the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.some kelp assemblages of Macrocystis integrifolia and Lessonia trabeculata around 24°S (Martínezet al. 2003, Vega et al. 2005). These effects would be related to the frequency and intensity of localcoastal upwelling (González et al. 1998, Lagos et al. 2002), which minimised the impact of warmingand retained high concentrations of nutrients within the coastal environment (Takesue et al. 2004).A long-term analysis of the structure and organisation of kelp communities in northern Chile(Vásquez & Vega 2004b, Vásquez et al. 2006), which included EN and LN events, showed that theabundance of Macrocystis integrifolia (1) increased significantly during EN 1997–1998, (2)decreased during LN 1999–2001, dropping nearly to zero in 2000, and (3) became reestablishedand recovered during a period of positive thermal anomalies in 2002–2003 (Figure 18). This patternFigure 18 (A) Upwelling index (Offshore Ekman Transport OET), (B) ENSO index (Southern OscillationIndex SOI-grey line and Multivariate ENSO Index MEI-black line), (C) temporal variability of Macrocystisintegrifolia, (D) Lessonia trabeculata, (E) benthic grazers, and (F) benthic predator densities, including El Niño1997–1998 and La Niña 1999–2000.260
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEwas different from that recorded on the California coast, where the rapid recovery of M. pyriferafollowing EN 1997–1998 was favoured by the establishment of a cold period (1998–2000) and thesurvival of sporophytes in deep environments (Ladah et al. 1999; Edwards 2004). In northern Chile,the recolonisation rate of kelp assemblages occurred comparatively slowly (Martínez et al. 2003;see also Population connectivity, p. 252ff. and Biogeography, p. 255ff.), even though cold conditionsprevailing during 1998–2000 enhanced the upwelling effect. In this regard, the slow recovery ofLessonia nigrescens after EN 1982–1983 (Castilla & Camus 1992) appeared more related to bioticconstraints: recruitment was strongly reduced by a combination of postsettlement grazing andinhibition by encrusting coralline algae, while erect coralline algae played a key role as facilitators,allowing the kelp some escape from grazers and space competitors (Camus 1994a).On the other hand, the decreased abundance of Macrocystis integrifolia was caused by asignificant reduction in the adult plant population and the lack of recruitment of juvenile sporophytes(Figure 18). Thus, the disappearance of the M. integrifolia population occurred 2 yr after EN1997–1998 and was inversely correlated with a population increase of the sea urchin Tetrapygusniger (Figure 18). In contrast, information from other areas of the southeastern Pacific during EN1997–1998 showed that superficial warming decreased the abundance of kelp on shallow bottoms,inducing migrations of grazers to deeper zones (Fernández et al. 1999, Godoy 2000, Lleellish et al.2001). In northern Chile, during EN 1997–1998 and LN 1998–2000, different events favoured theincrease of sea urchin populations during the cold phase, including (1) induction of mass spawningdue to increases in SST and persistence of upwelling events, (2) reduction in density of adultseastars, and (3) changes in the feeding behaviour of the seastar Heliaster helianthus, one of themost important benthic predators on Chilean and Peruvian coasts (Tokeshi & Romero 1995b,Vásquez et al. 2006) (Figure 18). Thus, the long-term study of subtidal communities suggests thatdifferent bottom-up and top-down factors might control ecosystem changes in northern Chile,including (1) the intensity and frequency of upwelling, which may buffer the positive thermalanomalies of SST and maintain high nutrient levels, favouring kelp persistence during EN events;(2) site-dependent oceanographic conditions, which may generate optimal conditions for spawning,larval development, and recruitment of echinoderms during and/or after EN events; (3) an overallabundance increase of carnivores which is correlated with an abundance decline of the mostconspicuous grazers; (4) population dynamics of adult seastars and sea urchins which may becomedecoupled during EN events; (5) species-specific population dynamics of some predator species(e.g., Luidia magellanica), and changes in dietary composition in others (e.g., H. helianthus), whichmay promote population increase of its prey, the urchin Tetrapygus niger, during EN events; and(6) changes in abundance of T. niger, which might be a key factor controlling the development oftwo alternate states: environments dominated by kelp beds versus barren ground areas.In a wider context involving both subtidal and intertidal environments, EN impacts can besummarised as a large-scale bottom-up effect influencing various (and as yet difficult-to-predict)levels of marine food webs. However, this is just the initial path for most impacts, and top-downeffects should not be neglected (e.g., see Nielsen & Navarrete 2004). Future research on EN impactscould consider at least five aspects related to the variability of biological effects, which may serveas guidelines or study framework: (1) the southward intensity attenuation of EN signals producesa latitudinal impact gradient, with reduced effects toward higher latitudes (e.g., Castilla & Camus1992, Martínez et al. 2003); (2) in the spatiotemporal context, many effects are episodic and/orlocal (e.g., abundance variability), and some others may propagate their effects to larger spatialscales (e.g., distribution changes, local extinctions), being highly persistent over time (e.g., seeCamus et al. 1994); (3) on a taxonomic basis, some taxa are recurrently affected (e.g., kelps), othersexhibit no significant impacts (e.g., chlorophytes), and some taxa can be more affected in theirreproduction while others in their recruitment (e.g., Camus 1994a, Navarrete et al. 2005, Vásquezet al. 2006); (4) the genetic and evolutionary consequences of recurrent phenomena such as mass261
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the humboldt current system of northern and central chile - figema

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