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Deep-sea ecosystems represent the largest biome in the biosphere, but its diversity is still the least explored and relatively poorly understood (Rex, 1981; Danovaro et al., 2010; Vanreusel et al., 2010; Ramirez-Llodra et al., 2010). Researches performed on a regional scale have provided insights on the structure of deep-sea communities and the forces that drive them (Levin et al., 2001). Contrary to what was previously thought, as being a homogeneous environment, both continental margins and mid-oceanic seafloors are much more complex ecologically, geologically, chemically and hydrodynamically (Vanreusel et al., 2010), and recent findings have a crucial role to play in proposals for exploiting the deep-sea for its resources such as minerals, petroleum, and waste disposals (Rex, 1981). Due to its sparsely occupied pattern in patches and often stochastic events, the key to understand deep-sea distributions lies in discerning the scales and documenting them, as well as the causes that operate to generate these distributions (Levin et al., 2001; Rex et al., 2005).
According to McClain et al. (2004), most of the deep-sea studies on the community level focus on the simple number of coexisting species without regarding their natural history. Only a fraction of them attempt to concentrate on the relation between taxonomy and adaptive properties, such as dispersal or feeding type. However, in the last 50 years, more research was done, aided by the development of new equipment, such as ROV's (Remotely Operated Vehicles) and AUV's (Autonomous Underwater Vehicles), combined with the discovery of new habitats, e.g., vents, seeps, and whale falls (Vincx et al., 1994; Levin et al., 2010; Ramirez-Llodra et al., 2010). For the first time these studies bring together geographically and taxonomically diverse data set in order to explore how habitats influence diversity patterns (Levin et al., 2010). Such research has transformed our knowledge of the deep-sea, a highly complex environment in terms of biodiversity patterns and community structures, and is extremely linked to surfaces processes (Levin et al., 2001).
Most of the deep-sea ecosystems are heterotrophic, i.e., depending on the organic matter flux from the surface waters that are provided through photosynthesis (Ramirez-Llodra, 2010). Other ecosystems have their productivity increased by physical factors, such as the seamounts, cold-water corals and canyons (Ramirez-Llodra et al., 2010). However, some of those particular deep-sea environments are being investigated to understand to which extent they contribute to total biodiversity, as the studies are mainly comparisons between them and the surrounding area on a local or regional scale (Vanreusel et al., 2010).
Spatio-temporal variation in deep-sea species diversity represents an integration of ecological and evolutionary processes that operate at different spatial and temporal scales (Levin et al., 2001). The biodiversity in the deep-sea is among the highest on the planet and its composition is mainly contributed by the macro and meiofauna, although they are still not well known (Ramirez-Llodra et al., 2010). In terms of average biomass, the importance of macro and megafauna declines dramatically with water depth while smaller size classes, such as bacteria and meiofauna, dominate the communities below 3000m depth (Smith et al., 2008).
Although there is a central paradigm for marine diversity stating that species richness follows a parabolic curve with increasing water depth with a maximum at mid-slope depths, this pattern mainly reflects the macro and megafauna distribution (Rex, 1981). Therefore, the hypothesis that enhanced levels of biodiversity at the slopes can act as a source for deeper basins (Rex et al., 2005) is not explained for all fauna since the studies were done only with gastropods and bivalves (McClain et al., 2004; Rex et al., 2005). Moreover, there are some hypotheses which state that isopods have suffered adaptive radiation from the abyss (Smith et al., 2008).
The causes of species distribution patterns have been proposed in many studies (Rex, 1981; Levin et al., 2001; Danovaro et al., 2010; Levin et al., 2010). They are driven by several factors with different importance according to depth (Rex, 1981) and can act in different combinations that can overcome other local or regional factors (Levin et al., 2001). The main factors are nutrient input, sediment grain size, substrate heterogeneity, oxygen availability, current regimes, productivity and stochastic events (Rex, 1981; Levin et al., 2001). The fitness of a population depends on ecological features, although physiological characteristics are of great importance, some of which clearly depend on organism size, such as metabolic rates, tolerance to chemical stress, the ability to move or migrate and vulnerability to predation (Soetaert et al., 2002).
