Fish are known conduits of metal exposure to humans and wildlife. Yet, there is a great deal of unexplained variation in metal levels found when fish from different lakes even within the same region are compared. Studies indicate that a large part of the lake-to-lake variation may be driven by as yet not understood differences among lower trophic levels of the food web. Our working hypothesis is that lakes characterized by small-bodied Vs. large-bodied zooplankton food webs transfer metals from algae to fish differently, and these differences can be predicted from taxa-specific properties such as metal exposure rates (via activity), burdens and feeding web relations. Our recent research suggests that two significant aspects of food web structure strongly influence metal movement into fish. First, we found that increasing the number of feeding connections within zooplankton trophic levels results in a reduction in movement of metals to fish. First, we found that increasing the number of feeding connections within zooplankton trophic levels result in a reduction in movement of metals to fish. Second, we find evidence that particular zooplankton taxa ("keystone conduits") profoundly influence the degree of bioaccumulation of metals suggesting that lakes dominated by different zooplankton taxa will differ in the level of metal trophic transfer to fish. Our end goal is to develop a mechanistic model to explain variation in metal burdens for 4 potentially toxic metals (As, Hg and MeHg, Zn and mechanistic model to explain variation in metal burdens for 4 potentially toxic metals (As, Hg and MeHg, , Zn and Cd) in plankton and fish across a variety of systems that arise from differences in the food web structure and the ability of particular taxa to accumulate, magnify or dilute metals. This proposal has four specific aims. Aim 1 is to characterize metal trophic pathways in the field and test whether the transfer to fish diminishes as zooplankton complexity increases. We will contrast metal transfer between lakes with large-bodied, less complex (LLC) Vs. small-bodied, more complex (SMC) webs and tests for temporal consistency across metals and taxa within lakes. Aim 2 is to determine the strength and consistency of specific zooplankton taxa as conduits of metals to fish. We hypothesize that larger bodied, metabolically active zooplankton will have higher total metal burdens than smaller, , less active zooplankton. Aim 3 is to test whether increasing algal productivity decreases the movement of different metals to top trophic levels. Novel stable metal isotope techniques will be used to compare trophic transfer in LLC and SMC food webs. Aim 4 is to develop general and specific biomarkers to investigate the environmental relevance responses to metal stress in natural field populations.