Stable Isotopes As Intrinsic Markers of Contaminant Dynamics in the Lake Winnipeg Food Web

Abstract

Lake Winnipeg, Manitoba, Canada is the tenth largest freshwater lake worldwide and has one of the largest catchment areas of any aquatic system in Canada. Intensive agriculture, livestock production, urban development and industrialization contribute excess nutrients and contaminants to the lake. Although nutrient-driven eutrophication, establishment of invasive fishes, commercial fishing pressures and avian predation may adversely affect fish community structure and function, trophic interactions among Lake Winnipeg’s biota have not been intensely examined. Similarly, the mechanisms governing the dietary transfer of mercury (Hg) and trace elements are unknown, despite the fact elevated Hg residues in fish tissues led to a commercial fishing ban in the 1970s. Therefore, the objectives of this study were to use a multi-stable isotope approach (δ15N, δ13C, δ34S and δ2H) to a) identify trophic interactions among fishes and piscivorous birds and b) track the dietary transfer of trace elements. An intensive field program was undertaken in 2009 and 2010. Muscle samples from fishes and Double-crested Cormorants (Phalacrocorax auritus; hereafter “cormorant”) were examined isotopically (δ15N, δ13C and δ34S) and the data were used to reconstruct the lake’s food web. The effects of trophic position (TP; δ15N), food source (δ13C and δ34S) and organism size on concentrations of aluminum (Al), arsenic (As), cadmium (Cd), copper (Cu), iron (Fe), manganese (Mn), Hg and selenium (Se) in muscle were examined according to an information-theoretic approach. The utility of scale sampling as a non-invasive method for monitoring contaminant concentrations in fillets of commercially-valuable walleye (Sander vitreus) was also investigated. Similarly, the concerted analyses of stable isotope ratios (δ15N, δ13C, δ34S and δ2H) and Hg concentrations in cormorant flight feathers were evaluated as a non-destructive method for tracking habitat use and contaminant biotransport. Muscle of fishes and cormorants collected from Lake Winnipeg’s north basin were generally depleted in 13C and 34S but enriched in 15N relative to individuals from the south basin. Stable isotopic mixing models also revealed much of the fish biomass consumed by cormorants (north basin: ≥ 59 %; south basin: ≥ 74 %) consisted of species with little or no direct commercial value. Trace element concentrations in biota were often species-specific; however the influence of TP and pelagic, benthic, or detrital nutrient sources were often evident at fine scales. Biomagnification (south basin) and bioaccumulation (north basin) of Hg were observed within the lake’s walleye populations. Fortunately, analysis of walleye scales demonstrated some utility in monitoring fillet-Hg concentrations. Mercury, as well as Cd, Fe, Mn, and Se increased with increasing TP in cormorants nesting on the north basin of Lake Winnipeg. Although the factors driving contaminant concentrations in cormorant muscle differed spatially, concentrations of Cd, Hg and Se in muscle did not differ among adult and hatch-year cormorants, or by nesting location. Measurements of stable isotopes (δ15N, δ13C, δ34S and δ2H) and Hg in primary feathers revealed cormorants accumulated more Hg from Lake Winnipeg than from winter habitats. This study is the first to demonstrate the utility of a multi-isotope approach in describing Hg and trace element trophodynamics for the Lake Winnipeg food web. The lake’s north and south basins appeared to differ in terms of trophic structure and the mechanisms underlying contaminant transfer. Mercury may be the element of greatest concern for Lake Winnipeg, as biomagnification, bioaccumulation and biotransport of this toxic element have been demonstrated.