versión On-line ISSN 0718-686X
Anales Instituto Patagonia (Chile) v.37 n.2 Punta Arenas 2009
Ana/es Instituto Patagonia (Chile), 2009. 37(2):17-27
THE MACROBENTHIC ECOLOGY OF THE STRAITS OF MAGELLAN AND THE BEAGLE CHANNEL
ECOLOGÍA MACROBENTÓNICA DEL ESTRECHO DE MAGALLANES Y CANAL BEAGLE
Sven Thatje1 & Alastair Brown
1 National Oceanography Centre, Southampton, School of Ocean and Earth Science, University of Southampton, European Way, Southampton, S014 3ZH, United Kingdom. email@example.com
The macrobenthic community of the Straits of Magellan and the Beagle Channel was investigated using a Reineck box corer at 22 stations during the Chilean "Cimar Fiordo 3" expedition in 1997. A total of 173 taxa represented by 2188 individuals were identified and are reported for the investigated area. Clear exponential relationships with depth were revealed by analysis of abundance, biomass, spe-cies richness, and evenness. These patterns coincide with posited theories of pelagic-benthic coupling and the source-sink hypothesis of colonisation-extinction dynamics. Polychaeta dominated macrobenthic community abundance and biomass, 67% and 38% respectively, therefore consideration of biogeographic affinities concentrated on this taxon. 13 species of polychaetes observed in the study area co-occur in Antarctica suggesting biogeographic or evolutionary affinities between these adjacent regions.
Key words: biogeography, diversity, polychaete, Antarctica, Subantarctic
En octubre 1997 se investigaron los ensambles macrozoobentónicos del estrecho de Magallanes y el canal Beagle con un "Reineck Box corer" en 22 estaciones durante la campaña chilena "Cimar Fiordo 3". Se identificó un total de 173 taxones representados por 2188 especímenes en el area de investigación. Se detectó una relación exponencial de profundidad dada por el análisis de abundancia, riqueza de especies y uniformidad. Estos patrones coinciden con teorías establecidas de flujos bento-pelágicos y las hipótesis de "source-sink". Los poliquetos dominan los ensambles macrobentónicos en abundancia y biomasa en 67% y 38% respectivamente, y por tanto el análisis biogeográfico presentado se basa en ellos. Trece de las especies de poliquetos identificados en el area investigada se conocen también por estar distribuidos en Antartica. Proponemos entonces afinidades biogeográficas y evolutivas entre ambas regiones.
Palabras clave: biogeografía, diversidad, poliqueto, Antartica, Subantártica
The Magellan region is geologically young. Although glaciation in the biogeographic Magellan region (sensu Camus 2001) did not reach the extent experienced by Antarctica, an extensive ice cap from 35 to 55°S did exist during the Late Glacial Máximum (LGM), approximately 21 ky ago (Benn & Clapperton 2000). The processes of gradual warming following the LGM explain much of the modern biogeographic pattern in the Magellan region (Arntz et al. 2005). During the glacial period sea level was between ca. 125 and 135m lower than it is today (Fairbanks 1989, Yokoyama et al. 2000). The earliest incursions of seawater into the Straits of Magellan occurred by the end of deglaciation around 8 ky ago. It has been proposed that the Straits did not fully open until approximately 7 ky ago (McCulloch & Davies 2001). All present species in Magellan waters therefore recolonised this region from adjacent Atlantic and Pacific areas (Montiel et al. 2005a). The area is a meeting place for water bodies from the Atlantic and Pacific, and is also partially influenced by the Southern Ocean (Panella et al. 1991).
The modern hydrologic regime in the Straits of Magellan and the Beagle Channel reflects the complex geomorphology and topography, and is highly variable (Brambati et al. 1991, Dávila et al. 2002). The regime is characterised by strong freshwater input from the runoff of high precipita-tion, sufficient to establish a strong and shallow pycnocline, mean temperature of 7-9 °C and salinity of 30 (Artegiani & Pachini 1991, Dávila et al. 2002). High sediment loads are associated with these inputs in regions of glacial action (Brambati et al. 1991). The regime varies significantly between narrows and basins, e.g. currents of 1 m s-1 on the Atlantic side of the Straits decrease to 0.2 m s-1 in the Paso Ancho (Michelato et al. 1991). This results in characterisation of these zones by coarse and fine sediments respectively (Brambati et al. 1991). Additionally, a wide depth range exists with depths reaching 1200 m at the western gateway to the Straits (Antezana et al. 1992). These factors have been identified as significant in structuring Magellan invertebrate communities (Montiel et al. 2005b, Moreno et al. 2008).
