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

The occurrence of macrophyte- and phytoplankton-dominated states has been s tudied in shallow Lake Muggelsee, which, due to its rather high average dep th and naturally eutrophic condition, differs from most typical shallow lak es. At the beginning of the 20(th) century, a diverse vegetation covered on e third of the lake with a maximum colonisation depth of 4 m. Secchi depths were high although cyanobacteria blooms, dominated by Aphanizomenon flos-a quae and Microcystis spec., occurred in late summer. Increasing eutrophicat ion finally led to disappearance of submerged plant stands around 1970 and a switch to phytoplankton dominance. Cyanobacteria were now mainly represen ted by Planktothrix agardhii and Limnothrix redekei, preventing a clear-wat er phase in spring. Turbidity increased significantly after disappearance o f submerged plants. Since 1990, external nutrient loads have been reduced. Cyanobacteria structure reversed to dominance of Aphanizomenon and Microcys tis resulting in a clear-water phase in spring and re-appearance of submerg ed macrophytes. Nevertheless, submerged plant stands still covered only 3 % of the total lake area in 1999. Average colonisation depth, biomass and co lonisation densities of the dominant species Potamogeton pectinatus are low . Hitherto, re-colonisation by other species formerly present in the lake h as not taken place. Exclosure experiments revealed that grazing by herbivor ous birds or fish hamper re-colonisation.