The Scotia Sea is a productive pelagic ecosystem in the Southern Ocean, which is rapidly changing as a consequence of global warming. Species range shifts are particularly evident, as sub-Antarctic species expand their range from North to South, potentially rearranging the structure of this ecosystem. Thus, studies are needed to determine the current extent of variation in food web structure between these two biogeographic regions of the Scotia Sea and to investigate whether the observed patterns are consistent among depth zones. We compiled a database of 10,888 feeding interactions among 228 pelagic taxa, underpinned by surveys and dietary studies conducted in the Scotia Sea. Network analysis indicated that the Northern Scotia Sea (NSS), relative to the Southern Scotia Sea (SSS), is more complex: with higher species richness (more nodes) and trophic interactions (more links) is more connected overall (greater connectance and linkage density). Moreover, the NSS is characterised by more groups of strongly interacting organisms (greater node clustering) than the SSS, suggesting a higher trophic specialisation of Antarctic compared to sub-Antarctic species. Depth also played a key role in structuring these networks, with higher mean trophic position and more dietary generalism in the mesopelagic and bathypelagic zones relative to the epipelagic zones. This suggests that direct access to primary producers is a key factor influencing the trophic structure of these communities. Our results suggest that under current levels of warming the SSS ecosystem will likely become more connected and less modular, resembling the current structure of the NSS.
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We are grateful to the SeaDNA project team for the fruitful discussions during the development of this work, particularly to Laura Balcells and Stefano Mariani. We are also thankful to Jennifer Dunne, Susanne Kortsch, Lawrence N. Hudson, and Richard J. Williams, who kindly responded to our queries regarding methods for network analyses. We also acknowledge the work of the British Antarctic Survey during the Discovery project and, generally, the work of marine polar ecologists over recent decades; without their dedicated research, studies like ours would simply not be possible. The work was supported by NERC Highlight Topic Grant NE/N005937/1, awarded to M.J.G. and E.J.O.G. and a NERC Fellowship (NE/L011840/1) awarded to E.J.O.G.
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