Phytoplankton dynamics in a subtropical lake dominated by cyanobacteria: cyanobacteria 'Like it Hot' and sometimes dry

An 18-year data set (1993-2010) of water column variables from Lake George, Florida (USA), was used to examine relationships between the phytoplankton community (composition and biomass) and physical variables (temperature and hydrologic discharge rates). Lake George is a large shallow eutrophic lake located in the subtropical environment of north Florida and is subject to frequent and intense blooms of cyanobacteria. Cyanobacteria dominated the phytoplankton community during warmer months of the year (May-September). Other phytoplankton taxa increased their relative contributions to phytoplankton biomass during the colder months (November-March), when cyanobacterial biomass was low. Increased discharge rates during the peak flushing season (September-December) were correlated to diminished cyanobacterial biomass despite elevated nutrient levels. Analysis of time series data revealed higher warm season peaks in cyanobacteria biomass during years of relatively low discharge, whereas other phytoplankton groups showed the opposite trend with peaks during the colder months. These observations indicate a need for lake management strategies that consider both hydrologic and nutrient loadings within the context of possible future changes in temperature and rainfall regimes.