Distribution and modification of diatom assemblages in and around a warm core ring in the western North Pacific Frontal Zone east of Hokkaido

Spatial distribution relationships between diatom assemblages and water types were investigated in and around a 2-year-old warm core ring in the western North Pacific Polar Frontal Zone off Hokkaido, Japan. From the oceanographic and diatom data, principal component analysis and a clustering technique identified four water types and four diatom assemblages. The Background Assemblage had a low standing stock and was distributed over the entire study area, including the core water of the ring. This assemblage was associated with water conditions that were not highly suitable for diatom growth. A Cold Assemblage, probably linked to submerging Oyashio Water, was found in sub-surface waters. Its standing crop was rather high. The two other assemblages also had relatively high standing stocks: one, the Stratified Assemblage, was found around the nutricline in the sub-surface layer and the other, the Warm Streamer Assemblage, in a warm streamer flowing along the outer edge of the warm core ring. We argue here that the Stratified Assemblage emerged from the Background Assemblage because of favorable local conditions (nutrient level), and, conversely, that in the downstream of the warm streamer, the Warm Streamer Assemblage reversed to the Background Assemblage because the nutrient supply was depleted.