Trends and explanatory variables for the major phytoplankton groups of two southwestern Florida estuaries, U.S.A.

Multi-year phytoplankton abundance data from two estuaries in southwest Florida were combined into taxonomic groups, assessed for trend, and compared with water quality data using multivariate techniques. Interannual flow variations were linked to climatic indices and were relatively large. Trends in water quality included declining biomass, N and P signals in Tampa Bay, and declining P and a complex but slightly increasing N in Charlotte Harbor. Trends were superimposed on flow-dominated seasonal and annual variations in nutrient loads. Trends in phytoplankton abundance were both gradual and abrupt, were not always monotonic, and were most noted for cyanobacteria. One species of cyanobacteria, Schizothrix calcicola sensu Drouet could be considered an indicator species. At specific stations, nutrient concentrations and seasonal parameters were significant explanatory variables for variations in community structure. Clustering of community structure by taxonomic groups did not align well with known trends in nutrient loadings or biomass.