the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.Abstract The Humboldt Current System (HCS) is one of the most productive marine ecosystemson earth. It extends along the west coast of South America from southern Chile (~42°S) up toEcuador and the Galapagos Islands near the equator. The general oceanography of the HCS ischaracterised by a predominant northward flow of surface waters of subantarctic origin and bystrong upwelling of cool nutrient-rich subsurface waters of equatorial origin. Along the coast ofnorthern and central Chile, upwelling is localised and its occurrence changes from being mostlycontinuous (aseasonal) in northern Chile to a more seasonal pattern in southern-central Chile.Several important upwelling centres along the Chilean coast are interspersed with long stretchesof coast without or with sporadic and less intense upwelling. Large-scale climatic phenomena(El Niño Southern Oscillation, ENSO) are superimposed onto this regional pattern, which resultsin a high spatiotemporal heterogeneity, complicating the prediction of ecological processes alongthe Chilean coast. This limited predictability becomes particularly critical in light of increasinghuman activities during the past decades, at present mainly in the form of exploitation of renewableresources (fish, invertebrates and macroalgae). This review examines current knowledge of ecologicalprocesses in the HCS of northern and central Chile, with a particular focus on oceanographicfactors and the influence of human activities, and further suggests conservation strategies for thishigh-priority large marine ecosystem. Along the Chilean coast, the injection of nutrients into surfacewaters through upwelling events results in extremely high primary production. This fuels zooplanktonand fish production over extensive areas, which also supports higher trophic levels,including large populations of seabirds and marine mammals. Pelagic fisheries, typically concentratednear main upwelling centres (20–22°S, 32–34°S, 36–38°S), take an important share of thefish production, thereby affecting trophic interactions in the HCS. Interestingly, El Niño (EN) eventsin northern Chile do not appear to cause a dramatic decline in primary or zooplankton productionbut rather a shift in species composition, which affects trophic efficiency of and interactions amonghigher-level consumers. The low oxygen concentrations in subsurface waters of the HCS (oxygenminimumzone, OMZ) influence predator-prey interactions in the plankton by preventing somespecies from migrating to deeper waters. The OMZ also has a strong effect on the bathymetricdistribution of sublittoral soft-bottom communities along the Chilean coast. The few long-termstudies available from sublittoral soft-bottom communities in northern and central Chile suggestthat temporal dynamics in abundance and community composition are driven by interannual phenomena(EN and the extent and intensity of the OMZ) rather than by intra-annual (seasonal) patterns.Macrobenthic communities within the OMZ are often dominated in biomass by sulphide-oxidising,mat-forming bacteria. Though the contribution of these microbial communities to the total primaryproduction of the system and their function in structuring OMZ communities is still scarcely known,they presumably play a key role, also in sustaining large populations of economically valuablecrustaceans. Sublittoral hard bottoms in shallow waters are dominated by macroalgae and suspension-feederreefs, which concentrate planktonic resources (nutrients and suspended matter) andchannel them into benthic food webs. These communities persist for many years and local extinctionsappear to be mainly driven by large-scale events such as EN, which causes direct mortalityof benthic organisms due to lack of nutrients/food, high water temperatures, or burial underterrigenous sediments from river runoff. Historic extinctions in combination with local conditions(e.g., vicinity to upwelling centres or substratum availability) produce a heterogeneous distributionpattern of benthic communities, which is also reflected in the diffuse biogeographic limits alongthe coast of northern-central Chile. Studies of population connectivity suggest that species withhighly mobile planktonic dispersal stages maintain relatively continuous populations throughoutmost of the HCS, while populations of species with limited planktonic dispersal appear to featurehigh genetic structure over small spatial scales. The population dynamics of most species in theHCS are further influenced by geographic variation in propagule production (apparently caused bylocal differences in primary production), by temporal variation in recruit supply (caused by upwelling196
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEevents, frontal systems and eddies), and topographically driven propagule retention (behind headlands,in bay systems and upwelling shadows). Adults as well as larval stages show a wide rangeof different physiological, ecological and reproductive adaptations. This diversity in life-historystrategies in combination with the high variability in environmental conditions (currents, foodavailability, predation risk, environmental stress) causes strong fluctuations in stocks of bothplanktonic and benthic resources. At present, it remains difficult to predict many of these fluctuations,which poses particular challenges for the management of exploited resources and the conservationof biodiversity in the HCS. The high spatiotemporal variability in factors affectingecological processes and the often-unpredictable outcome call for fine-scale monitoring of recruitmentand stock dynamics. In order to translate this ecological information into sustainable use ofresources, adaptive and co-participative management plans are recommended. Identification of areaswith high biodiversity, source and sink regions for propagules and connectivity among localpopulations together with developing a systematic conservation planning, which incorporates decisionsupport systems, are important tasks that need to be resolved in order to create an efficientnetwork of Marine Protected Areas along the coast of northern-central Chile. Farther offshore, thecontinental shelf and the deep-sea trenches off the Chilean coast play an important role in biogeochemicalcycles, which may be highly sensitive to climatic change. Research in this area shouldbe intensified, for which modern research vessels are required. Biodiversity inventories must beaccompanied by efforts to foster taxonomic expertise and museum collections (which shouldintegrate morphological and molecular information). Conservation goals set for the next decadecan only be achieved with the incorporation of local stakeholders and the establishment of efficientadministrative structures. The dynamic system of the HCS in northern-central Chile can only beunderstood and managed efficiently if a fluent communication between stakeholders, administrators,scientists and politicians is guaranteed.IntroductionThe deep-blue colour of the water observed for a long time past gave place to a green colour, and onthe whole there was a great change in the general character of the surface fauna, pointing to the nearnessof a great continent, similar to what was observed off Japan and elsewhere. On November 18 [1875],the water was very green in colour, and the ship occasionally passed through large red or brown patches,which the tow-net showed to be due to immense numbers of red copepods, hyperids, and other Crustacea.Murray (1895), on approaching Valparaíso aboard H.M.S. ChallengerSuch a vivid language was rarely used by John Murray to refer to the abundance of planktonicorganisms in surface waters. Only for the surface plankton from the Agulhas Bank off South Africadid he employ similar colourful language, referring to “myriads of Zoeae and a few larger Megalopae”.Scientists studying the plankton ecology of the Eastern Boundary Currents (EBCs) are usedto the sight of these dense accumulations of zooplankton, which are often found in sharply definedpatches. It is the intense upwelling of nutrient-rich waters in the EBCs that fuels the extraordinaryhigh primary production (PP) in the EBCs, which forms the basis of the food web supporting someof the largest fisheries of the world. However, the frequency and intensity of upwelling within theEBCs varies, mainly depending on large-scale climatic forcing, latitudinal/seasonal signals andlocal factors, such as the width of the shelf, coastal topography, and sources of upwelled waters(Thomas et al. 2004). Although the overall importance of upwelling in these large marine ecosystemsis relatively well known, the effects of temporal and spatial variability of upwelling on theecology and productivity of the planktonic and benthic communities remain poorly understood.Herein these effects are explored, using the Humboldt Current System (HCS), one of the most197
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the humboldt current system of northern and central chile - figema

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