Spatiotemporal effects of interacting water quality constituents on mercury in a common prey fish in a large, perturbed, subtropical wetland.

We present results of a multiyear study of the Everglades (Florida, USA) detailing how differences in environmental variables can alter mercury concentrations in the food web. About 1000 random locations throughout the freshwater Everglades marsh have been sampled for the United States Environmental Protection Agency's Everglades Regional Environmental Monitoring and Assessment Program ("REMAP") since 1995. REMAP sampling is synoptic and multimedia, including an abundant prey fish (eastern mosquitofish, Gambusia holbrooki) as an indicator of mercury bioaccumulation. Amplifying an approach we reported to Everglades National Park, we used Generalized Boosted Models on the REMAP data to estimate how much of the mercury concentration in mosquitofish could be explained by water quality constituents or indicators of ecological health (covariates). The resulting model accounts for 60% of the environmental influence on variation in mosquitofish mercury, a robust outcome for a large, disturbed ecosystem such as the Everglades, given its seasonal, annual, and spatial differences. Of the eight most influential covariates, two were methyl mercury in periphyton and water, two can be indicators of trophic state (alkaline phosphatase and chlorophyll-a), one can be a marker of stormwater transport (conductivity), and two can be enablers of mercury methylation (sulfate in soil and water). While these covariates had an average individual influence ranging from 4.0% to 10.1%, together they accounted for 52.2% of the total relative influence. Water with low phosphorus, but with sulfur and carbon above background, moved into the less disturbed parts of the Everglades via modifications to the existing water management system, could increase mercury bioaccumulation in those parts of the marsh.

Enhancing protection for vulnerable waters.

Governments worldwide do not adequately protect their limited freshwater systems and therefore place freshwater functions and attendant ecosystem services at risk. The best available scientific evidence compels enhanced protections for freshwater systems, especially for impermanent streams and wetlands outside of floodplains that are particularly vulnerable to alteration or destruction. New approaches to freshwater sustainability - implemented through scientifically informed adaptive management - are required to protect freshwater systems through periods of changing societal needs. One such approach introduced in the US in 2015 is the Clean Water Rule, which clarified the jurisdictional scope for federally protected waters. However, within hours of its implementation litigants convinced the US Court of Appeals for the Sixth Circuit to stay the rule, and the subsequently elected administration has now placed it under review for potential revision or rescission. Regardless of its outcome at the federal level, policy and management discussions initiated by the propagation of this rare rulemaking event have potential far-reaching implications at all levels of government across the US and worldwide. At this timely juncture, we provide a scientific rationale and three policy options for all levels of government to meaningfully enhance protection of these vulnerable waters. A fourth option, a 'do-nothing' approach, is wholly inconsistent with the well-established scientific evidence of the importance of these vulnerable waters.

Do geographically isolated wetlands influence landscape functions?

Geographically isolated wetlands (GIWs), those surrounded by uplands, exchange materials, energy, and organisms with other elements in hydrological and habitat networks, contributing to landscape functions, such as flow generation, nutrient and sediment retention, and biodiversity support. GIWs constitute most of the wetlands in many North American landscapes, provide a disproportionately large fraction of wetland edges where many functions are enhanced, and form complexes with other water bodies to create spatial and temporal heterogeneity in the timing, flow paths, and magnitude of network connectivity. These attributes signal a critical role for GIWs in sustaining a portfolio of landscape functions, but legal protections remain weak despite preferential loss from many landscapes. GIWs lack persistent surface water connections, but this condition does not imply the absence of hydrological, biogeochemical, and biological exchanges with nearby and downstream waters. Although hydrological and biogeochemical connectivity is often episodic or slow (e.g., via groundwater), hydrologic continuity and limited evaporative solute enrichment suggest both flow generation and solute and sediment retention. Similarly, whereas biological connectivity usually requires overland dispersal, numerous organisms, including many rare or threatened species, use both GIWs and downstream waters at different times or life stages, suggesting that GIWs are critical elements of landscape habitat mosaics. Indeed, weaker hydrologic connectivity with downstream waters and constrained biological connectivity with other landscape elements are precisely what enhances some GIW functions and enables others. Based on analysis of wetland geography and synthesis of wetland functions, we argue that sustaining landscape functions requires conserving the entire continuum of wetland connectivity, including GIWs.

Evaluation of the Possible Sources and Controlling Factors of Toxic Metals/Metalloids in the Florida Everglades and Their Potential Risk of Exposure.

