Exchange of planktonic biomass through the Strait of Gibraltar in late summer conditions

In order to estimate plankton biomass transport through the Gibraltar Strait, plankton biomass and velocity profiles were measured at three stations located in the eastern side of the Strait as a part of the CANIGO project. Abundance and biomass measurements were carried out for autotrophic (Prochlorococcus, Synechococcus and eukaryotic pico- nano- and microplankton) and heterotrophic (bacteria and nanoflagellates) organisms, in September 1997. Biomass and velocity decreased from the surface to deeper water. Highest biomass concentration was observed at the northern station (0.12 g C m−3), whereas maximum mean velocities (80 cm s−1) were found at the central and southern stations. Biomass transport is estimated with a approach with a 10 -m resolution in the vertical and three subareas of approximately 5 km in the horizontal direction. Estimate of plankton biomass transports towards the Mediterranean and the Atlantic are 5570 and 1140 tonnes C day−1, respectively. The former is co-dominated by heterotrophic bacteria (37%) and autotrophic nanoplankton (42%), while the latter is dominated by heterotrophic organisms like bacteria (75%) and heterotrophic nanoflagellates (14%). The variation during a one-day period of the biomass transport estimate at the central part of the Strait was explored. Also, in order to estimate the influence of spatial distribution of both biomass and velocity in the transport estimates, a comparison of our results with other possible estimates performed with less spatial resolution is carried out. The results confirms that both temporal and spatial resolution are key factors for transport estimates of inhomogeneous distributed variables through the Strait.