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Oceanographic surveys were performed in the coastal and offshore regions of the southwestern Okhotsk Sea off Hokkaido, Japan during the sea ice-free period (i.e., April to December), and 39 on-deck incubations for primary production were conducted using a simulated in situ method with 13C tracer. Temperature and salinity at the sea surface differed significantly between the coastal and offshore regions, whereas the concentrations of chlorophyll a (Chla) and nitrates did not. Seasonal changes in vertical profiles and standing stocks of Chla within the euphotic layer (21–70 m depth) in the coastal region differed from those in the offshore region, showing a subsurface Chla maximum during summer and autumn. The vertical profiles of primary production and Chla-specific primary production (PB) indicate that PB is dependent on PAR (photosynthetically available radiation) and that peak primary production seems to be observed around the depth of the subsurface Chla maximum. No regional differences in PB or primary production were observed. PB in the surface layer exhibited clear seasonality. However, the monthly mean of primary production integrated within the euphotic layer over both regions was relatively stable from 400–600 mgC m−2 day−1 from April to November. In conclusion, the primary productivity in the southwestern Okhotsk Sea is stably high from spring to autumn, although the phytoplankton bloom in early spring immediately after the sea ice break was likely missed.

Seasonal changes of primary production in the southwestern Okhotsk Sea off Hokkaido, Japan during the ice-free period