the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.biomass (Palma et al. 2005). On average, pooling the data for the three transects, abundance (~500ind. m −2 ) and species number (~25) peaks were located between 1000 and 1500 m depth.Temporal patterns of variability in shelf communitiesAs explained in previous sections of this review, the coastal zone off northern and central Chile isstrongly influenced by seasonal wind-driven upwelling, giving rise to one of the areas with thehighest PP rates known worldwide (Fossing et al. 1995, Daneri et al. 2000). Since water columnoxygen conditions and sediment organic loadings fluctuate at both intra- (i.e., seasonal) andinterannual scales (i.e., ENSO cycle), some degree of coupling with the dynamics of benthiccommunities is expected (Tomicic 1985, Arntz & Fahrbach 1991). This has been demonstrated forsouthern-central Chile (~36°S), where seasonal and interannual changes in upwelling intensity canlead to changes in bottom-water dissolved oxygen concentration, in the amount of OM reachingthe bottom (Gutiérrez et al. 2000), in the quality and lability of deposited OM (Neira et al. 2001,Sellanes & Neira 2006) and in the sediment nitrogen fluxes (P. Muñoz et al. 2004b). During thelast strong EN event (1997–1998), important insights were gained by examining the effects ofchanging environmental conditions on local bacterial, meiofaunal (Neira et al. 2001, Sellanes et al.2003) and macrofaunal (Gutiérrez et al. 2000) communities off central Chile. The largest biomassesof the bacterial component have been observed after several years of upwelling-favourable conditions,which are associated with cold LN phases of the ENSO (V.A. Gallardo et al. unpublisheddata), while the bacterial biomass is effectively depressed during warm EN phases. A decreasingtrend in macrofaunal density, as well as the presence of deeper-burrowing infauna, evolved towardthe end of EN 1997–1998, mainly due to the decrease of the polychaete Paraprionospio pinnata(Gutiérrez et al. 2000). It appears that more oxygenated bottom waters and oxidised sediment duringEN caused P. pinnata to fail in its summer recruitment. In addition, it is probable that increasedcompetition and predation by other species have contributed to its decline. Indeed, it has beenreported that during EN, many subtropical predators invade the coastal areas (Arntz et al. 1991),negatively affecting the surface-feeding polychaetes (Tarazona et al. 1996). In central Chile,P. pinnata recovered its numerical dominance only in summer 2003, i.e., 5 yr after the end of EN.Severe hypoxic and sulphidic conditions that developed during summer 2003 probably eliminatedor precluded possible competitors and/or predators, triggering the explosive increase of theP. pinnata population during this period (Sellanes et al. 2007).In northern Chile (20–30°S) few datasets extending over at least 12 months are available fromshallow benthic communities (20–80 m). For the time period 1990–1995, relatively high abundancesof polychaetes have been reported from several stations in northern Chile (23°50′S) at water depthsof 50–60 m (Carrasco 1997). This author remarked on the absence of a clear seasonal signal inabundance changes of the main polychaete species, and he suggested that the observed variationsrather reflected long-term patterns. At a long-term monitoring station near 28°S, abundances ofpolychaetes were high in 1995 (Figure 12), comparable to those found by Carrasco (1997). Highabundance, biomass and species diversity at 28°S were associated with the warm period 1993–1995and followed by a strong decline in 1996, coincident with LN conditions, which continued duringthe EN 1997–1998 (Lancellotti 2002, D. Lancellotti & W. Stotz unpublished data). Gradual disappearanceof spionids, as observed in Huasco in 1996 during LN (Figure 12), was also observedduring the same time period in Iquique (20°S) between 9 and 30 m depth (Quiroga et al. 1999).In northern-central Chile, during EN events, increased wave activity and freshwater runoff arefrequent, in contrast to calmer periods recorded during LN events. Turbulence and runoff, in a zonewhere rains and strong storms are uncommon, probably oxygenate the water column, resuspendOM and/or provide terrigenous material, thus favouring reproduction and settlement of macrobenthicspecies living below the zone of direct wave and sediment deposition impact (>20 m234
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILE60005000400030002000Upper sublittoral zone28°S6050403020100Abundance (ind m −2 )100006000500040003000200010000Lower sublittoral zone (OMZ)36°S6050403020100Species richness93 94 95 96 97 98 99 2000 01 02 03 04 05YearsFigure 12 Temporal variations in the abundance of soft-bottom macrofauna in the upper sublittoral zone(northern Chile, 28°S) and the lower sublittoral zone (central Chile, 36°S); average abundance and speciesrichness (S) are given; data from the upper littoral zone are taken by Smith-MacIntyre grab (1995–1996) orwith sediment cores by divers (1997–2005) (D.A. Lancellotti & W. Stotz unpublished data); data from thelower sublittoral zone are taken with a multicorer, and several samples from each year were pooled, thus notallowing intra-annual variation to be seen (J. Sellanes unpublished data).water depths). Effects of EN (and other) events on the temporal variability of benthic soft-bottomcommunities at present are difficult to evaluate because very few long-term datasets from benthichabitats are available from the HCS along the Chilean coast. It is herein suggested that long-termmonitoring programmes should be implemented, sampling on a seasonal or bimontly basis, followingexamples in Peru (Tarazona et al. 2003, Arntz et al. 2006, Peña et al. 2006) and the NorthernHemisphere (Frid et al. 1996, Kroencke et al. 1998, Salen-Picard et al. 2002).Intertidal and subtidal hard-bottom communitiesHard bottoms along the coast of northern-central Chile are generally restricted to a narrow fringeextending from the intertidal zone to shallow sublittoral waters. The rock substratum is composedof rock of volcanic, granitic or sedimentary origin (Fariña et al. in press). The extensive rockyshores between 18°S and 40°S are mostly exposed to strong wave action, and they are onlyinterrupted by short stretches of sandy beaches, which increase in extent toward the south (seeSandy beaches, p. 227, Figure 9), thereby leading to a wider separation of neighbouring hardbottomenvironments. Communities on intertidal and subtidal hard bottoms are dominated bymacroalgae and suspension-feeding animals that form large patches (occasionally extending overto neighbouring soft bottoms) or belt-like stretches (running parallel to the shore at a certain tidallevel). Most intertidal and subtidal hard bottoms are covered by one or a few dominating habitatformingorganisms. Patches may persist for many years at a given location (Durán & Castilla 1989,Fernández et al. 2000, Vega et al. 2005), and they offer abundant microhabitat and food for associatedorganisms (Moreno & Jara 1984, Vásquez et al. 1984, Núñez & Vásquez 1987, Buschmann1990, Vásquez 1993a, López & Stotz 1997, Sepúlveda et al. 2003a,b). Here these habitat-forming235
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