Nematode assemblages from the Kandalaksha Depression (White Sea, 251–288 m water depth)

Abstract

The shallow-water nematodes of the White Sea are relatively well studied; however, information on the nematode fauna inhabiting the deepest part of this sea is very scarce. The composition of the nematode assemblages (at species and genus level) was studied in samples collected during four sampling occasions in the deepest part of the Kandalaksha Depression (the White Sea) in July 1998, October 1998, May 1999, and November 1999. Samples were collected from a depth of 251–288 m with the aid of a multicorer. In total, 59 nematode morphotypes belonging to 37 genera and 18 families were distinguished. The genera Sabatieria and Filipjeva dominated at all stations, followed by Aponema, Desmoscolex, and Quadricoma. The composition of the dominant genera can be considered typical for this depth range in temperate and Arctic waters, although Filipjeva and Aponema were among the dominant genera for the first time. The most abundant species were Sabatieria ornata, Aponema bathyalis, and Filipjeva filipjevi. In general, diversity of the nematode assemblages was lower than in the temperate and Arctic continental shelf and slope with reduced evenness and species richness. The evenness of nematode assemblages and other diversity indices decreased with increasing sediment depth. Based on the valid species and genera recorded, the nematode fauna of the Kandalaksha Depression showed a higher resemblance to that found in the shallow waters of Kandalaksha Bay.

Introduction

The White Sea is a small marginal shelf sea separated from the Arctic Ocean by the shallow and narrow Gorlo Strait (Filatov et al. 2005; Berger et al. 2001), resulting in relatively unusual conditions in its deepest regions (small depressions with a maximum depth of 343 m): the deep water is very cold (about −1.5 °C) and not fully saline (29.5–30.0 ‰) (Loeng 1991; Berger and Naumov 2000). This type of marginal shelf depression, detached from the main oceanic water body by shallow sills, is called the “pseudo-bathyal” (Andriashev 1977).

The macrobenthic fauna has been well studied in this sea (for the most comprehensive and up-to-date catalog of the White Sea biota, see Tchesunov et al. (2008)). However, information on the diversity and distribution of meiobenthic organisms is rather scarce, and mostly concerns the tidal and subtidal zones.

The first nematode species recorded from the White Sea were described by Filipjev (1927), although more regular studies of the White Sea nematofauna did not start until the 1970s (Frolov 1972; Galtsova 1976, Aminova and Galtzova 1978; Galtsova 1982; Belogurov and Galtzova 1983; Tchesunov and Krasnova 1985; Mokievsky 1990). In the tidal zone, studies on nematode assemblages and species distribution (Galtsova 1991), vertical distribution of nematode species (Galtsova 1982), and seasonal dynamics of nematode species and assemblages (Galtsova and Aminova 1978; Mokievsky 1990; Krasnova 2007) have been undertaken.

The nematofauna from the upper subtidal zone of the White Sea has been found to be more diverse than the tidal zone, but has been studied far less: the portion of unknown species among the subtidal nematodes is still much higher that among the tidal. Nematode studies in the upper subtidal have been mainly performed at two Russian biological stations belonging to the Moscow State Universty and the St. Petersburg Zoological Intstitute of the Russian Academy of Science. Most of these studies were taxonomic (see, e.g., Tchesunov and Krasnova 1985; Tchesunov 1987, 1988a, b, 1989, 1990a, b, c, 1993, 1996, 2000a, b; Platonova and Mokievsky 1994; Decraemer and Tchesunov 1996; Okhlopkov 2002; Tchesunov and Miljutina 2002, 2008; Kovalyev and Tchesunov 2005; Tchesunov and Milyutin 2007). In total, 127 nematode species were reported from the tidal and upper subtidal zones near the White Sea Biological Station (Moscow State University) in the Kandalaksha Bay (Tchesunov and Walter 2008).

