Effects of frontal processes on marine aggregate dynamics and fluxes: An interannual study in a permanent geostrophic front (NW Mediterranean)

Abstract

This study provides and discusses the spatial distributions of abundances and sizes of marine-snow aggregates across the Ligurian Sea frontal system. A cross-front transect was sampled 34 times between 1992 and 1996, using the Underwater Video Profiler (UVP). Atlantic Water flows parallel to the Ligurian coast in the NW Mediterranean Sea, where that current creates a quasi-permanent front that separates the central and coastal waters. The horizontal distribution of aggregates (> 150 μm ESD, Equivalent Spherical Diameter) in the upper 1000 m shows two main features. First, the smaller aggregates (150 μm < ESD < 1 mm) are more abundant in coastal waters, as a result of continental input, cross-slope export, and re-suspension along the slope. The layers that contain very high concentrations of small aggregates are observed from surface down to 1000 m, and extend from the continental slope to the front. Second, the concentrations of large aggregates (ESD > 1 mm) are highest in and under the frontal zone, probably as a result of physical coagulation, and/or biological transformations. The seasonal intensity of large aggregate accumulations in and under the frontal structure seems to be more related to the autumn–winter increase in sub-mesoscale and mesoscale activity of the current flow than to the surface phytoplankton biomass. Interestingly, the horizontal distribution of aggregates is affected not only in the frontal zone (0–300 m depth), but also deeper down to 1000 m, probably as a consequence of rapid sinking or vertical transport. Results suggest that the settling of large aggregates under the frontal zone may limit the cross-slope transport of fine-grained particles by coagulation due to differential settling between the small particles suspended in the continental nepheloid layer and the large aggregates. This process, which takes place in sub-mesoscale zones (5–10 km wide), was also observed in one other front in the Western Mediterranean Sea. This led us to hypothesize that the impact of frontal processes on particle and aggregate dynamics might be generalized. Since fronts exist in many other coastal regions, the vertical fluxes at sub-mesoscale may have consequences for the transport of continental particles to the ocean's interior.