Submarine canyons: hotspots of biodiversity?
Submarine canyons form part of the deep-sea landscape and are considered widespread features present on most of the ocean's continental and island margins (Levin et al., 2010). They are defined as narrow, deep depressions with steep sides and the bottom has a continuous slope (Shepard & Dill, 1966). Their cross sections are mainly V-shaped along the upper course, reflecting the prevalence of erosion, and U-shaped in the lower course, due to accumulation processes (Danovaro et al., 2010). A new global inventory suggests that at least 660 submarine canyons are distributed worldwide (De Leo et al., 2010), against 96 mapped in 1996 (Shepard & Dill, 1966). Quantitative sampling from a canyon floor in the New Zealand margin points these environments as one of the most productive benthic habitats so far in the deep-sea (De Leo et al., 2010).
Submarine canyons are formed during low sea-levels, when slides and dense sediment-laden flows erode in the continental shelf and slope (Canals et al., 2006). They can act as a connection between the slope and the abyss or often act as a sink for particulate materials that come from river inflows and moved along the shore and across shallow water platforms (Levin et al., 2010). Canyons are also characterized by bottom currents that can modify the structure and composition of benthic faunas, although the mechanisms involved are not well understood (Levin et al., 2001). Besides the normal particle transport across the slope, canyons can form an alternative route for organic material export from shelf to deep-sea (Duineveld et al., 2001). This transport is enhanced in some canyons when there is winter heat losses and evaporation in hard winters. The surface waters cool down and become denser than the surrounding waters, causing cascading events that enhance the carbon flux and recruitment of some species (Canals et al., 2006; Company et al., 2008; Sardà et al., 2010).
Canyons reveal significant habitat heterogeneity and harbour species that are present only there, hence increasing beta diversity and supporting the hypothesis that they can function as hotspots of biodiversity (Levin et al., 2010). Moreover, different parts of the canyon can harbour different communities. In the study done by Ramirez-Llodra et al. (2010), polychaetes had a different distribution in the canyon head, wall and slope.
There are some species that spend their whole life cycle in the canyon (Levin et al., 2010; Gili et al., 2000). Some hydromedusae populations were studied in four Mediterranean canyons and revealed different rates of endemism related to the topography of the canyon and also nutrient input. Their life cycles are regulated by these factors and by other external factors, such as circulation and interaction of water masses and biological production (Gili et al., 2000). They also seem to be sensitive to climate variability (Richardson et al., 2009). This success in the colonization can be enhanced by the fact that these hydromedusae have a hydroid stage, a benthic feeder that can takes advantage of the organic carbon inflow provided during the winter (Gili et al., 2000). Hence, the heterogeneity seen in canyons acts as a great influence on the biotic diversity and emphasizes the fact that each canyon has its unique environmental characteristics that drive community structure (Levin et al., 2010).
Duineveld et al. (2001) reported a decrease in macrofauna biomass and density with increasing distance from the head of the Whittard Canyon. The macrofauna deposit and filter feeders account for at least 81% of the total community and they increased in importance with increasing depth in the canyon. Ophiuroids was the most common group found, in contrast to other canyons, where holothurians often dominate both in terms of abundance and biomass (De Leo et al., 2010; Amaro et al., 2009). When compared to the slope in terms of density, no significant differences were found, although the macrobenthic biomass in the Whittard canyon was higher. The same pattern was found for meiofauna, where canyons and trenches overlapped with the slope to a larger extent (Vanreusel et al., 2010). These results contrast with the study of Amaro et al. (2009), who reported that the densities are twice as high compared to the slope. These contrasting results suggest that more research is needed in canyons to clarify and establish general trends.
