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

Abstract

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. delta C-13 and delta N-15 values of primary producers and dominant invertebrates, along with their biomass or abundance, were monitored monthly from December 2004 to November 2005. S. triqueter flourished 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-rush S. triqueter biomass, the expected seasonal change in the isotopic signatures of the 7 dominant macroinvertebrates was not found. A large gap (3.5 to 12.3%o) in the delta C-13 signature between S. triqueter (-27.9 +/- 1.0 parts per thousand) and the marsh-dwelling macroinvertebrates (means from -24.4 to - 15.6 parts per thousand) indicates little utilization of this marsh-derived organic matter by the animals. Surf ace grazers and deposit f eeders (e. g. Hediste japonica, Heteromastus filiformis, Angustassiminea castanea, flyoplax pusilla and Paranthura sp.) had delta C-13 similar to VN and about 4 %. higher than those (- 16.5 +/- 2.1 and 8.7 +/- 1.1 parts per thousand, respectively) of microphytobenthos, revealing that mainly microphytobenthos-derived carbon contributed to their diets. Two suspension-feeding bivalves (e.g. Laternula marilina and Corbicula sp.) showed a similar delta C-13 to that of suspended particulate organic matter (POM) in the channel (-23.0 +/- 0.7 parts per thousand) and the Nakdong River (-24.5 +/- 1.9 parts per thousand). However, their delta N-15 values are about 4 %. higher than that of channel POM (5.7 +/- 0.6 parts per thousand), but lower compared to that of river POM (-11.6 +/- 2.3 parts per thousand), 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 the S. triqueter-marsh invertebrates are generally more dependent on organic matter derived from algal sources than on S. triqueter or terrestrial sources.