Dynamic sedimentary environments of an Arctic glacier-fed river estuary (Adventfjorden, Svalbard). II: Meio- and macrobenthic fauna

Estuarine, Coastal and Shelf Science 74 (2007) 274e284 www.elsevier.com/locate/ecss

Dynamic sedimentary environments of an Arctic glacier-fed river estuary (Adventfjorden, Svalbard). II: Meio- and macrobenthic fauna Maria W1odarska-Kowalczuk a,*, Maria Szymelfenig b, Marek Zaja˛czkowski a b

a Institute of Oceanology PAS, ul. Powstan´co´w Warszawy 55, 81-712 Sopot, Poland Institute of Oceanography, University of Gdan´sk, Al. Piłsudskiego 46, 81-378 Gdynia, Poland

Received 21 December 2006; accepted 21 April 2007 Available online 13 June 2007

Abstract The paper examines the meio- and macrobenthic responses to physical disturbance and sediment instabilities in a small Arctic glacier-fed river estuary. Zaja˛czkowski and W1odarska-Kowalczuk [Zaja˛czkowski, M., W1odarska-Kowalczuk, M., 2007. Dynamic sedimentary environments of Arctic glacier-fed river estuary (Adventfjorden, Svalbard). I. Flux, deposition, and sediment dynamics. Estuarine, Coastal and Shelf Science 74(1e2), 285e296] distinguished three zones in Adventfjorden (west Spitsbergen) estuary: the tidal flat, the slope (high sedimentation, frequent sediment slides), the central basin (low sedimentation, stable sediments). The numbers of individuals and species of meio- and macrofauna were very low on the tidal flat. The total densities of meio- and macrofauna were significantly lower on the slope of the glacio-fluvial delta than in the central basin. Only the macrofauna responded to sediment instabilities on the slope by a significant decrease in total biomass. Nematodes inhabiting the slope sediments were larger than those in the central basin, although there was no significant difference in the size of harpacticoids in the two zones. The frequently disturbed, resuspended, and redeposited slope sediments were colonized by the opportunistic polychaete Capitella capitata agg. and by the high sedimentation resistant polychaetes Chaetozone setosa agg. and Cossura longocirrata. Tube-dwelling, sedentary, or suspension-feeding fauna only occurred at the central basin stations. The species richness and ratio of surfacedwelling to burrowing deposit-feeders in the macrobenthic communities decreased towards the river mouths. The differences in the taxonomic composition of communities inhabiting the sediments of the slope and the central basin were less pronounced in meiofauna (studied at a higher taxonomic level) than in the macrofauna (identified to the species level). Nevertheless, the differences were significant for both benthic compartments (ANOSIM test, P < 0.05). The simultaneous survey of meio- and macrobenthic communities in an Arctic glacier-fed river estuary shows that both benthic compartments are sensitive to sediment instabilities and physical disturbance caused by high sedimentation and frequent sediment gravity flows. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: macrobenthos; meiobenthos; sedimentation; sediment gravity flows; disturbance; biomass; biodiversity; Arctic; Svalbard; Adventfjorden

1. Introduction Small high-latitude Arctic rivers are often glacier-fed and transport large amounts of mineral suspensions. These rivers are oligotrophic and nutrient-poor, and production in estuaries is driven by marine-derived nutrients (Gross et al., 1988). High loads of glacio-fluvial materials produce deltaic deposits at the

* Corresponding author. E-mail address: [email protected] (M. W1odarska-Kowalczuk). 0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.04.017

river/sea interface. When a river terminates in a fjord the submarine delta slopes are steeply inclined and consequently the slope sediments are susceptible to gravitationally induced landslides and mass movements (Naidu and Klein, 1988). Zaja˛czkowski and W1odarska-Kowalczuk (2007) described three zones of contrasting sedimentary dynamic characteristics in an Arctic fjord with a glacier-fed river mouth: (1) tidal flat; (2) delta slope (steeply inclined, high sedimentation, frequent small-scale sediment slides); (3) central fjord basin (relatively flat deep bottom, stable homogenous sediments, low mineral sedimentation).

M. Włodarska-Kowalczuk et al. / Estuarine, Coastal and Shelf Science 74 (2007) 274e284

The mineral sedimentation, sediment disruptions, and instabilities control macrobenthic standing stocks, diversities, and taxonomic and functional group composition (e.g., Rhoads et al., 1978; Posey et al., 1996; Newell et al., 1998; W1odarska-Kowalczuk et al., 2005). Warwick et al. (1990) hypothesized that meiofauna is less sensitive to sediment instabilities and physical disturbance than macrofauna, but meiobenthic responses have never been studied as extensively as those of the macrobenthic compartment. A deep-sea experimental study of mechanical sediment disturbance reported a significant decrease of meiobenthic standing stocks and changes in taxonomic composition at impacted sites (Ingole et al., 1999), while Somerfield et al. (2006) described similar effects in meio- and macrobenthic communities exposed to glacial disturbance. The authors of the present study wanted to determine if the contrasting disturbance regimes in the three zones of the glacio-fluvial estuary described by Zaja˛czkowski and W1odarska-Kowalczuk (2007) are accompanied by differences in meio- and macrobenthic standing stocks, taxonomic composition, macrobenthic diversity, and functional group composition. The Adventfjorden benthos was previously studied by Holte et al. (1996), W˛es1awski et al. (1999) and W˛es1awski and Szymelfenig (1999). However, this is the first Arctic fjord benthic study covering macro- and meiobenthic compartments that is supported by an extensive survey of sedimentary

275

processes (described in the accompanying paper by Zaja˛czkowski and W1odarska-Kowalczuk, 2007). It is also one of the few studies that simultaneously examines meio- and macrobenthic responses to physical disturbance and sediment instabilities.