Nematodes distribution and depth-related patterns
Nowadays it is well documented that nematodes are the most abundant metazoans in the biosphere, accounting for 80-99% of total meiofauna, followed by the copepods (Wigley & McIntyre, 1964; Fenchel, 1978; Heip et al., 1985; Riemann, 1988; Coull, 1988; Soetaert & Heip, 1989; Vincx et al., 1994), and with its relative abundance increasing with depth (Vincx et al., 1994). Before 1960 no studies were available on deep-sea nematodes (Heip et al., 1985). The first research was conducted in 1964 and was just quantitative of the area, describing the taxa that lived there, and considered groups at higher levels (Wigley & McIntyre, 1964).
The availability of food is one of the most important factors that explain the distribution of deep-sea nematodes, as they are separated from the euphotic zone and depend on the organic matter produced there, resulting in a bentho-pelagic coupling (Heip et al., 1985; Soetaert et al., 2002). Moreover, they show a high degree of food specialization (Wieser, 1953; Moens et al., 1999a; Moens et al., 1999b). As the food decrease within depth, the food-web structure is represented mainly by deposit-feeders (Heip et al., 1985), although nematodes without mouth or gut, such as Parastomonema, suggest a direct assimilation of the organic matter or a chemosynthetic association with bacteria that provide their food (Ingels et al., 2009a).
One important trend observed for deep-sea nematodes is the composition and variation of nematode body size as a function of the depth (Soetaert et al., 2002). Gigantism and dwarfism can both occur in the deep sea (Soetaert & Heip, 1989; Ramirez-Llodra et al., 2010), although the evolution of small body size seems to be a trade-off between metabolic rate and reproductive efficiency (Ramirez-Llodra et al., 2010). Most of the explanations rely on the "food supply limitation" hypothesis and ignore other aspects that could also be important (such as the granulometric composition) which can lead to a decrease in reliability by underestimation of the larger (more rare) specimens, and pressure (Udalov et al., 2005). Moreover, depending on the depth studied, some variables may be more important than others. For instance, Udalov et al. (2005) verified that the relationship between nematode size with granulometry and organic matter changed among different depth ranges. The correlation with the sediment is stronger on the shelf and upper slope and becomes insignificant on lower slope, abyssal plains and trenches, where the sediment is homogenized. The effect of nutritious sediments was weaker for abyssal nematodes than for bathyal ones. In fact, in his analyzes, the sediment composition and trophic conditions were considered better predictors for average nematode size than the depth, which acted more as a co-variable.
Observations on nematodes confirmed that they become smaller with increasing water depth (Soetaert & Heip, 1989; Soetaert et al., 2002). Soetaert & Heip (1989) found that, for the Mediterranean, the lower limit of nematode body length was 300µm. They also observed that there is a variation in the distribution within the sediment which is more important than the one observed due to water depth, a pattern that has been confirmed in other studies (e.g. Ingels et al. 2009b). Nematode shape shifts from plumper forms in superficial sediment layers to slender forms at depth, however for stations in the Nazare canyon no pattern was established for body width, just for body length (Soetaert et al., 2002). Considering lower taxonomic levels in the last study, the plump nematodes were represented by aberrant forms, different from the worm-like shape of most nematodes. They comprised the genera Tricoma, Desmoscolex and Richtersia, with the two first having different feeding behaviour from the last, allowing an easier coexistence. Although and appealing explanation is food availability, comparable degrees of dwarfism occur also in the surface of eutrophic sediments on the shelf and are in contradiction with Udalov et al. (2005),which state that the size structure of nematodes becomes more uniform as deeper you go in the sediment column.
Nowadays, there are some observations on nematodes from canyons and it was seen that communities from canyons, abyss and trenches relatively overlap with the slope communities (Vanreusel et al., 2010; Ingels et al., 2009b). The high dominance of certain genera in canyons are mainly explained by the harsh conditions found there, which have a negative impact on the establishment of rich nematode communities (Ingels et al., 2009b). The main dominant genera are represented by Daptonema, Sabatieria, Acantholaimus, Paralongicyatholaimus, Pomponema, Dichromadora, Elzalia and Halalaimus, which are also dominant at the slope (Vanaverbeke et al., 1997; Botelho et al., 2007; Ingels et al., 2009b; Vanreusel et al., 2010).
