Transient pulses of primary production generated by undulatory processes in the western sector of the Strait of Gibraltar

Physical processes forced by tidal dynamics, such as bores and internal waves, have a high influence on biological patterns in the main channel of the Strait of Gibraltar. To analyse this connection in the case of photosynthetic processes, a set of high resolution profiles of physical (temperature, salinity, and light) and chemical (nutrients and chlorophyll) variables was acquired during a tidal cycle, together with high resolution profiles of physiological parameters (photosynthetic efficiency Fv/Fm and functional cross section of photosystem II, σPSII) quantified by in situ fast repetition rate (FRR) fluorometry. Vertical profiles of variable fluorescence revealed two different chlorophyll maxima that had been previously described in the neighborhood of the Strait; Atlantic Maximum (AM) and Deep Maximum (DM). AM was present in the photic zone throughout the whole sampling period and the phytoplankton community associated to this maximum was responsible of the main production in the site until the arrival of the internal bore. DM only appeared after high water at 44 m depth, at the lower end of the photic zone and was associated to the Atlantic Mediterranean Interface (AMI). Phytoplankton associated to this maximum showed very low productivity values. FRR profiles revealed that the arrival of the internal bore happened concomitant with the appearance of a unique, deep and broad chlorophyll maximum associated to a water mass with different physico-chemical and optical characteristics. This maximum ascended quickly to the surface waters in the order of a few minutes. The internal bore's passage becomes a mechanism that supplies phytoplankton to the incoming Atlantic water and a transient increase of primary production in the site was then detected. In addition, FRR derived primary production profiles revealed a significant daily variability.

Research Highlights
► We quantify oceanic primary production by Fast repetition rate fluorometry.
► Physical processes influenced on biological patterns.
► Internal bores generated a transient increase of primary production in the study site.
► Primary production profiles also revealed significant daily variability.