Biodiversity and life strategies of deep-sea meiofauna and nematode assemblages in the Whittard Canyon (Celtic margin, NE Atlantic Ocean)

Highlights

• Meiofaunal assemblages differ between the Whittard Canyon and adjacent open slopes.

• Biodiversity is typically higher in slopes than in the canyon׳s branches.

• The gravity flows and sediment instability along with enhanced pulses of phytodetritus deposition favored the presence of opportunistic species in the canyon.

• Habitat types influence the biodiversity and life strategy of nematode assemblages.

Abstract

An extensive sampling strategy has been carried out along bathymetric transects in two branches (Eastern middle and Western) of the Whittard Canyon and in two adjacent open slopes to investigate meiofaunal assemblages (including nematode diversity and their life strategy) in the Celtic margin, Northern Gulf of Biscay. Our results show the presence of differences in terms of meiofaunal abundance and biomass among habitats even when located at the same depth. Meiofaunal abundance and biomass in the upper and middle (1000–2000 m) part of the branches of the Whittard Canyon are typically higher than those reported in adjacent open slopes while, in deeper sediments (ca. 3000 m), an opposite pattern is detected. The availability of food sources (both in term of quantity and quality) plays a key role in explaining such differences. Diversity expressed either as the richness of meiofaunal taxa and of nematode species is typically higher in slopes than in the branches of the Whittard Canyon. Turnover diversity is high (40–100% for meiofaunal rare taxa and 61–78% for nematode species, respectively) either among habitats and depths. Although a general dominance of deposit feeders, predators are more abundant in slopes (9–12% of total nematode abundance) than in both branches of the Whittard Canyon (4–7%). The higher fraction of predators of the nematode assemblages inhabiting the slopes determined higher values of the maturity index (i.e., more persisters, c-p=4/5). We hypothesize that the life strategies of nematode assemblages are influenced by gravity flows, sediment instability and the 2–3 times higher availability of phytopigments, which characterize the upper part of the Whittard Canyon, favoring opportunist/colonizer species. Our findings indicate that different deep-sea habitats are associated to different life strategies, thus contributing to increase the functional diversity of deep-sea ecosystems.

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?