the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.of meiobenthic organisms, mostly nematodes, have been recorded on the shelf off Concepción(Neira et al. 2001, Sellanes et al. 2003). OMZ macrofaunal assemblages thus have low diversityand are typically composed of organisms with morphological and metabolic adaptations and feedingstrategies suited to these conditions. A good example is the locally abundant polychaete Paraprionospiopinnata, an interface feeder organism, which is also able to switch to suspension feeding(Gutiérrez et al. 2000), with a complete set of enzymes for anaerobic metabolism (González &Quiñones 2002), and also bearing a set of highly branched and complex gills. Paraprionospiopinnata comprises about half of the ~14,000 macrofauna individuals m −2 reported from off Concepciónwithin the OMZ (Palma et al. 2005).Below the OMZ, the total abundance of macrobenthos decreases while biomass and diversityincrease in parallel with increases in oxygen and decreases in organic carbon (Levin 2003). In thebathyal area off Chile, important crustacean assemblages, mainly of commercially valuable galatheiddecapods (e.g., Pleuroncodes monodon and Cervimunida johni), thrive at the slope within andbelow the lower boundary of the OMZ. Below the OMZ, groups that are favoured by increasedoxygen concentrations are decapod crustaceans, gastropods and at greater depths echinoderms(mainly ophiuroids, asteroids and irregular echinoids). The large tubiculous onuphid polychaeteHyalinoecia sp. (tubes 10–20 cm in length) is particularly abundant at mid-slope depths offConcepción and Chiloé (J. Sellanes personal observations). Scleractinian corals and many speciesof gorgonarians are common on the slope off Chiloé, below 500 m water depth.Finally, though the three-layer scheme for benthic zonation is the general rule, another type ofchemosynthetic community, methane seep areas, has been reported from off Concepción at700–1400 m water depth (Stuardo & Valdovinos 1988, Sellanes et al. 2004, Sellanes & Krylova 2005).In these reducing systems, C is fixed locally by both free-living and symbiotic chemosynthetic bacteria.Seepage areas thus provide a suitable environment for the development of singular communitiesconsisting of sulphide oxidising bacteria (e.g., Beggiatoa), highly endemic endosymbiont-bearingclams (e.g., Vesicomyidae, Lucinidae, Thyasiridae and Solemyidae) and tubeworms (e.g., Lamellibrachiasp.). In addition, non-chemosymbiotic megafauna (e.g., crustaceans, gastropods, cephalopods,fish) are massively attracted to these deep-sea hot spots of biological activity. The attractionof these areas is due to both the abundance of locally produced organic material and the presenceof authigenic carbonate reefs (generated by microbial and chemical processes), which are avidlycolonised by a diverse benthic fauna (J. Sellanes unpublished data).Latitudinal and bathymetric patternsIn general, for the available data of macrofaunal species richness on the Chilean shelf, no clearlatitudinal pattern emerges. Published reports of species numbers range from 18 to 56 species atnorthern Chile (~23°S, 15–65 m water depth) (Jaramillo et al. 1998), 42 to 85 species at Huasco(~28°S, 20–50 m water depth) (Lancellotti & Stotz 2004), 10 to 41 species at Concepción (~36°S,15–65 m water depth) (Carrasco et al. 1988), and 24 to 34 species further south (~45°S)(D. Lancellotti & W. Stotz unpublished data). Highest abundances reported are for the shelf offConcepción (36,290–38,590 ind. m −2 ; Carrasco et al. 1988) and biomass values of almost 900 gwet wt m −2 are reported for south of Chiloé (D. Lancellotti & W. Stotz unpublished data).A crude comparison among the few available deeper studies (>100 m water depth) suggestshigher macrofaunal standing stocks and abundances also at the shelf off central and southern areas,both for macro- (body size 0.3–2 cm) and megafauna (body size >2 cm), compared with northernChile (Figure 11). Palma et al. (2005) reported macrofaunal biomass values for three transectscovering a depth range from about 100 m to 2000 m at Antofagasta, Concepción and Chiloé (about22°S, 36°S and 42°S, respectively). Though biomass trends with depth, beyond the OMZ, are ingeneral unimodal at all transects, with higher values at intermediate depths and lowest within the232
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEMegafaunal abundanceMegafaunal abundance (Log 10 ind 1000 m −2 ), Macrofaunal abundance (Log 10 ind m −2 )100000100001000100101100001000010001001011000010000100010010Antofagasta (∼22°S)98Concepción (∼36°S)Chiloé (∼42°S)Macrofaunal abundanceMacrofaunal biomassMegafaunal species numberMacrofaunal species number142 300 520 690 1350 1800120 365 535 800 1290 205070605040302010070605040302010070605040302010Macrofaunal biomass (g wet weight m −2 ), Megafaunal and macrofaunal species number1160 300 480 900Depth (m)1250 19600Figure 11 Macro- and megafauna depth-related patterns for three transects (22°S, 36°S and 42°S) across theshelf and upper bathyal zone of the Chilean margin. Data for macrofaunal abundance, biomass and speciesnumber from Palma et al. (2005) and for megafaunal abundance and species number from E. Quirogaunpublished data).OMZ and beyond 1350 m, average biomasses are lower off Antofagasta. Maximum values for thistransect (6.9 g wet wt m −2 ) are reported at 518 m water depth, while values about an order ofmagnitude higher (60.7 g wet wt m −2 ) are reported off Concepción at 784 m depth. For southernChile (~42°S) intermediate values (39.2 g wet wt m −2 ) are reported for a station located at 1250 mdepth. This also indicates a deepening of macrofaunal biomass maxima with latitude (Figure 11).For the megafauna observed at the same three transects, though biomass values are not available,abundances in general exhibited a similar pattern to that previously explained for macrofaunal233
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