The ‘Graveyard’ and ‘Andes’ seamount complexes were surveyed in 2015 as part of a broader study examining the impacts of fishing on seamount benthic communities and their recovery potential (Clark and Rowden, 2009; Clark et al., 2010a, 2019; Williams et al., 2010; Figure 1). Both seamount complexes are located on Chatham Rise, a submarine continental ridge extending approximately 1,000 km east from South Island, New Zealand. Situated below the Subtropical Front, Chatham Rise is a highly productive region (Murphy et al., 2001; Sutton, 2001) with high benthic biodiversity and biomass (Nodder et al., 2012). Commercial deep-water trawl fisheries for orange roughy (Hoplostethus atlanticus), oreo (Allocytus niger and Pseudocyttus maculatus), and alfonsino (Beryx splendens) target Chatham Rise seamounts (Clark and O’Driscoll, 2003; O’Driscoll and Clark, 2005; Clark et al., 2010b).

Each seamount complex examined here comprises features with varying fishing histories (i.e., untrawled, previously trawled, and actively trawled). The Graveyard Seamounts complex includes 28 seamounts ∼100-400 m in elevation, spanning depths from 750 to 1,600 m (Figure 1). The Andes Seamounts are a cluster of 12 seamounts ranging in depth from 500 to 1,300 m (Figure 1). One seamount with no known bottom trawling history was selected from each seamount complex to investigate variation in mega-epibenthic community structure within untrawled seamount habitats. These two seamounts were chosen to represent different physical, environmental, and biological characteristics on Chatham Rise. Ghoul (Graveyard complex) is a small seamount, or ‘knoll’, covering an area of 0.16 km² with a total height of ∼100 m and roughly conical shape (Figure 1). From summit to base, it ranges from 930 to 1030 m depth. Bottom currents near Ghoul flow at maximum speeds of 10-20 cm^–s (Nodder and Northcote, 2001), forming a moat around the base of the seamount (Clark and Rowden, 2010). Diamond Head (Andes complex) is a group of 3 rugose, domed peaks, though here data was only analysed from the central peak, which covers an area of 3.13 km² and has an elevation of 300 m spanning water depths of ∼600-900 m (Figure 1). Both Ghoul and Diamond Head, along with many other New Zealand seamounts, were formed by widespread Cenozoic intraplate volcanism (Gamble et al., 1986; Hoernle et al., 2006; Timm et al., 2010; Mortimer and Scott, 2020). Surface productivity near both seamounts is high, with estimated mean chlorophyll a around 0.5 and 0.4 mg Chl-a m^–3 at Ghoul and Diamond Head, respectively (Chiswell et al., 2013).

The study seamounts were sampled with the National Institute of Water and Atmospheric Research (NIWA) Deep Towed Imaging System (DTIS, Hill, 2009; Bowden and Jones, 2016) from the R/V Tangaroa. DTIS is a towed camera frame equipped for the 2015 voyage with a Canon EOS 400D used to capture downward-facing high definition still images automatically triggered every 15 s at 10-megapixel resolution. Simultaneous high definition (HD 1080i) video was recorded. Video and still images were scaled using paired 0.2 m spaced parallel lasers projected into the imaged area of each camera. For the present analysis, still images were used in preference to video because the still images are more appropriate for development of fine-scale quantitative data on benthic community structure as they have higher image resolution and their seabed areas can be calculated more accurately than from video.

Photographic transects were conducted from the peak of each seamount down to its base along the cardinal and intercardinal directions (N, NE, E, SE, S, SW, W, and NW), at a target speed of 0.2-0.5 m/s and target altitude of 2-3 m. The position of DTIS was monitored by an acoustic ultra-short baseline (USBL) transponder system (Kongsberg HiPAP) and recorded using OFOP (Ocean Floor Observation Protocol) software. DTIS position data was synced to the absolute time each image was taken to obtain precise positions for each seabed image. A Seabird CTD was attached to the DTIS frame to obtain temperature, salinity, and pressure data along each transect. Four DTIS transects (N, E, S, W) from the total eight were selected from Ghoul and Diamond Head seamounts for the present community analysis, while the remaining transects were only analysed to collect additional substrate composition data (Figure 1; see section “Random Forest Model Input Data”). For subsequent analyses, image samples were binned into “sectors” (i.e., the summit and north, east, south, and west sides) for a portion of the community analyses (see section “Community Structure Analysis”). The summit sector was defined by a circle with a radius equal to 0.3 × seamount elevation (Clark et al., 2019) and each side was defined based on the direction of the image sample transects.

