Journal of Experimental Marine Biology and Ecology 440 (2013) 35–41

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Journal of Experimental Marine Biology and Ecology

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Effect of a physical disturbance event on deep-sea nematode community structureand ecosystem function

Daniel Leduc a,b,⁎, Conrad A. Pilditch c

a Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealandb National Institute of Water and Atmospheric Research, Private Bag 14-901, Wellington 6021, New Zealandc Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand

⁎ Corresponding author at: National Institute of WatePrivate Bag 14-901, Wellington 6021, New Zealand. Tel

E-mail address: Daniel.Leduc@niwa.co.nz (D. Leduc)

0022-0981/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jembe.2012.11.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 September 2012Received in revised form 20 November 2012Accepted 20 November 2012Available online xxxx

Keywords:BiodiversityChatham RiseContinental slopeLaboratory experimentMeiofaunaResilience

Numerous studies have been conducted on the effect of physical disturbance on shallow water benthic commu-nities, but there is a paucity of data from deep-sea environments. We conducted a laboratory experiment usingundisturbed sediment cores from Chatham Rise (water depth=345 m), Southwest Pacific, to investigate the ef-fects of a physical disturbance event (resuspension of surface sediments) on sediment characteristics (sedimentgrain size, pigment content), nematode community attributes (abundance, diversity, community structure) andecosystem function (sediment community oxygen consumption (SCOC)) over a period of 9 days. Disturbancedid not have any noticeable impact on sediment characteristics, SCOC, or nematode species richness, but led tochanges in vertical distribution patterns and shifts in nematode community structure. The magnitude ofdisturbance-related effects was, however, much smaller than the effect of sediment depth (0–1, 1–3, and3–5 cm), and the main impact of disturbance on nematode vertical distribution patterns and communitystructure appeared to be related to a vertical re-shuffling of nematodes in the sediments rather than mor-tality. We did not observe substantial increases in the abundance of nematode genera generally regardedas disturbance-tolerant, such as Sabatieria. The worst-affected species belongs to the Stilbonematinae, agroup of typically long and slender nematodes that may be easily damaged by physical disturbance. Thelimited impact of physical disturbance on benthic community structure and function suggests that the Chat-ham Rise nematode community is relatively resilient to sediment resuspension. This resilience may havearisen from frequent exposure to disturbance in the field (e.g., from strong currents), or may be a morewidespread feature of nematode communities.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Disturbance has long been recognised as an important factor struc-turing communities (Connel, 1978; Petraitis et al., 1989). Numerousstudies have shown that disturbance can influence the abundance, di-versity, and community structure of faunal (e.g., Austen et al., 1998;Schratzberger et al., 2009) and microbial communities (e.g., Cowieand Hannah, 2007; Findlay et al., 1990) in marine benthic environ-ments. Changes in the structure of benthic communities are, in turn,likely to influence ecosystem processes such as secondary productivityand nutrient cycling (Danovaro et al., 2008; Karlson et al., 2010;Pusceddu et al., 2005). The relationship between disturbance, benthiccommunity structure, and ecosystem function, however, is likely to becomplex, and our knowledge of the processes involved remains patchy(Lee et al., 2001; Snelgrove, 1997).

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Physical disturbance, defined as the disruption of sediment particlescaused by abiotic (e.g., currents), biotic (e.g., bioturbation), and anthro-pogenic processes (e.g., bottom-trawling), is widespread in benthic sys-tems and occurs at a variety of spatial and temporal scales (Hall, 1994;Kaiser, 1998; Watling and Norse, 1998). Frequent and/or widespreaddisturbance, such as that caused by bottom trawling, has been linkedtomarkedly lower standing stocks and diversity of coastal benthic com-munities (Hinz et al., 2008; Reiss et al., 2009), as well as pronouncedshifts in function of soft sediment habitats (Pilskain et al., 1998; Puiget al., 2012; Reiss et al., 2009; Thrush and Dayton, 2002; Watling andNorse, 1998). At the local scale, disturbance may alter sediment grainsize structure, biogeochemical gradients, and availability of food re-sources, thereby favouring some species and inhibiting others (Thistle,1981; Widdicombe and Austen, 2001). Relatively little is known, how-ever, about the short-term response of deep-sea infaunal communitiesto local physical disturbance (Anhert and Schriever, 2001). We couldexpect, given the stable nature of deep-sea ecosystems relative totheir shallow counterparts, that a high proportion of specieswill be neg-atively affected by disturbance, resulting in a short-term decline inabundance and species richness (Grassle and Sanders, 1973). Any

Fig. 1. Map of study area (east of New Zealand's South Island) showing position ofstudy site (circle) and location of Subtropical Front (dotted lines).

