Bioaccumulation and Trophic Transfer of Hg in Aquatic Food Webs
Celia Y. Chen, Ph.D.
Department of Biological Sciences
Our research has focused on the fate of toxic metals in aquatic food webs. 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 and transferred to higher trophic levels. 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 exposures of toxic metals to their bioaccumulation in aquatic food webs. In order to understand the factors that influence metal exposure, bioavailability, and uptake, we study patterns of mercury 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.
Specifically, our research has examined mercury in the environment, 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.
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.
Later, in an experimental study using large cattle tanks as our ecosystems, we discovered that when nutrients (nitrogen and phosphorus) were added to the tanks at a range of concentrations, MeHg concentrations in algae were lower in the tanks with higher nutrients and resulting higher algal densities. This was strong evidence for a mechanism called algal biomass dilution of MeHg at the base of the food web. 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.
Algal biomass dilution is one explanation for lower mercury in fish in more productive systems. However, organisms in aquatic ecosystems that grow more rapidly can undergo growth dilution, during which the speed and efficiency of the organism's growth results in lower tissue concentrations of MeHg in those individuals. We have conducted laboratory experiments with Daphnia, a freshwater crustacean, and field experiments with juvenile Atlantic salmon in streams, showing that animals that consume either higher quality food or more food and grow faster have lower MeHg in their tissues on a per biomass basis. This growth dilution likely accounts in part for the lower MeHg tissue concentrations in ecosystems that have higher concentrations of nutrients and plankton densities.
Recently, we have 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 concentrations of MeHg, not sediment concentrations. Our results to date indicate that organic carbon 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.
Celia Chen Pubmed
Carol Folt Pubmed
Robert Mason Pubmed