The initial objective of this work was to compare deep-ocean ecosystem dynamics from a number of habitats within the Atlantic Ocean Basin that are characterised by different environmental conditions (e.g., trophic regime) to try and assess the response of deep Atlantic ecosystems to anthropogenic climate change using a space-for-time approach. The Covid-19 pandemic, as well as alterations to ship schedules (caused by Covid-19 and sudden ship repairs), prevented us from visiting many of the study sites (e.g. Argentinian Basin, Santos Basin) that we initially planned to visit. Nevertheless, several studies in the Atlantic Basin were undertaken and the data gathered has allowed us to compare our results to similar studies conducted in other regions of the Atlantic, enabling a number of conclusions to be made about how climate change will potentially impact the deep Atlantic Ocean.
Comparing two datasets (in situ and modelled data) derived from samples collected from soft-sediment abyssal experiments conducted around Cabo Verde during the iMirabilis2 expedition (4,200 m), and the Porcupine Abyssal Plain (in 2,000 at 4,800 m) showed that short-term bacterial and meiofaunal processing efficiency (i.e., per unit biomass) is greater on the more mesotrophic Cabo Verde Abyssal Plain (CVAB) compared to the more eutrophic PAP. This suggests that declining productivity caused by climate change could lead to a reduction in benthic biomass as noted in earlier studies, but an increase in bacterial and meiofaunal carbon (C) processing efficiency and greater C-recycling at the seafloor. The greater importance of microbes under more oligotrophic conditions was also suggested when isotope data from cold-water coral (CWC) reefs from Lampaul Canyon in the eutrophic NE Atlantic were compared to the same species of CWCs from more oligotrophic environments (e.g., Mediterranean Sea).
In addition to facilitating experiments to quantify abyssal soft-sediment ecosystem function, the iMirabilis2 expedition allowed us to study how shifts from fish-dominated, upper ocean ecosystems to squid-dominated environments may impact demersal and benthic scavengers in the Atlantic. Using a baited camera system, we found a clear difference in the community composition of scavengers and feeding activity at squid and fish baits. Our results suggest that changes to more squid-dominated ecosystems will likely benefit faster swimming fauna at the seafloor, and clearly highlighted how changes to upper ocean ecosystems may cause effects thousands of meters below the ocean surface.
Finally, we compared the carbon turnover and framework composition of CWC reefs from 4 sites in the Atlantic, which included areas beneath eutrophic waters that were characterised by high O2 and pH conditions, as well as areas in more oligotrophic settings (Angola) naturally exposed to lower O2 concentrations and pH levels. Our findings showed a dominance of dead coral material at the majority of sites, and the total carbon turnover m-2 was highest in the Angola site compared to NE and NW Atlantic sites. Changing O2, pH conditions and a reduction in food availability could therefore lead to a loss of live and dead coral framework, which may have significant impacts on Atlantic Vulnerable Marine Ecosystems (VMEs). These results are complemented by analysis of skeletal densities of several coral species and theoretical growth models that simulate how reductions in food availability leads to a relative increase in dead coral framework.
Banner image courtesy © Prof. Andrew Sweetman / Lyell Centre – Heriot-Watt University / iMirabilis2
Download the full report
iAtlantic Deliverable 4.2: Baseline ecosystem functioning in selected deep-sea environments. Report by A.K. Sweetman et al. (February 2024) (PDF, 4.2MB)
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This project received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 818123 (iAtlantic). This output reflects only the author’s view and the European Union cannot be held responsible for any use that may be made of the information contained therein.