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Progress in Oceanography 75 (2007) 343–347

Progress inOceanography

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Preface

The structure and functioning of the coastal upwelling system offcentral/southern Chile

1. Introduction

Coastal upwelling zones in eastern boundary current systems (EBCs) comprise a relatively small portion ofthe world oceans (<2%) but, due to their very high biological production, they sustain most of the global fish-eries (>23% of fish catch) and, thereby, the economy of several countries. At the same time, these zones act askey areas for carbon exchange between the coastal zone and the open ocean, as well as between the ocean andthe atmosphere. Among these zones, the Humboldt Current System (HCS) is one of the most productive ones(mean primary production of ca. 750 mg C m�2 d�1) and supports a strong fishery off Peru and Chile (Alheitand Bernal, 1993; Kudela et al., 2005). Also, the HCS contains one of the shallowest, and most extensive, oxy-gen minimum zones (OMZs) located in the coastal areas of EBC systems (Morales et al., 1999; Escribanoet al., 2004). Our knowledge of the main biological and biogeochemical processes governing the productivityof this large marine ecosystem is, until now, clearly not sufficient to provide a basis for predicting its responsesto a variety of natural large-scale phenomena affecting the eastern South Pacific region, such as El NinoSouthern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), other multi-decadal signals, and ongo-ing climate change.

The upwelling systems along the Chilean coast have remained the least investigated areas of the HCS (Mor-ales and Lange, 2004). In particular, the central/southern region between 35 and 38�S (Fig. 1) has receivedlittle attention in terms of long-term studies despite extremely high primary production values (10–20 g C m�2 d�1; Fossing et al., 1995; Daneri et al., 2000), and a large fisheries of Jack Mackerel in the easternSouth Pacific which is based in this region, with landings of 3 million tonnes between 1990 and 1996 (Arcoset al., 2001). This region is characterized by strong seasonal upwelling, with intensive events taking place dur-ing the austral spring and summer period (Caceres and Arcos, 1991; Figueroa and Moffat, 2000) along a rel-atively wide continental shelf (>50 km) interrupted by submarine canyons (Sobarzo and Djurfeldt, 2004).Over the continental shelf, extended periods of hypoxia affect the benthic environment and this condition pro-motes the development of high biomasses, in the form of mats, of the giant bacterium Thioploca (Gallardo,1977). The high productivity of this ecosystem exhibits a strong inter-annual variability related to the ENSOcycle (Escribano et al., 2004) causing uncertainty in the sustainability of the resources derived from this eco-system and in the potential ecosystem responses to ongoing climate change.

The lack of baseline information on the structure of the components of the lower trophic levels in theupwelling system off central/southern Chile, as well as on their changes over time and the degree of associationof these changes with variation of the physical and chemical environment, lead the Center for OceanographicResearch in the eastern South Pacific (COPAS) to launch in 2002 a time series study in the coastal zone offConcepcion (36�S). The goal of this long-term program has been focused on obtaining an integrated viewof the scales of variability in the physical, chemical and biological components of this upwelling system.

0079-6611/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pocean.2007.08.020

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Fig. 1. The coastal upwelling zone off central/southern Chile in the eastern South Pacific, showing the fixed stations, where the COPASoceanographic center has launched and maintains the physical–chemical–biological integrated time series study since August 2002.

344 Preface / Progress in Oceanography 75 (2007) 343–347

For this purpose, two fixed observational stations were selected (Fig. 1): Station 18 (90 m depth), located overthe continental shelf at ca. 30 km from shore, and Station 40 (1000 m), over the continental slope at 75 kmfrom shore. Large research efforts of COPAS have been devoted to Station 18 and, after 3 years of continuousstudies, the main findings are presented in this special issue of Progress in Oceanography. The volume has beenorganized into three inter-connected themes as described below.

