Composition and interannual variability of phytoplankton in a coastal upwelling region (Lisbon Bay, Portugal)

Abstract

From July 2001 to May 2005, at a fixed station located in Lisbon Bay (Cascais: 38° 41′ N, 09° 24′ W), surface seawater samples were collected on a weekly basis. We aimed to describe at different temporal scales, short-term to interannual, the phytoplankton community in relation to hydrographic conditions. Maxima of the main phytoplankton groups varied according to the seasonality of upwelling/downwelling cycles and nutrient availability and were associated with particular hydrological mesoscale structures highlighted by satellite images. Short succession cycles were identified dependent on coastal upwelling events. Intermittent and weak pulses allowed the coexistence of species from different succession stages and groups, although having consecutive maxima. The interannual differences observed in the phytoplankton community, in Lisbon Bay, varied according to both the duration and strength of the upwelling events and to precipitation and Tagus river flow regimes. Diatoms developed and were dominant, during spring–summer under prevailing upwelling conditions and silicon availability. Short upwelling pulses appeared to be unfavourable for diatoms maintenance. When upwelling weakened and SST increased due to onshore advection of warmer waters, coccolithophores dominated. This assemblage was the second most abundant during the study, in particular during the short transition period from upwelling (summer) to downwelling seasons (autumn) distributing in the largest range of hydrographical conditions between diatoms (maximum turbulence) during early spring and dinoflagellates (maximum stratification) during summer to further dominate during autumn and winter. Nitrites and nitrates seemed to favour greater developments of this group. Dinoflagellates peaked mainly during summer and were the less abundant through the four years due to the decrease of lasting convergence periods. Like coccolithophores, a preference for warmer waters emerged but this group seemed to have a narrow tolerance to turbulence and temperature changes.