EFFECTS OF FISH ON AQUATIC FOOD WEBS AT HIGH ELEVATION

Detmer, Thomas

The research reported here focuses on the role of fish in controlling food web structure and the flow of matter within lakes as well as the export of plankton from lakes at high elevation. The first objective of the present study is to evaluate the effects of fish on nutrients, primary producers, and consumers in lakes at high elevation. This was accomplished by quantifying the effects of fish on nutrients and primary producers in addition to the effect of fish on benthic and planktonic invertebrate body size, biomass, and production. 1) Lakes with fish have lower concentrations of dissolved nutrients and higher algal abundance. 2) Lakes with fish have smaller mean benthic invertebrate body size and lower invertebrate biomass. 3) Lakes with fish have smaller mean zooplankton body size and lower zooplankton biomass. 4) Lakes with fish have lower littoral and pelagic invertebrate production than fishless lakes. These hypotheses were tested by comparing dissolved nutrient concentrations, chl-a concentrations, and benthic macroinvertebrate and zooplankton samples from lakes with and without fish. The second objective of the study was to determine the effect of fish on the relative importance of terrestrial organic matter to lake food webs. 1) Fish increase the relative contribution of terrestrial material used by littoral and pelagic food webs of lakes. This hypothesis was tested through the combination of production data and concentration of naturally abundant carbon and nitrogen stable isotopes in biomass of food web components. The final objective of the study was to determine whether or not changes caused by fish within lakes perpetuate in streams below lakes. 1) The export of plankton biomass and body size is lower in lakes with fish than lakes without fish. 2) Plankton export from lakes at night that do not contain fish is greater than below lakes that do contain fish. These hypotheses were tested through comparison of zooplankton export to streams and the mass flux in streams below lakes with and without fish.