MERTZ-DIVA involves a relatively short but intensive fieldwork session (January to April 2017) during which most observations will be done and two moorings will be deployed. These will be retrieved in January 2019 during a second field campaign. Each fieldwork session will be followed by a phase during which the data and the samples collected will be analysed and the results published and communicated to the scientific community as well as the public. The first cruise will take place on board the research icebreaker Akademic Tryoshnikov, the latest of the Russian oceanographic fleet. Figure shows the seafloor topography at the study site and the locations of the sampling stations.

Map of the study area presenting the 5 dive sites, the CTD (green circles), and full stations (empty and full triangles - sediments ; circles and crosses – Nets and trawls). Black rectangles : glider deployment.

WP1. Biodiversity

Zooplankton assemblages – Krill – Pelagic fishes and higher predators:
A UVP5 will be deployed using the Rosette and will provide information about the size and the abundance of particles at each of the Rosette CTD stations. At full stations, we will also deploy vertical and horizontal nets (bioness, tucker, hydrobios) to quantify and describe the zooplanktonic diversity. For krill and fishes such as Pleuragramma antarcticum, we will deploy an Isaak-Kidds mid-water trawl. This trawl will also be deployed on an opportunistic basis at sites where krill schools will be observed at the echosounder. Each fish or fish larvae will be identified, measure and weighed. Teleosteans will be preserved in formalin (large specimens) or ethanol for a detailed morphological, molecular and systematic analysis in the laboratory. We will also use the otoliths to determine age and growth rates of each fish. The analyses of the otoliths from this predominant “forage” fish of the Antarctic will allow for determining if changes in the growth rates occurred before and after the Mertz calving. For krill samples a representative aliquot will be sorted in order to determine the “age” of the population, identify growth stages (e.g. nauplius, metanauplius, calyptopis and furcilia) for each species commonly observed in the area (Euphausia superba, crystallorophias, frigida…) and obtain samples for biomarker analyses (Goutte et al. 2013). One aliquot will be stored in ethanol for phylogenetic analysis. In addition to these net deployments, zooplankton stocks will be evaluated at each station using the multi-frequency echosounder installed on board. Higher predator (birds and marine mammals) abundances will also be estimated by sight and using sounders (in collaboration with 2 projects from the ACE expedition: Monitoring of Threatened Albatrosses and penguins –PI H. Weimerskirsch, CNRS France & Accoustic Mapping of Endangered Southern Ocean whales – PI Brian Miller, AAD Australia) and also by using UAVs equipped with
hyperspectral and IR cameras (www.survey-copter.com/en/produits/copter-4/ - collaboration Airbus industries).

 

Benthic Assemblages:
Composition and structure of benthic assemblages will be determined at the 5 ROV dive sites (Fig. 3) by combining benthic trawls for mega-faunal assemblages, and box cores for infaunal populations (4 replicates of 0,25m-2). We will also integrate small scale spatial variability by performing 4 sub-stations. A particular attention will be paid to polychaetes that are usually dominating the assemblages (between 25 and 75% of the
species composition, Montiel et al., 2005). Syllidae constitutes most of the species richness and abundance in the Weddel sea (up to 10000 ind.m-2, Montiel et al., 2005). Syllidae are very sensitive to changes in their environment (Grant et al., 2012) and are not only characterized by their large diversity but also very diverse ecologies (feeding and reproductive traits; San Martin, 2003). Although we will pay a particular attention to the polychaetes, all the species that will be collected will be used for our biodiversity assessments. In addition to this detailed analysis, we will incubate each of the interface cores that will be collected using ROPOS in order to compare the functioning of the sedimentary ecosystem along an environmental gradient (5 dive sites, 4 cores at each site). These incubations will allow for determining biogeochemical fluxes (N, C, P, carbon re-mineralisation according to a method adapted from Link et al. 2013a, b) at the water sediment interface. The detailed taxonomic investigation that will be carried out on the sediment cores collected which will allow to develop the relationship between biodiversity and ecosystem functioning that will be used to identify “hot spot” and “cold spots” in the Mertz area (Link et al., 2013b). Once the relationship evaluated, we will combine it with a detailed analysis of the physical, geological and biochemical environments of the assemblages (WP2) allowing for developing predictive models (Moritz et al. 2013, Cobbs et al. 2014, see WP3) for expected climate change scenario. Finally, we will test the usefulness of both polychaetes and benthic assemblages as ecological indicators of transition for the habitats that will be studied in this project.

