Trophic Transfer of Metals in Estuarine Food Webs

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Coastal Ecosystem and Public Health


Ata Bilgili Dartmouth College Co-Principal Investigator
Celia Chen Dartmouth College Principal Investigator


Forty-four states in the U.S. have instituted fish consumption advisories pertaining to mercury due to the rising concern over health risks from exposure to this neurotoxin. Mercury is listed as the number two agent of environmental and human health concern in the U.S and internationally, second only to arsenic in drinking water, according to the U.S. EPA, the Centers for Disease Control and Prevention, and the World Health Organization. It is the number one agent of concern among contaminants that bioaccumulate and biomagnify. The Gulf of Maine is of particular interest for mercury (Hg) bioaccumulation due to the strikingly high levels of atmospheric deposition of Hg in the region. Current atmospheric models of Hg transport show that some of the highest deposition occurs in the northeastern U.S. and Canada (NESCAUM, 1998). Moreover, coastal sites receive significantly higher atmospheric Hg deposition than inland sites (VanArsdale et al., 1998).

Human exposure to mercury is largely from consumption of fish which bioaccumulate both inorganic and organic forms from the water and from their food. To date, research on the transfer of mercury in aquatic food webs to fish has focused on freshwater systems where concentrations in fish are known to be related to a variety of biotic and abiotic factors (Watras et al., 1998; Chen et al., 2000; Mason, 2002; Pickhardt et al., 2002). However, consumption of fish by people in the U.S. and their exposure to mercury is largely from marine systems. Mercury concentrations in commonly consumed species such as tuna, shark and swordfish are at levels that can cause neurological and developmental effects in young and unborn children. Predatory marine species obtain their mercury burdens largely from their prey food, which reside in productive coastal and estuarine waters.

Estuaries have long been considered important nursery grounds for many marine species that are consumed directly by coastal predatory species including humans (Boesch and Turner, 1984; Deegan et al., 2000). Estuaries are also subject to the greatest inputs of pollutants due to the high population densities inhabiting coastal regions and the tunneling of contaminants from adjacent watersheds (O'Connor, 1996). The intertidal and subtidal regions of estuaries comprise the interface between terrestrial and coastal marine systems. It is here that contaminants are deposited in sediments or transported out to the deeper estuary and eventually to coastal waters. This interface is also the region from which productivity is thought to be transported horizontally across the estuarine landscape to the deeper estuary via the "nekton trophic relay" (Kneib, 1997, 2000). It is the mechanism by which different groups of resident and transient nekton species feed in the marsh vegetation and actively transport intertidal production from the marsh to the estuary. This conceptual model is also applicable to the transfer of contaminants in these coastal margins. Here, mercury may be taken up by resident nekton, transferred to transient species, and trophically transferred to estuarine and eventually coastal species that become prey for oceanic predators and humans.

Objective of Research

The objective of our overall research project is to couple the complex ecological processes involved in bioaccumulation and transfer of contaminants in estuarine food webs with the physical processes that influence the fate of contaminants in these inherently dynamic systems. This development grant will support initial pilot work towards the longer term objective. Surprisingly, little work has been done on this problem, despite the importance of these processes to the problem of contaminants in marine fish.

Food webs in the intertidal/subtidal regions of estuaries are both vertical in trophic structure and horizontal in spatial extent. As consumers transfer carbon, energy and contaminants up the food web, these elements are also being moved farther from the intertidal to the subtidal. Moreover, this horizontal transfer is affected by the temporal dynamics of these systems that are largely driven by the tidal cycle. In order to make predictions about the transfer of biomagnified contaminants such as mercury from the terrestrial/marine interface to the coastal ocean, both ecological and physical models need to be constructed and related to one another. The initial work proposed in this pilot project will involve field sampling in three Gulf of Maine estuaries in order to determine the mercury concentrations in sediment and biota. These data will ultimately contribute to a larger research effort involving the development and coupling of ecological and physical models.

In the summer of 2003, we will characterize the intertidal food webs in three different Gulf of Maine sites (Great Bay, N.H., Wells Estuary, Maine, and Salisbury Cove, Maine) that differ in hydrology, physical transport and mixing, as well as contaminant and nutrient inputs. In each site, we will investigate the bioaccumulation and trophic transfer of mercury (inorganic and methyl-mercury), particularly in the resident and transient benthic, epibenthic and nektonic species inhabiting the intertidal and subtidal portions of these systems. We will measure metal bioaccumulation in multiple trophic levels and in the surface sediments. We will also relate the metal bioaccumulation to trophic position of resident intertidal and transient subtidal nekton species using stable isotopes.

Part of the longer term objective is to investigate the physical dynamics of Hg transport and interface the ecosystem dynamics to an existing finite element numerical model of the Great Bay Estuarine System (Erturk et al., 2002, McLaughlin et al., 2002, Bilgili et al., 2002). This physical model is already a part of an ongoing one-year Sea Grant project to quantify estuarine/coastal ocean mixing in the Great Bay Estuary. In support of this modeling effort, data will be collected in summer of 2003 on local mercury sources (including both land and atmospheric) as well as current mercury distributions in the sediment. In addition, sampling efforts in Great Bay will include surface sediment samples in areas where data are unavailable. These samples will be analyzed for total mercury.