Patterns and Limitations in Limnoplankton Size Spectra

Abstract

The hypothesis that biomass is uniformly distributed over logarithmic size classes was evaluated with plankton samples from 15 temperate lake sites in southern Quebec. Over the size range from 0.2 to 1600 μm equivalent spherical diameter (ESD), biomass tended to increase between logarithmic size classes at a median rate of 7%, and numerical abundance declined strongly with increasing size. The slope of the normalized biomass spectrum (reflecting overall trends in the distribution) becomes significantly steeper as total phosphorus decreases and total biomass declines. Hence, more oligotrophic systems have a more uniform biomass distribution and proportionately more small organisms. Over the observed size range, most samples were dominated by a phytoplankton mode between 20 and 50 μm ESD. Because of this dominance and because of the variability of the samples, most sample distributions could not be shown to differ significantly from unimodal lognormal distributions. Efforts to fit bimodal and trimodal distributions met with limited success, likely because only 39 size classes could be used. Simple mathematical descriptors of size spectra are insensitive to substantial departures from both linear and lognormal models, and therefore, variaion in the parameters of such descriptions are ineffective correlates of departures from average patterns.