The aim of this study was to develop the existing characterisation of the macrobenthic ecology of the Magellan region by examining a high-level taxonomic abundance and biomass data for over the entire region, following a diversity approach. Due to the low number of replicates at each sampling station community analysis was considered inappropriate. Biogeographic affinities of the polychaetes, found as the dominant taxon in the Magellan region, with the Antarctic are discussed.
MATERIAL AND METHODS
Sampling was undertaken on the Chilean "Cimar Fiordo 3" expedition in 1997, from AGOR Vidal Gormaz (Thatje & Mutschke 1999a). A total of 22 stations were sampled: 18 stations located within the Strait of Magellan, adjacent channels and fjords, and 4 stations located within the eastern part of the Beagle Channel, adjacent channels and fjords (Table 1). Depths of sample stations ranged between 35 and 571m. Sampling was performed using a Reineck box corer (core area 0.017 m2), and 2 or 3 cores were taken at each location. Samples were sieved through 0.5mm mesh and preserved in 4% hexamethylenetetramine-buffered formalin prior to sorting. Animáis with calcareous shells were transferred to 70% ethanol following fixation.
Species-level identification was conducted by specialists and abundance (ind. m-2) was determined from pooled cores per station. Colonial hydrozoan and bryozoan were only considered as present (1 ind. m-2) or absent for calculation of abundance alues. Abundance data for station 5 were not preserved. Other univariate analyses were calculated from all remaining species-level abundance data using Margalef's d for species richness, Pielou's J for evenness and the Shannon-Wiener H' (based on loge) index for diversity. Analysis was implemented using the DIVERSE routine in PRIMER v6 (Plymouth Rou-tines in Multivariate Ecological Research; Clarke and Gorley 2006).
A total of 173 species/morphotypes were identified across all taxa (Table 2). Low-level taxonomic analysis indicated that Polychaeta dominated abundance (67%), followed by Arthropoda (17%), Mollusca (5%) and Echinodermata (4%) (Fig. 1a). Polychaeta also dominated biomass (38%), followed by Echinodermata (23%), Arthropoda (21%) and Mollusca (10%) (Fig. 1b). Average abundance of the Magellan region was 2179 ind. m-2 (range 313 -10168 ind. m-2), and average biomass was 36.8 g ww m-2(range 2.4 -142.3 g ww m-2). Polychaete abundance was dominated by the families Cirratulidae (271 ind. m-2), Ampharetidae (262 ind. m-2), Spionidae (227 ind. m-2) and Paraonidae (157 ind. m-2) with the most speciose families the Spionidae (6), Lumbrineridae (5), Nereididae (4) and Orbiniidae (4).
Abundance, biomass and species richness all decreased exponentially with depth (Fig. 2a, b and c). Linear regression analysis of natural log transformed abundance and biomass data over log transformed depth indicated that both measures co-varied significantly with natural log transformed depth (F1,19 = 17.69, P < 0.05, and F1,20 = 14.37 P < 0.05, respectively). Linear regression analysis of species richness data over natural log transformed depth indicated that species richness co-varied significantly with depth (F1,19 = 7.92, P < 0.05). These analyses yielded r2 values that indicated that the natural log depth model explained 48.2% of the variation in abundance and 41.8% of the variation in biomass and that the natural log depth model explained 29.4% of the variation in richness. Evenness increased exponentially with depth (Fig. 2d), and linear regression analysis of evenness data over natural log transformed depth indicated significant co-variation (F1,19 = 6.13, P < 0.05) with the natural log depth model explaining 24.4% of the variation in evenness. No trend was observed in diversity with depth (Fig. 2e) and linear regression analysis of diversity data over natural log transformed depth indicated that there was no significant co-variation (F1,19 = 1.55, P > 0.05). Visual inspection of the residuals for all linear regressions showed approximately homogenous variances and minimal deviation from normality, confirming validity of the test.
Grab sampling has been shown to underestimate epibenthic species that are large, highly motile or rare (Dahm 1996). For example, although large decapod species such as Peltarion spinosulum or motile species such as Munida subrugrosa are known to occur in large numbers in the Magellan region (Gorny 1999, Gutt et al. 1999) they were not found in the present study. A variety of grab cores using the same mechanical principies constitute the dominant benthic sampling method reported for this and comparable regions (Gerdes et al. 1992, Brey & Gerdes 1999, Gerdes & Montiel 1999, Thatje & Mutschke 1999b, Piepenburg et al. 2002) and are therefore the most appropriate method to facilítate contribution to, and comparison with, the existing literature.
