Description: Due to the increasing challenge of meeting human demands for metals from land-based resources, interest in extracting mineral ores from the deep sea has gained momentum in recent years. Targeted mining of deep-seabed minerals could have adverse effects on the associated ecosystems, but knowledge on the biological communities found there, their structure and functions is still limited. The focus of this study is to provide an overview on isopod crustaceans from the Clarion Clipperton Fracture Zone (CCFZ), an area well-known for its abundance of high-grade polymetallic nodules. Isopods generally comprise an important part of the macrofaunal communities of soft deep-sea sediments and indeed are one of the most dominant macrobenthic groups in the CCFZ. In this review, we have compiled all available data and information on isopod diversity and distribution in the CCFZ in a hybrid manner, which includes published data from the literature as well as the analysis of previously unpublished sources and newly collected data. Although isopods are one of the more prevalent and better-known groups of the CCFZ fauna, this study shows that it is still remarkably difficult to obtain a clear perception of isopod diversity and distribution, as well as the factors that could be responsible for the observed patterns. In many places, knowledge remains incomplete, which is largely due to the low sampling and taxonomic effort, non-standardised sample protocols and the limited taxonomic inter-calibration between studies. The latter is pivotal due to the high proportion of undescribed and presumably new species that typically occur there. An important starting point would therefore be to increase sampling effort and its spatial and temporal coverage in a standardised way, to intensify (integrative) taxonomic work as well as to facilitate sample and data exchange between scientists and contractors. These are fundamental requirements to improve our understanding of the biodiversity of isopods, but also of other faunal groups, in the CCFZ, before mining operations begin.

Description: Imaging is increasingly used to capture information on the marine environment thanks to the improvements in imaging equipment, devices for carrying cameras and data storage in recent years. In that context, biologists, geologists, computer specialists and end-users must gather to discuss the methods and procedures for optimising the quality and quantity of data collected from images. The 4 th Marine Imaging Workshop was organised from 3-6 October 2022 in Brest (France) in a hybrid mode. More than a hundred participants were welcomed in person and about 80 people attended the online sessions. The workshop was organised in a single plenary session of presentations followed by discussion sessions. These were based on dynamic polls and open questions that allowed recording of the imaging community’s current and future ideas. In addition, a whole day was dedicated to practical sessions on image analysis, data standardisation and communication tools. The format of this edition allowed the participation of a wider community, including lower-income countries, early career scientists, all working on laboratory, benthic and pelagic imaging. This article summarises the topics addressed during the workshop, particularly the outcomes of the discussion sessions for future reference and to make the workshop results available to the open public.

Description: Antarctic meiofauna is still strongly understud- ied, and so is its trophic position in the food web. Primary producers, such as phytoplankton, and bacteria may repre- sent important food sources for shallow water metazoans, and the role of meiobenthos in the benthic-pelagic coupling represents an important brick for food web understanding. In a laboratory, feeding experiment 13C-labeled freeze- dried diatoms (Thalassiosira weissflogii) and bacteria were added to retrieved cores from Potter Cove (15-m depth, November 2007) in order to investigate the uptake of 3 main meiofauna taxa: nematodes, copepods and cumaceans. In the surface sediment layers, nematodes showed no real difference in uptake of both food sources. This outcome was supported by the natural delta 13C values and the community genus composition. In the first centimeter layer, the dominant genus was Daptonema which is known to be opportunistic, feeding on both bacteria and diatoms. Copepods and cumaceans on the other hand appeared to feed more on diatoms than on bacteria. This may point at a better adaptation to input of primary production from the water column. On the other hand, the overall carbon uptake of the given food sources was quite low for all taxa, indicating that likely other food sources might be of relevance for these meiobenthic organisms. Further studies are needed in order to better quantify the carbon requirements of these organisms.