All selected images (494 on Ghoul, 999 on Diamond Head) were reviewed and analysed using the online image analysis tool BIIGLE 2.0 (Langenkämper et al., 2017). Images of insufficient quality (e.g., obscured by sediment disturbance) were removed from further analysis, reducing the total number of suitable images to 1,233 (380 on Ghoul, 853 on Diamond Head). All visible epi-benthic megafauna (animals living on or above the seafloor with sizes ≥ 2 cm) were identified to the finest possible taxonomic resolution and counted. Taxonomic guides were referenced (e.g., Tracey et al., 2011, 2014; Opresko et al., 2014) and representative images of observed taxa were sent to taxonomists to ensure correct and consistent identifications of individuals from analysed imagery (see section “Acknowledgements” for taxonomists referenced). Taxon names were designated based on the best practice recommendations for image-based identifications provided by Horton et al. (2021).

In order to quantify substrate composition, 15 dots were randomly overlaid on each image and the substratum at each point classified as one of 13 substrate types (bedrock, pebble, cobble, boulder, rocky outcrop, mud, sand, soft overlay, coral rubble, dead intact stony coral matrix, live intact stony coral matrix, urchin spines, shell hash) based on classifications by Wentworth (1922) and classifications used by previous studies (Clark and Rowden, 2009; Clark et al., 2019). Live coral matrix was re-categorised as bedrock (the substrate to which the live corals are attached) for subsequent analyses, to prevent the issue of including reef-forming coral species in both the substrate and abundance data. This treatment reduced the number of substrate types to 12. Image areas were calculated by reference to the paired lasers and if these could not be detected fauna of known consistent size were used as the scale reference. If neither of these methods was possible, a moving average of image area was calculated and plotted against time along the video transect to extract approximate areas for images of unknown size. That is, based on known areas of other images along each transect where DTIS was at a similar height above the seafloor.

The spatial extent of each seamount (basal perimeter) was delineated to ensure only images from the seamounts were included in subsequent analyses. First, a grid of 25 m cells (the same grid size used to generate bathymetric data from previously collected multibeam echosounder data; see section “Environmental Analysis”) was overlaid on the seamount bathymetry in the Geographic Information System (GIS) ArcGIS Pro 2.6.1. Then a polygon describing the extent of the seamount (along the outer extent of each grid cell) was defined by the point at which concave slope values increase to >20°, which is a typical slope for small seamounts (Grosse et al., 2012; Sánchez-Guillamón et al., 2018). These polygons were then visually compared in GIS with substrate and bathymetry data to help ensure non-seamount communities (e.g., communities of mud sediment on relatively low slope seabed) were not included (Supplementary Figure 1). This process resulted in a total number of 996 images within the seamount basal perimeters.

The influence of image area on the numbers of individuals and taxa observed was examined to evaluate the potential bias due to the influence of image area on the number and sizes of organisms reliably identified from imagery. Images included in the analyses were thus limited to those with visible seabed areas between 1 and 5 m², a range in which there was no apparent relationship between number of individuals/taxa and sample area (Supplementary Figure 2). This image treatment step reduced the total number of images included for analysis to 818 (262 images on Ghoul, 556 images on Diamond Head). Raw counts of individual taxa from each image were standardized as individuals per 5 m².

Substrate and faunal abundance data from individual images were allocated to 25 m grid cell ‘samples’ based on the multibeam bathymetry data (see section “Environmental Analysis”), to ensure the biological data matched the finest resolution of the environmental data. This data treatment step yielded 181 total samples (44 on Ghoul, 137 on Diamond Head). Mean standardised taxon abundances and substrate composition were then calculated for each sample. Individual counts and taxon richness were highly variable among samples, with no apparent influence of the number of image replicates per sample on the number of individuals/taxa. However, variability in the number of individuals and taxa was particularly high among samples with one image replicate, therefore, these samples were removed from the analysis, reducing the total sample number to 159 (40 on Ghoul, 119 on Diamond Head).

Detailed bathymetry data were obtained over the study areas in 2002 and 2009 using a Kongsberg EM300 multibeam echosounder (Clark and Rowden, 2009). Backscatter data collected during these surveys unfortunately did not encompass the entirety of the Ghoul study area and could therefore not be used to obtain continuous substrate proxy data for this study. Bathymetry data were gridded at a cell size of 25 m and exported as ESRI gridded formats for use in GIS (ArcGIS Pro 2.6.1). These data were then used to generate seafloor terrain variables: mean, standard deviation, and range of depth; aspect, converted to northness and eastness values (Walbridge et al., 2018); seafloor ruggedness, calculated as the vector ruggedness measure (Sappington et al., 2007; Walbridge et al., 2018); mean, plan, and profile curvature; and mean and standard deviation of slope (Table 1). All seafloor terrain variables except mean depth were gridded at the native 25 m spatial scale and four focal mean sizes (3 × 3, 5 × 5, 7 × 7, and 15 × 15 cells) to allow examination of the influence of spatial scale on community structure.

