the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.the winter, these amphipods live higher up on the beach, mainly to avoid being washed out byhigher wave activity. During summer, the abundances of O. tuberculata decrease and they are foundcloser to the flotsam (Sánchez et al. 1982, McLachlan & Jaramillo 1995), possibly to avoiddesiccation risk in the supralittoral zone or to get close to their food. The cirolanid isopods(Excirolana spp.) have similar abundances in summer and winter, but they shift their position inthe intertidal zone, moving higher during the winter months (Sánchez et al. 1982). On the otherhand, Emerita analoga in the northern (22°S) and Mesodesma donacium in northern-central Chile(30°S) reach highest abundances in summer (Sánchez et al. 1982, Contreras et al. 2000). At present,the reasons for the apparent inverse population dynamics of species from the upper versus thosefrom the lower intertidal zone are speculative, and seasonally varying offspring (larval) survival orfood supply could be invoked. One very interesting species from the supralittoral zone, Tylosspinulosus, which has a restricted distribution in northern-central 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), and it appears a promising enterprise to examine its population dynamics.The vicinity to upwelling centres plays an important role in the succession and structure ofhard-bottom communities (Broitman et al. 2001, Narváez et al. 2006), but there is no clear indicationthat the macrofauna composition of sandy beaches is influenced by their proximity to upwellingareas (Jaramillo et al. 1998). Contreras et al. (2000) concluded that growth rates of Emerita analogafrom a beach near the upwelling centre of Mejillones (22°S) were within values reported for otherareas, suggesting only limited or no direct effects of upwelling on sandy beach inhabitants. Communitydynamics of sandy 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 of seaweeds on hard bottoms (Camus & Andrade 1999,Wieters 2005) and thereby the community succession (Nielsen & Navarrete 2004), which clearlyis of no importance on exposed sandy beaches where algae are usually imported from neighbouring(or distant) hard-bottom habitats. Furthermore, the interplay between upwelling and subsequentrelaxation events strongly affects the recruitment of hard-bottom organisms with planktonic larvae(Narváez et al. 2006), but seems to be of minor importance on sandy beaches, where the mostcommon organisms feature direct development.The effect of ENSO has been intensively studied in hard-bottom environments, but its role inthe dynamics of sandy beach communities is not well known (Arntz et al. 1987). EN events mayhave deleterious effects on the organisms from the lower intertidal zone of sandy beaches (e.g.,Mesodesma donacium; see also Artisanal benthic fisheries, p. 278ff.). On the other hand, it is knownthat EN provokes mass mortalities of seaweeds and animals, many of which eventually will strandon sandy beaches (Arntz 1986). This high supply of OM may represent an important food sourcefor scavenging animals of the supralittoral zone of sandy beaches (e.g., Orchestoidea tuberculata).In this way, it can be suggested that intertidal organisms are distinctly affected by EN: species fromthe lower shore (suspension feeders with planktonic larvae) may be negatively affected by EN,while those from the supralittoral zone (scavenging animals with direct development) might benefitfrom the higher food supply and more benign climate (lower desiccation risk) during EN.Subtidal soft-bottom communitiesZonation patternsRecent studies suggest that the bathymetric distribution of subtidal benthic communities off theChilean margin seems to be controlled mainly by bottom water oxygen conditions and sedimentorganic loading. OMZs are significant mid-water features in the eastern Pacific Ocean (Wyrtki1973, Kamykowski & Zentara 1990) that strongly influence the distribution and diversity of230
