Variability in the spatio-temporal distribution and size-structure of phytoplankton across an upwelling area in the NW-Alboran Sea, (W-Mediterranean)

A north–south transect crossing the quasi-permanent upwelling area and the front which usually occupies the north-western part of the Alboran Sea (Western Mediterranean) was sampled 16 times during four cruises, covering an annual cycle in order to study the coupling between hydrodynamics and phytoplankton spatial distribution. On a spatial scale, considerable changes in biomass, size-structure and composition of the phytoplankton community were found at both sides of the front. The upwelling area was characterized by higher cell abundance, higher mean integrated chlorophyll values, intense subsurface chlorophyll maximum (SSCM) dominated by the size fraction >10 μm, as well as significantly less negative slopes of the size-spectra. Prokaryotic phytoplankton cells were more abundant in the oligotrophic waters off the front. These qualitative and quantitative differences of the phytoplanktonic spatial distribution were maintained in spite of the north–south displacement of the front that might occur even within days, in association with changes in position and size of the Western Alboran Gyre. On a temporal scale, the mean chlorophyll concentrations at the SSCM were similar in July and December (3.0±0.4 and 3.5±1.0 mg l−1 respectively), while abundance and biomass decreased in winter. The resulting lower C:Chl a and higher fluorescence cell−1 ratio indicate photoacclimation in winter. Implications of the different SAS at both sides of the front on the fate of phytoplankton biomass and importance of field measurements for satellite image interpretation are discussed.