Trophic importance of benthic microalgae to macrozoobenthos in coastal bay systems in Korea: dual stable C and N isotope analyses

Abstract

The role of benthic microalgae as the source of primary organic matter for the macro- zoobenthos community was examined in 3 bay systems of the south coast of the Korean peninsula using stable carbon and nitrogen isotope ratios (δ 13 C and δ 15 N) as a dual tracer. The δ 13 C and δ 15 N of riverine particulate inputs, primary producers and macrozoobenthos collected from the intertidal and subtidal habitats of the bays were measured and compared with those from adjacent offshore waters. Extensive intertidal flats occurred with wide marsh plains (primarily Phragmites australis) at the uppermost part of the intertidal zone in all 3 bay systems, but different hydrologic features were exhibited among bay systems. There were no systematic variations in δ 13 C for the same plant taxa, but different producer groups were clearly separated by their δ 13 C values. Benthic microalgae and macroalgae, which had similar δ 13 C values, were separated by their δ 15 N values. The δ 13 C values of macrozoobenthos collected from the offshore areas were similar to those of phytoplankton. Macro- zoobenthos collected from all 3 bay habitats had similar δ 13 C ranges, and their δ 13 C values (means of all consumers, feeding types and species) were less negative than those of the offshore macro- zoobenthos. Isotopic linkage between sources and benthic consumers, based on the documented trophic fractionation for C and N, indicated that benthic microalgae and phytoplankton are the major sources of organic matter supporting macrobenthic consumers in the bay systems. A 2-source mixing model revealed that pelagic feeders in both the intertidal and subtidal habitats subsisted in equal shares on marine phytoplankton and benthic microalgae, whereas the food source of benthic feeders was derived predominantly from benthic microalgae. Phragmites or riverine particulate inputs appeared to have made little contribution to the diets of consumers in the bay systems. Our collective isotopic data set suggested that benthic microalgae, along with phytoplankton, constituted the organic matter that forms the trophic base of pelagic as well as benthic food webs in the coastal bays, despite differing vegetational composition and hydrology.