Mercury in biota from Florida’s freshwater lakes and rivers: a review of current research and emerging challenges

A notable amount of aquatic mercury (Hg) research has been done in northern ecosystems but less is known about Hg dynamics in the sub-tropics. Our collective understanding of Hg bioaccumulation is particularly lacking in freshwater lakes and rivers across Florida, USA, which is dominated by uniquely shallow and polymictic solution lakes, as well as black-water rivers. To understand the state of information and direct future research and management efforts, we conducted a literature review of the primary research available on Hg dynamics in biota from inland freshwater ecosystems across Florida. We notably excluded research from the greater Everglades National Park (which includes the upgradient water conservation areas), a unique wetland system that has been well researched. In total, 42 peer-reviewed papers, conference proceedings, and student theses were found with data on methyl or total Hg in biota from Floridian lakes, rivers, or wetlands. These sources were notably dated, and much of the data used, even in more recent research, was >20 years old. Nevertheless, the collective research report elevated Hg concentrations across biotic groups, when compared to other regions in the United States, but lower than in the greater Everglades region. There was also significant spatial and/or temporal variability in the concentrations of both total Hg (THg) and methyl Hg (MeHg). Communal drivers of this variability included measures of biological productivity, precipitation, and the abundance of wetlands influencing a given waterbody. Notably, sulfate and dissolved organic matter, well studied drivers of Hg methylation and bioaccumulation in northern systems and the greater Everglades region, were absent from much of this past modeling; the limited research available suggest complex effects that warrant further study. Broader knowledge gaps and future challenges (e.g., the impact of urbanization, lake management practices, and invasive species) are also discussed herein, specifically focusing on how they may impact Hg cycling and direct future monitoring, research, or mitigation efforts. Such challenges are impacting freshwater ecosystem globally. This, coupled with the potential for other lakes to resemble Florida’s warm and productive systems due to climate change, make the findings of our review broadly applicable.