[A-CG15] Coastal Ecosystems - 2. Coral reefs, seagrass meadows, and mangroves

[ACG15-P03] Dynamics of terrestrial materials in coastal areas: evaluation using multiple stable isotope signatures of H₂O, DIC and POM

*Naoko Morimoto¹, Yu Umezawa², Atsushi Watanabe³, Maria Lourdes San Diego- McGlone⁴, Charissa M. Ferrera⁴, Genevieve L. Regino⁴, Kazuo Nadaoka³, Toshihiro Miyajima¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Faculty of Fisheries, Nagasaki University, 3.School of Environment and Society, Tokyo Institute of Technology, 4.Marine Science Institute, University of the Philippines)

Keywords:terrestrial input, stable isotope ratio, dissolved inorganic carbon, particulate organic matter, tropical coastal area

Evaluation of terrestrial loading of anthropogenic materials in coastal marine environments has become essential given the serious degradation of coastal habitats such as seagrass beds and coral reefs from human activities. Inputs of terrestrial materials change coastal water quality directly and indirectly, and local multiple organic sources such as mariculture fish feeds, resuspended sediment, and seagrass and mangrove detritus, and hydrodynamic characteristics complicate those influences. In order to assess the effect of allochthonous inputs in coastal areas, isotope signatures of water, dissolved inorganic carbon (DIC), and particulate organic matter (POM) were examined to identify sources and loading processes. Where freshwater simply mixes with seawater, δ¹⁸O-H₂O, a conservative tracer of freshwater input in coastal areas, linearly decreases with the decrease of salinity, hence can be used to calculate the mixing ratio as a basis. δ¹³C-DIC also linearly decreases with salinity since δ¹³C-DIC of river water is lower than that of seawater. But δ¹³C-DIC is also affected by photosynthesis and respiration in seawater through isotopic fractionation especially of CO₂ absorption. Where POM is dominated by phytoplankton, δ¹³C-POC is affected by δ¹³C-DIC which the phytoplankton used for photosynthesis, so river water inputs decrease the δ¹³C-POC. Terrestrial POM usually has lower δ¹³C than phytoplankton. In the study sites, Bolinao (mariculture area) and Banate Bay (area affected by siltation) in the Philippines, δ¹⁸O-H₂O positively correlated with salinity in the wet season over the pycnocline layer, indicating freshwater inputs, and a similar pattern was also observed in δ¹³C-DIC, suggesting that large terrestrial DIC inputs overwhelmed local biological processes as the determinant of δ¹³C-DIC. On the other hand, δ¹³C-DIC correlated with the apparent oxygen utilization (AOU) in the bottom layer of the mariculture area in the wet season and all layers in the dry season in Bolinao, suggesting accumulation of CO₂ with low δ¹³Cin the bottom layer in both seasons. Such CO₂ could have been generated by respiration and decomposition of sediment organic matter and excess fish feeds. In shallow seagrass beds, δ¹³C-DIC was mainly controlled by primary production. In Banate Bay, the variation of δ¹³C-DIC was small, and correlations with salinity and with AOU were not clear in the dry season, which is attributed to limited biological activity. δ¹³C-POC reflected lower δ¹³C-DIC in the wet season, but varied even when POC/Chl ratio was low. We tried to unravel underlying multiple processes by using mixing model of terrestrial water and seawater and focusing on the difference between model and measured values.