the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.Conversely, it is predicted that taxa with low dispersal potential will exhibit pronounced geneticstructure and will be the most affected by the oceanographic variations. Genetic studies of themacroalga Lessonia nigrescens show that gene flow is limited among nearby populations (Martínezet al. 2003, Faugeron et al. 2005). Additionally, these authors found that 20 yr after the EN1982–1983 event, which caused a massive mortality of L. nigrescens on 600 km of the coastline,northward recolonisation had only advanced 60 km (Martínez et al. 2003). Lessonia nigrescens isa good example of a species very vulnerable to oceanographic changes, specifically EN, and thatmay be continuously recovering from drastic population reductions and local extinction, neverreaching migration–drift equilibrium. The genetic structure found for L. nigrescens corresponds tochaotic patchiness at a small geographical scale (tens of metres), reflecting recent populationdynamic processes (years to tens of years) and life-history traits such as very low distance dispersalof propagules (Faugeron et al. 2005). Other species that show genetic differentiation at a smallspatial scale are the alga Mazzaella laminarioides (Faugeron et al. 2001) and the edible andoverexploited snail Chorus giganteus that has a low larval dispersal potential (Gajardo et al. 2002).For the edible and also overexploited scallop Argopecten purpuratus, Moragat et al. (2001) foundboth genetic and morphological differentiation between populations at the two protected sides ofthe Mejillones Peninsula (50 km apart) and discuss that it is probably due to currents that restrict thegene flow between the two localities.It can further be predicted that biogeographic breaks will reflect strong barriers to dispersaland thus gene flow for species with low dispersal potential, leading to breaks in the geographicstructure of the genetic diversity of species. It has been shown that along the northern Chileancoasts, habitat discontinuities can cause gene flow interruptions (e.g., Faugeron et al. 2001, 2005).Species with lower dispersal potential will be more vulnerable to breaks, while species with highpotential of dispersal may not show evidence of a genetic break associated with a biogeographicbreak, as is the case of Cancer setosus (Gomez-Uchida et al. 2003). Even though for the HCS ithas not yet been demonstrated that recognised biogeographic breaks correspond with the geographicdistribution of the genetic diversity, it has been shown to be the case for other biogeographic regionssuch as Point Conception in the California Current System (e.g., Burton 1998, Wares et al. 2001).Rafting may be a very advantageous dispersal mechanism for populations that suffer recurrentextinctions and recolonisations, mostly in the extent of the HCS where macroalgae with highfloatability are very abundant. Once organisms are in a raft that has the potential to be in voyagefor weeks or months, the rafting-mediated gene flow resulting may not be strictly associated withgeographic distance and the resulting pattern of connectivity will depend on the intensity of geneflow, that is, if the rafting route is frequent, intermittent or episodic (see Thiel & Haye 2006). Wepredict that given the abundance of floating macroalgae, rafting routes along the Chilean coast maybe intermittent or frequent, leading to patterns of genetic diversity ranging from chaotic patchinessto homogeneity. Ongoing studies of the isopod Limnoria chilensis may contribute to the validationof this prediction. These organisms have the potential to persist in rafts for long periods of timebecause they are brooders, have local recruitment and feed on the raft. It is interesting to mentionthat even though Lessonia nigrescens shows high genetic differentiation even at small spatial scales,the geographic distribution of the genetic diversity does not follow an IBD pattern, suggesting thatsome long-distance dispersal may occur, although it is not known whether it could be via freelivingspores or on drifting fragments of mature thalli (Faugeron et al. 2005).The HCS appears to be an interesting model for studying marine connectivity patterns invariable environments. Despite the general lack of such studies, recent and still unpublished resultson pelagic fishes such as anchovies and sardines, and commercially exploited benthic marineinvertebrates like the gastropod Concholepas concholepas and the bivalve Mesodesma donacium,support the existence of genetic homogeneity at large geographical scales as a consequence of theabsence of contemporary biogeographical barriers along the HCS for species with high dispersal254
