Dynamics of particulate organic matter in the estuarine turbidity maximum of the Chikugo [Japan] River estuary, Ariake [Japan] Sea, in summer: Influence of the fluctuation of freshwater discharge

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 cubic m/s) occurred in the middle of the observation period. Before the flood ( 100 cubic m/s), high concentrations of POC and pheophytin ( 3 mg/l and 10 micro g/l, respectively) were observed in the ETM, upstream from which chlorophyll a concentrations were high ( 50 micro g/l). 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, 3 micro g/l, and 5 micro g/l, 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 (deltasup(13)C; ca. -25 to -23 permille) and higher C/N atomic ratios ( 9), in contrast to the POM in chlorophyll a maxima (deltasup(13)C, ca. -30 to -19 permille; 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 deltasup(13)C values of living phytoplankton were variable spatially and temporally.