Benthic macrofauna  from experimental wood and rock substrates deployed for 10 months (2020-2021) in the Southern California Borderland.

Keywords: Samplingevent
Temporal coverage: 2020/2021

The study area for this project was located in the Southern California Borderlands (SCB), situated off the coast of Southern California. Two different sites were chosen for this experiment: 40-Mile Bank (representing a nearshore site likely to experience higher POC flux to the seafloor) and San Juan Seamount (representing an offshore site likely to experience lower POC flux to the seafloor). Each site had experimental deployments at two different depths: ~700m situated in the core of a well-developed Oxygen Minimum Zone (OMZ), representing lower oxygen exposures (7-9 μMol O2/kg), and ~1100m, at the lower boundary of the OMZ (22-23 μMol O2/kg) representing higher oxygen exposures.
To conduct the colonization experiment for this project, defaunated rocks and untreated douglas fir wood blocks were deployed at each location and depth (4 total deployments sets).  Rocks were defaunated by removing visible fauna and drying for &gt; 10 months. To facilitate deployment and recovery, experimental substrates were individually wrapped in thin black plastic garden mesh (1.2 cm) and affixed to polypro nylon rope loops with duct tape to allow for handling by ROV manipulators. Each nylon loop was labeled with the substrate’s type and replicate number. Wood blocks were soaked in filtered seawater for 48 hours before deployment to reduce buoyancy, and duct tape-covered four-pound lead weights were affixed to the plastic mesh with nylon rope to ensure the wood blocks would remain stationary once deployed at each site. Hard substrates were placed approximately 25-50 cm apart at sites to reduce the likelihood of direct contamination. 
Hard substrates included rock (ferromanganese, sedimentary, phosphorite, carbonate) and wood (uniform 9.1 x 9.1 x 25 cm untreated douglas fir blocks). Each deployment set included two wood blocks, two carbonate rocks, and one or more other rocks of varied types. The experiment was deployed October 30, 2020 (San Juan Seamount) and November 3, 2020 (40-Mile Bank) by the ROV Hercules during the E/V Nautilus 2020 Southern California Borderland cruise (NA124). The experimental substrates deployed at the four sites for approximately 10 months, and were collected on July 27, 2021 (San Juan Seamount) and July 31, 2021 (40-Mile Bank) during the Schmidt Ocean Institute’s 2021 Biodiverse Borderlands Expedition on board the R/V Falkor (FK210726). Four to seven in situ rocks were collected within 10 m of each colonization deployment site during the same dives to provide information about surrounding assemblages at the start and end of the experiment. The experimental and in-situ rocks were retrieved via the ROV SuBastian, equipped with two high-resolution video cameras, a CDT to measure environmental data, and a manipulator, which was used to gently pick up experimental hard substrates by the nylon loops affixed to them or in situ rocks directly. Each experimental unit or in situ rock was placed individually in clear isolated biobox compartments within a large delrin biobox to avoid contamination between samples. 
Upon recovery to the ship,  the containers with hard substrates were placed in a refrigerated cold room until processing, which usually took place within a few hours of their return to the ship. Each colonization unit or in situ rock was then photographed with a scale and label. All large, visible fauna were removed using forceps. Some were set aside for stable isotope or genetic analyses and recorded, the remainder were preserved in 70% ethanol. The surface area of all hard substrates collected (both experimental and background) was measured by wrapping the substrate in aluminum foil, weighing the aluminum foil, and calculating the surface area using the weight per unit area of aluminum foil. The exception was the wood blocks, which were originally each a uniform 1075.62 cm2 in surface area.  Surface area at the end of the experiment was not measured, although it may have increased in those heavily bored by Xylophagaid bivalves.  
The residue water contained in each biobox compartment was washed through a 45-micrometer and 300-micrometer mesh to collect meiofauna and macrofauna, respectively. Hard substrates were left in buckets of room-temperature seawater overnight to allow the remaining fauna to fall or crawl out of the crevices of the hard substrates. The substrates and the residue water in the buckets were then also washed through a 45-micrometer and 300-micrometer mesh to recover meiofauna and macrofauna. All macrofauna sorted upon recovery or retained on the 300-micrometer mesh were combined and preserved in 70% ethanol for later sorting and identification and are the subject of this dataset. 
After the cruise, all macrofauna (fauna retained on a 300-micrometer screen) from the experimental and in situ hard substrates were sorted, counted, and identified to the lowest taxonomic group feasible. Each sample was washed over a 300-micrometer mesh sieve and collected in petri dishes to be analyzed under a dissecting microscope at 25x magnification. Each animal was then sorted into major phyla and subsequently identified to the level of species or morphospecies. 
Due to the high density of xylophagaid bivalves and the inability to extract these animals without damage, wood blocks were subsampled to estimate xylophagaid bivalve counts. Wood blocks were sliced into one-inch sections and all visible boreholes on the interior wall of four randomly selected slices were measured for depth. Boreholes up to the average depth (generally 2-3mm) were then counted on the interior wall of each of the four slices and averaged. The average number of boreholes counted per wood slice face was then multiplied by the length of the wood blocks (250mm) divided by the average depth of the boreholes across the selected slices of wood to achieve an estimated number of adult xylophagaid bivalves across the whole block of wood. The exception to this method was for SCB-058 (Wood sample 3 from San Juan Seamount at 694m) due to the wood splitting lengthwise during processing. One side of the block was treated as the subsample wood slice for this sample and boreholes were counted via the same process described above, but multiplied by width (91mm) rather than length in the calculation.