Characterising mapping units (biotopes)

There is a discrepancy between the faunal assemblages identified using community analysis methods and what is required from a practically applicable mapping unit used in producing necessarily generalised maps of variation in the biological composition of the seabed. To characterise practical mapping units which can be mapped at a scale appropriate to that of the acoustic data, those clusters identified as faunally distinct (as assessed using the criteria described Sect. 2.5.1) using standard cluster analysis techniques were assessed against a second set of criteria to determine their use as mapping units. Only those clusters which met these criteria were further analysed in terms of their faunal composition and diversity. To function as a mapping unit assemblages must 1) occur at a scale relevant to the resolution of the acoustic data and

58

the scale of existing, widely accepted benthic communities such as cold water coral reefs (e.g. 10 m scale), and 2) be easily identified from video data.

Mapping units, hereinafter referred to as ‘biotopes’, were defined in terms of their characterising species, as determined by SIMPER analysis, together with the range of environmental conditions over which they occurred in this study. A 1-way Analysis of Similarity (ANOSIM) was undertaken on a normalised, Euclidean distance matrix of environment data (depth and temperature) to test if environmental conditions were different between biotopes.

To assess biotopes which could be considered of conservation concern, identified biotopes were compared with current definitions of OSPAR and the EC Habitats Directive listed habitats. To identify specifically those which are listed as VMEs, the guidelines of the FAO and current OSPAR definitions were used.

3.2.4.i Diversity indices

Diversity in terms of species richness and dominance were measured to compliment the characterisation of biotopes, and allow a more complete description of the assemblages.

Simpson’s Reciprocal Index [1/D] was measured using the DIVERSE routine in Primer v6 (Clarke and Gorley 2006) to give Simpson’s diversity index (λ) and the reciprocal form taken by 1/D. Count and cover data were measured separately for each sample image and then averaged to give a single Simpson measure per image, and expressed as the mean Simpson’s Reciprocal Index per biotope (as described in Sect.2.5.2).

Species richness was measured using two methods: firstly, simply by measuring species richness per sample image and expressing as mean species richness per biotope; and

59

secondly, due to variation in numbers of sample images (sample size) between the biotopes, standardised species richness was measured using rarefaction curves to calculate expected number of species (Mao tau Sobs) and incidence-based species richness estimators (ICE, Chao 2, Jackknife 1 and 2, and bootstrap) using EstimateS 8.3.

One-way Analysis of Similarity (ANOSIM) tests [Primer v6 (Clarke and Gorley 2006)]

on Euclidean distance resemblance matrices were undertaken to test for significant differences in diversity between biotopes (H^o: no significant difference in diversity between biotope). Univariate ANOSIMs were undertaken to compare mean species richness and dominance between biotopes and a multivariate ANOSIM was also undertaken, using a suite of normalised diversity measures [Simpson’s Reciprocal Index, expected species richness (Sobs) and the five incidence-based estimators] to give a holistic view of the diversity measure.

3.2.4.ii Distribution of biotopes Megahabitat scale

Given the potential difference in the hydrographic conditions on NW and SE sides of the seamount, analysis was undertaken to test the difference between species assemblages from the NW and SE side of the seamount. A full factorial type I SS covariate PERMANOVA (PERmutational MANOVA, Anderson 2008) test was performed using the previously calculated Bray-Curtis similarity matrix, first testing for variance explained by the covariate depth, then for differences between ‘locations’

(fixed main effect), substratum (random factor nested within location) and finally the interactions.

60 Mesohabitat scale

Video transects were reviewed and visually classified (guided by the sample image analysis cluster output) using the newly defined biotopes, and changes of biotope type within a transect were mapped using ArcGIS 9.3. Biotope mapped video data were overlaid on an interpreted geomorphology (undertaken by H. Stewart, BGS) polygon layer in ArcGIS and used to qualitatively describe the distribution of biotopes in relation to meso-scale geomorphology, particularly focusing on those biotopes identified as VMEs. Abiotic data were also extracted from the mapped data to define the environmental range of the distribution of each biotope.

