Distribution and fluxes of dissolved nutrients in the Strait of Gibraltar and its relationships to microphytoplankton biomass

This study presents the distribution and fluxes of dissolved nutrients in t he Strait of Gibraltar, as well as their relationship to the microphytoplan kton biomass distribution in the zone. In June and September 1997, the uppe r Atlantic inflowing waters west of the sill showed low nutrient concentrat ions. A significant increase was observed at the east of the sill. This inc rease can be attributed to mixing events at the sill between nutrient-rich Mediterranean Outflowing Waters (MOW) and the impoverished upper Atlantic I nflowing Waters (AW). Physical phenomena induced by tides at the sill act l ike an intermittent upwelling system and a part of the MOW nutrients re-cir culate to the euphotic zone. Another factor is the injection of nutrient-ri ch North Atlantic Central Water (NACW) into the upper layer induced by the tides at the sill. These features along with the shallower position of the Atlantic-Mediterranean Interface ((AMI) towards the north-east contribute t o support high microphytoplankton biomass on the eastern side of the Strait . In the north-east, the injected nutrients are consumed by a high microphy toplankton standing stock. An accumulation of biomass is possible because t he Atlantic-Mediterranean Interface (AMI) is shallower and the advection of cells is lower. In the south-east, AMI is deeper and the advection is high er. Nutrients are exported into the Alboran Sea with low consumption by phy toplankton. Nitrate, phosphate and silicate fluxes between the Mediterranean Sea and At lantic Ocean showed that Atlantic nitrate, phosphate and silicate concentra tions compensate for about 21, 39 and 17% respectively of the outflowing lo sses. The results, extrapolated on an annual basis, show a net loss budget of nitrate, phosphate and silicate from the Mediterranean Sea of 3.00, 0.24 and 4.82 x 10(6) tons year(-1), respectively. The evaluation of the flux o f nutrients in the upper Atlantic current shows high variability due to bio logical and vertical mixing processes. (C) 2000 Academic Press.