 

WP2. Analysis of environmental variables - stressors

Cryosphere:
Sea ice concentration will be determined using satellite imagery. We will combine SSMI observations processed using the « NASA Team » algorithm and providing a 25*25km spatial resolution (Cavalieri et al., 1995) to those obtained over a more recent period with the AMSR radiometer. These allow a higher resolution (6.25 km). Average concentrations and anomalies (vs 1978-2009) will be determined at each site and over the polynya area (Campagne et al. 2015). For the glacier, in order to accurately follow movements and deformation of the ice tongue, we will install high accuracy GPS at four sites (Legrésy et al, 2004). These will be installed on posts in order to avoid burial by snow and therefore allow for data to be transmitted over few years. In addition to the GPS data, we will obtain high resolution SPOT7 images of the glacier over the project duration (January 2107, 2018 and 2019 – Collaboration Airbus Defense and Space). Finally, we will install a hydrophone array on the western flank of the glacier (HTI 92 combined to a RTSYS recorder) to listen to the sounds created by the biota and the surrounding geosystems (frequency 100-70000 Hz). This system will allow for determining direction and distance of the sounds and therefore to “visualise” sea ice and glacier movements, as well as the foraging activity of the Emperors from the colony located nearby. In order to better understand the interactions between the ocean and the glacier (e.g. Heat fluxes, basal melting, etc.), we plan two ROV dives at the front and under the glacier. The base of the ice will be mapped and we will perform 3D reconstruction (3D at depth LIDAR system) in order to estimate roughness and refined ice ocean interaction models (Mayet et al., 2013). In the water column turbulence profiles will be obtained (SCAMP and ADCP) and samples will be collected using the 8 Niskin bottles available on the ROV (Salinity, micro and macro-elements, microorganisms, isotopes and noble gases).

 

Hydrological and biogeochemical environments:
For each CTD station, we will document in great detail the water column physical properties, the biogeochemical stocks, and phytoplankton (and other micro-organism) diversity and abundance. Salinity, temperature, particles, organic and inorganic carbon concentration, chlorophyll and oxygen profiles will be obtained using the probes/instruments from the Rosette. Current profilers and scientific echosounders (Rosette and Hull mounted) will also be used to obtain information about currents and acoustic measurements along the profiles. Each profile will be calibrated (Salinity, O2) using the Niskin bottles from the rosette, and we will also quantify nutrients (NOX-, NH4+, PO43-, Si(OH)4), and determine water isotopic composition (δ18O, δD) and noble gases concentrations. These discrete samples will also be analysed using the Counting/imaging FlowCytobot to illustrate the phytoplankton diversity. We will also determine the relative abundance of phytoplankton groups using pigments (Coupel et al, 2015) and large cells (e.g. diatoms) will be identified using inverted light microscopy techniques (Różańska et al., 2009) while small cell abundance will be determined by flow cytometry (Tremblay et al., 2009). Finally, some aliquots will be preserved in RNA later to carry out metatranscriptomic analyses (Gilbert et al., 2010). Size and abundance of the particles presents in the water column will be determined using the UVP5 from the rosette. The data from this instrument will allow for determining the vertical fluxes and the export of carbon from the surface to the bottom. This, combined with the results obtained from the sequential sediment trap (technicap PPS3) deployed on the moorings, will allow for estimating the amount of organic matter that is available for benthic assemblages (e.g.Lalande, 2009). In order to precisely document, (1) the water masses at the bottom, (2) movements of the water masses in the water column (3) icebergs (Lacarra et al. 2014) the 2 moorings will be equipped with salinity and temperature probes (SBE47) and an ADCP (RDI 75Khz Long Ranger). We also plan to deploy a profiling float (Bio- Argo float) to obtain year-round temperature, salinity, nutrient, and chlorophyll profiles along the water column.

