the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.1995, Kinlan & Gaines 2003, Palumbi 2003, Sotka & Palumbi 2006). Similarly, trace elementmicrochemistry (elemental fingerprinting) (Swearer et al. 1999, DiBacco & Levin 2000, Zacherlet al. 2003) or stable isotope ratios (Herzka et al. 2002, Levin 2006) hold promise for identifyinglarval origin under specific environmental conditions. However, these techniques do not yet providea quantitative measure of the fate of all propagules released from a focal location (i.e., the ‘dispersalkernel’). Therefore, spatially explicit connectivity among local populations, the type of informationneeded regarding the location of MPAs (Botsford et al. 1994, Lockwood et al. 2002, Kaplan 2006),remains tractable only through the combination of biophysical models linking larval attributes withadvection-diffusion physical processes (Marín & Moreno 2002, Largier 2003, Siegel et al. 2003,Guizien et al. 2006, Kaplan 2006, Levin 2006, Aiken et al. 2007).Studies of dispersal in HCSStudies of dispersal within the HCS are scarce at best. Santelices (1990a) provides a review ofdispersal in marine seaweeds and points out that most information comes from laboratory studiesconducted under idealised hydrographic conditions or from rather anecdotal evidence of colonisationof new habitats. Studies conducted by incubating seawater samples have demonstrated theexistence of a multispecific ‘spore cloud’ which is present year-round in coastal waters of centralChile (Hoffmann & Ugarte 1985, Hoffmann 1987). These studies showed the patchy and temporallyvariable nature of the spore cloud, but the dispersal distances and mechanisms involved are unclear.Considering the small size of spores (5–150 µm) and their short duration (a few hours, but it canbe up to few days; Santelices, 1990a), stochastic turbulence diffusion probably plays a major rolein shaping the dispersal kernels in algal dispersal. Nearshore advective currents within the dispersalscale of spores (e.g., tidal currents, breaking waves, internal tidal bores) cannot be ruled out,however. Recent studies by Bobadilla & Santelices (2005) conducted by sampling the water columnwith a semi-automated sampling device (Bobadilla & Santelices 2004), illustrate the great temporalvariability in multispecific dispersal kernels for major algal groups and dispersal distances exceeding100 m.The most direct studies of dispersal of invertebrates in the HCS are restricted to species withshort larval duration, such as tunicates (Castilla et al. 2002a,b, 2004a). These studies sampled larvaldistribution at distances from a unique adult population source. Quantitative aspects of dispersalfor species with long-lived larval stages are virtually unknown for any invertebrate or coastal fishspecies in the HCS. Several physical processes that can increase offshore and alongshore advection,or instead increase retention of larvae near shore, have been described for the coast of Chile, suchas upwelling filaments, cyclonic circulation in embayments, topographically controlled eddies andupwelling shadows and traps (Vargas et al. 1997, Marín et al. 2001, Castilla et al. 2002a, Escribanoet al. 2002, Wieters et al. 2003, Narváez et al. 2004). These features undoubtedly influence dispersalof coastal species, potentially increasing self-recruitment (Swearer et al. 2002), but their effect onconnectivity among adult populations of any species is hard to demonstrate. A few high-resolution3-dimensional numerical models of currents in the coastal ocean have been developed and testedagainst physical data for different sections of the coast of Chile (Mesías et al. 2001, Aiken et al.2007). Coupled with Lagrangian larval-tracking techniques these biophysical models can generatetestable hypotheses about dispersal and connectivity in real biological systems (e.g., Aiken et al.2007).There is an urgent requirement to develop highly variable, neutral molecular markers such asmicrosatellites for algal and invertebrate species inhabiting the HCS system to improve the abilityto infer dispersal distances over ecological timescales and to test hypotheses about connectivity (seealso Population connectivity, p. 252ff.) derived from theoretical models.248