61 3.3 Results

3.3.1 Geomorphology

In total 215 line km of multibeam echosounder data were collected over the NW and SE survey areas on Anton Dohrn Seamount covering 220.5 km². Interpreted multibeam data was used to identify a number of geomorphological features (Fig. 3.3 and 3.4), and interpretation was undertaken by H. Stewart from the BGS (see sect. 2.6.1). The geomorphology of the seamount can broadly be divided into the summit and flank (megahabitat scale of kilometres to tens of kilometres; sensu Greene et al. 1999) with meso-scale features associated with each (Table 3.1).

Summit Flank

Cliff-top mounds Flute Cliff edge Escarpment

Parasitic cone Radial ridge Landslide/Rockfall Furrow/moat

Table 3.1: Meso-scale geomorphological features identified from the summit and flank of Anton Dohrn Seamount. Interpretation undertaken by H. Stewart from the BGS.

62

Fig. 3.3: Plan (a) and 3D perspective view (b) of multibeam bathymetry acquired over the NW flank of Anton Dohrn Seamount, meso-scale geomorphic features labelled. Fig 3.3a shows polygons of meso-scale geomorphological features interpreted by BGS. 3.3b is visualised in Fledermaus^TM software looking south, for scale of features see Fig. 3.3a and bathymetry colour bar see Fig. 3.3b.

a b

63

Fig 3.4: Plan (a) and 3D perspective view (b) of multibeam bathymetry acquired over the SE flank of Anton Dohrn Seamount, meso-scale geomorphic features labelled. Fig 3.4a shows polygons of meso-scale geomorphological features interpreted by BGS Fig 3.4b is visualised in Fledermaus^TM software looking west, for scale of features see Fig. 3.4a and bathymetry colour bar see Fig.3.4b.

a b

64

A prominent cliff edge encircles the whole seamount and was imaged in both survey areas, below which is a steep escarpment with slopes up to 40°. On the NW side of the seamount, almost vertical flute-like features, or ridges, were visible on the cliff face (flanks) (Fig. 3.3b). These flutes probably reflect variability in the erodability of the bedrock which may be due to the emplacement of igneous dykes in fissures extending from the Anton Dohrn igneous centre (Long et al. 2010). Radial ridges were imaged in both the NW and SE survey areas (Fig. 3.3 and Fig. 3.4); these radial ridges comprise extensive areas of rock and coarse material at seabed and are up to 100 m in height extending radially from the seamount. In the NW study area the largest radial ridge observed was almost 1 km long and ~ 0.5 km wide with slopes of up to 40°; and extending from 1260 m to 1550 m at its deepest point. A radial ridge on the SE side was similar in size and slope (almost 1.5 km long and 0.5 km wide with slope up to 42°) but occurred deeper at 1500 m at its shallowest point to almost 1800 m.

Cliff top mounds were identified from both study areas although these features were less significant in terms of their size [interpreted from the multibeam bathymetry data]

on the SE flank compared to the NW flank. In the NW study area, cliff top mounds were visible along the cliff edge with each mound up to 50 m in height and 250 m wide (Fig. 3.3). In the SE study area cliff top mounds were less frequent and smaller being up to 20 m in height and 70 m wide. Parasitic cones had been identified on the lower flanks of Anton Dohrn Seamount from existing lower resolution multibeam echosounder data.

Two previously identified features were located in the NW study area (Fig. 3.3), the largest of which was broadly conical in shape, ~400 m high and ~1300 m in diameter with slopes up to 45° and ranging from a depth of 1420 m to1750 m. These features have been interpreted as parasitic cones due to their shape and proximity to the Anton Dohrn igneous centre. Parasitic cones form from volcanic material erupting from lateral

65

fractures rather than the central, main vent of a volcano (Allaby and Allaby 1990).

Moats have formed around the base of the parasitic cones in the NW study area, probably due to accelerated currents caused by the obstructive presence of the cones themselves (Long et al. 2010).

An area of uneven topography located at the base of the cliff at the western end of the NW study area has been identified as a submarine landslide or rockfall (Fig. 3.3) and appears to relate to a gully, or shute, on the cliff wall above. The distance from the headwall to the toe of the slide is 3.5 km and falls a height of 850 m. The clear evidence for this slide suggests that it is a young feature as it has not been reworked; however, the general uneven topography observed on the flanks suggests that there are multiple, probably small, slope failures (Stewart et al. 2009).