Community structure and diversity of scavenging amphipods from bathyal to hadal depths in three South Pacific Trenches

Author’s Accepted Manuscript Community structure and diversity of scavenging amphipods from bathyal to hadal depths in three South Pacific Trenches Nichola C. Lacey, Ashley A. Rowden, Malcolm R. Clark, Niamh M. Kilgallen, Thomas Linley, Dan J. Mayor, Alan J. Jamieson www.elsevier.com

PII: DOI: Reference:

S0967-0637(15)30161-8 http://dx.doi.org/10.1016/j.dsr.2016.02.014 DSRI2597

To appear in: Deep-Sea Research Part I Received date: 30 October 2015 Revised date: 22 February 2016 Accepted date: 23 February 2016 Cite this article as: Nichola C. Lacey, Ashley A. Rowden, Malcolm R. Clark, Niamh M. Kilgallen, Thomas Linley, Dan J. Mayor and Alan J. Jamieson, Community structure and diversity of scavenging amphipods from bathyal to hadal depths in three South Pacific Trenches, Deep-Sea Research Part I, http://dx.doi.org/10.1016/j.dsr.2016.02.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Community structure and diversity of scavenging amphipods from bathyal to hadal depths in three South Pacific Trenches. Nichola C. Lacey1, Ashley A. Rowden2, Malcolm R. Clark2, Niamh M. Kilgallen3,4, Thomas Linley1, Dan J. Mayor5, Alan J. Jamieson1* 1

Oceanlab, Institute of Biological and Environmental Sciences, University of Aberdeen, Main Street,

Newburgh, Aberdeenshire, AB41 6AA, Scotland 2

National Institute of Water & Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington

6021, New Zealand 3

Australian Museum, Sydney, Australia

4

Present address: No current academic affiliation

5

National Oceanography Centre, Southampton, SO14 3ZH, UK

**Corresponding Author: Jamieson, A.J. email: [email protected] Highlights 

Bathyal to hadal Amphipod diversity and community structure were examined.



Hadal community structures differed from abyssal and bathyal depths.



Hydrostatic pressure best explained intra-trench hadal assemblages composition.



Patterns of diversity was best explained by hydrostatic pressure and temperature.



Biogeographic regions for trench amphipoda are defined.

Abstract There are few biological datasets that span large bathymetric ranges with sufficient resolution to identify trends across the abyssal and hadal transition zone, particularly over multiple trenches. Here, scavenging Amphipoda were collected from three trenches in the South Pacific Ocean at bathyal to hadal depths.

Diversity and community structure were examined from stations within

the Kermadec Trench (1490 to 9908 m) and New Hebrides Trench (2000 to 6948 m) and additional data were included from the South Fiji Basin (400 0 m) and Peru-Chile Trench (4602 to 8074 m). The hadal community structure of the Kermadec and New Hebrides trenches were distinct from the surrounding abyssal and bathyal depths and correlated to hydrostatic pressure and POC flux. Low POC flux in the New Hebrides Trench and South Fiji Basin best explained the dissimilarity in abyssal community structure from those of the disparate Kermadec and Peru-Chile trenches. POC flux also best explained patterns in hadal community structure with the Kermadec and New Hebrides Trench communities showing greater similarity to each other than to the eutrophic Peru-Chile Trench. Hydrostatic pressure was the strongest driver of intra-trench assemblage composition in all trench

environments. A unimodal pattern of species diversity, peaking between 4000 and 5000 m, was best explained by hydrostatic pressure and temperature. Keywords: Amphipoda; Community structure; Bathyal; Abyssal; Hadal; Kermadec Trench; New Hebrides Trench; Peru-Chile Trench

Running head: Bathyal - Hadal amphipod community composition

1. Introduction The deep sea is the largest biome on Earth and yet largely unexplored. Extensive abyssal plains represent the single largest contiguous feature of our planet, constituting 75 % of the seafloor, but less than 1 % has been investigated (Ramirez-Llodra et al., 2010). Beyond the continental shelf, the deep sea is punctuated with geological features including submarine canyons, seamounts and larger features such as mid-ocean ridges and subduction trenches that can introduce habitat heterogeneity and therefore alter generalised community structure (Smith et al., 2008; Levin and Dayton, 2009; Jamieson et al., 2010). The deepest trenches extend to the hadal zone (>6000 m depth) (Belyaev, 1972; Wolff, 1960) that although constitutes <2 % of the global seafloor area, accounts for the deepest 45 % of the full bathymetric range of the oceans. Despite the large bathymetric range covered by trenches, there is limited understanding of the ecology of these environments. In places, the hadal trenches interrupt the bathymetric cline from the continental margin to abyssal plain, and these trenches of varying degrees of isolation and connectivity, are likely to disrupt generalised depth-related trends in animals without pelagic larval stages. Deep-sea benthic communities are largely reliant on particulate organic carbon (POC) supply from surface waters which supports the base of the food chain (Ruhl et al., 2008). POC flux diminishes greatly with depth, resulting in <1 % reaching the abyssal plains (Tyler, 1995). The reduction in POC flux at bathyal and abyssal depths has been recognised as a driver of the widely observed unimodal pattern of diversity (species richness) with depth (Stuart et al., 2003; Carney, 2005; Tittensor et al., 2011). However the bathymetric trends observed for continental margin and abyssal plains may not simply continue into the trench environment. The input of food supply to the trench floor is likely influenced by the characteristic V-shape cross-section of trenches that is expected to cause a topographic funnelling of nutrients downslope towards the trench axis (Ichino et al., 2015) enhanced by frequent seismic activity (Oguri et al., 2013) and possibly internal tides (Turnewitsch et al., 2014). The deep-trench sediment is therefore expected to have a higher organic matter composition than neighbouring abyssal plains, as found in the Mariana Trench (Glud et al., 2013).

Understanding the influence of POC flux on trends in deep sea biodiversity is complicated by the interacting effects of pressure and temperature, thought to be strong determinants of the bathymetric distributions of organisms. The amount and quality of POC available to benthic communities is expected to be trench specific because both surface productivity and the depth at which labile POC is remineralised are intrinsically linked to temperature and so vary with latitude (Marsay et al., 2015). Variations in regional oceanography (currents and other hydrographic features) also affect the range and bathymetric profile of temperature in trenches (Jamieson et al., 2010). Species vertical limits are at least partially controlled by physiological tolerances and metabolic processes along gradients of temperature (Rowe and Menzies, 1969; Yasuhara and Danovaro, 2014), pressure (Somero et al., 1983) and the synergistic effects of both (Cossins and Macdonald, 1989), modulating biodiversity ( Rex, 1981; Huston, 1994; Rex and Etter, 2010). The dominant environmental driver of bathymetric trends of species diversity is not clear, different studies have concluded temperature (O’Hara and Tittensor, 2010) and POC flux (Carney, 2005; Tittensor et al., 2011; McClain et al., 2012) best correlate with observed trends. Understanding bathymetric diversity trends in the ocean is further hampered by the infant state of hadal science relative to shallower studies (Jamieson and Fujii, 2011). Studies investigating diversity in the hadal environments are limited in the number, spatial replication and bathymetric resolution of samples that have been taken to date. It is therefore not possible to predict, with confidence, trends or patterns in the diversity of hadal communities through extrapolation from shallower regions. Investigating bathymetric trends across the abyssal-hadal transition zone is often difficult as many studies have concentrated sampling efforts solely within the trench environment without comparative data from surrounding regions (e.g. the Danish Galathea and Russian Vitjaz expeditions of the 1950s, summarised in Belyaev 1989), while others make comparisons from a hadal site to sites thousands of metres shallower (e.g. Danovaro et al. 2003; Glud et al. 2013) and/or thousands of km distant (e.g., Tietjen 1989). To date, the few studies that have provided quantitative data suggest that diversity in trenches is lower than at sites at adjacent bathyal and abyssal depths (e.g. Gambi et al. 2003; Fujii et al. 2013; Kitahashi et al. 2013). However, no study so far has compared diversity from trench sites that receive modelled or empirically higher levels of organic matter. Encouragingly, some recent work is beginning to address these issues but ecosystem level studies remain difficult due to the technical challenges in sampling a broad range of organisms. As such, the most extensive bathymetric datasets that include the hadal environment have been produced for a single taxonomic group, for example, scavenging amphipods (Fujii et al., 2013), sediment dwelling harpacticoid copepods (Kitahashi et al., 2013), and microbial communities (Nunoura et al., 2015)

