Trends in Primary Production in the Canary Current Upwelling System—A Regional Perspective Comparing Remote Sensing Models

Markel Gómez-Letona,Antonio G Ramos,Javier Arístegui,Josep Coca

doi:10.3389/fmars.2017.00370

Markel Gómez-Letona, Antonio G Ramos + Show 2 more

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https://doi.org/10.3389/fmars.2017.00370

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Abstract

After Andrew (Bakun, 1990) formulated his hypothesis of upwelling intensification caused by increasing global warming, contradictory results have been published on whether primary productivity is increasing or decreasing in Eastern Boundary Upwelling Ecosystems (EBUE). The present work is focused in comparing three net primary production (NPP) models -the VGPM (Vertically Generalized Production Model), the Eppley-VGPM and the CbPM (Carbon-based Production Model)- in the Canary Current (CanC) EBUE during the 1998-2015 period, making use of both SeaWiFS (Sea-viewing Wide Field-of-view Sensor) and MODIS (MODerate-resolution Imaging Spectroradiometer) derived data. We looked for the first time for seasonal to interannual trends of NPP under a regional perspective, with the aim of searching for temporal patterns that could support or reject the intensification hypothesis. According to previous studies based on the seasonality of the upwelling regime, the CanC EBUE was divided into three subregions: a seasonal upwelling zone (SUZ; 13-20oN), a permanent upwelling zone (PUZ; 20-26oN) and a weak permanent upwelling zone (WPUZ; 26-33oN). Our analyses do not show significant increasing trends in NPP with any of the productivity models used, challenging Bakun’s hypothesis. Nevertheless, differences in the output of the models are important, both at regional and subregional scales, questioning the accuracy of the models. The comparison made in this work clearly shows the disagreements between some of the best-known NPP models and calls for a validation effort in this region. Contrary to the open-ocean, temperature trends show significant decreases in certain areas over the shelf waters, which might indicate an increase of cool, upwelled waters in those regions. Seasonal to decadal anomalies of NPP and sea-surface temperature (SST) are estimated and analyzed in relation to selected climate indices, yielding only significant correlations between SST and the North Atlantic Oscillation (NAO) indices.

Highlights

Over recent decades increasing trends in global warming have been evident in most oceanic regions, both at surface and deep layers (Levitus et al, 2000, 2005; Gille, 2002; Ihara et al, 2008; Hansen et al, 2010; Gouretski et al, 2012; Nieves et al, 2015)
Trends in sea surface temperatures (SST) Trends computed with SST data show a significant (p < 0.001) overall warming tendency for the Canary Current (CanC) eastern boundary upwelling ecosystems (EBUE) during the 1993–2014 time period (Figure 2), the increase in temperature is more evident in the southern part of the study area rather than in the northern one
OC-CCI chlorophyll a (Chl-a) shows no significant change over the shelf, apparently due to a marked increase in ∼2002 and generalized decreases during the last part of the studied period, a fact confirmed by trend analyses of individual the SeaWiFS and MODIS datasets

Summary

Introduction

Over recent decades increasing trends in global warming have been evident in most oceanic regions, both at surface and deep layers (Levitus et al, 2000, 2005; Gille, 2002; Ihara et al, 2008; Hansen et al, 2010; Gouretski et al, 2012; Nieves et al, 2015). The global ocean is projected to warm under climate change, the surface waters of eastern boundary upwelling ecosystems (EBUE)—contributing nearly 25% to global fish production—may lead to cooling rather than warming, causing enhanced productivity. Bakun (1990) proposed that EBUEs would tend to cool, as the result of the intensification of wind-driven upwelling According to his hypothesis, the stronger continental land mass warming compared to the ocean would cause an increase in the cross-shore atmospheric pressure gradient, intensifying coastal upwelling, enhancing primary productivity. It is unclear to what extent wind stress can offset the increased stratification in upwelling regions due to surface warming, and how other ecosystem drivers (like the progressive acidification and deoxygenation) could affect organisms’ responses and ecosystem productivity in EBUEs

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