LIGHT AVAILABILITY AS A POSSIBLE REGULATOR OF CYANOBACTERIA SPECIES COMPOSITION IN A SHALLOW SUBTROPICAL LAKE

1. Variations in the relative biovolumes of dominant cyanobacterial ta xa were evaluated in the context of environmental conditions using can onical correlation analysis (CCorrA) and Redundancy Analysis (RDA). Th e objective was to test a conceptual model in which underwater irradia nce determines dominance by bloom-forming (high light adapted) or non- blooming (low light adapted) taxa. 2. The data set consisted of 404 co ntiguous observations, collected over a 3-year period at eight pelagic sites, in shallow Lake Okeechobee, Florida, U.S.A. Data included spec ies biovolumes, total phosphorus (TP) total nitrogen (TN), dissolved o xygen (DO) and chlorophyll a concentrations, as well as two indices: u nderwater irradiance (Secchi depth) and the ratio of Secchi:total dept h. 3. The first environmental canonical variable was strongly correlat ed with the two light-related indices, and negatively correlated with TP. This reflects the predominant role of resuspended P-rich lake sedi ments in controlling underwater irradiance in the shallow lake. The fi rst species canonical variable displayed a strong negative correlation with Lyngbya limnetica and L. contorta, and positive correlations wit h Anabaena circinalis, Aphanizomenon flos aquae and Microcystis spp. T he results support the conceptual model; the first pair of canonical v ariables explained 55% of the variation in the species-environmental d ata set. RDA results provided further support for the hypothesis that irradiance was the major force controlling community structure. 4. One unexpected result was a positive association between Oscillatoria spp . dominance and indicators of high irradiance. This conflicts with pas t research indicating that Oscillatoria is a low light adapted taxon, and the finding that it is the most abundant taxon in Lake Okeechobee. This may reflect the fact that the two Lyngbya taxa were more strongl y associated with low light conditions than Oscillatoria. CCorrA resul ts indicated that Oscillatoria densities are strongly controlled by wa ter temperature. There is a need for more detailed studies of cyanobac teria ecophysiology in order to explain fully the seasonality of phyto plankton in this and other shallow subtropical lakes.