Parsing the influence of geography, vegetation, and bottom water dissolved organic matter fluorescence characteristics on sediment total- and methylmercury concentrations in the St. Louis River Estuary

Background/Question/Methods

Mercury is a potent neurotoxin that bioaccumulates in animal tissue, causing human health issues and weakening the economies of regions that rely on fisheries as a main source of income. Industrial and mining activities deposit mercury into environments in an elemental or ionic species, but deposited mercury must be methylated before it is readily bioavailable. This methylation process is mediated by microorganisms and is regulated by complex abiotic factors. In particular, dissolved organic matter (DOM) may either increase methylation by providing substrate for the microorganisms that catalyze methylation or prevent methylation by binding to inorganic mercury compounds. Moreover, the chemical composition of DOM may impact the effect of DOM on mercury methylation. Here, we examined correlations between bottom water DOM fluorescence characteristics and total- and sediment methylmercury (THg and MeHg) concentrations at vegetated and unvegetated sites spanning a geographical gradient in the St. Louis River Estuary. Sediment THg and MeHg levels were determined at the USGS Mercury Lab, and bottom water DOM fractions were determined using fluorescence spectroscopy and PARAFAC analysis. Data were analyzed with ANOVA, t-tests, and linear regressions, as appropriate.

 

Results/Conclusions

Results demonstrated a strong geographical influence on DOM fractions and sediment MeHg, THg, and MeHg:THg, with a secondary influence of vegetation. In general, sediment MeHg, THg, and MeHg:THg were positively correlated with concentrations of microbially-derived DOM and DOM freshness and negatively correlated with the humic index; however, there were no relationships between DOM characteristics and mercury concentrations at one site. Stronger relationships between DOM characteristics and mercury concentrations were observed in vegetated sediment than in unvegetated sediment. The observed geographical differences in DOM-Hg relationships suggest than underlying factors such as runoff and hydrology may be important considerations in managing mercury levels. This study also demonstrates that DOM characteristics, rather than total DOM concentrations, may be important factors regulating total- and methylmercury concentrations, as fresh DOM putatively provides substrate for microbial activity while humic matter may physically inhibit methylation. These results may aid local managers to mitigate mercury toxicity and sustain healthy fisheries.