the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.in these hydrodynamically variable but highly productive environments, coastal and offshoreorganisms have developed a series of reproductive strategies that enable them to counteract theeffects of offshore advection in the surface Ekman layer or take advantage of other oceanographicprocesses to return to the coastal zone (see also Coastal oceanography, p. 205ff.). For instance, on aseasonal timescale, small pelagic clupeiform fish tend to synchronise their reproductive timing withprocesses that induce shoreward transport and coastal retention. Probably the best-documented speciesin the HCS of central Chile are the anchovy (Engraulis ringens) and common sardine (Strangomerabentincki) that reproduce during winter, when the wind-driven Ekman layer is directed shoreward andthus eggs and larvae are retained in the coastal area (Castro et al. 2000, Castro 2001, Cubillos et al.2001, Hernández-Miranda et al. 2003). Shelf-break, slope-demersal and mid-water fish species, suchas Chilean hake (Merluccius gayi), big eye flounder (Hippoglossina macrops), and the mesopelagicMaurolicus parvipinnis, instead seem to prefer early spring reproduction when subsurface watersdrive their eggs and larvae to the coast during the upwelling season, where they develop in the seasonof higher production (Vargas et al. 1997, Vargas & Castro 2001, Landaeta & Castro 2002, Landaetaet al. 2006). This strategy, originally described for some large offshore copepods such as Rhincalanusnasutus, and more recently observed also in the galatheid Pleuroncodes monodon and the majidLibidoclaea granaria, is currently accepted as a common feature among several types of organismsthat have in common larvae inhabiting subsurface waters in central Chile (Castro et al. 1993, Yannicelliet al. 2006a,b). Other species of decapod crustaceans also synchronise their reproduction with theseasonal changes in hydrodynamics for their transport or retention (Yannicelli et al. 2006b). In thisgroup, however, several species reproduce during the upwelling season in spring and summer andtheir horizontal distribution seems to be associated with their behavioural ability for verticalmigration (i.e., Neotrypaea uncinata, Pagurus sp.). Other species with protracted larval periodssuch as Emerita analoga and Blepharipoda spinimana, which reproduce late in summer and earlyspring and then reside in the upper water column without signs of vertical migration, probably usemore than a single retention process, as yet unknown, because their reproduction occurs over periodsof contrasting hydrographic processes (Yannicelli et al. 2006b). Among molluscs, there is scarceinformation; for instance, the larval retention and shoreward transport mechanism of the gastropodConcholepas concholepas are the only information reported for the HCS. In this case, avoidanceof the surface Ekman layer by competent larvae appears to be accomplished by an inverse verticalmigration that reduces their time exposed to seaward flow and keeps the larvae between the coastand an upwelling front (Poulin et al. 2002a,b). Coastal oceanographic processes such as upwellingshadows have also been reported (Escribano et al. 2002) as larvae retention mechanisms and suchmight constitute an understudied coastal larval retention system along the HCS.Tidal transport of larvae, associated with frontal structures near the coast, the entrance ofestuaries or large bays, has also been reported recently for central and southern Chile. Internal tidalbores associated with semi-diurnal temperature changes coincided with bivalve, gastropod andcrustacean settlement, suggesting that coastward larval transport occurred during these events insummer (Vargas et al. 2004). In Corral Bay (~40°S), changes in larval fish distributions coincidentwith changes in the estuarine front position in different tidal phases have been reported by Vargaset al. (2003) as an indication of potential larval tidal transport. More recently, with the aid of fineresolutioncurrent profiles and stratified larval collections over 24-h cycles, semi-diurnal changesin water flow patterns and of decapod and larval fish fluxes in and out of the Gulf of Arauco havebeen estimated (Yannicelli et al. 2006a, R. Veas unpublished data). The overall larval fluxes weremodified by their vertical position in the water column, the diurnal vertical migration patterns andthe tidal cycle. Interestingly, although clearly associated with tidal phases, the larval fluxes experiencedby these decapod and fish larvae in the Gulf of Arauco did not correspond exactly toselective tidal stream transport (STST) as it has been reported at the entrance of other bays andestuaries in other coasts of the world (Forward et al. 1998).272