36 D. Leduc, C.A. Pilditch / Journal of Experimental Marine Biology and Ecology 440 (2013) 35–41

negative impact on species richness, could, in turn, influence ecosystemfunctioning (Danovaro et al., 2008).

Most studies on the effect of physical disturbance on marine ben-thic communities have been conducted in relatively shallow waters(Hall, 1994; Thrush and Dayton, 2002). Quantifying the effect of dis-turbance on deep-sea benthic communities, however, remains achallenging task due to logistical reasons. The relatively low macro-andmegafaunal densities in deeper waters require obtaining a great-er number of samples of relatively large volume in order to detectecologically meaningful change (Gage and Bett, 2005; Rogers et al.,2008; Schratzberger et al., 2000), and conducting experiments indeep-sea settings is difficult and costly. Field experiments on the im-pact of deep-sea mining on the benthos have yielded some insightsinto the response of deep-sea communities to physical disturbance(e.g., Anhert and Schriever, 2001; Miljutin et al., 2011; Thiel andSchriever, 1990), but few laboratory experiments have been conductedon the response of deep-sea organisms/communities to physical distur-bance (Gallucci et al., 2008a).

Meiofaunal organisms, and nematodes in particular, are well suitedfor laboratory experiments because of their small size, high abundanceand wide distribution (Giere, 2009). Experiments can be conducted ondeep-sea nematode communities comprising hundreds to thousandsof individuals using relatively small volumes (b1 L) of sediment(Gallucci et al., 2008a). This approach is less costly than conducting ex-periments in situ and provides greater control over experimental condi-tion. Laboratory experiments are among the best tools for improvingour understanding of the short-term impacts of disturbance on commu-nity dynamics and ecosystem function at the scale of the disturbedpatch, and can provide new insights into the mechanisms involved(Schratzberger and Warwick, 1999; Schratzberger et al., 2009). Suchexperiments could, in addition, help identify which taxa are more sus-ceptible to disturbance (e.g. Schratzberger andWarwick, 1999), therebyunderpinning the use of nematodes for monitoring of deep-sea benthicecosystems (Miljutin et al., 2011).

The aim of the present study is determine the short-term (b10 days)effect of a one-off physical disturbance event on the abundance, spe-cies richness, and community structure of a bathyal nematode com-munity using a laboratory experiment. More specifically, we test thehypothesis that physical disturbance leads to mortality in a majorityof potentially disturbance-sensitive nematode taxa, resulting inlower abundance, species richness, and altered community struc-ture. The effect of disturbance on ecosystem function was also inves-tigated by comparing sediment community oxygen consumption(SCOC) between disturbed and undisturbed treatments.

2. Methods

2.1. Study area and sampling

The study site (43° 20.1′ S, 178° 17.5′ E) was located at 345 mwaterdepth on the crest of Chatham Rise, a submarine ridge extending east-wards from the South Island of New Zealand (Fig. 1). The rise encom-passes water depths from ca. 250 to 3000 m and lies beneath theSubtropical Front (STF), a region associated with heightened primaryproductivity (Bradford-Grieve et al., 1997; Murphy et al., 2001) andhigh levels of mixing and current activity (Nodder et al., 2007; Sutton,2001). Sediments on the crest of the rise are characterised by depositsof authigenic and biogenic silty sands with localised accumulations ofphosphorite nodules (Cullen, 1987; Orpin et al., 2008), which arebeing targeted by extractive industry.

Samples were collected on February 20, 2011 during National Insti-tute ofWater and Atmospheric Research (NIWA) cruise TAN1103 usingan Ocean Instruments MC-800Amulticorer (MUC; core internal diame-ter=9.5 cm). Faunal samples were obtained from three cores (from 2MUC deployments) and processed on board ship as described belowto provide field controls. An additional twelve cores from eight MUC

deployments were obtained for the disturbance experiment. Theupper 8–11 cm of sediment and the overlying water from each corewere transferred into incubation units (internal diameter=8.4 cm,height=30.0 cm) by subcoring (Nodder et al., 2003). Incubation unitswere then sealed, kept in the dark at bottom water temperature(8.0 °C±0.5 °C,), and aerated to ensure adequate oxygen supply. Incu-bation units were transferred to a water bath (8.0 °C±0.5 °C) in thelaboratory 5 days after sampling, and were connected to a recirculatingseawater system at same temperature for two additional days prior tothe beginning of the experiment. Constant seawater flow ensured ade-quate oxygenation of the incubation units (D. Leduc, unpublished data)but did not disturb the sediment surface.