Does dispersal affects the local dynamics?
The dispersal process influences the landscape at local and regional level. It determines the potential for membership in local assemblages and plays a critical role in the dynamics of metapopulations and species persistence (Levin et al., 2001). The water-column processes have proved to influence meiofaunal recruitment and colonization of different areas (Palmer, 1988). The ability of species to detect and move along environmental gradients was already explored in some studies (Fenchel, 1978). Gray (1967) did some experiments with the archiannelid Protodrilus rubropharyngeus and verified that they can actively choose were they land. In this case, they were attracted to spots that were already colonized by other individuals, implying that these are able to produce a chemical substance. Moreover, bacteria were inoculated with and without individuals and had a positive response on the settling of the nematodes. Moens et al. (1999a) also observed that nematodes can be influenced in choosing their food by the presence or absence of individuals from the same or different species.
The rate of dispersal may influence the community significantly at local levels. Predation and competition are some examples of factors that can provide it (Palmer et al., 1996). Palmer et al. (1996) created a model that takes in account the importance of regional and local controls to characterize certain kinds of communities relative to disturbance and dispersal. Although stating that the deep-sea communities are mainly controlled by local variables and is characterized by low disturbance and low dispersal, this can be different for canyons, which are considered dynamic environments.
Concerning nematodes, few studies focus on dispersal (Palmer, 1988; Palmer et al., 1996; Ullberg and Ólafsson, 2003) and all of them used organisms from shallow waters. There are numerous cosmopolitan nematode species (De Jésus-Navarette, 2007), although only 21% of all deep-sea genera recorded are restricted to a single habitat (Vanreusel et al., 2010).Despite the fact that nematodes do not have planktonic larval stages (Fenchel, 1978), the water-column processes might influence their recruitment and dispersal (Palmer, 1988). Palmer (1988) described 3 factors responsible for recruitment: availability of recruits; ability to reach new suitable sites, and ability to settle in these new areas. The continuous flux of individuals from the source to the sink and their residence time in the substrate can deeply impact the dynamics of local assemblages (Palmer et al., 1996).
The active ability to select a new place also depends on chemically recognizing the attractive spot (Ullberg & Ólafsson, 2003), as nematodes can be passively or actively present in the water column or move along the sediment to some extent (Palmer, 1988). The way in which they disperse is not well known, as they are poor swimmers and avoid the superficial sediment layers, moving deeper into the sediment when there is high flow, although moving upwards when the flow is reduced (Palmer, 1988). The presence of marine snow at the deep-sea bottom probably influences the recruitment of some species, as it creates disturbance and patchiness.
It is known, as previously mentioned, that nematodes can form aggregates due to food availability. Although intensively studied, patterns of their feeding ecology are quite contradictory or scant (Wieser, 1953; Jensen, 1987; Moens & Vincx, 1997; Moens et al., 1999a; Moens et al., 1999b). In the studies done by Moens et al. (1999a, b), it was verified that nematodes can be chemically attracted to bacteria and microalgae. These studies also demonstrated that one type of food may be, depending on its condition, attractive, unattractive or repulsive to the same nematode species, revealing their highly species-specific response and probably influencing the settlement of these individuals.
It was observed by Ullberg & Ólafsson (2003) that the sediment can also influence the settling of the nematodes and that smaller individuals are more active in choosing their preferred spot, while genera represented by larger individuals showed no distinct choice of habitat. Ullberg & Ólafsson (2003) stated that for the big nematodes the viscosity is not sufficient to let the sinusoidal wave propagation that nematodes use for swimming. This effect can increase in importance with the dwarfism observed in deep-sea.
The main aim of the project is to evaluate, through ex-situ experiments, if the nematodes from the Whittard canyon area have the ability to select their habitat (substrates with different food sources) when descending to the seafloor after a resuspension event, and to assess their ability to disperse actively towards different food sources. These experiments are combined with the analysis of medium-scale cross-canyon samples to investigate nematode heterogeneity, diversity and eco-functioning of this highly dynamic canyon environment.
