Hydrodynamics and the spatial distribution of plankton and TEP in the Gulf of Cadiz (SW Iberian Peninsula)

Abstract

During a summer cruise in the Gulf of Cadiz area (Southern Iberian Peninsula), an upwelling episode off the eastern coast of Cape Santa Maria was noted. The observed temperature field data suggested an anticyclonic circulation from NW to SE and a velocity of the associated surface currents ranging between 10 and 15 cm s ‐1 was estimated. Research was carried out on the physical‐biological coupling by analysing the influence of these hydrodynamic patterns on the mesoscale distribution of several chemical and biological variables. During the cruise, nitrate concentration, total chlorophyll, chlorophyll (>20 µm), phaeopigments and TEP bulk concentration were measured. The distribution of biological variables appears to be organized by physical forcing. The upwelling area acts as a notable disturbance in the area. A sequence of maxima of different variables classified as bloom phases was observed, arranged in the same direction as dominant currents. A clear frontal structure in radial transects was also apparent. Discussion focusses on the variability of the hydrodynamic patterns observed in the framework of previous observations and the role of physical forcing, moulding the biological distributions and altering their spatial correlations. Special reference is made to TEP, as this variable is scarcely analysed in extensive field studies. The distribution of TEP patches in the area was heterogeneous, which has been discussed in the framework of current theory. Chlorophyll (>20 µm), phaeopigment ratio and TEP concentration maxima had no direct spatial correlation with chlorophyll maxima. The sequence of typical processes in phytoplankton bloom phases and the influence that hydrodynamics has both on environmental suitability for growth and on displacement of patches, could explain the distribution of these biological variables in the area. Subsequent studies on hydrodynamics variability are suggested as a valuable tool for the future understanding of biological processes in the region.