2. Material and methods 2.1. Study area Adventfjorden is a marine inlet (8.3 km long, 3.4 km wide) in Isfjorden, the largest fjord on the west coast of Spitsbergen (Fig. 1). Depths exceed 100 m only in the outermost part of the fjord (Fig. 1). The mouths of two braided rivers, the Adventelva and the Longyearelva, are situated in the innermost part of Adventfjorden. The rivers transport melt waters from glaciers situated several kilometres from the coast. The melting season is restricted to 122 days (W˛es1awski et al., 1999). The hydrology, mineral suspensions, sedimentation rates, composition of sediments, and turbidity currents in Adventfjorden are described in detail by Zaja˛czkowski and W1odarska-Kowalczuk (2007). In summer the 1 m brackish water layer (salinity of 10) can be detected 400 m from the tidal flat. Its salinity decreases to 28 and its thickness increases to 5 m at a distance of 1.5 km. The fjordic water masses are

Fig. 1. The location of sampling stations in Adventfjorden. Stations are referred to according to depth. The symbols represent the three zones distinguished in the Adventfjorden estuary by Zaja˛czkowski and W1odarska-Kowalczuk (2007): the tidal zone, the slope, the central basin.

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influenced by warm, saline Atlantic waters that enter the fjord from the shelf (Berge et al., 2005). A tidal flat (0.9 km wide) is formed at the river mouths. The tidal flat sediments are disturbed by regular mixing by tidal pumping, catastrophic slides caused by storms, and extensive river flow and scouring by ice-cover in winter. Mineral sedimentation is very high at the tidal flat edge and in the water column over the slope and can reach 1000 g m2 24 h1. The sediments at the delta slope are extremely unstable as a result of the intense supply of glacio-fluvial sediments and the steepness of the slope (inclined at angles of 15e19 ). The gravity driven processes of sediment transport (debris flows) frequently resuspend and redeposit the slope sediments. Prior et al. (1981) first observed numerous chutes on side-scan sonograph images of the slopes of the delta in Adventfjorden and hypothesized that the material deposited on the river delta is transported downfjord via frequent small scale mass movements along actively prograding delta slopes. Bottom depth in the central part of the fjord varies from 60 to 80 m with a seaward inclination of 1e3 . Mineral sedimentation does not exceed 5 g m2 24 h1. 2.2. Sampling and laboratory analysis Thirteen stations situated along the fiord axis were named after their depths and located in the three estuarine zones (the tidal flat, the slope, the central basin) that were identified in Adventfjorden by Zaja˛czkowski and W1odarska-Kowalczuk (2007; Fig. 1). Stations 0, 0.5, 5A, 10A, 23, 40, 60, and 80 were sampled in July 2001, while the other stations were sampled in July 2002. Sampling at depths ranging from 23 to 100 m was carried out from the R/V Oceania. From three to five 0.1 m2 van Veen grabs were taken at each station. The grab was lowered gently to the sea bottom to minimize the bow-wave which can wash away the surficial sediments (Blomqvist, 1991). One core (inner diameter 2.1 cm, surface of 3.8 cm2) was pushed down to a depth of 5 cm into the sediment in a grab to sample meiofauna. Sampling from a small motor boat was done at shallower depths of 23 m, 10 m, and 5 m (10A and 5A, stations in the main stream of the Adventelva; and 10B and 5B, stations north of the main stream of the Adventelva) and in the littoral zone (0.5 m, 0 m). Three meiofauna samples were taken using a core sampler with an inner diameter of 2.1 cm and a surface of 3.8 cm2 (P1ocki and Radziejewska, 1980) at each station. Five replicates of macrofauna were taken using a Petite Ponar grab sampler (sampling surface of 0.045 m2) at each station. Two different grabs were used for macrobenthic sampling. To determine if this methodological difference could distort results, samples were taken using both grabs at station 23 (four van Veen grab replicates and five Petite Ponar grab replicates). Macrofauna samples were sieved on a 0.5 mm sieve. Macro- and meiobenthic samples were fixed in formalin. The meiobenthic samples were stained with Bengal Rose. In the laboratory, the decantation technique was used to extract the animals from the sediment (Pfannkuche and Thiel, 1988). The meiofauna that passed through a 1 mm sieve and were

