Abstract
The Southern Ocean is an important region for identifying ecoregions, both to assist in conservation efforts as well as for paleoecological studies. In this paper we use the Southern Ocean Radiolarian Dataset (SORAD), a compilation of surface sediment radiolarian census data, to determine radiolarian ecoregions throughout the Southern Ocean within the Atlantic, Indian and Southwest Pacific Sectors. The distribution of radiolarian species is explored using the unconstrained non-Metric Multidimensional Scaling to identify gradients in species assemblage variability, as well as distinct and sharp changes in assemblage composition. Ecoregions are assigned to groups of sites using Multivariate Regression Tree analysis, and environmental variables are assessed for their explanatory power. Radiolarian distribution was found to be heterogeneous throughout the Southern Ocean, with each Sector hosting a unique group of radiolarian ecoregions. A total of 6 ecoregions were identified for the Atlantic Sector, 8 for the Indian Sector and 6 for the Southwest Pacific Sector. Ecoregions were generally latitudinally distributed, but also appear to be influenced by significant oceanographic features, such as plateaux and islands, large eddy fields, hydrological fronts and sea ice. This study highlights the need for further exploration of radiolarian ecology, and for caution when using radiolarian assemblage variability in paleo-environmental studies.
Highlights
The Southern Ocean (SO) is an important region globally for ocean circulation and climate (Rintoul and Naveira Garabato, 2013)
This was evident in the 3D nMDS, indicating assemblages in each Sector do not vary in the same fashion and are likely responding to different drivers
A general north-south trend was apparent in the 2D nMDS, with Subantarctic Zone (SAZ) sites furthest from Antarctic Zone (AZ) sites for all sectors, indicating greatest dissimilarity
Summary
The Southern Ocean (SO) is an important region globally for ocean circulation and climate (Rintoul and Naveira Garabato, 2013). Ecoregionalisation partitions a large ecosystem into smaller ecoregions using assemblage data, such as relative species abundances (Koubbi et al, 2010) It can highlight heterogeneity in the broader ecosystem, potentially driven by differences in biogeochemical characteristics and environmental processes. 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. An assessment of how closely ecoregions track bioregions provides an evaluation of the usefulness of a certain taxonomic group for reconstructing past environmental conditions
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