Growth environment of diatoms in turbid water in the inner western part of Ariake Bay during winter

Abstract

To evaluate nutrient and light conditions as environmental factors that affect diatom growth in coastal areas of the inner western part of Ariake Bay, observations were conducted in the Kashima River and Shiota River estuary system. During high tide, nutrient concentrations increased between the upper and intermediate areas of the estuary due to river discharge. In these areas, cell densities of genus Skeletonema and Asteroplanus karianus tended to be high, reaching 22,350 and 6850 cells mL−1, respectively, whereas the euphotic depth (zeu)-to-water depth (z) ratios were low (0.1–0.3). In the lower estuary, where high salinities (> 28) were observed, zeu/z ratios were high (0.5–0.9) due to decreased turbidity, but nutrients were depleted. These results show that the diatom growth environment at high tide was characterized by low zeu/z ratios in the upper and intermediate areas of the estuary and low-nutrient conditions in the lower estuary. Time series observations in the lower estuary, performed to investigate the movement of water mass and variability in the growth environment with the tidal cycle, show a decrease in salinity during ebb tides and increases in nutrient concentrations and diatom cell densities, indicating movement of the water mass from the upper and intermediate areas to the lower estuary as the tide ebbs. Furthermore, the reduced water depth resulted in high zeu/z ratios. These results imply that nutrient and light conditions were improved in the lower estuary at low tide, and those better conditions made this area suitable for diatom growth.