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILESettlement studies in the HSCA more tractable and directly related problem is the settlement and recruitment of species to agiven habitat. Settlement is the process through which a spore or larva makes permanent contactwith the benthic habitat (Keough & Downes 1982). Since most adults of benthic organisms aresessile or have limited movement, settlement marks the end of the effective dispersal phase. Forbrooding organisms with mobile adults or those that use rafting as a secondary dispersal mechanismthis is of course not the case (Thiel & Haye 2006). Recruitment is the input of new individuals tothe benthic population measured at some arbitrary time after settlement. Therefore, while settlementis expected to reflect the arrival or supply of propagules, recruitment can be substantially modifiedby postsettlement mortality. Of the considerable number of studies of supply ecology conducted inthe HCS over the past decades, very few have come close to measuring settlement (Hoffmann &Ugarte 1985, Hoffmann 1987, Moreno et al. 1993a, 1998, Martínez & Navarrete 2002, Vargas et al.2004, Lagos et al. 2005, Narváez et al. 2006), and most have actually examined recruitment atvarying time intervals after settlement. The paucity of studies of settlement of marine algae, duelargely to the enormous difficulties of identifying sporelings to species level (Hoffmann 1987,Santelices 1990a), has not permitted the identification of mechanisms involved in algal settlementpatterns. On the other hand, using intertidal barnacles, mussels and several gastropod species asmodel organisms, a few larval transport mechanisms have been demonstrated in the central andsouthern HCS. At the locality of Las Cruces in central Chile, which has been characterised as anupwelling shadow (Kaplan et al. 2003, Wieters et al. 2003, Narváez et al. 2004), the onshore dailysettlement of several invertebrate species is associated with conditions that favour the occurrenceof internal tidal bores, which appear to be common in the area when the water column is wellstratified (Vargas et al. 2004). These results suggest that, for a number of intertidal invertebrates,internal tidal bores (Pineda 1991, 1994a) can be an important mechanism of onshore transport. Incontrast with results at some sites in the California upwelling ecosystem (e.g., Farrell et al. 1991,Wing et al. 1995a), studies at Las Cruces showed that settlement of invertebrates was not directlyassociated with upwelling-relaxation events, which occur throughout spring and summer oversynoptic timescales. The suggestion here is not that the upwelling-relaxation transport model (e.g.,Roughgarden et al. 1988, 1991, Wing et al. 1995a,b) plays no role in settlement and recruitmentin central Chile, but rather that at Las Cruces larval transport toward the shore does not seem tobe dominated by these mechanisms. Indeed, spatially intensive studies over a region of about120 km around Las Cruces showed a clear mesoscale spatial pattern in barnacle settlement, apparentlyimposed by the topographic variability in upwelling intensity (Lagos et al. 2005). Thus, thespatial variability in upwelling intensity typical of central Chile (Strub et al. 1998, Broitman et al.2001, Halpin et al. 2004) might influence the spatial position of the larval pool and probably affectsthe scales of dispersal of invertebrates, as suggested also by the study of spatial synchrony inrecruitment of species with contrasting dispersal potential (Lagos et al. in review). Topographiceffects on patterns of settlement and recruitment have also been reported for a variety of brachyurancrab species (A.T. Palma et al. 2006). Buoyancy fronts produced by river plumes, common fromabout 30°S to the south in the HCS, in conjunction with wind stress can also play a role in deliveringlarvae to shore (Vargas et al. 2006c). Narváez et al. (2006) also report on the effect of what theycalled ‘large warming events’, which occurred a few times in spring–summer in association withdownwelling-favourable (northerly) winds. During these specific large warming events these authorsobserved significant synchrony in recruitment of several invertebrate taxa (decapods, gastropods,polychaetes, mussels and sea urchins), suggesting that larvae could be entrained in these advectivefronts and delivered onshore. A roughly similar phenomenon has been observed around Valdiviain southern Chile, where southward and onshore movement of warm waters produced by winter249
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