Seasonal studies on the relative importance of different size fractions of phytoplankton in Narragansett Bay (USA)

The composition and productivity of four different size-fractions (<20, 20 to 60, 60 to 100, >100 μm) of the phytoplankton of lower Narragansett Bay (USA) were followed over an annual cycle from November, 1972 to October, 1973. Diatoms dominated the population in the winter-spring bloom and in the fall, the summer population was dominated by flagellates. The nannoplankton (<20 μm) were the most important, accounting for 46.6% of the annual biomass as chlorophyll a and 50.8% of the total production. The relative importance of the different fractions showed a marked seasonality. During the winter-spring and fall blooms the netplankton fractions (>20 μm) were the most important. Nannoplankters domnated in the summer. The yearly mean assimilation numbers for the different fractions were not signfficantly different. During the winter-spring bloom, however, the assimilation numbers for the netplankters were significantly higher than those for the nannoplankton fraction. Temperature accounted for most of the variability in assimilation numbers; a marked nutrient stress was observed on only two occasions. Growth rates calculated from ¹⁴C uptake and adenosine triphosphate (ATP)-cell carbon were generally quite high; maxima were >1.90 doublings per day during blooms of a flagellate in the summer and of Skeletonema costatum in the fall. The series of short cycles observed in which the dominant species changed were related to changes in the physiological state of the population. Higher growth rates were generally observed at times of peak phytoplankton abundance while lower growth rates were observed between these peaks. The high growth rates and assimilation numbers usually found suggest that the phytoplankton in lower Narragansett Bay was not generally nutrient-limited between November, 1972 and October, 1973. Nutrient regeneration in this shallow estuary, therefore, must be very rapid when in situ nutrient levels are low.