Biophysical interactions at the Almeria–Oran front

Abstract

The Almeria–Oran front forms where Atlantic origin waters meet Mediterranean surface waters at the eastern end of the Alboran Sea. Following a multidisciplinary cruise on RRS Discovery in December 1996, analysis of temperature and salinity on density surfaces at the front showed that periodic subduction of Mediterranean surface waters (MSW) influenced the distribution of phytoplankton and zooplankton in the frontal region. A layer of fluorescence coincident with the subducted MSW indicated that phytoplankton were drawn down and along isopycnals to depths of 200 m. The study of scattering layers, identified with acoustic backscatter data from the shipboard ADCP and a SIMRAD EK500 echosounder, revealed that a layer of zooplankton was also found coincident with the drawn-down phytoplankton. This layer persisted during and despite diel vertical migration. Smaller zooplankton, not undertaking diel vertical migration, were concentrated in the fast-flowing frontal jet as seen in high-resolution OPC data. EK500 target-strength data indicate that the subducted communities may constitute different size classes to those in the surrounding waters. Samples from Longhurst Hardy plankton recorder tows across the front provide additional ground truthing for our acoustic observations. [Work supported by EU MAST, NERC, and DERA.]