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILESeaward and alongshore exportation of larvae in river plumes, filaments and eddies seems tobe a common feature along the HCS. In northern Chile, a mixture of coastal and offshore larvalfishes, both in the coastal and adjacent offshore areas, has been repeatedly reported in differentseasons and years (Loeb & Rojas 1988, Rojas et al. 2002, Rodríguez-Graña et al. 2005). Besidesthe existence of a narrow continental shelf, at least three oceanographic processes involving larvaltransport have been advocated to explain such patterns: (1) seaward surface Ekman transport andupwelling plumes, (2) the presence of filaments, and (3) warm-water intrusion during EN years,all well-documented processes in northern Chile (González et al. 2000b, Sobarzo & Figueroa 2001).In central Chile, surface Ekman transport and cold-water filaments have also been reported to exportchl-a, meroplankton and ichthyoplankton from the coastal zone (Cáceres 1992, Morales et al. inreview) and they have been used to explain part of the larval mortality rate estimations in the coastalzone (Castro & Hernández 2000, Landaeta & Castro 2006). Larval transport associated with riverplumes, mesoscale processes not occurring in northern Chile, has also been proposed as a determinantof barnacle larval transport (Vargas et al. 2006c). Such plumes are also potential areas ofincreased larval food availability at the frontal area. Other still-unknown oceanographic processescapable of transporting larval forms alongshore probably occur in the HCS. In fact, it is worthnoting that almost no reports exist on the role of the Humboldt Current itself or the Chile-Perupoleward undercurrent as a means of alongshore latitudinal larval transport (except for potentialtransport of larval Pleuroncodes monodon in subsurface waters off central Chile; Yannicelli 2005).In summary, a number of larval transport processes have been described along the central andsouthern part of HCS. Three of them seem particularly important when considered from anecological perspective: (1) larval transport (seaward and shoreward) in the surface Ekman layer,(2) exportation from the coast in filaments (especially off the Mejillones Peninsula in northernChile and off the Talcahuano area of central Chile) and (3) the coastward subsurface larval transportin upwelling waters in spring and summer with the concomitant mixing of offshore and coastalspecies near shore. Examples of important differences between northern and central Chile are theinfluence of oceanographic processes such as EN events (much stronger in northern Chile) and ofriver plumes (absent in northern Chile). Overall, populations of invertebrates and fishes along theHCS develop multiple strategies to cope with the intense periods of transport during early lifestages. Timing the reproductive seasons with specific oceanographic events is most common.However, specific reproductive timing depends on the local sites of reproduction, capability oflarvae to move vertically in the water column, length of larval life history/cycle and other not sowell studied processes (i.e., retention in upwelling shadows) or those occurring during the adultlife stage (seasonal growth, energy accumulation, oogenesis) that may finally affect the larvalcharacteristics mentioned above.Life-history adaptations of macroalgaeThe macroalgal flora along the HCS is characterised by the presence of endemic species (32%) mixedwith species of different biogeographical affinities, including subantarctic (34%), widely distributed(23%), tropical (3.5%) and amphi-equatorial (7%) species (Santelices 1980). In general, this speciescomposition, which is comparatively poorer than that reported for other regions and increases innumber toward higher latitudes (Lüning 1990, Santelices & Marquet 1998), responds to the relativebiogeographic isolation as a consequence of the predominant direction of the water circulation regimein the HCS (Santelices et al. 1980, Meneses & Santelices 2000). However, an increasing number ofinvasive species as a result of human activities (ship transport, aquaculture, etc.) has recently beenreported with concern about their impact on indigenous species (Castilla et al. 2005a). For example,recent surveys indicate an increase of subtropical elements, a decrease in endemic species and twobreak points in species composition at 12°S and 42°S (Meneses & Santelices 2000).273
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the humboldt current system of northern and central chile - figema

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