The Florida Everglades is an environmentally sensitive wetland ecosystem with a number of threatened and endangered fauna species susceptible to the deterioration of water quality. Several potential toxic metal sources exist in the Everglades, including farming, atmospheric deposition, and human activities in urban areas, causing concerns of potential metal exposure risks. However, little is known about the pollution status of toxic metals/metalloids of potential concern, except for Hg. In this study, eight toxic metals/metalloids (Cd, Cr, Pb, Ni, Cu, Zn, As, and Hg) in Everglades soils were investigated in both dry and wet seasons. Pb, Cr, As, Cu, Cd, and Ni were identified to be above Florida SQGs (sediment quality guidelines) at a number of sampling sites, particularly Pb, which had a level of potential risk to organisms similar to that of Hg. In addition, a method was developed for quantitative source identification and controlling factor elucidation of toxic metals/metalloids by introducing an index, enrichment factor (EF), in the conventional multiple regression analysis. EFs represent the effects of anthropogenic sources on metals/metalloids in soils. Multiple regression analysis showed that Cr and Ni were mainly controlled by anthropogenic loading, whereas soil characteristics, in particular natural organic matter (NOM), played a more important role for Hg, As, Cd, and Zn. NOM may control the distribution of these toxic metals/metalloids by affecting their mobility in soils. For Cu and Pb, the effects of EFs and environmental factors are comparable, suggesting combined effects of loading and soil characteristics. This study is the first comprehensive research with a vast amount of sampling sites on the distribution and potential risks of toxic metals/metalloids in the Everglades. The finding suggests that in addition to Hg other metals/metalloids could also potentially be an environmental problem in this wetland ecosystem.

Legacy and fate of mercury and methylmercury in the Florida Everglades.

Mass inventories of total Hg (THg) and methylmercury (MeHg) and mass budgets of Hg newly deposited during the 2005 dry and wet seasons were constructed for the Everglades. As a sink for Hg, the Everglades has accumulated 914, 1138, 4931, and 7602 kg of legacy THg in its 4 management units, namely Water Conservation Area (WCA) 1, 2, 3, and the Everglades National Park (ENP), respectively, with most Hg being stored in soil. The current annual Hg inputs account only for 1-2% of the legacy Hg. Mercury transport across management units during a season amounts to 1% or less of Hg storage, except for WCA 2 where inflow inputs can contribute 4% of total MeHg storage. Mass budget suggests distinct spatiality for cycling of seasonally deposited Hg, with significantly lower THg fluxes entering water and floc in ENP than in the WCAs. Floc in WCAs can retain a considerable fraction (around 16%) of MeHg produced from the newly deposited Hg during the wet season. This work is important for evaluating the magnitude of legacy Hg contamination and for predicting the fate of new Hg in the Everglades, and provides a methodological example for large-scale studies on Hg cycling in wetlands.

Spatial variability in mercury cycling and relevant biogeochemical controls in the Florida Everglades.

Spatial patterns in mercury cycling and bioaccumulation at the landscape level in the Everglades were investigated by collecting and analyzing multimedia samples for mercury species and biogeochemical characteristics from 228 randomly located stations. Higher total mercury (THg) in environmental compartments (surface water, soil, flocculent detrital material (floc), and periphyton) generally occurred in the northern and central Everglades, but higher THg in water and periphyton in the Everglades National Park was an exception. Multiple biogeochemical characteristics, such as surface water dissolved organic matter (DOC(sw)), pH, chloride, and compositional properties of solid compartments (soil and floc), were identified to be important factors controlling THg distribution. Methylmercury (MeHg) was also higher in the northern Everglades for water, soil, and floc, but not for periphyton. Higher mosquitofish THg and bioaccumulation factor were observed in the central and southern Everglades, partially in accordance with periphyton MeHg distribution, but not in the "hot spot" areas of water, soil, or floc MeHg. The discrepancy in mercury bioaccumulation and mercury distribution in environmental compartments suggests that in addition to MeHg production, biogeochemical controls that make MeHg available to aquatic organisms, such as DOC(sw) and compositional properties of soil and floc, are important in mercury bioaccumulation.

Mercury mass budget estimates and cycling seasonality in the Florida Everglades.

We estimated the mass budget for mercury (Hg) seasonally deposited into the Florida Everglades and investigated seasonality of Hg cycling by analyzing data obtained for water, soil, flocculent detrital material (floc), periphyton, and mosquitofish collected throughout the Everglades freshwater marshes in the 2005 dry and wet seasons. Higher wet season total Hg (THg) in soil, floc, and periphyton agreed with greater Hg amounts entering these compartments during the wet season, probably owing to substantially greater Hg deposition in the wet season than in the dry season. Seasonal differences were absent for THg in surface water. Methylmercury (MeHg) showed mixed seasonal patterns, with higher water and soil MeHg and lower periphyton MeHg in the dry season but no seasonality for floc MeHg. Seasonal variations in Hg deposition, MeHg production and transport, and mass of ecosystem compartments could be responsible for the seasonality of MeHg cycling. Higher mosquitofish THg, higher bioaccumulation factors, and higher biomagnification factors from periphyton to mosquitofish were observed in the wet season than in the dry season, indicating that the wet season is more favorable for Hg bioaccumulation. The mass budget estimation agreed with this result.

Distribution of total and methylmercury in different ecosystem compartments in the Everglades: implications for mercury bioaccumulation.

We analyzed Hg species distribution patterns among ecosystem compartments in the Everglades at the landscape level in order to explore the implications of Hg distribution for Hg bioaccumulation and to investigate major biogeochemical processes that are pertinent to the observed Hg distribution patterns. At an Everglade-wide scale, THg concentrations were significantly increased in the following order: periphyton