MARTIN THIEL ET AL.<strong>the</strong> winter, <strong>the</strong>se amphipods live higher up on <strong>the</strong> beach, mainly to avoid being washed out byhigher wave activity. During summer, <strong>the</strong> abundances <strong>of</strong> O. tuberculata decrease <strong>and</strong> <strong>the</strong>y are foundcloser to <strong>the</strong> flotsam (Sánchez et al. 1982, McLachlan & Jaramillo 1995), possibly to avoiddesiccation risk in <strong>the</strong> supralittoral zone or to get close to <strong>the</strong>ir food. The cirolanid isopods(Excirolana spp.) have similar abundances in summer <strong>and</strong> winter, but <strong>the</strong>y shift <strong>the</strong>ir position in<strong>the</strong> intertidal zone, moving higher during <strong>the</strong> winter months (Sánchez et al. 1982). On <strong>the</strong> o<strong>the</strong>rh<strong>and</strong>, Emerita analoga in <strong>the</strong> nor<strong>the</strong>rn (22°S) <strong>and</strong> Mesodesma donacium in nor<strong>the</strong>rn-<strong>central</strong> Chile(30°S) reach highest abundances in summer (Sánchez et al. 1982, Contreras et al. 2000). At present,<strong>the</strong> reasons for <strong>the</strong> apparent inverse population dynamics <strong>of</strong> species from <strong>the</strong> upper versus thosefrom <strong>the</strong> lower intertidal zone are speculative, <strong>and</strong> seasonally varying <strong>of</strong>fspring (larval) survival orfood supply could be invoked. One very interesting species from <strong>the</strong> supralittoral zone, Tylosspinulosus, which has a restricted distribution in nor<strong>the</strong>rn-<strong>central</strong> Chile (17°–30°S) (Schmalfuss &Vergara 2000), is little known aside from a few data on its population density (Sánchez et al. 1982,Jaramillo et al. 2003), <strong>and</strong> it appears a promising enterprise to examine its population dynamics.The vicinity to upwelling centres plays an important role in <strong>the</strong> succession <strong>and</strong> structure <strong>of</strong>hard-bottom communities (Broitman et al. 2001, Narváez et al. 2006), but <strong>the</strong>re is no clear indicationthat <strong>the</strong> macr<strong>of</strong>auna composition <strong>of</strong> s<strong>and</strong>y beaches is influenced by <strong>the</strong>ir proximity to upwellingareas (Jaramillo et al. 1998). Contreras et al. (2000) concluded that growth rates <strong>of</strong> Emerita analogafrom a beach near <strong>the</strong> upwelling centre <strong>of</strong> Mejillones (22°S) were within values reported for o<strong>the</strong>rareas, suggesting only limited or no direct effects <strong>of</strong> upwelling on s<strong>and</strong>y beach inhabitants. Communitydynamics <strong>of</strong> s<strong>and</strong>y beaches may be influenced by upwelling to a lesser degree than thosepertaining to hard-bottom communities. For example, higher nutrient availability near upwellingareas positively influences growth rates <strong>of</strong> seaweeds on hard bottoms (Camus & Andrade 1999,Wieters 2005) <strong>and</strong> <strong>the</strong>reby <strong>the</strong> community succession (Nielsen & Navarrete 2004), which clearlyis <strong>of</strong> no importance on exposed s<strong>and</strong>y beaches where algae are usually imported from neighbouring(or distant) hard-bottom habitats. Fur<strong>the</strong>rmore, <strong>the</strong> interplay between upwelling <strong>and</strong> subsequentrelaxation events strongly affects <strong>the</strong> recruitment <strong>of</strong> hard-bottom organisms with planktonic larvae(Narváez et al. 2006), but seems to be <strong>of</strong> minor importance on s<strong>and</strong>y beaches, where <strong>the</strong> mostcommon organisms feature direct development.The effect <strong>of</strong> ENSO has been intensively studied in hard-bottom environments, but its role in<strong>the</strong> dynamics <strong>of</strong> s<strong>and</strong>y beach communities is not well known (Arntz et al. 1987). EN events mayhave deleterious effects on <strong>the</strong> organisms from <strong>the</strong> lower intertidal zone <strong>of</strong> s<strong>and</strong>y