retained on a 328 mm sieve were counted and identified to major taxon levels under a stereomicroscope. The volumetric method, together with conversion factors, was used to determine the wet weight of the meiobenthic organisms (Feller and Warwick, 1988). The macrobenthic animals were sorted, identified to the lowest possible taxonomic level, and counted. The organisms were weighed, and the wet formalin biomass of the phyla was determined. The molluscs were weighed with shells and the polychaetes without tubes. Foraminifera and planktonic organisms (Copepoda, crustaceans of Thyssanoessa, Themisto, Decapoda larvae) were excluded from the analyses. The average individual biomass (AIB) was calculated as biomass divided by abundance in each macrobenthic sample. AIB was used as an estimate of the average organism size in a sample. 2.3. Data analysis One-way analyses of variance (ANOVA) was used to determine significant differences in density (D), the number of species per sample (S ), and the Hurlbert rarefaction index (ES[50]) between samples collected with different grabs. The non-parametric ManneWhitney U test was used to check for differences in biomass (B) since the normality of data distributions could not be assessed after transformations. Oneway analyses of similarities (ANOSIM; Clarke and Green, 1988) were used to check the effect of the sampler on species composition and relative abundances. ANOSIM’s statistic R estimates the difference between average rank similarities among pairs of replicates within each of two groups and the average rank similarity of replicates between groups. It can range from 1 (pairs consisting of replicates from two groups are more similar to each other than pairs of replicates from the same group) to 1 (all similarities within groups are less than any similarity between groups, Clarke, 1993). R equals 0 when replicates within and between groups are equally similar. Groups of samples collected in the three zones of the glacio-fluvial estuary (the tidal flat, the slope, the central basin) were compared with regard to meio- and macrobenthic characteristics. Differences in meio- and macrobenthic D and B, macrobenthic AIB, and individual biomass (IB) of Nematoda and Harpacticoida were tested using the non-parametric KruskaleWallis test since even after data transformations the homogeneity of variance could not be assessed. Post-hoc testing was conducted using pairwise ManneWhitney U tests. Non-parametric multivariate analysis was performed on two data matrixes: meiobenthic major taxa and macrobenthic species abundances in samples. The data were double root transformed, which gives a ‘balanced’ view of community structure by reducing the influence of numerically dominant taxa. The similarities between samples were calculated using the BrayeCurtis index and viewed with the ordination technique of non-metric multidimensional scaling (nMDS). Oneway ANOSIM was used to test if there were differences in meio- and macrobenthic composition between three a priori

M. Włodarska-Kowalczuk et al. / Estuarine, Coastal and Shelf Science 74 (2007) 274e284

set groups of samples representing the different zones of the glacio-fluvial estuary. The macrofauna species richness and species diversity were estimated. Species richness is the total number of species in a given area, while species diversity is the number of species in a given number of individuals and takes into account both species richness and evenness (e.g., Magurran, 2004). Species richness is expressed as the number of species per sample (S ). Species diversity was measured using the Hurlbert index (Hurlbert, 1971) calculated for 50 individuals (ES[50]) and the ShannoneWiener loge based index (H ). The Pielou index (J ) was calculated as the measure of evenness. One-way ANOVA was used to check for differences in S and ES[50] between groups of samples collected on the slope and in the central basin. Non-parametric ManneWhitney U tests were performed for H and J since the homogeneity of variance was not assessed after data transformation. The composition of functional groups in the macrofauna was analyzed. All species were classified by their feeding mode and comparative mobility according to Fauchald and Jumars (1979), Feder and Matheke (1980), Kuznetsov (1980), and unpublished observations. Fifteen guilds representing combinations of five feeding types (carnivores, herbivores, suspension feeders, surface detritus feeders, subsurface (burrowing) detritus feeders) and three mobility types (sessile, discretely motile, mobile) were considered. The percentage of each functional group in the total number of animals was calculated for each station.

277

Table 1 Results of KruskaleWallis test comparing meiobenthic and macrobenthic density (D), biomass (B), individual biomass (IB) of Nematoda and Harpacticoida and average individual biomass (AIB) of macrofauna in samples collected on the tidal flat (TF), on the slope (SL) and in the central basin (CB). Pairwise contrasts determined by ManneWhitney U tests. B/Pc, biomass; AIB/Pc, average individual biomass without Pelonaia corrugata H

P

Significant pairwise contrasts (at P < 0.05)

Meiofauna D B IB Nematoda IB Harpacticoida

28.4 13.4 17.4 7.3

0.000 0.001 0.025 0.000

CB > TF, SL SL, CB > TF SL > TF, CB TF > CB

Macrofauna D B B/Pc AIB AIB/Pc

27.0 15.3 21.0 6.7 12.2

0.000 0.000 0.000 0.035 0.000

CB > SL, TF; SL > TF CB > SL, TF; SL > TF CB > SL, TF; SL > TF CB, SL > TF CB, SL > TF

0.1 m2) or in the central basin (mean 0.44  0.34 g ww 0.1 m2; Table 1). Nematoda IB was significantly higher at the slope (mean  SD 5.4  1.4 mg ) than on the tidal flat (2.2  1.9 mg) or in the central basin (2.2  1.9 mg). Harpacticoida

3. Results 3.1. Comparison of van Veen grab and Petite Ponar grab macrobenthic samples No significant difference at P < 0.05 (ANOVA) was detected in macrobenthic D (ind. 0.1 m2), S, or ES[50] between samples taken with van Veen grab and the Petite Ponar grab. Neither was there any significant difference in B (g ww m2) detected with the ManneWhitney U test (at P < 0.05). The low value of the R statistic and P exceeding 0.05 (ANOSIM, R ¼ 0.1, P ¼ 0.18) indicated that groups of samples taken with different grabs at the same site do not differ with regard to macrobenthic species composition. 3.2. Meio- and macrobenthic density and biomass There were significant differences in the meiobenthic D and B as well as in the IB of Nematoda and Harpacticoida (Table 1) between groups of samples collected in different zones of the glacio-fluvial estuary. The standing stock of meiofauna was very low in the tidal zone. Minimal D was recorded on the tidal flat (1258 ind. 0.1 m2) (Fig. 2), while maximum D (641812 ind. 0.1 m2) was observed at 78 m. Densities were significantly higher in the central basin than in the tidal zone or on the slope (Table 1). The B of meiofauna on the tidal flat (mean  SD 0.02  0.01 g ww 0.1 m2) was significantly lower than on the slope (0.27  0.17 g ww

