Coupling between physical and biological fields in the North Atlantic subtropical front southeast of the Azores

The physical and biological properties of the North Atlantic subtropical region southeast of the Azores were studied during an oceanographic cruise carried out in March 1992. The main patterns of physical variability were defined by the presence of (a) a thermohaline front (subtropical front; STF) located at 34°N–35°N and extending from 15°W to 28°W, which separates warmer more saline Western Atlantic Water from colder and fresher Eastern Atlantic Water, and (b) a strong eastward flow running along the subtropical front (Azores Current; AC) extending down to at least 250 m and showing velocities of 50–70 cm s−1 at the core of the current. Overall a close linkage was observed between the STF-AC physical feature and high levels of chlorophyll a. A high resolution survey showed chlorophyll a fluorescence associated with the southern frontal boundary (consisting of chain forming diatoms and flagellates) and with the AC (made up of cells in the less than 2 μm size-class), respectively. Primary production rates measured in the frontal high-chlorophyll region (> 1 mg C m−3 h−1) were much higher than previous measurements carred out in the same area in late spring and summer and about 2 times higher than modelling estimates for the region. The large spatial extension of the biological signature associated with the STF-AC system suggests that carbon fixation within the frontal structure could be significant for regional carbon budgets of the subtropical northeast Atlantic.