This study aimed to assess the macrobenthic ecology through analysis of a large-scale high-level taxonomic abundance and biomass data set. Com-munity analysis was not envisaged for this study with limited sampling undertaken due to time and weather constraints. Despite the low number of stations sampled for the overall area under investiga-tion, clear patterns were established by the diversity analysis. Both the range of abundance and biomass and the means calculated for these parameters strongly resemble other alues reported for the Straits of Magellan and the Beagle Channel, and for Antarctica (see Table 2 in Arntz et al. 2005). This is consistent with observations on the absence of latitudinal trends in this region (Brey & Gerdes 1999, Gerdes & Montiel 1999, Piepenburg et al. 2002). Similarly, the observed decrease in abundance and biomass with depth is a common pattern and has been reported from numerous other regions, e.g. for the high Antarctic Weddell and Lazarev Seas (Brey & Gerdes 1998). Pelagic-benthic coupling has been suggested as the dominant factor causing these patterns (Cattaneo-Vietti et al. 1999), with the flux of organic matter from the pelagic to the benthic being the major factor structuring these communities.
Decreasing species richness and increasing evenness with depth have also been reported from numerous regions, e.g. for the Gulf of Mexico (Pérez-Mendoza et al. 2003). Analysis of these patterns in the polychaetes of the Pacific coast of South America suggests that a source-sink hypothesis of colonisation-extinction dynamics, where shallower "sources" maintain deeper "sinks", provides a conceptual and methodological framework that explains patterns of diversity (Moreno 2008). The absence of any correlating pattern in diversity in the study region is consistent with other studies (Gutt et al. 1999) and may result from variation in hydrologic regime and sedimentation processes between sta-tions and/or the low number of replicates.
Moreno et al. (2006) identified a pattern of decreasing endemism in benthic polychaete species of the Magellan region with increasing latitude. Although species endemic to the Straits of Magellan and the Beagle Channel are therefore less likely to have been observed, the absence of available bio-geographic information in the literature prevented an assessment of endemism within the study region. Interestingly, a total of 13 of the 78 polychaete spe-cies/morphotypes identified in the samples from this study are known from the Antarctic shelf (e.g. Phylo felix, Leanira quatrefagesi, Nereis eugeniae, Glycinde armata, Idanthyrsus armatus). Clear distribution patterns have been identified between polychaete communities of the Magellan region and Weddell Sea shelves (Montiel et al. 2005b). Polychaete reproductive strategy commonly involves a meroplanktonic larval stage (Giangrande 1997) and Montiel et al. (2005a) suggest that the dispersal of Antarctic species through larval transport in easterly circumpolar currents plays an important part determining in the existing distribution patterns of the fauna around the Magellan region (for discussion see also Thatje & Fuentes 2003). This is supported by a greater proportion of species with high Antarctic affinities to the Pacific coast compared to the relatively small proportion of species with affinities to the Atlantic side (Montiel et al. 2005a). The durations of larval stages of polychaete species can be extremely short but at lower temperatures have been reported at several months (Bhaud 1998). Reduced densities of larvae observed in the Antarctic and the less obvi-ous seasonality of these larvae may be attributable to further protracted larval development resulting from low temperatures (Stanwell-Smith et al. 1997). However, the key criteria for establishment of a suc-cessful population in a new habitat are recruitment conditions and the substrate choice of settling larvae (Raguá-Gil et al. 2004), and therefore the spreading potential of polychaete larvae does not necessarily predict species' adult distribution (Bhaud 1998). Ultimately, given the glacial history of the region, the presence of common species on both sides of the Drake Passage strongly suggests that dispersion is an important process for faunal exchange between the Magellan region and Antarctica, and therefore that for many species the polar front does not necessarily function as a strict barrier (Thatje & Fuentes 2003, Montiel et al. 2005a).
The macrobenthic ecology identified in this study provides evidence of patterns in the Magellan region that have been reported for numerous other regions, additionally indicating exchange processes by which this region may have been recolonised following glaciation and which may still occur. This is a first study of high taxonomic level analysis providing information that may be of use in f uture biogeographic studies. Further high resolution sampling is required to develop community analysis of the region and elucídate Magellan-Antarctic connections.
The corresponding author would like to thank Erika Mutschke, Carlos Rios, Matthias Gorny, Dieter Gerdes, and Wolf Arntz for supporting his work during Cimar Fiordo 3. We are grateful to the captain and crew of AGOR Vidal Gormaz for assistance at sea. This work would not have been possible without the help in taxonomic identifications by several colleagues (in no particular order): Angelika Brandt, Anja Schmidt, José Saiz-Salinas, Iván Cañete, Claude De Broyer, Martín Rauschert, Ute Mühlenhardt-Siegel, Karin Riemann-Zürneck. We thank Gustavo Lovrich for revising an earlier version. The authors declare that they have no conflict of interest.
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Received: Sept, 10, 2009
Accepted: Nov., 8, 2009