Spatial dietary shift in bivalves from embayment with river discharge and mariculture activities to outer seagrass beds in northwestern Philippines
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To investigate the spatial variation in bivalve food sources along a pollution gradient and assess bivalve contribution to biogeochemical cycles in tropical coastal ecosystems, the δ13C and δ15N values of bivalves and their potential food sources were studied in northwestern Philippines. In a semi-enclosed embayment affected by river discharge and mariculture activities, bivalves depended primarily on 13C-depleted suspended particulate organic matter such as phytoplankton and/or fish feeds. However, toward the relatively oligotrophic seagrass beds, the bivalve food source gradually shifted to more 13C-enriched resuspended and/or settled particles. Furthermore, in the outer seagrass beds exposed to the open ocean, bivalves mainly relied on similar food sources, such as detritus of microalgae, regardless of the distance from the embayment. These trends appear to reflect the ready availability of the food sources. Especially in the outer seagrass beds, a semi-closed material cycle within the vicinity of the sea bottom likely emerged between bivalves and algae, but not between the phytoplankton in the overlying water column. This resulted in a relatively weak benthic-pelagic coupling for bivalves. These cycles would need to be taken into account when estimating the biogeochemical cycles in eutrophicated coastal areas.
KeywordsPhytoplankton Bivalve Particulate Organic Matter Particulate Organic Carbon Sedimentary Organic Matter
We thank K. Nadaoka and M.D. Fortes for leading this project. We are also grateful to C.M. Ferrera, T. Tsuchiya, E.C. Herrera, J.C. Rengel, M. Atrigenio, D.L. Mancenido, T. Yamamoto, L. Bernardo, A.S.J. Wyatt and staff of University of the Philippines (UP), and UP Marine Science Institute Bolinao Marine Laboratory (BML) for supporting field work. We are also sincerely thankful to the JICA Philippines office and to Y. Nagahama, Y. Geroleo, and the CECAM coordinating office for all logistic arrangements.
Compliance with ethical standards
The present study was supported by the Japan International Cooperation Agency (JICA) and Japan Science and Technology Agency (JST) through the Science and Technology Research Partnership for Sustainable Development Program (SATREPS) under the project “Integrated Coastal Ecosystem Conservation and Adaptive Management under Local and Global Environmental Impacts in the Philippines (CECAM)”, and by JSPS Grant-in-aid for Overseas Scientific Research Grant Number 23405002 (Granted to T.M.).
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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