Nematodes from the lower sublittoral and pseudobathyal of the White Sea have been little studied, and few new species have been described from these depths (Tchesunov 1988a; Tchesunov and Miljutina 2008; Kovalyev and Miljutina 2009). Galtsova and Platonova (1988) and Galtsova (1991) analyzed nematode species distributions from the tidal zone to 300 m water depth and detected three main nematode assemblages, each inhabiting a specific depth range and sediment type.

In 1998, a cooperative German–Russian scientific program “The investigation of the deep-sea ecosystem of the White Sea” was initiated with support from the International Association for the Promotion of Co-operation with Scientists from the New Independent States of the Former Soviet Union (INTAS) (Rachor 2000). Through this project, significant effort was made to collect qualitative and quantitative data on the deep-sea meiofauna of the Kandalaksha Depression. Four cruises (July 1998, October 1998, May 1999, and November 1999) collected mini-corer samples for meiobenthic studies from a depth of 251–288 m, and initial findings (based on the first cruise) were reported by Mokievsky et al. (2009). Miljutin et al. (2012) used the whole set of data (four cruises) to describe the patterns in density, relative abundance, and size spectrum of the major meiobenthic taxa. The most abundant meiobenthic group was Foraminifera (59 %), followed by Nematoda (26 %) and Harpacticoida (7 %). These relative and absolute abundance values were comparable with those from the same depth interval in Arctic and temperate regions. The densities of foraminiferans and nematodes were higher in Autumn and lower in Summer, reflecting a mass propagation event dependent on the influx of primary production from surface waters. The size range of the meiobenthos in the deepest part of the White Sea was comparable to that of deep-sea meiobenthos, in which the 63–125-μm-size class and 125–250-μm-size class were most dominant.

The aim of the present work was to describe the diversity and composition of nematode assemblages from the above-mentioned deep-sea site of the Kandalaksha Depression, since nematodes were the most abundant meiobenthic Metazoan in the samples.

Materials and methods

The sediment in the sampling area was a liquid, clayey mud. A more detailed description of the sampling area is given by Miljutin et al. (2012).

Meiobenthos was collected during four cruises of the RV “Kartesh” and the RV “Professor Kuznetsov” (Zoological Institute, Russian Academy of Sciences). Samples were obtained in July and October 1998 and in May and November 1999 (Table 1, Fig. 1). For more detailed information on location of stations, see Miljutin et al. (2012). One station was sampled on each cruise (i.e., four stations in total).

Table 1 Sample details

Fig. 1

Map of region and site of sampling (marked with black circle)

Samples were taken using a mini-corer bearing four plastic corers with an internal diameter of 5.4 cm. In total, nine samples (three deployments, three cores from each deployment) were collected in July 1998; six samples in October 1998 (two deployments, three cores from each deployment); five samples in May 1999 (from three deployments); and three samples (one deployment, three cores) in November 1999. It was not possible to acquire triplicate samples for every deployment at station CBB-23 owing to technical problems.

Meiobenthos was collected from the corers using cutoff syringes with an internal diameter of 2 cm (one syringe per core). Each syringe was divided into five subsamples representing 1-cm-thick layers from the surface down to 5 cm sediment depth (0–1, 1–2, 2–3, 3–4, and 4–5 cm). All subsamples were fixed a solution of 10 % formalin in seawater.

In the laboratory, subsamples were washed over a 32-μm mesh sieve, stained with 1 % rose bengal, and sorted under a stereo microscope using a Bogorov counting chamber. Nematodes were picked out, processed in glycerin using the Seinhorst’s method of slow evaporation (Seinhorst 1959), and mounted on permanent glycerin–paraffin slides. The nematodes were then examined under a light microscope. In total, 3,145 nematode individuals were examined (Table 1).

The software packages PAST (Hammer et al. 2001) and PRIMER v6 (Clarke and Gorley 2006) were used for statistical analysis.

