Development of submerged macrophytes in shallow Lake Müggelsee (Berlin, Germany) before and after its switch to the phytoplankton-dominated state

Abstract

The occurrence of macrophyte- and phytoplankton-dominated states has been studied in shallow Lake Muggelsee, which, due to its rather high average depth and naturally eutrophic condition, differs from most typical shallow lakes. At the beginning of the 20 th century, a diverse vegetation covered one third of the lake with a maximum colonisation depth of 4 m. Secchi depths were high although cyanobacteria blooms, dominated by Aphanizomenon flos-aquae and Microcystis spec., occurred in late summer. Increasing eutrophication finally led to disappearance of submerged plant stands around 1970 and a switch to phytoplankton dominance. Cyanobacteria were now mainly represented by Planktothrix agardhii and Limnothrix redekei, preventing a clear-water phase in spring. Turbidity increased significantly after disappearance of submerged plants Since 1990, external nutrient loads have been reduced. Cyanobacteria structure reversed to dominance of Aphanizomenon and Microcystis resulting in a clear-water phase in spring and re-appearance of submerged macrophytes. Nevertheless, submerged plant stands still covered only 3 % of the total lake area in 1999. Average colonisation depth, biomass and colonisation densities of the dominant species Potamogeton pectinatus are low. Hitherto, re-colonisation by other species formerly present in the lake has not taken place. Exclosure experiments revealed that grazing by herbivorous birds or fish hamper re-colonisation.