Short-term dynamics of late-winter phytoplankton blooms in a temperate ecosystem (Central Cantabrian Sea, Southern Bay of Biscay)

The Spring Phytoplankton Bloom takes place in the Central Cantabrian Sea (Southern Bay of Biscay) from late-winter to spring as a series of blooms with variable biomass accumulation. In late-winter of 2004 and 2005, phytoplankton blooms occurred in this area following a change in the weather. In order to describe the dynamics of these late-winter blooms, two oceanographic cruises which involved high-frequency sampling (every 2-3 days) were carried out. Meteorological conditions during the cruises showed similar changes in variables relevant to phytoplankton physiology and population dynamics. Before the bloom, phytoplankton started to grow actively when underwater photosynthetic active radiation (PAR) increased. However, biomass accumulation did not occur until wind, and hence turbulence levels in the water column, decreased. The observations presented here suggest that before the onset of a late-winter bloom a preliminary physiological activation phase is necessary driven by increased availability of underwater PAR. Afterwards, biomass accumulation can take place provided wind-derived water column turbulence decays. The development of the bloom is reinforced by the shoaling of the surface mixing layer depth. The timing of this sequence of events can be altered by meteorological disturbances, such as an increase of wind speed. The composition of the bloom differed across-shelf: phytoplankton larger than 5 mu m in equivalent spherical diameter (ESD) dominated on the coast and inner shelf, whereas smaller phytoplankton (< 5 mu m ESD) were more important in the oceanic area, markedly when a frontal structure separating both domains developed at the mid-shelf.