Fig. 2. D (1000 ind. 0.1 m2), B (g ww 0.1 m2), and IB (mg) of Nematoda and Harpacticoida (mean and 95% confidence intervals) in samples collected on the tidal flat, on the slope, and in the central basin. Percentages of major taxa in biomass: black, Nematoda; white, Harpacticoida; grey, other taxa.

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IB was significantly higher on the tidal flat (mean 10.6  4.4 mg) than in the central basin (mean 5.1  5.0 mg). There were no differences in Harpacticoida IB between the slope samples (mean 8.2  4.5 mg) and the samples collected in the two other zones. No macrobenthic animals were found at station 0 or 5B. Density was very low at stations 0.5 (10 ind. 0.1 m2) and 5A (80 ind. 0.1 m2). Density was lower in samples collected on the slope (mean 249.6  167.9) than in the central basin (700.6  193.3) (Fig. 3.; Table 1). The maximum density was noted at 60 m (994 ind. 0.1 m2). Polychaeta dominated the fauna at all subtidal stations and comprised 65e99% of the fauna. The next most numerous group was Mollusca, which comprised up to 34% of the fauna. The biomass of macrofauna was very low in the intertidal zone (0.02 g ww 0.1 m2 on average) and was significantly lower in the slope samples (13.8  24.2 g ww 0.1 m2) than in the central basin samples (17.5  10.8 g ww 0.1 m2) (Fig. 3; Table 1). At a few slope stations the biomass was very high due to the presence of a few specimens of the large tunicate Pelonaia corrugata. Six individuals of P. corrugata were present in a sample at station 23 and weighed 48 g ww, which comprised 92% of the total biomass, while six individuals in a sample at station 10A weighed 10.6 g ww (91% of the total macrobenthic biomass). When P. corrugata was excluded from biomass estimates, the contrast between slope

Fig. 3. Mean  0.95 CI of D (ind. 0.1 m2), B (g ww m2), and AIB (g) of macrofauna in samples collected on the tidal flat, on the slope, and in the central basin. Percentages of major taxa in total density and biomass: black, Polychaeta; grey, Mollusca; white, other taxa. Mean B and AIB after exclusion of Pelonaia corrugata from slope samples are plotted with a dotted line.

and central basin biomass was even more pronounced. The mean biomass in the slope samples without P. corrugata was 5.4 6.8 g ww 0.1 m2. AIB was low in the intertidal samples, and there was no significant difference in AIB between slope and central basin samples regardless of the inclusion or exclusion of P. corrugata in the estimates. 3.3. Trends in meio- and macrobenthic taxonomic composition Eleven major meiofaunal taxa and three larval stages were recorded in the study area (Table 2). From two to ten meiobenthic taxa were noted per station. Only two groups, Nematoda and Harpacticoida, were present at every station. Permanent meiobenthic groups (Nematoda, Harpacticoida, Turbellaria, Kinorhyncha, Tardigrada, Acari, Ostracoda) contributed from 98 to 100% of the total meiobenthic density. Temporary meiofauna (Bivalvia, Oligochaeta, Polychaeta, Priapulida adults and larvae, Copepoda nauplii, Cirripedia cypris) appeared occasionally in very low quantities. The tidal flat meiobenthic samples at stations 0 and 0.5 were very poor and only consisted of Nematoda and Harpacticoida (Table 2). They differed strongly from central basin samples but were not significantly different from the slope samples (Fig. 4; Table 3). Five taxa on average were recorded in the slope samples (from 1 to 9 taxa per sample). Copepoda nauplii (mean density 250 ind. 0.1 m2) and Oligochaeta (111 ind. 0.1 m2) were the most common and abundant following Nematoda (42219 ind. 0.1 m2) and Harpacticoida (3509 ind. 0.1 m2). ANOSIM R statistics indicate that the slope and the central basin samples are separable in terms of meiobenthic taxa composition (Table 3). The meiofauna in the central basin was dominated by Nematoda (mean density 239112 ind. 0.1 m2), Harpacticoida (2261 ind. 0.1 m2), and Kinoryncha (2515 ind. 0.1 m2). One hundred and three macrobenthic species were identified, mostly Polychaeta (46 species), Mollusca (31 species), and Crustacea (17 species). No macrobenthic individuals were found in samples collected at stations 0 and 5B. The samples collected at another intertidal station (0.5) were very poor in terms of the numbers of macrobenthic animals; only a few specimens of the lyssianasid amphipod Onisimus littoralis were found (Table 3). The intertidal macrobenthic samples were very different from those collected on the slope or in the central basin (Fig. 5; Table 3). The ANOSIM test indicated that the samples collected on the slope are significantly different from those from the central basin (Table 3). However, the samples do not form clearly separated groups on the nMDS plot. Instead of sharp discontinuity between the depths of 40 m and 60 m, there is a gradual change in fauna as one gets closer to the river mouths (Fig. 5). The slope samples were dominated by the polychaetes Capitella capitata agg., Ophryotrocha sp., Scoloplos armiger, Polydora spp., and Spio filicornis (Table 4). The polychaetes Chaetozone setosa agg., Cossura longocirrata, and the gastropod Cylichna occulta were most numerous in the central basin samples (Table 4).

