Summer diatom and dinoflagellate blooms in Lisbon Bay from 2002 to 2005: Pre-conditions inferred from wind and satellite data

Abstract

Sea surface temperature (SST) and chlorophyll ( Chl) maps derived from satellite data, together with wind and phytoplankton cell concentration time-series, are used to characterize the wind forcing and the spatio-temporal SST/ Chl variability for periods preceding the summer (June–September) diatom and dinoflagellate blooms. The blooms (concentrations of cells higher than 200 × 10 3 cells l - 1 ) were identified in the weekly samples collected at the long-term phytoplankton study site in Lisbon Bay, from 2002 to 2005. It is shown that bloom detection at the site occurs either during upwelling events, i.e. when offshore northerly wind speed is greater than 5 m s - 1 for more than 5 days, or up to 4 days after wind relaxation. However, no direct relationship was found between the blooms and either the intensity or duration of the wind pulses. The satellite image sequences show that the Chl patterns in Lisbon Bay are strongly linked to the orientation and branching of the upwelling filament rooted at Cape Roca and the westward progression of the warm-core cyclones shed from the slope off Setúbal Bay. High Chl values are typically maintained throughout the upwelling cycle along the inshore boundary of the Roca filament, covering the whole bay during the intense wind phase. During the relaxation phase, there is a Chl increase concurrent with the contraction of the high Chl area to the northern part of the bay, associated with an inshore poleward current. It is proposed that the earlier setup of the offshore stratification conditions and the shedding of a slope eddy favored the bloom of dinoflagellates in July 2004, while the stable position of the main circulation features in July 2002 favored a diatom bloom. The results add further evidence that, in summer, the ocean circulation in the embayments south Cape Roca is dominated by the upwelling dynamics and plays a key role in the development of phytoplankton blooms off central Portugal.