Temperature, salinity, and pressure values obtained from along-transect CTD measurements were averaged for each 25 m grid cell. Substrate diversity of each sample was calculated using Hill’s N2 metric (Hill, 1973). Percent compositions of each substrate type were averaged per sample. A principal component analysis (PCA) was performed on substrate composition values to reduce the total number of environmental variables for subsequent regression analyses (Ellingsen et al., 2007). The first three principal component (PC) axes accounted for 85.1% of the cumulative variation between samples. The coordinates of each sample on the first three PC axes, termed the PC scores, were thus used to describe sample substrate composition instead of the image-derived composition values. PC 1 (41.2% variation) broadly represented sand and bedrock composition (coefficients: 0.755 and -0.619). PC 2 (33.6% variation) was mostly represented by coral rubble (coefficient: 0.850). PC3 (10.3% variation) accounted for pebble and sand composition (coefficients: 0.648 and -0.474) (Supplementary Figure 3). PC 1, 2, and 3 are hereafter referred to as “sand/bedrock,” “coral rubble,” and “pebble/sand” substrates.

Taxon abundance and environmental data were separated by seamount and analysed with multivariate statistical routines in the software package PRIMER v7 (Clarke and Gorley, 2015) with PERMANOVA + (Anderson et al., 2008). Image analyses identified 101 taxa from Diamond Head and 78 from Ghoul (Supplementary Table 1). Faunal count data were square-root transformed to down-weight the effect of highly abundant taxa and ensure rarer taxa still influenced patterns in community structure. Despite this transformation, the abundances of Stylasteridae spp. (spiny morphotypes) on Diamond Head remained substantially higher than all other taxa in nearly all samples. Therefore, a more severe fourth-root transformation was applied to this taxon for the Diamond Head dataset.

Bray-Curtis resemblance matrices were generated from each transformed dataset to quantify dissimilarity among samples within seamounts. Group-average hierarchical clustering (CLUSTER) with similarity profile (SIMPROF) tests were performed on the resemblance matrices to identify samples with similar community structure within each seamount. A p-value of 0.01 was used for the SIMPROF test to help ensure that clear evidence of heterogeneity in faunal abundance and composition was identified between groups before separating groups with few samples (Clarke et al., 2008). Non-metric multidimensional scaling (nMDS) plots were generated to visualize relationships within and between SIMPROF groups, hereafter referred to as “communities.” Similarity percentage (SIMPER) analysis was performed using a cumulative cut-off of 70% to identify taxa which distinguished between communities and contributed to overall within-community similarity. Locations of the identified communities were overlaid on digital terrain models of each seamount using GIS to examine spatial patterns in community structure.

Beta diversity analyses were conducted to further examine spatial variation of mega-epibenthic taxa within each seamount. The R package “betapart” was used to calculate the Jaccard dissimilarity indices turnover (β_JTU), nestedness (β_JNE), and total beta diversity (β_JAC) from taxon presence-absence (see Baselga and Orme, 2012 for calculations). Beta diversity was measured at three spatial scales: (1) whole seamount; (2) among seamount sectors (i.e., the summit and north, east, south, and west sides); and (3) within each sector.

The potential influence of habitat and other environmental parameters on community structural variation within each seamount was analysed using distance-based linear models (DistLM routine in PERMANOVA+). DistLM analyses relationships between predictor variables and a dissimilarity matrix using a regression or multiple regression model (McArdle and Anderson, 2001; Anderson et al., 2008) and thus can be used to identify environmental variables that correlate most strongly with community structure (Anderson et al., 2008). When two variables were highly correlated (Pearson’s r > 0.85), the variable which was correlated with a greater number of additional variables was removed. Depth, however, was retained despite its high correlation with temperature, salinity, and pressure because it was directly measured over each seamount’s extent, while the others were derived from modelled relationships and may not be as accurate. All focal-mean scales were also retained for the selected variables. This selection yielded a total of 39 environmental variables (see bolded variables in Table 1).

Distance-based linear models were run with step-wise selection and the Adjusted R² criterion (Anderson et al., 2008), with separate analyses for each seamount and for each focal mean scale (i.e., 1 × 1, 3 × 3, 5 × 5, 7 × 7, and 15 × 15 cells), resulting in 5 models for each seamount. Environmental data derived from imagery, including substrate diversity and substrate composition PCs 1, 2, and 3, were included as starting conditions in all models. Distance-based redundancy analysis (dbRDA) plots were produced to visualize environmental variables which contributed most to variation between communities for the best DistLM models.

Hierarchical clustering with SIMPROF analysis identified 6 communities on Ghoul and 16 communities on Diamond Head. Of these, community v from Ghoul and community k from Diamond Head each comprised single samples. The nMDS plots showed that Ghoul communities generally formed relatively dispersed but distinct clusters (Figure 2A). In contrast, Diamond Head communities were more tightly clustered, but with an apparent separation between the more dispersed clusters for communities p and o and the remaining 14 communities (Figure 2B).

