Phytoplankton, bacterioplankton and nitrate reductase activity distribution in relation to physical structure in the northern Alborán Sea and Gulf of Cadiz (southern Iberian Peninsula)

The IctioAlboran-Cadiz 95 cruise (July 1995) covered the northwest sector of the Alboran Sea, the Straits of Gibraltar and the Gulf of Cadiz. We present and discuss the distribution of phytoand bacterioplankton abundance, nitrate concentration and nitrate reductase (NR) activity according to circulation patterns described previously in the literature. The water-column integrated values of chlorophyll-a in the Alboran Sea were higher and more variable (12 to 318 mg m‐2) than in the Gulf of Cadiz (3 to 49 mg m‐2). This reflects the higher dynamism of the Alboran sector, related to the existence of characteristic mesoscale physical structures. The plankton (2-80 µm equivalent spherical diameter) in the upwelling region of the Alboran Sea had a biovolume 30 times higher than in the nutrient-poorer water of the Gulf of Cadiz. Large organisms were abundant and dominated the size spectrum of one coastal station in the Alboran zone, whereas the slope of the size spectrum became more negative at the Gulf of Cadiz station, as a consequence of the dominance of small cells. Bacterial abundance maxima (> 3 � 106 bact ml‐1) also occurred in the upwelling region. We found relatively high nitrate and chlorophyll a concentrations in Cape Trafalgar’s cold surface waters, probably due to the interaction between topography and strong tidal currents. The spatial pattern of the NR activity is discussed according to chlorophyll a, nitrate and bacteria distributions. The existence of high NR activity but low chlorophyll a and bacterial abundance suggests that this could be related with dissimilatory activity in large-sized particles.

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