Antarctic deep-sea food webs: Understanding Ecological and Environmental drivers using South Sandwich Islands and South Georgia regions as case-studies

The Southern Ocean is a unique and extreme environment that is changing fast due to climate change, with shifts recorded from the surface to deep-sea ecosystems. By evolving in such a specific environment, Southern Ocean biodiversity generally shows low capacity to cope with this changing environment. Like climate change, toothfish fisheries also pressure this ecosystem, especially deep-sea demersal communities where they operate, with these two factors having reciprocal/accumulating impacts. The Southern Ocean deep sea presents a major role on the World’s Ocean circulation and its biodiversity presents high levels of endemism. Nonetheless, it remains inadequately sampled, with the biodiversity and the influence of environmental conditions in these communities being poorly understood. This is off particular concern because deep-sea species have low productive life cycles, e.g. long lifespan, slow growth, and late maturation, which make them more vulnerable to disturbance. The way communities respond to climate change and fisheries depends on the food-web structure, with the food chain length being a feature that largely influences the response to these stressors. However, a lack of knowledge persists regarding the food-web structure of the Southern Ocean deep sea, especially when considering the demersal communities and the benthopelagic coupling. Therefore, this thesis, using the South Sandwich Islands (SSI) and South Georgia (SG) (Scotia Sea) as study regions, aims to study: 1) potential impacts of toothfish fisheries in the deep-sea communities at SSI and how climate change can impact this fishery; 2) the biogeography of bathyal communities at SSI; 3) the interannual variability in the food-web structure at SSI; and 4) the structure of the food web in a latitudinal gradient at the Scotia Sea (from SSI to SG). Results show that toothfish fisheries are well managed and not substantially impacting the populations of target and non-target species at SSI. However, the influence of environmental conditions in the capture of these species indicate that this fishery will be potentially impacted by climate change in the future. Furthermore, this thesis shows that two main bathyal communities exist at SSI, one at north and one at south of Saunders Islands, with temperature and upwelling being the main environmental conditions influencing the biodiversity. Regarding the deep-sea food webs at SSI and SG, both food webs consist of five trophic levels, with a fourth trophic level mainly composed of fish and a third trophic level of cephalopods and crustaceans. The top predators at SSI deep-sea ecosystem are both Antarctic (Dissostichus mawsoni) and Patagonian toothfish (Dissostichus eleginoides), while at SG the highest trophic position belongs to the Bigeye 20 grenadier (Macrourus holotrachys), though D. eleginoides also occupies the fifth trophic level in this archipelago. The benthopelagic coupling at SSI mainly occurs from the third to the fourth trophic level, while at SG both downward and upward exchanges continuously happen in these trophic levels. The food chain length varied between locations at the Scotia Sea, with the longest food web being found in the northern area of the SSI, and between consecutive years. Interannual changes in the food chain length at SSI are driven by changes in the productivity at the archipelago. The influence of environmental conditions in fisheries, biogeography and food-web structure highlight the vulnerability of these deep-sea ecosystems to climate change. Predicted environmental changes for the Southern Ocean suggest that deep-sea food webs may be longer in the future. Longer food webs are, among other things, less stable and recover slower from disturbances. Furthermore, higher assimilation losses and more fluctuations in populations of these communities are observed. Changes in the length of the food web may also alter ecosystem processes, such as carbon and nutrient cycles, that can impact not only the Southern Ocean but all the World oceans. This thesis provides critical scientific knowledge regarding the structure of Southern Ocean deep-sea ecosystems at SSI and SG and how environmental conditions influence this structure and the toothfish fisheries in the Southern Ocean, providing a better comprehension needed to sustainably manage and protect these ecosystems.

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