Influence of River Inflow and Microbial Activity on Distribution of Dissolved Organic Carbon in the Northern Part of Ariake Sea, Kyushu, Japan

Abstract

To identify the factors controlling the distribution of dissolved organic carbon (DOC) in the northern Ariake Sea (Japan), we measured DOC, salinity and chlorophyll a (Chl. a) along transect lines from the largest river discharging into the Ariake Sea (the Chikugo River) to the opposite shore. The DOC concentration was significantly correlated with salinity, although no correlation was found between the Chl. a concentration and DOC. Thus, river inflow is the primary source of DOC. However, the expected concentration of riverine DOC, represented by the y-intercept of the regression curve between salinity and DOC, was much higher than the DOC concentration of the Chikugo River, suggesting the presence of additional DOC sources to the Ariake Sea. We conducted particulate organic matter (POM) decomposition experiments and observed DOC production after incubation. Thus, microbial POM decomposition may be a source of excess DOC. This study is the first to show that river inflow and microbial decomposition of POM affect the DOC distribution in the northern Ariake Sea.

Copepod community structure in the highly turbid innermost part of Ariake Bay in the 2000s with special reference to high abundances of <i>Oithona davisae</i>

“…Eutrophication in the innermost part of Ariake Bay originates mostly from the inflow of the Chikugo River (Tabata et al 2015), which is not polluted by urban or industrial drainage but is highly turbid with mud particles suspended from bottom sediments. Microbial decomposition of particulate organic matter may contribute substantially to the eutrophic waters of the innermost part of Ariake Bay (Uchino et al 2021). Uye (1994) assumed that the change in phytoplankton flora from microphytoplankton in the 1940s to pico-and nanoflagellates in the 1980s might have been a cause for the increase in Oithona davisae in Tokyo Bay.…”