the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.AutotrophsHeterotrophsHighchlorophyllSpatial/temporal heterogeneityPicoautotrophsAutotrophicnanoflagellatesmid-sizepreyDiatomssmall preyDiatomsAutotrophicnanoflagellateslarge preyBacteriaHeterotrophicnanoflagellatesMicroprotozoaMicroprotozoaHeterotrophicnanoflagellatesFood sizeCopepodstagesFood size+−+Physical processesAffecting both food and larvaeTurbulenceInternal waves, tidesRiver plume motionFrontal structuresFilamentsUpwelling/downwellingLowchlorophyllPicoautotrophsAutotrophsBacteriaHeterotrophs−Figure 20 Conceptual scheme of main pathways of interaction in coastal food webs involving invertebrate(i.e., veliger competent larvae and barnacle nauplii) and fish larvae under spatial/temporal variation in chlorophylllevels in the Humboldt Current System. The thickness of the arrows represents main predator-preyinteractions. The sizes of the boxes or circles represent the dominance in terms of biomass of a specific fooditem (both autotrophic and heterotrophic prey) during each condition. Physical processes discussed in thischapter, which affect both larvae and food distribution, are also included. Arrows directed to food items attop or bottom boxes were included for convenience.nature, in the coastal upwelling area off central Chile (36–37°S), the changes in the phytoplanktonprotozoan and microplankton community that constitute the food supply for larval fish also occuron a seasonal basis (Vargas et al. 2006b). Accordingly, larvae produced in the middle of winter inthe HCS, when PP and seawater temperatures are low and wind-induced turbulence in the upperpart of the water column is high, are probably not going to face the same prey as those producedin summer, when upwelling and PP are at maximum (Figure 20). Most of the studies of larval fishfeeding have focused on changes in prey field, diet overlap, and their influence on larval feedingover short timescales, between adjacent areas, or have attempted to investigate whether evidenceof starvation occurred in wild-collected larvae (Llanos et al. 1996, Balbontín et al. 1997, Pizarroet al. 1998, Llanos-Rivera et al. 2004). For the anchovy, Engraulis ringens, in northern Chile(20–21°S), scarce incidence of starvation was even observed during autumn, a season of reduced270
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEplankton production (Pizarro et al. 1998). In northern Chile, the diet of the myctophids Diogenichthyslaternatus and Triphoturus oculeus, which feed diurnally and share the upper water column (0–200m depth), was shown to vary according to prey availability in the field. The diets of both speciesoverlap in periods and areas where food is more abundant (e.g., copepods, copepodids, nauplii,invertebrate eggs and ostracods), but differ under conditions of low prey availability (Rodríguez-Graña et al. 2005). Other studies in central Chile have shown that either microplankton concentrationsdid not appear limiting in winter (Castro et al. 2000, Hernández & Castro 2000, Castro 2001)or larval diet overlap occurred among several species but during periods of high food abundance(Llanos et al. 1996, Balbontín et al. 1997, Llanos-Rivera et al. 2004). Hence, the few studies carriedout on feeding of larval fish along the HCS in Chile suggest that, although the larval food abundancemay vary among seasons and localities, starvation due to limiting food availability alone does notseem to be such a common feature, even in seasons of lower production (autumn and winter). Forstarvation to occur, other factors may be necessary, such as increased turbulence, which may playa concomitant role during the low productivity seasons, at least in southern Chile (Cury & Roy1989, Castro et al. 2002). Scarce dietary overlap and local morphological and physiological adaptations(i.e., increased reserves in fish eggs and larger yolk size in fish larvae; Llanos-Rivera &Castro 2004, 2006) seem to exist in larval periods when ontogenetically food is highly necessary(i.e., onset of feeding) or when food becomes less abundant in the environment (Balbontín et al.1997, Llanos-Rivera et al. 2004, Rodríguez-Graña et al. 2005).For benthic invertebrates, sharp spatial transition in phytoplankton biomass associated withupwelling dynamics has been assumed to have important effects on larval condition (Wieters et al.2003). This assumption is based on the idea that phytoplankton is the most important food itemfor PL. A major impediment to understanding how food quantity and quality influence larval lifeunder natural conditions is that virtually all published information comes from laboratory rearingstudies in which larvae are offered a monospecific or controlled-mix algal culture. One of the firststudies done in the HCS analysing feeding preferences in larval invertebrates, by Vargas et al.(2006a), found evidence that barnacle nauplii (Jehlius cirratus and Notobalanus flosculus) andveligers (Concholepas concholepas) exhibited high consumption of heterotrophs (i.e., ciliates anddinoflagellates), but the size spectrum of food particles removed by barnacle nauplii was in contrastwith those for C. concholepas veligers. Barnacle nauplii preyed heavily on small picophytoplankton(20 µm). Theability of barnacle nauplii to feed on small prey hinges on the small spaces between their limbsetules (Stone 1989). This important finding indicates that omnivorous larval feeding should be thenorm in the pelagic ecosystem and might explain why larvae maintain positive growth in environmentswhere phytoplankton is thought to be limiting (Crisp et al. 1985). Therefore, the scarcepublished information for the HCS suggests that the inference of patterns of larval condition andrecruitment over large scales from chl-a biomass, now easily measured from satellite images (e.g.,Thomas et al. 2001b), has to be regarded with caution. The scenario suggests that a large spectrumof food particles is available for larval feeding, and species may adapt their feeding preferences inrelation to temporal/spatial food distribution along the HCS as well as their physiology and energeticreserves to counteract the spatial and temporal variations in food quality and quantity (Vargas et al.2006b) (Figure 20).Upwelling and larval transport processesThe wind-driven seasonal upwelling, besides its paramount effect on PP in the coastal zone, inducesprofound changes in the dynamics of coastal waters that directly affect the distribution and abundanceof organisms in the nearshore areas as well as over the continental shelf and slope. To reside271
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