ON FLORIdA BAy HypERSALINITy ANd WATER ExCHANgE

Florida Bay is made up of a collection of shallow basins separated by mud banks and mangrove islands situated between the Florida mainland and the Florida Keys. The bay is located downstream of the Everglades discharge that has been altered over the past century due to South Florida land use practices, leading to reduced water delivery to Florida Bay and elevated salinities. The reduced freshwater flow has had the strongest impacts in the north-central region of the bay, in the vicinity of Whipray basin (WB), where extreme hypersalinity can develop along with degradation of water quality and seagrass die-off. We use direct measurement of water exchange between Whipray and surrounding regions for dry and wet seasons of 2001 together with detailed salinity surveys, sea level measurements, and freshwater flux estimates to evaluate water and salt balances, and to estimate basin water renewal rates and residence times. Water renewal of WB is strongly regulated by local wind forcing. Winds toward the east from the passage of cold fronts during the winter/spring dry season resulted in a mean eastward flow through Whipray of 11 m3 s−1, with inflows over the wide western mud banks, and outflows through the eastern and southern channels. Conversely, winds toward the southwest and west typical of the summer/fall wet season produced a mean throughflow of 3 m3 s−1, with inflows through the eastern channels and outflows over the western banks. The time required for complete renewal of WB waters is estimated at 6–12 mo. Water balances are used to estimate a weak seasonal average groundwater input to Whipray of 1.7 m3 s−1 during the dry season and a negative groundwater flow or downwelling of −4.7 m3 s−1 for the wet season. Hypersalinity development was found to be caused by the combination of reduced freshwater inputs during the dry season combined with weak basin water renewal rates. Hypersalinity development could be greatly reduced by diversion of freshwater to WB via McCormick Creek during dry seasons. Hypersaline conditions (salinity > 40) develop in the north-central region of Florida Bay as part of the seasonal cycle of freshwater fluxes. Concerns have been raised that prolonged or extreme development of hypersalinity may make the ecosystem more vulnerable to seagrass die-off and contribute to the development of blue-green algal blooms that could have larger impacts on marine ecosystems of Florida Bay, the southwest Florida shelf, and the coral reefs of the Florida Keys National Marine Sanctuary. As part of the Comprehensive Everglades Restoration plan (CERp; WRdA, 2000 and FFA, 2000), efforts are underway to restore the spatial and temporal delivery of freshwater to Florida Bay to the more natural flow conditions which occurred prior to the construction of dikes and canals that diverted freshwater from the Everglades as part of previous water management practices (Light and dineen, 1994). Comprehensive modeling efforts have begun to evaluate the fate of different freshwater discharge scenarios (Hamrick and Moustafa, 2003). However, until recently no direct measurements of water exchange between the interior basins of Florida Bay were available to quantify basin renewal times and verify models. previous residence time estimates were based on long-term salinity data sets using water volume and salt budget methods (Nuttle et al., 2000). Our purpose is to present the BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 2, 2006 302 results of new direct observations of water exchange between Whipray basin (WB) and adjacent basins in the central region of Florida Bay, from which we estimate basin residence time, water and salt budgets, and describe the controlling role of local wind forcing on bay circulation and exchange. Florida Bay physical processes. —Florida Bay is located at the southern end of Everglades National park (ENp), between the Florida Keys and the mainland. The bay is made up of a complex network of shallow basins with depths ranging from 1 to 3 m, separated by shallow mud banks and mangrove islands (Fig. 1). Connection between basins occurs through narrow channels and over the shallow banks. The mud banks are largely covered by seagrass that can reach to the water surface. Water depths over the banks are typically < 0.3 m. during periods of low sea level (e.g., winter dry season or strong winds toward the southwest), the banks can become exposed, causing further isolation of the interior basins. Although the bay is openly connected to the southwest Florida shelf