Journal of Oceanography, Vol. 55 (No. 4), pp. 493-503, 1999
Akihiro Shiomoto1 and Hiroki Asami2
1National Research Institute of Far Seas Fisheries, 7-1 Orido 5-chome, Shimizu, Shizuoka 424-8633, Japan
2Hokkaido Fish Hatchery, Kitakashiwagi 3-373, Eniwa, Hokkaido 061-1433, Japan
(Received 2 November 1998; in revised form 16 February 1999; accepted 17 February 1999)
Abstract: We have determined chlorophyll a(Chla) concentration, primary productivity, cell density and species composition of diatoms, and the number of microzooplankton at the surface in the subarctic North Pacific in January 1996. The wet weight of copepods obtained by vertical tows from 150 m to the surface was also measured during the cruise. Chla concentration and primary productivity tended to be higher in the region west of 180°, the western subarctic North Pacific (WSNP), than east of 180°, the eastern subarctic North Pacific (ESNP). The same results were observed for the total diatom cell densities and for the genera Thalassiosira and Denticulopsis. Significant linear relationships were observed between the Thalassiosiracell density and Chlaconcentration and primary productivity, indicating that Thalassiosiracontributes to the high-WSNP and low-ESNP distribution patterns of Chlaconcentration and primary productivity. Moreover, naked ciliate abundance tended to be lower in the WSNP than in the ESNP, whereas copepod biomass showed an inverse trend. Significantly negative Spearman rank correlations were found between the Thalassiosiracell density and the number of naked ciliates and between the number of naked ciliates and the wet weight of copepods. These results indicate that copepod grazing indirectly controls Thalassiosiracell density via predation on the naked ciliates. We conclude that the high copepod biomass in the WSNP is a factor controlling the high-WSNP and low-ESNP Thalassiosiraabundance and hence Chlaconcentration and primary productivity patterns.