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ARTICLE IN PRESS Continental Shelf Research 29 (2009) 2222–2233 Contents lists available at ScienceDirect Continental Shelf Research journal homepage: www.elsevier.com/locate/csr Small-scale, patchy distributions of infauna in hydrodynamically mobile continental shelf sands: Do ripple crests and troughs support different communities? Patricia A. Ramey , Judith P. Grassle, J. Frederick Grassle, Rosemarie F. Petrecca Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, 71 Dudley Road, New Brunswick, New Jersey 08901, USA a r t i c l e in f o a b s t r a c t Article history: Received 17 March 2009 Received in revised form 21 August 2009 Accepted 27 August 2009 Available online 19 September 2009 Spatial variability of infauna with respect to distribution of topographic habitat features was examined in hydrodynamically mobile sandy sediments on the inner continental shelf off New Jersey, USA (391 27.690 N, 741 15.810 W). Sediment cores for infauna were taken by SCUBA divers at multiple spatial scales over time at 12-m depth in the LEO-15 research area on Beach Haven Ridge. Crests, troughs and less consistently flanks of sand ripples 5–15-cm in height, were characterized by different infaunal community patterns at spatial scales of centimeters to kilometers on several sampling dates. Overall, infaunal community differences among ripple crests, troughs, and/or flanks within areas o 1-m2 were greater than those found for each of these habitats (i.e., either crests, troughs, or flanks) that were separated by distances of 2 m–4 km. Infaunal density and species richness were consistently higher in troughs compared to crests. Indirect measures of food resources such as particulate organic carbon, chl a, and pheophytin were associated with ripple crests and troughs. Troughs contained significantly higher levels of particulate organic carbon ( 1.2 times higher) associated with finer sediments, compared with crests and flanks. Various combinations of taxa had higher densities in either crests or troughs of sand ripples depending on date, and the relative abundances of three taxa, the depositfeeding polychaete Polygordius jouinae, the suspension-feeding surfclam Spisula solidissima, and predatory nemerteans were important in distinguishing between crests and troughs on most dates. Thus, a priori knowledge of whether a benthic sample comes from a crest or trough helped to explain small-scale infaunal patchiness in relatively homogeneous, subtidal sandy sediments. Consideration of such topographic features in sampling designs can help in explaining variation in species’ distributions at several spatial and temporal scales. & 2009 Elsevier Ltd. All rights reserved. Keywords: Rippled beds Topography Habitat Macrofauna Polygordius Spisula 1. Introduction Infaunal organisms are intimately associated with the sediment in which they live. Therefore one would expect strong relationships between sediment properties, and species distributions and abundances (reviewed by Snelgrove and Butman, 1994). In the present study, community variability is examined in relation to multiple topographic habitat features and sediment properties at a variety of spatial scales over time. Habitat characteristics influencing the structure and dynamics of populations and communities have been emphasized for both terrestrial and marine environments (e.g., Able et al., 2003; Diaz et al., 2003; Morris, 2003; Zajac et al., 2003; Zajac, 2008). Availability, quality, size, and spacing of habitat patches have direct effects on populations or communities by providing Corresponding author. Tel.: + 1732 932 6555x311; fax: + 1732 932 8578. E-mail addresses: [email protected] (P.A. Ramey), jgrassle@marine. rutgers.edu (J.P. Grassle), [email protected] (J.F. Grassle), petrecca@ marine.rutgers.edu (R.F. Petrecca). 0278-4343/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2009.08.020 suitable living conditions (often species specific) in some areas but not others (Chapman, 1994; Underwood et al., 2000). Indirect effects include interactions among individuals or species (Menge et al., 1985; Fairweather, 1988). It is well known that the scale of sampling, relative to the distributional pattern of organisms, can influence both the precision and interpretation of data (Thrush et al., 1994). Studies of population and community patterns within the context of defined habitats occurring at repeated intervals in space, have been more common in terrestrial and intertidal landscapes where species distributions and habitats are readily visible (e.g., Kent et al., 1997; Underwood and Chapman, 1998). Due to logistical constraints, research on subtidal macrofaunal communities has more often focused on scale-dependent factors without adequately defining or quantifying habitat(s) over time or space (Zajac et al., 2003; Zajac, 2008) (but see Rogal et al., 1978). Most commonly, spatial distributions and composition of subtidal habitats and infaunal communities have been inferred from point samples randomly collected via remote or ‘‘blind’’ bottom grabs with little explicit a priori consideration given to habitat type other than sediment grain size distributions (Zajac ARTICLE IN PRESS P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 et al., 2003). An outstanding