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The uptake of dissolved nitrogen by different assemblages of phytoplankton was deter- mined during 4 seasonal surveys carried out in the northwest sector of the Alboran Sea. The study area was characterized by stratification of the water column from spring to autumn, leading to very low nutrient concentrations in the euphotic zone; in particular, nitrate was below the detection limit at numerous stations in autumn. In winter and spring, the potential limitation of phytoplankton growth by nitrate was alleviated at some stations by upwelling of the nutrient-enriched Levantine Intermediate Water (LIW). The values of relative NO3 - uptake (the proportion of nitrate to total N uptake) were highly variable (ranging from 0.01 to 0.8), even during single surveys. At most of the sampling stations during the stratification period, the relative nitrate uptake was <0.2 at the surface, but increased at the depth of the sub-surface chlorophyll a fluorescence maximum (SFM). At the surface, relative nitrate uptake and nitrate concentrations were strongly correlated, and about 82% of the relative nitrate uptake variabil- ity was accounted for by simultaneous changes in the nitrate concentrations and the micro-plankton chlorophyll a size fractions. These changes were related to modifications in upwelling intensity, as determined by variability in surface salinity. In contrast, relative nitrate uptake was not correlated with nitrate concentrations in the SFM samples, where the mean nitrate concentration increased 3-fold with respect to the surface value. On average, the micro-phytoplankton in these samples collected at the SFM was dominated by diatoms, the abundance of which ranged from 24 to 960 cells ml -1 . This variability in diatom cell abundance explained about 52% of the variability of relative nitrate uptake at this depth level. Therefore, our data demonstrate that the proposed relationship between relative nitrate uptake and large phytoplankton was most evident at non-limiting nitrate concentrations.

Partitioning the effects of changes in nitrate availability and phytoplankton community structure on relative nitrate uptake