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279

Table 2 Mean densities of meiobenthic major taxa at stations (100 ind. 0.1 m2)

Nematoda Harpacticoida Turbellaria Kinorhyncha Acari Ostracoda Tardigrada Bivalvia Oligochaeta Polychaeta Priapulida Cirripedia cypris Copepoda nauplii Priapulida larvae

0

0.5

5A

5B

10A

10B

23

40

60

70

78

80

100

116 17

23 3

204 104 5

405 41

448 32 6

741 29 4

54 1

681 3

1836 12 2 10

1494 38 2 28 1

4675 9 5 7

2460 21 2 43 2 2

1491 33 2 39

1 1 2 3 1 3 21 1

1 1

3 2

5

3.4. Macrobenthic diversity Only one species was found in all the samples collected on the tidal flat at 0.5 m. In other samples, S varied from 2 (station 5A) to 37 (station 100). ES[50] was the lowest at station 5A (5) and the highest at station 23 (15). The minimal value of H was found at station 5 (0.18), while the maximum of this index was recorded at station 100 (2.68). J varied from 0.26 to 0.89, and the lowest and the highest values were recorded at station 5A. S, ES[50], and H were significantly higher in the central basin samples than in the slope samples (ANOVA F ¼ 56.68, P < 0.001 for S, F ¼ 19.64, P < 0.001 for ES[50], Manne Whitney U test U ¼ 107.0, P < 0.001 for H ). There was no significant difference in J between the slope and central basin samples (P > 0.05, ManneWhitney U test) (Fig. 6). Only one macrofaunal species (Onisimus littoralis), and thus only one functional group (mobile scavengers/carnivores), was present in the tidal flat samples. From 7 to 14 functional groups were observed at the remaining stations. Deposit feeders comprised from 67 to 87% of the total density. The percentages of surface deposit feeders increased and the

3 1

2

3 2

1 1 5 4

1

1

1 3

17

1 2

1 1

1

percentages of sub-surface (burrowing) deposit feeders decreased towards the fjord mouth (Fig. 7). Suspension-feeders were either absent or very rare at most stations; only at station 100 did they comprise 5% of the fauna. Carnivores comprised up to 20% of the macrofauna. Macrofauna was dominated by mobile organisms, and at most stations they made up more than 90% of the fauna. Only at stations 10 and 23 did discretely-mobile fauna make up 7e12% of the fauna, and at station 100 sedentary fauna made up 25% of the community. 4. Discussion The three zones of the Adventfjorden glacio-fluvial estuary differed significantly with regard to meio- and macrobenthic characteristics. The most distinct feature of an Arctic estuary is the extremely poor life in the intertidal zone. Only a few specimens of one macrobenthic species (the mobile scavenging lyssianasid amphipod Onisimus littoralis) and very low densities of only two meiofaunal taxa (Nematoda and Harpaticoida) were recorded in the Adventfjorden tidal flat. Severe disturbance by winter ice cover has an overwhelming effect on the benthic biota in the intertidal zones of Spitsbergen fjords. Fast ice, which is up to 1.5 m thick, covers the waters of the inner basins of west Spitsbergen fjords from November to June (Wiktor, 1999). The persistence of ice cover can result in under-ice anoxic conditions (Kvitek et al., 1998), while in spring the ice melts and removes the upper 10 cm of sediment Table 3 Results of ANOSIM test comparing within group and between group similarities of samples collected on the tidal flat (TF), on the slope (SL) and in the central basin (CB) Meiofauna

Fig. 4. nMDS plot of BrayeCurtis similarities of double root transformed densities of major meiofaunal taxa in samples. Samples collected in different zones of the glacio-fluvial estuary are circled with a dotted line.

Macrofauna

R

P

R

P

Global test

0.508

0.001

0.732

0.001

Pairwise contrasts CBeSL CBeTF SLeTF

0.482 0.934 0.175

0.001 0.001 0.075

0.615 1 0.968

0.001 0.001 0.001

280

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basin sediments. Sediment stability and sedimentation processes are the most important factors influencing the distribution of benthic fauna in the subtidal sediments. Salinity fluctuations here appear unimportant since the freshwater input from Adventelva is relatively low and the salinity in water layers deeper than a few meters remains high and is similar to that along the adjacent coast (Zaja˛czkowski and W1odarskaKowalczuk, 2007). High sedimentation can be destructive to benthic fauna because it can alter sediment texture and stability, impede animals from maintaining the optimum position in the sediments, bury larvae and adult animals, and clog the feeding and respiratory organs of macrobenthic animals (Moore, 1977; Ahrens and Morrisey, 2005). On the steep slope of the glacio-fluvial delta in the Arctic fjord in the present study, the high sedimentation rate is accompanied by frequent sediment gravity flows (Zaja˛czkowski and W1odarska-Kowalczuk, 2007), during which the surface sediment and associated fauna are physically removed, dragged along the slope, and redeposited elsewhere. The effects of the gravity sediment flows must be comparable to the effects of dredging and dredged spoils disposal described in a number of impact assessment studies and reviewed by Newell et al. (1998). The benthic communities of the glacio-fluvial delta slope form a mosaic

Fig. 5. nMDS plot of BrayeCurtis similarities of double root transformed densities of macrobenthic species in samples. Samples collected in different zones of the glacio-fluvial estuary are circled with a dotted line.

