the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.poor oxygen conditions, such as those described in González & Quiñones (2002). It has beenobserved that several abundant epipelagic species concentrate in the upper 50 m without exhibitingdiel vertical migration (DVM) (Escribano 1998, Escribano & Hidalgo 2000a), although someeuphausiids, such as Euphausia mucronata, may temporarily enter the OMZ (Antezana 2002), orsome others like the copepod Eucalanus inermis may even reside in it (Hidalgo et al. 2005a). Thus,the OMZ cannot be considered as just a constraint for vertical distribution because several speciesmay use it as their habitat, either temporarily or permanently. The ecological and biogeochemicalconsequences of entering and living within the OMZ should be considered as relevant issues. Futurestudies of species life cycles and behavioural or metabolic adaptations may provide novel findingsfor life under low oxygen, as described for benthic organisms inhabiting these systems (Helly &Levin 2004). Cycling and vertical fluxes of C and N mediated by zooplankton migration and verticaldistribution may be substantially modified by the low-oxygen and highly reduced environment.These issues have received little attention for marine zooplankton associated with OMZ systems(e.g., Wishner et al. 1998).In the upwelling region off northern Chile, the most abundant species are usually closely relatedto coastal upwelling plumes (Escribano et al. 2000, Giraldo et al. 2002). These species have beenwell identified (Heinrich 1973, Hidalgo & Escribano 2001). Among dominant ones, the studies ofhorizontal and vertical distribution have been focused on the calanoids Calanus chilensis (Escribano1998), Centropages brachiatus (González & Marín 1998) and Eucalanus inermis (Hidalgo et al.2005a) and on the euphausiid Euphausia mucronata (Escribano et al. 2000, Antezana 2002). Theavailable information indicates that Calanus chilensis and Centropages brachiatus are mostlyrestricted to the upper layer without performing substantial DVM (Escribano 1998, Escribano &Hidalgo 2000a). By contrast, Eucalanus inermis, the dominant species among a complex of fourto five species of the genus Eucalanus that coexist in this region, may remain in the upper boundaryof the OMZ with limited excursion into surface waters (Hidalgo et al. 2005a). Meantime, Euphausiamucronata has been suggested as actively and daily migrating into the OMZ (Antezana 2002). Asummary of the vertical extent of species habitats for zooplankton in this region has been recentlyconstructed from several cruises (Escribano et al. accepted) and is illustrated in Figure 5. Zooplanktoncan indeed occupy the entire water column despite the presence of an intense OMZ.Occurrence and vertical movements of various species may ensure a substantial contribution ofzooplankton to the vertical export of OM.Zooplankton life cycles and population dynamicsLife cycles and population dynamics are certainly important issues for understanding the biogeochemicaland ecological role of zooplankton in this region. When examining annual life cyclesthere are some important factors to consider. Since the entire upwelling region is subjected tointerannual variability due to the ENSO cycle, different years might induce changes in populations.Water column warming, an abrupt thermocline and oxycline deepening characterises the ENconditions in northern Chile (Ulloa et al. 2001, Escribano et al. 2004a), in contrast to a typicallyshallow thermocline and OMZ indicating a ‘normal’ (LN condition) upwelling situation. However,despite oceanographic variability some species, such as Calanus chilensis, seem to have a ratherstable annual cycle from year to year independent of ENSO variation (Escribano & Hidalgo 2000b).Dominant zooplankton species have been suggested to be strongly associated with upwelling centres(González & Marín 1998, Escribano & Hidalgo 2000b), and they can thus complete their life cycleswithin the upwelling zone, growing at temperature-dependent rates under non-limiting conditionsof food (Escribano & McLaren 1999, Giraldo et al. 2002). Lack of food shortage has even beensuggested during EN conditions (Ulloa et al. 2001). Continuous reproduction and multiple generationsthroughout seasons characterise life cycles of copepods in this region (Escribano & Rodríguez216
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEDepth (m)020406080100120140160180200220240260280300320340360380400420Eucalanus subtenuisSubeucalanus crassusEucalanus attenuatusEucalanus inermis (1)Paracalanus parvus (1)Oncaea conifera (2)Oithona similisCorycaeus typicus (1)Calanus chilensis (1)Scolecithrichella bradyiAcartia tonsa (2)Euaetideus bradyiPleuromamma gracilis (3)Centropages brachiatus (1)Candacia sp.Euchaeta sp.Candacia rostrataEuphausia mucronataEuphausia distinguendaEuphausia eximia (3)Euphausia recurvaEuphausia teneraStylocheiron affinisNematoscelis megalops?Nematoscelis flexipes?Euphausia sp.Figure 5 Dominant habitat and daily movements of zooplankton species in and out of the main core of theoxygen minimum zone (OMZ) (shaded area) during coastal upwelling at northern Chile. Day (white dots)and night (black dots) positions represent the depth distribution of each one of the species listed and the arrowthe extent of their vertical movement. These positions were estimated from depth-weighed averages ofabundances. Data are from MinOx cruise carried out in March 2000 (modified from Escribano et al. accepted).(1) Dominant species found in upwelling centers, (2) inshore species, (3) species associated with El Niñoevents.1994, Hidalgo et al. 2005b). However, strong variation in population size can be observed at sometimes of the year (Escribano 1998, Ulloa et al. 2001, Hidalgo et al. 2005b). The populations ofannual species typically show a sudden collapse that tends to occur by the end of the summer. Thispattern has been described for Eucalanus inermis (Hidalgo et al. 2005b), Calanus chilensis andCentropages brachiatus (Hidalgo & Escribano submitted) and Euphausia mucronata (Escribanoet al. accepted). The conceptual model of population dynamics for these species (Figure 6) considersa two-stage population, which may grow exponentially during the spring and early summer at lowmortality rate and then exhibit an abrupt decay at high mortality rate due to increased predation.In this model the increase in predation pressure coincides with the rise of the OMZ, which producesa habitat that is vertically constrained to surface waters by low oxygen concentrations, therebyresulting in increased interactions among prey and predators. This model does not preclude thepossibility that changing food quality associated with the rise of the OMZ, either through deleteriouseffects of diatoms on copepods (Ianora et al. 2004) or by low nutrition, might negatively impactthe population. Indeed, a recent study (Vargas et al. 2006b) has shown that available food resourcesmay strongly affect reproduction and recruitment of zooplankton in the coastal zone of the HCS.Future perspectives in zooplankton researchMuch of the current research devoted to understanding the ecological and biogeochemical role ofzooplankton consumers in the upwelling region off northern Chile is focused on establishing217
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the humboldt current system of northern and central chile - figema

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