Isotopic and enzymatic analyses of planktonic nitrogen utilisation in the vicinity of Cape Sines (Portugal) during weak upwelling activity

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Using measurements of 15N uptake and activities of nitrate reductase and glutamine synthetase, the utilization of nitrogenous nutrients by microplankton in the Portuguese upwelling area was investigated. During this cruise the euphotic zone of coastal waters was in most cases bisected by a nitracline forming two layers. Total inorganic nitrogen uptake rates (NH 4 + + NO 3 −) in the upper mixed and nitrate-impoverished layer ranged from 0.1 to 0.8 nM h −1 and were primarily supported by regenerated (ammonium) nitrogen (62–97%), whereas they varied between 0.9 and 10.4 nM h −1 in the deep nitrate-rich layer and were mainly driven by new (nitrate) nitrogen (52–82%). Depth profiles of Chl a-specific uptake rates for ammonium and nitrate paralleled those of absolute uptake rates, i.e. values of V NH 4 + Chl were highest (up to 16.1 nmol μg −1 h −1) in nitrate-poor surface waters while values of V NO 3 − Chl were maximum (up to 8.4 nmol μg −1 h −1)within the nitracline. This latter vertical ordering of planktonic nitrogen nutrition was consistent with an aged upwelling situation. However, applying several indices of cell metabolism and nutritional status, such as 15N uptake/enzyme activity, surge uptake internally controlled uptake, and V max Chl K t ratios, we were able to demonstrate that the phytoplankton assemblages inhabiting the nutrient-impoverished upper layer still bore the signature of physically mediated nitrogen (nitrate) supply generated by active upwelling that had occurred during the week before our visit to the area. This signature was the most evident in samples from the station furthest inshore and faded with distance from shore as a result of the deepening of the nitrate isopleths (weakening of upwelling activity), which showed the same offshore trend. The appearance of nitrate-rich waters at the surface, after a strong pulse of upwelling favourable winds just before the end of the cruise, led to a five-fold increase in average (over the euphotic zone) absolute and Chl a-specific nitrate uptake rates (10.4 nM h −1, 7.5 nmol μ −1 h −1) compared to the mean rates during weak upwelling (1.7 nM h −1, 1.5 nmol μ −1 h −1). From a comparison with the neighbouring Moroccan upwelling, it is assumed that new production in the Portuguese upwelling averages 50 nM h −1. Thus, this upwelling would rank with the northwest African upwelling system off Cape Blanc or with the Californian upwelling at Point Conception for the capacity of new production, but seems to be much less efficient (seven-fold) than the highly permanent Peru upwelling.