Physical and Biogeochemical Regionalization of the Southern Ocean and the CCAMLR Zone 48.1

Abstract

The Southern Ocean plays a major role in the Earth’s climate, provides fisheries products and help the maintenance of biodiversity. The degree of correspondence between physical and biogeochemical spatial variability and regionalization were investigated by calculating the main physical factors that statistically explained the biogeochemical variability within the Southern Ocean and the 48.1 zone of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The mean value of physical and biogeochemical variables was estimated during austral summer within a grid of 1° × 1° south of 50°S. The regionalization was developed using both non-hierarchical and hierarchical clustering method, whereas BIO-ENV package and distance-based redundancy analysis (db-RDA) were applied in order to calculate which physical factors primarily explained the biogeochemical spatial variability. A total of 12 physical and 18 biogeochemical significant clusters were identified for the Southern Ocean (alpha: 0.05). The combination of bathymetry and sea ice coverage majorly explained biogeochemical variability (Spearman rank correlation coefficient: 0.68) and db-RDA indicated that physical variables expressed the 60.1% of biogeochemical variance. On the other hand, 14 physical and 16 biogeochemical significant clusters were identified for 48.1 CCAMLR zone. Bathymetry was the main factor explaining biogeochemical variability (Spearman coefficient: 0.81) and db-RDA analysis resulted in 77.1% of biogeochemical variance. The correspondence between physical and biogeochemical regions was higher for CCAMLR 48.1 zone with respect to the whole Southern Ocean. Our results provide useful information for both Southern Ocean and CCAMLR 48.1 zone ecosystem management and modeling parametrization.

Ecoregionalisation of the Southern Ocean Using Radiolarians

“…Previous ecoregional studies of the SO using phytoplankton, pelagic fish and echinoid species (Koubbi et al, 2010;Ardyna et al, 2017;Fabri-Ruiz et al, 2020) generally agree with the habitat complexity found by Testa et al (2021), particularly along the Antarctic coast and shelf region. The permanently open ocean zone (POOZ), on the other hand, displays more homogeneity, which is thought to be related to the connectivity of habitat provided by the continuous eastward flow of the Antarctic Circumpolar Current (ACC) (Linse et al, 2006).…”

“…The results of this study support the complexity presented by the latter. However, radiolarian distribution appears less complex than the patterns observed by Testa et al (2021) suggesting that radiolarians are not as sensitive to the fine-scale variability used to define the biogeochemical regions.…”

“…However, our results indicated that the SO can be divided by Sectors and further subdivided by the latitudinal zones (Figures 3, 4). Testa et al (2021) presented physical and biogeochemical regionalization models, the former resulting in homogeneous latitudinal bands, while the latter presented a complex pattern of regions that varied significantly between Sectors. The results of this study support the complexity presented by the latter.…”

“…However, due to the sparsity of species data in some oceanic regions, ecoregionalisation is not always possible. Therefore, partitioning of large regions is typically done using abiotic parameters such as temperature, nutrients or salinity, to develop bioregions of relatively predictable and homogeneous environmental conditions (e.g., Longhurst, 1998;Testa et al, 2021). Combining these two approaches of ecoregions and bioregions will allow for a clearer understanding of regional processes, with insight into the biogeochemical and environmental thresholds of species or ecosystems.…”

“…Both classifications describe the SO as bands of relatively homogeneous environmental conditions, punctuated by subtle environmental variations. However, recent work by Testa et al (2021) using abiotic partitioning of the SO basin suggested that the interaction of bathymetry with oceanographic and biogeochemical processes produces much more complex mesoscale bioregionalisation of the SO.…”