Dynamics of particulate organic matter in the Tamagawa Estuary and Inner Tokyo Bay

The distribution of carbon isotope ratio (delta(13)C) in suspended particle 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 delta(13)C of POM (delta(13)C(POM)) in the intertidal river waters and seawaters are explained clearly in the delta(13)C(POM)-salinity figure. The delta(13)C(POM) of land-derived organics showed an almost constant value (-26.9 +/- 1.1 parts per thousand) throughout the year. However, delta(13)C(POM) and 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, delta(13)C(POM) was highly variable in the Tamagawa Estuary and Inner Tokyo Bay. Growth of phytoplankton was notable, and the POM with high chlorophyll a concentration could be categorized into two groups with light (around -30 parts per thousand) and heavy (around -15 parts per thousand) delta(13)C(POM), which appeared in the low- and high-salinity areas, respectively. The former light delta(13)C(POM) value was caused by the light delta(13)C(POM) value (-13.7 parts per thousand) of substrate CO2 under the normal algal growth rate. The heavy delta(13)C(POM) values 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 to the CO2 uptake process in this type of blooming. This is because the substrate delta(13)C(POM) value was kept almost constant (0.0 parts per thousand) with its high dissolved inorganic carbon concentration (2 mM) in the seawater. Due to the light delta(13)C value of autochthonous particles in the high-salinity area in winter, no significant difference in the delta(13)C(POM)-salinity figure was successfully constructed, and a framework assessing the dynamics of POM involving the phytoplankton growth rate was proposed for the highly eutrophic estaurine and inner bay areas. The delta(13)C of sedimentary organics in Inner Tokyo Bay reflected the heavy delta(13)C(POM) in the overlying water. The delta(13)C value tended to decrease from the shallow coastal areas to the deep open bay. This trend clearly corresponded to the annual mean delta(13)C(POM) in the water column of overlying seawater. Variations in the delta(13)C value of the coastal sediments from the estuary to the open bay were illustrated by a dynamic model rather than a two-source model. (C) 1997 Academic Press Limited.