Comparing effects of oligotrophication and upstream hydropower dams on plankton and productivity in perialpine lakes

In recent decades, many perialpine lakes have been affected by oligotrophication due to efficient sewage treatment and by altered water turbidity due to upstream hydropower operations. Such simultaneous environmental changes often lead to public debate on the actual causes of observed productivity reductions. We evaluate the effects of those two changes by a combined approach of modeling and data interpretation for a case study on Lake Brienz ( Switzerland), a typical oligotrophic perialpine lake, located downstream of several hydropower reservoirs. A physical k-epsilon scheme and a biogeochemical advection-diffusion-reaction model were implemented and applied for several hypothetical scenarios with different nutrient loads and different particle input dynamics. The simulation results are compared to long-term biotic data collected from 1999 to 2004. The analysis shows that enhanced nutrient supply increases the nutritious value of algae, stimulating zooplankton growth, while phytoplankton growth is limited by stronger top-down control. Annually integrated productivity is only slightly influenced by altered turbidity, as phosphorous limitation prevails. Simulations indicate that the spring production peak is delayed because of increased turbidity in winter caused by upstream hydropower operation. As a consequence, the entire nutrient cycle is seasonally delayed, creating an additional stress for zooplankton and fish in the downstream lake.