Mercury Bioaccumulation and Methylation in Stream Ecosystems

Mercury is a neurotoxin and anthropogenic activities such as coal burning substantially elevate its concentrations in different environmental compartments. Through long-range atmospheric transport and deposition, mercury can be dispersed over the globe and thus contaminates virtually all terrestrial and aquatic ecosystems. In particular, aquatic ecosystems are very vulnerable to mercury contamination because of the elevated production of highly bioavailable form, methylmercury, in the waters and the watersheds. In fact, methylmercury is efficiently taken up by the base of aquatic food webs and magnified along the food chain, leading to unsafe level in many freshwater and marine fish species worldwide.;In the past three decades, many studies examined the biogeochemical cycling of mercury in inland lakes and wetlands. However, our understanding on the movement, transformation and bioaccumulation of mercury are very deficient in the flowing waters, despite the similar magnitude and frequency of mercury contamination in streams and rivers as in the lakes and wetlands. Therefore, my thesis research is focused on the bioaccumulation and methylation of mercury in stream ecosystems. Specifically, I examined two important environmental gradients of stream ecosystems in my research. First, I studied how the land cover types in the watersheds can mediate mercury bioavailability in stream food webs and methylation of mercury associated with leaf litter accumulation. Second, I investigated how stream size and primary productivity can influence mercury bioaccumulatiom in stream food webs and in-situ mercury methylation within stream channels.;The results showed that watershed land cover determine the concentrations and speciation of mercury in the water. Interestingly, dissolved organic carbon was found to attenuate the incorporation of dissolved methylmercury into the sestonic particles consumed by the filter-feeding marcroinvertebrates. Therefore, additional factors beyond the total dissolved pool of methylmercury are also crucial in mediating mercury contamination in stream food webs.;Stream size can have influences on mercury bioavailability in a stream network during summer baseflow in northern California. The results revealed that as stream size increased methylmercury in water and biota also increased, which was attributed to insitu mercury methylation in sunlit and algae rich streams. In order to determine whether in-situ mercury methylation is substantial at the ecosystem scale, I used a mass balance study in an algae rich stream reach and found that a significant amount of methylmercury was derived from within the channels, but not from the watershed. Therefore, internal processes in stream ecosystems can mediate stream mercury cycling, in addition to various watershed sources.;Organic matter availability is important in driving mercury methylation but the environmental controls on mercury methylation within litter of terrestrial plants that accumulate in stream ecosystems are poorly understood. This study found that different water chemistry (from different streams and watersheds) significantly influenced release of litter mercury and the subsequent mercury methylation during in-bottle incubation of leaf litter. I found that stream water with high level of suspended solids, nutrients, sulfate and algae can release relatively more litter mercury and more importantly methylate more mercury during the incubations. Therefore, whether litter mercury is transferred to biota by methylatation or transported as inorganic mercury depends in part on the chemical characteristics of the stream waters that the litter enters.;Overall, this research sheds new light on the in-situ biogeochemical cycling of mercury in stream ecosystems and improves our current understanding of the environmental controls on mercury bioavailability in streams. The current findings also suggested that stream characteristics are an important factor in determining mercury bioaccumulation and thus the diversity of the world's stream types need to be examined for better understanding the environmental controls on mercury contamination.