beaches (e.g.,Mesodesma donacium; see also Artisanal benthic fisheries, p. 278ff.). On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, it is knownthat EN provokes mass mortalities <strong>of</strong> seaweeds <strong>and</strong> animals, many <strong>of</strong> which eventually will str<strong>and</strong>on s<strong>and</strong>y beaches (Arntz 1986). This high supply <strong>of</strong> OM may represent an important food sourcefor scavenging animals <strong>of</strong> <strong>the</strong> supralittoral zone <strong>of</strong> s<strong>and</strong>y beaches (e.g., Orchestoidea tuberculata).In this way, it can be suggested that intertidal organisms are distinctly affected by EN: species from<strong>the</strong> lower shore (suspension feeders with planktonic larvae) may be negatively affected by EN,while those from <strong>the</strong> supralittoral zone (scavenging animals with direct development) might benefitfrom <strong>the</strong> higher food supply <strong>and</strong> more benign climate (lower desiccation risk) during EN.Subtidal s<strong>of</strong>t-bottom communitiesZonation patternsRecent studies suggest that <strong>the</strong> bathymetric distribution <strong>of</strong> subtidal benthic communities <strong>of</strong>f <strong>the</strong>Chilean margin seems to be controlled mainly by bottom water oxygen conditions <strong>and</strong> sedimentorganic loading. OMZs are significant mid-water features in <strong>the</strong> eastern Pacific Ocean (Wyrtki1973, Kamykowski & Zentara 1990) that strongly influence <strong>the</strong> distribution <strong>and</strong> diversity <strong>of</strong>230
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEplanktonic <strong>and</strong> benthic marine communities. Where OMZs intercept <strong>the</strong> continental margin (bottomwaterdissolved oxygen < 0.5 ml L −1 ), strong gradients are formed in both bottom-water oxygenconcentration <strong>and</strong> OM input (Levin et al. 1991, Levin et al. 2000). These gradients influence <strong>the</strong>biogeochemical properties <strong>of</strong> sediments (Cowie et al. 1999) <strong>and</strong> <strong>the</strong> distribution <strong>and</strong> diversity <strong>of</strong>bacteria, meio-, macro- <strong>and</strong> megabenthic organisms (S<strong>and</strong>ers 1969, Mullins et al. 1985, Wishneret al. 1990, Tyson & Pearson 1991). Off Chile, oxygen-deficient waters are in general associatedwith <strong>the</strong> ESSW, which partially covers <strong>the</strong> continental shelf <strong>and</strong> upper bathyal area. The intensity<strong>and</strong> vertical extent <strong>of</strong> <strong>the</strong> OMZ suggest a latitudinal gradient, <strong>the</strong> effect disappearing at about 41°S(Br<strong>and</strong>horst 1971). Off nor<strong>the</strong>rn Chile, sediments affected by <strong>the</strong> OMZ extend from a few tens <strong>of</strong>metres below <strong>the</strong> surface to 300–400 m water depth. Between Huasco <strong>and</strong> Valparaíso (~28–32°S),<strong>the</strong> OMZ seems to intercept <strong>the</strong> sea floor deeper than 100 m (D. Lancellotti & W. Stotz unpublisheddata), <strong>and</strong> due to <strong>the</strong> narrowness <strong>of</strong> <strong>the</strong> shelf <strong>and</strong> steepness <strong>of</strong> <strong>the</strong> slope, <strong>the</strong>re are zones that probablyare not severely affected. This is also corroborated by <strong>the</strong> presence <strong>of</strong> a fauna atypical for oxygendeficientareas (e.g., diverse species <strong>of</strong> gastropods) <strong>and</strong> <strong>the</strong> absence <strong>of</strong> bacterial mats (Lancellotti& Stotz 2004). The shelf widens southward <strong>and</strong> when upwelling prevails, during spring–summer,<strong>the</strong> OMZ again can be found only a few metres from <strong>the</strong> surface even in sou<strong>the</strong>rn-<strong>central</strong> Chile(~36°S), within Concepción Bay (Ahumada et al. 1983), <strong>and</strong> extending down to 200–300 m (seealso Arntz et al. 2006). However, <strong>the</strong> OMZ intensity here is probably <strong>the</strong> result <strong>of</strong> many localfactors (e.g., <strong>the</strong> high PP that leads to high remineralisation rates in <strong>the</strong> water column <strong>and</strong> sea floor,consuming oxygen <strong>and</strong> generating sulphidic conditions within <strong>the</strong> sediment) (P. Muñoz et al.2004b). In this way, it is probably possible to visualise <strong>the</strong> OMZ impinged sea floor, from nor<strong>the</strong>rnto <strong>central</strong> Chile, as a wedge-shaped b<strong>and</strong>, getting narrower southward, but with two foci <strong>of</strong> mostintense oxygen-deficient conditions, one <strong>of</strong>f nor<strong>the</strong>rn Chile <strong>and</strong> <strong>the</strong> o<strong>the</strong>r at <strong>the</strong> shelf <strong>of</strong>f Concepción.The continuity between <strong>the</strong>se two foci may be interrupted by better-oxygenated sediments, atcomparable depths, <strong>of</strong>f <strong>central</strong> Chile. Reports <strong>of</strong> low bivalve abundances between 80 <strong>and</strong> 120 mdepth in Valparaíso Bay (31°S) are suggestive <strong>of</strong> OMZ effects (Ramorino 1968), but additionaldata are required to resolve <strong>the</strong> intensity <strong>and</strong> extent <strong>of</strong> <strong>the</strong> OMZ <strong>and</strong> its effect on benthic communitiesbetween 25°S <strong>and</strong> 35°S.Considering <strong>the</strong> general effect <strong>of</strong> <strong>the</strong> OMZ on benthic communities, <strong>and</strong> based on <strong>the</strong> limitedamount <strong>of</strong> biological sampling available at that time, Gallardo (1963) proposed <strong>the</strong> existence <strong>of</strong>basically three main benthic zones for <strong>the</strong> local eukaryotic communities: (1) an upper sublittoralzone, up to 50 m depth, with favourable conditions for <strong>the</strong> development <strong>of</strong> ‘normal’ benthiccommunities, (2) a lower sublittoral zone, from 50 to 300–400 m (varying with latitude <strong>and</strong>coinciding with <strong>the</strong> extent <strong>of</strong> <strong>the</strong> OMZ), in which only those organisms highly adapted to copewith oxygen deficiency <strong>and</strong> high organic loadings are able to thrive (basically small polychaetes,oligochaetes, nematodes <strong>and</strong> a few molluscs), <strong>and</strong> (3) a bathyal area, associated mainly with AntarcticIntermediate Waters, with a diverse <strong>and</strong> rich fauna (dominated by annelids, crustaceans, molluscs<strong>and</strong> echinoderms) that benefits from enhanced oxygen <strong>and</strong> good quality <strong>and</strong> quantity <strong>of</strong> sedimentOM. How this general pattern differs in sou<strong>the</strong>rn areas (>41°S) where <strong>the</strong> OMZ dissipates is stillpoorly known.One <strong>of</strong> <strong>the</strong> most distinguishing features <strong>of</strong> benthic shelf communities within OMZ-impingedsediments is <strong>the</strong> presence <strong>of</strong> extensive mats <strong>of</strong> <strong>the</strong> filamentous, sulphide-oxidising bacteria Thioploca<strong>and</strong> Beggiatoa (Gallardo 1963, 1977, Schulz et al. 2000, Arntz et al. 2006). These bacteriaare <strong>the</strong> most conspicuous component <strong>of</strong> <strong>the</strong> benthos also in <strong>the</strong> <strong>central</strong> <strong>and</strong> sou<strong>the</strong>rn Peruvian shelf(Rosenberg et al. 1983). Bacterial biomasses <strong>of</strong> up to 1 kg m −2 wet wt have been reported fromshelf sediments <strong>of</strong>f Iquique (~21°S) (Gallardo 1963) <strong>and</strong> <strong>of</strong>f Concepción (~37°S) (Gallardo 1977)at depths between 50 <strong>and</strong> 100 m. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, within <strong>the</strong> OMZ eukaryotes are in generalsmall-size forms, like mei<strong>of</strong>auna, calcareous foraminiferans <strong>and</strong> nematodes (Gooday et al. 2000,Neira et al. 2001, Levin 2003). Very high densities, on <strong>the</strong> order <strong>of</strong> 10,000 individuals (ind.) 10 cm –2 ,231