Similarity percentage showed that within-community similarity ranged from 43-71% on Ghoul and 43-75% on Diamond Head. Three to nine taxa contributed to 70% of the within-community similarities on Ghoul, while one to nine taxa contributed to 70% on Diamond Head (Table 2). Representative images of each community were selected to help visualise the SIMPROF/SIMPER results and distinguish communities (Figure 3). The seamount benthic communities were assigned categories according to the most important characterising taxa identified by SIMPER analysis to further distinguish communities within the two seamounts. These community categories include: stony coral reef communities; octocoral communities; mixed communities characterised by demosponges, hexactinellids, stylasterids, and comatulid crinoids, and communities dominated by pagurids (hermit crabs) (see colour coding in Figure 3).

The Ghoul stony reef community x (Styl.Sol) was mostly characterised by stylasterid hydrocorals, the reef-building coral Solensomilia variabilis, and reef-associated taxa including Polychaeta sp. 1, demosponges, and the crinoid Comatulida Sp. 1. Stylasterid hydrocorals, Comatulida sp. 1, Solenosmilia variabilis, and ophiuroids contributed most to the within-group similarity of the other stony reef community w (Sty.Com1). The pagurid community t (Pag.Dem) had the lowest within-group similarity and was characterised by only three taxa: pagurid hermit crabs, demosponges, and the crinoid characterized. 1. The mixed community u (Dem.Com1) was mostly characterised by demosponges, Comatulida sp. 1, pagurids, and Actinaria sp. 2, whereas the other mixed community y (Com.Sty) was characterised by Comatulida sp.1, stylasterid hydrocorals, Ophiurida spp., and demosponges.

Pagurid hermit crabs and spiny stylasterid hydrocorals were identified as characterising taxa in 13 of 15 Diamond Head communities included in the SIMPER analysis. The mixed communities on Diamond Head differed in their most important characteristic taxa. Comatulida Sp. 1 contributed most to within-group similarity of communities h (Com1.Pec), g (Com1.Pag), c (Com1.Dem), and e (Com1.Pag), whereas Pectinidae spp. Scallops contributed most to community i (Pec.Pag) and d (Pec.Com1) similarities. Demosponges contributed most to community b (Dem.Com) similarity. Farrea spp. Were among the characterising taxa of the mixed communities i (Pec.Pag), d (Pec.Com1), j (Sty.Pag), h (Com1.Pec), g (Com1.Pag), c (Com1.Dem), e (Com1.Pag), and b (Dem.Com1). The octocoral communities p (Ant2.Sty), n (Ant2.Pag), and m (Alc1.Ant) were characterised by the soft coral Anthomastus sp. 2. In contrast, primnoid octocorals were characteristic of community f (Pri6.Com1) and also contributed to within-group similarity of the mixed community c (Com1.Dem). The stony reef community l (Alc1.Ena) was characterised most by Alcyonacea sp. 1 and the reef-building coral Enallopsammia rostrata.

Spatial distribution patterns of the benthic communities varied between the seamounts (Figure 4). Communities on Ghoul were distributed by depth and aspect (side) of the seamount: community × (Sty.Sol) occurred on the summit; community y (Com1.Sty) on the upper to mid-flanks on all four sides; community u (Dem.Com1) mostly on the lower flanks on the north and west sides; community t (Pag.Dem) on the lower flanks of the south/east sides, and communities w (Sty.Com1) and v only occurred on the upper flank of the north side. The benthic communities on Diamond Head were also distributed by depth and broad-scale aspect, but depth-related patterns among communities were less distinct than on Ghoul. The summit of Diamond Head was mostly occupied by communities p (Ant2.Sty), m (Alc1.Ant2), n (Ant2.Pag), and l (Alc1.Ena). Community distribution on the flanks was more variable. Community o (Pag.) mostly occurred on the upper flanks along a concave rim on the north, south, and west sides. The distributions of several communities were limited to one seamount side: community d (Pec.Com1) on the lower flanks of the east side; community h (Com1.Pec), f (Pri6.Com1), and c (Com1.Dem) on a small pinnacle on the west side; community b (Dem.Com1) and g (Com1.Pag) on the lower flanks of the north side. Community i (Pec.Pag) was distributed on the east, south, and west sides at varying depths along the seamount. Community k comprised a single sample which occurred on a pinnacle on the south side.

Total beta diversity was higher among samples (β_JAC > 0.9) than among sectors (β_JAC ∼ 0.6) for both seamounts (Table 3 and Supplementary Figures 8, 9). Within sectors, beta diversity was highest for the east flank of Ghoul (β_JAC = 0.868) and the west flank of Diamond Head (β_JAC = 0.95), and lowest for the summits of both seamounts (Ghoul β_JAC = 0.675, Diamond Head β_JAC = 0.812). In all comparisons, beta diversity was primarily attributed to species turnover (β_JTU 0.5-0.97), rather than nestedness (β_JNE 0.01-0.17).