that are readily recoverable using baited traps, sediment cores and water sampling bottles respectively. The Amphipoda (Crustacea, Malacostraca), are an important component of the scavenging fauna in the deep sea and found in high abundances, particularly at hadal depths (De Broyer et al., 2004; Hessler et al., 1978, Kamenskaya, 1995). Amphipods play an important role in the degradation and redistribution of organic matter (Christiansen and Diel-Christiansen, 1993) as a result of specialist adaptations for locating, intercepting, consuming and utilising food falls (Dahl, 1979; Kaufmann, 1994). They also display extreme pressure tolerance (Yayanos and Nevenzel, 1978; Yayanos, 1981) and trophic plasticity (Blankenship and Levin, 2007; Kobayashi et al., 2012). Members of the superfamily Lysianassoidea in particular are prolific scavengers and readily recovered in simple baited traps. Their catchability makes them an ideal taxonomic group to collect sufficient data, in terms of numbers and diversity, over large enough bathymetric ranges to permit meaningful study. Despite their benthic lifestyle and lack of dispersive larval stage, many deep sea scavenging species have cosmopolitan distributions, such as the Alicellids Paralicella caperesca and P. tenuipes (Chevreux, 1908) found in the Pacific, Atlantic and Southern Oceans at abyssal depths ( Jamieson et al., 2011b; Cousins et al., 2013; Horton et al., 2013). Others such as the Eurytheneid Eurythenes gryllus (Lichenstein in Mandt 1822) are known from all the world’s oceans, over considerable depth ranges, and may represent many different species-level lineages with different geographic and bathymetric distributions (Havermans et al., 2013; Thurston et al., 2002). While there is little evidence of specific trench endemism, the geographic range of hadal species does appear limited. For example, the hadal species Hirondellea dubia Dahl 1959 (Hirondelleidae) has only been recorded in the Kermadec and Tonga Trenches, southwest Pacific, whereas H. gigas (Birstein and Vinogradov, 1955) has been recorded in all northwest Pacific trenches studied to date and three further species of Hirondellea have been identified from the Peru-Chile Trench in the southeast Pacific (Kilgallen, 2014). Recent attempts to examine the patterns in amphipod community structure across the abyssal-hadal transition as part of HADEEP (HADal Environment and Educational Program) (Jamieson et al., 2009c) have identified an abrupt shift in faunal composition between the hadal communities and those in the surrounding abyssal zones of the Kermadec and Peru-Chile Trenches (Jamieson et al., 2011b; Fujii et al., 2013). This boundary sits between 6007 and 6890 m and is unlikely to be due to a disjunct in pressure or temperature as both increase linearly with depth, but may be due to a change in depositional food input caused by the change in topography between the abyssal and trench slopes (Jamieson et al., 2011b). These studies however are limited in the number of sampling locations, bathymetric resolution and the two trenches are extremely distant (~10,000 km apart). Differences

in hadal community structure between these trenches may be expected because of their relative physical isolation, as much as differences in the productivity of the overlying water masses (Fujii et al., 2013). The most recent extension of the HADEEP project (2007 – 2013) has included additional sampling in the Kermadec Trench and new sampling in the New Hebrides Trench (and across the neighbouring abyssal plain) which now permits a comparison of data from three trench regions in the Pacific Ocean. These three trenches are positioned beneath different water masses that vary in their overlying surface productivity and temperature characteristics, and thus the flux of POC to the seafloor (Lutz et al., 2007). The present study uses the bathymetrically and geographically extensive HADEEP dataset to examine the factors influencing community structure and diversity of bait-attending amphipods, both within and between hadal trenches (>6000 m) and their surrounding bathyal (1000 - 3500 m) and abyssal (3500 - 6000 m) environments. Specifically, we aim to (1) describe patterns in amphipod community structure and species richness in and around the Kermadec and New Hebrides Trenches, and compare them to those of the Peru-Chile Trench region, and (2) determine if any observed differences in community structure and species richness between the study regions and from bathyal to hadal depths are driven by POC flux, pressure and/or temperature. 2. Materials and Methods 2.1 Study sites The Kermadec Trench lies ~120 km off the coast of North Island, New Zealand in the southwest Pacific Ocean and runs parallel to the Kermadec Ridge (Fig. 1A). The Kermadec Trench is the fifth deepest trench in the world, reaching 10,047 m depth (Angel 1982), and is approximately 1500 km long and 60 km wide. The trench has the characteristic V-shaped cross section topography, formed by tectonic subduction of the Pacific Plate under the Australian Plate. The New Hebrides Trench is ~1000 km northwest of the Kermadec Trench and is entirely partitioned from the Kermadec Trench by the Kermadec Fore Arc and South Fiji Basin at bathyal and abyssal depths respectively (Fig. 1B). It is a subduction trench where the Australian plate subducts northeastward beneath the overriding Vanuatu archipelago. The trench is ~ 2000 km in length with a maximum depth of 7156 m. The South Fiji Basin, sits between the Kermadec and New Hebrides Trenches and is bounded to the west by the Three Kings Ridge, Loyalty Ridge and Cook Fracture Zone, to the east by the Colville-Lau Ridge and to the south by the Northland Plateau (Mortimer et al., 2007). The predominantly abyssal seafloor slopes gently to a maximum depth of 3000 – 4000m.

The Kermadec Trench and New Hebrides Trench underlie different biogeographical provinces (South Pacific Subtropical Gyre (or SPSG) and the Western Pacific Archipelagic Deep Basins (or ARCH)), which have similar surface primary production rates of 87 and 100 g C m-2 y-1 respectively (Longhurst et al. 1995). Additional data are included from the Peru-Chile Trench in the southeast Pacific Ocean, as reported by Fujii et al. (2013) (Fig. 1B). This trench (4602 to 8074 m) lies 11,000 km and 10,000 km east of New Hebrides and Kermadec Trenches, respectively and is in a different biogeochemical province (Chile-Peru Current Coastal (or CHIL)) with a higher surface primary production rate of 269 g C m-2 y-1 (Longhurst et al., 1995). 2.2 Sampling Twenty-one trap deployments were made in the Kermadec Trench region from 1488 m to 9908 m at an average depth interval of 420 m. Twelve deployments were made in the New Hebrides Trench region between 2000 m and 6948 m with an average interval of 450 m. In the South Fiji Basin, 5 deployments were made at two stations (~4100 m). Additional data used in the present study were from 7 and 5 deployments between 4329 – 7966 m and 4602 – 8074 m in the Kermadec Trench and the Peru-Chile Trench regions, respectively, using the same or similar gear (Fujii et al., 2013; Jamieson et al., 2011b). Details of all the baited trap deployments are summarised in Table 1. Samples used in this study were taken using small baited funnel traps and cages of varying size mounted upon a series of deep-sea lander vehicles; LATIS, a Fish Trap, OBULUS and the HadalLanders A, B and C. LATIS (described in Jamieson et al. 2013, Jamieson 2015 p. 58) was baited with ~500 g of Jack mackerel (Trachurus sp.). Within the trap were four small baited invertebrate funnel traps, 12 cm diameter (ø) x 30 cm long, baited with ~100 g of mackerel and secured to a 1 m2 aluminium frame at the base of the trap which made contact with the seafloor upon landing. The Fish Trap comprised one large fish cage, 200 x 200 x 100 cm cuboid, with four square funnel openings of 14 x 14 cm recessed 25 cm into the trap. The trap was lined with 1 cm mesh and ~500 g of Jack mackerel were secured in proximity to each funnel opening. Two invertebrate funnel traps, 12 cm ø x 30 cm long, baited with ~500 g of Jack mackerel were secured inside the Fish Trap, positioned centrally at the base. The OBULUS lander consisted of two invertebrate traps as described for the Fish Trap, attached to a 43 cm ø glass sphere housing an acoustic burn wire release.

Data for 7 of the stations within the Kermadec Trench (Jamieson et al., 2011b) were collected using Hadal Lander A and B. Three small invertebrate funnel traps (12 cm ø x 30 cm) with a funnel opening of 2.5 cm ø and baited with ~100 g of Jack mackerel were attached to the feet of each lander. Samples in the New Hebrides Trench were obtained using a replacement for Hadal-Lander A, HadalLander C, with three invertebrate funnel traps, 12 cm ø x 30 cm long. Other samples were taken from the Abyssal-Lander: a 6000 m rated version of Hadal-Lander B. Data for the Peru-Chile Trench were collected using Hadal-Lander B with 3 traps at 0 metres above bottom (mab), 1 trap at 1 and 2 mab, further traps attached to the buoyancy modules every 10 metres between 20 and 60 m and one trap at 90 mab. Each trap was baited with ~200 g of locally sourced tuna (Thunnus sp.) (Fujii et al., 2013). Each lander was equipped with a temperature and pressure sensor (SBE-39; SeaBird Electronics, USA) which recorded at 30 s intervals throughout each deployment. Temperature and pressure data were averaged and pressure converted to depth following Saunders (1981). Conductivity was also measured at 10 sec intervals during most voyages (up until KAH1301) using a CTD Sensor (SBE 19PlusV2, SeaBird Electronics, USA). Estimated annual flux of particulate organic carbon to the seabed (POC g C m-2 year-1) was calculated for each station based on the remote-sensing model of Lutz et al. (2007). The Lutz model estimates the annual mean POC flux at depth from annual mean surface net primary production but does not account for lateral slope transport of sediment nor potential remineralisation in the hadopelagic zone. Here POC flux is assumed to be a useful proxy measure for delivery of organic matter to the deep-sea floor, including the carcasses of large organisms on which scavenging amphipods typically rely for food. Biomass in the photic zone and, though less well characterised, the deeper water column generally reflect levels of surface productivity ( Ryther, 1969; Priede et al., 1994; Angel, 2003; Sommer et al., 2002) and the quality and quantity of carbon flux is known to influence benthic biomass and community structure (Grebmeier and McRoy, 1989; Ruhl and Smith, 2004). The output from the model was derived on a scale of 1 km2 in ArcGIS ArcMap 10 using the ESRIS Spatial Analyst programme. 2.3 Sample processing Amphipod specimens were preserved in 99 % ethanol within 1 hour of recovery. On return to the laboratory samples were transferred to 70 % ethanol for species identification and counting. A subset of ~200 were attained from the pooled traps of the Kermadec Trench samples, between 7291 – 9908 m (voyages KAH1109 and KAH1202), using a Folsom Plankton Splitter where numbers of individuals exceeded 300. Individuals were identified to species level where possible following

