the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.productivity observed along the northern and central coast of the HCS (23–37°S) is probablyfertilising oceanic and oligotrophic areas through the formation of eddies and filaments that transportcoastal water offshore (Marín et al. 2003a, Hormazábal et al. 2004, Letelier et al. 2004). Finally,much of what is known about upwelling dynamics, phytoplankton ecology and their role inbiogeochemical cycles along the Chilean coast has been mainly gathered from selected upwellingareas (i.e., Antofagasta, Coquimbo, Concepción, Valparaíso) with scarce or no information fromareas in-between.Zooplankton consumersZooplankton consumers are considered the link between primary producers and higher trophiclevels in the pelagic realm of the world ocean. This function also implies a role to regulate the rateat which phytoplankton-C can be transferred through the food web or retained as zooplanktonbiomass. This rate greatly depends on population turnover rates, which vary widely among zooplanktontaxa. Thus, the knowledge of the population biology of dominant species, the communitystructure and its variation, become key issues in understanding the ecological and biogeochemicalrole of zooplankton. In the HCS substantial progress has been made in the last decades about someof these issues. In the following sections we summarise the major advances in understandingpopulation, community and ecosystem processes involving zooplankton in the upwelling regionoff northern Chile. Much less progress is being made in molecular, genomic and genetic biologyof zooplankton in this region. However, the need for a better understanding of zooplankton ecologyin the region will most likely motivate the use of molecular tools in the near future.Biogeographic and biodiversity issuesIn the upwelling region off northern Chile most species or species assemblages are considered tobe part of the subantarctic fauna. This fauna originates in the Austral region, but it is then advectednorthward. In northern Chile and off Peru, it mixes with some species of tropical and equatorialorigin. The most studied taxa are Copepoda (Heinrich 1973, Vidal 1976, Hidalgo & Escribano2001) and Euphausiacea (Antezana 1978, D. Fernández et al. 2002). Nearly 60 species of copepodshave been identified, of which the dominant in the coastal zone are Calanus chilensis, Centropagesbrachiatus, Paracalanus parvus, Acartia tonsa, Eucalanus inermis, Oithona similis, Oncaeaconifera and Corycaeus typicus. Calanus chilensis is an endemic species of the HCS (Marín et al.1994), whereas Centropages brachiatus, Paracalanus parvus, Acartia tonsa and Oithona similisare cosmopolites and widely distributed along the Chilean coast. Eucalanus inermis is a typicaltropical species, although the genus Eucalanus may contain about six species, which have not yetbeen clearly defined. Among euphausiids, the most abundant and endemic species of the HCS isEuphausia mucronata (Antezana 1978). This species is closely associated with upwelling centresoff northern Chile (Escribano et al. 2000) and performs an extensive vertical migration into theOMZ. Euphausia eximia is another abundant species, which increases in number during EN(Antezana 1978, González et al. 2000b). Gelatinous zooplankton, on the other hand, has receivedmuch less attention, although it may form dense aggregations at times of the year in the coastalupwelling zone (Pagès et al. 2001), and it can exert a strong predation pressure on copepods(Giesecke & González 2004, H.E. González et al. 2004b, Pavez et al. 2006) and possibly on fishlarvae.Species replacements seem to occur in the zooplankton associated with EN versus LN regimes(Hidalgo & Escribano 2001). Dominant species alternate between copepods (mainly Calanus andEucalanus genera) and euphausiids during upwelling conditions and cyclopoid copepods, Paracalanus214
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEand Acartia during EN events (González et al. 2002). Warm conditions during an EN may alsocause reduced size of copepods at maturity (Ulloa et al. 2001). All these changes in structure ofthe pelagic system may have profound implications on the functioning and productivity of thisregion (see, e.g., Alheit & Niquen 2004) and should also be considered in future ecosystem studies.Zooplankton and coastal upwelling in northern ChileIn the coastal zone off northern Chile, wind-driven upwelling is the principal driver of biologicalproductivity. Pelagic organisms benefit by high productivity in upwelling sites. However, upwellingzones comprise strongly variable environments and zooplankton must cope with changing conditionsin this zone. The understanding of these physical and biological interactions in the watercolumn of upwelling systems may give much insight into the key processes that control productionand biological diversity of pelagic assemblages. In northern Chile, examples of physical–biologicalinteractions taking place during coastal upwelling are restricted to particular conditions and sites(e.g., Escribano & Hidalgo 2000a, Escribano et al. 2001, 2002, Giraldo et al. 2002). Other studies(Marín et al. 2001) have shown some relevant findings that may help understanding mechanismsthrough which pelagic populations are able to maintain their populations in the food-rich coastalzone. Off Mejillones Peninsula (23°S), the interaction between a poleward flow and cold upwellingplumes may generate large eddies by which non-migrant plankton can be maintained nearshore.This type of circulation may act as a retention mechanism to avoid offshore advection of zooplankton(Giraldo et al. 2002). Thus, most species maintain their population in the food-richupwelling centres being recirculated by near-surface currents. In other upwelling systems, such asthe Benguela Current, zooplankton is maintained nearshore by vertical migration behaviour(Verheye et al. 1992, Roy 1998). Organisms advected offshore migrate to deep water and then arereturned to shore by a compensatory flow. In northern Chile, however, vertical migration does notseem to be an important or widely used behaviour. In fact, most dominant species appear stronglyconstrained to the upper layer (
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