Further studies on decomposition of dissolved organic matter in coastal seawater

Experiments were made on decomposition of dissolved organic matter (DOM) in coastal seawater (Sagami Bay and Tokyo Bay, Japan). Decomposition of DOM seems to proceed in two steps: a rapid decrease in DOM during the first several days, followed by a slow decrease after this time. The apparent rate constant for decomposition of the total DOM was calculated as 0.01 to 0.09 day^-1 for the first step and 0.001 to 0.009 day^-1 for the second step, assuming that such decomposition proceeds as a first-order reaction. A part of such utilizable fractions of DOM may be derived from excretion and decomposition products of phytoplankton, especially “red-tide” organisms occurring in summer. Molecular weight distribution of DOM changed with advancing decomposition. The low-molecular-weight DOM (mainly less than 500) in the surface water of Tokyo Bay was 0.67 mgC/l (24% of the total DOM) at the initiation of incubation; this decreased to 0.36 mgC/l (16% of the total DOM) after 2 days incubation at 20°C in the dark. Low-molecular-weight DOM was thought to be firstly utilized by micro-organisms. After 6 to 10 months incubation, the proportion of low-molecular-weight DOM decreased and that of high-molecular-weight DOM increased.