2.2. Disturbance experiment

Six incubation units were randomly allocated to an undisturbedtreatment (operational controls) and six were allocated to a disturbedtreatment. Three incubation units from each treatment were destruc-tively sampled at 2 and 9 days after disturbance. Physical disturbanceof the sedimentswas achieved by re-suspending the upper 5 cmof sed-iments with a plastic paddle for 5 seconds. Sediments were left to settlefor 8 hours before reconnecting incubation units to recirculating seawa-ter system.

Samples for estimation of nematode community attributes (i.e.,abundance, species richness, and community structure) were obtainedfrom disturbed and undisturbed treatments as well as from fieldcontrols. Each sample consisted of one subcore of internal diameter29 mm taken to a depth of 5 cm. Subcores were sliced into 0–1, 1–3,and 3–5 cm depth layers and fixed in 10% formalin and stained withRose Bengal. Samples were subsequently rinsed on a 1 mm sieve to re-move large particles and on a 45 μm sieve to retain nematodes. Nema-todes were extracted from the remaining sediments by Ludox flotationand transferred to pure glycerol (Somerfield and Warwick, 1996). Be-tween 110 and 150 randomly chosen nematodes (or all individuals iffewer were present in the sample) were mounted on permanent slidesand identified to genus and putative species using the descriptions inWarwick et al. (1998), as well as the primary literature. Monhystrellaand Thalassomonhystera were treated as one genus (“Monhysteridae”)because they are sometimes difficult to distinguish based onmorpholo-gy (Fonseca and Decraemer, 2008). The remaining fauna in the samplewere mounted in glycerol on a separate slide and the abundance ofnematodes was quantified using a compound microscope (100× mag-nification). Nematode abundance was compared between treatmentsand sediment depths as nematode density (ind. cm−3) to take into ac-count the different volumes of the sediment depth layers. Nematodespecies richness was quantified using Hurlbert's expected number ofspecies for a sample of 51 individuals (ES(51), Hurlbert, 1971).

37D. Leduc, C.A. Pilditch / Journal of Experimental Marine Biology and Ecology 440 (2013) 35–41

Samples for the analysis of sediment parameters were obtainedfrom disturbed and undisturbed treatments using a small subcore(19 mm internal diameter) to a depth of 5 cm. Subcores were slicedinto 0–1, 1–3, and 3–5 cm depth layers and kept frozen at −20 °Cuntil analysis. The following sediment parameters were determined:chloroplastic pigment equivalents (CPE; sum of chlorophyll a andphaeopigments), geometric mean grain size, particle sorting, and %sand. Methods for the analyses of sediment parameters are given inNodder et al. (2003) and Grove et al. (2006).

Sediment community oxygen consumption (SCOC) was mea-sured immediately prior to disturbance (t=0), 2 days after distur-bance (t=2), and 9 days after disturbance (t=9). Water flow wasstopped and the incubation units were sealed prior to SCOC determi-nation. Incubations were conducted in the dark for a period of24 hours, at which point oxygen levels had decreased by less than10% of initial levels. A magnetically driven impellor (~60 rpm) fittedto the chamber lids gently circulated water during the incubation.Oxygen concentration was measured with a pre-calibrated PreSensMICROX I microoptode inserted through a sampling port in thechamber lid. Three more O2measurements were made during the in-cubation period. SCOC was estimated from the decline in O2 concen-tration with time (linear regression R2>0.9).

