Spatial heterogeneity of fish larvae across a meandering current in the northwestern Mediterranean

Abstract

Physical and biological coupling in frontal zones presents a high spatio-temporal variability as a result of both the hydrographic complexity of such systems and the biological activity of the organisms. A permanent frontal system (density front and associated current), parallel to the bathymetry, is present in the shelf-slope region in the Catalan Sea (NW Mediterranean). This study presents the results obtained from a detailed examination of the frontal system and larval fish distribution in a small area in the Catalan Sea, over the span of just a few days. A repeated transect of 11 stations, spaced 2.5 miles apart, was sampled across the front. The shelf-slope front presented a meandering structure with a wavelength of about 50–60 km and an amplitude of about 10–12 km. The position of the frontal system on successive transects was shifting gradually, from quite close to the edge of the shelf to about 10 km towards the open sea. This shift may reflect a frontal oscillation due to the passage of a half wave of a meander. During the phase in which the front draws nearer to the shelf, slope to shelf exchanges take place, with fish larvae accumulating on the coastal side of the front along with very low concentrations from there oceanwards. During the phase in which the front moves away from the shelf, shelf to slope exchanges take place, and fish larvae were beginning to be dispersed so that high larval concentrations were shifted in an oceanwards direction. While there were still relative accumulations on the coastal side of the front, these accumulations were not nearly as pronounced. On that occasion there was heightened upwelling on the oceanic side of the front with local peaks in chlorophyll biomass in this region. The results presented here have revealed not only that the fertilization mechanisms are associated with the front but have also shown that oscillations in the front have a decisive role in supplying nutrients to the photic zone on an intermittent basis. The temporal scale at which the frontal system oscillates also exerts a decisive influence on processes affecting concentration/dispersal of larval fish populations.