the humboldt current system of northern and central chile - figema

THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEnot readily explained simply by the contact between warm and cold biotas, and proper explanationswill require a multivariate, integrative approach and an exploration of possible external forcings.The role of past and present processes in northern-central Chile (18–35°S)Different lines of historical and ecological evidence suggest that northern-central Chile constitutesa very complex and dynamical biogeographic scenario. Clearly, present-day patterns are notdivorced from physical changes related to the origin and installation of the cold HCS during theTertiary or from subsequent Quaternary fluctuations (e.g., see Villagrán 1995, Hinojosa & Villagrán1997, Villa-Martínez & Villagrán 1997, Maldonado & Villagrán 2002). The establishment of theHCS had involved both the northward advance of the subantarctic biota and the northward retractionof a former tropical/subtropical biota (Brattström & Johanssen 1983, Camus 2001), with consequencesthat may persist until the present, reflected in the heterogeneous character of the northernbiota. For instance, 10 of the nowadays most common bivalves in northern Chile exhibit upperthermal tolerances exceeding the highest temperatures recorded during EN events in the past century(Urban 1994), which is unexpected for species evolving in a cold upwelling system. At least oneof them (Argopecten purpuratus) is thought to be a relict of the Miocene tropical/subtropical fauna(Wolff 1987). Such physiological ‘anomalies’ suggest the presence of an inertial faunistic componentwithin the modern northern biota (i.e., remnants of the former warm fauna that escaped the forcedretraction to lower latitudes, and maintained their warm-water characteristics facilitated by recurrentpost-Miocene warming events such as EN) (Wolff 1987). In comparison, the marine flora appearsmore homogeneous and dominated mainly by subantarctic species, while tropical/subtropical speciesare virtually absent (Santelices & Meneses 2000). For instance, some common and ecologicallyimportant kelp species are not only more sensitive to warming episodes but also their upper thermaltolerance varies in accordance with the thermal latitudinal gradient (Martínez 1999).In this regard, the northern fauna underwent repeated distributional alterations in the pastassociated with climatic fluctuations. Some of them involved the simultaneous range retraction andexpansion of different species (e.g., Ortlieb et al. 1994), but more often the occurrence of tropical/subtropical fauna related to warming (EN-like) events in the Pleistocene (e.g., Ortlieb 1995) andHolocene (e.g., Llagostera 1979). Moreover, Neogene processes related to the establishment of themodern upwelling system in the HCS (e.g., shallowing of the OMZ) provoked a mass extinctionof bivalve molluscs (>75% of species), with lasting impacts on their current distribution patternsand biological characteristics (Rivadeneira 2005), and similar effects have been suggested for thepolychaete fauna (R.A. Moreno et al. 2006a). Notwithstanding, the causal relationships betweenhistorical events and current distribution patterns in northern Chilean waters remain largely unexplored,although their importance may be overwhelming.On the other hand, modern processes also have strong influences in northern Chile, particularlyinterannual fluctuations related to ENSO, which, however, should be looked at retrospectively,acknowledging the frequency and importance of EN-like events throughout the Holocene (e.g, seeMaldonado & Villagrán 2002). Strong EN events can modify the taxonomic composition of littoralcommunities (e.g., Arntz 1986, Castilla & Camus 1992, Camus et al. 1994, Vega et al. 2005,Vásquez et al. 2006) and the geographical occurrence of many species including key structuralcomponents such as intertidal and subtidal kelps (e.g., Lessonia nigrescens, L. trabeculata andMacrocystis pyrifera). Short-term modifications of community composition during EN occurthrough local extinctions and invasions, depending also on local conditions, which either favour orprevent their occurrence (Arntz 1986, Camus et al. 1994, Martínez et al. 2003, Castilla et al. 2005a).Moreover, the impacts on key species may scale up to produce long-term biogeographic changes,as exemplified by the dramatic effects of the 1982–1983 EN event on the intertidal kelp Lessonia257