Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Journal of Geophysical Research. Oceans Année : 1998

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

Résumé

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-•) 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-Provenqal front are interpreted as an illustration of these numerical results. Computed primary production rates are compared with measurements conducted in the Almer/a-Oran front during the Almofront i cruise (April 1991) in a similar situation. In both fronts, upward advections of 1-2 m d-• 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. 1. Introduction The Liguro-Provengal front [B•thoux et Prieur, 1983; Sournia et al., 1990] and the Almerfa-Oran front [Cheney and Doblar, 1982; Tintor• et al., 1988] are permanent geostrophic structures in the Western Mediterranean. Both fronts are areas of high-plankton biomass, in contrast with two adjacent oligotrophic-type systems [Lohrenz et al., 1988; Tintor• et al., 1988; Sournia et al., 1990; Prieur et al., 1993; Claustre et al., 1994b], and show net change in both the specific composition of the autotrophic [Claustre et al., 1994b] and the heterotrophic [Boucher et al., 1987; Thibault et al., 1994] communities. Several features indicate that the two fronts are sites of enhanced production in response to geostrophic frontal dynamics. In addition to the main geostrophic flow, a secondary circulation exists, i.e., the ageostrophic flow, which is characterized by enhanced vertical advections as shown for example by Dewey et al. [1991] in the California Current. It is thought that upward advections in the frontal zone upwell nutrients into the photic zone and enhance primary production [Lohrenz et al., 1988; Fideau et al., 1994]. The resulting high biomass is then downwelled by the conver-Paper number 98JC01537. 0148-0227/98/98 JC-01537509.00 gent part of the ageostrophic circulation along the isopycnals [Claustre et al., 1994a, b; Fideau et al., 1994]. However, physical evidence for upward advection in the two fronts is not well-documented (see, however, Tintor• et al. [1991] and l?iudez and Tintor• [1996]) and poorly understood. This paper defines criteria from the vertical distribution of parameters involved in primary production, which can be used to localize, with some confidence, the upward motions in such frontal zones. In a previous modeling study [Zakardjian and Prieur, 1994], two types of phytoplankton-nutrient systems resulting from high-and low-turbulent regimes (HTR and LTR, respectively) were described, with the LTR representing the ecological conditions in the waters adjacent to the Liguro-Provengal and Almerga-Oran fronts, where the vertical structure is controlled by diffuse vertical fluxes. We use a modified version of this previous one-dimensional vertical model (presented in section 2) to test the sensitivity of the LTR system when an upward advection term is added. In section 3 we describe the steady state vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen obtained with upward advection values ranging from 0 to 5 m d-•. Effects of upward advection on the vertical distribution of primary production, on phytoplankton growth, and, finally, on the productivity indexes of the water column are detailed. In section 4 we present multiparametric 27,849
Fichier principal
Vignette du fichier
98JC01537.pdf (1.59 Mo) Télécharger le fichier
Origine : Fichiers éditeurs autorisés sur une archive ouverte
Loading...

Dates et versions

hal-02142615 , version 1 (12-11-2020)

Identifiants

Citer

Bruno Zakardjian, Louis Prieur. Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean. Journal of Geophysical Research. Oceans, 1998, 103 (C12), pp.27849-27866. ⟨10.1029/98JC01537⟩. ⟨hal-02142615⟩
60 Consultations
65 Téléchargements

Altmetric

Partager

Gmail Facebook X LinkedIn More