Species richness, trophic efficiency, and energy resources in lakes: Fish exploitation of littoral primary production across an ecosystem size gradient

Background/Question/Methods

Lake ecosystems are fueled by autochthonous primary production originating in both near-shore littoral and open water pelagic habitats. The function of littoral primary production in lake food webs remains poorly resolved due to a scientific emphasis on the more homogeneous and often more spatially extensive pelagic zone. We compiled literature data from 81 lakes worldwide to explore the extent to which fish are energetically dependent on carbon fixed by littoral attached algae. Lake surface area ranged from 0.017 to 82000 km² and mean depth ranged from 1.5 to 740 m. ¹³Carbon end members for littoral and pelagic primary production were combined with ¹³C data for fish to determine the relative reliance of each fish species on littoral carbon sources. We used information on water column chlorophyll, light attenuation, trophic status, and morphometry to model the contribution of littoral algae and phytoplankton to whole-lake primary production.
Results/Conclusions

In the vast majority of lakes, the average reliance of fish on benthic algal carbon exceeded the relative contribution of benthic algae to ecosystem primary production. Across lakes, benthic primary producers contributed an average of 38% of whole-lake primary production, but the average percent reliance of fish on littoral algal carbon was 56%. This does not indicate that 56% of fish production was supported by benthic algae, because each fish species was weighted equally due to a lack of information on fish abundances. The results may reflect a greater efficiency of energy transfer between trophic levels in littoral compared with pelagic food chains. However, an analysis of habitat use by fishes indicated that most fish species inhabit the littoral zone, while relatively few species inhabit the pelagic zone exclusively.  Although littoral zones represent an increasingly smaller proportion of total lake habitat as lake size increases, the total extent of littoral zone habitat is a positive function of lake size. There was a strong positive correlation between ecosystem size and fish species richness, but the reliance of fish species on benthic primary production was not correlated with ecosystem size.  The consistently high energetic reliance of fish on littoral primary production appears to reflect the concentration of fish diversity (but not necessarily fish production) in the littoral zone. This pattern is independent of lake size and indicates that it takes less primary production per unit area to support a single species in the littoral zone than is required to support a species in the unstructured pelagic zone.