Influences of fish on food web structure and function in mountain lakes

Introduction of new fish species to lakes that contain fish is known to disrupt food webs, but few studies have quantified the effects of introduced fish on flow of organic matter and mechanisms governing food web structure in historically fishless lakes. Contrasting flow of organic matter and food web structure in macroinvertebrate communities in lakes with and without fish provides an opportunity to characterise resiliency of macroinvertebrate food web function when there are changes in community composition and reductions in biomass. Organic matter flow and food web structure were characterised in six lakes with and six lakes without fish at high elevation in the southern Rocky Mountains of Colorado, U.S.A. Carbon (delta C-13) and nitrogen (delta N-15) were used to determine the proportionate contribution of organic matter from primary organic matter sources (periphyton, phytoplankton, terrestrial plants) to consumers including herbivores and higher trophic levels. Contributions of organic matter sources to macroinvertebrates were combined with estimates of littoral macroinvertebrate production in quantifying the contribution of each primary source to total invertebrate production. Although introduced fish altered community composition, body size, and biomass, they did not alter contributions of the organic matter sources to production of macroinvertebrates. The dominant resources contributing to macroinvertebrate production were periphyton and phytoplankton, which combined contributed to 80% of macroinvertebrate production. Despite occupying the highest trophic position (difference in delta N-15 relative to primary sources), fish did not alter trophic structure of littoral macroinvertebrate communities in maximum macroinvertebrate trophic position and community trophic position. Community trophic position weighted by production for macroinvertebrates was related to macroinvertebrate production per unit area but was independent of lake area. In lakes with fish, maximum macroinvertebrate trophic position was similar to that of lakes without fish because large predators (predaceous beetles and dragonflies) were replaced by small predators (tanypod chironomids). The present study indicates that macroinvertebrate food webs can exhibit resilience to introduced or invasive fish even when there are changes in community composition and biomass. This suggests that changes in structure (e.g. biomass, community composition) associated with introduced or invasive species do not necessarily equate to changes in food web function (e.g. flow of organic matter, maximum trophic position). Further, the present study suggests that although maximum trophic position has been shown to increase with lake size for lakes containing fish, the pattern is unlikely to extend to lakes without fish. The relationship between lake size and maximum trophic position for lakes containing fish may occur because fish have expandable niches associated with high motility and indeterminate growth, in contrast to macroinvertebrates.