feature of soft-sediment infaunal communities is that both abundances and species composition vary greatly at a variety of spatial scales (cm to km) (e.g., Volckaert, 1987; Morrisey et al., 1992), and patchy distributions are very common at small scales (cm–m) even when sediment grain size appears homogeneous (e.g., Sandulli and Pinckney, 1999). Such patchiness can confuse interpretation of community pattern, especially if patterns are documented with samples collected at scales larger than those relevant to the infauna. Hall et al. (1994) observed that samples from subtidal, soft-sediments are not generally taken with sufficient spatial resolution to examine small-scale differences in community structure. Advances in imaging techniques have shown that habitats in subtidal, soft-sediment landscapes are heterogeneous and complex (e.g., Able et al., 2003; Diaz et al., 2003; Zajac et al., 2003; Zajac, 2008). In shallow continental shelf environments, such as the LEO-15 research site (Long-term Ecosystem Observatory at 15-m depth) on the inner shelf off New Jersey, waves, storms, and stronger than average currents (average 5 cm s 1) create a sandy, rippled bed with varying ripple wavelengths and heights and over space and time (e.g., Traykovski et al., 1999; Styles, 1998; Able et al., 2003). The morphology of ripples may be important in explaining the spatial distribution of benthic microalgae and infauna in sediments disturbed by waves (Kendrick et al., 1998), and heterogeneity created by ripples may be a source of microhabitat specialization and resource partitioning (Hogue and Miller, 1981; Kendrick et al., 1998; Ramey and Bodnar, 2008). One goal of the present study was to determine whether community patterns at relatively small scales (i.e., cm–m) in hydrodynamically mobile, sandy continental shelf sediments can be associated with defined topographic features, and what features of these habitats (e.g., sediment properties) are related to observed infaunal patterns. Camera and video images from a single inshore station at LEO-15 (Station 9) were used to define and characterize habitats that were frequently encountered at small scales of o1 m along a 44-m transect. Defined habitats included ripple crests, flanks, troughs with bivalve shell hash in them (+ SH), and troughs without shell hash (–SH). A suite of sediment properties including grain size and indirect measures of food availability (e.g., chl a, pheophytin, particulate organic carbon, and nitrogen) were also quantified for each habitat coincident with infaunal sampling to aid in understanding observed patterns. To determine whether crest and trough community patterns were detectable at larger spatial scales (m–km) and whether observed patterns were consistent over time (sampling dates), a previously collected, comparable infaunal dataset was re-analyzed to specifically address community patterns in rippled beds for the first time. Specific questions asked were: (1) do infaunal communities inhabiting crests of ripples differ from those in troughs (–SH and + SH) or flanks?; (2) at what scale(s) ( o1 m–4 km) are community patterns (i.e., species composition and abundance) observed?; (3) what species and sediment properties are most important in driving observed differences in community structure?; (4) are community patterns and the species responsible for them consistent over time and space? 2223 Haven Ridge (391 280 N, 741 150 W), a shore oblique sand ridge typical of the 71 such ridges found off the New Jersey coast (McBride and Moslow, 1991). Research stations examined in the present study included Station 9 located inshore on the southern landward side of the ridge at 12-m depth (391 27.690 N, 741 15.810 W) and Station 30 located on the offshore flank of the Ridge at 16-m depth (Station 30: 391 28.80 N, 741 13.290 W), 4 km northeast of the inshore station (Fig. 1). Comparison of sediment grain size between these two stations in 1990 indicated that they were similar, with slightly coarser sediments offshore at Station 30 (mean grain size Station 9: 500 mm [F =1.0]; Station 30: F = 0.9) (Craghan, 1995). Like other physically active shelf areas, rippled beds are the predominant habitat feature (Twichell and Able, 1993; Able et al., 2003). 2.2. Sampling design 2.2.1. Small-scale (cm–m), single date June 2005 In July and August 2004, a SCUBA diver-operated, handheld video camera was used to record the benthic environment at Station 9 (Fig. 1). Habitats present within 1-m2 plots were defined and characterized. On 9 June, 2005 a 44-m transect was set up at Station 9 perpendicular to the ripples and prevailing current. Along the transect a nested sampling design was used with a series of successively smaller spatial scales nested within larger scales (Fig. 2A). Spatial scales of interest included o1, 2, and 4 m. Four infaunal core samples were taken from each of 12, 1-m2 quadrats on either the left or right side of the transect (determined randomly by a coin toss), using a handheld corer (7-cm diameter, 10-cm deep, 38.5 cm2) (Fig. 2A). Infaunal cores were taken from four of five habitats present in 2004 (1 core habitat 1 =4 cores quadrat 1, n= 48). Habitats sampled 2. Materials and methods 2.1. Study site This research was conducted on the inner continental shelf at the Rutgers University Long-term