Resources supporting the food web of a naturally productive lake

Background/Question/Methods

Organic resources often cross habitat and ecosystem boundaries before they are assimilated by consumers in recipient habitats. In lakes, consumer production can be subsidized by terrestrial (allochthonous) organic matter, but the extent of allochthony varies among consumers and lakes. Recent studies suggest that consumer allochthony is high in unproductive lakes that are highly subsidized, but low in productive lakes that receive few subsidies.  Furthermore, differences in habitat usage and feeding abilities may drive differences in consumer allochthony. 

Although lake trophic status is thought to influence consumer allochthony, few studies have assessed allochthony in productive lakes.  Moreover, despite resource use often being a relative measure, the contribution of aquatic macrophytes to lake food webs is rarely evaluated in the context of allochthony. In lakes where macrophytes are abundant, assuming that they are not consumed may cause food-web resource use to be estimated inaccurately, especially for littoral consumers that are capable of feeding on macrophytes.

A Bayesian mixing model and stable isotopes of carbon, nitrogen, and hydrogen were used to evaluate the extent to which six consumers (three fishes, two zooplankton, and a snail) in a naturally productive lake used terrestrial resources, epilimnetic and metalimnetic phytoplankton, benthic algae, and macrophytes.

Results/Conclusions

Resource use varied with consumer habitat usage and feeding ability, but allochthony was consistently low (averaging 15% among consumers).  The pelagic invertebrates Skistodiaptomus oregonensis and Chaoborus spp. relied on phytoplankton from the epilimnion (59% and 49%, respectively), and to a lesser extent from the metalimnion (28% and 26%, respectively); terrestrial resources comprised 9% and 18% of the diet of these consumers, respectively.  The snail Helisoma trivolvis relied mainly on littoral resources (floating-leafed macrophytes; 68% of diet), but terrestrial resources also constituted a substantial portion of its diet (21%).  The fishes integrated among habitats more evenly than the other consumers, but pelagic resources formed the largest portion of their diets (Pimephales promelas = 64%, Lepomis gibbosus = 47%, and Perca flavescens = 47%). L. gibbosus was the most allochthonous fish (23%). The consumers of this productive lake were not highly allochthonous, and tended to rely most heavily on local resources, including macrophytes.