Microbes as food for sediment-ingesting detritivores: low-density particles confer a nutritional advantage

Detritivorous fish often selectively feed on organic-rich sediment particles, yet there is a paucity of data on the nutritional quality of different sediment fractions. A saturated cesium chloride solution was used to separate sediment from a freshwater reservoir into low- and high-density sediment fractions to mimic selective feeding on an organic-rich sediment fraction. Microbial biomass and community structure of sediment fractions were determined by phospholipid analysis. Carbon and nitrogen content and isotopic signatures were determined by elemental analysis gas source mass spectrometry. The low-density sediment fraction was enriched in organic carbon and C-12, had higher microbial biomass and relative abundance of microeukaryotes, and contributed less microbial carbon to total carbon when compared to the high-density sediment fraction. Stable isotope values were unable to resolve the source of organic matter present in the sediment density fractions but were able to provide a foundation to test models of gizzard shad nutrition in Acton Lake, Ohio, USA. Combined, biomimicry of selective feeding and microbiological and isotopic assessment of density-partitioned sediment indicate that deposit-feeding fishes gain a nutritionally superior food source by selectively feeding on low-density particles in freshwater habitats.