Implications of body size, trophic position, species identity, and lake water quality for crayfish mercury burdens in and near Voyageurs National Park

Background/Question/Methods

High mercury concentrations in fish and aquatic wildlife are common in the northern lakes of Voyageurs National Park. Crayfish are a key food web component, but little is known about their mercury burdens. In a series of studies from 2002-2006, we examined 1) factors influencing mercury burdens in the native crayfish Orconectes virilis, 2) potential differences in mercury burdens in O. virilis vs. the invasive O. rusticus, and 3) the potential for crayfish mercury concentrations to serve as an indicator of mercury burdens in other biota of interest. Using baited modified minnow traps, O. virilis were collected from seven interior lakes at Voyageurs National Park (VOYA) in 2002 and 2003. Additional crayfish were collected from two nearby lakes in 2006: O. virilis were collected from Little Johnson Lake, and O. rusticus were collected from Johnson Lake. Each crayfish was analyzed for total mercury (THg), and a subset of the 2002 samples was analyzed for methyl mercury (MeHg, n=30 crayfish). Crayfish trophic position was determined using carbon and nitrogen stable isotope analysis (n=29 crayfish). Watershed, morphometric and water chemistry data were assembled from previous studies. 
Results/Conclusions

Crayfish total mercury concentrations averaged 51 ng/g among the seven VOYA lakes and were dominated by the methylated form (mean=88%). The concentrations spanned an order of magnitude and differed significantly among lakes (p < 0.001). Crayfish THg was not significantly correlated with crayfish body weight (r=-0.047, p=0.623) among lakes. ¹³C and ¹⁵N signatures of crayfish differed little among lakes, and neither was significantly correlated with crayfish THg burden or body weight (α=0.1). Few watershed, morphometric or chemical attributes explained much variation in crayfish THg; however, total organic carbon, which likely originated in catchment wetlands, independently explained 78% (p=0.009) of the variation in mean crayfish THg. After accounting for between-lake variation in THg, mercury concentration in native O. virilis did not differ significantly from that of invasive O. rusticus. Crayfish THg explained significant variation in northern pike THg over the nine lakes for which data on both species were available (r²=0.92), suggesting that crayfish may be a useful proxy for mercury burdens in game fish. Overall, our results suggest that in this region lake-to-lake differences, particularly organic carbon concentrations, influence crayfish THg more than crayfish size, trophic position, or species identity.