The best DistLM model (highest adjusted R² value) for Ghoul included all 11 environmental variables at the 25 m spatial scale and explained 51.2% of the sample variation (Table 4). Seven variables included in the best model were significantly correlated with community structure (p-Value ≤ 0.05): depth; sand/bedrock; standard deviation of slope; pebble/sand; profile curvature; coral rubble, and slope (Table 5). The best model for Diamond Head included all 11 environmental variables at the 15 × 15 scale, which explained 45% of the total sample variation (Table 4). Nine variables were significant: depth; sand/bedrock; eastness; substrate diversity; pebble/sand; coral rubble; slope, and profile curvature (Table 5). Depth explained the largest proportion of variation on both seamounts (Ghoul: 21%, Diamond Head: 18.3%), followed by sand/bedrock substrate composition.

The first two axes of the dbRDA plots (dbRDA1 and dbRDA2) explained 42.6 and 40.4% of total community variation on Ghoul and Diamond Head, respectively (Figure 5). For Ghoul, dbRDA1 was largely correlated with coral rubble substrate, while dbRDA2 was correlated mostly with depth, standard deviation of slope, and substrate diversity (Figure 5A and Supplementary Table 2). Sand/bedrock and pebble/sand substrates mostly distinguished community t (Pag.Dem) from the other five communities. Higher substrate diversity distinguished community u (Dem.Com1) from communities t (Pag.Dem), v, x (Sty.Sol), and w (Sty.Com1). Community x (Sty.Sol) was mostly separated from the other communities by increased ruggedness. For Diamond Head, dbRDA1 was mostly correlated with sand/bedrock substrate composition, while dbRDA2 was correlated largely with depth and standard deviation of slope (Figure 5B and Supplementary Table 2). Sand/bedrock substrate distinguished communities o (Pag) and p (Ant2.Sty) from the other communities, while higher values of substrate diversity distinguished community d (Pec.Com1). Communities m (Alc1.Ant2), n (Ant2.Pag), and l (Alc1.Ena) were mostly distinguished by lesser depths.

Our analysis identified six communities on Ghoul Seamount and sixteen on Diamond Head Seamount. Although both study features are considered small seamounts (elevations ≤ 1000 m), the difference in the number of mega-epibenthic communities identified on the two seamounts is likely due to differences in the size and morphology of these features; Diamond Head being larger and having greater seafloor topographic variation than Ghoul Seamount. The number of benthic communities identified on both seamounts is comparable to similar studies of large seamounts (elevation ≥ 1,000 m) in the Pacific Ocean, which have identified between 8 and 14 mega-epibenthic communities on a single seamount (Boschen et al., 2015; Du Preez et al., 2016; Morgan et al., 2019). The range in the number of such communities so far identified on seamounts is likely due to a combination of factors, including differences in the size and physical characteristics of the seamounts, as well as the sampling density, taxonomic resolution of the data and clustering method used to identify communities. Most of the benthic communities identified in this study were characterised by a high abundance of one to several taxa: predominately hydrocorals, comatulid crinoids, galatheid squat lobsters, alcyonacea corals, and sponges. Similarly, analyses of benthic communities from other seamounts showed that a small number (∼1 to 5) of species predominated and characterised distinct communities (Henry et al., 2014; Boschen et al., 2015; Serrano et al., 2017; De la Torriente et al., 2018; Morgan et al., 2019).

Although the taxon compositions within the four main community types (coral reef, octocoral, mixed, and hermit crab) differed between the two seamounts examined, similar spatial distribution patterns were observed. Coral reef communities were primarily distributed on the summits. Octocoral communities were only identified from Diamond Head but were also primarily distributed on the summit. In contrast, the mixed communities (e.g., communities characterised by demosponges, comatulid crinoids, and stylasterids on Ghoul and comatulid crinoids, pectinid scallops, pagurids, demosponges, and stylasterids on Diamond Head) were distributed along the seamount flanks. On both seamounts, the hermit crab communities mostly occurred at the seamount base and/or deeper flanks. These differences in spatial distribution between community types are likely to be partially attributable to habitat variation with position on the seamount. For example, the summit is likely to be characterised by accelerated currents which provide increased trophic input for large suspension-feeding corals (Clark and Bowden, 2015; Serrano et al., 2017; Lapointe et al., 2020), whereas rocky outcrops and boulders on the steeper seamount flanks may provide filter-feeders in mixed communities (e.g., stylasterid hydrocorals, comatulid crinoids, pectinid scallops, alcyonacean corals, bryozoans, demosponges, glass sponges) with patches of suitable substrate for attachment and access to above seabed currents (Beaman et al., 2005; Du Preez et al., 2016; Lacharité and Metaxas, 2017). Areas near the seamount base typically have a greater proportion of unconsolidated sediments, which provide suitable habitat and food for mobile scavengers (e.g., hermit crabs) and infauna (Beaman et al., 2005; Du Preez et al., 2016; Serrano et al., 2017).

