Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean

Abstract

Upward motions are often invoked to explain the high productivity of permanent geostrophic fronts in the Western Mediterranean while physical evidence of such upward advections is seldom reported. The goal of this study is to define biological and chemical criteria, which can be used to localize such upward motions zones. We use a one‐dimensional, time‐dependent model of phytoplankton dynamics to test the effects of upward advection on the vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen. Simulations also include the effects of advective motions of the phytoplankton cells in the light field on phytoplankton growth. In conformance with the continuity equation, boundary conditions were defined to allow horizontal flow of the upwelled water within the upper mixed layer. Low upward advections (≤3 m d−1) led to a shallowing and sharpening of the nitracline, oxycline, and deep maxima of phytoplankton biomass and oxygen and to an increase in phytoplankton biomass. By confining the phytoplankton‐nutrient system in the surface mixed layer, higher upward advections lead to homogeneous phytoplankton biomass and oxygen vertical distributions in the upper mixed layer, the nitracline and the oxycline being then at the top of the pycnocline. Data collected during the Prolig 2 cruise (May 1985) on the heavy side of the Liguro‐Provençal front are interpreted as an illustration of these numerical results. Computed primary production rates are compared with measurements conducted in the Almería‐Oran front during the Almofront 1 cruise (April 1991) in a similar situation. In both fronts, upward advections of 1–2 m d−1 would be sufficient to account for the observed vertical distributions and the increased primary production. Ecological implications for the phytoplankton‐nutrient system are discussed, particularly the spatial uncoupling of phytoplankton biomass and primary production in permanent geostrophic fronts.