Effects of terrestrial organic matter on food sources for the sulf clam Mactra veneriformis by stable isotope analysis in the Geum-river estuary, Korea

Abstract

Stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) was utilized to address the potential food sources and origins for the Bivalvia Mactra veneriformis in the Geum-river estuary under the influence of offshore discharge from the Geum-river. M. veneriformis, waterborne particulate organic matters (phytoplankton, zooplankton, and detritus including suspended organic matter), and benthic organisms (microphytobenthos and meiofauna) were sampled from three sites in Geum-river (two sites adjacent to in and out of estuary bank, one site far distanced). Sampling surveys were carried out twice (May and August) and subjected to stable isotope analysis as well as general analysis of abiotic environmental parameters. M. veneriformis was classified into two groups by size (shell length) and the data was interpreted in terms of seasonal variations, say spring vs. summer variation. The δ13C and δ15N signatures indicated that M. veneriformis seemed to undergo size-dependant selective feeding. Also, variations in spatial distribution and isotopic composition of food sources cross the season seemed to indicate that the clams use food sources along the lotic estuarine system. Apparently, the food sources for the clams collected adjacent to the estuary was found to be terrestrial particulate matters. Overall, the signatures of δ13C and δ15N indicated site- and/or size-specific feeding strategy of the sulf clam in the GeumGeum-river. M. veneriformis, waterborne particulate organic matters (phytoplankton, zooplankton, and detritus including suspended organic matter), and benthic organisms (microphytobenthos and meiofauna) were sampled from three sites in Geum-river (two sites adjacent to in and out of estuary bank, one site far distanced). Sampling surveys were carried out twice (May and August) and subjected to stable isotope analysis as well as general analysis of abiotic environmental parameters. M. veneriformis was classified into two groups by size (shell length) and the data was interpreted in terms of seasonal variations, say spring vs. summer variation. The δ13C and δ15N signatures indicated that M. veneriformis seemed to undergo size-dependant selective feeding. Also, variations in spatial distribution and isotopic composition of food sources cross the season seemed to indicate that the clams use food sources along the lotic estuarine system. Apparently, the food sources for the clams collected adjacent to the estuary was found to be terrestrial particulate matters. Overall, the signatures of δ13C and δ15N indicated site- and/or size-specific feeding strategy of the sulf clam in the Geum-river estuary. Our study provides insight into benthic food web structure and function in the estuarine ecosystem as influenced by terrestrial organic inputs, with particular emphasis on the irregular freshwater discharged through estuary bank, even on a local scale.