2.3. Data analyses

The PERMANOVA routine in PRIMER was used for the analysis ofunivariate and multivariate data (Anderson et al., 2008). PERMANOVAis a semi-parametric, permutation-based routine for analysis of vari-ance based on any similarity measure (e.g., Euclidean, Bray–Curtis). Be-cause the sediments in four of our incubation units were obtained fromthe same MUC deployment (i.e., they are pseudoreplicates), we testedfor the potential effect ofMUCdeployment on nematode community at-tributes prior to the main analyses. No statistically significant differ-ences were found (Bray–Curtis similarity measure of squareroot-transformed data, P>0.1). For the analysis of nematode communi-ty attributes, we began by comparing variables between field and oper-ational controls (day 2 and 9) to evaluate the impact of laboratoryconditions on the nematode community. These analyses wereconducted using a repeated measure design (to take into account thelack of independence between sediment depths) using the fixed factorControl (three levels: field control and operational controls at day 2and 9), the fixed factor Depth (three levels: 0–1, 1–3, and 3–5 cm),and with replicates nested within Control but not Depth (Quinn andKeough, 2009). The effect of disturbance on sediment parameters, nem-atode community attributes, and SCOC was investigated by comparingvariables in the disturbed and undisturbed treatments 2 and 9 daysafter disturbance whilst omitting field controls from the analyses.These analyses were conducted using the fixed factor Disturbance(two levels: disturbed and undisturbed), the fixed factor Time (twolevels: day 2 and 9), the fixed factor Depth (three levels: 0–1, 1–3,and 3–5 cm), and with replicates nested within Disturbance and Timebut not Depth (repeated measure design, Quinn and Keough, 2009).Because nematode community attributes are often correlated with envi-ronmental factors such as food availability and sediment granulometry(e.g., Leduc et al., 2012a), environmental parameters (CPE, mean grainsize, sorting, and %sand) were fitted first as covariates, followed by thefactors listed above. Sequential sum of squares was used to take into ac-count the sequential order of terms (i.e., covariates first, followed by thefactors; Anderson et al., 2008).

Similarity matrices for univariate variables (i.e., nematode densi-ty and species richness, sediment parameters, SCOC) were builtusing Euclidean distance of untransformed data, and similaritymatrices for multivariate data (nematode community structure)were built using the Bray–Curtis similarity measure of squareroot-transformed data (Anderson et al., 2008). P-values for individualpredictor variables were obtained using 9999 permutations, and

P-values for pairwise comparisons were obtained using Monte-Carlosampling due to the low number of possible permutations (Andersonet al., 2008). The square root of estimates of components of variationwas used for comparing the amount of variation attributable to differ-ent terms in the multivariate PERMANOVA models (Anderson et al.,2008). The SIMPER routine in PRIMER was used to identify the speciescontributing most to within-group similarity and between-group dis-similarity (Clarke and Warwick, 2001).

3. Results

A total of 4412 nematodes belonging to 362 species and 146 gen-era were identified. Eighty one (22%) of these species were recordedonly once, and 54 (15%) were recorded twice. The genera withhighest overall mean abundance were: Microlaimus (41 ind.10 cm−2), Sabatieria (35), Molgolaimus (27), Daptonema (20),“Monhysteridae” (19), Halalaimus (19), and Desmoscolex (19). Noobvious vertical gradient in sediment colouration was observed inany of the experimental units throughout the experiment, whichsuggests that the RPD layer depth exceeded the height of the sedi-ments in the experimental units (8.0–10.5 cm). Live macrofauna, in-cluding polychaetes, amphipods, and gastropods, were present inthe experimental units throughout the experiment. Some mortality,however, appears to have occurred, as suggested by the develop-ment of small black patches of anoxic sediments below the surfaceof most cores by the end of the experiment. No obvious differencesin the number or size of these patches were observed between dis-turbed and undisturbed experimental units.

3.1. Effect of experimental conditions

Total, depth-integrated (0–5 cm) nematode abundance in the fieldand operational controls ranged from 1311 to 2902 ind. 10 cm−2

(mean=1969 ind. 10 cm−2). Total abundance was significantly lowerat the end of the experiment relative to 2 days after disturbance andfield controls (1492 vs. 1937 and 2494 ind. 10 cm−2, respectively,one-way PERMANOVA and pairwise comparisons, Pb0.05). Nematodedensity (ind. cm−3) differed significantly between sediment depths(two-way PERMANOVA, Pb0.05, Table 1) but not between controls.Pairwise comparisons showed that this difference between sedimentdepths was due to higher nematode density in the 0–1 cm relative tothe 3–5 cm layer (mean 108 vs. 11 ind. cm−3 respectively, Pb0.05).Nematode density in the 1–3 cm layer was intermediate and did notdiffer significantly from the other two layers (33 ind. cm−3, P>0.05).

