Nitrate uptake by size-fractionated phytoplankton on the Scotian Shelf (Northwest Atlantic): Spatial and temporal variability

New (nitrate) phytoplankton production was estimated monthly during 1 year (March 1991-March 1992) at three stations on the Scotian Shelf, Northwest Atlantic. Samples were size fractionated to assess the uptake of nitrate by small (<5 mu m) and large (>5 mu m) phytoplankton. The biomass of small phytoplankton remained relatively constant over the year, whereas that of the large size fraction was high in early spring and low during the remainder of the year. Monthly variations in nitrate uptake were similar for the two size fractions, suggesting that both small and large phytoplankton used nitrate when available. It follows that, outside the spring bloom, new production was largely due to the small fraction. Our results do not support the notion that new production is associated with large phytoplankton and regenerated production with small phytoplankton.