Control of the phytoplankton distribution in the Strait of Gibraltar by wind and fortnightly tides

CTDefluorescenceeturbidity profiles from two surveys conducted in June and September 1997 were examined in order to evaluate the influence of the predictable fortnightly (spring/neap) tidal cycle and the less predictable wind on the phytoplankton distribution in the Strait of Gibraltar. The injection of the nutrient-rich North Atlantic Water (NACW) in the Atlantic inflowing current is associated with initial phytoplankton blooming conditions (high fluorescence, low turbidity). In September, a thick layer of NACW was recorded in the euphotic zone of the Atlantic side of the Strait, but biological variables showed post-bloom conditions. This is interpreted as the previous development of a phytoplankton bloom favoured by the ascent of NACW into the euphotic zone during the neap tide period. The passage of NACW through the Strait into the Mediterranean Sea was initially hindered by easterly winds and later by the spring tide. The neap tide period favoured the injection and passage of nutrient-rich North Atlantic Water (NACW) into the Atlantic inflowing current, enhanced during the westerly winds regime. During the spring tides, the injection and passage of NACW is hindered, especially when the easterlies blow. The contribution of the NACW to the primary production was estimated as 100e150 kg carbon s 1 (enhanced during the neap tides). The influence of the wind-driven upwelling along the NW Albora´n Sea on the northeastern side of the Strait is discussed.