Mercury and other trace elements in a pelagic Arctic marine food web (Northwater Polynya, Baffin Bay) (original) (raw)

Abstract

Total mercury (THg), methylmercury (MeHg) and 22 other trace elements were measured in ice algae, three species of zooplankton, mixed zooplankton samples, Arctic cod (Boreogadus saida), ringed seals (Phoca hispida) and eight species of seabirds to examine the trophodynamics of these metals in an Arctic marine food web. All samples were collected in 1998 in the Northwater Polynya (NOW) located between Ellesmere Island and Greenland in Baffin Bay. THg and MeHg were found to biomagnify through the NOW food web, based on significant positive relationships between log THg and log MeHg concentrations vs. _δ_15N muscle and liver . The slope of these relationships for muscle THg and MeHg concentrations (slope

0.197 and 0.223, respectively) were similar to those reported for other aquatic food webs. The food web behavior of THg and _δ_15N appears constant, regardless of trophic state (eutrophic vs. oligotrophic), latitude (Arctic vs. tropical) or salinity (marine vs. freshwater) of the ecosystem. Rb in both liver and muscle tissue and Zn in muscle tissue were also found to biomagnify through this food web, although at a rate that is approximately 25% of that of THg. A number of elements (Cd, Pb and Ni in muscle tissue and Cd and Li in seabird liver tissue) were found to decrease trophically through the food web, as indicated by significantly negative relationships with tissue-specific _δ_15N. A diverse group of metals (Ag, Ba, La, Li, Sb, Sr, U and V) were found to have higher concentrations in zooplankton than seabirds or marine mammals due to bioconcentration from seawater. The remaining metals (As, Co, Cu, Ga, Mn, Mo and Se in muscle tissue) showed no relationship with trophic position, as indicated by _δ_15N values, although As in liver tissue showed significant biomagnification in the seabird portion of the food web.

Introduction

The occurrence of heavy metals in Arctic marine food webs is of concern despite limited anthropogenic activity in this region. Mercury (Hg) and other metals have been detected in various biota and environmental samples across this region, and there has been concern about sources and concentrations in sample food items frequently consumed by humans and wildlife (Dietz et al., 1996, Fisk et al., 2003). In fact, concentrations of many metals in Arctic biota are similar to concentrations measured in temperate regions (Muir et al., 1999). While Hg and other metals are found naturally in the environment, the origin of elevated concentrations observed in biota from across the Arctic have been attributed to anthropogenic sources in southern regions (Muir et al., 1999). Of particular concern are lead (Pb), cadmium (Cd) and Hg compounds, which have reached concentrations in many Arctic biota that exceed Health Canada and World Health Organization guidelines for human consumption (Muir et al., 1999). Pb in Arctic environments has been declining, partially due to increased global limits on alkyl lead additives in gasoline (Jensen et al., 1997). Cd in seal livers and other dietary items is of concern since it has been found that traditional hunters and their families can potentially ingest Cd above Health Canada guidelines of 450 mg Cd/week (Jensen et al., 1997, Johansen et al., 2004). Despite their importance in the Arctic, the fate and dynamics of metals and other elements in Arctic marine systems remain largely unknown.

Polynyas are persistent regions of open water surrounded by sea ice and are a vital component of Arctic marine ecology (Stirling, 1980). Polynyas are also significant from a socio-economic perspective, since the Inuit from Canada and Greenland hunt traditionally important animals such as ringed seals Phoca hispida that congregate in polynyas. The Northwater Polynya (NOW) is the largest (50,000 km2) and most productive polynya in the Canadian Arctic (Fig. 1). As such, it supports large populations of seabirds and mammals in northern Baffin Bay.

The dynamics of carbon and anthropogenic persistent contaminants in the NOW polynya food web have previously been characterized using stable nitrogen (_δ_15N) and carbon (_δ_13C) isotopes (Fisk et al., 2001, Hobson et al., 2002). Stable isotopes have increasingly been used in aquatic ecosystems to evaluate food web structure and energy pathways. Stable nitrogen isotope ratios (_δ_15N) increases 2‰ to 4‰ with each trophic level, providing a means to assess trophic level (Peterson and Fry, 1987). In contrast, _δ_13C values often demonstrate lower trophic fractionation (0.8‰ to 1‰), and are useful for evaluating sources of primary production within a food web (Peterson and Fry, 1987). Stable isotopes can also be used to estimate the rate of biomagnification of a chemical across the entire food web. The slope of contaminant levels against trophic levels as determined by _δ_15N, often called trophic magnification factors (TMFs) of contaminants, have been applied in Arctic food webs to determine the rate of biomagnification through the food web (Atwell et al., 1998, Borgå et al., 2004).

The objective of this research was to assess the trophic transfer of 22 elements and methylmercury in the NOW marine food web by determining metal and stable isotopes of nitrogen and carbon values in a wide range of species.

Section snippets

Sample collection

Samples were collected during May 1998 from the CCGC Pierre Radisson in the Northwater Polynya located in northern Baffin Bay (Fig. 1). Zooplankton were collected from vertical tows from bottom to surface using a 1-m diameter zooplankton net with 520-μm mesh. After collection, zooplankton were either unsorted (“mixed”) or sorted by species, including the amphipod Themisto libellula, the calanid copepod Calanus hyperboreus and the mysid, Mysis oculata (Table 1). Arctic cod Boreogadus saida (

Food web structure

The food web structure of the North Water Polynya, based on nitrogen and carbon stable isotope values was consistent with that reported by Hobson et al. (2002), which included a much larger sample set and more species. As expected, a continuum of increasing _δ_15N values was observed from ice algae to zooplankton to Arctic cod (B. saida) to seabirds and ringed seals (P. hispida) (Fig. 2). Seabirds occupied a trophic gradient from the dovekie (Alle alle), which feed on zooplankton and fish (

Biomagnification of metals

Total mercury and MeHg were both found to biomagnify in the NOW Arctic marine food web, consistent with other studies on freshwater and marine food webs in the Arctic and sub-Arctic (Atwell et al., 1998, Power et al., 2002) and other regions (Cabana and Rasmussen, 1994, Kidd et al., 1995, Thompson et al., 1998, Bowles et al., 2001). This study found log concentration–_δ_15N relationship slopes of 0.197 and 0.223 for THg and MeHg, respectively, providing further evidence that MeHg is the main

Acknowledgements

Funding for the collection and analysis of stable isotopes was provided in part by a Natural Science and Engineering Research Council (NSERC) Network grant on the Northwater Polynya, Canadian Wildlife Service, Polar Continental Shelf Project and the National Science Foundation (NSF). Funding for the analysis of metals was provided by Northern Contaminants Program, Department of Indian and Northern Affairs and the Northern Ecosystems Initiative, Environment Canada. The authors would like to