Phytoplankton size distribution and growth rates in the Alboran Sea (SW Mediterranean): short term variability related to mesoscale hydrodynamics

The short term variability of the size distribution and growth rates of the phytoplankton community was studied during several visits at three stations located across the anticyclonic gyre of the Western Alboran Sea in May 1998. Chlorophyll a (Chl a) concentration was fractionated in three size classes, picoplankton (<2 μm), nanoplankton (2–20 μm) and microplankton (>20 μm). Biovolume and cell carbon estimations, based on cell cytometry and microscopy counts, image analysis and published conversion values, were carried out for seven groups of autotrophic organisms. Carbon:chlorophyll a (C:Chl a) ratios in combination with 14C uptake experiments were used to calculate phytoplankton growth rates. Temporal differences among stations in the size structure and composition of the phytoplankton community were related to hydrographic phenomena. At the beginning of the survey, the proportion of the three chlorophyll size fractions ranged between 20 and 40% for all the stations; this situation changed dramatically after a nutrient upwelling event which occurred at the station closest to the coast, located at the northern edge of the anticyclonic gyre, and was associated with an increase in the biomass of the whole phytoplankton community (mainly due to diatoms) and with a shift in the microplankton proportion from 35 to 60% of the integrated Chl a. In contrast, the pico- and nanoplankton fractions increased their dominance in the oligotrophic waters of the centre of the gyre (between 75 and 95% of the integrated total Chl a, mainly accounted for by Synechococcus and small flagellates). At all the stations, picoplanktonic cells were generally more abundant in the upper layers of the photic zone while nano- and microplanktonic organisms tended to be more important with depth. Maximum phytoplankton growth rates ranged from 0.45 to 1.41 day−1 and did not present significant differences among stations, in spite of their different hydrographic and nutrient conditions. In particular, average duplication times of 0.9 day at the oligotrophic central station indicate that phytoplankton was actively growing there. Growth rates averaged for the photic layer were not associated with the phytoplankton composition and size distribution but were positively correlated with the available irradiance. These findings suggest a physiological acclimation of the phytoplankton community to the light conditions and may be relevant for the parameterization of primary production models. The C:Chl a ratio decreased with depth and increased from high to low nutrient stations and with the fraction size.