英虞湾におけるサイズ別クロロフィルa濃度の季節分布

抄録

Concentrations of size-fractionated chlorophyll a (>20μm, 2–20μm, <2μm) and physicochemical parameters, i.e. temperature, salinity, σt, transparency and nutrients (DIN, PO₄-P and SiO₂-Si) were investigated in four different periods (April, August and November 2000, and January 2001) in Ago Bay, Japan, a renowned pearl oyster aquaculture ground. Nutrients were low in the entire bay throughout the year. Total chlorophyll a concentration was high (<2.0 mgm^-3) in the mouth of the bay in spring and in the innermost area that includes the shallow tributary bays in summer. In contrast, chlorophyll a concentration was low in autumn and winter (>2.0mgm^-3) in the entire bay. However, small-sized phytoplankton (>20μm) was predominant in all cases, i.e. in the entire bay and throughout the year. Thus, although the main size class of plankton found in the bay fits with the prey size usually taken by pearl oysters, their scarcity within the heavily cultured areas of the innermost and tributary bays suggest that food may not be sufficient, especially during the summer when the feeding activity of the oysters reaches a peak.