(W˛es1awski and Szymelfenig, 1999). Thus, each year in late spring the fauna recolonizes the intertidal sediments, and only a seasonal community is present during the summer and fall (W˛es1awski and Szymelfenig, 1999). There were significant differences in various characteristics of the benthic communities inhabiting the slope and central

Table 4 Mean densities of the most abundant macrobenthic species at stations (ind. 0.1 m2). Values are rounded to the nearest integer values: 0 indicate species occurring at a station with mean density <0.5 ind. 0.1 m2. Ten most abundant species at each station are presented. Species which were among the five most abundant at any station are printed in bold 0.5 Onisimus littoralis Spio sp. Paroediceros lynceus Eteone foliosa Microspio sp. Spio cf. Armata Eteone foliosa Nephasoma diaphanes Spio cf. filicornis Scoloplos armiger Polydora sp. Capitella capitata agg. Ophryotrocha sp. Leitoscoloplos mammosus Chaetozone setosa agg. Aricidea suecica Diastylis rathkei Cylichna occulta Cossura longocirrata Levinsenia gracilis Thyasira gouldi Aphelochaeta sp. /Chaetozone sp. Frigidoalvania cruenta Axionice flexuosa Maldane sarsi Eteone flava/longa Heteromastus filiformis Terebellides stro¨emi agg. Ennucula tenuis Pectinaria hyperborea Macoma calcarea Lumbrineris cf. mixochaeta

5

10 1 0 0 1 1 0 8 2 17 45 0 3

10A

10B

1

0 2

3 0 4 1 37 15 124 11 22 7 5 0 3 9 2 20

1 2

1

17 1 9 173 20 1 7 7 0

40

60

5 0 13 2 71 28 12 28 18 1 12 38 11

78

80

100

1 9

2

0

1

13

1

0 0

14 1

1

1

1

2

0

70

10 2

30 10 2 89 17 2 48 285 13 3 12

0 0

23

5 3 34 220

1 0 22 214

8 4 12 160

1 3 2 22 174

5 163 292 37 25 2

3 141 61 4 27 79 10 12 9 10 8 22 8 4 4 0

3 167 3 16 278 1 1 9 5 6 1 8 14 13 7

3 87 65 7 41 131 1 1 20 10 10 21 7 7 7 1

1 7 5 1 2 3 1

34 65

2 2 1 0 31 1 41 9 6 16 6 5 5 18

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Fig. 6. Mean and 95% confidence intervals of diversity measures in samples collected on the slope (SL) and in the central basin (CB). S, number of species per sample; ES[50], Hurlbert rarefaction index for 50 individuals; H, ShannoneWiener index; J, Pielou index.

of patches of different successional stages following defaunation caused by sediment gravity failures. These communities are also continuously influenced by the prevailing high turbidity and high mineral sedimentation rates. Meiobenthic densities were much lower in unstable slope sediments than in the central basin. This concurs with the results of Vivier’s (1978) study of meiobenthic response to waste sediment disposal or those published in Ingole et al. (1999) on the effects of physical disturbance on the meiofauna of deepsea sediments. Experimental studies showed that the resilience of meiobenthic communities to single events of small scale mechanical disturbance is very high. Although meiobenthic numbers drop dramatically immediately after a disturbance, only one tidal cycle is required to recolonize disturbed sediments and after 12 h densities of major groups return to predisturbance levels (Sherman and Coull, 1980). Nevertheless, continuous or frequently repeated disturbances may have a deleterious effect on meiobenthic communities. Significant differences in meiobenthic composition between the slope and the central basin were observed even at the low level taxonomic resolution of major group analyses. The analyses at such coarse taxonomic levels were often

Fig. 7. Mean percentages (SD) of mobile sub-surface (burrowing) detritus feeders (BM) and mobile surface detritus feeders (SM) in the total macrobenthic density at stations collected on the slope and in the central basin.

