Publications: Research reports and publications

Land use changes in the Tukituki River catchment: Assessment of environmental effects on coastal waters

18 August, 2013
Cawthron Report 2392. Prepared for Hawke's Bay Regional Council.

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 outwelling 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 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 data sets 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).