1.1. Physical and chemical characteristics

The first theme is devoted to the variability in the physical and chemical environment over the continentalshelf off central/southern Chile. Three articles provide information on the scales of variability in the water col-umn and the air–sea interface. The study by Sobarzo et al. (a) provides a detailed account of the strong sea-sonal upwelling process in the region. Coastal upwelling is forced by seasonally variable winds but two otherseasonal processes also modify the stratification of the water column, the mixed layer’s heat balance, which isdominated by solar radiation (maximum in January), and the freshwater balance, which is dominated by riverdischarge and precipitation (maxima in June and July). A second paper by Sobarzo et al. (b) describes thecoupling and interaction between wind forcing and inertial motions over the continental shelf. The authorsfocused on the temporal and spatial variability of the near-inertial band (0.045–0.055 cph), which accountsfor 3–62% of the total variance in the current field. Common features of the near-inertial motion, especiallythe 180� phase difference between the upper and the lowers layers, and the intermittency of this motion, aredescribed for the first time for the Chilean shelf. A third paper, by Cornejo et al., describes the temporal var-iability in N2O distribution in the water column and its association with the OMZ, and reveals the link be-tween environmental forcing driven by upwelling and the biogeochemical mechanisms controlling thedistribution of this important greenhouse gas.

Preface / Progress in Oceanography 75 (2007) 343–347 345

1.2. Plankton variability

A second theme in this volume considers plankton variability in waters of the central/southern Chileupwelling system. Here, seven studies describe the temporal variability (mainly at Station 18) and, some ofthem, the spatial (mesoscale) variability of planktonic organisms, from small nanoplankton components tolarge-sized meso- and macroplanktonic components. The whole of this size spectrum (ca. 2–2000 lm) consti-tutes the basis for primary and secondary production of the upwelling ecosystem and, clearly, are the leaststudied compartments in this region. The study by Anabalon et al. deals with the temporal variation in thefunctional structure of nanoplankton and microplankton communities on the shelf station and evaluatesthe impact of physical forcing and nutrients on this structure. The dominant components were found through-out the study period, with maxima in abundance/biomass co-occurring under similar environmental condi-tions; the variation in the main macro-nutrient concentrations did not explain the occurrence of maxima orof functional group replacements. Bottjer and Morales’ study focuses on the nanoplankton components atStation 18, its temporal variability during a two year period, and the potential grazing impact of nano-hetero-trophs on bacterioplankton and cyanobacteria abundances. Their results reveal a small impact of seasonalhydrographic variability on the abundance and biomass of nanoplanktonic assemblages but suggest that graz-ing by small dinoflagellates might control the prokaryotic picoplankton populations in this area.

The paper by Gonzalez et al. analyzed the seasonal variation in the composition and biomass of diatoms,tintinnids, and dinoflagellates, and evaluated the contribution of microplankton and faecal material to the ver-tical flux of particles at Station 18, including an analysis of the spatial distribution of microplanktonic com-ponents in waters over the continental shelf and slope off Concepcion. It was found that, the biologicallymediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf offConcepcion depend upon key groups of phytoplankton (Thalassiosira spp. and Chaetoceros spp.) and zoo-plankton (euphausiids) through the export of either cells or faecal material, respectively. Morales et al. focusedon the spatial variability of chlorophyll-a, particulate carbon and planktonic components in the coastal tran-sition zone off Concepcion, their variation under spring and winter conditions, and their relation to mesoscaleoceanographic features. They describe cyclonic eddies and large filaments coinciding with higher chlorophyll-acontents (�1 mg m�3) and plankton abundance than the surrounding waters, suggesting that these mesoscalefeatures, in combination with upwelling, play a role in potentially increasing the biological productivity of thecoastal transition zone.

The temporal variability of the meso- and macrozooplankton, and their response to environmental varia-tions are covered by three papers. The work of Escribano et al. deals with the temporal variation in the bulk ofzooplankton C and C/N ratios, and the community structure, with emphasis on the dominant group of cope-pods. Main results suggest continuous production of zooplankton year-round and strong seasonal variationsin C/N contents, possibly related to shifts in food quality coupled to upwelling variation. The paper by Castroet al. details the fine-scale spatial variation in the distribution of the dominant copepod species in the coastalupwelling zone and its relation to vertical migration and water column structure. Vertical distribution-migra-tional patterns of dominant species appeared greatly influenced by thermocline and oxycline distribution,which in turn are modulated by upwelling-forced variability of the OMZ and water column structure. Finally,Hidalgo and Escribano selected two abundant copepods species as an attempt to elucidate the mechanismscoupling upwelling variability and species life cycles, which in turn determine zooplankton variation overthe seasonal scale. Indeed, they found that life cycles and population dynamics of zooplankton seem stronglycoupled to the seasonal variable upwelling process.

