Contributions of primary organic matter sources to macroinvertebrate production in an intertidal salt marsh (Scirpus triqueter) ecosystem

The trophic importance of different primary organic matter sources to macroinvertebrates was studied in the intertidal salt marsh (Scirpus triqueter) habitat of the Nakdong River estuary (Korea) using carbon and nitrogen stable isotope ratios. δ13C and δ15N values of primary producers and dominant invertebrates, along with their biomass or abundance, were monitored monthly from December 2004 to November 2005.S. triqueterflourished from spring to summer and vanished in late fall to winter. Biomass of microphytobenthos peaked in late spring to summer. Of 25 taxa of macrozoobenthos collected, 7 were found throughout the year. Despite a dramatic seasonal fluctuation in the marsh club-rushS. triqueterbiomass, the expected seasonal change in the isotopic signatures of the 7 dominant macroinvertebrates was not found. A large gap (3.5 to 12.3‰) in the δ13C signature betweenS. triqueter(–27.9 ± 1.0‰) and the marsh-dwelling macroinvertebrates (means from –24.4 to –15.6‰) indicates little utilization of this marsh-derived organic matter by the animals. Surface grazers and deposit feeders (e.g.Hediste japonica,Heteromastus filiformis,Angustassiminea castanea,Ilyoplax pusillaandParanthurasp.) had δ13C similar to δ15N and about 4‰ higher than those (–16.5 ± 2.1 and 8.7 ± 1.1‰, respectively) of microphytobenthos, revealing that mainly microphytobenthos-derived carbon contributed to their diets. Two suspension-feeding bivalves (e.g.Laternula marilinaandCorbiculasp.) showed a similar δ13C to that of suspended particulate organic matter (POM) in the channel (–23.0 ± 0.7‰) and the Nakdong River (–24.5 ± 1.9‰). However, their δ15N values are about 4‰ higher than that of channel POM (5.7 ± 0.6‰), but lower compared to that of river POM (–11.6 ± 2.3‰), suggesting that organic matter derived from coastal phytoplankton, not river discharge, is a major nutritional source for these bivalves. Overall, our isotopic data suggest that theS. triqueter-marsh invertebrates are generally more dependent on organic matter derived from algal sources than onS. triqueteror terrestrial sources.