Nematode diversity in the uppermost 5-cm sediment layer at species and genus level was measured using Margalef’s (d), log₂- and log_e-based Shannon-Wiener (H′), Hill′s (N _∞ ), and Pielou’s (J′) indices, and richness was also estimated as ES(51) and ES(100) for species level, and EG(51) and EG(100) for genus level (51 and 100 being a standardized number of specimens). For the latter index, only samples containing not less than 51 and 100 individuals, respectively, were analyzed. For the comparison of nematode diversity from different 1-cm-thick sediment layers, ES(31) for species was calculated (owing to the small sample size), as well as d, J′, and H′ indices.

Analysis of similarity (ANOSIM) based on Bray-Curtis similarity distances was used to analyze the multivariate data (square-root-transformed relative abundances of nematode species in each samples). One-way and two-way nested ANOSIM tests were used to compare differences between stations, deployments, and sediment layers. Similarity percentage analysis (SIMPER) based on Bray-Curtis similarity distance was used to assess which taxa were primarily responsible for any observed differences between groups of samples. The significance of differences in the ratio juveniles/adults and diversity indices was tested using two-way ANOVA, Kruskal–Wallis, and Mann–Whitney U tests. The significance of the deviation of the ratio males/females from 1 was tested by a χ ² test. Two factors were used for the two-way ANOVA: “Year” (1998 and 1999) and “Season” (Summer and Autumn).

The vertical distribution of nematode assemblages was studied for the 0–1-cm, 1–2-cm, and 2–3-cm sediment layers only because of the limited number of specimens in the 3–4-cm and 4–5-cm sediment layers. In order to enlarge the sample volume, and therefore the number of specimens available for analysis, all samples from the same deployment were merged.

The pairwise comparison of the nematode taxa found in the Kandalakscha Depression (presence/absence matrix of valid species and genera) with other nematofaunas described from the White Sea was undertaken using Simpson’s resemblance index (Simpson 1960):

$$C/N_{\hbox{min} } \times 100,$$

where C is the number of taxa common to both nematofaunas; N _min is the number of taxa in the nematofauna with the shorter list of taxa. To this aim, a list of tidal and subtidal nematode species found in the area of the White Sea Biological Station (Moscow State University, Russia) in the Kandalaksha Bay (Tchesunov and Walter 2008) was created, as well as lists of tidal nematofauna from other area of the Kandalaksha Bay and nematofaunas from 2, 5–15, and 18–300 m depths from the central part of the White Sea (Galtsova 1991).

During the examination of nematode specimens, the life stage (juvenile or adult) and gender (for adult specimens) were recorded. Patterns in the proportions of juveniles, males, and females were studied separately for the three most abundant species, Filipjeva filipjevi (211 individuals in total), Aponema bathyalis (234 individuals), and Sabatieria ornata (619 individuals), and pooled for all other species (2,149 individuals).

Results

Nematode assemblage composition

In total, 59 nematode morphotypes were distinguished, which could be allocated to 37 genera from 18 families. The most species-rich families were Xyalidae and Diplopeltidae (ten species in each), followed by Desmoscolecidae and Chromadoridae (six and five species, respectively). About 55 % of genera were represented by only one species, and 34 % of genera had 2 species. The most species-rich genera (each containing three species) were Desmoscolex, Campylaimus, and Filipjeva (Table 2). The average number of species per genus and per family was 1.6 and 3.2, respectively.

Table 2 List of identified taxa and the number of species (morphotypes) determined

Nineteen morphotypes were identified as known species (Table 3). Of these, three were described from the present samples (Aponema bathyalis, A. minutissima, and Intasia monhystera) (Tchesunov and Miljutina 2008; Kovalyev and Miljutina 2009). Three valid species (Sabatieria ornata, Filipjeva filipjevi, and A. bathyalis) were the most abundant in the nematode assemblages described here. Four valid species (Aegialoalaimus elegans, Filipjeva arctica, Gerlachius lissus, and S. ornata) were described from the White Sea for the first time (Habitat “A”). About one half (nine) of the valid species were previously known from the tidal and/or subtidal zone of the Kandalaksha Bay.