Like the studies of Victorero et al. (2018), Morgan et al. (2019), we found that beta diversity within both seamounts was mostly attributed to faunal turnover, rather than nestedness. The turnover component of beta diversity on Ghoul and Diamond Head was always greater within seamount side sectors than between sectors, indicating that faunal turnover occurred mostly over changes in depth, rather than over horizontal gradients at similar depths. In contrast, Morgan et al. (2019) reported high faunal turnover between sides on Mokumanamana Seamount. This difference is likely attributable to the small sizes of Ghoul and Diamond Head (elevations 100-300 m) relative to Mokumanamana (elevation > 700 m), meaning there is little variation in niche-defining parameters between seamount sides (e.g., food availability, substrate suitability, current speed) while on Mokumanamana oceanographic conditions can differ greatly among seamount sides.

Our distance-based regression analysis revealed that depth had the greatest explanatory power for the community structure observed on both study seamounts. Variation in megabenthic community structure has been related to depth on other seamounts (Lundsten et al., 2009; Boschen et al., 2015; McClain and Lundsten, 2015; Du Preez et al., 2016; De la Torriente et al., 2018; Victorero et al., 2018; Auscavitch et al., 2020a, b). Many of these other seamount studies were conducted along comparatively large depth gradients, which would be expected to exhibit strong corresponding environmental gradients and consequently vertical zonation of benthic communities. The narrow depth ranges of the seamounts examined here (Ghoul: 930-1,030 m, Diamond Head: 600-900 m) mean that variables such as temperature did not vary much (5.2-6.1 and 6.6-8.0°C for the Ghoul and Diamond Head samples, respectively). It is thus surprising that depth was more strongly correlated with community structure here than other variables which were included in the analysis. However, bottom temperature variations of 1-2°C have been shown to influence distributions of deep-sea coral and sponge species (Dijkstra et al., 2021). High levels of benthic community variation have also been observed on a Hawaiian seamount over comparatively narrow depth ranges of 320-530 m (Long and Baco, 2014). Depth is likely a proxy for one or more other variables which could influence fine-scale species distributions, such as seabed current strength and organic matter flux, measures of which it was not possible to include as variables in the analysis. Substrate type along a gradient from sand to bedrock substrate (PC 1) had the second-highest explanatory power for community structure on both study seamounts, supporting other studies which have observed that benthic communities show variation as substrate changes across a range of spatial scales within a seamount, with particularly noticeable differences in faunal composition between consolidated hard substrate (e.g., boulders) and unconsolidated soft substrate (e.g., sandy mud) (McClain et al., 2010; Sautya et al., 2011; Henry et al., 2014; Boschen et al., 2015; Clark and Bowden, 2015; Du Preez et al., 2016; Serrano et al., 2017; Victorero et al., 2018; Morgan et al., 2019; Lapointe et al., 2020).

Slope characteristics [standard deviation of slope, slope (both correlated with depth range), and profile curvature (correlated with plan curvature)] were more influential environmental predictors of communities on Ghoul than those on Diamond Head, which were correlated more with eastness, substrate diversity, and ruggedness (correlated with curvature). This pattern is reflected in the observed distribution of the seamount communities, with Diamond Head communities varying more with broadscale aspect (i.e., seamount side) than those on Ghoul Seamount. Small pinnacles are present on the west and south sides of Diamond Head, but not on the north or east sides, which may contribute to fine-scale spatial variation in seabed current speed and/or direction, and consequently community variation with aspect on this feature through the unequal partitioning of food (Genin et al., 1986; Dower et al., 1992; Mohn et al., 2014; Morris et al., 2016).

The best DistLM regression models accounted for 51 and 45% percent of the total variation among the Ghoul and Diamond Head samples, respectively. Similar seamount studies have also found that modelling only explained up to half of the observed benthic community variation, even with the inclusion of additional explanatory variables, such as magnetivity, backscatter, oxygen, surface productivity, and current strength (Henry et al., 2014; Long and Baco, 2014; Boschen et al., 2015; Clark and Bowden, 2015; De la Torriente et al., 2018; Morgan et al., 2019). These findings suggest that seamount community assembly processes are complex and difficult to model using environmental variables alone, or that these variables were not obtained at the most suitable spatial scale for examining ecological processes at the within-seamount scale. Furthermore, because taxa respond to environmental parameters differently (e.g., Dijkstra et al., 2021), it can be difficult to link environmental and habitat variation to community-level dissimilarities. Community structure is also influenced by biological factors which are difficult to sample and include in analyses, such as predation or competition, and/or exhibit a degree of stochasticity that are not easily accounted for in linear regression analyses. Even with these limitations, the results of our analysis indicate that environmental predictors of within-seamount benthic community structure are somewhat similar between the two seamounts. That is, depth, substrate composition, profile curvature, and slope were significantly correlated with community structure on both features. Finally, although our analysis suggested that the spatial scales of important community structuring variables may vary among seamounts (based on our finding that the best regression model for Ghoul included the 25 m environmental data, while the best regression model for Diamond Head included the 15 × 15 focal mean data), the adjusted R² values did not vary greatly between the 25 m and 15 × 15 focal mean regression models for Diamond Head, indicating that the selected spatial scale does not have a major influence on model performance for this seamount.

