region, usually during late summer. Seasonal variations of salinity appear to be related to the 
rainfall conditions in South Florida, where highest rainfall occurs during the summer and early 
fall. Freshwater drainage into the Bay is limited to runoff from Taylor Slough, from the coastal 
wetlands south of Shark River Slough, and seepage of groundwater from the mainland (Schomer 
and Drew, 1982). 
Florida Bay opens to the Gulf of Mexico in the southwest. Open water effects on the Bay, 
however, are dampened by interconnecting mudbanks which cordon the Bay into a series of 
internal basins or ’lakes'' (Merriam, 1989) (Figure 2). Major morphological changes in the Bay 
occur during major storms such as hurricanes. The intense runoff and increased rainfall that 
accompanies these storms appear to be very significant in maintaining the Florida Bay 
ecosystem (Meeder and Meeder, 1989; Craighead and Gilbert, 1962; and others). Storms that 
affect the Bay bottom and coastline occur about once every 3 - 5 yrs, and those that produce 
extreme freshwater runoff occur once every 6 - 7 yrs. 
There are 237 muddy islands with areas smaller than 100 m 2 unevenly distributed in Florida 
Bay (Enos, 1989). Most of these islands are connected by narrow mudbanks (Merriam and 
Quinn, 1989). They are most common in the central region of the Bay. In general, these islands 
are larger in the northeast region adjacent to the mainland than in the rest of the Bay, and the 
mudbanks are thicker and wider in the western part adjacent to the Gulf of Mexico. These 
islands are composed of soft carbonate mud accumulated over the Miami Limestone 
(Pleistocene) bedrock during the sea level rise through the past 5,000 yrs. Florida Bay is a 
source of biogenic carbonate sediments (Merriam, 1989; Bosence, 1989; and others). The 
principal habitats of the small islands are associated with red and black mangrove swamps, 
algal and halophyte marshes, grass ’prairies', and hardwood-buttonwood hammocks (Enos, 
1989). The islands are dynamic and the habitats are subject to sudden catastrophic alterations. 
The islands also migrate through erosion on exposed margins and lateral accretion on sheltered 
margins. Geological stratigraphy of the islands as determined by cores showed no obvious 
relationship to habitat. The largest keys in Florida Bay are listed in Table 1. 
The bottom of the Bay is dominated by seagrasses, especially Thalassia testudinum (turtle 
grass). There is a gradient in seagrass communities from the enclosed northeast region of the 
Bay to the open western regions (Fourqurean, 1992). The northeast region is dominated by 
sparse T. testudinum with denser cover on localized areas of increased sediment accumulation. 
The seagrass communities in this region are nutrient limited (Powell et al., 1989a; Lapointe, 
1989; and others). Seagrass cover increases towards the west where T. testudinum is 
intermixed with Halodule wrightii (shoalgrass) and Syringodium filiforme (manatee grass). 
Large bird colonies in some of the mangrove islands in the northeast Bay increase nutrient 
availability and therefore seagrass density in the vicinity of the colonies. Since 1987, a major 
die-off of seagrass and benthic macrophytes has been observed in Florida Bay (Zieman et al., 
1988). Anomalies in the recent climate record, including excessively warm waters during the 
summer and fall of 1986 - 1988 and 1990, and a reduction of tropical storm frequency, may 
have contributed to the die-off. Recent reports of seagrass die-offs on the Atlantic Ocean side 
of the Florida Keys between Long and Grassy Key have been reported in the media. 
The Florida Bay ecosystem supports a variety of species, some of which are threatened or 
endangered. Endangered and common species are listed in Table 2. 
3 
