Blooms of the picoplanktonic cyanobacterium Synechococcus in Florida Bay, a subtropical inner-shelf lagoon

Seventeen sites in Florida Bay were sampled on a monthly basis for 51 month s to describe the spatial and temporal patterns of phytoplankton blooms. Th e study focused on the picoplanktonic cyanobacterium Synechococcus. The gre atest frequency and intensity of blooms was observed in the north-central r egion of Florida Bay, where cellular biovolumes of this species regularly e xceeded 10 x 10(6) mu m(3) ml(-1) and chlorophyll a concentrations were fre quently >20 mg m(-3) Synechococcus blooms were often restricted to this reg ion of the bay, in part because of the network of shallow mudbanks and isla nds that restrict water exchange with other regions and outlying waters of the Atlantic Ocean and Gulf of Mexico. The most severe blooms occurred in t he summer and fall (May-December). High concentrations of Synechococcus als o appeared during the fall in the south-central region of the bay. The appe arance of blooms in this region coincided with the onset of seasonal cold f ronts, whose strong northerly and northwesterly winds appear to drive bloom -laden water from the north-central region into adjacent parts of the bay. A number of physical and chemical factors appear to contribute to the remar kably high phytoplankton biovolumes observed in the north-central region of Florida Bay. Physical factors include the shallowness and hydrological iso lation of the region. The dominance of Synechococcus in the center of the b ay may be attributable to several of the unique physicochemical characteris tics of this species, including its small size, cyanobacterial metabolism, euryhaline character, buoyancy, and tolerance to high light intensity.