Detailed Project History
Our research has focused on the fate of toxic metals in aquatic food webs as conduits to human exposure. Our study systems have ranged from lakes, ponds, and reservoirs to coastal estuaries. In all of our research, we are interested in how metals are taken up by organisms at the base of the food web from water and sediments and then transferred to higher trophic levels including fish that humans consume. We have focused on mercury, a global contaminant and potent neurotoxin, as well as other metals such as arsenic, lead, and cadmium, and are interested in understanding the relationship between environmental levels of toxic metals to their bioaccumulation in aquatic food webs. In order to understand the factors that influence metal exposure, bioavailability, and uptake, we have studied patterns of metal bioaccumulation in biota across broad gradients of sites from contaminated to pristine. We also use experimental approaches to determine the processes that underlie these patterns of metal bioaccumulation.
Our studies of mercury in the environment are significant, as consumption of contaminated fish is a serious public health concern, and fish are the most important agents of exposure for humans to the neurotoxin, methylmercury (MeHg). Mercury in the environment is present in several forms: elemental Hg (Hg0) and Hg2+, the two inorganic forms, and MeHg, the organic and most toxic form of mercury. Although elevated inputs of inorganic mercury to ecosystems are generally thought to result in high concentrations in fish, there are many factors that mediate the ultimate fate and trophic transfer of mercury in the environment. Elevated MeHg bioaccumulation in fish and piscivorous birds and mammals results from a complex sequence of biotic and abiotic mechanisms that control the transport and availability of Hg2+, MeHg production, bioaccumulation, or biomagnification. Through our studies in lakes, streams, rivers, and estuaries, we seek to understand these mechanisms that result in elevated MeHg in the fish that humans consume.
The current goal of our research project is to investigate the effects of multiple environmental factors associated with climate change on MeHg production and bioaccumulation in coastal ecosystems. Current climate change models predict increases in temperature, stream flows, and sea level in coastal waters that will impact the thermal conditions, nutrient and carbon loading, and salinity of these marine ecosystems. We are using experimental approaches, field studies, and modeling to examine the combined and interactive effects of temperature, salinity and organic carbon on MeHg production and fate.
In the early years of our project we studied 20 lakes in the northeastern US that ranged widely in size and depth and differed in their chemical characteristics. Our results, like those of others, showed increasing concentrations of MeHg with increasing trophic level (biomagnification). However, we also found that lakes with greater human disturbance (adjacent human land use, higher nutrient inputs, and higher concentrations of other chemicals) had lower concentrations of mercury in fish. This was a surprising and counter-intuitive result. We also found that lakes with higher plankton densities also had lower concentrations of mercury in fish tissue. These field patterns suggested that lake productivity and nutrient inputs might be a driver in reducing mercury transfer in aquatic food webs. These results were corroborated by field mesocosm experiments where we found strong evidence for a mechanism called algal biomass dilution of MeHg at the base of the food web where high algal biomass induced by nutrient enrichment resulted in lower concentrations of MeHg in algae and in zooplankton. In a meta-analysis of 150 lakes, we also discovered that lakes with higher zooplankton densities across more than 150 lakes in the northeast regions, had lower mercury concentrations in fish. Therefore, in lakes containing fewer individual plankton, each plankton has more MeHg, but in lakes with higher plankton numbers, each plankton has less MeHg. These findings suggest that the indicators of human disturbance relating to lower mercury in fish were likely linked to increased nutrients and productivity, resulting in biomass dilution of mercury as it progressed up the food web. There is evidence to suggest that this is true of marine ecosystems as well.
Recently, we investigated the fate of MeHg in coastal food webs, particularly coastal marshes in northeastern estuaries. We have again taken a gradient approach in studying estuarine sites that range from contaminated to relatively undisturbed. We measured inorganic mercury and MeHg in sediments, water, and a variety of estuarine organisms including species that feed in different compartments: in the mud (worms, amphipods), on the mud surface (crabs, snails), and in the water column (fish, mussels). Here, we have asked the question Do sites that have high levels of MeHg in their sediments have higher MeHg in the organisms that live there, and does the organism's feeding strategy make a difference in their level of bioaccumulation?
By studying 10 coastal sites from central Maine to New Jersey, we found that while concentrations of MeHg in sediments varied by more than 200X, the concentrations in the animals associated with those sediments varied by less than 10X. Something was reducing the bioavailability of the MeHg in the more contaminated sites. It became clear that the more impacted sites had higher organic carbon in their sediments as well; therefore, the organic carbon in those sites reduced the bioaccumulation of MeHg by the organisms living there either by creating conditions that reduce Hg methylation or by reducing the bioavailability of the MeHg to the organisms. Moreover, we found that even though there were much higher concentrations of MeHg in sediments than in the water above, the organisms that were pelagic feeding (feeding from algae in the water column) had higher concentrations of MeHg than organisms feeding directly on detritus in the mud. We have not found this to be true of other toxic metals such as arsenic, lead, and cadmium. In addition, fish concentrations in these coastal sites were related to water column particulate concentrations of MeHg, likely made up of live algal cells and detritus, not sediment concentrations. Our results to date indicate that organic carbon, likely the result of nutrient enrichment, plays an important role in decreasing the bioavailability of MeHg in contaminated sites, and that the flux of MeHg from the sediments to the water column may be more important than sediment concentrations in determining the concentrations of MeHg contamination in the fish that humans consume.
By studying a range of ecosystems, from freshwater to marine, we see that some parallel mechanisms are at work in determining the bioaccumulation of MeHg into aquatic food webs and that these factors ultimately impact human exposure to MeHg in the environment.