Table 3 Average density and relative abundance of valid nematode species found in the deepest part of the Kandalaksha Depression (averaged across 4 cruises), with known habitat indicated

According to the analysis of Simpson’s resemblance (based on a presence/absence matrix of found valid species and genera), the nematofauna from the Kandalaksha Depression was most similar to the subtidal nematode fauna from the Kandalaksha Bay in the area of the White Sea Biological Station (Fig. 2, Table 4). This resemblance is not strong (ca. 28 % at species level, ca. 53 % at genus level); however, its similarity to other nematofaunas from the White Sea was even less (Fig. 2).

Table 4 Site information and number of valid nematode taxa recorded at different locations in the White Sea used for resemblance analysis (Simpson’s resemblance index)

Fig. 2

Cluster analysis ordination to compare nematode assemblage presence/absence data from the current study with that from other White Sea sites. Based on average Simpson’s resemblance index. Abbreviations: CP Central pWhite Sea, KB Kandalakscha Bay. Sources: KB 251–288 m (present data); KB tidal zone 1 and KB subtidal zone (Tchesunov and Walter, 2008); CP 2 m, CP 5–12 m, CP 18–300 m, and KB tidal zone 2 (Galtsova 1991). For more detail on the literature sources, see Table 4

The nematofauna from the Kandalaksha Depression did not resemble the other deep-sea (18–300 m) nematofauna (Galtsova 1991); it was only 4 % similar at species level and 33 % at genus level. The relative abundances of genera differed markedly too (Table 5). The genera Metadesmolaimus and Aegialoalaimus were most abundant in the Galtsova’s study, whereas these genera were not numerous or not found at all in the present study. In contrast, the genera Sabatieria and Filipjeva dominated in the present study, but were much less abundant or not mentioned in the Galtsova’s study.

Table 5 Proportions (averaged over all full cores) of the 10 most abundant genera and their contribution to dissimilarities (%, result of SIMPER test) between the relative abundance of nematode genera from the present study and from a similar depth range in the White Sea Basin (data from Galtsova 1991)

The three most abundant species, S. ornata, A. bathyalis, and F. filipjevi, comprised 19.2, 7.3, and 6.6 % of total nematode abundance, respectively. The maximum density of these species was recorded in October 1998 (176.6 ind./10 cm², 85.0 ind./10 cm², and 72.6 ind./10 cm², respectively) (Table 6).

Table 6 Density (inds/10 cm², mean ± standard error) and relative abundance (%, mean ± standard error) of the 10 most abundant nematode species, genera, and families inhabiting the uppermost 5-cm sediment layer

At genus level, Sabatieria and Filipjeva were dominant at all stations, followed by Aponema, Desmoscolex, and Quadricoma. The families Comesomatidae and Xyalidae dominated at three of the four stations, while Xyalidae and Desmoscolecidae dominated at CBB-34 with Comesomatidae being the third most abundant family (Table 6).

A two-way nested ANOSIM test showed significant differences in relative abundance of nematode species between stations (p = 0.02), but there was no difference between deployments (p = 0.22). The differences between assemblages at the stations mainly reflected differences in the proportions of the most abundant species (A. bathyalis, S. ornata, F. filipjevi, Desmoscolex sp.).

The Kruskal–Wallis tests indicated no significant differences in diversity indices between different stations for the top 0–5 cm of the sediment (p values in all pairwise comparisons were >0.05), and therefore, the values were averaged across stations. The diversity indices are given in the Table 7.

Table 7 Diversity indices (mean ± standard error) for the nematode assemblage from the deepest part of the Kandalaksha Depression at species and genus level

Proportions of life stages and genders

The proportion of juveniles in the top 0–5 cm of the sediment varied from 36 to 88 % for all species (average 52.8 %), with no dependence on season or year of sampling detected (Table 8).