Ecosystem Observatory at 15-m depth (LEO-15), located 9 km off southern New Jersey (391 27.690 N, 741 15.810 W) (Fig. 1). This area is dominated by Beach Fig. 1. Bathymetry of Beach Haven Ridge at LEO-15 showing sampling stations Station 9 on inshore side of ridge and Station 30 offshore flank of the ridge (modified from Twichell and Able (1993); same as in Weissberger and Grassle (2003)). ARTICLE IN PRESS 2224 P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 Fig. 2. Nested sampling designs (A) spatial scales o 1, 2, and 4 m at Station 9 (June 2005) where core samples were collected from 12 1-m2 quadrats, randomly placed on either side of a 44-m long transect. Four infaunal cores (by open circles) and two sediment cores were collected within each quadrat from ripple crests, flanks, troughs + SH, and troughs SH (infaunal core:1 core habitat 1 =4 cores quadrat 1, n= 48; sediment cores: 2 syringe cores habitat 1 = 8 syringe cores quadrat 1, n= 96). Top insert is a digital photograph of quadrat 1 indicating core samples and habitats, (B) spatial scales o 1 m, r 60 m, r 200 m at Stations 9 and 30 (multiple dates in 1994 and 1995) where triplicate paired crest and trough samples o1 m apart were nested within three 60-m diameter circles (substations), nested within a 200-m diameter circle at each station. Stations 9 and 30 are 4 km apart. Large filled central circle denotes permanent station mooring, filled circle in center of substation indicates randomly located permanent substation mooring, and ovals with circles inside represent paired crest (i.e., open) and trough (i.e., filled) samples (n= 6 cores substation 1 [3 crests, 3 troughs]; 18 cores station 1) on each sampling date. included ripple crests, flanks, troughs + SH, and troughs SH (Fig. 2A). Separate sediment cores were also taken for grain size and sediment analyses (i.e., carbon, nitrogen, chl a, and pheophytin) using weighted, modified syringe cores (5.3 cm2 and 8-cm long). Sediment cores were taken as close as possible to each infaunal core (2 syringe cores habitat 1 = 8 syringe cores quadrat 1, n= 96), (Fig. 2A). 2.2.2. Large-scale (cm–km), multiple dates 1994–1995 A previously collected dataset originally used to provide baseline information on the spatial and temporal distribution of infaunal communities at LEO-15 (Grassle et al., accepted) was examined (only Stations 9 and 30) in the present study to address questions regarding the persistence of ripple crest and trough community patterns across relatively large spatial scales, and over successive sampling dates. Infaunal data in the 1994/1995 study were highly comparable to those taken in 2005 with respect to sampling location (LEO-15, Station 9), methods (i.e., both employed nested designs; paired crest and trough samples; utilization of the same diver-collected hand corers; and a 300mm mesh screen) and effort (n =9 cores habitat 1 station 1 date 1 [1994/1995] and n = 12 cores habitat 1 [2005]). Infaunal core samples were collected on 20 July 1994, 20 September 1994, and November/December 1994 (14 November [Station 9], 2 December [Station 30]), 5 June 1995, and 12 October 1995. At each station a nested, partially randomized sampling design (habitat specified: crest or trough) was used where within each station (each station: 200-m diameter), three 60-m diameter areas called ‘‘substations’’ were set up, and within each of these a pair of infaunal samples including one from a crest and the other from a trough o1 m apart, were collected at three points (6 cores [3 crests and 3 troughs] substation 1 = 18 cores station 1; n = 180 cores over all dates sampled) (Fig. 2B). Thus, spatial scales of interest included o1 m, r60, r200, and 4 km. Separate sediment cores for grain size and sediment properties were not examined in 1994/1995. ARTICLE IN PRESS P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 2225 Fig. 3. Means 795% confidence intervals, with n= 12 for (A) total density of infauna June 2005, (B) richness, (C) Shannon diversity H0 , and (D) evenness J0 . The same letters (a, b) indicate no significant differences between habitats (p 40.05, Mann–Whitney U Test). Scales on y-axes differ among panels. 2.3. Infaunal samples 2.5. Data analysis All infaunal samples (samples from 2005 and 1994/1995) were sieved over a 300-mm mesh screen, fixed in 10% formalin and seawater mixture, and then promptly transferred to 70% ethanol with rose bengal. Infauna were identified to the lowest taxonomic level possible (i.e., genus or species). In some cases, small juveniles could not be identified and were grouped by family or some higher level of classification and referred to as spp. Community composition was compared among habitats on individual infaunal cores (i.e., cores were not averaged or pooled) and separate analyses were conducted for the June 2005 data as well as for each sampling date in 1994/1995 using Chord Distance Normalized Expected Species Shared (CNESS) (described by Trueblood et al., 1994). CNESS is an extension of Orlóci’s (1978) chord distance and Grassle and Smith’s (1976) Normalized Expected Species Shared (NESS). CNESS constructs a dissimilarity matrix from a sample by species matrix, that depends on the number of expected species shared in a random draw of n individuals from two samples. This index is sensitive to rare