THE SIZE DISTRIBUTION OF PLANKTON BIOMASS IN A LARGE LAKE AND ITS SEASONAL VARIABILITY

Biomass size spectra were calculated from comprehensive microscopic assessment of plankton organisms in large, deep Lake Constance (Bodensee). They revealed an approximately constant size distribution of biomass from bacteria (10(-14) g C cell-1) to crustaceans (10(-4) g C ind.-1), resembling distributions found in open marine systems (seasonal average). Size ranges without detectable biomass as reported from other lakes did not exist. External perturbations, and size and depth of the pelagic zone, but not the type of habitat (marine vs. limnetic) were suggested as crucial factors influencing the shape of the biomass size distribution. The fit of the normalized biomass size spectrum to a straight line with a slope of -1.00 was very close (r2 = 0.98), indicating a decrease of abundance per size class N proportional to the increase of body weight w (N is-proportional-to w-1). Seasonal variation of biomass per size class was strongly related to body weight. It was largest for organisms with generation times of about 1 yr. Slopes of normalized biomass size spectra were steep in early spring (-1.16) indicating a dominance of small organisms and a low transfer efficiency to larger organisms. From spring to early summer, the size distribution of organisms shifted toward larger organisms (slope up to -0.94). The seasonal changes of the size spectra support the idea that, due to differences in the intrinsic reaction times of differently sized organisms, varying environmental conditions produce less regularly shaped actual size spectra, if not averaged over a period longer than the external fluctuations.