Patterns in the size structure of the phytoplankton community in the deep fluorescence maximum of the Alboran Sea (southwestern mediterranean)

The Alboran Sea (southwestern Mediterranean) exhibits strong horizontal and vertical gradients associated with macroscale and mesoscale physical structures due to the input of surface Atlantic waters into the Mediterranean basin. During the summer of 1992, two anticyclonic and two cyclonic areas were found with fluorescence maxima (DFM) below the seasonal thermocline (ST). Although the depth of the ST is fairly constant, the position and intensity of the DFM is more variable, with a tendency to deepening and smoothing in the anticyclonic gyres. The position of the Atlantic–Mediterranean interface (AMI) can be used as a tracer for cyclonic or anticyclonic dynamics and their potential biological effects. A shallow AMI indicates divergence or upwelling dynamics and coincides with the highest fluorescence intensity, chlorophyll concentration and phytoplankton biovolume in the DFM. Under the conditions typical of the two anticyclonic gyres, the contrary is found. The size structure of phytoplankton shows significant differences between cyclonic and anticyclonic structures. Log-transformed size-abundance spectra can be adequately described by linear models with slopes of −0.78 for cyclonic and −0.93 for anticyclonic structures. The integration of size-abundance spectra indicates that picoplankton biovolume in the DFM is independent of the type of circulation or dynamics, whereas nanoplankton and, particularly, microplankton increase their absolute and relative presence in the DFM under cyclonic or upwelling dynamics.