Aspects of Phytoplankton Communities Response to Climate Changes

Climate changes concerning shifts in seasonal dominant winds and increases in atmospheric CO2 are underway. Thus, changes are occurring in coastal upwelling regimes as well as increases in CO2 absorption by the ocean/acidification impacting phytoplankton communities in terms of abundance and diversity. In order to illustrate these apparent changes we present results obtained from coastal waters adjacent to the Tagus estuary under different hydrological conditions. Upwelling events prevailed in winter 1994, were absent in winter 2001 and were present in summer 2002. Chemical and biological properties are examined: in March 1994, a strong bloom of phytoplankton developed (chlorophyll a up to 40 mg m(-3)) which was attributed to the combined effect of intense freshwater runoff and upwelling, leading to the establishment of a strong frontal boundary and a supply of a considerable amount of nutrients. In March 2001, under an extremely intense river discharge and absence of upwelling, only values up to 1.5 mg m(-3) of chlorophyll a were measured. On the other hand, in June 2002, when Tagus river inflow was reduced and nutrient levels were quite low, chlorophyll a levels attained 5 mg m(-3) despite the occurrence of upwelling. Over the same study area, potential impacts of acidification on phytoplankton communities are discussed. Actually, blooms of calcifying organisms occur often in Iberian coastal waters linked to upwelling events. In June 2002, a bloom episode of the Coccolithophore, Coccolithus braarudii, developed attaining up to 60 cells mL(-1), being responsible for production of 11.2 mmol CaCO3 m(-2) d(-1). In the context of actual lowering of seawater pH, the expected calcification slow-down as well as the reduction of buffering seawater ability, a shift of such a phytoplankton group to other groups is likely to be induced. Thus, the maintenance of biogeochemical time-series is crucial for the detection of future changes in the structure and functioning of this marine ecosystem.