Bull. Jpn. Soc. Fish. Oceanogr. 73(3), Page 149-160, 2009   Dynamics of particulate organic matter in the estuarine turbidity maximum of the Chikugo River estuary, Ariake Sea, in summer: Influence of the fluctuation of freshwater discharge Keita W. Suzuki1†, Ryo Sugimoto2, Akihide Kasai3, Kouji Nakayama4 and Masaru Tanaka4 1 Laboratory of Marine Stock-enhancement Biology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan 2 Research Center for Marine Bioresources, Fukui Prefectural University, 49-8-2 Katsumi, Obama, Fukui 917-0116, Japan 3 Laboratory of Fisheries and Environmental Oceanography, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan 4 Laboratory of Estuarine Ecology, Field Science Education and Research Center, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan †e-mail: keita@kais.kyoto-u.ac.jp To elucidate the influences of semi-lunar tidal periodicity and freshwater discharge on the dynamics of particulate organic matter (POM) in the estuarine turbidity maximum (ETM), environmental observations (13 cruises) were conducted along the macrotidal Chikugo River estuary in the Ariake Sea, Japan, from 8 June to 9 August 2005. The semi-lunar tidal periodicity was generally observed during the period; the ETM (turbidity>100 NTU) was more developed close to and at salinity 1 during spring tides, corresponding approximately with the maxima of particulate organic carbon (POC) and pheophytin concentrations. However, the fluctuation of freshwater discharge drastically influenced distributions and properties of POM, since a large flood (freshwater discharge>1700 m3 s−1) occurred in the middle of the observation period. Before the flood (<100 m3 s−1), high concentrations of POC and pheophytin (>3 mg l−1 and >10 μg l−1, respectively) were observed in the ETM, upstream from which chlorophyll a concentrations were high (>50 μg l−1). In contrast, the ETM and chlorophyll a maximum were completely flushed downstream by the flood, which resulted in low concentrations of POC, chlorophyll a, and pheophytin (<3 mg l−1, <3 μg l−1, and <5 μg l−1, respectively). The ETM almost recovered three weeks after the flood, as the brackish water front gradually moved upstream with increases in turbidity, POC and pheophytin concentrations. The POM in the ETM had relatively constant stable carbon isotope ratios (δ13C; ca. −25-−23‰ and higher C/N atomic ratios (>9), in contrast to the POM in chlorophyll a maxima (δ13C, ca. −30-−19‰; C/N <8). The present study showed that the POM maximum incidental to the ETM consisted mainly of composite detritus of phytoplankton that was distributed from the river to the sea, whereas the δ13C values of living phytoplankton were variable spatially and temporally.