[A-CG15] 沿岸海洋生態系──２．サンゴ礁・海草藻場・マングローブ

[ACG15-P03] 沿岸域における陸源物質動態：水、DIC、POM安定同位体比による評価

*森本 直子¹、梅澤 有²、渡邉 敦³、McGlone Maria Lourdes San Diego⁴、Ferrera Charissa M.⁴、Regino Genevieve L.⁴、灘岡 和夫³、宮島 利宏¹ (1.東京大学 大気海洋研究所、2.長崎大学 水産学部、3.東京工業大学 環境・社会理工学院、4.フィリピン大学 海洋科学研究所)

キーワード：陸源物質流入、安定同位体比、溶存無機炭素、懸濁態有機物、熱帯沿岸域

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.