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Published: 2015-02-18 00:00:00

Two new reports on nutrients that can support algal growth in the Waikato and Waipa rivers indicate some potentially positive developments for protecting and restoring water quality in the region.

The information - presented to a recent meeting of Waikato Regional Council’s environmental performance committee - will be looked at by the Healthy Rivers: Plan for Change/Wai Ora: He Rautaki Whakapaipai project, which is considering a regional plan change to better protect water quality in the rivers.

The reports focus on the nutrients nitrogen (N) and phosphorus (P) that can stimulate algal growth, which causes water clarity to decline, interferes with aquatic life, and can stimulate unsightly and potentially toxic algal blooms.

N and P in the river come from a range of sources, including factories, wastewater treatment plants, and run-off from farms.

The first report to the committee dealt with a council study of nutrients in the Waikato and Waipa rivers, while the second was a council-commissioned NIWA study related to the Waikato River.

Some 61 per cent of N and 45 per cent of P in the rivers was likely to come from agricultural sources, the report on the council’s study of sources of the nutrients in the two rivers from 2003-12 said.

The report indicated that overall P was about 12 per cent lower than in earlier years.

Downstream of Hamilton, this P improvement was probably due to reductions in discharges from the city’s wastewater plant, said council water scientist Bill Vant, while reductions at sites further up the river may be due to better dairy effluent management and erosion prevention measures.

This information underpins an indication in a second report by NIWA for the council that the threat of increased nitrogen alone causing algal growth in the rivers may be less than previously feared under current conditions, Mr Vant said.

Council monitoring had shown average concentrations of N in the Waikato River had increased in recent years, raising fears of more algal growth. However, P – which also supports algal growth - had decreased at several locations and it seemed this had contributed to observed reductions in concentrations of algae. Overall, said Mr Vant, although there have been recent incidents of algal blooms, over the period of the study it is noted that algae was down recently at five of 10 Waikato River monitoring sites and had not increased at any of them.

The report to the committee on NIWA’s study said recent testing of water samples showed algal growth often responded to the experimental addition of both N and P, and occasionally to just P.

“But there was little or no response to the addition of N alone,” the report said.

This may indicate, generally speaking, that waterways now don’t need more N to produce algae and that recent falls in the amount of P in the river had contributed to actual reductions in algae.

The report also noted that zooplankton in the river, including an introduced North American species, could “graze” on algae, thereby helping reduce further the impacts of N and P on algal growth. Mr Vant stressed, however, that this North American zooplankton couldn’t be counted on to stay in our waterways.

“This new information will be assessed by the multi-agency technical leaders group of experts which is assisting the Healthy Rivers/Wai Ora process,” said the council’s science and strategy director Tracey May.

“The data is potentially very important for deciding how we best go about establishing the need for any limits on land-based activities, such as farming, to help protect and restore the health of the Waikato River.”