Interannual variability in trophic relationships of fish in a pristine northern temperate lake

Background/Question/Methods

The study of trophic interactions in freshwater lakes is often complicated by an abundance of species to characterize and anthropogenic impacts such as nutrient loading and invasive species.  We used ¹³C and ¹⁵N stable isotopes to describe trophic relationships of fish at three times over the course of seven years in Lake Hövsgöl, a pristine north temperate lake in northern Mongolia.  The lake, the seventeenth largest in the world, is deep, dimictic, oligotrophic, and home to only eight species of fish.  This paucity of fish species in the lake offers a unique opportunity to characterize trophic dynamics of all of the fish species in a system with minimal or no human impacts.  In addition, these data provide an important baseline for aquatic systems in the face of climate change; northern Mongolia has experienced unusually rapid warming over the last four decades, and this study system provides an exceptional opportunity to assess the effects of climate change on aquatic systems in the absence of other confounding anthropogenically mediated factors.  

Results/Conclusions

δ¹⁵N values indicated that the lake has a food chain length (FCL) of 3.4-3.9, with all fish occupying a trophic level between 2.8 and 3.9.  This low FCL in a very large lake provides yet another counterexample to the proposed positive relationship between FCL and ecosystem size in addition to suggesting substantial omnivory within the lake.  Benthic and pelagic primary consumers in the lake demonstrated distinct δ¹³C signatures, which allowed inferences to be drawn as to the feeding habits of the fish species.  Two fish species indicated a strong reliance on benthic productivity (burbot and Eurasian minnow), one on pelagic productivity (roach), and the rest a mix of the two (lenok, Hövsgöl grayling, perch, Siberian stone loach and Siberian spiny loach).  Fish dependent on benthic productivity showed less variability in their δ13C values than did those dependent on pelagic and mixed sources.  δ15N increased with body size in some but not all species.  Lenok shifted from pelagic to benthic productivity with increasing body size, while minnows did the reverse.  The data from this lake provide an overall picture of a short food chain characterized by some omnivory, interannual variability and ontogenetic shifts in feeding.