Nitrogen uptake and dissolved organic nitrogen release in planktonic communities characterised by phytoplankton size–structure in the Central Atlantic Ocean

Ammonium, nitrate and urea uptake rates, as well as the release of dissolved organic nitrogen (DON), were measured along a latitudinal transect (49°N–33°S) in the Atlantic Ocean during the 2000 summer-to-autumn transition period and subsequently related to phytoplankton-size structure. The transect included upwelling areas, oligotrophic gyres and temperate regions and, as a result, euphotic layer integrated uptake rates of ammonium (18–355 mmol m−2 h−1), nitrate (5–2830 mmol m−2 h−1) and urea (26–1212 mmol m−2 h−1) covered a wide productivity spectrum. The percentage of extracellular DON release relative to total nitrogen uptake (PER) was highest when nitrate was used as the substrate (mean 42±SE 3.1%) whereas the uptake of ammonium and urea produced lower PER values (22±2.1 and 26±1.6%, respectively). Small cells (<2 μm) contributed between 20% and 80% of the total primary production; however, they accounted for 50–80% of total chlorophyll-a. In upwelling areas of the ocean, dominated by large phytoplankton, PER accounted for less than 30% of total nitrogen uptake and could be predicted from a regression equation relating the contribution of <2 μm phytoplankton to either total biomass or production. In contrast, in oligotrophic areas, small cells formed the bulk of the phytoplankton biomass, but not of primary production. In these areas, PER often exceeded 50% and was very variable, displaying a low correlation with phytoplankton size, which suggests that processes not directly related to the size of the primary producers influence DON release in the oligotrophic ocean.