Assessing the impact of multiple environmental stressors on Atlantic Ocean deep-sea sediment ecosystems functioning.

Abstract

Earth is a blue planet, and the deep seafloor covers more than 60% of its surface, with the greater majority comprising of soft sediments. Deep soft sediments host a unique and diverse seafloor community and are of major importance to global biogeochemical cycles. As the ocean acts as a major buffer for the increasing atmospheric CO2 concentration and temperature, we observe temperature increases, expansion of oxygen minimum zones, ocean acidification, and shifts in pelagic communities and processes resulting in altered composition of organic matter (OM) reaching the seafloor. This thesis contributes to closing knowledge gaps in relation to how multiple stressors affect the functioning of deep-sea sediments, utilizing benthic landers, a multi-stressor experiment, and modelling. Field work was conducted at the Cabo Verde Abyssal Basin (CVAB), providing the first baseline data on sediment functioning in the tropical Atlantic. A comparison of abyssal functioning at sites of differing trophic regimes (mesotrophic CVAB vs. the eutrophic Porcupine Abyssal Plain [PAP]) suggests reduced food availability under climate change may lead to reduced overall seafloor processing of OM but happening more efficiently due to a shift towards dominance of smaller organisms (chapter 2). Closer analysis of the food webs at these sites validates these results, and further indicates that eutrophic regions with greater current food availability may be less resilient to disturbances as they have not developed the tightly organized food-web structure seen at CVAB which facilitates efficient cycling of scant resources (chapter 3). The community dynamics and composition of demersal scavengers was found to differ between squid vs. fish bait, indicating that proliferation of squid under climate change may alter demersal scavenger communities disadvantaging those which are slower and/or are more dependent chemical cues for feeding (chapter 4). Finally, upper bathyal sediment community oxygen consumption was sensitive to warmer temperatures, almost doubling with +2oC, with a significantly lower processing rate of degraded phytodetritus, both likely caused by the response of the bacterial community (chapter 5). Although no significant interactions were observed in the multi-stressor experiment, this thesis does identify some potential cumulative effects on deep-sea sediment ecosystem functioning. The extent and direction of knock-on effects of these predicted changes will be region-specific but will likely affect ecosystem services such C sequestration and nutrient regeneration.

Date of Award	4 Mar 2025
Original language	English
Awarding Institution	University of the Highlands and Islands
Sponsors	EU Horizon2020
Supervisor	Andrew Sweetman (Supervisor) & Bhavani Narayanaswamy (Supervisor)