Located at the Celtic margin, the Whittard canyon is considered one of the most important canyons apart its size (Duineveld et al., 2001; Otto & Balzer, 1998). It extends from the shelf break of the Celtic Shelf south of Goban Spur to the abyssal plain and contains deeply incised branches with side walls bigger than 500m (Van Rooij et al., 2010) and is influenced by strong tidal currents (Holt & Thorpe, 1997). As the distance from the slope increases, the sediment grain size decreases, consisting of fine to very fine sand and silt, and the CaCO3 content increases due to a growing marine input (Duineveld et al., 2001). According to Reid & Hamilton (1990), the Whittard system seems to be inactive in terms of massive sediment transport and the sedimentation is mainly hemipelagic. Despite this, the canyon can still act as a trap for organic material by semi-diurnal tides and seems to be enriched locally compared to the open slope (Goban Spur), and can act as a connection to the abyss (Duineveld et al., 2001).
Ex-situ dispersal experiments
Azoic (500°C, overnight), homogenised sediment collected during the R/V Belgica campaign ST0613 in 2006 was used to fill the petri dishes used in the experiments.
The nematodes used in the experiments were collected at 800 (48°46.4944'N; 10°37.9565'W) and 812m (48°46.5009'N; 10°38.4987'W) depth of the Belgica expedition (10/17b) realized in june/2010. They were separated from the sediment (sieving through 1000 µm and on 32 µm mesh) to use for the experiments. Three replicates (considered pseudoreplicates in terms of sampling) were used for each experiment.
Passive settling experiment
A settling chamber was built, which comprised a plastic cylinder of 150L capacity. The cylinder was filled with filtered sea water and the settling distance from the surface was approximately 50cm.
For each replicate, 5 plastic petri dishes were used, 1 with algae, 1 with bacteria, 1 consisting of a sulphidic medium and 2 controls (one with and one without azoic sediment). The algae treatment contained a fine layer of azoic sediment and 0.9g of pelagic diatoms aggregations previously frozen in the Marine Biology department of Gent University. The bacteria treatment contained a fine layer of azoic sediment together with 20mL of benthic bacteria solution (0.001537 g/1.5ml dry weight). The sulphidic medium consisted of 50mL of sulphidic agar The control containers were filled with a fine layer of azoic sediment or kept empty. The settling containers were placed equidistantly from each other in the plastic tray, which was perforated to allow the water run off after the experiment.
At the beginning of the experiment the plastic cylinder was filled with sea water and the nematodes were spread over its surface. The cylinder was left for four hours and afterwards the water was siphoned off and the individuals of each treatment were transferred to Borax-buffered formalin (4% end concentration)
Active dispersal along the sediment
A squared plastic box with external dimensions of 400x300x270mm and internal dimensions of 367x269x216mm was used for this experiment. For each replicate, 10 Cut-off sampling bottles red cap (5cm high, 6cm inner diameter) were used, 2 with algae, 2 with bacteria, 2 consisting of a sulphidic medium and 4 controls (2 with and 2 without azoic sediment) . The algae treatment contained azoic sediment together with 1.6g dwt microalgae (Tetraselmis suecica, dark green freeze dried powder, particle size after resuspension 10-15 µm, cell density per g dwt 2-5 x 10^9). The bacteria treatment was composed by azoic sediment together with 45mL (0.001537 g/1.5ml dry weight) of unidentified bacteria strains from Paulina samples cultured in rich broth (2x2L) for 36 hours on a shaker of 120rpm then centrifuged and washed several times (5000 rpm 15min) to concentrate the microbial solution. The Sulphidic treatment contained 50mL of sulphidic agar. One control contained azoic sediment and the other was empty. The petri dishes were placed equidistantly from each other and randomly displayed inside the tray.