species as well as to abundant ones. Distribution patterns were clustered using weighted, pair-group mean average sorting of CNESS dissimilarities along with a metric scaling of CNESS using COMPAH 96 and Matlab programs written by Gallagher, University of Massachusetts, Boston (http://alpha.es.umb.edu/fa culty/edg/files/edgwebp.htm). The hypergeometric probability matrix (H) that was produced by metric scaling of CNESS was then examined by principal components analysis (PCA-H). The PCA-H plot is very similar to that produced by non-metric multidimensional scaling (NMDS) (Trueblood et al., 1994; Snelgrove et al., 2001), but the benefit of the metric scaling is that CNESS distances among samples are conserved and therefore species that contributed 45% to CNESS variation among samples can be displayed in a Gabriel Euclidean distance biplot overlay (Gabriel, 1971). In the biplot, the length and angle of species vectors indicate the contribution of the species to the PCA-H axes. 2.4. Sediment properties (June 2005) Sediment grain size analysis was conducted using stacked sieves (Z2, 1 mm, 500, 250, 125, 32, r32 mm). For analytical purposes sediments were described as medium-to-coarse-grained ( Z250 mm) or fine-grained (o250 mm). A small amount of sediment was removed from the top layer ( 1-cm deep) of syringe cores taken for sediment analysis and frozen for later determination of ambient concentrations of total carbon, nitrogen, chlorophyll-a, and pheophytin. Sediment for carbon and nitrogen analysis was first dried, ground, and acidified in silver cups to remove carbonates and then measured using a Fisons NA1500N elemental analyzer with acetanilide as a calibration standard. Cchl a and pheophytin was extracted from sediment (3–11 g) in 90% acetone and determined by fluorometric analysis on a Hitachi F2000 spectrofluorometer (modified from Strickland and Parsons, 1972; APHA, 1992). ARTICLE IN PRESS 2226 P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 Fig. 4. Means7 95% confidence intervals for (A, B) total density of infauna in ripple crests compared to troughs on each sampling date (20 July 1994, 20 September 1994, 14 November/2 December 1994, 5 June 1995 and 12 October 1995; n= 90) at Stations 9 and 30 respectively, (C, D) richness, (E, F) Shannon diversity H0 , and (G, H) evenness J0 . Note scale on y-axes differs among panels. Community measures including density of total infauna, taxon richness, Shannon diversity H0 , and evenness J0 were also compared among the four habitats by plotting means with 95% confidence intervals for each sampling date and station. To determine whether there were significant differences in any of these measures among the four habitats examined in 2005 a Kruskal–Wallis test was used. For significant results, post-hoc testing was conducted using pair-wise Mann–Whitney U tests in SPSS 10.0. A Mann–Whitney U test was also performed (pooled across sampling dates) to determine if there were significant differences between crests and troughs at Stations 9 and 30 in 1994/1995. Principal components analysis (PCA) determined differences among the four habitats based on sediment properties (June 2005 only) including sediment grain size (i.e., medium to coarser grained Z250 mm and finer sediments o250 mm) and indirect measures of food availability (i.e., total particulate organic carbon, chlorophyll-a, and pheophytin) (SPSS 10.0). Values for nitrogen were not used in this analysis since they were close to the detection limits of the elemental analyzer. Prior to analysis, variables were standardized to z-scores, thus weighting all variables equally. To determine whether there were significant differences in environmental variables among the four habitats a Kruskal–Wallis test was conducted followed by a pair-wise Mann– Whitney U tests. 3. Results 3.1. Community measures In June 2005, infaunal density, richness, and diversity were higher in ripple troughs compared with the other three habitats ARTICLE IN PRESS P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 crest trough -SH trough +SH 0.8 flank relative to 1994/1995, which may be related to larger scale changes in productivity in the Mid-Atlantic Bight. Spisula solidissima PCA-H axis 2 (16%) 0.6 3.2. Spatial patterns 0.4 0.2 0.0 Tellina agilis -0.2 -0.4 Oligochaeta spp. Nemertea spp. Acanthohaustorius sp. -0.6 -0.8 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 0.8 Spisula solidissima 0.6 0.4 PCA-H axis 2 (16%) 2227 0.0 -0.2 4 76 1 10 85 61 5 0.2 3 8 6 9 -0.4 Nemertea spp. 11 10 -0.6 -0.8 -0.8 -0.6 -0.4 -0.2 7 10 3 7 9 9 2 2 8 Tellina agilis 1 11 312 4 5 10 6 4 2 12 8 4 7 1 11 0.0 12 Oligochaeta spp. Acanthohaustorius sp. 2 0.2 0.4 0.6 0.8 PCA-H axis 1 (24%) Fig. 5. PCA-H metric scaling ordination of infaunal core assemblage spatial patterns for June 2005 at Station 9 based on CNESS (n= 5 individuals). First two axes explain 24% and 16% of the variance in the data respectively. Solid arrows indicate taxa contributing to 45% of CNESS variation in biplots. Taxa in bold indicate those with heavily weighted factor loadings (7 0.5). (A) Individual cores are labeled according to habitat and by stippling/shading (for illustration purposes only), (B) quadrat (numbers correspond to those used in the sampling design shown in Fig. 2). (Figs. 3A–C). Kruskal–Wallis tests showed