Benthic megafaunal and demersal fish assemblages on the Chilean continental margin: The influence of the oxygen minimum zone on bathymetric distribution

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Abstract

Benthic megafaunal and demersal fish assemblages were sampled in three areas off Chile during the German–Chilean Expedition PUCK (SO-156) onboard the R/V Sonne from March to May 2001, at depths ranging from 120 to 2201 m. These samples, taken with an Agassiz trawl, are among the deepest ever taken in Chilean waters. A total of 147 species were recorded, mainly decapod crustaceans (Galatheidae, Pandalidae, Crangonidae), gastropods (Trochidae, Muricidae, Volutidae), ophiuroids (Asteronychidae, Gorgonocephalidae, Ophiolepididae, Ophiurinae), asteroids (Pterasteridae, Solasteridae, Goniopectinidae), polychaetes (Onuphidae, Aphroditidae, Maldanidae), and demersal fish (Macrouridae, Ipnopidae, Squalidae). Species richness and rarefaction analyses suggest that the fauna was undersampled. From the 147 species identified in this study, 36 species (24.5%) occurred only once and another 24 species occurred only twice (16.3%). Depth and dissolved oxygen levels were found to be the main factors influencing megafaunal changes along the continental shelf and in bathyal areas, as indicated by principal component and Pearson's correlation analyses. Some species appear to be limited to distinct areas in the upper and lower bathyal zones, whereas other species have a wider range, extending from the continental shelf to lower bathyal zones. Biogeographic relations exist with the Pacific, South Atlantic, and Southern Oceans, but the latter seem to be weaker than would be expected considering the connection by Antarctic intermediate water.

Introduction

Oceanic oxygen minimum zones (OMZs) are ubiquitous and persistent features on the eastern continental margins of ocean basins, where dissolved oxygen concentrations decrease to low levels (Kamykowski and Zentara, 1990; Levin, 2003; Arntz et al., 2006). These OMZs develop beneath highly productive surface waters associated with upwelling, and result from the depletion of oxygen in the water column due to high biological and biochemical demand (Wyrtki, 1962; Rowe, 1981). OMZs intercept the continental margin, producing strong gradients of bottom water dissolved oxygen and organic matter input (Levin et al., 2000) and unique biogeochemical processes (Karl and Knauer, 1984; Smith et al., 2000). The vertical distribution of benthic populations and community structure are greatly influenced by the presence of OMZs (Menzies et al., 1973; Rosenberg et al., 1983; Arntz et al., 1991; Wishner et al., 1995; Levin, 2003). In general, benthic fauna of the OMZs are dominated by nematodes, oligochaetes, and small-bodied polychaetes, probably associated with high food availability and reduced predation pressure (Levin et al., 2002).

The existing literature on megabenthic communities along the continental margin off Chile and Peru is restricted mostly to the continental shelf and upper slope (e.g., Gallardo, 1963; Brattström and Johanssen, 1983). In fact, except for the results of the R/V Anton Bruun cruise in the eastern South Pacific (Garth and Haig, 1971; Menzies et al., 1973; Tommasi, 1976), the Russian Expeditions (Andriashev et al., 1958; Romanova, 1972; Mironov and Rudjakov, 1990), and general studies of archibenthal fauna (Frankenberg and Menzies, 1968; Rowe, 1971; Andrade, 1986, Andrade, 1987), there are no detailed ecological studies on bathyal megabenthic communities. More recently, Sellanes et al. (2004) and Sellanes and Krylova (2005) described diverse chemosymbiotic assemblages, including various bivalve species (Calyptogena, Lucinoma, Thyasira, Conchocele, Acharax) associated with a methane seepage in the bathyal zone off central Chile (∼36°S). Notwithstanding, the species composition, diversity, and function of the megafaunal assemblages in bathyal and abyssal depths along the continental margin of the Humboldt Current system (HCS) off Chile and Peru still remain largely unexplored.

Benthic megafaunal assemblages are an important component in bathyal ecosystems. In fact, the scarce available data suggests that benthic megafauna comprise a major fraction of the deep benthic biomass. Most of the studies on megabenthic communities have focused on the fish fraction and knowledge of invertebrates is still limited (e.g., Kallianiotis et al., 2000; Company et al., 2004; Pirtle, 2005; Wicksten and Packard, 2005). These studies have identified demersal fish and invertebrates as one the most important components of biodiversity in deep marine ecosystems; however, their specific habitat requirements, functional roles, and diversity patterns are not well understood.

Due to logistical constraints, megabenthic community investigations have focused on the continental shelf off northern (Gallardo, 1963) and central Chile (Gallardo et al., 1996). More recently, however, quantitative macrobenthic surveys have been extended onto the continental slope (Palma et al., 2005; Quiroga et al., 2005), including the jointly organized Chilean–German Expedition PUCK (SO-156 onboard R/V Sonne) (Hebbeln et al., 2001). One of the main objectives of the PUCK Expedition was to describe the bathyal benthic communities (i.e. mega, macro, meiofauna) in relation to the extension of the OMZ along the continental margin off Chile in specific areas with different local production. This paper aims: (1) to characterize the species composition, diversity, community structure, and functional aspects of the demersal fish and megabenthic fauna; (2) to determine the effect of the OMZ on the megabenthic community; and (3) to identify the megafaunal assemblages in association with the water masses in the eastern South Pacific region off Chile.

Section snippets

Study area

The regional hydrography of the HCS was recently described by Schneider et al. (2003). The OMZ core consists of equatorial subsurface water (ESSW), characterized by high salinity (34.4–34.9 psu) and low dissolved oxygen (<1.0 mL L−1), and is transported southward by the Gunther Current (also known as the Peru–Chile Undercurrent). On the continental margin off Chile, during normal cold (non-El Niño) regimes, the upper border of the ESSW mass is located between 50 and 100 m, extending to about 350 to

Environmental variables

The lowest bottom water dissolved oxygen concentrations (<1 mL L−1) were recorded at stations located on the continental shelf and upper slope off Antofagasta and Concepción, i.e. within the OMZ associated with ESSW. Oxygen concentrations increased below 632 m off Antofagasta and below 365 m off Concepción. No OMZ was detected off Chiloé, where oxygen concentrations varied between 1.3 (shelf stations) and 2.8 mL L−1 (upper bathyal stations) (Table 1).

The sediments sampled off both Antofagasta and

Influences of the OMZ on megafaunal assemblages

The decrease of megafaunal species diversity, abundance, and biomass in the ocean with water depth is well documented (e.g. Haedrich et al., 1975, Haedrich et al., 1980; Smith and Hamilton, 1983; Lampitt et al., 1986; Gage and Tyler, 1991). However, some coastal upwelling ecosystems present an exception due to excessive surface production and the ensuing formation of low oxygen zones on the shelf and upper slope. The major hydrographic feature of the Humboldt Current region is the presence of a

Acknowledgments

We thank all participants of the R/V Sonne 156 Cruise (PUCK Expedition 2001) who contributed to the success of this study. The expedition was funded by the German BMBF (No. 03G0156A) and organized in the framework of the FONDAP Humboldt Program 1997–2000 (CONICYT, Chile). Ongoing cooperation between the AWI and UdeC was funded by the International Bureau of the BMBF (Project No. CHL01/010). This research was funded by the COPAS center (CONICYT, Chile). The CIEP center (Gobierno regional de

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