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GLERL 1997 Milestone Report
OBJECTIVE: PROTECT, CONSERVE AND RESTORE COASTAL RESOURCES, HABITATS, AND THEIR BIODIVERSITY
PM: Number of studies that improve Assessment/understanding of select nonindigenous species in the Great Lakes
Milestone: Complete field study of the effects of the zebra mussel on food-web transfer of contaminants in the Great Lakes
Scientist: Patricia Van Hoof
A study of the impact of a recent exotic species invader, the zebra mussel (Dreissena polymorpha), on the accumulation of organic contaminants (PCBs and DDT) within the food web of Saginaw Bay, Lake Huron was completed at GLERL. The mussel's large filtering capacity and high lipid content suggest that this organism could potentially accumulate organic contaminants from water, and if consumed expose upper trophic organisms to higher levels of contamination. In addition, zebra mussels could have a large impact on the transport and fate of particle-bound contaminants. Enhanced removal of particulate matter from the water column and subsequent deposition to the benthic region in the form of feces/pseudofeces could increase the exposure of other bottom-dwelling organisms to aquatic contaminants. Field collections of biota, sediment, and water were obtained seasonally from 1993 through 1996. Findings:
- Zebra mussels do not appear to be enhancing the exposure of other benthic organisms to organic contaminants due to feces/pseudofeces production. PCB levels in benthic invertebrates at an uninfested site were comparable to, or greater than, those of benthic organisms (including zebra mussels) at mussel infested sites.
- Trophic transfer of PCBs within the food web of Saginaw Bay was examined quantitatively using stable isotope enrichment of 15N to characterize trophic levels. Using this nondiscrete measure of trophic status a strong exponential relationship between trophic level and PCB concentration is observed for both benthic and pelagic components of the Saginaw Bay food web. White sucker, a fish which feeds almost exclusively on zebra mussels, has no higher PCB content than would be predicted for a pelagic fish residing at the same trophic position. The degree to which PCBs were enriched within the Saginaw food chain was comparable to that observed in Lake Ontario and Lake Baikal. This relationship supports the concept that trophic level and structure are important determinants of bioaccumulation of hydrophobic organic contaminants through food webs. Despite being a very perturbed ecosystem, food web transfer of PCBs in Saginaw Bay follows a biomagnification relationship found in other large lake ecosystems. The consistency of these relationships will result in improved modeling of toxics cycling within aquatic ecosystems and thus better predictions of how reductions in pollution loadings will benefit environmental resources.
(Click on figure to retrieve EPS file)
In order to determine if zebra mussels are enhancing biomagnification, characterization of food web trophic levels is necessary. Trophic levels are often defined in discrete terms: producers, and primary and secondary consumers. Intermediate levels (i.e. organisms that consume from two or more different trophic levels), however, are usually less precisely described. Stable isotope ratios of 15N/14N in organisms have been used with some success in characterizing trophic levels and tracing trophic pathways. Heavier isotopes of nitrogen are enriched by metabolic processes of organisms, such as, assimilation and excretion, relative to lighter isotopes. The numeric difference between del 15N (a derivative of the isotope ratio) of two organisms is a measure of the distance between their trophic levels. Using this nondiscrete measure of trophic status a strong exponential relationship (r2 =0.91) between trophic level and PCB concentration is observed for benthic and pelagic components of the Saginaw Bay food web.
(Click on figure to retrieve EPS file)
Relationships of log total PCB concentration vs. trophic level (del 15N) are shown for PCBs in large freshwater bodies, and the pesticide toxaphene in two small Canadian lakes. The strong relationships (r2=0.62-0.91) between trophic level and contaminant concentration points to the importance of food-chain uptake relative to other routes of environmental exposure. The degree of PCB biomagnification in Saginaw Bay (as described by the slope) is comparable to that observed in Lake Ontario and Lake Baikal. In addition, another complex mixture of organochlorines, toxaphene, was observed to transfer through two subarctic lake food webs (Laberge & Kusawa) to a similar degree as PCBs in temperate lakes.
Current GLERL Zebra Mussel Research Projects can be found at: Non-Indigenous Species Program Page
Current GLERL Aquatic Contaminants Research Projects can be found at: Aquatic Contaminants Program Page
Last updated: 2002-03-26 mbl