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEpotential. In general, it is expected that further studies of different biological systems will showthat all the patterns of connectivity (Figure 16) are present along the HCS as a result of theinteraction of present and past environmental conditions with species life-history traits.BiogeographyLarge-scale patterns in the HCSThe pioneer work by S.P. Woodward in 1856, which is probably the earliest biogeographicalclassification involving the southeast Pacific (Camus 2001, Harzhauser et al. 2002), was followedby a series of foundational studies (e.g., Dall 1909, Ekman 1953, Stuardo 1964, Viviani 1979,Santelices 1980, Brattström & Johanssen 1983, among others) that provided a consistent view ofthe major biogeographic features of the HCS temperate area (south of the tropical PanamianProvince), based on physical gradients and patterns of endemism, richness and spatial turnover ofspecies, and supported by subsequent studies (see reviews by Fernández et al. 2000 and Camus2001). Overall, two main biotic replacements along the coast differentiate three biogeographicalunits (see Brattström & Johanssen 1983 and Camus 2001 for reviews on available classifications):(1) a warm-temperate biota extending from northern Peru (4–6°S) toward a variable, taxondependentlimit in northern Chile (usually 30–36°S), often designated as Peruvian Province, anddominated by subtropical and temperate species; (2) a cold-temperate biota (also present in southernArgentina) extending along the fragmented coast of the Chilean archipelago from 54°S to about41–43°S, corresponding to the Magellanic Province dominated by subantarctic and temperatespecies, exhibiting reduced wave exposure and an estuarine condition due to the dilution causedby high rainfall levels, glaciers and rivers (Ahumada et al. 2000); and (3) a transition zone betweenboth provinces, characterised by strong numerical reduction of subtropical and subantarctic speciesat its southern and northern borders, respectively, rather than by diffusive overlap of biotas. However,many species occurring throughout this transition zone have a subantarctic affinity and a widedistribution in Chile (e.g., Menzies 1962, Castillo 1968, Alveal et al. 1973, Santelices 1980),probably facilitated by the HCS transporting cool water masses toward the north, which is alsoconsidered to be the main reason why the area lacks a definite biogeographic character.Traditionally, the important physical changes around 42°S are considered to be external forcingsthat act as effective filters for dispersal, and with few exceptions, this zone represents the steepestinduced transition along the HCS coast. Contrastingly, the northern limit of the transition zone isremarkably diffuse for the whole coastal biota (Figure 17) and highly variable depending on thetaxon examined (Camus 2001), which has been attributed so far to the apparent absence of majorphysical discontinuities between northern Peru and Chiloé Island (e.g., Brattström & Johanssen1983, Jaramillo 1987). Such variation mirrors a typical pattern of transitions (Brown & Lomolino1998), due to differential attenuation rates among taxa related with their different dispersal abilityand physiological tolerance. In fact, some particular taxa (e.g., peracarid crustaceans; Thiel 2002)show a well-defined overlap of northern and southern species with a gradual replacement pattern.On a wider taxonomic basis, however, the breaking points for different taxa do exhibit somelatitudinal scattering throughout northern Chile, but they are significantly concentrated around 30°Sand 33°S (see comparative analyses of animal and macroalgal taxa in Brattström & Johanssen1983, Lancellotti & Vásquez 2000, Meneses & Santelices 2000, Santelices & Meneses 2000, Camus2001). Notably, these multiphyletic breaks include southern and northern limits of species withvery different life forms and ecological requirements, even involving pelagic groups (e.g., Antezana1981, Hinojosa et al. 2006). This information strongly suggests that such breaks are not a passiveoutcome of dispersal and tracking of key environmental variables. For instance, recent work showsthat latitudinal patterns of SST (the main causal factor invoked in most studies) fail to explain255
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the humboldt current system of northern and central chile - figema

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