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 semilunar 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>1700m3 s-1) occurred in the middle of the observation period. Before the flood (<100m3 s-1), high concentrations of POC and pheophytin (>3mgl-1 and>10 μgl-1, respectively) were observed in the ETM, upstream from which chlorophyll a concentrations were high (>50μgl-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 (<3mgl-1, <3μgl-1, and <5μgl-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.
