Lake food webs: Biological and chemical perspectives

I observed no relationship between fish growth rate and mercury concentration in a lake trout (Salvelinus namaycush) population and conditional support for an inverse relationship in a smallmouth bass (Micropterus dolomieu) population in two Maine lakes. A bioenergetics model indicated that mercury concentration was more responsive to dietary mercury intake than to growth rate. Because fish obtain mercury primarily from their diet, these findings suggest that food web structure and individual diet preferences may influence fish mercury contamination.; I found mercury levels in lake trout, lake whitefish (Coregonus clupeaformis), and their major diet items increased with site depth in Flathead Lake, Montana. These findings imply that individual fish have some long term preferences in foraging depth, and strongly suggest that habitat use can affect fish contamination.; I found that δ13C and δ15N of lake trout, lake whitefish, and their major prey items were generally related to site depth in Flathead Lake. I used δ13C as a proxy for average foraging depth in the fishes and found a significant relationship between δ13C and mercury contamination. I used δ 15N as a measure of fish trophic position and found a positive relationship between mercury contamination and δ15N. However, the interpretation of the nitrogen results is obscured by correlations among δ 15N, depth, and mercury.; δ15N signature of Mysis relicta from the isotope study suggests they obtain substantial quantities of pelagic nitrogen during their diurnal migration to feed on pelagic zooplankton. Using a multi lake study I found lakes with Mysis had a lower proportion of cladocerans (the preferred prey of Mysis) in the vicinity of the thermocline. These finding illustrate how Mysis predation in the water column shunts zooplankton production from the water column to the bottom, allowing deep water fishes to proliferate.; Using otoliths I compared lake trout growth pre and post Mysis in Flathead Lake. Widths of annual increments 1 to 3 were similar, but generally declined in increments 4 to 10 post Mysis. These results suggest that increased growth was not the mechanism by which lake trout populations expanded post Mysis. However, the energetic shuttle of zooplankton production by Mysis foraging may have increased deep water foraging opportunities for small lake trout.