A day of field work in the Florida Everglades will humble even the most seasoned field scientist. This iconic wetland system, where over $26B has been spent by federal and state partners to restore the quantity and quality of water of Everglades National Park, is critical for maintaining drinking water for south Florida communities and resides at the interface of agriculture and rising seas. A typical day in the field consists of launching airboats at dawn under blue, cloudless skies. Airboats have a big-block V8 car engine fitted with an airplane propeller, which enables them to operate in as little as a half meter of standing water. This is critical as the depth of the wetlands changes daily based on rainfall and water management. The airboat drivers fire up the engine, we don ear protection, and they navigate the network of ridges, sloughs, and tree islands of the wetlands to get us to the sites where we sample the water released from canals across the sheet-flow wetlands.

As we approach our first wetland site, where high nutrient canal waters are released into the wetlands, cattails (also known as bulrush or Typha) grow in dense blocks with stalks that reach 3 meters tall, preventing sunlight from reaching the surface waters. Towering over us, we blast the airboat through the cattail stands, risking getting stuck, matting down a “runway” for our exit, and shut the airboat down. With chest waders, we carefully inspect the surroundings for wildlife and proceed out of the boat to sample. By 11 a.m., the “honeymoon” phase of the day is over. Thunderheads develop at an alarming rate in South Florida in summer, reaching beyond the troposphere as the atmosphere warms. Heat exhaustion is a real threat to both us and our equipment (e.g., GPS units and field meters have been fried during this study from the heat radiating off the aluminum hull of the boat) – and don’t ask about ants and other insects. By noon, our airboat driver warns us that we have less than three hours before the thunder and lightning will start, a dangerous place to be waist-deep in water with no cover in sight. We huddle at the front of the airboat, consult our field notes, and make decisions on which of our remaining sites to prioritize for sampling based on our study goals. By 2:30 pm, a sheet of rain falls with little warning, delivering rain and mercury to the wetland from the upper troposphere. That is why we are here. Because of the mercury.

Mercury is a trace metal contaminant and global pollutant that poses severe risks to wildlife and humans. The atmosphere has been enriched five- to-sevenfold in mercury due to anthropogenic activity, and the Florida Everglades receive a daily delivery of rainwater high in mercury. The sources of the mercury are coal combustion, industrial operations, and artisanal and small-scale gold mining, all largely occurring far from Florida. However, mercury can travel long distances in the atmosphere and is deposited into aquatic environments far from its original sources. The prevalence of high mercury levels in fish prompted global action by the United Nations, which adopted the Minamata Convention on Mercury in 2013 to decrease environmental mercury releases and human exposure.

Wetlands are critical environments for the biogeochemical cycling and ecological uptake of mercury in the aquatic food web. However, not all wetlands show elevated levels of mercury in fish. The most important step in the process is the microbial conversion of inorganic mercury, benign at low concentrations, into methylmercury. Methylmercury bioaccumulates and biomagnifies in the aquatic food web. A subset of anaerobic bacteria and archaea in wetlands are responsible for the formation of methylmercury. For over three decades, sulfate has been identified as an important constituent in wetlands that can either exacerbate or diminish the formation of methylmercury, attributed to how sulfur interacts with mercury and is metabolized by microorganisms. Our research team studied the Florida Everglades, which is effectively a “living laboratory,” to better understand how sulfate governs the formation and uptake of methylmercury.  The northern Everglades has been drastically altered for agriculture, with sulfur applied to sugar cane that is eventually transported via canals to wetlands, eventually reaching Everglades National Park. Further, sea level rise is threatening coastal wetlands with sulfate from marine waters. The freshwater wetlands are like the bellows of an accordion, responding to water management in the north and sea level rise in the south.

Between 2012 and 2019, we conducted seven field campaigns that followed the flow of water from points of canal water discharge across three distinct wetlands of the freshwater Florida Everglades. We sampled the surface water of the wetlands, the water in the sediments (termed “pore water”), and mosquito fish (Gambusia holbrooki). Mosquito fish were selected because they have a short life span (≤ 6 months) and reflect the availability of methylmercury to the aquatic food web of the wetland.

In Everglades wetlands with low sulfate, sampled far from where canals discharge agricultural waters, the concentrations of methylmercury were low in surface water, pore waters, and fish; often below detection limit in the waters. Further, the wetland surface waters showed the lowest concentrations of dissolved organic matter, which is an important component of the water influencing the bioavailability of the mercury to anaerobic microorganisms. Across all the wetlands, a consistent response was observed with increasing sulfate and high levels of dissolved organic matter and methylmercury up to a certain sulfate concentration, above which the concentration of methylmercury declined. Regardless of when and where we sampled, the concentrations of methylmercury in surface waters correlated positively with methylmercury concentrations in mosquito fish, providing strong support that sulfate promotes methylmercury formation and fish uptake in the wetlands. The concentration of methylmercury in the small prey fish was as high as 10 million times greater than the water they were living in, highlighting the high bioaccumulation potential of methylmercury. The consistency in our observations, in the context of complementary studies by our group and others in this system, allowed us to propose a framework for methylmercury risk that accounts for sulfate, dissolved organic matter, and proximity to the aquatic food web.

Natural resource managers are tasked with balancing short- and long-term strategies to decrease methylmercury formation and uptake in the aquatic food web. Our study provides evidence that reductions in sulfate loading to the wetlands may offer an attractive local approach that also has other positive benefits on ecosystem health. The rain will continue to fall in South Florida and the mercury concentrations in the rain will respond to changes in atmospheric emissions of mercury. However, there are local solutions that can be employed to mitigate the threat of this contaminant to wildlife and humans.