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.

中文翻译：

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有明湾内西部混浊水中硅藻冬季生长环境

为了评估作为影响有明湾内西部沿海地区硅藻生长的环境因素的营养和光照条件，在鹿岛河和盐田河河口系统进行了观察。在涨潮期间，由于河流的排放，河口上游和中游区域之间的养分浓度增加。在这些区域中，Skeletonema 属和 Asteroplanus karianus 的细胞密度往往较高，分别达到 22,350 和 6850 个细胞 mL-1，而透光深度 (zeu) 与水深 (z) 的比率较低 (0.1–0.3) ）。在观察到高盐度（> 28）的河口下游，由于浊度降低，zeu/z 比值高（0.5-0.9），但营养物质被耗尽。这些结果表明，高潮时硅藻生长环境的特点是河口上中区低zeu/z比和下河口低营养条件。下河口的时间序列观测，用于研究潮汐周期中水体的运动和生长环境的变化，显示退潮期间盐度降低，营养浓度和硅藻细胞密度增加，表明水的运动随着潮汐退潮，质量从上游和中间区域到下游河口。此外，降低的水深导致较高的 zeu/z 比率。这些结果意味着低潮时河口下游的营养和光照条件得到改善，这些更好的条件使该地区适合硅藻生长。