Description: The meiobenthic community of Potter Cove (King George Island, west Antarctic Peninsula) was investigated, focusing on responses to summer/winter conditions in two study sites contrasting in terms of organic matter inputs. Meiofaunal densities were found to be higher in summer and lower in winter, although this result was not significantly related to the in situ availability of organic matter in each season. The combination of food quality and competition for food amongst higher trophic levels may have played a role in determining the standing stocks at the two sites. Meiobenthic winter abundances were sufficiently high to infer that energy sources were not limiting during winter, supporting observations from other studies for both shallow water and continental shelf Antarctic ecosystems. Recruitment within meiofaunal communities was coupled to the seasonal input of fresh detritus for harpacticoid copepods but not for nematodes, suggesting that species-specific life history or trophic features form an important element of the responses observed.

Description: The West Antarctic Peninsula is one of the fastest warming regions on Earth. Faster glacier retreat and related calving events lead to more frequent iceberg scouring, fresh water input and higher sediment loads, which in turn affect shallow water benthic marine assemblages in coastal regions. In addition, ice retreat creates new benthic substrates for colonization. We investigated three size classes of benthic biota (microbenthos, meiofauna and macrofauna) at three sites in Potter Cove (King George Island, West Antarctic Peninsula) situated at similar water depths but experiencing different disturbance regimes related to glacier retreat. Our results revealed the presence of a patchy distribution of highly divergent benthic assemblages within a relatively small area (about 1 km**2). In areas with frequent ice scouring and higher sediment accumulation rates, an assemblage mainly dominated by macrobenthic scavengers (such as the polychaete Barrukia cristata), vagile organisms and younger individuals of sessile species (such as the bivalve Yoldia eightsi) was found. Macrofauna were low in abundance and very patchily distributed in recently ice-free areas close to the glacier, whereas the pioneer nematode genus Microlaimus reached a higher relative abundance in these newly exposed sites. The most diverse and abundant macrofaunal assemblage was found in areas most remote from recent glacier influence. By contrast, the meiofauna showed relatively low densities in these areas. The three benthic size classes appeared to respond in different ways to disturbances likely related to ice retreat, suggesting that the capacity to adapt and colonize habitats is dependent on both body size and specific life traits. We predict that, under continued deglaciation, more diverse, but less patchy, benthic assemblages will become established in areas out of reach of glacier-related disturbance.

