Biological activity in the wake of an island close to a coastal upwelling

arXiv:0802.3532v1 [nlin.CD] 24 Feb 2008 Biological activity in the wake of an island close to a coastal upwelling Mathias Sandulescu1, Crist´obal L´opez2, Emilio Hern´andez-Garc´ıa2, and Ulrike Feudel1 1 Carl-von-Ossietzky Universit¨at Oldenburg D-26111 Oldenburg, Germany 2 IFISC, Instituto de F´ısica Interdisciplinar y Sistemas Complejos (CSIC - Universitat de les Illes Balears) E-07122 Palma de Mallorca, Spain November 3, 2018 Abstract Hydrodynamic forcing plays an important role in shaping the dy- 1 namics of marine organisms, in particular of plankton. In this work we study the planktonic biological activity in the wake of an island which is close to an upwelling region. Our research is based on nu- merical analysis of a kinematic flow mimicking the hydrodynamics in the wake, coupled to a three component plankton model. We use parameter values of relevance for the Canary wake, and the main results for a realistic range of parameters in this area area are: a) Primary production is enhanced in the region of the wake opposite to the upwelling zone. b) There is a strong dependence of the pro- ductivity on the inflow conditions of biological material entering the wake transported by the main current. Finally c) we show that under certain conditions the interplay between wake structures and biolog- ical growth leads to plankton blooms inside mesoscale hydrodynamic vortices that act as incubators of primary production. Keywords: plankton; island wake; primary production; upwelling; vortex dynamics 2 1 Introduction Understanding the influence of hydrodynamic motions on the growth, pro- ductivity and distribution of marine organisms, especially in the context of plankton dynamics, is a major challenge recently addressed from a variety of perspectives (Mann & Lazier, 1991; Denman & Gargett, 1995; Abraham, 1998; Peters & Marras´e, 2000; K´arolyi et al., 2000; L´opez et al., 2001a,b; Martin et al., 2002; Martin, 2003). Vertical transport processes of nutrients are recognized as key factors controlling plankton productivity (Denman & Gargett, 1995). In particular, upwelling areas in the world’s oceans are of fundamental im- portance for the growth of phytoplankton which is the base of oceanic food webs. They are characterized by nutrient rich waters coming to the sur- face from depths of over 50 meters. Nutrient enrichment enhances phyto- plankton growth close to the upwelling regions, giving rise to an increase in zooplankton and fish populations in the area. More recently, the impor- tance of horizontal fluid motion has also been pointed out (Abraham, 1998; L´opez et al., 2001b; Hern´andez-Garc´ıa et al., 2002; Hern´andez-Garc´ıa et al., 2003; Martin, 2003). Mesoscale stirring redistributes and mixes plankton and nutrients laterally, giving rise also to enhanced productivity (Martin et al., 2002), or to bloom initiation (Reigada et al., 2003), and affects species com- petition and coexistence (K´arolyi et al., 2000; Bracco et al., 2000). Satellite images illustrate the interaction between horizontal mesoscale motions and 3 plankton dynamics. Vertical upwelling and strong mesoscale activity occur simultaneously in several places of the globe. A stronger impact and a high complexity of the physical-biological interactions are expected there. Some of these areas are the Benguela zone, the Humboldt Current, or the Canary islands. Though the phenomena we discuss are rather general, we illustrate them by using the Canary islands, which are close to the northwestern African coast, as a specific example. There, upwelling occurs at the African coast because of Ekman pumping induced by the dominant winds, and in addition, the Canary islands constitute an obstacle for the main ocean current in the area, flowing from Northeast to Southwest, originating a strong mesoscale hydrodynamic activity in their wake. The interaction between the vortices in the wake and the Ekman flow transporting nutrient-rich waters from the coastal upwelling seems to be at the heart of the observed enhancement of biological production in the open Atlantic ocean close to the Canary region. Motivated by this situation, the aim of this paper is to study, in a more gen- eral framework, the role of wake vorticity in redistributing upwelled nutrients and influencing phytoplankton growth. To this end we combine the kinematic model flow introduced in Sandulescu et al. (2006) with a simple model of a Nutrient-Phytoplankton-Zooplankton (NPZ) trophic chain, and study the impact of the flow characteristics on the bio- 4 logical dynamics, particularly on the primary production (PP). We will use mainly parameter values of relevance in modeling the Canary wake, but we expect our results to have broader application. Only horizontal transport is explicitly taken into account in the flow, the upwelling is modelled as a source term in the nutrient equation. We address questions such as (i) whether the island wake is a barrier for the upwelled nutrients, or (ii) if rather the gene

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