Phytoplankton traits over an annual cycle in the NW Mediterranean
Phytoplankton traits over an annual cycle in the NW Mediterranean
Abstract:
In spring 2013, an intense phytoplankton bloom was observed in North Western Mediterranean (NWM) from ocean color images and in situ data (ships, profiling floats and gliders). The bloom was spatially structured in two distinct regions (or bioregions), where the seasonal evolutions of the mixed layer and of the chlorophyll-a concentrations follow two different dynamics. In the bioregion associated to the center of the NWM (hereafter named CB), the winter mixed layer and the spring bloom were more intense (respectively 2350 m and 4.8 mg m-3) than for the bioregion located at the periphery (PB, respectively 300 m and 2.3 mg m-3). Nitrates data are however homogenous over the whole area, and they cannot then explain the observed differences in the bloom spatial structure. HPLC pigment data (collected over the four seasons of the 2012-2013 period) indicate that the phytoplankton communities in the two bioregions are identical in summer, and diverge in winter and in spring. Higher proportion of diatoms (and lower proportion of nano-flagellates) in CB than in PB are observed. Therefore, the intense bloom in the CB region was a diatom bloom, while the less intense bloom in PB was dominated by nano-flagellates. We hypothesized that the observed differences in phytoplankton spring communities between both bioregions are essentially explained by a difference in the silicate availability, a consequence of the contrasting winter mixing intensities.