Seagrass Die-off and How Freshwater from the Everglades gets into Florida Bay

Seagrass Die-off and How Freshwater from the Everglades gets into Florida Bay

Categories: Special Report: Florida Bay

Dr. Stephen E. Davis, Ecologist
The Everglades Foundation, Palmetto Bay, FL 33157
Dr. Joseph N. Boyer, Professor & Director
Department of Environmental Science & Policy, Center for the Environment
Plymouth State University, Plymouth, NH 03264
Dr. James, W. Fourqurean, Professor & Director
Department of Biological Sciences & Marine Education & Research Initiative for the Florida Keys
Florida International University, Miami, FL 33199
 (collectively, authors have more than 75 years of research experience in & around Florida Bay)

In the past, salinity throughout Florida Bay was much lower, before freshwater flow from Lake Okeechobee was disconnected from the Everglades during the first half of the 20th Century. This is widely accepted and supported by our understanding of the hydrology of the historic Everglades (Light and Dineen 1994; McIvor et al. 1994). The changes that ensued are also evident in the paleo-record of organisms preserved in bay sediments over the past century and fluorescent banding of coral heads, suggesting that freshwater flows were reduced by about 59% and present-day salinities are between 5 and 20 parts per thousand (ppt) higher than historical conditions (Smith et al. 1989; Brewster-Wingard and Ishman 1999; Marshall et al. 2009). We also know that reduced freshwater flows from the Everglades and the subsequent rise in salinity led to a change in the bay’s seagrass communities with Thalassia testudinum becoming more dominant (Zieman 1982).

Reduced freshwater delivery to Everglades National Park has been a documented concern since before its establishment in 1947 (Craighead 1966). Because the large watershed that fed the park has largely been cut-off, flows in Shark River Slough have dramatically declined. Current average annual flows are about one third of the historical flows (CISRERP 2016, page 110), which is consistent with paleo-based estimates (Brewster-Wingard and Ishman 1999; Marshall et al. 2014). In Florida Bay, at the southern terminus of the park, droughts are known to exacerbate the problem through enhanced evaporation and chronic hypersalinity (Fourqurean and Robblee 1999). Some of Florida Bay’s highest salinity values were recorded during the drought of the late 1980’s and coincided with a major seagrass die-off that began in 1987. It is worth noting that the 2015 seagrass die-off in Florida Bay, which was also associated with a drought-induced lack of freshwater flow through Everglades National Park, was just as severe as the 1987 event, if not more so in terms of the magnitude of hypersalinity.

The scientific consensus that emerged following the 1987 die-off was that the abnormally high salinity was the catalyst that initiated a complex series of processes resulting in the loss of tens of thousands of acres of seagrasses in Everglades National Park (McIvor et al. 1994, Zieman et al. 1994, Fourqurean et al. 1999, Zieman et al. 1999, Koch and Erskine 2001, Borum et al. 2005, Koch et al. 2007a, Koch et al. 2007b). This understanding and the growing recognition of other hydrological and ecological problems across the greater Everglades (summarized in Sklar et al. 2005) were the impetus for the Comprehensive Everglades Restoration Plan (CERP), which, when implemented, will improve freshwater flow and drought-resistance across the Everglades including ecologically and economically important estuaries like Florida Bay.

So, how does freshwater get from the Everglades to Florida Bay? The Florida Bay Scientific Review Panel, which was assembled by the National Fish and Wildlife Foundation, the National Park Service and the South Florida Water Management District to investigate the causes of the 1987 seagrass die-off, stated that “Freshwater is delivered to Florida Bay through Taylor and Shark River Sloughs and coastal drainage outside of these sloughs” (Boesch et al. 1993; our emphasis added). Since that time and with several monitoring and modeling studies from which to re-consider the question, the opinion of the scientific community remains unchanged. The contention by the South Florida Water Management District (SFWMD 2017) that Shark River Slough has no significant role in providing water to Florida Bay is outside of the scientific community consensus.

While it is important to recognize the value of any project that increases the flow of freshwater to Florida Bay, the justification for the South Florida Water Management District’s current Taylor Slough project should not minimize the value of other important sources of freshwater inflow to Florida Bay. In terms of volume, Taylor Slough inflows to Florida Bay are relatively small yet direct and therefore an important component of the freshwater budget of the bay (Nuttle et al. 2000). Although Shark River Slough is much larger, spanning more than 200,000 acres of Everglades National Park and passing more than 700,000 acre-feet of freshwater each year toward Whitewater Bay and the Southwest coast, its contribution to Florida Bay salinity is relatively unquantified. Still, we know a significant connection exists, and given that the volume of freshwater flow in this massive slough will nearly double with Everglades restoration, its contribution will be even more important.

