Distribution of meiobenthos in the Nazaré canyon and adjacent slope (western Iberian Margin) in relation to sedimentary composition

Abstract

Abundance of metazoan meiofauna and foraminifera, and biomass and community structure of nematodes, were investigated in the benthic zone along the Nazaré Canyon and adjacent continental slope in relation to concentration of organic matter and its suitability as a food source for the meiobenthos. The Nazaré canyon sediments were richer in organic carbon (Corg), total nitrogen, and phytopigments than the adjacent open slope. In addition, phytodetritus was fresher in the canyon sediment than on the slope (higher chlorophyll a: phaeopigments). Nevertheless, the abundance of polychaetes, copepods, bivalves, nematodes, total metazoans, and nematode biomass were not always higher in the canyon than on the adjacent open slope. Lower densities occurred in the upper and middle canyon, and living benthic foraminifers were significantly more abundant on the adjacent slope. The stations in the upper and middle canyon contained infinitesimal numbers of foraminifers. Reduced diversity and evenness and high K-dominance of the nematode assemblages in the upper part of the canyon indicated environmental stress, perhaps related to high Corg content and sediment disturbance. Non-selective deposit-feeders dominated the nematode assemblages of the upper and middle parts of the canyon, whereas a more diverse trophic structure was found in the deeper parts and the open slope. Conditions in the upper and middle areas of the Nazaré canyon are harsh, and only opportunistic organisms can survive there. KEY WORDS: Nazaré canyon · Meiobenthos · Foraminifera · Sedimentary organic carbon · Physical disturbanceResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 340: 207-220, 2007 Corg content than in the open continental shelf and slope (Epping et al. 2002, Van Weering et al. 2002. Enhanced remineralisation rates have been shown as a result of enrichment with Corg (Epping et al. 2002).Meiofaunal communities are responsible for a significant amount of sediment remineralisation and support significant trophic pathways (Leguerrier et al. 2003). According to Tietjen (1992), on average, bacteria comprise 86%, macrofauna 7% and meiofauna 5% of living carbon in deep-sea sediments. Hence, despite comprising a relatively minor component of the living carbon compared to bacteria, macro-and meiofauna play an important role in the total carbon turnover of deep-sea environments. The meiofauna communities in the Nazaré canyon have rarely been studied: a few records of metazoan meiobenthos are available for the Western Iberian Margin (see Soltwedel 2000, Flach et al. 2002), while Flach (2003 reported very high densities of meiofauna in the canyon at all depths.Benthic animal communities from canyon systems can reflect the unstable and organically enriched conditions of these systems. Indeed, the fauna communities inhabiting submarine canyons have been found to differ in structure from those on adjacent abyssal plains, continental shelf, or slope. Faunal densities and biomasses in canyon systems have...

An insight into the feeding ecology of deep-sea canyon nematodes — Results from field observations and the first in-situ 13C feeding experiment in the Nazaré Canyon

“…In contrast, Garcia et al (2007) found that non-selective deposit feeders were more important in canyons than on the continental slope. The localities studied by Garcia et al (2007) were much more disturbed (near canyon axis) compared to the sediment draped terrace studied here and so we suggest that intense recurrent hydrodynamic activity at their sites is (partially) responsible for this discrepancy.…”

“…In contrast, Garcia et al (2007) found that non-selective deposit feeders were more important in canyons than on the continental slope. The localities studied by Garcia et al (2007) were much more disturbed (near canyon axis) compared to the sediment draped terrace studied here and so we suggest that intense recurrent hydrodynamic activity at their sites is (partially) responsible for this discrepancy. In addition, non-selective deposit feeders have been known to exhibit recolonisation ability after severe disturbance, but in less disturbed environments they are less dominant (Lee et al, 2001a).…”

“…However, because the canyon intersects a large part of the shelf it entraps large quantities of sediment moving along the Portuguese coast and so is recognised as a major sediment pathway among European canyons (Tyler et al, 2009). Besides high rates of sedimentation, OM transport, entrapment, and burial, the Nazaré canyon is hydrodynamically very 20 active and its heterogeneous topography contributes to a complex current regime within, and even beyond, the canyon's borders (Tyler et al, 2009) with significant impacts on the canyon fauna (Curdia et al, 2004;Garcia et al, 2007;Ingels et al, 2009;Koho et al, 2007). Terraces in the middle canyon area are draped in thick layers of muddy turbidites creating a distinct depot centre with little sediment passing beyond the middle canyon section .…”

“…The abundance of meiobenthic communities in canyons reflects the prevailing current regime to an extent, with often very low 20 densities in areas where sediments are disturbed regularly and hydrodynamic activity is greatest, e.g. in the axis of a canyon (Bianchelli et al, 2008;Garcia et al, 2007;Ingels et al, 2009;Van Gaever et al, 2009). Although strong near-bottom flow can reduce the abundance and alter the assemblage of both meiofauna and macrofauna in the deep sea (Thistle and Levin, 1998), meiofauna appear to recover more quickly than macrofauna with regards to physical disturbance (Lee et al, 2001a;Lee et 25 al., 2001b;Thistle et al, 1999).…”

“…The Nazaré Canyon is characterised by complex and variable hydrodynamic conditions Tyler et al, 2009), which affect meiofauna abundances and assemblage composition (Garcia et al, 2007;Ingels et al, 2009;Soetaert and Heip, 1995), but increased food supply may act to increase abundances . The abundance of meiobenthic communities in canyons reflects the prevailing current regime to an extent, with often very low 20 densities in areas where sediments are disturbed regularly and hydrodynamic activity is greatest, e.g.…”