Chlorophyll maxima and water mass interfaces: Tidally induced dynamics in the Strait of Gibraltar

A conceptual model proposing the distribution and dynamics of water masses and chlorophyll maxima according to tidal cycle is presented for the Strait of Gibraltar. The proposal is based on the analysis of data registered in a section along the strait at different stages of the tidal cycle. Basic water characteristics were established from CTD casts combined with bottle sampling. Composition and nature of the phytoplankton assemblage were studied by analysis of chlorophyll, pulse amplitude modulated fluorometry, flow cytometry and light microscopy analyses. The usual occurrence of deep chlorophyll maximum (DCM) linked to pycnoclines and/or water mass interfaces draws complicate patterns in the Gibraltar region, as three water masses (SAW, MOW and NACW) meet in the upper layers of the water column. The results indicate that the chlorophyll maxima found in the channel of the strait are not homogeneous and could be classified into at least three types: Atlantic maxima, linked to the SAW–NACW interface; suction maxima, present only in the SAW–MOW interface and deep maxima, associated with the transition NACW–MOW on the eastern side of the strait. These chlorophyll maxima are strongly associated with the different water mass interfaces and, even more, to a specific water density following the movement of these water mass interfaces, which is forced mainly by the tidal amplitude in this area. This study also adds new elements supporting the hypothesis that some of the pulsating chlorophyll patches observed in the strait could have a coastal origin, as their characteristics were clearly different from those of other chlorophyll maxima detected in the channel but originating in the open sea.