Djian, V., C. Cotté and P. Koubbi (2025), Hydrologic regionalisation from Crozet to Kerguelen and subtropical Southern Indian Ocean

The islands of Crozet and Kerguelen are located at the junction of the Southern Indian Ocean and the Southern Ocean, an area of major environmental contrasts due to the juxtaposition of subtropical warm and salty waters and polar cold waters. Frontal structures generated by the Antarctic Circumpolar Current divide these oceans into several oceanographic zones. Several regionalisations have been carried out in recent decades, but only on the epipelagic or mesopelagic zones, without considering both together. As these frontal zones have a strong influence on environmental conditions and the distribution of biodiversity, it is important to be able to identify their spatial distribution and boundaries. The aim of this study was to identify the different hydrologic zones in the Indian sector of the Southern Ocean and the Southern Indian Ocean from oceanographic open access data. Temperature and salinity profiles from the surface to 1000 m depth between 2010 and 2020 were retrieved from Copernicus reanalyses. A functional PCA was applied to these data to account for the defined depth of the water column, followed by k-means clustering to identify regions with common hydrologic profile. We also calculated the mean and standard deviation of the cluster value for each geographical cell to identify stable areas and areas of high hydrologic variability. Each region was then linked to environmental data from the whole study area to characterise it. We were able to identify 7 regions, 4 in the Southern Indian Ocean and 3 in the Southern Ocean. The regions in the Southern Ocean are strongly explained by the frontal zones such as the Subantarctic and Polar fronts. Conversely, the regions in the Southern Indian Ocean are explained by differences in salinity, particularly in subsurface waters, associated with the action of the South Equatorial Current.

Djian, V., C. Merland, M. Thellier, B. Leroy, C. Cotte and P. Koubbi (2025), Species assemblages of key prey taxa from the Southern Ocean and the Southern Indian Ocean using biogeographic networks

Plankton plays an important role in the functioning of marine ecosystems, for example, through their role as prey. It is therefore important to understand the spatial distribution of assemblages of key taxa. This study aimed to determine the effect of hydrologic and topographic features on two major macrozooplankton groups: euphausiids and amphipods, which are important prey of seabirds and marine mammals. The biogeography of these taxa’s species between the Southern Indian Ocean and the North Indian sector of the Southern Ocean will be assessed by network analyses on species abundances. Our results from the REPCCOAI surveys from 2017 to 2019 between Crozet, Kerguelen and St Paul and New Amsterdam revealed a strong biogeographic separation between the subtropical and subantarctic zones. Species assemblages for each major taxon revealed a distinction between off shelf areas and the neritic zone and between high and low productivity areas, underlining the role of the subantarctic islands and their effect on primary production in the biogeography of the southern plankton. In the subtropical Indian Ocean, no significant major distinction was observed with the network analysis, even if some sites seem to show an influence of the Agulhas Return Current.

Merland, C., C. Azarian, F. d'Ovidio and C. Cotté (2025), Physical and biogeochemical regionalisations and climate velocities in the epipelagic and mesopelagic Southern Indian Ocean

Environmental regionalisations are useful tools for spatial planning and are mostly performed at the global scale to define biogeochemical provinces from surface data or in the mesopelagic zone. However, these regionalisations often lack separations at scales relevant for conservation as they do not always consider regional patterns such as island effects or regional dynamic patterns. Our study aimed to overcome these limitations by defining coherent physical and biogeochemical regions within the Southern Indian Ocean (20°W-160°E; 30°S-60°S), considering both spatial and temporal dynamics of environmental parameters. For this purpose, two complementary approaches have been used: one focusing on surface environmental data, and the other considering space-time dimensions through their vertical profiles. The first classical method, based on multivariate analyses, allowed us to delineate regions latitudinally due to the existing temperature and oxygen concentration gradient and revealed regional patterns such as highly energetic regions or productive areas. The newly developed second approach used functional analyses and provided additional information, including subdivision in the Subtropical Zone dominated by mesopelagic patterns. This subdivision results from temperature differences. It separates longitudinally the subtropical region with warmer waters found in the western area, likely transported from lower latitudes by the Agulhas Return Current. Climate velocities of temperature (i.e. speed of isotherm drift) were also computed for both epipelagic and mesopelagic regions to investigate their potential shift due to climate change. This environmental regionalisation brings relevant information to understand the distribution of the pelagic diversity and abundance and highlights the importance of accounting for vertical structures in a context of climate change.

Koubbi, P., M. Vacchi, V. Djian, A. Walters, B. Serandour and E. Tavernier (2025), Atlas of Myctophids from the Southern Indian Ocean and around the subantarctic islands of Crozet and Kerguelen

This paper summarises myctophid distribution from surveys from the Crozet to Kerguelen regions of the Southern Indian Ocean. These fish play a crucial role in the marine ecosystem trophic web as they are important prey for seabirds and marine mammals. This study integrates both the subtropical and Southern Ocean species from surveys from 2017 to 2019. In these surveys, 62 species of myctophids were observed across different oceanographic regions from the subtropical zone to the Subantarctic and the Antarctic zone. These species exhibited high abundances in the Southern Ocean, with concentrated patches in the Subantarctic zone between the Crozet and Kerguelen archipelagos, as well as the eastern part of the Kerguelen Plateau. The fronts at the northern boundary of the Southern Ocean explained the separation of subtropical and Subantarctic faunas.