Nematode species richness (ES(51)) did not differ significantlybetween controls, but differed significantly between sedimentdepths (Pb0.05, Table 1). Pairwise comparisons showed that this dif-ference was due to significantly lower species richness in the 3–5 cmlayers relative to the 0–1 and 1–3 cm layers (ES(51): 27.8 vs. 33.4–33.7). Nematode community structure differed significantly be-tween controls, sediment depths, and their interaction (Table 1).The presence of a significant interaction suggests that differencesin community structure among controls were not consistent amongsediment depths (Fig. 2); pairwise comparisons, however, did not re-veal any significant differences between any combinations of controlsand sediment depth (P>0.05, data not shown). Differences in commu-nity structure between controls were the result of relatively minorshifts in the abundance of species (SIMPER, Appendix 1). Comparisonof the square-root of estimates of components of variation indicatedthat the effect of sediment depth on nematode community structurewas 3.0 and 1.9 times stronger than the effects of control type and inter-action term, respectively. Values of SCOC ranged from 170.2 to364.7 μmol m−2 h−1 (mean=266.1 μmol m−2 h−1) and did notvary significantly between operational controls at 2 and 9 days (datanot shown, P>0.05).

Table 1Results of PERMANOVA analyses testing for the effect of control (field controls and op-erational controls at day 2 and 9), sediment depth (0–1, 1–3, and 3–5 cm) and their in-teraction on nematode community attributes. Variables that are significantlycorrelated with the response variable are shown in bold.

Source df MS Pseudo-F P

Density Control 2 1113 4.22 0.093Depth 2 23242 65.21 0.001Replicate (Control) 6 264 0.74 0.677Control×Depth 4 168 0.47 0.754Residuals 12 356Total 26

Species richness (ES(51)) Control 2 3 0.41 0.702Depth 2 101 5.88 0.026Replicate (Control) 6 8 0.46 0.818Control×Depth 4 15 0.88 0.539Residuals 12 17Total 26

Community structure Control 2 3294 1.41 0.020Depth 2 10249 5.86 0.001Replicate (Control) 6 2340 1.34 0.012Control×Depth 4 2568 1.50 0.009Residuals 12 1748Total 26

Table 2Sediment parameters (mean (standard deviation)) at 0–1, 1–3, and 3–5 cm sedimentdepth in the incubation units (N=12). Values followed by different letters are signifi-cantly different between sediment depths. See Appendix 1 for results of PERMANOVA.CPE: chloroplastic pigment equivalents.

CPE (ng cm−3) Mean grain size (μm) Sorting %Sand

0–1 cm 6.14 (1.44)a 43.9 (16.3)a 4.01 (0.27)a 45.9 (13.4)a

1–3 cm 4.60 (1.33)b 59.4 (22.2)b 4.09 (0.51)a 56.6 (13.1)b

3–5 cm 4.61 (1.72)b 54.7 (14.1)b 4.29 (0.47)a 55.1 (9.0)b

38 D. Leduc, C.A. Pilditch / Journal of Experimental Marine Biology and Ecology 440 (2013) 35–41

3.2. Effect of disturbance

There was no significant effect of disturbance or sampling time (2vs. 9 days after disturbance) on sediment parameters (Appendix 2).There was, however, a significant effect of sediment depth on CPE,mean grain size, and % sand (PERMANOVA, Pb0.05). Pairwise com-parisons showed that CPE values were greater in the surface(0–1 cm) layer relative to subsurface (1–3 and 3–5 cm) layers, whilstmean grain size and %sand were lower in the surface layer relative tosubsurface layers (Pb0.05, Table 2). There were no differences inSCOC between disturbance treatments and controls, or between sam-pling times (data not shown, P>0.2).

Fig. 2. Two-dimensional multidimensional scaling configuration for nematode speciesabundance (square-root transformed data) showing differences between (A) sedimentdepths (0–1, 1–3 and 3–5 cm), and (B) field controls and operational controls 2 and9 days after the beginning of the experiment.

Nematode density (ind. cm−3) was significantly correlated with thecovariates CPE and mean grain size (PERMANOVA, Pb0.01, Appendix3). There was a significant effect of disturbance, sediment depth, andtheir interaction on nematode density (Pb0.05). Nematode density insurface (0–1 cm) sediments was significantly lower in disturbed exper-imental units relative to undisturbed units, whilst the opposite trendwas observed in the deepest (3–5 cm) sediment layer (pairwise com-parisons, Pb0.05, Fig. 3). Total, depth-integrated abundance (ind.10 cm−2), however, did not differ between the disturbed treatmentand the controls (data not shown, P>0.05).

