Abstract
Many lakes in Florida are eutrophic and experience water quality problems derived from extensive algal development. The existence of smooth gradients of change in phytoplankton community structure with increasing lake trophic status was demonstrated based on examination of phytoplankton communities from 165 lakes located throughout Florida. The lakes were sampled twice between spring and summer 1980. At each lake, subsurface (approximately 0.5 m) samples for phytoplankton (biomass, density and size) were collected from three midlake locations. On the larger lakes , samples were collected from nearshore, open-water areas. A Nikon phase-contrast microscope was used to examine at least 20 random microscopic fields per sample at 400 magnifications. Phytoplankton community structure was examined at two levels, the major (biomass > 10% of total biomass in any other one sample) broad taxonomic categories, which corresponded to diatoms, green algae, and cyanobacteria, and the dominant (present in > 5 samples with biomass > 3% of total community biomass in any one sample) genera within each taxonomic group. This change involves a gradation from a tendency toward the dominance of green algae in oligotrophic lakes to dominance of cyanobacteria in eutrophic and hypereutrophic lakes, with a peak in diatom abundance in mesotrophic lakes. The mean biomass of the dominant genera within these groups was not related to the biomass of the communities where they occurred; instead, it was strongly related to their size, the (geometric) mean biomass of the genera increasing as the 0.74 power of their cell volume. These patterns provide a clear target for experimental efforts to elucidate the dominant mechanisms responsible for the differences in phytoplankton communities across a gradient of lake trophic status, thereby helping to fill the gap between experimental and comparative phycology. (Brunone-PTT)