Using aerial images from the US Department of Agriculture National Agriculture Imagery Program and the US Geological Survey National Hydrology Dataset, we estimated 577 654 farm ponds with surface areas from 0.005 to 1 ha in a 229 489 km 2 region of the southeastern Great Plains (2.52 ponds/km 2 ). Ponds with surface areas from 0.005 to 0.1 ha were the numerically dominant size class in the study region. The distribution of farm pond sizes followed an inverse power law relationship. We estimated 376 209 permanent ponds and 201 445 temporary ponds were in our study area. The ratio of temporary to permanent ponds within a pond size class was inversely related to pond surface area; 47% of ponds with surface areas of 0.005-0.1 ha were temporary, whereas only 13% of ponds with surface areas of 0.91-1 ha were temporary. Because permanent and temporary farm ponds are abundant and have different physicochemical properties and ecological communities, assessments of regional biogeochemical processes and biodiversity in the Great Plains must consider both types of ecosystems.

Consumption of methylmercury (MeHg)‐contaminated fish is the primary source of MeHg in humans and poses a hazard to human health. Because of widespread atmospheric deposition of inorganic mercury (IHg), all water bodies in the United States have been contaminated with Hg. In aquatic ecosystems, IHg is converted to MeHg, which biomagnifies, reaching high concentrations in piscivorous fish. It is not possible for governmental agencies to monitor fish from every waterbody to determine if concentrations of MeHg in fish are hazardous to human health. To help government agencies focus their monitoring efforts, it is critical that we develop the ability to predict regions where waterbodies are most likely to contain fish with hazardous concentrations of MeHg. The objective of the present study was to examine the relationship between MeHg contamination of largemouth bass (Micropterus salmoides), a popular piscivorous gamefish, and land cover in 24 ecoregions across 15 states in the southeastern United States. In our study we demonstrate for the first time that 72% of the variance in average concentrations of MeHg in largemouth bass between ecoregions of the southeastern United States can be explained by the percentage coverage by evergreen forests, emergent herbaceous wetlands, and pasture/hay. Land cover determines the sensitivity of freshwater systems to atmospheric IHg deposition, and the present study suggests that at the ecoregion scale, MeHg bioaccumulation in piscivorous gamefish, and ultimately the health hazard that these MeHg‐contaminated fish pose to humans, can be in part predicted by land‐cover type. Environ Toxicol Chem 2022;41:2386–2394. © 2022 SETAC

Fish consumption advisories are used to inform citizens in the United States about noncommercial game fish with hazardous levels of methylmercury (MeHg). The US Environmental Protection Agency (USEPA) suggests issuing a fish consumption advisory when concentrations of MeHg in fish exceed a human health screening value of 300 ng/g. However, states have authority to develop their own systems for issuing fish consumption advisories for MeHg. Five states in the south central United States (Arkansas, Louisiana, Mississippi, Oklahoma, and Texas) issue advisories for the general human population when concentrations of MeHg exceed 700 ng/g to 1000 ng/g. The objective of the present study was to estimate the increase in fish consumption advisories that would occur if these states followed USEPA recommendations. The authors used the National Descriptive Model of Mercury in Fish to estimate the mercury concentrations in 5 size categories of largemouth bass-equivalent fish at 766 lentic and lotic sites within the 5 states. The authors found that states in this region have not issued site-specific fish consumption advisories for most of the water bodies that would have such advisories if USEPA recommendations were followed. One outcome of the present study may be to stimulate discussion between scientists and policy makers at the federal and state levels about appropriate screening values to protect the public from the health hazards of consuming MeHg-contaminated game fish.

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