Makhado, A.B., F.E. Dakwa, P.G. Ryan, M.J. Masotla, B.M. Dyer, S.M. Seakamela, F.W. Shabangu, S. Somhlaba and R.R. Reisinger (2025), Seabird assemblages, abundance, and distribution in the African sector of the southern Indian Ocean

Seabird distributions in the Southern Ocean are influenced by the location and accessibility of suitable breeding sites, but also by the environmental factors that influence the distribution and availability of their prey. For example, oceanic fronts, concentrate prey at their surface and therefore present important foraging areas for many seabirds. This study investigated the latitudinal distribution and abundance of seabirds in the African sector of the Southern Ocean. In particular, we investigated the relationship of seabird assemblages and densities to key biophysical environmental parameters (SST, sea surface height, bathymetry) and the main oceanic fronts.
There was a high density of seabirds north of the Subtropical Convergence (STC), which is situated at approximately 39°S, with declining densities farther south. There was latitudinal segregation between several species, e.g. black-browed albatross (Thalassarche melanophris) occurred north of the STC, and grey-headed albatross (T. chrysostoma) occurred to south of it. The Subantarctic Front (SAF) and the Antarctic Polar Front (APF) had less influence on seabird populations than the STC. Latitude was the greatest predictor of seabird assemblages and densities, reflecting environmental gradients in physical and biological parameters and their influences on prey distributions. Of the environmental parameters, sea surface temperature and bathymetry were the most important physical features influencing seabird assemblages. In particular, the density of seabirds north of STC declined with increasing sea surface temperature and had a negative relationship with bathymetry, with most seabirds occurring in shallower waters. In contrast, seabird density had a positive linear relationship with sea surface height. Relationships with other environmental parameters, such as wind, salinity and chlorophyll concentration (as a proxy for productivity), were less well-defined.

Merland, C., M. Thellier, V. Djian, C. Cotté and P. Koubbi (2025), Macrozooplankton from Crozet to Kerguelen and Subtropical Southern Indian Ocean

This paper investigates the spatial distribution of macrozooplankton across various hydrological regions within the Southern Indian Ocean and the Southern Ocean, encompassing the Crozet and Kerguelen archipelagos in the Southern Ocean, and the islands of Saint-Paul and New Amsterdam in the Southern Indian Ocean. Data used in this study were gathered during the REPCCOAI surveys conducted during the summers from 2017 to 2019 aboard the R/V Marion Dufresne II. Macrozooplankton was collected using an Isaacs-Kidd Midwater Trawl. Taxonomic analysis revealed 248 taxa of zooplankton belonging to 20 orders. Marked differences in taxa composition were observed between the Subtropical region of the Southern Indian Ocean and the Southern Ocean because of the significant influence of hydrological fronts, such as the Subtropical Front (STF) and the Subantarctic Front (SAF), in structuring zooplankton assemblages. These differences are due to the closeness of these fronts, notably North of Crozet, where a triple front is observed, and North of Kerguelen, where the transition between the Polar Frontal Zone (PFZ) and the Subtropical Zone (STZ) is narrow.

Reisinger, R.R., A.B. Makhado, K. Delord, C.A. Bost, M.A. Lea and P.A. Pistorius (2025), Towards higher predator ecoregionalisation of the pelagic zone in the subantarctic and subtropical Indian Ocean

Numerous seabird and marine mammal species occur, often in high abundance, in the subtropical and subantarctic zones of the southern Indian Ocean. They breed, moult and rest at French, South African and Australian islands in the region and forage at sea in areas including the Exclusive Economic Zones of these nations, Areas Beyond National Jurisdiction and also the area to which the Convention on the Conservation of Antarctic Marine Living Resources applies. Information on the at-sea distribution of these species is important for understanding ecosystem patterns and processes in the region, and is a key component of any spatial conservation and management frameworks. Information on the distribution of marine predators is widely used to define priority areas for conservation and management, with several such initiatives already having been implemented in the extended Southern Ocean. Seabird and marine mammal distribution has been studied mainly using two methods: animal-borne biotelemetry/biologging and at-sea sighting records. Here, we give an overview of data on the distribution of marine mammals and seabirds – collected mainly through long-term tracking programmes at French and South African islands – with the aim of identifying data sources that can inform pelagic ecoregionalisation to support spatial conservation and management planning in this region. We identify priorities for further distribution data collection related to life-history stages, sites and species and we identify broader challenges for understanding marine predator distribution in this region, towards spatial conservation and management planning.