We used the environmental variables identified by the distance-based regression analysis as being best correlated with observed community structure as predicator variables in our RF community distribution models. The five most influential explanatory variables included in the Ghoul model in order of importance were: coral rubble, depth, slope, standard deviation of slope, and sand/bedrock; and for Diamond Head: depth, ruggedness, eastness, sand/bedrock, and slope. Depth and substrate composition explain community variation within both seamounts, while aspect and ruggedness more greatly influence Diamond Head community structure.

Similar predictive seamount community distribution studies include those by Du Preez et al. (2016) on Cobb Seamount and Serrano et al. (2017) on Galicia Bank. Cobb Seamount is a guyot in the Cobb-Eickelberg Seamount chain which spans an area of 824 km² and depths 20-3,000 m in the northeast Pacific Ocean. Galicia Bank is a non-volcanic, isolated seamount located ∼220 km northwest of Spain in the northeast Atlantic with an area of 1,844 km² and depth range of 600-2,000 m. Du Preez et al. (2016) identified depth and rugosity as the most important predictors of mega-epibenthic community structure on Cobb Seamount. Serrano et al. (2017) found that depth and substrate type were included in the best models for every mega-epibenthic community on Galicia Bank. Despite differences between Cobb Seamount, Galicia Bank, Ghoul, and Diamond Head in terms of overall size, isolation from surrounding features, and depth range, there is a general consistency in selecting depth, substrate, and some measure of seafloor terrain (i.e., rugosity, slope, curvature) as variables that make the best community distribution models on seamounts at fine spatial scales.

Performance of the predictive distribution models for Ghoul and Diamond Head was very good (multi-class AUC 0.85 and 0.9, respectively), and comparable to models made by Serrano et al. (2017) and Du Preez et al. (2016). The overall OOB error rates obtained here were higher than those of Du Preez et al. (2016) for Cobb Seamount (30 and 39% OOB error compared to 21%). Bathymetric variation on Ghoul and Diamond Head is substantially less than on Cobb Seamount, resulting in some explanatory variables having smaller ranges, which may make accurately distinguishing between communities based on these variables alone more difficult. The inclusion of hydrodynamic and/or productivity variables could potentially improve the performance of the community distribution models (Rengstorf et al., 2012, 2014; Pearman et al., 2020) by helping to describe fine-scale environmental variation but these data are difficult to obtain for deep, offshore areas at the spatial scales examined here. Additionally, the exclusion of the minority communities (<5% samples) may have contributed to some of the high OOB error rates we observed.

The predicted distributions of the identified mega-epibenthic communities on Ghoul generally formed bands which partially or fully encircled the seamount, similar to community distribution patterns predicted on Cobb Seamount, a large shallow seamount in the northeast Pacific (Du Preez et al., 2016). However, it is important to emphasise that the summit depth of Cobb Seamount is much shallower than that of Ghoul (∼20 m compared to 930 m), suggesting that the similarity in patch characteristics between these two features may be due to a general pattern of vertical zonation due to high species turnover on seamounts (Rowden et al., 2010), rather than similarities in topography or depth of Ghoul and Cobb. On Diamond Head, community distributions patterns were predicted to vary more, forming patches of varying shape, size, and distribution, which more closely resembled patterns predicted by Serrano et al. (2017) for the large deep northeast Atlantic seamount known as the Galicia Bank. Differences are likely due to differences in habitat requirements and feeding modes by the representative taxa of each community. Octocoral and stony coral reef communities are each characterised by suspension feeders and were predicted to occur in small patches on the summits of both seamounts, where currents are likely to be accelerated and thus provide these taxa with suitable food. This pattern is similar to predicted spatial distributions of S. variabilis on the Louisville Seamount Chain, whereby higher probabilities of S. variabilis presence occurred along the summit-slope breaks (Rowden et al., 2017). On both Ghoul and Diamond Head, the hermit crab communities comprised multiple small patches distributed on the deeper flanks. These areas are characterised by low relief and a high proportion of soft sediment, and thus may provide suitable habitat for a community dominated by scavenging fauna. Mixed communities were predicted to occur across broader depth ranges than the other community types, indicating that the dominant taxa of these communities, including Pectinidae spp., Stylasteridae spp., and Comatulid sp. 1, may have broader realized niches than the dominant taxa of the other community types.