Barnard & Karaman (1991) with updates and some changes to family level, sub-family level or informal groupings after Lowry & Stoddart (1990, 1994, 1995, 2010, 2011), Stoddart & Lowry (2004), Lowry & De Broyer (2008), Horton & Thurston (2013) and Kilgallen & Lowry (2015). Identification was undertaken using a Wild Heerbrugg Wild M8 dissection microscope. 2.4 Data analysis 2.4.1 Community structure Patterns in community structure were analysed using routines in the multivariate statistical software package PRIMER 6 (Clarke & Gorley 2006) and the add-on package PERMANOVA + (Anderson 2008). Amphipod species identities and their abundances at each station were converted into Bray-Curtis similarity matrices (Bray-Curtis 1957) for each trench, and a single matrix for all three trenches and the South Fiji Basin. To account for the difference in sample size, and both very high abundances (and related sub-sampling) and rare species, species abundances were standardised and fourth-root (√√) transformed prior to analysis. In order to assess the potential confounding influence of sampling effects, a RELATE test was conducted to determine the correlation between the amphipod multivariate structure and both time spent by the traps on the sea floor (effective sampling effort) and sample size. Patterns within the multivariate structure were determined using group average cluster analysis (CLUSTER) and similarity profile analysis (SIMPROF) permutation tests (significance <0.05) to identify ‘community’ groups. Patterns in community structure in the similarity matrices were illustrated using non-metric multi-dimensional scaling (nMDS) 2-D ordinations. Similarity of percentage (SIMPER) analysis was used to reveal the percentage contributions of species which accounted for ≥90 % of the similarity within, and dissimilarity between, cluster groups. Relationships between the environmental variables (estimated POC flux, pressure and temperature) and amphipod community structure were investigated using distance-based linear model (DistLM) analysis. The environmental variables that best explained the multivariate assemblage data were selected using a permutated (p = 9999) forward selection procedure using adjusted R2 (R2(adj)) criterion and then visualised using distance based redundancy analysis (dbRDA). The contribution of each individual variable to the model was determined via marginal tests, followed by sequential tests to determine the cumulative effect of the variables. 2.4.2 Species richness Bathymetric trends for estimated species richness (Sest) (rarefied species richness using Chao 1 estimator (Chao 1984)) were calculated from raw abundance data for each station using the DIVERSE routine in the PRIMER 6 + software, illustrated using simple x-y plots, and the nature of the trend determined by quadratic line fitting using the “vegan” package in R v.3.1.3 (R Core Team, 2015).

Relationships between the environmental variables (estimated POC flux, pressure and temperature) and estimated species richness were investigated using DistLM as described above. To examine patterns in both intra- and inter-trench community structure and species richness, these analyses were performed for three separate scenarios: 1) the Kermadec Trench, 2) the New Hebrides Trench and 3) the trans-South Pacific (Kermadec, New Hebrides, and Peru-Chile Trenches, and the South Fiji Basin). 3. Results 3.1 Environmental conditions Both the New Hebrides and Kermadec Trench regions were sampled over a sufficient range to observe the effects of adiabatic heating (Fig. 2) in which temperature rises due to increasing pressure (Brydon 1973). The lowest temperature in the New Hebrides Trench was recorded at 3400 m at 1.81 °C and at 4329 m in the Kermadec Trench at 1.06 °C. The bottom temperatures of the Kermadec Trench were ~0.7 – 1 °C lower than at equivalent depths in the New Hebrides and PeruChile Trenches >4000 m. Estimated POC reaching the seafloor (g m-2 yr-1) was markedly higher in the Peru-Chile Trench region (3.9 – 16.1 g m-2 yr-1) than in the Kermadec Trench (1.2 – 3.3 g m-2 yr-1) and New Hebrides Trench (1.8 – 2.6 g m-2 yr-1) regions. There was a reduction in estimated POC flux with depth in all regions (Table 1). 3.2 Species composition A total of 2085 individuals were counted from the Kermadec Trench region comprising 20 species in the following families (Table 2, S1): Alicellidae (3), Eurytheneidae (1), Hirondellidae (1), Lysianassidae (1), Scopelocheiridae (2), Uristidae (7), Valettiopsidae (2), and Lysianassoidea (3). Five of these species are potential new to science. These data were combined with the Kermadec Trench data detailed in Jamieson et al. (2011) including two further species recorded in the families Cyclocaridae and Eusiridae.

The most numerically dominant species at bathyal – abyssal depths were

Orchomenella gerulicorbis (n = 1586, total range: 1488 – 7568 m) and Paralicella caperesca (n = 927, 1488 – 6968 m) while at hadal depths Hirondellea dubia (n = 1644, 6000 – 9908 m) and Bathycallisoma schellenbergi (= syn. Scopelocheirus (Kilgallen and Lowry, 2015)) dominated the trap catches (n = 720, 6007 – 8487 m). Sampling in the New Hebrides Trench region yielded 3065 individuals, comprising 19 species, of which nine may be new to science and were not recovered from the other areas (Table 2, S2). These

species were distributed among the following families: Alicellidae (6), Cyclocaridae (1), Eurythenidae (1), Eusiridae (1), Hirondellidae (2), Lysianassidae (1), Scopelocheiridae (2), Uristidae (3) and Valettiopsidae (2). The two most numerically dominant species were Abyssorchomene abyssorum (n=841, 2000 – 5600 m) and B. schellenbergi (n = 792, 5600 – 6948 m), which dominated the samples at the bathyal-abyssal boundary (2500-3400 m) and hadal depths (6000-6948 m), respectively. From the abyssal depth of 4100 m in the South Fiji Basin, 491 individuals were counted, comprising seven species, three of which may be new to science (also found in the New Hebrides Trench), from the following six families (Table 2, S3): Alicellidae (2), Eurythenidae (1), Cyclocaridae (1), Scopelocheiridae (1), Hirondellidae (1) and Uristidae (1). A. abyssorum and P. caperesca were the most abundant numerically (n = 167, n = 143 respectively). No single species dominated over all five of the samples taken in the basin; P. caperesca dominated two samples whereas A. abyssorum and Cyclocaris sp. dominated one sample each. Data for the Peru-Chile Trench were taken from Fuji et al. (2013), but are reported here for completeness. From this trench, 1536 individuals were counted from species in the following 7 families: Alicellidae (3), Hirondelleidae (3), Uristidae (2), Lysianassidae (2, 1 putative), Eurytheneidae (1), Valettiopsidae (1) and Pardaliscidae (1). The most numerically dominant species was Eurythenes gryllus, found at all depths and comprised more than twice the number of individuals than any other species. P. caperesca dominated the catch at 5329 m (n = 174). Abyssorchomene chevreuxi and P. caperesca each comprised 33 % of the catch at 6173 m (n = 44, n= 33 respectively). E. gryllus dominated at 7050 m (n = 261) and Hirondellea thurstoni dominated at 8074 m (n = 104).

3.3 Community Structure 3.3.1 Kermadec Trench region Three groupings or amphipod communities were identified in the Kermadec Region (Fig. S1). Group 1 included all bathyal and abyssal stations, and the shallowest hadal station (1488 – 6097 m); groups 2 and 3 comprised hadal samples, with the former group generally comprising samples from shallower stations (6709 – 7884 m) than the latter group (6890 - 9908 m). On the nMDS plot, the distribution of the stations generally followed a depth gradient increasing from left to right. The stations of group 1 clustered together and clearly separated from groups 2 and 3, which were more closely clustered together (Fig. 3A). SIMPER analysis revealed the species and their percentage contributions which accounted for the similarity within and dissimilarity between cluster groups. Within group similarities increased between groups 1 to 3 (47 – 83 %); increasing with depth (Table S4). The number of species responsible for within group similarities reduced with increasing depth. Hirondellea dubia was the

sole species responsible for the similarity within group 3 and, along with B. schellenbergi, contributed to the similarities within group 2. Paralicella caperesca contributed the most (41%) of the similarity for group 1. The average dissimilarity was highest at 94 % between groups 1 and 3, followed by 84.2 % between the groups 1 and 2. The average dissimilarity between the two hadal groups (groups 2 & 3) was lowest at 54.6 %. The relative proportions of B. schellenbergi and H. dubia were responsible for the majority of the average dissimilarity between these two hadal groups. H. dubia also showed the greatest contribution to the difference in community structure between groups 1 and 3. While B. schellenbergi accounted for the greatest contribution to the difference between groups 1 and 2, P. caperesca and O. gerulicorbis each contributed >10 % to the differences between groups 1 and 2 and groups 1 and 3. Pressure and estimated POC flux individually accounted for significant proportions of the total variation within amphipod community structure observed in the Kermadec Trench region; pressure accounted for 48 % and POC flux accounted for 34 % of the variability (Table 3). Sequential tests showed that in addition to pressure, temperature explained an extra 9 % of the total variation and POC flux an additional 4 %. Visualisation of this analysis using dbRDA shows the stations distributed mainly along axis 1 reflecting the primary influence of pressure. The distribution of stations along axis 2 reflects the additional influence of temperature and POC flux, particularly for bathyal and abyssal (group 1) stations (Fig. 4A). 3.3.2 New Hebrides Trench region Three groupings of amphipod communities were identified in the New Hebrides Trench region (Fig. S2). Group 1 comprised all bathyal and abyssal stations between 2000 and 5600 m depth, group 2 comprised the deepest abyssal station and one hadal station at 6000 and 6948 m, and ‘group’ 3 consisted of a single hadal station at 6228 m. On the nMDS plot, the distribution of the samples generally followed a depth gradient left to right (Fig. 3B). All bathyal and abyssal stations (group 1) were clustered together (with shallower stations generally towards the bottom of this cluster) and were clearly separated from the deeper stations (groups 2 and 3), which were clustered relatively close together. SIMPER analysis revealed an average community similarity of 57 % for the bathyal-abyssal sample group (1) and high similarity of 82 % for the deep abyssal and hadal sample group 2 (Table S5). Within group similarity could not be calculated for group 3 as it comprised only one station. The number of species responsible for within group similarity was highest in group 1 (n = 5). P. caperesca

