Spring distribution of diatom assemblages in the East China Sea

The distribution of diatom assemblages in the East China Sea in late spring was investigated in relation to hydrographical characteristics. Using prin cipal component analyses, 3 water types and 4 diatom assemblages were defin ed. The distribution pattern of water types showed a 2-layer structure. The nutrient-depleted Surface Water Type forms a layer above the nutrient-lade n Mid-Layer Water Type. In addition, the low-salinity eutrophic Coastal Wat er Type was found near the Changjiang river plume. In general, the distribu tion of diatom assemblages matches that of water types. The Upwelled Assemb lage and the Coastal Assemblage existed respectively in the upwelled Subsur face Kuroshio Water (the Mid-Layer Water Type) and the Changjiang river plu me (the Coastal Water Type). A Background Assemblage with low standing stoc k was found over the entire study area in both the Surface Water Type (nutr ient limited) and the Mid-Layer Water Type (light limited). The Shelf-Break Assemblage was observed near the shelf break, where variations in physical or chemical environments were insignificant. This probably resulted from a n upwelling event that had not previously been observed. Our results indica te that a widespread Background Assemblage is the basic diatom distribution pattern in the East China Sea. At locations where a nutrient source become s available, the Background Assemblage will change into a new assemblage su ch as the Upwelled Assemblage or the Coastal Assemblage.