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Primary Productivity in a Polymictic Lake – Temporal Dynamics, Controlling Factors and Trophic State
The seasonal and interannual dynamics of phytoplankton biomass, underwater light climate and pri- mary productivity were studied in a phytoplankton-dominated, polymictic lake (Müggelsee, Berlin, Ger- many) from 1994 to 1998. Turbidity was relatively high (k 0 between 0.4 and 2.5 m –1 ) and the mixing depth often exceeded the euphotic depth. Therefore light was the most important factor limiting primary productivity. 82% of the observed variability in photosynthetic carbon uptake could be explained by a linear regression of carbon uptake against the product of chlorophyll a concentration and mean under- water irradiance. Annual productivity showed a rather large interannual variability (carbon uptake: 233–358 g C m –2 y –1 ; net oxygen productivity: 730–1150 g O 2 m –2 y –1 ) that was clearly related to algal biomass and global radiation. Compared to the 1980s, the mean annual primary productivity has de- creased by 33%.
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