Depth-specific variation in carbon isotopes demonstrates resource partitioning among the littoral zoobenthos

Benthic animals readily consume benthic algae, phytoplankton and terrestrial organic matter and are themselves a major component of fish diets. However, the effect of variation in resource availability on resource use by littoral macroinvertebrates remains poorly resolved. Using stable isotopes of carbon, we quantified depth-specific resource use by zoobenthic functional feeding groups in five lakes in northern Wisconsin. The littoral zoobenthos was collected from soft sediments at several depths in conjunction with samples of bulk periphyton (top 5 mm of sediment and biofilm) and measurements of benthic algal primary productivity. Periphyton δ¹³C was positively correlated with depth-specific benthic algal primary productivity, but grazer δ¹³C was consistently higher than that of the periphyton. The δ¹³C of infaunal collectors and predators was tightly correlated with, and nearly identical to, bulk periphyton δ¹³C (collectors: R², 0.88; slope, 0.97; P < 0.0001; predators: R², 0.78; slope, 0.88; P < 0.0001). Reliance of collectors and predators on benthic algal carbon varied between 43 and 100%, depending on whether grazers or bulk periphyton was used as the benthic algal end-member. Despite the apparent homogeneity of the sediments, benthic grazers and collectors partitioned resources in a consistent way in our study lakes, indicating either selective ingestion or assimilation of different components of the biofilm.