Mesoscale subduction at the Almeria–Oran front: Part 2. Biophysical interactions

The Almeria–Oran front forms where surface waters of Atlantic and Mediterranean origin meet at the eastern end of the Alboran Sea. A multidisciplinary field experiment on RRS Discovery in December 1996, in the second observational phase of the EU-funded Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motions (OMEGA) project, observed the biological impact of mesoscale frontal instability of the Almeria–Oran frontal jet. It is concluded that periodic vertical velocities of 20 m/day, associated with the propagation of wave-like meanders along the front, have a significant effect on the vertical distribution of zooplankton across the front despite their ability to migrate at greater speeds. Observations of a layer of fluorescence coincident with subducted surface waters indicated that phytoplankton were drawn down and along isopycnals, by cross-front ageostrophic motion, to depths of 200 m. From the study of sound-scattering layers (SSL) identified in acoustic backscatter data, a layer of zooplankton was found coincident with the drawn-down phytoplankton. This layer persisted during and despite diel vertical migration. High-resolution optical plankton counter (OPC) data showed smaller zooplankton, which did not undertake diel vertical migration, remained concentrated near the surface in the fast-flowing frontal jet.