There was a significant effect of sediment depth on species richness,with values in the 3–5 cm layer significantly lower than in the 0–1 and1–3 cm layers (Pb0.05, Table 3, Appendix 3), but disturbance did nothave a significant effect (P>0.05). Nematode community structurewas significantly correlated with the covariate CPE, but not with sedi-ment grain size parameters (Appendix 3). There were significant differ-ences in community structure between disturbed treatments andcontrols, between sediment depths, but not sampling times (Pb0.05,Fig. 4). Comparison of the square-root of estimates of components ofvariation for disturbance (11.3) and sediment depth (27.8) indicatedthat sediment depth accounted for two and a half timesmore of the var-iation in nematode community structure than disturbance.

About a third of the within-group similarity of surface sediments(0–1 cm) was accounted for by species of the family Desmoscolecidae(Desmolorenzenia sp. 1, Desmoscolex sp. 1, and Hapalomus sp. 1),Calomicrolaimus sp. 1, and Chromadorita sp. 1 (Appendix 4).Molgolaimus sp. 1, Nannolaimoides sp. 1, and Stilbonematinae sp. 2accounted for much of the within-group similarity of the 1–3 cm sedi-ment depth layer, whilst Stilbonematinae sp. 2, Neotonchus sp. 2, andLaimella sp. 1 were the highest contributors to within-group similarityof the 3–5 cm sediment depth layer. Species contributing most towithin-group similarity also explained much of the dissimilarity be-tween sediment depth layers (Appendix 5). Nannolaimoides sp. 1,Stilbonematinae sp. 2, Southernia sp. 1 explained much of thewithin-group similarity for both the disturbed and undisturbed treat-ments. Disturbed sediments, however, were characterised by lowerabundance of Stilbonematinae sp. 2, Laimella sp. 1,, and Southernia sp.1 than undisturbed sediments, and higher abundance of Neotonchussp. 2, Nannolaimoides sp. 1, Desmoscolex sp. 1, and Calomicrolaimus sp.1 (Appendix 4 and 5).

4. Discussion

4.1. Experimental conditions

The experimental units did not allow for immigration of nema-todes from nearby sediments or from the water column, eventhough these processes are likely to be important in natural set-tings (e.g., Le Guellec, 1988; Sun and Fleeger, 1994). The aim ofour experiment, however, was to investigate the short-term impactof disturbance on local community dynamics at the scale of the dis-turbed patch. The severity of the artificial disturbance could becompared to bioturbation induced by benthic megafauna or bottomfeeding fish, or a resuspension event caused by strong currents.

The artificial conditions created within the experimental units didnot affect nematode species richness or density (or their vertical

Fig. 3.Mean nematode density (ind. cm−3) in disturbed and undisturbed treatments atdifferent sediment depths (0–1, 1–3, and 3–5 cm) and at different sampling times (t=2 and 9 days after beginning of experiment). Error bars are standard deviations fromthe mean.

Fig. 4. Two-dimensional multidimensional scaling configuration for nematode speciesabundance (square-root transformed data) showing differences between (A) sedimentdepths (0–1, 1–3 and 3–5 cm), and (B) disturbed and undisturbed treatments.

39D. Leduc, C.A. Pilditch / Journal of Experimental Marine Biology and Ecology 440 (2013) 35–41

gradients), but led to lower total nematode abundance and shifts incommunity structure at the endof the experiment. The changes in nem-atode community attributes we observed in the disturbed treatmentwere, therefore, an integrated response to both the conditions insidethe experimental units and the physical disturbance itself. Declines innematode abundance inside microcosms relative to the field havebeen observed in shallow water-based laboratory experiments, andare usually associated with marked declines of specific taxa (e.g.,chromadorids; Schratzberger and Warwick, 1999; Schratzberger et al.,2000). The changes in community structure we observed, however,were relatively complex (as suggested by the presence of a significantinteraction between sediment depth and control type), and resultedfrom small changes in species relative abundances rather than changesin species composition (see Appendix 1). Reasons for the decline innematode abundance over the experimental duration remain unclear,but are unlikely to be related to changes in food availability since wedid not observe any changes in pigment concentrations over the courseof the experiment. Predation bymacrofaunamayhave played a role, butthis process was not quantified in the present study.

4.2. Effect of disturbance

Disturbance did not influence pigment content and sedimentgranulometry in the experimental units, or their vertical distribution.This finding was unexpected given the relatively severe nature of thedisturbance which led to complete resuspension of the top 5 cm of sed-iments. These results could indicate that the sediments at the ChathamRise study site are subjected to regular disturbance events. Periodic

Table 3Nematode species richness (mean (standard deviation))in disturbed and undisturbed treatments, and at 0–1,1–3, and 3–5 cm sediment depths in the incubationunits (averaged across sampling times). Values followedby different letters/number are significantly different be-tween treatment/sediment depths. See Appendix 3 forresults of PERMANOVA.

