Abstract
Mercury (Hg) is a widespread environmental pollutant anthropogenically released into aquatic ecosystems where it is tightly retained and cycled through a combination of abiotic and biotic processes. The organic form of Hg, Methylmercury (MeHg), binds to organic carbon chains and efficiently bioaccumulates through aquatic food webs to toxic levels in consumers. For these reasons, understanding the transport and fate of mercury, including the role of organisms in these cycles, is important. The premise of the Somatic Growth Dilution (SGD) concept is that bioaccumulation of contaminants will become more dilute within an organism as it displays higher growth efficiency. While multiple studies have substantiated the effects of SGD on reduced MeHg concentrations at the individual level, the implications of SGD to populations and overall food web dynamics is uncertain. Herein, we examined the influence of SGD on total MeHg standing stock and flux in stoneroller (Campostoma oligolepis) populations at five sites in East Fork Poplar Creek (EFPC), a stream contaminated by Hg on the Oak Ridge Reservation. We assessed patterns in periphyton and stoneroller MeHg concentrations in relation to relative growth and population density to determine evidence of SGD and density-dependent growth, respectively. We then calibrated a biodynamic model relating MeHg concentration in stonerollers to growth rate, assimilation efficiency, uptake, and elimination of MeHg. Using over 30 years of population data, individual biodynamic models were scaled to the population level to analyze the effects of population density and growth on MeHg standing stock (ug m-2) in stoneroller populations and flux (ug m-2 d-1) from periphyton communities to stonerollers. We found evidence of both density-dependent and “boom-and-bust” growth patterns for stonerollers in EFPC. In congruence with the SGD concept, sites with higher average relative growth had lower MeHg concentrations. However, total MeHg standing stock and flux was strongly related to population density, irrespective of changing growth patterns. Additionally, higher relative growth was positively related to a higher total MeHg flux. However, the implications of growth on the transfer of MeHg in food webs is complicated and dependent upon population size, age structure, and growth-at-age. Our results also suggest that stonerollers may play significant roles in contaminant cycles in EFPC by mobilizing considerable quantities of Hg, storing significant reservoirs of hg and MeHg, and transferring MeHg to higher trophic levels.