and A. abyssorum were responsible for >50 % of the within group similarity of group 1, whereas B. schellenbergi and H. dubia accounted for ~80 % of the similarity for group 2. Average dissimilarity was lowest between the two deepest groups 2 and 3 (44 %) due to the mutually high proportion of B. schellenbergi at stations in both groups (Table S5). The contributions to the community dissimilarity of the 6 species ranged from 19 to 12 %. Average dissimilarities between groups 1 with 2 and 3 were high (88 and 77 % respectively). The absence of B. schellenbergi from all but one group 1 station dominated the dissimilarity between both groups 1 and 2 and 1 and 3. The dissimilarity between groups 2 and the single group 3 station at 6228 m was due to the presence of Paralicella spp., V. anacantha and Eusiridae sp. in the 6228 m sample. Pressure alone accounted for a significant proportion of the variation in amphipod community structure within the New Hebrides Trench region (R2(adj) = 0.35, p < 0.01) (Table 3). Sequential tests showed that in addition to pressure temperature accounted for a further 13 % of the variation, though this was non-significant. Visualisation of this result using dbRDA showed the influence of pressure on the separation of abyssal group 1 and then hadal groups 2 and 3 (Fig. 4B). 3.3.3 Trans-South Pacific regional comparison Five distinct community groupings were identified across all regions when considered together (Fig. S4). Group 1 comprised the Kermadec Trench region stations; both bathyal stations (1488 – 2197 m), two abyssal stations (4192, 6000 m) and the two shallow hadal stations (6007, 6097 m). Group 2 contained one deep bathyal and the three remaining abyssal samples from the Kermadec Trench region (3268 – 5242 m), two abyssal stations (4602 - 5329 m) and one shallow hadal station (6173 m) from the Peru-Chile Trench region and one abyssal New Hebrides Trench region sample (5300 m). Group 3 comprised all remaining bathyal and abyssal samples from the New Hebrides Trench (2000 – 5600 m) and all of the South Fiji Basin samples (4100 m). Group 4 comprised the two deepest hadal samples from the Peru-Chile Trench region (7050 - 8074 m). Group 5 consisted of one deep abyssal station (6000 m) and the hadal samples from the New Hebrides Trench region (6228 – 6948 m), as well as the deepest hadal samples from the Kermadec Trench region (6709 – 9908 m). The nMDS plot illustrates that the majority of hadal samples of the Peru-Chile Trench and from the New Hebrides/Kermadec Trench regions are clearly distinct from one another and the remaining samples from shallower samples. (Fig. 3C). Within group similarities for the cross-region analysis showed an average of ~55 % (Table S6). Only two species contributed to the similarities of each of the hadal groups; H. dubia and B. schellenbergi in the New Hebrides/ Kermadec Trench deep abyssal/ hadal community (group 5) and E. gryllus and Hirondellea thurstoni in the Peru-Chile Trench hadal community (group 4). The numbers of species

contributing to the similarities within the shallower groupings were higher, 4 species in group 1 and 5 species in groups 2 and 3. E. gryllus contributed to the similarities in groups 1 – 4, P. caperesca contributed to groups 1 – 3 and Abyssorchomene distinctus contributed to groups 1 and 2. The average dissimilarity level between community groups was highest (96 %) between the PeruChile Trench hadal community and the New Hebrides/Kermadec Trench deep abyssal/hadal community (groups 4 and 5) reflecting the absence of E. gryllus and H. thurstoni from the latter community and H. dubia and B. schellenbergi from the former. There was ~93 % average dissimilarity between the bathyal/abyssal/hadal community of the Kermadec/Peru-Chile/New Hebrides Trench regions (group 2) and bathyal/abyssal community of the New Hebrides Trench/South Fiji Basin (group 3) with the deep abyssal/hadal community of New Hebrides/Kermadec Trench regions (group 5). This difference was largely attributable to the absence of H. dubia and presence of P. caperesca in groups 2 and 3. Groups 2 and 3 showed the lowest average community dissimilarity (60 %), with the majority of the dissimilarity accounted for by the presence of A. abyssorum in group 3 and presence of A. chevreuxi in group 2. Group 1, which contained a mixture of bathyal, abyssal and hadal samples from the Kermadec Trench region, also showed a relatively low level of dissimilarity (62 %) with group 2, largely due to the relative absence of O. gerulicorbis in group 2 and absence of A. chevreuxi in group 1. Separate DistLM analyses were conducted on the bathyal-abyssal groups 1 to 3 and the predominantly hadal groups 4 and 5 to try and identify what environmental drivers were responsible for community structure within these two major groupings of samples (Table 3). DistLM analyses revealed that pressure and estimated POC flux accounted for 12 % and 11 % respectively of the total variation in groups 1 to 3, while temperature accounted for just 4 %. Sequential tests revealed that the best model included all three variables and accounted for 26 % of the total variation (R2(adj) = 0.16). Visualisation by dbRDA of this result revealed relatively little separation of the three bathyal/abyssal groups along the two axes that each represented an equal amount of the variation (Fig 4C). The distribution of samples along axis 1 reflected both the influence of POC flux and temperature on the community structure of the bathyal/abyssal communities. The influence of POC flux on community structure was evident from separation of samples from the Peru-Chile Trench region and the Kermadec/New Hebrides Trench regions along axis 1, which reflects the much higher fluxes of POC estimated for those sample stations in the former region. Temperature also correlated with axis 1, but the strong effect of POC flux on community structure prevented the complete clustering of the similarly warm New Hebrides and Peru-Chile Trench region stations relative to those of the colder

Kermadec Trench region. The distribution of samples along axis 2 generally reflected a relationship with pressure and the overriding influence of depth on community structure. DistLM analyses of the data for the two predominantly hadal communities (groups 4 and 5) revealed that estimated POC flux accounted for 43 %, pressure accounted for 20 % and temperature accounted for 18 % of the variation in observed community structure in the marginal tests (Table 3). Sequential tests revealed that the best model included all three variables and accounted for 68 % of the total variation (R2(adj) = 0.60). In addition to estimated POC flux, pressure explained 21 % of the variation and temperature accounted for a further 4 %. Visualisation of this result by dbRDA revealed the separation of the two hadal community groups along axis 1, reflecting the higher estimated POC flux values, and temperature of the hadal stations in the Peru-Chile Trench versus those from the New Hebrides and Kermadec Trenches (Fig. 4D). Pressure correlated well with axis 2 of the plot, and the distribution of the stations along this axis clearly reflected the influence of depth on community structure. 3.4 Species richness Within the Kermadec Trench region there was an overall decline in estimated species richness from bathyal to hadal depths, with regression analysis indicating a peak in species richness at abyssal depths (R2 = 0.51, F = 9.232, df = 18, p < 0.01) (Fig 5A, Table S7). The maximum estimated species richness (Sest) was 11 species at 5173 and 5242 m and 10 at 2197, 6007 and 6709 m. Deeper than 7000 m, species richness did not exceed 4 species. Only 1 species was estimated at 7966, 9053 and 9908 m. For the New Hebrides Trench region, there was a unimodal pattern of estimated species richness from bathyal to hadal depths, with regression analysis indicating a peak in species richness at abyssal depths, (R2 = 0.63, F = 7.511, df = 9, p < 0.05) (Fig. 5B, Table S7). Highest estimated species richness was at 4835 m (n = 13) and was lowest at 6000 and 6948 m (Sest = 3). When data from all South Pacific regions were considered together, estimated species richness exhibited a unimodal trend with depth, with regression analysis indicating a peak in species richness at abyssal depths (4000 m - R2 = 0.41, F = 13.84, df = 40, p < 0.001) (Fig 5C, Table S7). Although limited, data from the greater depths of Peru-Chile Trench region appear to fit well with those data at the same depths from the other two trench regions. Conversely, species richness from shallower abyssal depths in the Peru-Chile Trench region is estimated to be lower than corresponding depths in the New Hebrides Trench region. Data from the South Fiji Basin agree well with the range of

species richness values estimated at the approximately equivalent depths in the Kermadec Trench region. Pressure accounted for 35 %, estimated POC flux accounted for 2 % and temperature accounted for 1 % of the variation in estimated species richness in the marginal tests (Table 4). Sequential tests revealed that the best model included pressure and temperature only and accounted for 37 % of the total variation (R2(adj) = 0.34). In addition to estimated pressure, temperature accounted for 2 % of the variation. Visualisation of this result by dbRDA showed samples mainly arranged along axis 1, reflecting a strong influence of hydrostatic pressure on the pattern of estimated species richness across bathyal to hadal depths in all regions. Separation of samples from the Kermadec Trench region from those of the other regions was apparent along axis 2 of the plot, reflecting the influence of the low temperatures in this region on species richness compared to the other regions (except at the shallowest stations in the Kermadec Trench region) (Fig. 6). 4. Discussion The data collected and compiled in the present study provided for a comprehensive analysis of community structure and diversity of scavenging amphipods across bathyal, abyssal and hadal depths. Below we discuss the major findings of this analysis in the context of previous studies: first, describing the patterns in biodiversity observed in the Kermadec and New Hebrides Trench regions as a result of new sampling; second, highlighting the differences in community structure and species richness observed among the study regions; third, identifying and considering the role that environmental variables play in accounting for the observed biodiversity patterns; and finally, concluding with some recommendations for future research aimed at further revealing biodiversity patterns at bathyal to hadal depths. 4.1 Community structure The Kermadec Trench region analysis, which included new data, revealed an abrupt change in amphipod species composition between a predominantly bathyal/abyssal community and two hadal communities at depths of between 6097 and 6709 m. This depth range agrees well with the ecotone identified by Jamieson et al., (2011) using some of the same data. The predominantly bathyal/abyssal community was primarily composed of P. caperesca, O. gerulicorbis, E. gryllus and Abyssorchomene species, which are abyssal cosmopolitan species with a trans-Pacific distribution (Brandt et al., 2012). The split of the hadal fauna into relatively shallow and deep hadal communities reflects the changing proportions of B. schellenbergi and H. dubia. Both species have been previously sampled from hadal