Description: For the determination of sediment properties and biogenic sediment compounds, sediment was sampled with 3.6 cm diameter cores in five replicates by SCUBA divers. Sediment subsamples were taken with cut-off syringes (cross-sectional area = 1.65 cm²) and sliced in 1 cm intervals down to 5 cm sediment depth. Each interval was analyzed for various parameters including median grain size, porosity, photosynthetic pigments, total carbon, total organic carbon and total nitrogen. Sediment samples for photosynthetic pigments were stored at -80°C. Sediment samples of other parameters were stored at -20°C. The median grain size was determined with a Malvern Mastersizer 2000G, hydro version 5.40. Sediment porosity was determined after drying sediment samples over minimum two days at 105°C. The sediment porosity was calculated following Burdrige (2006, Geochemistry of marine sediments, Princeton University Press). Chlorophyll a (Chl a), phaeophytin (Phaeo) and fucoxanthin (Fuco) pigment concentrations were determined by HPLC The total carbon (TC) and total nitrogen (TN) was measured by combustion using an ELTRA CS2000The total organic carbon (TOC) was measured using the same method, but after acidifying the sample (3 ml of 10 M HCl). For prokaryotic density determination, five replicate sediment sub-samples were taken with cut-off syringes (cross-sectional area = 1.65 cm²), sliced at 1 cm intervals down to 5 cm sediment depth, fixed in a 2% formaldehyde/seawater filtered solution (9 ml) and stored at 4°C. The acridine-orange-direct-count method after Hobbie (1977, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC170856/) was used to stain prokaryotes in the sub-samples and subsequently counted with a microscope (Axioskop 50, Zeiss) under UV-light (CQ-HXP-120, LEj, Germany). For each sample, single cells were counted on two replicate filters and for 30 random grids per filter (dilution factor 4000). Prokaryotic biomass was estimated by the determination of the mean prokaryotic cell volume in the first two centimetres with a "New Portion" grid (Graticules Ltd, Tonbridge, UK), converted into biomass using a conversion factor of 3.0 x 10-13 g C pm-3 after Børsheim et al. (1990, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC183343/) and multiplied with the prokaryotic density. For the determination of meiofauna density, biomass and identification of meiofauna taxa, five sediment samples were taken with small sediment cores (Ø 3.6 cm). Sediment samples of the first five centimeters were stored in filtered seawater buffered 4% formaldehyde solution at 4°C till the start of the analyses. The samples were sieved over a 1 mm and 32 µm mesh. The sample was then centrifuged three times in a colloidal silica solution (Ludox TM-50) with a density of 1.18 g cm-3 and stained with Rose Bengal after Heip et al. (1985, The Ecology of marine nematodes, Oceanography and Marine Biology - An Annual Review; vol. 23). Afterwards, the taxa were identified and counted. Calcifying organisms (except nematodes and polychaetes) were acidified prior the total organic carbon content of single taxa were analyzed with a FLASH 2000 NC Elemental Analyzer (0.01% detection limit) The benthic macrofauna was sampled by using a Van Veen grab (530 cm² surface area). At each station, four recovered sediment samples were sieved over a 1 mm mesh, stored in seawater buffered 4% formaldehyde and stained with Rose Bengal after Heip et al. (1985). In the laboratory, the taxa were identified to the lowest possible taxonomic level (at least family level), counted, weighted, and the Shannon-Wiener diversity index (H') calculated in Primer v6.0. Ash-free dry weight (AFDW) was determined by subtracting the ash weight (after combustion at 500°C) from the dry weight (dried for 48 h at 60°C). AFDW was converted into carbon by assuming that 50% of the AFDW is carbon after Wijsman et al. (1999, http://www.jstor.org/stable/24849594). The Van Veen grab sampling results in a strong underestimation of the density of the important Antarctic bivalve Laternula elliptica. Therefore, two rows of eight grids (45 cm x 45 cm) were randomly placed on the seafloor by scuba divers and photos were taken (Nikon D750 with a rectilinear Nikon 16-35 mm lens in a Nauticam underwater housing and two Inon Z-240 strobes). The photos were used to count siphons of L. elliptica to determine the density and to measure the siphon width (maximum distance between outer edges of the two siphons of one individual) at the three sites. Assuming a linear relationship between siphon width and AFDW, a conversion from the siphon width to estimated biomass of L. elliptica was performed. The calculation of the conversion relationship of the siphon width to AFDW was performed on data from the same L. elliptica population. The community bioturbation potential was calculated following formula Queiros et al. (2013, doi:10.1002/ece3.769). Three transparent and three black chambers (inner diameter 19 cm, height 33 cm) were carefully pushed into the sediment at each station by SCUBA divers, who took special care to not disturb the sediment surface during the procedure. About 15 cm of sediment and 18 cm of overlying water was enclosed. During the incubation, which lasted 20-22 h and encompassed light and dark hours, cross-shaped stirrers powered by a 12 V lead-acid battery kept the overlying water homogenous. HOBO Pendant loggers (Onset, Bourne, USA) were placed both in situ and on land to record the amount of radiation (150-1200 nm) during the incubation with a temporal resolution of 5 minutes. The enclosed overlying water in the chambers was sampled through valves attached to the chamber lids at the start and end of the chamber incubation, using gas-tight glass syringes. The water samples were kept at in situ temperature and in darkness until further processing, which took place within 1.5 h after the samples were taken. Subsamples were taken to either determine the oxygen concentration, the concentration of dissolved inorganic carbon (DIC) and the concentration of phosphate, ammonium, nitrite, nitrate, and sulfate. Winkler titration was used to immediately determine the oxygen concentration in the water sample in technical duplicates. For DIC analyses technical triplicates were poisoned with HgCl2 and stored at 4°C until measurement after 6 months. DIC samples were analyzed using an autosampler (Techlab, Spark Basic Marathon) with a digital conductivity measuring cell (VWR, digital conductivity meter, Germany). For nutrient analyses technical triplicates were filtered through a GF/F filter and stored at -20°C until analysis. The samples were analyzed with an autosampler (CFA SAN-plus, Skalar Analytical B.V., Netherlands) for ammonium, phosphate, nitrite and [nitrate + nitrite] concentrations. The nitrate concentration was determined by subtracting the nitrite concentration from the [nitrate + nitrite] concentration. The resulting total fluxes were calculated following Glud et al. (2008, doi:10.1080/17451000801888726). High-resolution in situ oxygen profiles were measured using a microprofiler. The microsensors were driven from the water phase into the sediment with a spatial resolution of 100 µm and a temporal resolution of 30 seconds. On the profiler electronic unit, three custom made electrochemical O2 microsensors after Revsbech (1989, doi:10.4319/lo.1989.34.2.0474) were mounted and calibrated before deployment in oxygen saturated and oxygen depleted water. The microprofiler was programmed, so microsensors penetrated the SWI around noon at the same or the following day after the deployment. Running average smoothed profiles (https://doi.pangaea.de/10.1594/PANGAEA.885472) were used to calculate the diffusive oxygen uptake (DOU) over the SWI using Fick's first law. For the calculation of the diffusive flux of sulfate, DIC, and nutrients, sediment was sampled with cores (10cm diameter) with predrilled holes at 1 cm depth intervals that were sealed with diffusion-tight tape. The porewater was extracted using Rhizons (type: core solution sampler, Rhizosphere Research Products, filter pore diameter of 0.1 mm) connected to 10 mL Luer lock syringes. The Rhizons were horizontally inserted into the cores and by creating a permanent vacuum in the syringes, porewater was extracted. The first drops were used to rinse the syringe and then discarded. The extracted pore water was split for sulfate analyses (sample fixed in 5% ZnAc, stored at 4°C), DIC analyses (sample fixed in HgCl2, stored at 4°C) and nutrient analyses (frozen at -20°C). DIC and nutrients were analyzed as described above. Sulfate was analyzed by using non-suppressed ion chromatography with the Methrom 761 Compact IC equipped with a Metrosep A SUPP 5 column (Methrom, Herisau, Switzerland). From the resulting depth profiles, diffusive fluxes were calculated using the same formula as for the DOU calculation, but with Ds of the specific molecule.

