The sensitivity of phytoplankton in Loch Leven (U.K.) to changes in nutrient load and water temperature

1. Loch Leven is a shallow, eutrophic lake in Scotland, U.K. It has experienced much change over the 30 years that it has been studied; this has primarily been due to reduced nutrient loads to the lake through active catchment management. Its recovery has been slow and, therefore, we used a phytoplankton community model (PROTECH) to test its sensitivity to changing nutrient loads and water temperature. 2. PROTECH was initialized to simulate the observed phytoplankton community in 1995 and was then repeatedly run through a combination of step-wise changes in water temperature and nutrient load (two treatments were simulated for nutrient load: one changing both nitrate and phosphorus, and one changing just phosphorus). The effect on total chlorophyll-a concentration, cyanobacteria abundance and phytoplankton diversity was examined. 3. Whilst changes in temperature had little effect, variations in the nutrient load produced a range of responses. Increasing only the phosphorus load caused a large increase in Anabaena abundance and total chlorophyll-a concentration. However, the opposite response was recorded when nitrate load was changed as well, with Anabaena increasing its biomass under reduced nutrient load scenarios. 4. The key factor determining the type of response appeared to be nitrogen availability. Anabaena, a nitrogen fixer, could exploit the phosphorus resource of Loch Leven under limiting nitrogen conditions, allowing it to dominate under most of the scenarios tested apart from those supplying extra nitrogen to the lake. The model predictions agree with the observed data, which show that Anabaena continues to dominate the summer phytoplankton bloom in Loch Leven despite the considerable reduction in phosphorus supply from the catchment. This research provides a possible explanation for this.