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

Abstract

The distribution of carbon isotope ratio (δ 13C) 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 δ 13C of POM ( δ 13C POM) in the intertidal river waters and seawaters are explained clearly in the δ 13C POM-salinity figure. The δ 13C POMof land-derived organics showed an almost constant value (−26·9±1·1‰) throughout the year. However, δ 13C POMand 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, δ 13C POMwas 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‰) δ 13C POM, which appeared in the low- and high-salinity areas, respectively. The former light δ 13C POMvalue was caused by the light δ 13C POMvalue (−13·7‰) of substrate CO 2under the normal algal growth rate. The heavy δ 13C POMvalues 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 CO 2uptake process in this type of blooming. This is because the substrate δ 13C value was kept almost constant (0·0‰) with its high dissolved inorganic carbon concentration (2 mM) in the seawater. Due to the light δ 13C value of autochthonous particles in the high-salinity area in winter, no significant difference in the δ 13C POMlevel was found from the upper to lower tidal zones, regardless of changes in salinity. Consequently, the δ 13C 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 estuarine and inner bay areas. The δ 13C of sedimentary organics in Inner Tokyo Bay reflected the heavy δ 13C POMin the overlying water. The δ 13C value tended to decrease from the shallow coastal areas to the deep open bay. This trend clearly corresponded to the annual mean δ 13C POMin the water column of overlying seawater. Variations in the δ 13C value of the coastal sediments from the estuary to the open bay were illustrated by a dynamic model rather than a two-source model.