Seventeen size-fractionation experiments were carried out during the summer of 1979 to compare biomass and productivity in the < 10, <8 and <5 μm size fractions with that of the total phytoplankton community in surface waters of Narragansett Bay. Flagellates and non-motile ultra-plankton passing 8 μm polycarbonate filters dominated early summer phytoplankton populations, while diatoms and dinoflagellates retained by 10 μm nylon netting dominated during the late summer. A significant number of small diatoms and dinoflagellates were found in the 10–8 μm size fraction. The > 10 μm size fraction accounted for 50% of the chlorophyll a standing crop and 38% of surface production. The <8 μm fraction accounted for 39 and 18% of the surface biomass and production. Production by the < 8 μm fraction exceeded half of the total community production only during a mid-summer bloom of microflagellates. Mean assimilation numbers and calculated carbon doubling rates in the <8 μm (2.8 g C g Chl a−1 h −1 ; 0.9day −1 )and <5 μm(1.7 g C g Chl a−1 h −1 ; 0.5day −1 ) size fractions were consistently lower than those of the total population (4.8 g C g Chl a−1 h −1 ; 1.3 day −1 ) and the <10 μm size fraction (5.8 g C g Chl a−1 h −1 ; 1.4 day −1 ). The results indicate that small diatoms and dinoflagellates in fractionated phytoplankton populations can influence productivity out of proportion to their numbers or biomass.
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Miles J. Furnas (1983) studied this question.