Limnol. Oceanogr., 44(4), 1999, 1166–1175

Seventeen sites in Florida Bay were sampled on a monthly basis for 51 months to describe the spatial and temporal patterns of phytoplankton blooms. The study focused on the picoplanktonic cyanobacterium Synechococcus. The greatest frequency and intensity of blooms was observed in the north-central region of Florida Bay, where cellular biovolumes of this species regularly exceeded 10 3 106 mm3 ml21 and chlorophyll a concentrations were frequently .20 mg m23. Synechococcus blooms were often restricted to this region of the bay, in part because of the network of shallow mudbanks and islands that restrict water exchange with other regions and outlying waters of the Atlantic Ocean and Gulf of Mexico. The most severe blooms occurred in the summer and fall (May–December). High concentrations of Synechococcus also appeared during the fall in the south-central region of the bay. The appearance of blooms in this region coincided with the onset of seasonal cold fronts, 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 remarkably high phytoplankton biovolumes observed in the north-central region of Florida Bay. Physical factors include the shallowness and hydrological isolation of the region. The dominance of Synechococcus in the center of the bay may be attributable to several of the unique physicochemical characteristics of this species, including its small size, cyanobacterial metabolism, euryhaline character, buoyancy, and tolerance to high light intensity. The importance of picoplanktonic cyanobacteria in oceanic phytoplankton communities is well established (Stockner and Antia 1986). Unicellular organisms of the genus Synechococcus are some of the major forms of cyanobacteria in the open ocean, particularly in warmer waters (Murphy and Haugen 1985). In contrast, diatoms and dinoflagellates are more commonly regarded as the dominant phytoplankters in estuaries and lagoons, reaching bloom proportions in regions with requisite bioavailable nutrients and light (Paerl 1988; Kennish 1990). Recent studies of estuarine and lagoonal ecosystems in the tropics have, however, encountered blooms of small single-celled cyanobacteria, e.g., the Mundau-Manguaba (Oliveira and Kjerfve 1993), Guarapina (Knoppers and Moreira 1990), and Patos lagoons of Brazil and the Davies Reef lagoon of Australia (Ayukai 1992). It has been hypothesized that cyanobacteria may be especially prevalent in tropical and subtropical coastal environments (Kennish 1990), particularly in systems where water exchange is restricted (Oliveira and Kjerfve 1993; Knoppers 1994). This suggestion must be tempered by recent observations of summer blooms of Synechococcus in boreal environments, such as the Baltic Sea (Jochem 1988; Kousa 1988). A more generalized hypothesis that cyanobacterial abundance is encouraged by high temperatures may be warranted (Murphy and Haugen 1985). Coastal waters of Florida range from the warm temperate climate of the panhandle to the Florida Keys at the southern tip of the peninsula, which are within the northern boundary of the tropics. Diatoms and dinoflagellates have been reported as dominant phytoplankton groups in some of Florida’s estuarine ecosystems (Vargo et al. 1987; Bledsoe and Phlips in prep.). Red tides, characterized by high densities of toxic dinoflagellates, are a recurring phenomenon along the west coast of Florida (Vargo et al. 1987). Blooms of diatoms are common on both the Atlantic and Gulf coasts. In recent years, the picoplanktonic cyanobacterium Synechococcus has been added to the list of bloom-forming phytoplankton, with reports from Florida Bay and the Florida Keys (Phlips and Badylak 1996). The proliferation of these blooms in the only region of the continental United States containing sensitive coral reef ecosystems has caused widespread concern among scientists and water managers (Boesch et al. 1993). Intensive cyanobacterial blooms may represent a direct threat to the well-being of coral reefs in the Florida Keys (Lapointe and Clark 1992). Cyanobacteria blooms in Florida Bay have also been implicated in mass mortalities of sponges (Butler et al. 1995), loss of lobster habitat (Butler et al. 1995), and potential alteration of food webs (Vargo et al. 1996). Here, we describe the spatial and temporal extent of cyanobacterial blooms in Florida Bay and discuss the physical, chemical, and biological factors that may support these blooms. Florida Bay is a restricted 1,600-km2 inner-shelf lagoon centered on latitude 258009N and longitude 808459W. Depths within the bay are generally ,3.0 m. A network of very shallow mud banks (Fig. 1A), located throughout the bay, restrict water exchange between inner regions of the bay and the Atlantic Ocean and Gulf of Mexico. In our study, water was collected monthly at 17 sampling sites (Fig. 1A) from August 1993 through October 1997. In situ measurements included depth and salinity (ppt, salinity refractometer). Water samples were collected with a water column integrating tube that captured water from the surface to 0.1 m from the bottom. For chlorophyll a (Chl a) determinations, 0.1–1 liter of station water was filtered onto A/E glass fiber filters and frozen. Tests were carried out to confirm that picoplanktonic cyanobacteria were retained on the filters. Chl a concentrations were determined using the acetone extraction method (American Public Health Association [APHA] 1989). Nutrient preservation, storage, and analyses were carried out using standard methods (APHA 1989) modified according to our Environmental Protection Agency-approved QA/QC (Comp QAP 910157) and included soluble reactive phosphorus (SRP), nitrate 1 nitrite, and ammonium. Picoplankters were enumerated with fluorescence microscopy (Phlips et al. 1995). Subsamples of station water were