Attached microalgae contribute to planktonic food webs in bays with fish and pearl oyster farms

Abstract

Planktonic food webs are primarily dependent on organic matter derived from phytoplankton. In coastal areas, aquaculture has accelerated in recent decades, and attached algae and invertebrates proliferate on the farming cages. We hypothesized that the organic material on the farm structures is important to planktonic food webs and that the effects of aquaculture differ between fish (fertilized) and pearl oyster farms (not fertilized). To test these hypotheses, we examined the planktonic food webs at fish and pearl oyster farms using stable isotopes in the Uwa Sea, Japan. We collected zooplankton, particulate organic matter (POM, predominantly phytoplankton), attached algae, and macroinvertebrates in July 2005 and February 2006. Based on the isotope mixing model results, the attached microalgae contributed up to approximately 70% of the copepod food sources, and the contribution of attached microalgae to cyclopoid copepods was similar to their contribution to attached macroinvertebrates. Amphipods mainly fed on planktonic detritus from microalgae that had detached from the farm structures. The contribution of attached microalgae to copepod biomass was lower on the pearl oyster farm than on the fish farm. Our results show that attached microalgae from sea farms are important food sources for planktonic food webs in areas with fish farms and that the 2 food webs, attached and pelagic, are coupled through zooplankton grazing. © Inter-Research 2008.