along its wide western boundary, exchange with the Atlantic coastal zone of the Keys is restricted to a few narrow tidal channels between the Keys island chain. The northern boundary of the bay is fringed with mangroves, and freshwater input is presently confined to the northeastern region through Taylor Slough and Trout Creek. The combined tidal harmonics of the gulf of Mexico and the Atlantic produce a mixed tide along the western boundary of the bay with a 1–1.5 m tidal range. Surprisingly, the largest tide on the U. S. eastern coast south of Brunswick, georgia, occurs nearby at the mouth of Shark River, where the tidal range can exceed 2 m during spring tides. Within Florida Bay, rapid fall-off of tidal range with distance from the western boundary (Wang et al., 1994) and dramatic increases in observed interior salinities (Fourqurean et al., 1993; Nuttle et al., 2000) indicate poor exchange of the northeast and central portions of the bay with adjacent subregions. Nuttle et al. (2000) estimated the seasonal cycle of freshwater fluxes to Florida Bay for the period 1970–1995. There are essentially two seasons defined by monthly average rainfall: the winter/ spring dry season and the summer/fall wet season. Rainfall and evaporation water flux for the bay as a whole was found to have large seasonal changes that tend to nearly balance annually. Freshwater discharge is geographically restricted to the northeast region of the bay except for periods of very high sea level and rainfall. Although river discharge represents only about 10% of the total annual freshwater supply to the bay, it has a controlling influence on salinity variability in the northeast region. Nuttle et al. (2000) also combined the available salinity data for the 31 yr period 1965–1995 to estimate mean seasonal cycles of salinity for the east, west, south, and central regions of the bay. All regions show a maximum monthly mean salinity at the end of the dry season and a minimum toward the end of the wet season. Largest amplitudes of seasonal cycles were found in the central and eastern regions, suggesting greater isolation from the moderating effects of southwest Florida shelf waters, as compared to the western and southern regions where greater exchange with the adjacent shelf waters is to be expected. Maximum salinities occurred in the central region, indicating not only a greater degree of isolation, but also, the absence of direct riverine inputs. Minimum salinities were found in the northeast region, where most of the river discharge occurs. Long-term salinity measurements from Florida Bay reveal considerable interannual variability of bay salinities, reversal of the seasonal salinity cycle during El Niño events, and decade-long trends of increasing/decreasing salinity during drought/wet LEE ET AL.: FLORIdA BAy HypERSALINITy ANd WATER ExCHANgE 303 Fi gu re 1 . L oc at io n of E ve rg la de s N at io na l P ar k m ar in e m on ito rin g st at io ns in F lo rid a Ba y (tr ia ng le s), C M A N /S ea K ey s w in d st at io ns (s qu ar es ), an d th e lo ca tio n of fr es hw at er d is ch ar ge p oi nt s ( ar ro w s) su pe rim po se d on F lo rid a Ba y ba nk /b as in c on fig ur at io n (y el lo w /w hi te ) a nd m an gr ov e isl an ds (g re en ). BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 2, 2006 304 periods (Orlando et al., 1998; Boyer et al., 1999; Nuttle et al., 2000; Johns et al., 2001). during prolonged or multiple dry seasons hypersalinity conditions in the central bay can exceed 70 (Fourqurean and Robblee, 1999; Swart et al., 1999). Hypersalinity development in Florida Bay has been associated with seagrass die-off, increased chlorophyll and turbidity levels, and loss of nursery function for important commercial species, such as pink shrimp (Zieman et al., 1988; Robblee et al., 1991; Nance, 1994; Fourqurean and Robblee, 1999; Boyer et al., 1999). Recent measurements indicate a drought occurred during 1989 and 1990, followed by a wet period through 1997. A strong El Niño event occurred between 1998 and 1999 that ended with a wet fall in 1999 due to the passage of several tropical storms. The years 2000 and 2001 represent another drought period followed by a new freshening with increased numbers of tropical storms from 2002 to the present (C. Kelble, NOAA/AOML, unpubl. data). Monitoring data suggest that not only do bay-wide salinities decrease during wet periods, but there is also a general trend for decreased turbidity and chlorophyll levels, all of which indicate improved water quality and ecosystem health (Boyer et al., 1999).