significant difference among these habitats only for infaunal density (p= 0.035, n =48). Pair-wise post-hoc tests indicated that density in troughs SH was significantly different compared with flanks (p= 0.003, n =24). Evenness values were similar across habitats (Fig. 3D). Infaunal density and richness were also consistently higher in troughs compared to crests at Stations 9 and 30 on all dates sampled (exception =richness in October 1995) (Figs. 4A–D), however this difference was only significant for density (Station 9: p= 0.027, n = 90; Station 30: p =0.004, n = 90). Infaunal diversity showed the opposite trend to that observed for June 2005 with higher diversity in crests compared to troughs for the majority of sampling dates (Figs. 4E, F) (Station 9: 0.030, n =90; Station 30: 0.012, n = 90). Evenness was also generally higher in crests than in troughs (Figs. 4G, H) (Station 30: p= 0.006; n =90). Overall, a total of 709 individuals were collected in June 2005 (n =48) encompassing 35 different taxa, whereas a total of 43,708 individuals were collected encompassing 140 different taxa in 1994/1995 (n = 180). Moreover, density and richness were much lower in June 2005 (n = 12) compared to June 1995 (n =9) (Figs. 3 and 4). Scuba divers at the study sites have also noticed reduced densities of epifaunal species at the study site in recent years PCA-H analysis of infaunal cores from June 2005 indicated that samples clustered together by habitat type (i.e., crests, flanks, and troughs [ +SH and SH]) rather than with respect to spatial arrangement along the transect (i.e., by quadrat) (Figs. 5A, B). Thus, samples collected close together (paired crest and trough cores taken o1 m apart) were more different from each other than samples from the same habitat type taken further apart at distances of 2–44 m (see Figs. 2A and 5A). At relatively larger scales, samples also generally grouped by habitat type in July 1994 (Station 30), September 1994, November/December 1994, and June 1995 (Figs. 6A–D). Clustering of cores based on habitat type, however, did not hold for samples taken in July 1994 (Station 9) or in October 1995 where paired crest and trough samples grouped together by station (Figs. 6A, E). In June 2005 five taxa were identified in Gabriel biplots as being important (contributing to 45% of CNESS variation) in distinguishing among the habitat groupings including the bivalves Spisula solidissima and Tellina agilis, the amphipod Acanthohaustorius sp., Nemertea spp., and Oligochaeta spp. (Fig. 5A, Table 1). Densities of species with high factor loading scores ( 70.5) significantly differed among the four habitats (Kruskal–Wallis test) including S. solidissima (p = 0.023, n =48; highest in troughs SH/+ SH), T. agilis (p= 0.039, n = 48; lowest in crests) and Nemertea spp. (p =0.035, n =48; highest in crests) (see Fig. 7 for pair-wise post-hoc comparisons). Depending on the date examined in 1994/1995, different combinations of 13 taxa (i.e., 8 polychaetes, 3 bivalves, Oligochaeta spp., and Nemertea spp.) were important in distinguishing among habitat and station groupings (3–6 taxa per date) (Table 1). Most taxa (7 out of 13) were important in distinguishing among groups on a single date, however S. solidissima and Nemertea spp., which were important in distinguishing among habitats in June 2005, were also important on three of the five dates examined in 1994/1995 (Fig. 7, Table 1). Moreover, the polychaete Polygordius jouinae was important on four dates in 1994/1995. Densities of P. jouinae and Tharyx acutus were consistently higher in troughs compared to crests, whereas density of Nemertea spp. was consistently higher in crests. Density of S. solidissima was highly variable between crests and troughs depending on station and date. Species with significant differences in density between crests and troughs at either Station 9 or 30 are given in Table 2. 3.3. Habitat related sediment properties (June 2005) Spatial patterns based on PCA analysis of sediment properties showed similar clustering of samples based on habitat type to those observed for the infaunal data (Fig. 8). Samples taken from ripple crests were the most distinct, trough samples ( + SH and SH) formed another group, and samples from ripple flanks overlapped considerably with those from crests and troughs. Principal components axis 1 was important in distinguishing among the habitat groupings described above and explained 45% of the variation in the data. Variation among samples within habitats generally occurred along axis 2 and explained 32% of variation in the data. A comparison of sediment properties among habitats significantly differed for coarse and fine-grained sediment (p = o0.0001, n= 43; p = o0.0001, n = 43) respectively, as well as sedimentary organic carbon (p = o0.0001, n = 48) (see Fig. 9 for pair-wise post-hoc comparisons). Generally crests contained coarser sediments ( Z250 mm) and higher amounts of ARTICLE IN PRESS 2228 P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 Fig. 6. PCA-H metric scaling ordination of infaunal core assemblage spatial patterns for each date sampled in 1994/1995 at Stations 9 and 30 based on CNESS (n= 10 individuals). First two axes explain 27–37% and 17–21% of the variance respectively depending on sampling date. Solid arrows indicate taxa