A comparative study of biological production in eastern boundary upwelling systems using an artificial neural network

Z Lachkar,N Gruber

doi:10.5194/bg-9-293-2012

Abstract

Abstract. Eastern Boundary Upwelling Systems (EBUS) are highly productive ocean regions. Yet, substantial differences in net primary production (NPP) exist within and between these systems for reasons that are still not fully understood. Here, we explore the leading physical processes and environmental factors controlling NPP in EBUS through a comparative study of the California, Canary, Benguela, and Humboldt Current systems. The NPP drivers are identified with the aid of an artificial neural network analysis based on self-organizing-maps (SOM). Our results suggest that in addition to the expected NPP enhancing effect of stronger equatorward alongshore wind, three factors have an inhibiting effect: (1) strong eddy activity, (2) narrow continental shelf, and (3) deep mixed layer. The co-variability of these 4 drivers defines in the context of the SOM a continuum of 100 patterns of NPP regimes in EBUS. These are grouped into 4 distinct classes using a Hierarchical Agglomerative Clustering (HAC) method. Our objective classification of EBUS reveals important variations of NPP regimes within each of the four EBUS, particularly in the Canary and Benguela Current systems. Our results show that the Atlantic EBUS are generally more productive and more sensitive to upwelling favorable winds because of weaker factors inhibiting NPP. Perturbations of alongshore winds associated with climate change may therefore lead to contrasting biological responses in the Atlantic and the Pacific EBUS.

Highlights

Eastern boundary upwelling systems (EBUS), i.e. the California, Humboldt, Canary, and Benguela upwelling systems, are among the most productive marine ecosystems in the world and have long been recognized for supporting some of the world’s major fisheries (Pauly and Christensen, 1995; Bakun, 1990, 1996; Carr, 2001; Carr and Kearns, 2003; FAO, 2009)
1% (37 obs) of the map to very high net primary production (NPP) in the bottom left corner. This gradual change in production regimes is associated with continuous changes in the NPP drivers across the map, with the upwelling index, the mixed layer depth and the shelf width reaching their maximum values in the bottom left, top right and bottom right corners, respectively
Our aim has been to identify the leading drivers and modulating factors for NPP and to provide an objective classification and comparison of Eastern Boundary Upwelling Systems (EBUS) based on their production regimes

Summary

Introduction

Eastern boundary upwelling systems (EBUS), i.e. the California, Humboldt, Canary, and Benguela upwelling systems, are among the most productive marine ecosystems in the world and have long been recognized for supporting some of the world’s major fisheries (Pauly and Christensen, 1995; Bakun, 1990, 1996; Carr, 2001; Carr and Kearns, 2003; FAO, 2009). The basic mechanism responsible for the high productivity in EBUS is well understood: equatorward winds along the eastern boundaries of the Atlantic and Pacific force an offshore Ekman transport, which drives upwelling of deep nutrient-rich water into the euphotic zone, where favorable light conditions sustain phytoplankton growth (Allen, 1973; Brink, 1983a). The comparison of different EBUS provides a framework for generalizing individual observations

Methods

Results

Discussion

Conclusion