depths within the Kermadec Trench ( Blankenship et al., 2006; Blankenship and Levin, 2007; Jamieson et al., 2011b; Fujii et al., 2013). H. dubia was the only species found in abundance > ~8000 m (Jamieson et al., 2011b). The inclusion of the relatively shallow 6890 m station in the deep hadal community was due to the high proportion of H. dubia, which reduced as the proportion of B. schellenbergi increased down to 7291 m. H. dubia were not recovered from depths <6000 m, consistent with their minimum known depth limit in this region (Jamieson et al., 2011b). The bathyal/abyssal community of the New Hebrides Trench region was composed largely of common abyssal species of the genera Paralicella, Abyssorchomene, Eurythenes (Shulenberger and Barnard, 1976; Thurston, 1979; Duffy et al., 2012, 2013; Fujii et al., 2013; Havermans et al., 2013;) and an undescribed species of Paracallisoma. A disjunct between the abyssal and hadal fauna occurred between 5600 - 6000 m and at greater depths the communities were largely composed of the common southwest Pacific hadal species B. schellenbergi and H. dubia (Belyaev, 1989; Blankenship et al., 2006; Jamieson et al., 2011b; Fujii et al., 2013; Kilgallen and Lowry, 2015). B. schellenbergi was abundant at the two hadal stations and both B. schellenbergi and H. dubia were abundant at the deepest station with a single E. gryllus. Within the southwest Pacific, E. gryllus has previously only been recorded at depths <6252 m in the relatively cold Kermadec and Tonga trenches (Blankenship et al., 2006; Jamieson et al., 2011b). Penetration of E. gryllus to maximum depths of the warmer New Hebrides and Peru-Chile trenches (Fujii et al., 2013) suggests that low temperature may be a limiting factor in depth penetration, contradicting the cold water stenotherm status of E. gryllus (Thurston, 1990). The separation of the 6228 m station from the stations of the deep abyssal/hadal community in this region was due to the combined presence of the largely abyssal P. tenuipes (n = 9) and hadal B. schellenbergi (n = 241) at this station. However, due to the relatively low sampling effort > 6000 m it is unlikely that this station represents a true change in community structure at this depth within the hadal zone of the New Hebrides Trench. The cross region analysis revealed five community groups. The first group contained the shallowest bathyal and intermediate abyssal stations and the three shallowest hadal stations from the Kermadec Trench. The remaining Kermadec bathyal and abyssal stations formed a separate second group with the two abyssal stations and one shallow hadal station from the Peru-Chile Trench. All stations from the South Fiji Basin grouped with those from the predominantly bathyal and abyssal group identified in the New Hebrides Trench analysis. All three groups showed a relatively high level of similarity to each other but were distinct from the remaining hadal stations which formed two groups, one combining the deepest stations from the New Hebrides and Kermadec Trenches and a distinct Peru-Chile Trench hadal group.

The similarity between the first three groups was due to the shared presence of cosmopolitan species with a trans-Pacific distribution, namely species of the genera Abyssorchomene, Paralicella and Eurythenes (Brandt et al., 2012). Orchomenella gerulicorbis was recorded at bathyal and shallow hadal depths, but was absent from intermediate depths within the Kermadec Trench. It is primarily due to this bimodal depth distribution of O. gerulicorbis that the shallow bathyal and shallow hadal Kermadec Trench stations were combined into a separate group from the remaining abyssal and hadal Kermadec Trench stations. The reason for the absence of O. gerulicorbis from abyssal depths of group 2 is unclear. There may be two distinct bathymetrically separated populations, as has been found in the bathymetric structuring and possible speciation of E. gryllus (Eustace et al., 2016), though no morphological diversification was evident to suggest speciation. Patchy distributions have been recorded for many abyssal benthic fauna ( Jumars, 1976; Belyaev, 1989; Howell et al., 2002; Schwabe et al., 2007). It is also possible that there is migration of O. gerulicorbis between the observed inhabited depths by individuals not attracted to the bait. Brooding lysianassoid females are known to avoid carrion falls and are rarely captured using baited traps (Blankenship et al., 2006; Kraft et al., 2013). The separation of the Kermadec and Peru-Chile primarily abyssal stations (group 2) from comparable depths of the New Hebrides Trench and South Fiji Basin stations (group 3) was primarily due to the relative abundances of different Abyssorchomene species; A. chevreuxi was the dominant species within group 2 and A. abyssorum within group 3. A. abyssorum has been reported from bathyal to hadal depths in the Atlantic Ocean (Duffy et al., 2013; Horton et al., 2013; Thurston, 2000, 1990) and is known to coexist with A. chevreuxi at bathyal and abyssal depths in that ocean ( Thurston, 1990; Horton et al., 2013), but apparently not in the New Hebrides Trench region. Elevated populations of A. abyssorum at abyssal depths of the Mid-Atlantic Ridge relative to catches from the Porcupine Abyssal Plain have been attributed to elevated food supplies and reduced competition from shallower continental margin species (Horton et al., 2013). However, POC flux to the seafloor was similar for the Kermadec Trench and New Hebrides Trench regions and so the possibility exists that the distribution of A. chevreuxi (and A. distinctus) does not extend into the latter region. There was a high degree of dissimilarity between the two hadal communities (groups 4 and 5). The similarity in the hadal community for the two southwest Pacific trenches was due to the dominant proportions of B. schellenbergi and H. dubia. The separation of the Peru-Chile hadal stations from this community was due to the dominance of E. gryllus and presence of two different species of Hirondellidae, H. thurstoni and H. sonne, in the Peru-Chile Trench which were absent from stations in the other two trenches.

Fuji et al. (2013) postulated that the similar feeding ecology of B. schellenbergi and E. gryllus (Blankenship and Levin, 2007; Ingram and Hessler, 1983) may prevent their coexistence, preventing E. gryllus from penetrating the hadal zone. E. gryllus is absent from depths >6252 m in the Kermadec and Tonga Trenches (Blankenship et al., 2006; Jamieson et al., 2011b) and was found in very small numbers at hadal depths in the New Hebrides Trench. B. schellenbergi has been reported from hadal trenches in the southwest and north Pacific (Birstein and Vinogradov, 1958, 1970; Dahl, 1959; Blankenship et al., 2006; Jamieson et al., 2009a; Kilgallen and Lowry, 2015), Atlantic (Schellenberg, 1955; Vinogradov and Vinogradov, 1993; Lacey et al., 2013) and Indian Oceans (Birstein and Vinogradov, 1964) but is absent from the Peru-Chile Trench. The absence of B. schellenbergi from the Peru-Chile Trench could have allowed E. gryllus to dominate the amphipod community at hadal depths in this trench. It is possible that such competitive exclusion could be facilitated by the difference in available organic matter on the seafloor of the Peru-Chile Trench versus the other two trenches, as has been suggested for other amphipod species (Horton et al. 2013). Contrary to the wide-reaching distributions of B. schellenbergi and E. gryllus, different species of Hirondellea appear to be endemic to trench regions. H. dubia have only been recovered in southwest Pacific trenches, H. gigas in northwest Pacific trenches, and the recent identification of H. thurstoni and H. sonne in the Peru-Chile Trench indicates a southeast Pacific Trench endemism for these species. This pattern is likely the result of allopatric speciation between these disparate regions (France, 1993; Jamieson, 2015). The cross region analyses paired the hadal communities of Kermadec Trench with those of the neighbouring New Hebrides trench, whereas the Peru-Chile Trench stations were grouped separately. It was expected that the amphipod communities of the two southwest Pacific trenches would be more similar due to their proximity and similar levels of organic matter input relative to the Peru-Chile Trench. However, while H. dubia were sampled in both the Kermadec and New Hebrides Trenches, and was the species predominantly responsible for the similarity of the hadal community that included both these trenches, it is unlikely that significant migration is maintained between these populations. A study of another species recorded in multiple trenches, H. gigas, has found phylogenetic differentiation in populations from the Philippine, Palau and Mariana Trenches in the northeast Pacific (France, 1993). Migration between the Kermadec and New Hebrides Trenches would have to occur at the depths of the abyssal plains between them. No H. dubia were identified shallower than 6000 m in the Kermadec Trench or 4700 m in the New Hebrides Trench and the species were entirely absent from the South Fiji Basin stations at 4100 m. The presence of H. dubia at 4700 m is, however, tantalising in that this is the shallowest record of the species to date. Despite not being found at stations on the adjoining South Fiji Basin, is could suggest the possibility

of an abyssal corridor for dispersal between trenches. If this is not the case, then populations of Hirondellea within trenches are indeed unable to maintain gene flow between them and it is likely that species have diverged in spatially disparate trenches. A detailed phylogenetic study of Hirondellea populations is required to elucidate the degree of speciation between them, and may reveal historical penetration of hadal environments. One of the most interesting results to emerge from the multiple-trench analysis of amphipod community structure relates to the definition of the hadal zone. The minimum depth of the hadal zone is traditionally considered to be 6000 m (Belyaev, 1989; Wolff, 1960), and was the definition used in the present study. Previous work in the study regions has identified a gradual change in community structure with increasing depth until an abrupt change between the abyssal and hadal communities of scavenging amphipods in the Kermadec Trench between 6007 – 6890 m (Jamieson et al., 2011b) and between 6173 – 7050 m in the Peru-Chile Trench (Fujii et al. 2013). Likewise a shift in community structure has been identified for Kuril-Kamchatka Trench copepods, between 5730 – 7000 m depth (Kitahashi et al., 2013). The results of the analyses conducted in the present study that includes new data, indicate that a separation of abyssal and hadal dominated communities occurs between 5600 - 6000 m in the New Hebrides Trench, and between 6097 - 6709 m in the Kermadec Trench. Apart from the finding for the New Hebrides Trench, these results suggest that the hadal zone probably begins beyond 6000 m, but shallower than ~6700 m. Thus, the recent definition of >6500 m used in a global marine biogeography is probably a more appropriate definition of the hadal zone (UNESCO, 2009; Watling et al., 2013). 4.2 Species richness Previous work has identified a decline in species richness along the abyssal to hadal depth gradient in the Kermadec Trench ( Jamieson et al., 2011b; Fujii et al., 2013;). The addition of bathyal data for this trench region revealed a unimodal trend in species richness, peaking between 3500 – 4000 m depth. No previous studies have reported upon bathymetric trends for species richness in the New Hebrides Trench region. A unimodal trend in species richness was revealed, peaking at ~ 4000 m depth. This estimate is similar to that found for the Kermadec Trench despite the fewer number of data points. The estimated species richness curves based on sample data from bathyal to hadal depths exhibited a unimodal relationship with depth, peaking at ~3500 - 4500 m, which was generally conserved between the geographically disparate regions. Studies which have focused on trenches, but have not included data from the bathyal zone, have to date only been able to describe the descending portion