ES(51)

TreatmentUndisturbed 31.6 (5.3)a

Disturbed 34.2 (3.9)a

Sediment depth0–1 cm 34.6 (3.2)1

1–3 cm 33.5 (4.6)1

3–5 cm 30.7 (5.6)2

resuspension of sediment particles would explain the presence of a sur-face layer with greater content of lighter fine particles than subsurfacelayers, as we observed in the experimental units at the beginningof the experiment. The presence of relatively high pigment concen-trations in the deepest (3–5 cm) sediment layer is also suggestiveof physically-mixed sediments (see Table 2; Sun and Dai, 2005).Near-bottom current speeds on and near Chatham Rise crest regularlyexceed erosion thresholds (Nodder et al., 2007), and lead to periodicresuspension of surface sediments. Bioturbation by macrofauna couldalso help explain the relatively high pigment concentrations in subsur-face sediments (Sun and Dai, 2005). Lack of impact of disturbance onsediment community oxygen consumption (SCOC) provides additionalsupport to the notion of a naturally disturbed community, although thisobservation may be better explained by the relatively coarse nature ofthe sediments with a deep RPD layer (>8 cm below sediment surface).Coarse sediments are typically well-oxygenated with weak vertical bio-geochemical gradients and are therefore not as severely impacted byphysical disturbance as finer sediment communities (Glud, 2008). Sim-ilar results were obtained in microcosm study by Braeckman et al.(2011) on a subtidal fine sand community, which showed no or onlylimited impact of a physical disturbance event on SCOC and pigmentconcentration distribution.

Disturbance did not influence nematode survival, as indicated bythe absence of disturbance effect on total depth-integrated nema-tode abundance. The presence of significant disturbance, sedimentdepth, and interaction effects on nematode densities, however, sug-gests that disturbance led to a vertical re-shuffling of nematodes.More specifically, physical disturbance appears to have led to theburial of nematodes from the surface sediment layer into subsurfacelayers (see Fig. 3). A similar response was observed for a physicallydisturbed subtidal nematode community (Braeckman et al., 2011).Experimental evidence suggests limited impact of physical distur-bance on nematode survival (Schratzberger et al., 2002), and that,when present, impacts are most pronounced in communitiesinhabiting muddy sediments than sandy sediments (Schratzberger

40 D. Leduc, C.A. Pilditch / Journal of Experimental Marine Biology and Ecology 440 (2013) 35–41

and Warwick, 1998). Our findings, based on an upper slope muddysand community from Chatham Rise, are consistent with the latterstudies. Our results, however, may partly reflect the low level of rep-lication (N=3) and high variability in nematode abundance be-tween experimental units, which limit the power of the presentstudy. Experiments on the effects of disturbance on nematode com-munities, in comparison, typically use homogenised and/or sievedsediments, which reduces variability between experimental units(e.g., Braeckman et al., 2011; Schratzberger and Warwick, 1999).This kind of manipulation, however, constitutes disturbance in itself,and may impact nematode communities before the beginning of theexperiment.

Clear shifts in nematode species richness were observed betweensediment layers, with lower values in the deepest (3–5 cm) layer rela-tive to the two uppermost layers (0–1 and 1–3 cm). This is a common,though not universal, trend observed in shallow water and deep-seasediments (e.g., Ingels et al., 2011; Leduc and Probert, 2011), and maybe related to the more challenging environmental conditions oftenfound in deeper sediment layers (e.g., low oxygen levels, high sulphideconcentrations; Vanaverbeke et al., 2011). Larger, more mobile taxathat do not feed selectively may also be better at exploiting the oftenlimited and low quality food sources in deeper sediments (Ingels etal., 2011; Soetaert et al., 2002). We observed greater species richnessin disturbed sediments relative to undisturbed sediments; this trendis most likely due to the vertical mixing of nematodes between sedi-ment layers (see above), which would lead to vertical homogenisationof species distributions, thereby increasing the pool of species in agiven layer (e.g., Leduc et al., 2010). The evidence available to date sug-gests that the effect of disturbance on local nematode diversity dependon the interaction of factors such as type/frequency of disturbance(Austen et al., 1998; Schratzberger and Warwick, 1998) and sedimentgrain size (Schratzberger and Warwick, 1998). The limited impact ofdisturbance on nematode species richness in the present study maybe related to the low frequency of disturbance (only once) and the rel-atively coarse sediments at the Chatham Rise study site.