Table 8 Two-way ANOVA for the factors sampling season (“summer” vs. “autumn”) and sampling year (1998 vs. 1999) effects on the ratio “adults/juveniles” across the whole dataset and for the three most abundant species separately

The proportions of life stages and genders were examined separately for the three most abundant species (F. filipjevi, A. bathyalis, and S. ornata) and pooled for all other species (Fig. 3). On average, F. filipjevi was represented by 55.2 ± 4.5 % juveniles, while A. bathyalis, S. ornata, and the remaining pooled species were 63.8 ± 10.9 %, 77.2 ± 2.8 %, and 45.7 ± 1.9 % juveniles, respectively. There was a significantly greater proportion of juvenile S. ornata than for both F. filipjevi and the remaining pooled species (Mann–Whitney U test: p = 0.03 for both pairwise comparisons).

Fig. 3

Proportions (%) of life stages and genders of nematode assemblages inhabiting the deep White Sea. Each of four cruises shown separately. Abbreviations used in the legend: J juveniles, F females, and M males

The average ratio of males/females in F. filipjevi, A. bathyalis, S. ornata, and the remaining pooled species was 1.88 ± 0.09, 2.39 ± 0.14, 0.84 ± 0.04, and 0.57 ± 0.01, respectively. Thus, it was significantly greater than 1 in F. filipjevi and A. bathyalis (χ ² test: p < 0.01 for both species) and significantly less than 1 for the pooled species (χ ² test: p < 0.01). There was no significant difference in the proportion of males and females in S. ornata (χ ² test: p = 0.23).

Vertical distribution

The one-way ANOSIM test separately preformed for “Station,” “Deployment,” and “Sediment layer” factors indicated no significant influence of the “Deployment” factor (p = 0.07) on the composition of nematode assemblages at species level, but the factors “Station” and “Sediment layer” were significant (p < 0.03 and 0.01, respectively). Of the most abundant species, S. ornata and F. filipjevi normally dominated the 1–2-cm and 2–3-cm sediment layers. All stations were dominated by the same species complex, but their ranks varied (Table 9).

Table 9 Relative abundance (%) of the 5 most dominant nematode species at each station, abundances shown for each sediment layer (0–1, 1–2, and 2–3 cm)

The similarity within samples from different sediment layers increased with increasing sediment depth: average similarity within samples from 0–1-cm, 1–2-cm, and 2–3-cm sediment layers was 29, 41, and 53 %, respectively (results of one-way SIMPER analysis for the factor “Sediment layer”).

The evenness of nematode assemblages decreased with increasing sediment depth (Table 10), as did other diversity indices. Kruskal–Wallis tests indicated no significant differences in diversity indices between nematode species assemblages from the 0–1-cm and the 1–2-cm sediment layers, but there were significant differences between these layers and the 2–3-cm sediment layer. Diversity indices were significantly lower in the 2–3-cm sediment layer (Table 11).

Table 10 Diversity indices for the nematode species assemblages by sediment layer (0–1, 1–2, and 2–3 cm)

Table 11 Pairwise comparison of diversity indices of the nematode species assemblages in each sediment layer (0–1, 1–2, and 2–3 cm)

There was also a significant difference (Kruskal–Wallis test: p < 0.04) in proportions of juveniles between the 0–1-cm and 2–3-cm sediment layers (50.4 and 61.2 %, respectively). The proportion of juveniles in 1–2-cm sediment layer was intermediate (56.6 %) and did not differ significantly from the upper or lower layer.

Discussion

Assemblage composition and structure

A significant difference in the composition of nematode assemblages between different stations (sampling occasions) was reported. However, the sampling strategy did not allow replication of stations over time (stations differed from each other not only in date of sampling, but also in their geographical position and depth), and this created difficulties in the interpretation of results, since apparent differences between sampling occasions could be affected not only by the time period but also by spatial parameters. Nevertheless, it was previously shown that a significant part of the variance in total meiobenthos density in these samples was explained by seasonality (Miljutin et al. 2012). Yet, other studies have found no significant seasonal variation in the composition of marine nematode assemblages under stable environmental conditions (Warwick and Buchanan 1971; Pavlyuk 2000). Temperature seems to be very stable in the Kandalaksha Depression, but significant seasonal variation in sedimentation from primary production has been recorded (Miljutin et al. 2012). This variability in sedimentation rates may induce seasonal mass propagation of particular species, resulting in changes to the composition of deep-sea benthic communities (Gooday 2002).

The genera Sabatieria and Filipjeva were the most abundant at all stations. Sabatieria has been characterized as the most common dominant genus on the shelf break (Soetaert et al. 1995). Often it has been recorded as one of the most abundant genera from the lower shelf to the medium slope and in canyons in the temperate and Arctic waters (Tietjen 1976; Vanreusel et al. 1992; Soetaert and Heip 1995; Soetaert et al. 1995; Vanaverbeke et al. 1997b; De Leonardis et al. 2008). Sabatieria was not found to be abundant over the same depth range in the Arctic Laptev Sea, however (Vanaverbeke et al. 1997a). Similar to slope and other shelf studies (Soltwedel et al. 2009; Vanreusel et al. 2010), the genus Desmoscolex was abundant at some stations. This is the first time, however, that the genera Filipjeva and Aponema have been noted as dominant or subdominant. Thus, the composition of the dominant nematode genera in the deepest part of the White Sea can be considered typical for this depth range in temperate and Arctic waters, but with a new record of dominance for the genera Filipjeva and Aponema.

The studied nematode assemblage showed the highest resemblance (presence/absence of found valid species and genera) with the assemblage from the subtidal zone in the area of the White Sea Biological Station (WSBS) in the Kandalaksha Bay. A possible reason for this is that the area of the WSBS is the most studied area of the White Sea, resulting in the longest list of nematode taxa in comparison with other studied areas of the White Sea (Table 4). It should be remembered, however, that only 19 of the 59 distinguished morphotypes were attributed to known species, and this may indicate that many species in the studied area do not also occur in the shallow waters of Kandalaksha Bay.

The relative abundance of genera differed markedly from that recorded for a comparable depth range in the White Sea Basin, 200 km from our sampling site (Galtsova 1991). It is difficult to give an unambiguous explanation of this discrepancy. Firstly, it might be explained by possible differences in environmental conditions in these two areas (they are about 200 km apart). The difference in depth ranges (251–288 m in the present study vs. 18–300 m in the Galtsova’s study) could also cause the difference between the nematode assemblage compositions. Unfortunately, having described the nematode assemblage from the 18–300 m material, Galtsova (1991) did not state which specimens were taken from which depth, only that the nematode assemblages were homogenous across this depth range. Alternatively, it might simply reflect a different methodology—Galtsova (1991) used a 90-μm mesh sieve, while a 32-μm sieve was used in the present study. It was shown that about 40 % of nematodes from the Kandalaksha Depression passed through a 125-μm mesh (Miljutin et al. 2012), and consequently, it is possible that Galtsova (1991) lost a considerable number of smaller specimens and species, thereby influencing the nematode assemblage composition recorded.

