Changes in shallow lake functioning: response to climate change and nutrient reduction

Abstract

Nutrient, phytoplankton and zooplankton dynamics have been monitored intensively at Loch Leven for 34 years. The data collected reveal a decline in phosphorus concentrations, following major reductions in external nutrient loading, and large changes in its seasonal availability, particularly in recent years. More striking has been an increasing trend in water temperatures, annual means increasing by about 1 °C over the 34 years, with even greater increases observed for winter and spring periods. In response to these changes, there has been a general pattern of decline in measures of phytoplankton biomass (chlorophyll_a concentrations). Closer inspection reveals that the detailed response is, however, not so consistent with the environmental trends. A significant decline in chlorophyll_a concentrations occurred early on in the time series, before major reductions in nutrient availability. Correlation analysis revealed that this decline was associated with the re-appearance of Daphnia grazers. Further declines are only apparent in very recent years; correlation analysis and comparison of trends suggest that these were associated with the observed decline in nutrient concentrations. There was little correlation and no consistent relationship between annual measures of chlorophyll_a and water temperature, but winter mean values did show a consistent positive relationship. Spring Daphnia densities showed an even stronger, and significant, positive relationship with spring water temperatures. It is clear from this that the scale of climate change predicted in the future will significantly alter the functioning of shallow lakes and seasonal patterns in water quality. This is a particular concern in Europe with the implementation of the EC Water Framework Directive, as if the net effect of global warming on annual measures of water quality is negative, the principle aim of `good status' in all surface waters by 2016 will become even more difficult to achieve.