We found a significant effect of disturbance on nematode com-munity structure, although the magnitude of this effect was smallrelative to the effect of sediment depth. This effect could be partlyexplained by the vertical mixing of individuals in the experimentalunits. The effect of disturbance was relatively subtle, and was dueto small shifts (in both directions) in the abundance of many species.Some genera, such as Sabatieria and Leptolaimus, are known to re-spond positively to disturbance in shallow water systems, whileothers, such as Desmodora, are often negatively affected (seemeta-analysis by Schratzberger et al., 2009). These genera werepresent in the experimental units but we did not observe substantialchanges in their relative abundances.

Of all the species present in our experimental units, Stilbonematinaesp. 2 was the most adversely affected by the disturbance event. This spe-cies, as well as all other species within the subfamily Stilbonematinae,have long, slender bodies that may be easily damaged by physical distur-bance. Species of this group are typically covered by a dense layer of bac-terial sulphur-oxidising ectosymbionts and are seldom reported fromdeep-sea habitats (but see Tchesunov et al., 2012; Van Gaever et al.,2006). Species of Stilbonematinae are usually found in shallow watercoarse sediments with high organic matter input, and concentrateat the interface between oxygenated surface sediments and deeper,suboxic sediments (Ott and Novak, 1989). Their presence in relative-ly high densities, particularly in the deepest sediment layer (wherethey represent up to 17% of nematode abundance), may suggestthe presence of reduced subsurface conditions, even though we didnot observe a RPD layer in the experimental units. Stilbonematinaesp. 2 has been recorded from several other sites along ChathamRise crest (D. Leduc, unpublished data), a distribution pattern prob-ably related to the proximity of the highly productive SubtropicalFront (Bradford-Grieve et al., 1997; Grove et al., 2006). Further

research is required to ascertain the impact of physical disturbanceon this species.

The Chatham Rise nematode community appears to be resilient todisturbance. This resilience may be due to frequent exposure to distur-bance in the field, or may be a common feature of nematode communi-ties in general. As discussed above, lack of change in sedimentparameters and SCOC following disturbance is consistent with periodi-cally disturbed sediments. Schratzberger and Warwick (1999, p.227)noted that “(nematode) assemblages are most affected by the kinds ofdisturbances that they do not normally experience naturally,” whichalso supports the notion of a naturally disturbed community. The nem-atode community at the study site, however, was characterised by rela-tively high diversity (e.g., Leduc et al., 2012b), whilst disturbedcommunities typically have low diversity with high dominance of afew resilient taxa such as Sabatieria (Schratzberger et al., 2009). Thisgenus was neither dominant nor particularly abundant in our samples.It is possible that the disturbance was not sufficiently severe (in inten-sity or frequency) to elicit a response. Several laboratory experiments,for example, were designed to investigate the effect of periodic physicaldisturbance on nematode communities (e.g., Schratzberger andWarwick, 1998, 1999), as opposed to a one-off physical disturbanceevent in the present study.

Studies in shallowwater environments have shown that nematodesare more resilient to disturbance than larger macrofaunal organisms(Schratzberger et al., 2002;Whomersley et al., 2009), butmore researchis needed to determine how resilient deep-sea nematode communitiesmay be to physical disturbance relative to shallow water communities.The relatively mild physical disturbance caused by bioturbation orresuspension events does not appear to have amajor influence on nem-atode diversity in deep-sea environments (Gallucci et al., 2008b), butthe more severe disturbance associated with mining may have pro-nounced and lasting impacts (Miljutin et al., 2011).

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jembe.2012.11.015.

Acknowledgments

Funding was provided by FRST through a postdoctoral fellowship toD. Leduc (UOOX0909), the programme “Coasts & Oceans OBI”(C01X0501), and the Department of Marine Science, University ofOtago. We are thankful to Scott Nodder (NIWA) for facilitating samplecollection, Keith Probert (University of Otago) for his support and guid-ance, and Anna Lawless (University of Waikato) for her help withprocessing of multicorer samples onboard RV Tangaroa. We also ac-knowledge the other participants of voyages TAN1103, the officersand crew of RV Tangaroa, and staff at NIWA's Mahanga Bay facilitiesfor their technical support.Wewould like to thank J. Ingels for construc-tive criticisms on an early draft of the manuscript. [SS]

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