The influence of Shark River Slough in freshening western Florida Bay, whether through groundwater or surface water connections, was recognized more than a half century ago (Tabb et al. 1962). More recent investigations with drifters have provided stronger evidence supporting the southern flow of Shark Slough discharge towards Florida Bay, and the use of current meters and physical modeling has shed light on the magnitude of volume that is brought into Florida Bay by tidal action and the evaporation of water from its interior (Smith 2000; Lee et al. 2002, Nuttle et al. 2000). Lee et al. (2016) also makes clear that “In addition, fresh water from the Shark River discharge can be transported around Cape Sable in a low-salinity plume and into Flamingo Channel”. In their 1993 Report, the Florida Bay Scientific Review Panel summarized data from Ned Smith showing that discharge of water through Flamingo Channel into northwestern Florida Bay is on the order of 1.2 million acre-feet/year or about 40 times the flow of freshwater through Taylor Slough into northeastern Florida Bay (Boesch et al. 1993).

Long-term salinity monitoring data from Florida Bay show even more compelling evidence of Shark River Slough’s influence in freshening western Florida Bay (Boyer et al. 1999, Kelble et al. 2007). Also, bay-wide and shelf salinity data collected by Florida International University through the 1990s until 2006 clearly illustrate the repeated influence of Shark River Slough on salinity in the western and north-central Florida Bay, including into the area of seagrass die-off—even during the dry season (Figure 3). See link for more examples of the effect of Shark River Slough discharge on salinity. These data and lines of evidence illustrate how outflows from Shark River Slough, at any given time of the year, can enter the bay from the northwest and, owing to their relatively large volume, work to moderate developing hypersalinity conditions during drought.

Despite the claims made by the SFWMD, restoring a small amount of freshwater flow into Taylor Slough will not be sufficient to fix Florida Bay’s problems that have been caused by decades of diversion of water from both Taylor and Shark River Sloughs. In fact, an analysis of their own modeling revealed most of the flow increases in their plan are realized in average to wet years, not during dry years when the bay experiences hypersalinity and seagrass die-off becomes a real threat (Paudel and Davis 2016). The real challenge is finding water to meet the needs of the Everglades and Florida Bay—not to mention the needs of agriculture and millions of South Floridians—during the next drought when freshwater is locally scarce. The historic Everglades, due to its size and connectivity, had massive amounts of internal dynamic storage from which to draw upon during drought. The CERP was designed to remedy this by increasing storage and sending more water south through both sloughs. In fact, the 1999 Re-Study states that “Improved water deliveries to Shark River Slough, Taylor Slough, and wetlands to the east of Everglades National Park will in turn provide improved deliveries of fresh water flows to Florida Bay” (USACE and SFWMD 1999). Has the science changed? No. If anything, the latest science supports this statement more than ever before.

Based on what we learned from long-term monitoring following the 1987 seagrass die-off and since the Florida Bay Scientific Review Panel’s report (Boesch et al. 1993), Florida Bay will take years, likely decades, to recover from this most recent die-off. Algal blooms that afflicted Florida Bay a few years after the 1987 die-off are just beginning to materialize and are anticipated to cause further damage to benthic habitats (Hall 2012). We need to restore the flow of freshwater to Everglades National Park—through both sloughs—so that these water masses can converge in the north-central region of the bay and help prevent future hypersalinity from reaching the detrimental levels we saw in the summers of 1987 and 2015.

There exists an innovative plan to fix Florida Bay and the myriad degraded habitats across the greater Everglades. It entails a reservoir south of Lake Okeechobee, and it is a key component of the Comprehensive Everglades Restoration Plan.

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Figure 1

Map of Everglades National Park showing how freshwater flows into Florida Bay from Shark River Slough and Taylor Slough (red arrows include the current, un-restored distribution of flow).  The zone in orange indicates the region where frequent hyper-salinity events occur and the epicenter of seagrass die-off.

Figure 2

Salinity plots for Florida Bay showing the development of dry season hyper-salinity conditions (above 35 parts per thousand) in 1993 and 1998 in the north-central region of the bay.  In both instances, the lighter blues indicate the influence of freshwater sources from Taylor Slough (in the eastern interior portions of the bay) and Shark River Slough (wrapping around Cape Sable from the west).  The approximate area of the current seagrass die-off is delineated by the white dashed line and reveals an equal importance of freshwater inflows from the east and west in influencing the salinity of this zone.

Figure 3

1995 and 2006 dry season salinity plots for Florida Bay the Florida Keys and Shelf showing the influence of Shark River Slough outflows in freshening Western Florida Bay.  Orange to yellow colors reflect the influence of freshwater sources from Taylor Slough and Shark River Slough (from the west) converging in central Florida Bay (TOP) or surrounding a developing zone of hypersalinity (BOTTOM).  There are numerous examples of this influence in the period of record: