The photosynthetic rates of phytoplankton and microphytobenthos were measured with a combustion method of
14C in a tidal flat of the Wakaura estuary, Central Japan. In addition, chlorophyll a concentra-tion and a set of environmental factors (water temperature, salinity, nutrient concentrations) in the water and the sediment were measured in order to investigate factors affecting their photosynthetic rates. The chlorophyll a concentration of microphytobenthos (average 20 mg chl a m
-2) was several times as high as that of phytoplankton (average 3.0 mg chl a m-2). The photosynthetic rate of microphytobenthos, which showed a significant positive correlation only with chlorophyll a concentration in the sediment, ranged from 1.1 to 19 mg C m
-2 hr
-1 (average 8.7 mg C m
-2 hr
-1), which was equal to or greater than the photosynthetic rate of phytoplankton at that same time. The photosynthetic activity of microphytobenthos (average 1.5 mg C mg chl a
-1 hr
-1) was lower than that of phytoplankton (average 2.3 mg C mg chl a
-1 hr
-1). During the observation period, the chlorophyll a concentration, photosynthetic rate and photosynthetic activity of phytoplankton dropped with decreasing water temperature, whereas those of microphytobenthos were almost unchanged. From these results, it was considered that microphytobenthos, particularly in winter, greatly contributed to primary production at the Wakaura tidal flat due to the large amount of biomass, though the growth rate of microphytobenthos was small.
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