Ingestion of microbially-synthesized organic aggregates and egestion of fecal pellets by marine harpacticoid copepods

Bacteria convert dissolved organic matter (DOM) into detrituslike particles and clump small particles into larger ones, which may then become available to higher consumers. Microbial aggregates produced in the laboratory from DOM and particles <100 µm (both sources derived from freshly collected North Sea macroalgae), were ingested by the epibenthic harpacticoid copepods Paramphiascella vararensis and Tisbe holothuriae in short-term laboratory experiments. The production of fecal pellets was used as an indicator of aggregate consumption. Results showed that between 1-3 pellets copepod-1 h-1 were produced by P. vararensis, independently of algal aggregate source and age. In contrast, T. holothuriae produced between 5-13 pellets copepod-1 h-1 depending on the type of algal aggregate source. Microscopical examination of ageing aggregates and pellets confirmed the presence of a rich bacterial flora as well as some protozoans that may provide nutrients for copepods. Enriched fecal material may also be disaggregated and transformed by microbial action into smaller particles and DOM ("fecal pellet loop"), demonstrating the various pathways for carbon flow within detritus-based nearshore food webs