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, stratified lake (Arendsee) and constructed size spectra for particles ranging in size from bacteria to herbivorous cladocerans. The Arendsee Lake annual mean spectrum extended over nearly 10 decades of body size and 2.1 decades of biomass per size class (minimum, autotrophic picoplankton; maximum, colonial cyanobacteria). The spectrum was continuous (no size class without associated biomass). These data indicate that continuous PSDs are not restricted to the plankton in large pelagic zones (e.g., central oceanic systems or large freshwater lakes). We conclude that differences in the biomass of the various size classes are a consequence of eutrophic conditions in Arendsee Lake, not of littoral or benthic influences. In addition, we evaluated the impact of daphnids on the shape of the PSD with a data set comprising single samples from each of 28 lakes in northern Germany plus seasonal data from Arendsee Lake. 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 northern lakes, but not in Arendsee Lake, there was greater variation about the fitted line when Daphnia biomass was high, indicating a less homogenous PSD. Discontinuities in the spectra were found only in samples from the 28 northern lakes and for higher Daphnia biomass. Our interpretation is that the feeding mode of large filter-feeders results in a higher transfer efficiency from small to larger individuals (less negative slopes), which produces changes to a nonhomogenous PSD.