Physical control of zooplankton distribution at the Strait of Gibraltar during an episode of internal wave generation

We analyzed the effect of physical forcing on zooplankton biomass distribution in the Strait of Gibraltar as a function of tidal phase. A set of 5 Longhurst-Hardy-Plankton-Recorder (LHPR) casts were made along the main channel of the Strait during different phases of the tidal cycle to determine how the hydrological processes taking place in the strait (mainly internal wave generation) influenced the distribution and taxonomic composition of zooplankton biomass. A CTD coupled to the LHPR net allowed a detailed study of the hydrological patterns created by the generation and development of the internal waves. Plankton samples were analyzed using a semi-automatic method for zooplankton discrimination (plankton visual analyzer) which is able to differentiate each individual within a sample into specific taxonomic groups. The results show that the internal wave generation on the main channel of the strait induces divergent processes causing a reduction of the zooplankton biomass. Flow patterns within the internal waves lead to alternating bands of high and low zooplankton biomass with varying taxonomic composition. Changes in taxonomic composition appear to be the result of spatial differentiation between smaller (weak-swimming) and larger (strong-swimming) taxa. This work demonstrates the necessity to consider and understand physical processes when addressing biological patterns, especially in regions with intense hydrodynamic regimes.