Biodiversity and life strategies of deep-sea meiofauna and nematode assemblages in the Whittard Canyon (Celtic margin, NE Atlantic Ocean)
Introduction
Submarine canyons are numerous, complex features with elaborate patterns of hydrographic flow, sediment transport and accumulation, as exiting routes for water and sediment escaping from the continental shelf (Canals et al., 2009; De Leo et al., 2010; Canals et al., 2013). Oceanographic conditions inside active canyons, such as accelerated currents and dense-shelf-water cascading, caused by a combination of topographic setting and climate-driven episodic events, alter suspended particulate matter concentrations and the transport of mass and organic matter from the coastal zones to the deep ocean (Bosley et al., 2004, Genin, 2004, Puig et al., 2004, Billett et al., 2006, Canals et al., 2006, Company et al., 2008, Pusceddu et al., 2013). This is particularly evident for canyons very close to the shorelines and thus directly influenced by river inputs or severe coastal storms or dense shelf water cascading (Canals et al., 2006, Tesi et al., 2010, Pedrosa-Pàmies et al., 2013). The variation in the frequency of these events and the pulses of material and energy influence the structure and functions of the benthic assemblages (Cunha et al., 2011, Duros et al., 2011, Paterson et al., 2011, Pusceddu et al., 2013, Ramalho et al., 2014). Previous findings suggest that increased habitat heterogeneity in canyons can be responsible for the presence of hotspots of biomass and consequently enhanced local fishery production (Rowe et al., 1982, Vetter, 1994, Tudela et al., 2003, Company et al., 2008, De Leo et al., 2010). Some canyons are also defined as “hotspots” of benthic diversity due to the presence of high levels of endemism and the occurrence of species with peculiar life-history patterns (Skliris and Djenidi (2006) and citations therein; Vetter et al., 2010; De Mol et al., 2010; Danovaro et al., 2014). However, the increasing number of investigations carried out worldwide on canyon systems has accumulated “contrasting” results on different paradigms regarding the biodiversity, trophic conditions and functions of these deep-sea habitats (Danovaro et al., 2009, Tyler et al., 2009, Bianchelli et al., 2010, De Leo et al., 2010, Cunha et al., 2011, Duros et al., 2011, Ingels et al., 2011, Paterson et al., 2011, Ingels et al., 2013, Ramirez-Llodra et al., 2013, Schlining et al., 2013, De Leo et al., 2014, Leduc et al., 2014, Ramalho et al., 2014, Amaro et al., 2015).
The Whittard Canyon is a wide and complex system consisting of a series of coalescing channels that incise the southern Celtic margin (Duineveld et al., 2001, Amaro et al., 2015). The head of the canyon is far from the shoreline, but Whittard Canyon and the adjacent slopes display a large presence of fresh organic material (i.e., phytopigments) and high concentration of organic matter (Duineveld et al., 2001, Duros et al., 2011, Amaro et al., 2015). While the Whittard Canyon has been mainly investigated for geological, oceanographic and trophic features, foraminifera and macro- and mega-fauna (Duineveld et al., 2001, Duros et al., 2011, Huvenne et al., 2011, Davies et al., 2014, Robert et al., 2014, Amaro et al., 2015, Gunton et al., 2015), metazoan meiofaunal assemblages still remain largely unexplored. Meiofaunal assemblages have been previously investigated at ca. 700 and 1000 m depth in the Western middle branch of the Whittard Canyon (Ingels et al., 2011). Here we investigated meiofaunal assemblages (including nematode diversity at species level and their life strategy) along bathymetric transects in the two branches (Eastern middle and Western) of the Whittard Canyon and in two adjacent open slopes to understand: (1) Are bathymetric patterns different in the two branches of the Whittard Canyon and in the two open slopes? (2) Are meiofaunal abundance, biomass and diversity higher in canyon when compared to the adjacent slopes? (3) Are the life strategies of the nematode assemblages different in canyon and open slope systems?
Section snippets
Study area and sampling strategy
The Whittard Canyon is a wide dendritic canyon system located in the northern Bay of Biscay continental slope between the Goban Spur and the Meriadzek Terrace and its features have been described elsewhere (Duineveld et al., 2001, Duros et al., 2011, Ingels et al., 2011, Huvenne et al., 2011, Davies et al., 2014, Robert et al., 2014, Amaro et al., 2015, Gunton et al., 2015). This region is characterized by high availability of organic matter in the sediments (Duros et al., 2011) mainly derived
Meiofaunal assemblages in the Whittard Canyon and the adjacent open slopes
In the W Whittard Canyon meiofaunal abundance (average value±standard deviation) ranged from 190.4±98.2 to 1231.2±202.4 ind 10 cm−2 at 3002 m and 1109 m-depth, respectively (Fig. 1a). The results of PERMANOVA analyses revealed the presence of significant effects of the interactions habitat×depth for meiofaunal abundance (Table 2). The pairwise tests comparison across different habitats showed that meiofaunal abundance was significantly higher in the W Whittard Canyon than in the W slope at 1000
Distribution of meiofaunal assemblages in the Whittard Canyon and the adjacent slopes
Most of the investigations carried out on benthic assemblages in canyon systems focused on canyon interior (i.e., axes or on their flanks), without considering the adjacent open slopes (Amaro et al., 2009, Tyler et al., 2009, Ingels et al., 2009, Ingels et al., 2011, Ingels et al., 2013; Ramalho et al., 2014; Kenchington et al., 2014). Available studies designed to compare meiofaunal assemblages inhabiting the canyons and the adjacent open slopes at similar depth, provided contrasting results,
Conclusions
Results reported here suggest that the complex structure of the Whittard canyon influences the spatial distribution of meiofaunal abundance, biomass and diversity both inside and outside the canyon' branches, with different effects at different bathymetric depths. The diversity of nematodes down to species does not show bathymetric patterns in all of the investigated systems, but the turnover diversity is high both among and within different habitats. Overall, meiofaunal assemblages are
Acknowledgments
This study has been conducted in the framework of the Project HERMES funded by the European Commission under the Framework Program VI (Contract No. GOCE-CT-2005-511234-1).
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