Ability of grazing fish to generate particulate organic matter derived from autochthonous primary production

Grazing fish play a role of controlling autochthonous primary production and converting it to secondary production by consuming benthic algae. Moreover, they will also provide autochthonous particulate organic matter (POM) for freshwater ecosystems by dislodging the benthic algal matrix while feeding. However, it has not been confirmed whether grazing fish induce POM production during their feeding on benthic algae. To verify this POM production during grazing by fish, short-term (90 min) feeding experiments were conducted by introducing grazing fish Plecoglossus altivelis into indoor aquaria with benthic algal mats. We found that the fish can generate POM by feeding on benthic algae. Although POM was generated in the aquaria where the fish had not grazed on the benthic algal mats at all, the concentration of POM in the aquaria increased with the frequency of the acts of scraping benthic algae by the fish. Furthermore, we observed that the fish egested faeces which corresponded to about 44% on average of the ingested benthic organic matter. In some cases, the fish generated POM that corresponded to 88% of the removed (dislodged and ingested) benthic organic matter through their feeding and defecation. This study suggested that grazing fish have a function of providing autochthonous POM to freshwater ecosystems both via dislodging benthic algae during feeding and through egestion of consumed benthic algae.