contributing to 45% of CNESS variation in biplots. Taxa in bold indicate those with heavily weighted factor loadings ( 7 0.5). Individual cores labeled according to station and habitat from which they were collected. Sample groupings discussed, indicated by shading (highlight habitats) and dashed lines (highlight stations), (for illustration purposes only). Table 1 Summary of 15 taxa/species contributing to 45% of CNESS variation among samples and displayed in Gabriel Euclidean distance biplots. Group Taxa/species Polychaeta Polygordius jouinae Tharyx acutus Tharyx kirkegaardi Syllides convoluta Aricidea catherinae Caulleriella sp. A Spionidae juv. sp. A Bivalvia Oligochaeta Spisula solidissima Nucula annulata Tellina agilis Peosidrilus spp. Oligochaeta spp. 9 June 2005 20 July 1994 + + + + 20 September 1994 14 November/ 2 December 1994 5 June 1995 12 October 1995 + + + + + + + + + + + + + + + + + Nemertea Nemertea spp. Nemertea sp. A + Amphipoda Acanthohaustorius sp. + + + + + + + Indicates taxon contributed to variation on a particular sampling date (i.e., June 2005, July 1994, September 1994, November/December 1994, June 1995 and October 1995). ARTICLE IN PRESS P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 Fig. 7. Mean densities 7 95% confidence intervals of taxa contributing to 45% of CNESS variation in Gabriel biplots with heavily weighted factor loadings ( 7 0.5) (n= 12) for samples taken June 2005. Same letters (a, b) indicate no significant differences in density between habitats (p 40.05, Mann–Whitney U Test). sediment chl a (i.e., benthic diatoms), whereas a greater proportion of finer sediments ( o250 mm), particulate organic carbon, and pheophytin were present in troughs ( SH and + SH). 4. Discussion Crests, troughs (including areas with and without shell hash), and to a lesser extent flanks of sand ripples were instrumental in identifying infaunal community patterns in highly mobile, wellsorted continental shelf sediments over scales of centimeters to kilometers on the majority of sampling dates examined. Infaunal samples primarily grouped by habitat type rather than spatial arrangement of the sampling design(s) (e.g., groups did not cluster by quadrat, substation etc.). Habitat-based community patterns varied at scales o1–200 m on most dates, with less variability at the between-station scale (4 km) on three of the five dates examined. Habitat-defined communities at the smallest scale examined ( o1-m2) in ripple crests, troughs, and/or flanks were more different from each other than communities within either 2229 crest, trough, or flank habitats 2m–4 km apart. In concert with our findings, Barros et al. (2004) also found less variability in infaunal communities (500-mm sieve; family level identification) from replicate to replicate within crests and within troughs than between crests and troughs (scales of r4–10 m) in Botany Bay, Australia at 7–8 m depth. Abundances of infauna inhabiting crests have been described as different from those in troughs in high-energy subtidal (e.g., nematodes: Sameoto, 1969; macrofauna 500-mm sieve: Barros et al., 2004) and intertidal sandy sediments (crustaceans: Grant, 1981) with some having significantly higher abundances in troughs than crests (Barros et al., 2004) and vice versa (Grant, 1981). In the present study, various combinations of taxa characterized the patterns described depending on sampling date. Given the diversity of reproductive strategies, feeding modes, food preferences, and motility of infaunal organisms, it is not surprising that different taxa exhibit different habitat associations and patterns. Community differences between ripple crests and troughs on the majority of dates, irrespective of station, were driven by differences in the relative abundance of three taxa: the deposit-feeding polychaete P. jouinae, the suspension-feeding surfclam S. solidissima, and predatory Nemerteans. Overall, infaunal density and species richness were consistently higher in troughs compared to crests. Qualitative observations by divers indicate lower densities of certain macrofauna (e.g., Echinarachnius parma, Cerianthiopsis americanus) in recent years compared to 1994/1995. Such changes may be linked to declines in the magnitude of the fall and winter phytoplankton blooms (Schofield et al., 2008) which in turn are likely to affect the level of particulate matter reaching the bottom. We found evidence for differences in food resources associated with crests and troughs of ripples. Ripple troughs contained significantly higher levels of particulate organic carbon ( 1.2 times higher) associated with finer sediments, compared with crests and flanks. Chl a and pheophytin concentrations were major factors in determining within-habitat variability. Although concentrations in June 2005 did not differ substantially among habitats in the present study, Ramey and Bodnar (2008) found concentrations up to five times higher in ripple troughs than crests at Station 9 and Node B following the spring phytoplankton bloom (i.e., in May 2006). Topography of sandy rippled beds creates pressure-driven transport of water through the interstices of the sediments augmenting the deposition, transport, and patchiness of organic matter in permeable shelf sands (Huettel et al., 1996; Pilditch et al., 1998; Precht and