of the bathymetric-diversity relationship observed (Jamieson et al., 2011b; Fujii et al., 2013). Bathyal to abyssal studies of macrofaunal diversity have shown a unimodal distribution with depth, with the peak typically occurring between 2000 and 3000 m ( Rex, 1981; Maciolek et al., 1987). Bathymetric trends in macrofaunal diversity do vary both within and among ocean basins, although a unimodal pattern is a common feature (Brown and Thatje, 2013; Rex and Etter, 2010), and the results of the present study using comparable data suggest that this unimodal pattern in sample diversity is extended into the hadal zone for the scavenging amphipoda. 4.3 Environmental controls on community structure and species richness Hydrostatic pressure was the primary driver of scavenging amphipod community structure and species richness across the trench regions examined in this study. However, hydrostatic pressure is directly related to depth and is in turn related to both temperature and POC flux (Suess, 1980; Lutz et al., 2007). Nonetheless, evidence that hydrostatic pressure itself (or potentially other depthrelated variables not considered by the study) is the most likely driver of biodiversity patterns was revealed in the conservation of depth-related community structure and species richness trends between trench regions with different temperature characteristics, maximum depths and levels of surface primary productivity/POC flux to the seafloor. The identification of distinct amphipod communities, with relatively high levels of community dissimilarity, above and below ~6000 - 6500 m water depth, across trench regions in the South Pacific, presents convincing evidence for a distinct hadal fauna. Pressure tolerance has been posited as a physiological barrier preventing abyssal species penetrating the hadal zone ( Wolff, 1970; Belyaev, 1972), and hadal species are not present in trenches (Danovaro et al., 2010) or canyons (Duffy et al., 2012) at abyssal depths. Thus, it is likely that physiological controls exerted by high pressures below ~6000 - 6500 m, irrespective of the potential ecological drivers related to the topographic or regional nature of the trench environment, are important in structuring faunal communities across the trench regions studied. High hydrostatic pressure decreases the fluidity of biological membranes ( DeLong and Yayanos, 1985; Behan et al., 1992; Somero, 1992; Winter and Dzwolak, 2005), the effects of which include disruption to respiratory and cardiac function (Brown and Thatje, 2011), the central nervous system ( Brauer et al., 1980; Brauer, 1984;), voluntary movement and feeding ( Thatje et al., 2010; Brown and Thatje, 2011;) and metabolic efficiency and growth (Smith et al., 2015). Typical responses to high hydrostatic pressure to maintain membrane fluidity are to increase the proportions of unsaturated fatty acids in the membranes, a process termed homeoviscous adaptation, which has been documented in bacteria (DeLong and Yayanos, 1985), deep sea fish (Cossins and Macdonald, 1989; Mayor et al., 2013) and copepods (Pond et al., 2014), an adaptation that we would expect in deep-sea amphipods. At what precise depth

physiological controls are responsible for an abrupt change in the community structure of scavenging amphipods is yet to be determined by fine-scale resolution sampling across the abyssalhadal boundary. Such sampling will need to be at the scale of <100 m water depth, and most likely achievable using ROV or submersible placement and recovery of traps. When multiple trench regions, with varying environmental conditions, were considered together in the present analysis the influences of additional environmental drivers on community structure were revealed. The difference between the bathyal/ abyssal community of the New Hebrides Trench/ South Fiji Basin regions and the predominantly bathyal/ abyssal community of the Kermadec Trench/ Peru-Chile Trench regions appear related to the estimated quantity of POC flux to the seafloor. In particular, the relationship between POC flux and the separation of the former two community groups, as well as stations from the Peru-Chile Trench region from those of the other regions in the mixed region abyssal community, suggests that there may be a POC flux threshold at ~2 g C m2 y-1 governing the structure of bathyal/abyssal scavenging amphipod communities. The New Hebrides Trench and South Fiji Basin stations of group 3 had estimated POC flux values ≤1.9 g C m 2 y-1, with the exception of the single shallowest New Hebrides Trench station at 2.3 g C m2 y-1. In contrast the abyssal Kermadec/ Peru-Chile Trench stations in group 2 had POC flux values ≥2.1 g C m2 y-1, with the exception of the single New Hebrides Trench station at 1.5 g C m2 y-1. The cross-region dbRDA analysis reveals a separation of the stations of the bathyal/abyssal communities along the first axis and stations nearest this separation point had POC fluxes of 1.07 – 1.19 g C m2 y-1on one side and 1.91 – 2.03 g C m2 y-1 on the other, corresponding to the suggested threshold for POC flux of ~2 g C m2 y-1. Differences in scavenging community composition have been identified between areas underlying differing levels of primary productivity (and hence POC flux) from bathyal to abyssal depths in the Atlantic (Merrett, 1987; Armstrong et al., 1992; Thurston et al., 1995; Christiansen, 1996; King et al., 2006; Horton et al., 2013), Mediterranean (Maynou and Cartes, 2000; Tecchio et al., 2011), Arabian Sea (Janßen et al., 2000) and North Pacific (Priede et al., 1990). POC flux was used to identify the bathyal and abyssal biogeographical provinces proposed by Watling et al. (2013). According to this scheme, the lower bathyal (defined as 800 – 3500 m) seafloor of the South Fiji Basin and Kermadec Trench regions are within single province (BY6, characterised by POC flux of 4 – 6 g C m2 y-1), separate to those of New Hebrides Trench (BY12, 2 – 3 g C m2 y-1) and Peru-Chile Trench (BY8, 6 – 8 g C m2 y-1). The abyssal (defined as 3500 – 6500 m) seafloor of the New Hebrides Trench, South Fiji Basin and Kermadec Trench regions are included in an extensive South Pacific province (AB10< 1 g C m2 y-1) separated to that of the Peru-Chile Trench (AB9, 2 – 6 g C m2 y-1 ) by the East Pacific Rise. The relationship between POC flux and community structure observed in the present study supports the proxy use of this variable to classify biogeographic provinces.

However, while the identification of separate lower bathyal and abyssal provinces in the southeast Pacific is supported by the present analysis, the boundaries of these provinces may need to be reviewed in the light of our results. That is, they suggest that the Kermadec Trench region should sit in a separate lower bathyal province characterised by a POC flux > 2 g C m2 y-1, and the South Fiji Basin and New Hebrides Trench region should be in the same province with POC values < 2 g C m2 y-1 (i.e. remove the South Fiji Basin from BY6 and combine it with BY12). In addition, the results suggest that the South Pacific abyssal province may need to be divided in some way. Increased sampling effort at bathyal and abyssal depths, particularly of the largely unexplored abyssal plains of the South Pacific, would further usefully inform a revision of the benthic biogeography of the region. The cross-trench region analysis identified distinct hadal communities, with the Kermadec and New Hebrides Trenches being different to those of the Peru-Chile Trench. Analysis revealed this community structure pattern was related to relative POC flux estimates. POC flux to hadal depths in the Peru-Chile Trench was ~2.5 times that estimated for the Kermadec and New Hebrides trenches. Many studies have noted the influence of overlying productivity regimes (and hence POC flux) on the density and composition of trench communities ( Wolff, 1960; Longhurst et al., 1995; Jamieson et al., 2011a), with comparative studies showing differences in diversity and community structure (Fujii et al., 2013; Gallo et al., 2015). The hadal provinces proposed by Belyaev (1989) and accepted in Watling et al. (2013) were based on the high levels of endemism from the limited sampling at the time. The findings of the present study clearly support the need for a separate hadal biogeography. However the identification of a single hadal community for the New Hebrides and Kermadec Trenches suggests that the largely single-trench defined provinces (which separate these trenches into provinces HD4 and HD5, respectively) may require revision based on values for POC flux and other variables used to define provinces at shallower depths (Watling et al., 2013). In addition, analyses such as the present study that compare data from multiple trenches, particularly for a range of taxonomic groups, will also help revise our understanding of hadal biogeography. Temperature had only a small, but significant, effect on the community structure in the cross-trench analysis that was dominated by the effect of POC flux. Temperature showed an inverse relationship with POC flux in the analysis to determine the environmental influences on the community structure of groups 1 – 3. The thermocline at bathyal depths coincided with the overlap between groups 1 and 3 at these depths but did not account for the differences between groups. Despite the large difference in temperature between the hadal depth stations of the Kermadec and New Hebrides trenches (~1 °C) temperature only had a marginal effect on the community structure at these depths, accounting for some of the within group 5 variation. The small effect of temperature on community structure in this study is perhaps not surprising given the known cosmopolitan

