0023-6152

AA00697606

10074

査読無

0023-6152

AA00697606

A thin-layer diatom bloom was examined during four surveys in July and August 2005 around the mouth of the Arakawa River, Japan. During two surveys, a thin layer of concentrated chlorophyll a (thickness 1-3m), was found under a layer of salinity 10-15 over 5km offshore. The dominant taxa in the thin layer were Skeletonema costatum, Thalassiosira binata, and Cryptomonadales. When the thin layer was found, an estuarine circulation pattern occurred around the mouth of the river. The direction of flow was seaward in the upper layer and landward in the lower layer. The seaward flux of chlorophyll a in the upper layer was nearly the same as the landward flux of chlorophyll a in the lower layer. In addition, a surface layer was found over 5km offshore. The surface layer (first upper layer) was formed above the halocline, which was at about 0.5-m depth. In the first upper layer, salinity was low, chlorophyll a concentration was low, and nutrient concentrations were high. Therefore, the thin layer of concentrated chlorophyll a was formed and maintained by phytoplankton remaining around the mouth of the river in response to the estuarine circulation pattern and the supply of nutrients from the first upper layer.