Carbon sources supporting fish growth in a north temperate lake

Lake consumers are supported by autochthonous organic matter produced by photosynthesis within the aquatic ecosystem and imported allochthonous material produced outside the ecosystem. To evaluate carbon sources that support fish growth, we enriched a 26 ha lake with inorganic 13C. This labeled the autochthonous primary production and allowed us to determine the extent to which invertebrates in fish diets were supported by this autochthonous carbon. Fish autochthony was defined as the proportion ranging from 0 to 1 of fish growth derived from aquatic primary production. This proportion was calculated using the weighted average of each diet taxa's contribution to fish growth along with the autochthony of diet taxa estimated with dynamic models of delta 13C time series. Age 0 bluegill (Lepomis macrochirus) and yellow perch (Perca flavescens) had the highest autochthony (0.56, 0.57, respectively) because of greater use of zooplankton that were highly dependent on phytoplankton. Older fishes (age 1 and above) of all species had similar estimates of autochthony (mean=0.47 standard deviation=0.04) derived from feeding on either benthic invertebrates or other fishes. Proportional contribution of terrestrial prey (primarily terrestrial beetles) to fish growth was highest for bluegill (0.07-0.22) and substantially lower (< 0.04) for largemouth bass (Micropterus salmoides) and yellow perch. Across species and ages 45% of fish growth could not be attributed to terrestrial prey items or current autochthonous primary production. This residual detrital carbon source is a mixture of allochthonous material and autochthonous material derived from primary production prior to the 13C addition. Fish growth and production in the study lake were not tightly coupled to the current season's primary production. Nearly half of the organic carbon supporting fish growth came from prior autochthonous primary production or allochthonous sources.