Limnetica 31

The latitudinal position of the north-wall of the Gulf Stream influences the climatic conditions in the Atlantic North and Western Europe. Southerly movements of the Gulf Stream (low Gulf Stream Index values, GSI) typically induce unstable meteorological conditions in this region, while north displacements of the oceanic current (high GSI values) are associated with stable weather. In the present article we explore a seven years data set including annual GSI, meteorological records and phytoplankton community composition and abundance to demonstrate that the year-to-year changes in the position of the Gulf Stream and its influence on the prevailing weather conditions have an effect on the long-term variability of phytoplankton community in El Gergal reservoir, an ecosystem located in the Atlantic coast of Andalusia (SW Spain). Furthermore, we describe the response of each considered phytoplankton functional groups to changes in the Gulf Stream position. Thus, northerly displacements of the north-wall increased the abundance of H +S₁ cyanobacteria through a non-linear function with two marked GSI thresholds. GSI and L_m dinoflagellates abundance depicted a significant positive linear correlation, while groups B + P diatoms abundance was negatively linear correlated with GSI. Groups Y cryptophytes and X₁ + J chlorophytes abundance remained nearly constant for most of the studied years but developed an exponential increase at high GSI years. Implications for water quality management are pointed out.