Stable isotope analyses of the trophic structure of macrobenthos on an artificial tidal flat developed using sediments dredged from pearl oyster [Pinctada fucata] farms in Ago [Japan] Bay

In a study organized by the Mie Prefectural Government, an artificial tidal flat was created on a sandy shore in Ago Bay using enriched sediments that had been dredged from pearl oyster farms. The aim of the artificial tidal flat (ATF) was to remove enriched sediments from pearl oyster farms and to re-mineralize the organic matter in the sediment through the activity by the biota. We investigated the trophic structure of the macrobenthos on the intertidal flat before (November, 2003) and after (November, 2004 and 2005) the ATF was constructed using the stable carbon and nitrogen isotope technique in order to evaluate the effect of the ATF. Before the ATF, the consumers were classified into four groups: group A, which was composed of suspension feeders and characterized by depleted deltasup(13)C values(-19.5 to -18.0permille), group B, in which members had intermediate deltasup(13)C values (-14.8 to -12.1permille), group C, in which members had enriched delta sup(13)C values (-11.5 to -10.7permille) and group D, in which members had enriched delta sup(15)N values (11.9 to 13.7permille). The members of groups A, B, and C are primary consumers and the food sources of each group are considered to have been a mixture of coastal phytoplankton and benthic micro algae, benthic micro algae, and a mixture of benthic microalgae and eelgrass or other unknown primary producers that have enriched deltasup(13)C values, respectively. The members of group D are secondary consumers. Benthic animals collected after ATF construction were also divided into four groups, however the number of species in groups C and D decreased due to the change of sediment composition from sand to sandy mud and to the short duration (8 months) after the completion of the ATF. After the ATF, the suspension feeding bivalves were enriched in sup(13)C by 0.6 to 1.4permille and were depleted in sup(15)N by 0.5 to 1.3permille, suggesting the increased dependency on benthic microalgae that had increased their biomass by utilizing the nutrients from the pearl-oyster farm sediments.