Huettel, 2003; Reimers et al., 2004). In oscillatory flows such as those found at the study site, ripples as small as 0.7-cm high (3-cm wavelength) can enhance pore–water exchange rates by factors of 6–15 (Precht and Huettel, 2003). Organic matter may also accumulate in troughs by settling there as a result of reduced shear stress. Current speeds, direction, and topography at the study area, can change on time scales from seconds to seasons (e.g., Traykovski et al., 1999) creating patchy distributions of food resources. Patchiness of resources coupled with the behavior of species can determine the distribution of individuals within communities (Thrush et al., 1989), and the heterogeneity generated by topographic features may be an important source of microhabitat specialization and resource partitioning (Hogue and Miller, 1981; Ramey and Bodnar, 2008). Several studies have suggested that active selection by post-larval stages may also be responsible for distribution patterns associated with crests and troughs (e.g., Sameoto, 1969; Grant, 1981). Flume experiments have shown that under realistic flows at the study site (free stream velocity of 5 cm s 1; u* =0.32 cm s 1; Styles, 1998) and flat bed conditions, adult P. jouinae selected for higher levels of particulate organic matter, and the worms can potentially traverse the wavelength of ARTICLE IN PRESS 2230 P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 Table 2 Mean density with 95% confidence intervals for taxa contributing to 45% of CNESS variation among samples and displayed in Gabriel Euclidean distance biplots for each sampling date at Stations 9 and 30 (i.e., July 1994, September 1994, November/December 1994, and June 1995). Date 20 July 1994 Species Polygordius jouinae Tharyx acutus Syllides convoluta Spisula solidissima Station 9 Station 30 Crest (no. ind. 38.5 cm 2) Trough (no. ind. 38.5 cm 2) Mann U, p-value n= 18 Crest (no. ind. 38.5 cm 2) Trough (no. ind. 38.5 cm 2) Mann U, p-value n= 18 0.00 26.447 13.8 10.0 (0.006) 0.0 0.11 7 0.3 1.567 1.7 0.3370.5 2.22 7 1.9 0.447 0.58 0.89 7 0.8 10.78 7 6.6 4.117 2.4 0.677 0.9 16.0 (0.031) 10.0 (0.006) 10.78 7 4.0 7.677 3.2 2.44 71.1 0.89 70.5 17.5 (0.040) 20 September 1994 Spisula solidissima 17.117 9.1 39.67716.0 13.0 (0.014) 5.567 3.19 7.78 7 4.6 4.5 (o0.001) 14 November/2 December 1994 Polygordius jouinae Spionidae juv. sp. A 46.44 7 26.6 146.67741.7 7.0 (0.002) 46.337 30.35 381.88 7 206.3 15.5 (0.046) 0.0 0.0 13.78 7 17.9 37.007 60.4 93.33764.9 23.78 7 25.3 360.22 7 230.5 8.677 3.67 9.007 8.91 11.56 7 6.1 21.227 18.81 5 June 1995 Polygordius 19.677 17.8 jouinae Nemertea sp. A 6.337 2.7 Tellina agilis 0.30 70.32 2.667 1.49 1.22 7 0.63 17.0 (0.036) 18.0 (0.032) 13.0 (0.015) Mann–Whitney test for differences in density between crest and trough habitats at a =0.05 are also given. Only species and corresponding dates with significant differences in density are listed. Mann–Whitney U values are followed by p-values in parentheses. Fig. 8. (A) Spatial patterns defined by principal components analysis (PCA) of sediment properties. First two axes explain 45% and 32% of variance in data respectively. Individual cores labeled according to (A) habitat and by stippling/shading (for illustration purposes only), (B) Plot of factor loadings for each environmental variable. Heavily weighted loadings (70.5) are bolded. a typical ripple (14–30 cm) in 35–75 min (Ramey, 2008; Ramey and Bodnar, 2008). Infauna may also be passively transported to (via re-suspension and bedload transport) and deposited in troughs (Eckman, 1979). There is also some evidence for larval and/or post-larval selection, and passive or active transport via re-suspension of bedload by P. jouinae and juvenile S. solidissima at the study site (Snelgrove et al., 1999). Inconsistent distribution patterns of S. solidissima in crests and troughs may have resulted from differences in susceptibility of individuals to re-suspension or bedload transport. The need for this suspension-feeding clam to remain in sediments with relatively higher levels of sedimentary organic matter is probably much less important than for a deposit feeder like P. jouinae. Moreover, juvenile S. solidissima are restricted to surface sediments due to their short siphon length (Zwarts and Wanink, 1989) which makes them susceptible to re- suspension and bedload transport as well as predation. A combination of these factors is likely to influence distributions of infauna between crests and troughs. The consistency and repeatability with which these small-scale species distributions occurred in this study, suggest that they are quickly re-established following sediment re-suspension and ripple migration. Differences in ripple characteristics such as height and wavelength may also be related to the spatial variability of infauna and finer scale patchiness in organic matter distribution in sandy continental shelf sediments. In Botany Bay, Australia Barros et al. (2004) found some evidence for a relationship between spatial variability of infaunal assemblages (500-mm sieve; family level identification) and the size of ripples, where dissimilarity between replicates (within crests and within troughs) and the dissimilarity among replicates (crests and troughs) were greater for larger ripples. Video and still images ARTICLE IN