Phytoplankton dynamics in a chain of subtropical blackwater lakes: the Upper St. Johns River, Florida, USA

Spatial and seasonal patterns in phytoplankton biovolume and community composition were examined for a chain of lakes in the Upper St. Johns River (USJR), Florida, USA. There was a general downstream trend in both increasing phytoplankton biovolume, and dominance of the algal community by cyanobacteria. Total algal biovolume increased from 0.7 x 10(6) mu m(3)/ml in the headwaters lake to 5.6 x 10(6) mu m(3)/ml downstream in Lake Winder. Cyanobacteria dominated the downstream lakes, accounting for approximately 50% of total algal biovolume, yet constituted only 2% of total biovolume in the headwaters lake. The diatom assemblage, as well as water quality data, suggests that these blackwater lakes are mesotrophic to eutrophic and neither nitrogen nor phosphorus limited growth. Fifteen months of cyanobacterial biovolume data were compared to water quality data to determine principal regulating factors. A regression model indicates that the major factors correlated with cyanobacteria in these lakes are temperature, total Kjeldahl nitrogen, water level, and color, with temperature alone accounting for 54% of the variability in cyanobacterial biovolume. This analysis demonstrates that multiple interacting factors need to be considered when attempting to explain spatiotemporal patterns in algal dynamics.