Methodology:

Oceanographic data were collected using RV METEOR's 24 x 10 L Niskin bottle rosette with a Seabird CTD equipped with conductivity, temperature, pressure, and an SBE 43 dissolved oxygen sensor. The near seafloor water data were used to report bottom temperature, salinity, and oxygen.

Seafloor sediment cores and macrobenthic assemblages were obtained with a multicorer (MUC) (OKTOPUS GmbH), comprising eight 10 cm diameter and four 6 cm diameter cores per deployment. On arrival on deck, cores were photographed to document sediment height, layering, and colour before being distributed for geochemical, sediment, and biological analyses. Water depth of sediment recovery ranged from 136 m (Cumberland East Bay station CB 07) to 369 m (Church Trough station CT 05).

MUC cores allocated for macrobiology (a total of 38) were sliced into 0-2 cm and 2-5 cm horizons using a core extruder, sieved through a 500 micrometer mesh, and preserved in 96 percent ethanol. Specimens were initially sorted to higher taxonomic levels onboard using a Meiji stereomicroscope, then further sorted to morphospecies on land under a Zeiss Stemi SV6 stereomicroscope. Photographs were taken using ScopeTek ScopePhoto 3.1.475, and taxonomic identifications were verified by experts using literature (Arntz and Rauschert, 2015). Due to identification challenges, nematodes were grouped into a single taxon, and indeterminate specimens were marked as 'indet.'. Only individuals with heads were counted to prevent duplication.

Abundance data were scaled to m squared for quantitative analyses. Patterns in species richness, abundance, and diversity were assessed by pooling cores per MUC deployment to obtain an assemblage per core. Abundance data were summed at the phylum level and scaled for comparisons between stations. Species richness was verified with a species accumulation curve to ensure observed richness was not an artifact of sampling effort.

Data have been combined with other comparable species/polychaete Southern Ocean non-seep assemblage datasets (see references for the publications used). MUC coring was also used for macrofaunal sampling in these studies - full details of sampling can be found in those publications.

Data collection:

1. RV METEOR's 24 x 10 L Niskin bottle rosette - Equipped with a Seabird CTD (Conductivity, Temperature, Depth) sensor.

Sensors:

Conductivity (psu)

Temperature (degCel)

Pressure - Depth (m)

SBE 43 dissolved oxygen sensor (mLL-10)

2. Multicorer (MUC) (OKTOPUS GmbH) - Used for seafloor sediment and macrobenthic assemblage collection.

Core sizes:

Eight 10 cm diameter cores

Four 6 cm diameter cores

3. Stereomicroscopes:

Meiji stereomicroscope - Used onboard for initial macrofaunal sorting.

Zeiss Stemi SV6 stereomicroscope - Used in the laboratory for further taxonomic sorting.

4. Core Extruder: Used for sediment slicing at 0-2 cm and 2-5 cm horizons.

5. 500 micrometer sieve - Used for sieving sediment samples on deck.

Data quality:

NA in Environmental_parameters.csv data - data were not collected

Indet - the abbreviation 'indet.' was used to denote an indeterminate specimen.

Species were identified to highest taxonomic level possible, and quality checking occurred for consistency in identification. Specialists were contacted to assist in identification for specific taxa.