[A-CG50] [JJ] Coastal Ecosystems -- 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG50-P01] Dissolved carbon dynamics in rivers and coastal areas of the Philippines: evaluation of terrestrial inputs using dissolved inorganic carbon stable isotopic composition

*Naoko Morimoto¹, Atsushi Watanabe², Yu Umezawa³, Maria Lourdes San Diego-McGlone⁴, Charissa M. Ferrera⁴, Toshihiro Miyajima¹ (1.Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Transdisciplinary Science and Engineering, School of Environment and Society, Tokyo Institute of Technology, O-okayama 2-12-1 W8-13 Meguro, Tokyo 152-8552, Japan, 3.Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo, Nagasaki 852-8521, Japan, 4.Marine Science Institute, University of the Philippines, Diliman, Quezon City 1101, Philippines)

Keywords:terrestrial inputs, stable isotopic composition, dissolved inorganic carbon, particulate organic matter, tropical coastal area

River load of anthropogenic materials is one of key sources for degradation of coastal habitats as with aquacultures. They change coastal water quality directly and indirectly, and local multiple organic sources such as mariculture fish feeds/feces, resuspended sediment and coral mucus complicate those influences. To assess the effect of allochthonous inputs, isotope signatures of dissolved inorganic carbon (DIC) and particulate organic matter (POM) were examined to identify sources and their loading processes. In Bolinao, where mariculture is densely deployed in semi-closed embayment, δ¹³C-DIC and δ¹³C-POC values of river water were almost similar between the wet and dry seasons, and were decreased as decreasing salinity. However, the relationship between δ¹³C-DIC and δ¹³C-POC was unclear. In the coastal area, large decrease of salinity was observed in the wet season. The negative correlation between salinity and each parameter suggests that river inputs mainly decreased δ¹³C-DIC (–5.8‰ ~) and δ¹³C-POC (–28.5‰ ~) in the wet season. In the dry season, mariculture, maybe fish feeds/feces, was mainly attributed to the decrease of δ¹³C-DIC values especially in the surface layer through their decomposition. In contrast, in Iloilo, where some rivers input to the strait among islands, the character of river δ¹³C-DIC and δ¹³C-POC was similar as Bolinao. In coastal area, the decrease of δ¹³C-DIC was not so serious in the wet season (–1.4‰ ~) compared to Bolinao, however low δ¹³C-POC value was observed in the bottom layer (–27.7‰ ~). It may be ascribed to resuspension of settled materials which was originated from river inputs. We try to unravel their underlying multiple processes and discuss the relationship between river and coastal area in terms of dissolved carbon dynamics in those areas.