1.3. Biogeochemical processes

A third theme contains six papers focused on biogeochemical processes occurring in the water column andsediments at Station 18. The study by Montero et al. reveals the seasonal and annual cycle of primary pro-duction (PP), explores the main connections between this rate process and environmental forcing over the sea-sonal scale, and assesses the role of diatoms and bacteria in the flux of freshly produced carbon. Primaryproduction rates were found to be extremely high during the upwelling season and strongly coupled to bac-terial secondary production. The annual cycle of solar radiation seems a key factor controlling the year cycle

346 Preface / Progress in Oceanography 75 (2007) 343–347

of PP. Linked with primary production at Station 18, the paper by Levipan et al. describes the variability ofprokaryote secondary production below the oxycline; they reveal the importance of both Bacteria and Ar-chaea in using the carbon produced in the photic zone and accumulated in the upper part of the OMZ. Thislow-oxygen layer may greatly impact biochemical processes in the water column and sediments during theupwelling season. For instance, Gonzalez et al. found that the microbial community inhabiting the OMZexhibits a strong metabolic activity, comparable or even greater than that of the surface oxygenated layer.The OMZ also influences biological-mediated processes in the sediments at Station 18, such as the cyclingof N2O under the effect of variable levels of oxygenation, as shown by Farıas and Cornejo. However, the influ-ence of the OMZ on sediments over the continental shelf in this region may strongly vary from year to year,depending on ENSO conditions. In this respect, the work by Contreras et al., using sedimentary proxies,establishes a link between ENSO variability and exported production and the structure of the benthic com-munity. A further analysis on sedimentary proxies by Munoz et al. provides estimates of the monthly fluxof 210Pb, along with phyto-detritus flux and macrofauna abundance in shelf sediments. This study suggeststhat variation in the organic flux promotes changes in the benthic faunal activity, which combined with sed-iment resuspension and water circulation over the shelf, produces a seasonal variation in the 210Pb inventory.

2. Concluding remarks

The findings reported in this volume on the physical, chemical and biological variability of the highly pro-ductive ecosystem in the coast off central/southern Chile reveal strong connections between wind-drivenupwelling and the ecological and biogeochemical responses of the system components/compartments. Thephysical–biological interactions might be transferred from low to higher trophic levels, causing variabilityin the productivity of fishery resources, and such system responses occur over a variety of temporal and spatialscales. Certainly, increased knowledge derived from regularly obtained data and experimental studies, is re-quired for a better understanding of the underlying mechanisms governing the structure and production ofthe pelagic and benthic realms. Further efforts should address the connections between upwelling variationand large-scale remote-forcing, and the role of mesoscale oceanographic features in further supporting biolog-ical production beyond the shelf area. Observational high-resolution measurements of the physical and chem-ical environment, as well as spatially resolved data, must be included in future studies. In the biologicalcompartment, several processes need further investigations, together with the use of highly technical observa-tional resources available today to monitor biological changes in marine systems. For example, trophic inter-actions and the different pathways of C and N fluxes, as modulated by upwelling and OMZ variation, must beconsidered in further detail. Modeling development should also become a major goal for this integrated effortof the COPAS Center as to explore and predict the impacts of natural and human-induced changes in the envi-ronment on the functioning of this highly productive ecosystem.

Acknowledgements

We are indebted to all reviewers, who devoted their precious time in assisting us and ensuring that the sub-mitted manuscripts resulted in high quality scientific papers. The COPAS Center and the COPAS Time SeriesStudy are funded by CONICYT (the Chilean Commission for Science and Technology) through its FONDAPProgram (Grant 15010007). Additional support for the Time Series Study has been provided by FIP (ChileanFund for Fishery Research) through its Grants FIP 2004-20 and FIP 2005-1. We thank the crew of the L/CKay Kay of the University of Concepcion and many students, who have been enthusiastic participants in thefield work. This Special Issue is a contribution of Chile to the GLOBEC International Program.

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Ruben EscribanoWolfgang Schneider

Center for Oceanographic Research in the eastern South Pacific (COPAS),

Department of Oceanography, University of Concepcion, Chile

E-mail address: rescribano@udec.cl (R. Escribano)

Available online 1 September 2007