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.