A bigger plastic container, 1 Cut-off sampling bottles Nalgene (5cm high, 7.7 cm inner diameter), was disposed in the middle of the petri dishes containing sediment together with nematodes. The plastic tray was filled with sea water and the experiment was left for 40 hours. After that, the water was siphoned off and the individuals of each treatment transferred to Borax-buffered formalin of 4%.
To extract the fauna from the sediment, each sample was separated in two sieves of 1µm (macrofauna) and 32µm (meiofauna) to separate the macrofauna from the meiofauna. The meiofauna was centrifuged with Ludox© (density =1.18) three times to separate the meiofauna from the sediment, and the supernatant transferred to formalin 4%.
The samples are going to be enumerated under a dissecting microscope at 50x magnification to the highest taxa. For the genera identification, nematodes will be selected under dissecting microscope and treated according to De Grisse (1969). Afterwards, the 100 first individuals are going to be mounted on permanent slides and identified at genus level according to Lorenzen (1994), De Ley et al. (2006) and pictorial keys of Platt & Warwick (1988).
Characterization of the meiofauna community and the environment
All samples (table 1) were sliced in 1cm slices down to 10cm sediment depth (when available). Faunal samples were preserved in Borax-buffered formalin lengthened with filtered (32 Î¼m) sea water to an end concentration of 4%. Cores suitable for environmental characterization were processed with rhizons to extract interstitial water and were subsequently stored at -20°C. 1ml of the water was injected in a Zinc Acetate solution (stored in a capped vial at -20°C) to capture the sulphide compounds whilst the left over water was stored at -20°C for nutrient analysis. Also molecular samples were collected and stored in 100% molecular grade ethanol for further study of connectivity and possible molecular identification of nematode-bacteria endosymbionts for certain nematode genera. Additional cores were used for experimental treatments or backup faunal characterization. The retrieved sediment cores gave visual confirmation of the heterogeneity of the seafloor in this area, as also observed by the ROV video footage. The efficiency at the canyon walls was higher due to the presence of softer sediments, enabling easier penetration by the corers. In the thalweg of the canyon branch, sediment penetration was hampered by the presence of compacted layers of grey clay mixed with sandy turbidites and anoxic strata. All other sediment cores (apart from the last deployment) from other stations showed typical soupy, muddy surface layers with more compacted mud in deeper layers. The last station visited was the shallowest at 574 m water depth. Here, a coarser grain size was noticed in comparison with previously taken samples.
The extraction of the fauna was the same as the one reported for the ex-situ experiments.
Table . Location of the sample sites.
Analyses are going to be performed using PRIMER/PERMANOVA v6 software (Clarke & Gorley, 2006) for the statistical multivariate analysis in order to compare treatments and look for significant differences and trends.
Justifications and hypotheses
Free-living marine nematodes are often encountered in the water column, especially in high energy areas. Strong currents and turbidity events can cause nematodes to be resuspended after which they may resettle as particles depending on the intensity of the hydrodynamic environment. Such dislodgement is important for less-mobile taxa to achieve dispersal. The Whittard Canyon along the Irish continental margin is a physically heavily disturbed environment that receives high amounts of sediments and organic material coming from the shelf or surface waters. The main aim of the project is to evaluate, through ex-situ experiments, if the nematodes from this canyon area have the ability to select their habitat (substrates with different food sources) when descending to the seafloor after a resuspension event, and to assess their ability to disperse actively towards different food sources. These experiments are going to be combined with the analysis of medium-scale cross-canyon samples to investigate nematode heterogeneity, diversity and ecofunctioning of this highly dynamic canyon environment.
This work is part of the Master in Nematology and is sponsored by the programme VLIR-UOS from the Flemish government.
It is part of a bigger project that combines geological, ecological and physical aspects, HERMES/HERMIONE, which aims to study deep-sea environments on the Irish Continental margin in the Bay of Biscay. This project had the samples collected in the Belgica cruise, in june 2010, and was coordinated by Prof. David Van Rooij.
The project had September 2010 as the starting point and intends to be concluded at September 2011.