Abstract
In the vicinity of the coastal area of the Okhotsk Sea an oceanographic survey was conducted to elucidate the relationship between oceanographic structure and primary productivity in２００３and２００４. Soya Warm Water（SWW）was found near the coast of Hokkaido during spring and summer. Okhotsk Surface Low Salinity Water（OSLSW）and Intermediate Cold Water（ICW）were present at the offshore surface and at mid-depth, respectively. Nutrient concentration was the highest in ICW at around ２０ μM of nitrate and low in SWW and OSLSW. Nutrient rich ICW showed a low chlorophyll a（Chl a）concentration. Incubation experiments were conducted to examine the effect of water mass mixing on primary productivity. No increase in Chl a and a decrease in nutrients was observed during a week-incubation in the bottles of individual seawater. However, the mixing of OSLSW and ICW, and SWW and ICW showed an increase in Chl a and a decrease in nutrients. A large increase in＞１０μm size Chl a was observed in the latter case. Diatom seemed to be the dominant phytoplankton because silicate also decreased in the bottle. The reason for this rapid growth of diatoms might be the supply of dissolved Fe from SWW because dissolved Fe was the
highest in this water mass at more than２nM. This result explains the persistence of a high Chl a zone near the boundary between SWW and ICW during spring and summer. Furthermore, we examined the relationship between oceanographic structure and diet supply for scallop culture on the seafloor under SWW. There was a tendency of higher growth of scallops during April and June when ICW approached near the farming ground of scallops. This approach of ICW supplied nutrients to SWW, accelerating phytoplankton production, especially large diatoms and as a consequence led to the good growth of scallops.
