Phytoplankton distribution during the winter convective season in Sendai Bay, Japan

Abstract

We investigated the elevated chlorophyll-a (Chl-a) zone found along the coast in winter in Sendai Bay, Japan, using hydrographic observations and a one-dimensional ecosystem model. Chlorophyll-a distribution was vertically homogeneous with a horizontal gradient and could be approximated as a power function of bottom depth; Chl-a concentration drastically increased with decreasing bottom depth, despite temperature and salinity being almost vertically and horizontally homogeneous. The observed results revealed significant correlations among Chl-a and nutrients concentrations proportional to the Redfield ratio. Diatoms accounted for more than 99% of the detected total cells, indicating the occurrence of a diato m bloom. A one-dimensional ecosystem model, which incorporated vertical mixing and the self-shading effect of phytoplankton, revealed that bottom depth was responsible for the occurrence of the bloom during the convective season in coastal area where vertical mixing reached the bottom and that there existed the critical bottom depth where the integrated Chl-a in the water column remained constant. A bloom could occur where the bottom depth is shallower than the critical bottom depth, not when the depth of the mixed layer is shallower than the classical critical depth and the stratification is established. From the observational and model results, it is suggested that the diatom bloom was induced by oceanic water intrusion, which transported nutrients to the bay and the elevated Chl-a zone was formed within a month after the intrusion.