distribution of some of the amphipod species sampled ( Thurston et al., 2002; Brandt et al., 2012; Cousins et al., 2013; Havermans et al., 2013; Horton et al., 2013; Kilgallen, 2014). Unlike its importance in shallow water systems, temperature has been found to have a far lesser effect than chemical energy on biological organisation at community and ecosystem levels in the deep sea (McClain et al., 2012). Although unimodal trends in species richness with depth are widely reported, the processes that definitively drive these patterns remain elusive (reviewed in Graham & Duda 2011). The unimodal relationship in sample species richness observed with depth across the study regions was explained to a relatively large extent by hydrostatic pressure (35 % of the variation observed), with only a marginal but significant effect of POC flux and temperature (≤ 2% of variation observed). The dominance of pressure as an explanatory variable supports the finding that bathymetric trends in species richness were generally conserved across regions. The metabolic theory of ecology predicts an increase in diversity with temperature (Allen et al., 2002), and therefore it is not surprising that temperature is also identified, but to a much lesser extent, as contributing to the observed patterns in species richness. The small influence of temperature on the observed species richness patterns is prevalent at the deeper depths, as evidenced by the comparatively lower species richness in the colder Kermadec Trench. Physiological adaptation to both increased hydrostatic pressure and reduced temperature are required by shallow water species to penetrate greater depths and this has been proposed as a driver of adaptive radiation at bathyal depths, contributing to the unimodal diversity-depth pattern (Brown & Thatje, 2013 and references therin). However, the mechanistic relationship between the physiological effects of pressure and temperature and the unimodal trend in diversity remains unresolved (Brown and Thatje, 2013). Despite the relative importance of pressure in explaining the patterns of species richness observed, we cannot confidently say that pressure per-se is driving species richness as many biotic and abiotic variables, beyond the remit of this study, vary with depth e.g. sediment characteristics (Etter and Grassle, 1992). Furthermore, as well as environmental factors, biological drivers could also be responsible for the unimodal patterns of diversity with depth. Lower diversity at bathyal depths may be due to a higher rate of biological interactions including increased predation risk from animals not present in the hadal zone (e.g. fish) ( Jamieson et al., 2009b, 2013; Fujii et al., 2010). Reduced diversity at maximum trench depths may in turn be driven by increased competitive exclusion amplified by high population densities of successful species in the smaller area of the trench axis, with lower nutritional input (Arrhenius, 1921; Currie et al., 2004; Evans et al., 2005; Kunstler et al., 2012; Segre et al., 2014).

5. Conclusions Taken together, the results of the present and previous studies suggest that the distinction between “abyssal” and “hadal” communities will vary between trenches, potentially due to ecologically related factors. Thus, while there is likely a boundary between the abyssal and hadal zones that is fundamentally set by a fauna’s physiological limitations, there are also likely fauna that are specific to the suite of environmental conditions of trenches. The concept of a benthic trench fauna has previously been proposed by Menzies & George (1967), who received criticism because they failed to also recognise the importance of the physiological limits on life imposed by pressure at depths beyond 6000 m (Wolff, 1970). Pressure and POC flux appear to drive the bathymetric trends in community structure within trench regions. POC flux best explained similarities in both abyssal and hadal community structure, with the scavenging amphipod fauna of abyssal regions with POC flux exceeding ~ 2 g C m2 y-1 (Kermadec and Peru-Chile Trench regions) showing a greater similarity than the those of the more oligotrophic abyssal regions (South Fiji Basin and New Hebrides Trench regions). Likewise the oligotrophic hadal regions (New Hebrides and Kermadec Trenches) had a more similar community than that of the eutrophic hadal region (Peru-Chile Trench) regardless of bottom temperature. A unimodal pattern of species richness, peaking at 3000 – 4000 m was conserved across trench regions. Pressure appeared to be the primary driver of species richness, while temperature played a marginal but significant role particularly at bathyal depths. Current perspectives on bathymetric trends of species diversity have been largely derived from continental slope studies. However, the identification of a deeper peak (3000 – 4000 m) for the unimodal pattern of amphipod diversity with depth, when multiple trenches were considered, indicates the need for similar investigations over the full bathymetric range of the deep sea. An increased focus on multi-region analyses, incorporating a range of other faunal groups is also required to produce a revised hadal biogeography. Acknowledgements This work was funded by the TOTAL Foundation (France) through the projects ‘Multi-disciplinary investigations of the deepest scavengers on Earth’ (2010-2012) and ‘Trench Connection’ (20132015). We thank the crew and company of the RV Kaharoa (KAH1109, KAH1202, KAH1301 and KAH1310), and NIWA Vessels Management, Wellington, New Zealand. We also gratefully acknowledge assistance by Dr Tammy Horton at the National Oceanography Centre, Southampton, U.K. N.C.L. and A.J.J. are supported by the Marine Alliance for Science and Technology for Scotland (MASTS) pooling initiative, whose support is gratefully acknowledged. A.A.R. and M.R.C. participated in the study through the New Zealand Foundation for Research, Science and Technology (now

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TABLES Table 1. Station and environmental data for the Kermadec Trench (KT), New Hebrides Trench (NHT), Peru-Chile Trench (PCT) and South Fiji Basin (SFB) regions. Data from cruises KAH0910/SO197 and SO209 are from Jamieson et al. (2011) and Fujii et al. (2013), respectively. FT: Fish Trap, HL: Hadal Lander, AL: Abyssal lander. Estimated POC flux in g m-2 yr-1 is from Lutz et al. (2007), Bottom time is in decimal hours.

Area

Depth (m)

Station

Date

Gear

Latitude

Longitude

Temp. (°C)

