Primary Production in the Northwestern Part of the Pacific off Honshu, Japan

Abstract

The primary productivity in the northwestern Pacific was discussed on the basis of the results obtained during the summer cruise in 1961. 1) Vertical stratification of phytoplankton was clearly recognized in the Oyashio area as well as in the Kuroshio area. Maximal chlorophyll concentration in water occurred at the depth where the light intensity reduced to ca. 10% or slightly less of the surface intensity. Chlorophyll amount for the entire euphotic zone in the Kuroshio area, the Oyashio area, and the mixed water region was 25-45, 50-70, and 40-55 mg/m², respectively, and that in a littoral region of the Kuroshio, ca. 120 mg/m². 2) Differentiation of photosynthetic pattern into sun and shade types, and light inhibition in photosynthesis were evidently observed, the facts being in good accordance with the results in 1960. 3) Photosynthetic rate for the surface samples from the Kuroshio, the Oyashio, and the mixed water region was at optimal light intensity 0.6-2.0, 3.5-6.0, and 2.2-4.0 mgC/chl. mg/h, respectively. 4) Daily gross production for the entire euphotic zone was estimated by the chlorophyll method. The values calculated for three weather conditions, clear, slightly clouded, and clouded, were respectively 0.36, 0.29, and 0.16 gC/m²/day in the Kuroshio area, and 0.83, 0.72, and 0.49 gC/m²/day in the Oyashio area. 5) The Kuroshio area differed strictly from the Oyashio area in the concentration of phosphate in waters. The ratio of nitrogen to phosphorus in the sea water was anomalously small over all the areas surveyed, especially in the Kuroshio area. Such nutrient conditions might be responsible in part for the difference between the Kuroshio and the Oyashio areas in photosynthetic rate and standing crop of phytoplankton, and consequently in the primary productivity.