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We identified phycoerythrin-pigment phenotypes of Synechococcus strains in samples obtained from the Uwa Sea, and investigated the seasonal changes in the pigment-type composition of the Synechococcus community in this sea from January 2001 to March 2002. Two Synechococcus strains (UT01 and UT02) with different flow cytometric signatures were isolated by sorting and subsequent culturing. Excitation spectra showed that these two strains possessed different ratios of phycourobilin (PUB) to phycoerythrobilin (PEB). Flow cytometric signatures of the two types were also different; UT01 showed a higher green to red fluorescence ratio than UT02. The ranges of green to red fluorescence intensity of the strains did not overlap, though the strains were cultured under various light intensities (7-160 mE m -2 s -1 ). In seawater samples, we could divide Synechococcus into lowerand higher-PUB types based on the ratio. There were two peaks of total Synechococcus abundance in June (2.8´10 5 cells ml -1 ) and August (1.6´10 5 cells ml -1 ) in the upper layer at a depth of 0 to 15 m. Major types during these two peaks were different; the higher-PUB type contributed 75.6% to the total Synechococcus community in June, and the lower-PUB type, 58.6% in August. This is the first report which describes the seasonal succession of two phycoerythrin-pigment types of Synechococcus in a coastal sea.

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Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water ͑bottom intrusion͒. No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001. These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton ͑diatoms and dinoflagellates͒ roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton ͑APP͒ cell density, which reached maxima in August of every year at very similar levels ͑4.0-5.0 ϫ 10 5 cells ml -1 ͒. Thus, the seasonal abundance of APP was apparently independent of the occurrence of bottom intrusions. Seasonal changes in cell densities of heterotrophic bacteria showed similar trends to the APP, and temperature-dependent growth of both was indicated. The present study suggests that the matter cycling in the bay varies as a result of shifts in the dominant food linkages, from a microbial food web to a herbivorous food web, due to intermittent nutrient supplies from bottom intrusions.

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