Nitrogen dynamics in lower Narragansett Bay, Rhode Island. I. Uptake by size-fractionated phytoplankton populations

Abstract

The contribution of nanoplankton (< 10 μm fraction) to winter – spring (1977 – 78) and summer (1978, 1979) phytoplankton nitrogen dynamics in lower Narragansett Bay was estimated from ammonium, nitrate and urea uptake rates measured by ¹⁵ N tracer methods. During the winter – spring, an average of 80% of chlorophyll a and nitrogen uptake was associated with phytoplankton retained by a 10 μm screen. In contrast, means of 51 – 58% of the summer chlorophyll a standing crops and 64 – 70% of nitrogen uptake were associated with cells passing a 10 μm screen. Specific uptake rates of winter – spring nanoplankton populations were consistently lower than those of the total population. Specific uptake rates of fractionated and unfractionated summer populations were not significantly different. Ammonium uptake averaged between 50 and 67% of the total nitrogen uptake for both the total population and the < 10μm fraction. The total population and the 10 μm fraction displayed similar preferences for individual nitrogen species. Though composed of smaller cells, flagellate dominated nanoplankton assemblages may not necessarily take up nitrogen at faster rates than diatom dominated assemblages of larger phytoplankters in natural populations.