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unable to trace the effects of even severe pollution (e.g., Warwick et al., 1990). While no major difference in harpacticoid numbers was observed in Adventfjorden, the decrease in the number of nematodes in the slope as compared that in the central basin samples was in excess of twofold. The good performance of harpacticoids in the frequently disturbed sediments of the slope might be connected to their good dispersal capabilities, which are advantageous in the recolonization of disturbed patches. In a stingray disturbance impact study, harpacticoid densities returned to pre-disturbance numbers 29 h after a stingray disturbance (Reidenauer and Thistle, 1981), but nematodes did not repopulate disturbed patches for more than 48 h (Sherman and Coull, 1980). The contrasting patterns of size distributions of these two dominant meiobenthic groups might be related to different dispersal modes employed during the recolonization of defaunated patches. Harpacticoid copepods occupy the surface layers of bottom sediments (Warwick and Gee, 1984), are easily resuspended, and recolonize disturbed sediments by passive and active migration through the water column (Chandler and Fleeger, 1983). Nematodes penetrate deeper layers of sediments and recolonize the defaunated sediments via active lateral interstitial migration (Schratzberger et al., 2004). Experimental studies showed that nematodes are able to actively migrate through sediments and in this way withstand high sedimentation and burial by uncontaminated sediments (Schratzberger et al., 2000). The hypothesis is put forward that only larger nematode individuals are observed on the slope since they are capable of more effective movement through the sediment and, thus, are more effective in recolonizing disturbed sea bed patches than are smaller nematodes observed in the stable sediments of the central basin. Experimental studies have demonstrated that the larger individuals of the nematode Metacromadora vivipara were more successful migrators than smaller ones (Schratzberger et al., 2004). The decrease in macrobenthic standing stocks in the Adventfjorden slope sediments concurs with a similar pattern observed in areas severely disturbed by dredge spoils disposal (Rhoads et al., 1978; Blanchard and Feder, 2003), increased fluvial and glacial sedimentation (Gorlich et al., 1987; Aller and Stupakoff, 1996; W1odarska-Kowalczuk et al., 2005), and deep-sea nodule mining (Ingole et al., 2001). Dredging disturbance can result in a 40e95% reduction in the numbers of macrobenthic individuals and a 60e90% reduction in their biomass (Newell et al., 1998). Surprisingly, no decrease was noted in the average macrofaunal size reported, for example, in the physically disturbed sediments of glacial bays (W1odarska-Kowalczuk et al., 2005). At the glacio-fluvial delta slope, the macrofauna responded to sediment instabilities with a decrease in numbers rather than the elimination of larger animals. The ANOSIM tests indicated there are significant differences in the taxonomic composition of the macrofauna of the slope and the central basin. However, we noted a gradual change in fauna rather than distinct discrimination between the two zones. No clear-cut discontinuity was observed in either the distribution of samples on the MDS plot or the

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distribution of dominant species in samples as presented in Table 4. Capitella capitata agg., the slope sediment dominant, is a common, successful colonizer of defaunated sediments. Capitella capitata was recorded in large numbers in sediments disturbed by organic enrichment (Pearson and Rosenberg, 1978), dredging waste disposal (Rhoads et al., 1978; Blanchard and Feder, 2003), and oil spills (Grassle and Grassle, 1974). Capitella capitata is also very resistant to sediment instabilities and intense sedimentation (McCall, 1977), which, together with the opportunistic dispersal characteristics of this species, support its domination in the unstable, frequently disturbed sediments of the slope. The polychaetes Chaetozone setosa agg. and Cossura longocirrata were noted in most samples but were most abundant close to the basal part of the slope at depths ranging from 40 to 70 m. Both taxa were among the dominants in west Spitsbergen glacial bays (W1odarskaKowalczuk and Pearson, 2004) and are obviously resistant to high sedimentation. Olsgard and Hasle (1993) described Ch. setosa and C. longocirrata as characteristic species in areas with intense non-toxic mine waste disposal. They do not, however, employ an opportunistic mode of dispersal (W1odarskaKowalczuk et al., 2005), and thus may be less adapted to survive repeated catastrophes resulting from frequent debris flows at the Adventfjorden delta slope. Tube-dwelling sedentary worms (e.g., Maldane sarsi or Pectinaria hyperborea) occur only in the stable sediments of the central basin. Tube dwellers are especially sensitive to sediment instabilities and high sedimentation that can bury the tubes, impede irrigation, and result in suffocation. The decrease of macrobenthic species richness and species diversity as one gets closer to a glacial or glacio-fluvial outflow was observed in the current study as well as in several other Arctic locations (Feder and Jewett, 1988; Schmid and Piepenburg, 1993; W1odarska-Kowalczuk et al., 1999; Kendall, 1994). Experimental studies indicated that the deposition of a layer as thin as 7 mm of sediments induced a 50% reduction in the number of species in a soft sediment benthic community (Lohrer et al., 2004). Newell et al. (1998) reported that physical disturbances of sediments during maintenance dredging eliminated from 30 to 70% of species. In Adventfjorden, a decrease in species richness (S ) and the indices combining species richness and evenness (ES, H ) were observed, but there was no significant difference in evenness as expressed by the Pielou index (J ). Magurran (2004) showed that increased dominance does not always accompany community perturbation and recommended the use of species richness measures rather than heterogeneity measures in environmental assessment studies. The macrofauna in Adventfjorden is composed almost entirely of mobile detritus-feeders and carnivores. Only the outermost fjord samples contained sedentary and suspensionfeeding fauna, species that are especially sensitive to sediment instabilities and high sedimentation (Moore, 1977; Feder and Matheke, 1980; Feder and Jewett, 1988). The slope and the central basin samples differed in the proportions of surface and sub-surface deposit feeders. Surface-dwelling fauna may be more sensitive to sediment disturbance than the sub-surface

