Nutrient regeneration from riverine high-molecular-weight dissolved organic matter through marine bacterial decomposition in a eutrophic coastal system: The Ariake Sea, Japan

Abstract

Because riverine dissolved organic matter (DOM) has been considered less bioavailable to coastal bacteria than marine DOM produced by marine phytoplankton, nutrient regeneration from riverine DOM through marine bacterial decomposition has rarely been quantified, especially in eutrophic coastal seas. Through incubation experiments, this study aimed to compare nutrient regeneration from the decomposition of riverine and marine high-molecular-weight (HMW) DOM by coastal bacteria in a eutrophic coastal sea, namely the Ariake Sea in Japan. As an indicator of the bioavailability of DOM, carbon contained in total dissolved amino acids relative to dissolved organic carbon (DOC) was higher in marine HMW-DOM than riverine HMW-DOM in July and August, although the opposite pattern was observed in September. Another indicator of the bioavailability of DOM, the degradation index of amino acids, showed more fresh organic matter in riverine HMW-DOM than marine HMW-DOM in August and September. These results suggest that the bioavailability of HMW-DOM was comparable between the river site and marine site. NH4+ and PO43− were released in seawater incubations treated with both riverine and marine HMW-DOM. In addition, amount of riverine DOC decomposition and NH4+ and PO43− regeneration in this study were higher than those in other eutrophic sea. In July, NH4+ and PO43− significantly increased in the riverine HMW-DOM treatment, despite slow DOC decomposition. To our knowledge, this study is the first to show that riverine DOM may be comparable to marine DOM as a source of nutrients in a eutrophic coastal ecosystem. In addition, the western Ariake Sea is one of the most serious red tides area, though NH4+ is always (NO3− + NO2− is occasionally) depleted. Thus, NH4+ supply through riverine DOM degradation may contribute to prolong red tides in this area.