The range of patch characteristics observed on Ghoul and Diamond Head can vary greatly by size, inter-patch distances, density, and contiguity. Predicted community patch areas ranged from 0.06 to 76 ha (0.0006 to 0.76 km²), though it is unclear how the patch areas reported here compare to other studies because previous seamount community distribution models have not described patch characteristics. Du Preez et al. (2016) did report the total surface area of each predicted community’s distribution on Cobb Seamount, which ranged from 1.6 to 70 km². On Cobb Seamount and Galicia Bank, the number of patches per community varied from 1 to > 10. The “pinnacle” and “crinoid” communities were predicted to form single contiguous patches near the summit on Cobb Seamount, similar to the predicted distributions of stylasterids-Solenosmilia-comatulid communities identified on Ghoul. The other Cobb Seamount communities appeared to exhibit varying patch characteristics, from large, mostly contiguous patches with limited distribution (e.g., the “Sand” and “Bare” communities) to highly irregular patch sizes distributed across the seamount extent (e.g., the “Soft Coral” and “Anemone” communities). On Galicia Bank, none of the predicted communities appeared to comprise a single contiguous patch but comprised smaller widely dispersed patches, similar to the patterns predicted on Diamond Head Seamount for stylasterid-pagurid-comatulid crinoid communities.

On both seamounts examined here, inter-patch distances were quite small, reaching a maximum of several hundred metres, indicating that patches of the same community class are potentially highly connected, and that environmental patch characteristics (e.g., substrate type, depth, and slope) may have a greater influence on patch colonization than inter-patch distance. However, the degree of connectivity may vary between communities, as patch contiguity ranged from 0.13 to 0.55 on Diamond Head and 0.21 to 0.77 on Ghoul. Contiguity was often higher for communities which covered a larger portion of the total seamount area, although this relationship was not consistent. Patch density was overall very low, mostly ranging from ∼1 to 6, but substantially higher for pagurid-demosponge-comatulid communities t (25.2) and u (44.1) on Ghoul, supporting the prediction that these communities are generally well-connected. There was no obvious relationship between the metrics inter-patch distance, contiguity index, or patch density with community composition, highlighting the need to further explore this relationship in marine environments, as this is a common practice in terrestrial studies (Wedding et al., 2011).

Our analysis showed that many of the communities identified on Ghoul and Diamond Head were characterised by at least one vulnerable marine ecosystem (VME) indicator taxon, such as stony corals, hydrocorals, anemones, sponges, and gorgonian corals (Parker et al., 2009). Although Diamond Head is protected from bottom trawling impacts as part of a Seamount Closure Area (SCA; Brodie and Clark, 2003), stony coral reef communities predominated by Solenosmilia variabilis on Ghoul may be particularly vulnerable to fishing disturbances. The S. variabilis reef community was predicted to form one patch on the summit, which is often targeted as the landing place of trawl gear before being towed downslope (Clark and O’Driscoll, 2003). This community was also predicted to cover a small area relative to the potential cumulative area swept by trawl gear in a single tow. Assuming a trawl width (i.e., door spread) of 100 m (Clark and Koslow, 2007) and tow length of 2 km (Clark and O’Driscoll, 2003), 0.2 km² seafloor area can be impacted in a single tow, or 20 times the total predicted area of the S. variabilis reef, 0.01 km². A fishery closure protecting the upper 20 m of the Ghoul summit could therefore effectively conserve the reef community. However, enforcement of fishing regulations in protected areas of this size could prove difficult and would require that trawlers have precise systems for determining the location of the trawl net on the seafloor. A more conservative whole-seamount fishery closure may therefore be warranted. The inclusion of Ghoul, whether partial or whole seamount, in a SCA could provide a refuge area for benthic fauna on other seamounts in the Graveyard Complex that are subject to bottom trawling, potentially aiding recovery in those areas.

Knowledge of a community’s patch characteristics (i.e., the predicted patch number, area, and connectivity of a given community type) may provide further important information to designate potential areas for protection from fishing or other human impacts within a seamount. For example, if a seamount benthic community considered for protection (e.g., one characterised by VME indicator taxa) is predicted to occur in multiple patches of varying area sizes, an effective protected area can be designed to balance the number and total area of protected patches to reach a particular conservation objective (e.g., number of individuals, biomass, area cover, percent total population). If the minimum distance needed to maintain population connectivity is known, this metric can also be used to ensure that a potential protected area will keep individuals of the same species inhabiting different patches connected via migration and/or spawning. This approach may be particularly applicable to seamounts in which the community of interest is limited to a particular depth range or area of the seamount, making a within-seamount spatial closure effective for conservation. Multiple within-seamount closures across seamounts in a chain or group may also be an effective conservation option for maintaining connectivity if the community of interest is known to occur in patches on several features in close proximity, or to provide potential areas for recolonisation and recovery from earlier fishing impacts.