More research is needed on the impact land use is having on the Tomahawk Lagoon, a Dunedin water scientist says.
The lagoon has recently experienced potentially harmful algal blooms, which an Otago Regional Council report found were caused by high nutrient levels in the lagoon due to past farming practices.
University of Otago department of zoology research fellow Marc Schallenberg said it was not clear whether nutrient inputs from the catchment had much impact on algal blooms.
He believed more research was needed before money was invested to restore the lagoon.
Dr Schallenberg agreed with the Otago Regional Council's finding that a lot of the nutrients in the lagoon would have come from past land-use practices. However, he said the ORC was not able to rule out continuing effects of catchment land use on the lagoon.
Dr Schallenberg believed monitoring was required to establish what proportion of the nutrient problem was caused by historical nutrients versus present-day nutrient leaching, especially after rain.
The ORC report was based on past research on the lagoon and there was no monitoring going on now except for public health issues around algal blooms, he said.
It was possible to put sensors in the lagoon to measure things like phytoplankton levels, salinity, oxygen levels, and temperature. That information would show if the appearance of algal blooms coincided with any other factors, such as heavy rains, or winds stirring up the lake bed. This would give researchers some idea of what was causing the blooms.
The sensors could cost a few thousand dollars and would require infrequent maintenance, he said.
Monitoring the lagoon catchment by sampling the inflowing stream would also make it possible to directly measure for nutrient input.
Dr Schallenberg believed it would be possible for an honours or masters student to do the work if funding was available.
ORC policy and resource planning director Fraser McRae said the type of monitoring suggested was not needed as the ORC already knew the problem was that the lagoon was nutrient rich. Land use had not changed significantly in recent years and external nutrients entering the lagoon were an ''ongoing process''.
However, Mr McRae believed Plan Change 6A, a recent change to the Otago Water Plan which placed limits on how much phosphorus, nitrogen, E. coli, and sediment landowners could discharge into water, would make a big difference in the amount of nutrients going into the lagoon.
- by Jonathan Chilton-Towle
