Dynamics of Particulate Organic Matter in the Tamagawa Estuary and Inner Tokyo Bay

The distribution of carbon isotope ratio (δ13C) in suspended particulate organic matter (POM) and sedimentary organics in the Tamagawa River and Inner Tokyo Bay was investigated. The objective was to assess the origin and fate of organic particles with emphasis on the growth of phytoplankton in the highly eutrophic estuarine and coastal environments. Variations in the δ13C of POM ( δ13CPOM) in the intertidal river waters and seawaters are explained clearly in the δ13CPOM-salinity figure. The δ13CPOMof land-derived organics showed an almost constant value (−26•9±1•1‰) throughout the year. However, δ13CPOMand salinity in the intertidal zone exhibited a significant difference between warm (summer and early autumn) and cold (late autumn and winter) seasons. In the warm season, δ13CPOMwas highly variable in the Tamagawa Estuary and Inner Tokyo Bay. Growth of phytoplankton was notable, and the POM with high chlorophyll aconcentration could be categorized into two groups with light (around −30‰) and heavy (around −15‰) δ13CPOM, which appeared in the low- and high-salinity areas, respectively. The former light δ13CPOMvalue was caused by the light δ13CPOMvalue (−13•7‰) of substrate CO2under the normal algal growth rate. The heavy δ13CPOMvalues were observed at sites where pH of the water was significantly high (around 8•5). It was thus suggested that a rapid algal growth rate such as a red-tide phenomenon was highly limited by the CO2uptake process in this type of blooming. This is because the substrate δ13C value was kept almost constant (0•0‰) with its high dissolved inorganic carbon concentration (2 mM) in the seawater. Due to the light δ13C value of autochthonous particles in the high-salinity area in winter, no significant difference in the δ13CPOMlevel was found from the upper to lower tidal zones, regardless of changes in salinity. Consequently, the δ13CPOM-salinity figure was successfully constructed, and a framework assessing the dynamics of POM involving the phytoplankton growth rate was proposed for the highly eutrophic estuarine and inner bay areas. The δ13C of sedimentary organics in Inner Tokyo Bay reflected the heavy δ13CPOMin the overlying water. The δ13C value tended to decrease from the shallow coastal areas to the deep open bay. This trend clearly corresponded to the annual mean δ13CPOMin the water column of overlying seawater. Variations in the δ13C value of the coastal sediments from the estuary to the open bay were illustrated by a dynamic model rather than a two-source model.