Lake-wide distributions of temperature, phytoplankton, zooplankton, and fish in the pelagic zone of a large lake

We studied three-dimensional distribution patterns of temperature, phyto- and zooplankton, and fish in the large, prealpine Lake Constance during spring 2007. A strong westerly wind induced an intense eastward displacement of epilimnetic water and upwelling of hypolimnetic water in the western part of the lake. This led to the formation of an internal front separating cold, hypolimnetic water depleted of chlorophyll in the western part from epilimnetic, warm water with high chlorophyll concentrations in the eastern part. Hydroacoustic detection of zooplankton (by Acoustic Doppler Current Profiler) and juvenile fish (by echosounding) revealed both to be passively transported by the wind. Consequently, zooplankton and fish showed comparable horizontal distributions as temperature and chlorophyll. During periods of low wind velocities (< 6 m s(-1)), water temperature was more evenly distributed, whereas phytoplankton distribution was still heterogeneous, probably because of local differences in resource supply. The relative influence of biotic factors for the distribution of organisms increased when external forcing was low. At periods with weak wind forcing, phytoplankton typically showed highest concentrations in the metalimnion, where zooplankton also aggregated in thin layers. In conclusion, we found spatial distributions of temperature and organisms to be strongly controlled by wind forcing when wind velocities were sufficiently high, whereas the importance of internal biotic factors for distribution of organisms increased when wind velocities were less strong. Abiotic factors appeared to act over relatively large spatial scales and affected distributions within the entire ecosystem, whereas biotic factors affected distributions of algae, zooplankton, and fish on a more local scale.