Diversity

According to the literature data on diversity of nematode assemblages (at species level) from the northern temperate and Arctic continental shelf, slope, and abyss, J′ varied from 0.93 to 0.96 in the samples from the Bay of Biscay (Dinet and Vivier 1979); from 0.85 to 0.95 in the Norwegian Sea (Jensen 1988), and from 0.88 to 0.95 at the NE Atlantic slope (Danovaro et al. 2009). The mean J′ values were 0.85–0.89 for the temperate subtidal zone and the slope (Boucher and Lambshead 1995), 0.83 for the Arctic abyss (Gallucci et al. 2008), and 0.87 for the Haterras Abyssal Plain (Tietjen 1989). In the present study, J′ was lower (mean, 0.79). Also, in the literature H′(log₂) was 5.23–6.67 (Dinet and Vivier 1979), 4.1 (Tietjen 1989), and 3.79–5.01 (Jensen 1988), and its mean values were 4.88–5.00 (Boucher and Lambshead 1995). Again, this index value was lower in the present study (3.54). Gallucci et al. (2008) reported the mean value of H′(log_e); this was 3.5 and was also higher than in the present study (2.43). Finally, Boucher and Lambshead (1995) and Lambshead et al. (2000) reported mean values of ES(51) from 24 to 31, while for the Arctic deep seas at the 2,000-m isobath Fonseca and Soltwedel (2009) reported ES(50) varied from 27 to 38 and Danovaro et al. (2009) reported ES(100) from 48 to 65. The ES(51) and ES(100) indices produced significantly lower values in the present study (16.1 and 21.9, respectively).

At genus level, the diversity indices were also lower compared to previous studies. Vanaverbeke et al. (1997a, b) indicated N∞ values of 5–10 for nematode assemblages from the NE Atlantic and the Laptev Sea (vs. 3.5 on average for the present study). On the Arctic Yemark Plateau, Soltwedel et al. (2009) reported values of H′(log₂) and J′ were 4.2–4.6 and 0.80–0.89, respectively (vs. 3.2 and 0.784, respectively, on average in the present study): H′(log_e) averaged 3.0 in the Arctic abyss (Guilini et al. 2011) versus 2.2 in the present study. Finally, EG(100) was 31–39 (Vanaverbeke et al. 1997b) and 24 − 32 (Soltwedel et al. 2009) in comparison with a mean value of 17.5 in the present study.

There were several exceptions to the above pattern, where the diversity of the nematode assemblages was comparable with the present data. These were the studies (performed at genus level) by Renaud et al. (2006) on nematode assemblages from the Arctic abyss and by Sebastian et al. (2007) on abyssal nematode assemblages from the Cape Verde Abyssal Plain and the Porcupine Abyssal Plain. In the first study, the J′ was 0.64–0.75, and H′(log_e) was 2.08–2.45, compared to mean values of 0.78 and 2.20, respectively, for the present data. In the latter study, the N∞ values were 3.0–4.2 compared to 3.5 for the present study, but the H′(log_e) was nevertheless higher (2.4–2.6). Thus, the nematode assemblage of the deepest part of the White Sea appeared to be reduced in diversity in comparison with most studied temperate and Arctic regions.

The diversity indices for nematode assemblages in the different sediment layers decreased with increasing sediment depth: nematode assemblages inhabiting the 2–3-cm sediment layer exhibited stronger dominance and significantly lower species diversity than in the 0–1-cm and 1–2-cm layers. Decreasing nematode diversity with sediment depth was previously shown by Fonseca et al. (2010) in more detail. Generally, the dominance of S. ornata and F. filipjevi was higher in the deeper sediment layers.

The examined samples were taken in different seasons during different years, and it is possible that the composition of the nematode assemblage was impacted by environmental factors that varied seasonally and/or annually. However, it has been shown that the major taxon composition of the meiobenthos in the deep White Sea does not change with time and only varies with sediment depth (Miljutin et al. 2012). Unfortunately, it was impossible to test this hypothesis here at the species level because nematodes were examined from a relatively small number of samples.

Life history traits

The proportion of juveniles was maximal in the 1–2-cm sediment layer. It has been already shown that density of juveniles also was the highest in the 1–2-cm sediment layer, while adult densities were similar in the 0–1-cm and 1–2-cm layers (Miljutin et al. 2012). Possibly, juveniles prefer inhabiting this subsurface sediment layer.

The proportion of juvenile S. ornata was significantly higher than for many other species. Indeed, the proportion of juveniles for the three most abundant species were higher than for other less abundant species. However, the size of nematode species or the age of juveniles could also influence the resulting proportion of juveniles in samples, with smaller larvae being lost to a greater extent than larger ones during sample processing.