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEplanktonic and benthic marine communities. Where OMZs intercept the continental margin (bottomwaterdissolved oxygen < 0.5 ml L −1 ), strong gradients are formed in both bottom-water oxygenconcentration and OM input (Levin et al. 1991, Levin et al. 2000). These gradients influence thebiogeochemical properties of sediments (Cowie et al. 1999) and the distribution and diversity ofbacteria, meio-, macro- and megabenthic organisms (Sanders 1969, Mullins et al. 1985, Wishneret al. 1990, Tyson & Pearson 1991). Off Chile, oxygen-deficient waters are in general associatedwith the ESSW, which partially covers the continental shelf and upper bathyal area. The intensityand vertical extent of the OMZ suggest a latitudinal gradient, the effect disappearing at about 41°S(Brandhorst 1971). Off northern Chile, sediments affected by the OMZ extend from a few tens ofmetres below the surface to 300–400 m water depth. Between Huasco and Valparaíso (~28–32°S),the OMZ seems to intercept the sea floor deeper than 100 m (D. Lancellotti & W. Stotz unpublisheddata), and due to the narrowness of the shelf and steepness of the slope, there are zones that probablyare not severely affected. This is also corroborated by the presence of a fauna atypical for oxygendeficientareas (e.g., diverse species of gastropods) and the absence of bacterial mats (Lancellotti& Stotz 2004). The shelf widens southward and when upwelling prevails, during spring–summer,the OMZ again can be found only a few metres from the surface even in southern-central Chile(~36°S), within Concepción Bay (Ahumada et al. 1983), and extending down to 200–300 m (seealso Arntz et al. 2006). However, the OMZ intensity here is probably the result of many localfactors (e.g., the high PP that leads to high remineralisation rates in the water column and sea floor,consuming oxygen and generating sulphidic conditions within the sediment) (P. Muñoz et al.2004b). In this way, it is probably possible to visualise the OMZ impinged sea floor, from northernto central Chile, as a wedge-shaped band, getting narrower southward, but with two foci of mostintense oxygen-deficient conditions, one off northern Chile and the other at the shelf off Concepción.The continuity between these two foci may be interrupted by better-oxygenated sediments, atcomparable depths, off central Chile. Reports of low bivalve abundances between 80 and 120 mdepth in Valparaíso Bay (31°S) are suggestive of OMZ effects (Ramorino 1968), but additionaldata are required to resolve the intensity and extent of the OMZ and its effect on benthic communitiesbetween 25°S and 35°S.Considering the general effect of the OMZ on benthic communities, and based on the limitedamount of biological sampling available at that time, Gallardo (1963) proposed the existence ofbasically three main benthic zones for the local eukaryotic communities: (1) an upper sublittoralzone, up to 50 m depth, with favourable conditions for the development of ‘normal’ benthiccommunities, (2) a lower sublittoral zone, from 50 to 300–400 m (varying with latitude andcoinciding with the extent of the OMZ), in which only those organisms highly adapted to copewith oxygen deficiency and high organic loadings are able to thrive (basically small polychaetes,oligochaetes, nematodes and a few molluscs), and (3) a bathyal area, associated mainly with AntarcticIntermediate Waters, with a diverse and rich fauna (dominated by annelids, crustaceans, molluscsand echinoderms) that benefits from enhanced oxygen and good quality and quantity of sedimentOM. How this general pattern differs in southern areas (>41°S) where the OMZ dissipates is stillpoorly known.One of the most distinguishing features of benthic shelf communities within OMZ-impingedsediments is the presence of extensive mats of the filamentous, sulphide-oxidising bacteria Thioplocaand Beggiatoa (Gallardo 1963, 1977, Schulz et al. 2000, Arntz et al. 2006). These bacteriaare the most conspicuous component of the benthos also in the central and southern Peruvian shelf(Rosenberg et al. 1983). Bacterial biomasses of up to 1 kg m −2 wet wt have been reported fromshelf sediments off Iquique (~21°S) (Gallardo 1963) and off Concepción (~37°S) (Gallardo 1977)at depths between 50 and 100 m. On the other hand, within the OMZ eukaryotes are in generalsmall-size forms, like meiofauna, calcareous foraminiferans and nematodes (Gooday et al. 2000,Neira et al. 2001, Levin 2003). Very high densities, on the order of 10,000 individuals (ind.) 10 cm –2 ,231
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