The Science Behind Florida Bay’s Yellow Fog

The Science Behind Florida Bay’s Yellow Fog

Categories: Blog, Foundation Science

Florida Bay
Left: healthy sea grass bed; right: summer 2015 photo showing yellow fog blanketing sea grass in central Florida Bay. Photo: Everglades National Park

Last month, the Miami Herald detailed a “yellow fog” spotted in Florida Bay suggesting the bay is on the verge of another massive sea grass die-off and extended algae bloom — similar to what many recall occurring in the late 1980s and early 1990s.

The trigger of the catastrophic event then and now is the same, a lack of freshwater flow in the Everglades combined with severe drought leading to hyper-salinity (when water salinity exceeds that of seawater, about 36 parts per thousand) in the bay. In late July 2015, we measured salinity that exceeded 60 parts per thousand in parts of Florida Bay — nearly twice the salinity of seawater.

In the 1990s the problem in Florida Bay became so severe that more than 50,000 acres of sea grass were impacted and harmful algae blooms persisted for years.

Some fishermen say that the fishing has never been the same.

Ultimately, this event became a rallying cry that led to the establishment of the Everglades Foundation and development of the Comprehensive Everglades Restoration Plan (CERP).

More than 20 years later and 15 years since CERP was authorized, we have seen some local successes in restoration; however, we have not realized any progress in moving water south from Lake Okeechobee into the heart of the Everglades. Until we do this, Florida Bay will continue to suffer.

Yellow fog — exactly what is it?

In addition to sodium chloride (table salt), seawater contains an abundance of other salts or ions including sulfate. Sulfate is about 1000 times more abundant in seawater than fresh water, and when oxygen is depleted in flooded saline soils like in sea grass beds, some bacteria use sulfate (because it contains oxygen) for respiration giving them an advantage over bacteria that require aerobic conditions to live.

This respiratory process is called sulfate reduction and leads to the production of hydrogen sulfide — the rotten egg smell that emanates naturally from coastal mangrove wetlands. An overabundance of hydrogen sulfide production in Florida Bay mud contributed to the formation of this yellow fog.

But how did it get there?

First, we need to understand that there are a lot of different kinds of bacteria in nature with very specific roles and unique abilities. Some bacteria can use hydrogen sulfide as an energy source to produce sugar and yellow-colored elemental sulfur as an end product. These sulfide-oxidizing bacteria live and grow in the zone between sources of hydrogen sulfide (Florida Bay mud) and oxygen (Florida Bay water).

Although these bacteria are always present in coastal water bodies such as Florida Bay, they thrive and grow to bloom-like proportions when conditions are greatly modified. Such was the case in the summer of 2015.

Many know that Florida Bay is starved of freshwater relative to pre-drainage Everglades conditions. In fact, Florida Bay only gets about half the freshwater it received historically from the River of Grass.

This has had negative consequences on everything from sea grass and sponge habitats to shrimp and spotted sea trout fisheries, and even larger predators like Roseate spoonbills and American crocodiles. Combined with reduced flow conditions and recent drought, the result is catastrophic.

The yellow fog, which we know to be a cloud of sulfur, is not the problem and it is not the cause. It is merely a symptom—like a runny nose to a cold—in the hyper-saline layer of water that blankets large areas of sea grass in central Florida Bay.

The real problem is the hyper-salinity combined with high water temperatures that have shifted the balance of oxygen in the water and mud in Florida Bay and led to such high concentrations of hydrogen sulfide.

As a plant toxin, hydrogen sulfide will kill sea grass when concentrations are high enough. This fuels a series of downward spirals with many paths that lead to sea grass die-off and algae blooms that are perpetually fed by the nutrients released from dead and decomposing sea grass.

What can we do about it?

Right now, there is nothing we can do but pray for rain and wind to flush affected areas of Florida Bay. In the near-term, we need to move forward with the Modified Water Deliveries Project that will help restore the pattern of flow into Everglades National Park. We also need to advance implementation of the C-111 Spreader Project that will increase flows to Taylor Slough and Florida Bay.

Finally, we need to move forward with the Central Everglades Planning Project (CEPP) that will provide almost immediate benefits for Florida Bay and the rest of the River of Grass.

If we wait too long, we may lose what remains of Florida Bay.