Phytoplankton Photosynthesis and Production

Productivity of aquatic vegetation determines the trophic level of any freshwater ecosystem. Phytoplankton photosynthetic rates are particularly relevant. Results are reported here on photosynthetic rates, primary productivity and associated parameters of phytoplankton from a polymictic, groundwater seepage lake in an urban environment before, during and after restoration measures. In addition, a simple regression model is presented to approximate daily column production from column integrated chlorophyll-a measurements. Calculated and estimates phytoplankton annual lake production is compared to production by submerged vascular plants. Results indicate that macrophytes played an essential role during the clear water phase preceding the eutrophication phase associated with intense algal productivity and vanishing submersed plant production. Internal restoration measures led to rapidly decreasing phytoplankton production and slowly re-appearing macrophytes. The rehabilitation phase following this period was characterized by declining phytoplankton productivity and re-establishing of macrophyte production. Total lake production as the sum of phytoplankton and macrophyte production declined from 626 t C to 186 t C during 1993–1996 mainly as a result of declining plankton production. During rehabilitation phytoplankton production further declined and macrophyte production regained importance. The interannual variability was ascribed to changes in the winter North Atlantic Oscillation Index. The index significantly correlated with lake production explaining 48% of the variability.