Assessment of the effects of multiple stressors on the functioning of hard-bottom VME ecosystems

The Covid-19 Pandemic significantly impacted this report (Deliverable 4.3) in terms of collection of key CWC species for experimentation, as well as impossibility or limitations to accessing aquaria infrastructure, leading to significant delays in experimental setups, preliminary testing, and experiment start dates. Nevertheless, long-term experiments examining the impacts of single and multiple stressors on key coral species were successfully completed, allowing a number of important conclusions to be drawn as to the potential future of CWC species in the Atlantic.

With regard to environmental stressor experiments to live coral framework, multiple stressor exposures on three key framework-forming species – Desmophyllum pertusum, Desmophyylum dianthus and Dendrophyllia cornigera –identified that these species can continue to survive multiple stressor (deoxygenation, acidification and increased temperature) environments over experimental timescales investigated here. While this was demonstrated over several months, it is unknown whether this would be sustainable over several years – and what consequences this could have on other biological functions, such as reproduction.

Exposure of live corals to some single stressors such as deoxygenation elicited species-specific responses. In those instances, decreases in respiration after several months may be linked to decreases in energetic reserves. All the experiments on live corals highlighted the importance of long-term studies, as responses may only appear during chronic exposure (not observable in short-term experiments), and that lack of food availability coupled with energetic reserves may be key drivers in the negative responses observed. Additional environmental stressors to live corals, such as salinity, which drove changes in D. dianthus growth rates, further highlight the need to consider local relevant environmental pressures in experimental work in addition to the broader deoxygenation, acidification, and increased temperature scenarios.

While live coral responses indicate that survival is possible under multiple stressor future ocean scenarios, ocean acidification exposure had significant detrimental impacts to dead coral framework, with dissolution apparent after 3 months. This leads to net loss of coral material in D. pertusum experiments. This is supported by in silico modelling evidence where ocean acidification leads to net loss of coral framework and habitat.

Exposure to sedimentation and metals from potential mining activities were generally detrimental, resulting in immediate or delayed mortality, or survival with loss of tissue. However, the rate of these detrimental results were species specific. Moreover, responses to sedimentation differed between the type of mineral resource extraction, with examples of mortality in response to toxic sediment exposures produced during mining of seafloor massive sulphides, and survival (but with loss of tissue from particle abrasion) upon exposure to non-toxic sediments produced during mining of ferromanganese nodules.

The above points highlight that while some coral species may be able to survive specific environmental changes, changes in growth and respiration rates (which can feed into ecosystem carbon turnover models) varies according to length of exposure and species. The addition of stressors from deep-sea mining plumes is generally detrimental to coral survival and condition in the short-term, and in a future ocean where both may occur, we are at risk not only of rapid coral mortality from plume exposure, but longer-term changes in net coral growth or loss leading to changes in habitat provision.