Quantitative sampling of plankton, largely composed of phytoplankton, has been carried out in Oshoro Bay, west coast of Hokkaido, from 1949 through 1969, and in Matoya Bay, Pacific coast of Central Honshu, from 1949 through 1985. In Oshoro Bay, plankton standing crops were very rich
in 1954, the year of low sea temperature, low salinity and large snowfall. In Matoya Bay rich plankton standing crops were observed in 1955, 1961 and 1981; and they were poor in 1956–1959, 1962–1980 and 1982–1985. The decline of standing crops from the 1960s to the 1980s
was due to the effect of increasing pearl oyster culture and expanding green laver cultivation in this bay. There wasa bimodal pattern (primary maximum in spring and secondary maximum in autumn) to the productivity of standing crops of plankton in Oshoro Bay. Onset of spring growth began with
reduced salinities before the sea temperature had risen. Large land drainage from melting snow supplied plenty of nutrients to thc bay. In Matoya Bay, the plankton production was bimodal (summer and winter) when averaged for 1948–1985. However, productivity was nearly unimodal with summer
maximum for 1949–1954 and nearly unimodal with winter maximum for 1961–1967. Disappearance of summer growth after 1955 could be explained by the cultivation of green laver that depleted nutrients from the water, resulting in poor plankton standing crops in summer. The rapid development
of pearl oyster culture after 1958 accelerated the decrease of plankton standing crops in summer.
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