PRESS a b b 25 b a b Flank 120 Crest P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 2231 bc c 100 Fines (%) 80 15 10 5 60 a b 0.03 0.02 0.01 0.00 8.0e-4 6.0e-4 4.0e-4 2.0e-4 0.0 3.5e-4 3.0e-4 Chl a (mg g-1) 1.0e-3 b Trough -SH a Pheophytin (mg g-1) Carbon (%) 0.04 0 Trough +SH Coarse (%) 20 Habitat 2.5e-4 2.0e-4 1.5e-4 1.0e-4 5.0e-5 Trough -SH Trough +SH Flank Crest 0.0 Habitat Fig. 9. Means 7SD for sediment properties, with n= 10–12 (A) medium–to-coarse grained (Z 250), (B) fines (grain sizes o 250 mm), (C) carbon, (D) pheophytin and (E) chlorophyll-a. Same letters (a, b) indicate no significant differences in concentration between habitats (p 40.05, Mann–Whitney U Test). Scales on y-axes differ among panels. taken along our study transect in 2005 indicated considerable variability in ripple morphology with ripple heights ranging from 5 to 15 cm. Some ripples were high, symmetrical, and with sharp crests, whereas others were lower, in a bed that was irregular and hummocky. These heights are comparable to those measured during a particularly active period from August to September 1995 at our study site (heights: 3–15 cm; wavelengths: 10–100 cm), where mean alongshore currents (measured 44 cm above the bottom) near Station 9 were 5–20 cm s–1 and cross-shore currents associated with the tides were generally r8 cm s–1 (Traykovski et al., 1999). Although habitat groupings in the present study were not well separated from each other in ‘‘PCA space’’ this is understandable since species differences among habitats were generally related to differences in density rather than species presence in, or absence from, a particular habitat. Moreover, habitats were close to each other, in a highly dynamic environment with no obvious barrier to species dispersal. Similarity in communities in troughs + SH and troughs SH was likely due to the fact that shell hash was present in relatively small quantities and consisted of small shell fragments rather than large surfclam valves, which also occur as transient features (Able et al., 2003). Shell hash was hypothesized to provide a different habitat because its presence affects the hydrodynamic regime in the surrounding sediments (i.e., velocity and turbulence of the benthic boundary layer; Newby, 2006), and it may also provide areas of refuge from predators (Kamenos et al., 2004). The only dates where communities did not differ between crests and troughs were in July 1994 (Station 9 only) and October 1995. Community differences on these dates were attributed to the largest spatial scale (4 km), i.e., between the inshore and offshore stations, and the sampling dates corresponded with seasons when larval recruitment for certain infaunal species is maximal (Ma and Grassle, 2004). Thus, community differences between the inshore and offshore station may have been a result of factors such as circulation patterns, upwelling and downwelling events, combined with differences in life history strategies, all of which can differentially influence larval supply and settlement at these two locations (Ramey, 2008; Ma, 2005; Ma et al., 2006; Snelgrove et al., 1999; Snelgrove et al., 2001; Weissberger and Grassle, 2003). Moreover, the October 1995 sample collection followed a particularly stormy, two-week period at the end of ARTICLE IN PRESS 2232 P.A. Ramey et al. / Continental Shelf Research 29 (2009) 2222–2233 September. Thus, stormy conditions may have resulted in substantial sediment re-suspension and transport, disrupting the community patterns established in crests and troughs during calmer periods. This study highlights the influence small-scale differences in habitat can have on the distribution and concentration of sedimentary food resources and infaunal species distributions, abundances, and related community measures even in relatively homogeneous, mobile shelf sediments. Several individual taxa showed significantly greater densities in either crests or troughs of sand ripples. Infaunal density and species richness were higher in ripple troughs compared to crests, whereas diversity showed the opposite pattern on the majority of dates examined. A priori knowledge of whether a benthic sample came from a crest or trough helped to explain some of the small-scale infaunal patchiness observed in relatively homogeneous, well-sorted sandy sediments. Ubiquitous and persistent features such as crests and troughs of sand ripples should be considered in future sampling designs when examining infaunal community patterns as they may help identify the scales over which particular processes and mechanisms are important, and aid in the design of experimental studies. Acknowledgements Special thanks to Char Fuller, and the Rutgers Institute of Marine and Coastal Sciences (IMCS) dive team for collecting the infaunal cores at the LEO-15 research site; Kenneth Elgersma for his help on the boat and fixing samples; to Kate Douglas and Tiara Johnson for their assistance in sorting samples in the laboratory; Carola Noji and two anonymous reviewers provided constructive comments on the manuscript. P.A. Ramey was supported by an IMCS graduate assistantship and graduate student research funds from the Rutgers University Marine Field Station, New Jersey. This is contribution 2009-6 from IMCS. 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