By: Larry E. Brand, Ph.D
Rosenstiel School of Marine and Atmospheric Science

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Introduction
Florida Bay and the Florida Keys are at the downstream end of the Kissimmee River-Lake Okeechobee-Everglades watershed (Fig. 1). Their ecological health depends on what happens upstream. Within the past 20 years, a number of ecological changes have occurred in South Florida coastal waters. In Florida Bay, large algal blooms have developed and persisted, large areas of seagrasses and sponges have died off, and major changes have occurred in fish populations (Zieman et al, 1994; Robblee et al., 1991; Boesch et al., 1993; Durako, 1994; Thayer et al., 1994; Butler et al., 1995; McPherson and Halley, 1997). In the Florida Keys, macroalgae have overgrown many coral reefs, coral diseases appear to be spreading, and many corals have died (Dustan and Halas, 1987; Porter and Meier, 1992; Ogden et al.,1994; Kuta and Richardson, 1996; Richardson, et al., 1996; Richardson, 1997). Many of these changes are classical indicators of nutrient eutrophication.

 

Figure 1. Map of South Florida. Dashed line shows boundaries of the Kissimmee-Lake Okeechobee-Everglades watershed.

The dominant hypothesis for explaining many of these changes, however, is that reduced water flow into Florida Bay from the Everglades led to hypersaline conditions, which then led to massive seagrass dieoff (Robblee et al., 1991; Durako, 1994; Carlson et al., 1994; Zieman et al., 1994). This hypothesis further proposes that the seagrass dieoff and subsequent organic decomposition and sediment resuspension released nutrients which then generated the algal blooms. This hypothesis has been used as a rationale for pumping more freshwater into Florida Bay as part of a large scale alteration of water management in South Florida (United States Army Corps of Engineers and South Florida Water Management District, 1999). The hypothesis is a reasonable one to begin with, but an examination of the data available leads to serious doubts about the validity of the hypothesis and the predicted ecological consequences of pumping more freshwater into Florida Bay.

Salinity and Seagrass Dieoff
While hypersaline water may generate physiological stress on seagrasses, there is very little temporal or spatial correlation between high salinity and the seagrass dieoff in 1987 in Florida Bay. McIvor et al. (1994) have documented that salinities up to 70 psu have occurred in Florida Bay in the past 50 years. The highest salinities observed in various studies conducted in Florida Bay, summarized by McIver et al. (1994), are presented in Table 1. The highest salinity observed during the time of the seagrass dieoff in 1987 was 46.6 psu, while no massive seagrass dieoffs were observed at previous times when salinities up to 70.0 psu were measured.

Although the spatial distribution of salinity in 1989-1990 (Fig. 2) has been used to explain the seagrass dieoff, freshwater flow through Taylor Slough and the South Dade Conveyance System (SDCS) into Florida Bay was greatly reduced in 1989 and 1990 due to drought, compared to 1987 (a non-drought year) when the seagrass dieoff occurred (Fig. 3). Rainfall data (Fig. 4) also show the major drought was 2 to 3 years after the seagrass dieoff and that freshwater input to Florida Bay in 1987 was similar to that observed in 1984 to 1986. Apparently there are no baywide salinity data for 1987, but it was likely not as saline in 1987 as in 1989 and 1990 when the drought occurred and probably not much different from 1984 to 1986. There is no evidence that the seagrass died off in 1987 as a result of increasing salinity.

Figure 2. Salinities in Florida Bay measured from June 1989 to August 1990. (redrawn from Fourqurean et al., 1992)

While some seagrass dieoff in 1987 did occur in areas of high salinity, much of the seagrass dieoff reported by Robblee et al (1991) occurred in areas (Fig. 5) that had near normal marine salinity even in the drought years of 1989 and 1990 (Fig. 2). Using satellite imagery, Stumpf et al. (1999) have shown that even larger areas of seagrass to the west of Florida Bay (Fig. 5) died off as well, as local fishers and other boaters had reported at the time (DeMaria, 1996). The fact that this area of seagrass dieoff has normal marine salinity and occasionally lower salinity because it is downstream of the Shark River outfall suggests that high salinity was not the dominant cause of the seagrass dieoff.

Figure 5. Location of the major seagrass dieoff in 1987 (redrawn from Robblee et al., 1991 and Stumpf et al., 1999)

The lack of significant spatial or temporal correlations between high salinity and seagrass dieoff suggests that simply pumping more freshwater into Florida Bay will not solve the ecological problem. Indeed, the data (Fig. 3) show that South Florida Water Management District began pumping more freshwater into Florida Bay from the Everglades well before the seagrass dieoff.

Charts and Graphics provided by Larry Brand
Larry Brand ©2000-2003