RELATIONSHIPS BETWEEN PLANKTON COMMUNITY STRUCTURE AND PLANKTON SIZE DISTRIBUTION IN LAKES OF NORTHERN GERMANY

To test existing hypotheses about the plankton size distribution (PSD) in lakes, we intensively sampled the plankton of a eutrophic, stratif ied lake (Arendsee) and constructed size spectra for particles ranging in size from bacteria to herbivorous cladocerans. The Arendsee Lake a nnual mean spectrum extended over nearly 10 decades of body size and 2 .1 decades of biomass per size class (minimum, autotrophic picoplankto n; maximum, colonial cyanobacteria). The spectrum was continuous (no s ize class without associated biomass). These data indicate that contin uous PSDs are not restricted to the plankton in large pelagic zones (e .g., central oceanic systems or large freshwater lakes). We conclude t hat differences in the biomass of the various size classes are a conse quence of eutrophic conditions in Arendsee Lake, not of littoral or be nthic influences. In addition, we evaluated the impact of daphnids on the shape of the PSD with a data set comprising single samples from ea ch of 28 lakes in northern Germany plus seasonal data from Arendsee La ke. In lakes of situations with a large Daphnia biomass (greater than or equal to 0.5 mg fresh weight liter(-1)), the slopes of the straight lines fitted to the normalized PSDs were less negative. In the 28 nor thern lakes, but not in Arendsee Lake, there was greater variation abo ut the fitted line when Daphnia biomass was high, indicating a less ho mogenous PSD. Discontinuities in the spectra were found only in sample s from the 28 northern lakes and for higher Daphnia biomass. Our inter pretation is that the feeding mode of large filter-feeders results in a higher transfer efficiency from small to larger individuals (less ne gative slopes), which produces changes to a nonhomogenous PSD.