Lingering impacts of summer 2015
Poor water conditions continue to plague Florida Bay. Muddy, pea-soup-colored water has persisted across western Florida Bay for months and has been shifting from basin to basin with winds and tides. Understandably, fishing guides who earn their living on the bay have wondered about what might be causing these stubborn blooms of algae.
The answer is clear. What we are seeing in the bay is symptomatic of a bay that is still in recovery from a massive seagrass die-off that impacted nearly 80 square miles of the central and western bay, beginning in the summer of 2015. How do we know this? Decades of science and monitoring in and around the bay.
Large-scale seagrass die-off in Florida Bay has occurred twice in documented history: a multi-year die-off that began in the summer of 1987 and continued throughout the late 1980s and the more recent event in 2015. Both die-offs were similarly triggered and have exhibited similar patterns of post die-off recovery.
Both events were initiated by hyper-salinity conditions in Florida Bay resulting from a lack of rainfall and diminished freshwater inflows from the Everglades. High summertime water temperatures combined with hyper-salinity contributed to oxygen depletion and sulfide toxicity, which initiated the rapid, overnight spread of die-off.
Following die-off, exposed bay bottom (from a lack of seagrass cover) is more vulnerable to sediment resuspension, leading to more turbid conditions during moderate-strong winds or strong tides (e.g., Snake Bight). Following the late 1980s die-off, algae blooms developed and intensified two years post die-off, peaking at seven to eight years (i.e., in 1994 and 1995) after die-off initiated. Following the 2015 event, we witnessed blooms in the die-off area one year later (peaking in November 2016), then Hurricane Irma occurred in September 2017. At seven years post die-off, the current blooms in western Florida Bay are reflective of the trend observed following the late 1980s event.
Florida Bay, like the Everglades, is limited by the availability of phosphorus. Because phosphorus in the Everglades is so low, the Gulf of Mexico represents the major, natural source of phosphorus to Florida Bay. This natural gradient in phosphorus availability leads to productivity gradients in mangrove forests from the Gulf (where the tallest trees are) inland toward the freshwater Everglades (where trees are short in stature). We see the same productivity gradients in seagrass meadows/banks from the Gulf edge (where the most productive seagrass banks are) to the interior of Florida Bay (where grasses are sparse).
Combined with the decay of below-ground seagrass biomass that died in 2015, phosphorus in exposed, re-suspended bay sediment and this natural source of phosphorus from the Gulf of Mexico, algae blooms will continue to flourish until enough seagrass recovers to sequester these locally sourced increases in water column phosphorus.
In summary, the current Florida Bay algae blooms are not triggered by the quality of water coming out of the Everglades or water quality along the Florida Keys. The bad water we are seeing is related to local and natural sources of phosphorus that have been exacerbated by the 2015 die-off.
Following the die-off of the late 1980s, we have learned that recovery of Florida Bay takes time — as much as 20 years. If the seagrasses continue to recover, we should see these bad water conditions subside over the next few years. The lesson to be learned from this is to not allow another seagrass die-off in Florida Bay. The nature of Florida Bay gives rise to its beauty and productivity. However, it also makes the bay highly vulnerable to hyper-salinity-induced die-off. We can prevent this from happening through Everglades restoration, especially projects like the Everglades Agricultural Area reservoir that send more clean freshwater south to Everglades National Park. This will better mimic the historic estuarine salinity conditions needed by Florida Bay.
Dr. Steve Davis is chief science officer of The Everglades Foundation.