Inorganic nitrogen uptake by marine picoplankton: Evidence for size partitioning

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Abstract
A series of size‐fractionation experiments was carried out in coastal waters around Georges Bank and in the oligotrophic NW Atlantic to assess the relative importance of picoplankton (< 1 µm) to community nitrogen utilization and to determine if there is a partitioning of the form of nitrogen used (NO3− or NH4+) based on organism size. Results indicate that picoplankton nitrogen uptake was substantial in coastal and oceanic waters, averaging >30% of the combined nitrate and ammonium uptake by the intact communities; the picoplankton portion of total community N uptake increased offshore. Of the summed nitrate and ammonium used across all sizes, ammonium was favored disproportionately by the picoplankton fraction. On average, however, the differences in preference for nitrogen form were relatively small. The size partitioning of photosynthetic activity and nitrogen uptake was similar.