 

Geological environments:
For each site, we will carry out a detailed analysis of the bottom topography and sediment type using the ship multibeam and sediment sounder. A survey of few hours will be performed before each ROV dive. For at least the 2 main sites, we will create a 3D map of the assemblage (and bottom) along the track of the ROV. At these full stations, we will complete a granulometric analysis of samples from the box cores and push cores from the ROV. The structure of these cores will also be analysed using X-ray computed micro-tomography in order to obtain a detailed “in-situ” overview of the sediment endo-faunal assemblages and the bioturbation potential. Finally, the inorganic fraction from the sediment trap samples will be analysed using laser granulometric techniques and XRD to obtain size frequencies and qualitative indices.

 

Environmental stressors, Reconstructions:
Large changes are currently observed in the Mertz area. Whether they originate from large (e.g. SAM) or more local (Mertz dynamics) processes, these have profound impacts on every compartments from surface to bottom. In order to better understand the impacts of these changes onto the ecosystems that will be studied in this project, we make large efforts to collect good sediment archives as well as carbonated (molluscs, echinoderms, corals, foraminifera) and siliceous (sponges) archives. Using a combination of granulometric, biomarker, micropaleontological, isotopic and sclerochronological analyses, the Mertz glacier dynamics, sea surface conditions (e.g. Campagne et al., 2015) and bottom water mass characteristics (T, salinity) will be determined at timescales ranging from seasons to millennia. Since Milam & Anderson (1981) reported a large diversity of benthic foraminifera, we will pay a particular attention to this group. Calcareous microfossils will therefore be counted, the species determined and their isotopic/major element composition determined to reconstruct bottom water masses characteristics. These data will be combined with those from the analyses of bivalve shells, coral skeletons and sponge spicules. Indeed, these bio-archives are routinely used for environmental reconstructions: (1) the growth of these invertebrates is (at least in part) is influences by the environments and changes in growth rates will provide information about the response of these organisms to environmental changes. (2) Isotopic or elemental analyses along sclerochronological profiles will allow for obtaining (and reconstruct) detailed information about the physical and chemical environment of the animals during their lifetime (Chauvaud et al., 2005, 2011, 2012).

 

WP3. Modelling. Toward prediction?

We will develop a meta-systemic approach to study diversity-productivity and biodiversity-functioning relationships at local and regional scale. In order to achieve this, we will model metaecosystems and identify ecological zones such as hot and coldspots (Link et al. 2013b). The spatial model that will be developed will help us to predict the occurrence and type of benthic assemblages across the area to generate probability maps at the regional scale. This approach was successfully used in the St-Laurent Gulf and we will adapt it to our study area (Moritz et al., 2013; Levesque et al., 2009). We will first examine the biodiversity in the samples collected during the cruise and will create groups that are based upon similarity indices. These indices will be obtained via multivariate statistical analyses. This will allow for delineating typical assemblages. A non-metric multidimensional scale (NMDS) framework which is based upon the Euclidean distance to the standardised Hellingers index (Legendre & Gallagher, 2001) will be built to spatially visualise species assemblages. A second step will consist in linking the typical assemblages to environmental conditions (WP2) using a linear ordination method (redundancy analysis; Legendre & Legendre, 1998). This method will allow for identifying combinations of environmental variables that are best explaining the variability observed amongst assemblages. This model will be evaluated against the data already available about the optimal distribution of the observed species. The link between assemblages and environmental conditions will be established using a generalize linear model (McCullagh et Nedler, 1989). This will also allow us to create maps occurrence probabilities. Using this approach, we will also map operating levels of the ecosystem and identify hot and coldspots. One potential original output will consist in forcing the model with precalving conditions (e.g. using CEAMARC data; Hosie et al., 2011) or those base on realistic predictions of the future evolution of environmental conditions. We will then be in a position to better predict potential modifications of these assemblages and ecosystem functioning. This, of course, is of crucial importance in terms of understanding the
potential impacts of climate change and will contribute to establishing future conservation strategies.