Common Food Sources for Macrobenthic Communities in Different Bottom Covers: Submerged Macrophytes and Benthic Cyanobacteria in a Japanese Temperate Lake

Macrobenthic communities in a lake are affected by the type of bottom cover such as macrophytes or algae. In the southern basin of Lake Biwa, mats of the benthic cyanobacteria (BC) <i>Microseira wollei</i> widely cover the lake bottom and are interspersed with submerged macrophytes (SMs). Because different macrobenthos species appear to occur at those bottoms, we investigated the composition of the communities. Furthermore, as <i>M. wollei</i> is supposed to be inedible to consumer organisms owing to its hard sheath and toxins, the food possibility of the cyanobacteria and macrophytes for macrobenthos was analyzed. In this study, macrobenthos were collected, identified molecularly, counted in the BC and SM zones, and analyzed for carbon and nitrogen stable-isotopic compositions. In the BC zone, asellids and amphipods dominated the macrobenthic community, while chironomids dominated the SM zone. The stable-isotope analysis revealed that <i>M. wollei</i> was a minor food source for macrobenthos and other higher-level consumers, with some macrophytes, particulate organic matter and bottom sediment potentially being a major source. The dominance of crustacean macrobenthos in <i>M. wollei</i> mats suggested that they provided a refuge from predation for crustaceans, whereas SMs were not sufficiently abundant to achieve this. Although different macrobenthic communities in the BC and SM zones were likely supported by common food sources, with the exeption of <i>M. wollei</i>, the present study was conducted over a short period and lacked advanced methods for gut content analysis. Therefore, further monitoring and food web analysis are required.