream Wairoa River Taehaenui River 10 m Nuhaka River 20 m Mohaka River Waitaha Stream 30 m Aropaonui River Pakuratahi Stream Esk River TNC Rivers Tukituki River Maraetotara River Total Nitrogen (tonnes/annum) , m 100 m Km Figure 4. 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. 11

20 AUGUST 2013 REPORT NO 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/m 3, peaking between 2 and 6 mg chl-a/m 3 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/m 3 and peaked at 2.3 mg chl-a/m 3. 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). 12

21 CAWTHRON INSTITUTE REPORT NO AUGUST 2013 Figure 5. 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. 13

22 AUGUST 2013 REPORT NO 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 km 2 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). 14

23 CAWTHRON INSTITUTE REPORT NO AUGUST 2013 Figure 7. 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) 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 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). 15

24 AUGUST 2013 REPORT NO 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). 16

25 CAWTHRON INSTITUTE REPORT NO AUGUST 2013 Table 2. 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 Catchment area (km 2 ) 2,502 2,058 River flows (m 3 /s) Mean Median Annual flood peak 1, Total N loading (tonnes/yr) 1, Nutrient concentrations (mg/m 3 ) Nitrite-Nitrate-N Ammonium-N Total-N Phosphorus (SRP) River 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) 17

26 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE Chl a mg/m 3 Nitrate mg/m 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 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 Cond mmho/cm 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 Water Temp C 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. 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. 18

27 CAWTHRON INSTITUTE REPORT NO AUGUST 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/m 3 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. 19

28 AUGUST 2013 REPORT NO CAWTHRON INSTITUTE 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 (NO 3 -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 20

29 CAWTHRON INSTITUTE REPORT NO 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/m 3 to 2,400 mg/m 3 during the winter months (when nutrient concentrations are highest), and from about 250 mg/m 3 to 330 mg/m 3 during summer months (Uytendaal & Ausseil 2013). Using these concentrations and either mean river flow (43 m 3 /s) or a high flood flow (average of 800 m 3 /s over 24 hours), estimates of downstream concentrations can be made assuming the plume has covered an area of 100 km 2 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 km 2 within 24 to 48 hours following a moderate flood (see Cornelisen et al for example). 21