Est. POC

Bottom Time

KT

1488

KAH1301/19

28.01.13

LATIS

39°00.19'S

178°34.50'E

3.26

2.697

07.2

KT

2197

KAH1301/04

21.01.13

FT

33°59.52'S

179°53.44'W

2.38

3.340

10.7

KT

3268

KAH1301/07

22.01.13

FT

33°59.99'S

179°20.35'W

1.44

2.883

13.0

KT

4193

KAH1301/10

23.01.13

FT

34°00.00'S

179°01.32'W

1.07

2.395

12.5

KT

4329

KAH0910/08

09.11.09

HL-B

36° 45.31’ S

179°11.52’W

1.06

2.902

12.6

KT

5173

KAH0910/02

05.11.09

HL-B

36°31.02’ S

179°12.03’W

1.09

2.753

09.5

KT

5242

KAH1301/13

24.01.13

FT

34°00.01'S

178°41.07'W

1.08

2.105

12.1

KT

6000

KAH0910/06

07.11.09

HL-B

36°10.07’ S

179°00.27’W

1.17

2.496

12.7

KT

6007

SO197/1a

07.07.07

HL-A

26°43.94’ S

175°11.33’W

1.16

1.234

17.5

KT

6097

KAH1202/04

19.02.12

LATIS

32°40.76'S

176°45.95'W

1.19

2.033

12.0

KT

6709

KAH1202/09

22.02.12

LATIS

32°22.70'S

177°05.62'W

1.27

1.806

11.2

KT

6890

SO197/2a

08.07.07

HL-A

26°48.73’ S

175°18.10’W

1.31

1.255

12.3

KT

6968

KAH1109/07

28.11.11

OBS

32°37.770'S

177°14.630'W

1.31

1.819

11.5

KT

7000

KAH1109/03

28.11.11

LATIS

32°33.428'S

177°14.594'W

1.31

1.826

12.7

KT

7291

KAH1109/13

01.12.11

LATIS

32°35.228'S

177°17.739'W

1.36

1.948

08.4

KT

7561

KAH0910/07

08.11.09

HL-B

35°45.10’ S

178°52.55’W

1.4

2.293

12.6

KT

7884

KAH1109/09

29.11.11

LATIS

32°36.985'S

177°21.493'W

1.45

1.839

09.0

KT

7966

SO197/3a

10.07.07

HL-A

26°54.96’ S

175°30.73’W

1.46

1.278

47.0

KT

8487

KAH1202/06

20.02.12

LATIS

32°39.92'S

177°27.93'W

1.56

1.823

12.1

KT

9053

KAH1202/08

21.02.12

LATIS

31°58.71'S

177°23.31'W

1.66

1.725

10.5

KT

9908

KAH1109/12

30.11.11

LATIS

32°01.594'S

177°22.255'W

1.82

1.773

14.4

PCT

4602

SO209/11

03.09.10

HL-B

06° 12.42’ S

81°40.13’W

1.80

16.135

20.3

PCT

5329

SO209/03

01.09.10

HL-B

04° 27.02’ S

81° 54.72’W

1.87

13.152

11.1

PCT

6173

SO209/19

05.09.10

HL-B

07° 48.04’ S

81° 17.01’W

1.98

7.889

18.4

PCT

7050

SO209/35

10.09.10

HL-B

17° 25.47’ S

73° 37.01’W

2.07

4.348

22.5

PCT

8074

SFB

4100

SFB

4100

SFB

4100

a

SO209/48

13.09.10

HL-B

23° 22.47’ S

71° 19.97’W

2.25

3.896

20.3

b

KAH1310/03

09.11.13

LATIS

25° 08.56'S

171° 04.16'E

1.85

1.810

12.6

b

KAH1310/04

09.11.13

FT

25° 08.62'S

171° 05.78'E

1.85

1.810

08.2

b

KAH1310/07

10.11.13

LATIS

24°58.24°'S

171° 03.42'E

1.85

1.753

14.3

b b

SFB

4100

KAH1310/08

10.11.13

FT

24° 58.32'S

171° 04.94'E

1.85

1.754

13.0

SFB

4100

KAH1310/39

29.11.13

FT

27°44.84’S

174°14.97’E

1.89

1.835

15.0

NHT

2000

KAH1310/31

23.11.13

FT

21° 16.58'S

168° 12.53'E

2.17

2.571

15.2

NHT

2500

KAH1310/37

26.11.13

FT

21° 13.15'S

168° 40.04'E

1.93

1.556

15.0

NHT

3374

KAH1310/24

20.11.13

HL-C

21° 07.68'S

168° 11.07'E

1.82

1.865

12.5

NHT

3400

KAH1310/23

20.11.13

FT

21° 06.80'S

168° 09.87'E

1.81

1.865

17.1

NHT

4700

KAH1310/29

22.11.13

FT

20° 56.08'S

168° 28.60'E

1.91

1.617

15.5

NHT

4835

KAH1310/28

22.11.13

AL

20° 57.00'S

168° 29.41'E

1.92

1.552

14.0

NHT

5180

KAH1310/16

17.11.13

FT

20° 54.72'S

168° 32.25'E

1.97

1.552

14.3

NHT

5300

KAH1310/26

21.11.13

FT

20° 49.99'S

168° 31.52'E

2.00

1.552

18.0

NHT

5600

KAH1310/34

25.11.13

FT

20° 45.52'S

168° 29.15'E

2.03

1.477

15.5

NHT

6000

KAH1310/33

24.11.13

FT

20° 47.67'S

168° 32.77'E

2.09

1.498

14.2

NHT

6228

KAH1310/20

19.11.13

HL-C

20° 49.31'S

168° 34.99'E

2.13

1.519

12.1

NHT

6948

KAH1310/27

21.11.13

HL-C

20° 38.91'S

168° 36.83'E

2.23

1.506

17.5

a

Estimated temperature extracted from regression of measured temperature and depth at other KT stations. b Sonar derived depth estimate. Table 2. Depth distributions of amphipod species recorded from the Kermadec Trench (KT), New Hebrides Trench (NHT), Peru-Chile Trench (PCT) and South Fiji Basin (SFB) regions. * indicates potentially new species arising from this study, ** denotes a new depth record: only applied to species level identifications. Family

Species

Alicellidae

Alicella gigantea

Cyclocaridae

Region KT

NHT

4700 -7000

c.f. Diatectonia*

NHT

3374

Diatectonia sp.*

NHT

3374

KT

NHT

PC

SFB

1488 - 6968

**

Paralicella tenuipes (Chevreux, 1908)

KT

NHT

PC

SFB

3400 - 7291

**

Tectovalopsis c.f. wegeneri*

NHT

Cyclocaris sp*

NHT

Cyclocaris tahitensis

KT

Eurythenes gryllus

KT

Eusiridae

Eusiridae sp.* Rhachotropis sp.

SFB

PC

SFB

2000 - 8074 6228

KT

5173 NHT

KT

2000 - 5180 6007

NHT

SFB

NHT

3400 - 5600 4700 - 9908

Hirondellea sonne (Kilgallen 2014)

PC

7050

Hirondellea thurstoni (Kilgallen 2014)

PC

6173 - 8074

PC

6173

Hirondellea wagneri (Kilgallen 2014) Orchomenella gerulicorbis (Shulenberger and Barnard,1976) Species A

KT

NHT

KT

6968

Species B*

KT

1488

Species C*

KT

8487

Pardaliscidae

Princaxelia sp.

Scopelocheiridae

Paracallisoma sp.*

KT

NHT

KT

NHT

Tryphosinae

Bathycallisoma schellenbergi (Birstein and Vinogradov,1958) aff.Tryphosella Tryphosella sp. 1

KT

1488 - 7561

PC SFB

KT

Abyssorchomene chevreuxi (Stebbing, 1906)

KT

PC

Abyssorchomene distinctus (Birstein and Vinogradov, 1960) Abyssorchomene musculosus (Stebbing, 1888)

KT

PC

Stephonyx sp.*

KT

**

8074 6007 - 6709

Abyssorchomene abyssorum (Stebbing, 1888)

KT

**

2000 - 5600 5600 - 8487

PC

Abyssorchomene sp.*

**

5329

PC

Tryphosella sp. 2 Uristidae

3374 - 5180

NHT

Hirondellea aff. Brevicaudata* Hirondellea dubia (Dahl, 1959)

Lysianassidae

New depth record

Paralicella caperesca

Eurytheneidae

Hirondelleidae

Depth (m)

7050

NHT

SFB

1488 - 5600

**

3268 - 6173

**

1488 - 5329

**

NHT

1488 - 5173

NHT

2000 - 5300 1488 - 2197

Valettiopsidae

Tectovalopsis sp.

PC

Valettietta anacantha (Birstein and Vinogradov, 1963)

KT

Valettietta c.f. gracilis*

2197 - 7000

NHT

4700 - 4835

Valettietta sp.

PC

Valiettetta cf lobate*

4602

NHT

KT

4600 2197

Table 3. Result of distance-based linear model (DistLM) analyses for the influence of the environmental variables (estimated POC flux, temperature and pressure) on the amphipod community structure. Results of the marginal tests show the influence of each variable in isolation, whereas results of the sequential tests show the effect of environmental variables on the community in the combined model (stepwise selection with R2(adj) criterion). Prop. = proportion of total variation explained; Prop. (cumul.) = cumulative proportion of total variation explained.

Prop. (cumul.)

R2(adj) (cumul.)

0.48 0.09 0.04

0.48 0.57 0.61

0.45 0.53 0.54

New Hebrides Trench region Marginal Pressure** 0.41 POC 0.15 Temperature* 0.31 Sequential Pressure** 0.41 + Temperature 0.13

0.41 0.54

0.35 0.44

0.12 0.22 0.26

0.08 0.16 0.16

Parameter Kermadec Trench region Marginal Pressure*** POC*** Temperature Sequential Pressure*** + Temperature** + POC

Prop.

0.48 0.34 0.05

Trans-South Pacific region 1 to 3 Marginal Pressure** 0.12 Temperature 0.04 POC** 0.11 Sequential 0.12 Pressure** 0.11 + POC** 0.04 + Temperature 4 and 5 Marginal

**

Pressure* Temperature* POC** Sequential POC** + Pressure** + Temperature * p < 0.05 ** P < 0.01 *** p < 0.001

0.20 0.18 0.43 0.43 0.21 0.04

0.43 0.64 0.68

0.39 0.58 0.60

Table 4. Result of distance-based linear model (DistLM) analyses for the influence of the environmental variables (estimated POC flux, temperature and pressure) on the estimated species richness for the groups identified by cluster and SIMPROF analysis across all regions. Results of the marginal tests show the influence of each variable in isolation, whereas results of the sequential tests show the effect of environmental variables on the community in the combined model (stepwise selection with R2(adj) criterion). Prop. = proportion of total variation explained; Prop. (cumul.) = cumulative proportion of total variation explained.

Parameter Marginal Pressure*** POC Temperature Sequential Pressure*** Temperature**

Prop.

Prop. (cumul.)

R2(adj) (cumul.)

0.35 0.37

0.33 0.34

0.35 0.02 0.01 0.35 0.02

FIGURE LEGENDS Figure 1. Maps showing the study stations in A) the relative positions of the study regions in the South Pacific, B) the South West Pacific and C) the South East Pacific. White circles: New Hebrides Trench, diamonds: South Fiji Basin, filled circles: Kermadec Trench and triangles: Peru-Chile Trench region. Figure 2. A) Temperature profiles showing the effect of adiabatic heating with increased depth > 3000 – 4000 m depth and B) Estimated POC flux (± S.D) between the bathyal (1000 – 3500 m), abyssal (3501 – 6000 m) and hadal (> 6000 m) depth zones for the New Hebrides Trench (white circles), Kermadec Trench (black circles), Peru-Chile Trench (triangles) and South Fiji Basin (diamonds) regions. Figure 3. nMDS plots of the amphipod species-abundance data (fourth root transformed) for A) the Kermadec Trench region, B) the New Hebrides region and C) the combined Kermadec Trench, New Hebrides Trench, South Fiji Basin and Peru-Chile Trench region stations. Symbols and encircling lines denote the community groupings identified by cluster and SIMPROF analysis, and are labelled by depth (m). A) Group 1 (squares), group 2 (inverted triangles), group 3 (triangles), B) Group 1 (squares), Group 2 (triangles), Group 3 (inverted triangles), C ) Group 1 (filled squares), Group 2 (open squares), Group 3 (grey diamonds), Group 4 (open circles), Group 5 (filled triangles). Kermadec Trench (KT), New Hebrides Trench (NHT), Peru-Chile Trench (PCT) and South Fiji Basin (SFB) regions. Figure 4. Results of the distance-based redundancy analysis of the amphipod species-abundance data (fourth root transformed) for A) the Kermadec Trench region, B) The New Hebrides Trench region, C) groups 1 to 3 and D) groups 4 and 5 of the combined Kermadec Trench, New Hebrides Trench, South Fiji Basin and Peru-Chile Trench region stations. Ordinations are overlaid with the significant explanatory variables (pressure, temperature and POC = particulate organic matter flux to seafloor) using a permutated forwards selection procedure based on R2(adj) in DistLM. Symbols denote the community groupings identified by cluster and SIMPROF analysis. A) Group 1 (squares), group 2 (inverted triangles), group 3 (triangles), B) Group 1 (squares), group 2 (triangles), group 3 (inverted triangles), C and D) Group 1 (filled squares), group 2 (open squares), group 3 (grey diamonds), group 4 (open circles), group 5 (filled triangles). Kermadec Trench (KT), New Hebrides Trench (NHT), Peru-Chile Trench (PCT) and South Fiji Basin (SFB) regions. Figure 5. Bathymetric trends in the number of estimated species (Sest) within the (A) Kermadec Trench region, (B) New Hebrides Trench region, and (C) for all regions. Kermadec Trench (filled circles), New Hebrides Trench (open circles), Peru-Chile Trench (inverted open triangles), and the South Fiji Basin (open diamonds) regions. Figure 6. Results of the distance-based redundancy analysis of the estimated species richness data for the Kermadec Trench (filled circles), New Hebrides Trench (open circles), Peru-Chile Trench (inverted open 39

triangles), and the South Fiji Basin (open diamonds) regions. Ordinations are overlaid with the significant explanatory variables (pressure and temperature) using a permutated forwards selection procedure based on R2(adj) in DistLM.

40

41

42

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