Description: To understand the fate of the increasing amount of macroalgal detritus in Antarctic shallow subtidal sediments, a mesocosm experiment was conducted to track 13C and 15N labelled macroalgal detritus into the benthic bacterial, meio- and macrofaunal biomass and respiration of sediments from Potter Cove (King George Island). We compared the degradation pathways of two macroalgae species: one considered palatable for herbivores (the red algae Palmaria decipiens), and one considered non-palatable for herbivores (the brown algae Desmarestia anceps). 15 sediment cores were taken from station Faro at 20m water depth. 13C and 15N labelled macroalgae were added to 10 cores: 5 cores received Desmarestia anceps, 5 cores received Palmaria decipiens. 5 cores did not receive any macroalgae and acted as a control. At different points in time, the cores were closed airtight for a dark incubation of 12h, during which oxygen was measured to calculate Total Oxygen Uptake (TOU), next to 13C-DIC, total nutrients and 15N-labelled nutrients (NH4, NOx and N2). The next day, the cores were sacrificed to determine the assimilation of macroalgae detritus in bacteria, microphytobenthos, meiofauna, macrofauna. Also the remaining large (〉1mm) macroalgae fragments were recovered and the bulk POC and PN of the sediment was measured. The sampling points are 1d after addition, and 7, 14 and 21 and 26 days after addition of macroalgae detritus.

Description: During 4 expeditions to Potter Cove, King George Island, Antarctica (Carlini base: February-March 2015, September 2015, December 2015 and December 2016), benthic chambers were deployed in situ at three locations with differing glacial melt impact (Faro, Isla D, Creek) to measure total oxygen exchange at the sediment-water interface. Dark chambers measured community respiration, while light chambers measured net community metabolism. Sediment parameters were measured in the top 5 cm at the same location as the benthic chambers (sediment physico-chemical parameters, as well as prokyarotic and meiobenthic density and biomass; density and biomass of macrobenthos and of the large burrowing bivalve Laternula elliptica were measured separately). A detailed record of climatology, meteorology and oceanography of Potter Cove over the entire period spanning between the first (February 2015) and last (December 2016) benthic chamber deployment is also provided.