deposit feeders. Posey et al. (1996) reported significant reduction in numbers of surface deposit feeders and no effect on burrowing fauna after the physical disturbance of coastal sediments caused by a storm event. In a deep-sea experimental study, mechanical sediment disturbance eliminated macrofauna from the upper 2 cm of the sediment and left large numbers of deeper-dwelling fauna (Ingole et al., 2001). Signs of perturbation in the meiobenthic and macrobenthic communities were noted when comparing the slope and the central basin in Adventfjorden. There are few comparative studies of meio- and macrobenthic responses to physical disturbances, and opinions on the sensitivity of the two main benthic compartments to physical sediment disturbance are inconsistent in the literature. Warwick et al. (1990) observed a decrease in macrobenthic diversity but no effect on meiobenthic diversity in sediments disturbed by large cruise ships. These authors hypothesized that meiofauna is less sensitive to sediment instabilities than macrofauna. The contrasts in response to physical disturbance were linked to different mechanisms for diversity maintenance and resource partitioning in the two groups; high feeding selectivity and specialization of feeding modes was attributed to meiofauna and spatial segregation to macrofauna (Warwick, 1984). Austen et al. (1989) studied both benthic compartments of intertidal sediments prone to mechanical disturbance by human digging for shellfish and found an undisturbed meiobenthic community and a disturbed macrobenthic community (as indicated by abundance/biomass distribution). Austen and Widdicombe (2006) summarized several experimental studies and concluded that the responses of macrofauna and meiofauna to physical disturbances were not consistent and varied across different experimental settings (e.g., varied with the presence of different species of bioturbators). Several other studies reported similar effects of physical sediment disturbance on both benthic compartments. Somerfield et al. (1995) reported that physical disturbances associated with dredging disposal clearly affected macro- and meiobenthic diversity and composition. Austen et al. (2003) found that small-scale patterns of distribution of macro- and meiofauna were highly correlated in an area of frequent megafaunal bioturbation. A decrease of both macro- and meiobenthic standing stocks was observed after the mechanical disturbance of deep-sea sediments (Ingole et al., 1999). Macrofauna and meiofauna were studied in Konsgfjorden, a glacial fjord off west Spitsbergen and the effects of glacial sedimentation on standing stocks, diversities, and taxonomic composition were found to be similar in both communities (W1odarska-Kowalczuk and Pearson, 2004; Kotwicki et al., 2004; W1odarska-Kowalczuk et al., 2005; Somerfield et al., 2006). 5. Conclusions As described by Attrill and Rundle (2002), estuarine macrobenthic communities form a two-ecocline system. They comprise two ecoclines, or sets of communities, that gradually change along the gradient of ‘‘environmental harshness’’ from a fresh water community towards mid estuary and from

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a marine community towards mid estuary. Only one ecocline can be identified in Adventfjorden, while ‘‘environmental harshness’’ is related to the stability of sediment and sedimentary dynamics rather than to changes in salinity (i.e., the ecocline from marine stable sediments to unstable sediments close to the river outflow). The other side of the ecocline (i.e., the fresh-water to estuarine transition zone) cannot be defined as there is almost no macrobenthic fauna in oligotrophic Spitsbergen rivers (W˛es1awski et al., 1999). Macro- and meiobenthic communities respond to sediment instabilities on the slope of the glacio-fluvial delta by dramatic decreases in total densities. A significant decrease in biomass was observed only for macrofauna. The differences in taxonomic composition of communities inhabiting the sediments of the slope and of the central basin are less pronounced in meiofauna (studied at the higher taxonomic level) than in macrofauna (identified to the species level). The decrease of macrobenthic biomass, diversity, and changes in functional group composition are consistent with the results of other studies of high sedimentation and the impact of sediment instability on benthic biota. Acknowledgments We would like to thank Mrs Agata Zaborska, Dr Wojtek Walkusz, and the crew of R/V Oceania for help with sampling in Adventfjorden. The authors would like to gratefully acknowledge the following colleagues for performing identifications: S1awomira Gromisz (Polychaeta); Prof. Jan Marcin W˛es1awski (Crustacea); Monika K˛edra (sipunculids and tunicates). We are very grateful to Prof. Howard Feder and an anonymous referee whose comments much improved the manuscript. References Ahrens, M.J., Morrisey, D.J., 2005. Biological effects of unburnt coal in the marine environment. Oceanography and Marine Biology Annual Reviews 43, 69e122. Aller, J.Y., Stupakoff, I., 1996. The distribution and seasonal characteristics of benthic communities on the Amazon shelf as indicators of physical processes. Continental Shelf Research 16, 717e751. Attrill, M.J., Rundle, S.D., 2002. Ecotone or ecocline: ecological boundaries in estuaries. Estuarine, Coastal and Shelf Science 55, 929e936. Austen, M.C., Parry, D.M., Widdicombe, S., Somerfield, P.J., Kendall, M.A., 2003. Macrofaunal mediation of effects of megafaunal bioturbation on nematode community structure. Vie et Milieu-Life and Environment 53, 201e209. Austen, M.C., Warwick, R.M., Rosado, M.C., 1989. Meiobenthic and macrobenthic community structure along a putative pollution gradient in southern Portugal. Marine Pollution Bulletin 20, 398e405. Austen, M.C., Widdicombe, S., 2006. Comparison of the response of meioand macrobenthos to disturbance and organic enrichment. Journal of Experimental Marine Biology and Ecology 330, 96e104. Berge, J., Johnsen, G., Nilsen, F., Gulliksen, B., Slagstag, D., 2005. ocean temperature oscillations enable reappearance of blue mussels Mytilus edulis in Svalbard after a 1000 year absence. Marine Ecology Progress Series 303, 167e175. Blanchard, A.L., Feder, H.M., 2003. Adjustment of benthic fauna following sediment disposal at a site with multiple stressors in Port Valdez, Alaska. Marine Pollution Bulletin 46, 1590e1599.

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