Mercury bioaccumulation and bioaccumulation factors for Everglades mosquitofish as related to sulfate: a re-analysis of Julian II (2013).

The Everglades, an ecosystem of international significance, has elevated biota mercury levels representing risk to human and wildlife consumers of fish. Given the critical role of sulfate in the methylation of mercury, and because there is a significant agricultural contribution, one potential means of reducing these mercury levels is reducing Everglades sulfate inputs. Julian II (Bull Environ Contam Toxicol 90:329-332, 2013) conducted regression modeling of the relationship between surface water sulfate concentrations and Gambusia spp. mercury bioconcentration factors across the major hydrologic subunits of the Everglades, and used those results to draw conclusions about the role of sulfate in the cycling of mercury in the Everglades. We however demonstrate a number of fundamental problems with the analysis, interpretation and conclusions. As a result, we strongly caution against using the results of Julian II (Bull Environ Contam Toxicol 90:329-332, 2013) to formulate management decisions regarding mitigation of the Everglades mercury problem.

Mercury cycling in aquatic ecosystems and trophic state-related variables--implications from structural equation modeling.

Structural equation modeling (SEM) provides a framework that can more properly handle complex variable interactions inherent in mercury cycling and its bioaccumulation compared to more traditional regression-based methods. SEM was applied to regional data sets for three different types of aquatic ecosystems within Florida, USA--lakes, streams, and the Everglades--to evaluate the underlying nature (i.e., indirect and direct) of the relationships between fish mercury concentrations and trophic state related variables such as nutrients, dissolved organic carbon (DOC), sulfate, and alkalinity. The modeling results indicated some differences in key variable relationships--for example, the effect of nutrients on fish mercury in lakes and streams was uniformly negative through direct and indirect pathways consistent with biodilution or eutrophication-associated effects on food web structure. Somewhat surprisingly, however, was that total phosphorus did not serve as a meaningful variable in the Everglades model, apparently because its effects were masked or secondary to the effects of DOC. What is perhaps a more important result were two key similarities across the three systems. First, the modeling clearly indicates that the dominant influence on fish tissue mercury concentrations in all three systems is related to variations in the methylmercury signal. Second, the modeling demonstrated that the effect of DOC on fish mercury concentrations was exerted through multiple and antagonistic pathways, including facilitated transport of total mercury and methylmercury, enhanced rates of methylation, and limitations imposed on bioavailability. Indeed, while the individual DOC pathways in the models were all highly significant (generally p<0.001), the net effect of DOC in each model was greatly reduced or insignificant. These results can help explain contradictory results obtained previously by other researchers in other systems, and illustrate the importance of SEM as a modeling tool when studying systems with complex interactions such as the aquatic mercury cycle.

Mercury bioaccumulation factors and spurious correlations.

While bioaccumulation factors (BAF) - the ratio of biota contaminant concentrations (Cbiota) to aqueous phase contaminant concentrations (Cw) - are useful in evaluating the accumulation of mercury (Hg) and other contaminants for various trophic levels in aquatic ecosystems, reduction of the underlying relationship between Cbiota and Cw to a single ratio (BAF) has inherent risks, including spurious correlation. Despite a long and rich history of remonstrations in the literature, several very recent publications evaluating Hg-related BAFs have suffered from false conclusions based on spurious correlation, and thus it seems that periodic reminders of the causes and risks of these errors are required. Herein we cite examples and explanations for unsupported conclusions from publications where authors using BAF-Cw relationships fail to recognize the underlying statistical significance (or lack thereof) of direct relationships between Cw and Cbiota. This fundamental error leads to other problems, including ascribing mechanistic significance (e.g., mechanisms related to biota contaminant uptake) to "inverse" BAF-Cw relationships that reflect nothing more than regressing the log transformed inverse of Cw (i.e., negative log) against itself (i.e., positive log transformed), and using such regression models of BAF-Cw relationships that appear significant for predictive purposes, but are misleading. Spurious correlation arising in the analysis of BAF relationships can potentially appear in more subtle forms as well, including regressing variables such as dissolved organic carbon (DOC) that are correlated with Cw. We conclude that conducting a direct analysis by examining the relationship between Cbiota and Cw (or Cbiota and other variables) rather than evaluating a ratio (BAF) is less ambiguous and subject to error, more easily interpreted, and would lead to more supportable conclusions.

A screening model analysis of mercury sources, fate and bioaccumulation in the Gulf of Mexico.

A mass balance model of mercury (Hg) cycling and bioaccumulation was applied to the Gulf of Mexico (Gulf), coupled with outputs from hydrodynamic and atmospheric Hg deposition models. The dominant overall source of Hg to the Gulf is the Atlantic Ocean. Gulf waters do not mix fully however, resulting in predicted spatial differences in the relative importance of external Hg sources to Hg levels in water, sediments and biota. Direct atmospheric Hg deposition, riverine inputs, and Atlantic inputs were each predicted to be the most important source of Hg to at least one of the modeled regions in the Gulf. While incomplete, mixing of Gulf waters is predicted to be sufficient that fish Hg levels in any given location are affected by Hg entering other regions of the Gulf. This suggests that a Gulf-wide approach is warranted to reduce Hg loading and elevated Hg concentrations currently observed in some fish species. Basic data to characterize Hg concentrations and cycling in the Gulf are lacking but needed to adequately understand the relationship between Hg sources and fish Hg concentrations.

Mercury in the Gulf of Mexico: sources to receptors.

Gulf of Mexico (Gulf) fisheries account for 41% of the U.S. marine recreational fish catch and 16% of the nation's marine commercial fish landings. Mercury (Hg) concentrations are elevated in some fish species in the Gulf, including king mackerel, sharks, and tilefish. All five Gulf states have fish consumption advisories based on Hg. Per-capita fish consumption in the Gulf region is elevated compared to the U.S. national average, and recreational fishers in the region have a potential for greater MeHg exposure due to higher levels of fish consumption. Atmospheric wet Hg deposition is estimated to be higher in the Gulf region compared to most other areas in the U.S., but the largest source of Hg to the Gulf as a whole is the Atlantic Ocean (>90%) via large flows associated with the Loop Current. Redistribution of atmospheric, Atlantic and terrestrial Hg inputs to the Gulf occurs via large scale water circulation patterns, and further work is needed to refine estimates of the relative importance of these Hg sources in terms of contributing to fish Hg levels in different regions of the Gulf. Measurements are needed to better quantify external loads, in-situ concentrations, and fluxes of total Hg and methylmercury in the water column, sediments, and food web.