Spatial distribution of the toxic dinoflagellate Alexandrium tamarense in summer in the Okhotsk Sea off Hokkaido, Japan

Abstract

To investigate the mechanisms influencing the spatial distribution of the toxic dinoflagellate Alexandrium tamarense in the Okhotsk Sea off Hokkaido, Japan, intensive field surveys were conducted at 34-37 stations in late July every year from in 2002-2007. Alexandrium tamarense occurred every year. However, the abundance of A. tamarense fluctuated year by year, with extremely low cell densities of A. tamarense in 2005. High abundances of A. tamarense were found frequently in the oceanic area of the surface low-salinity water (LSW, salinity 32.5) and the mixed water (MW). Low abundances were found along the coastal area of the Soya Warm Current (SWC, salinity 33.6) and in the dichothermal water (DTW, temperature 2°C) in the layer of oceanic areas deeper than 30m. The PO4-P concentration in each water mass was in the order DTW MW LSW SWC and the lowest PO4-P concentration that occurred in the SWC is considered to be a potential limiting factor for the growth of A. tamarense. The reason for the low A. tamarense abundance in the DTW is considered to be the low water temperature and low light intensity. It is concluded that the water mass of LSW has favorable conditions for the growth of A. tamarense. The relative frequency of each water mass fluctuated every year and the results suggest that the frequency of occurrence of the LSW and MW is one of the most significant factors controlling the abundance of A. tamarense in the area. © The Japanese Association of Benthology.