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Ocean ridges form where new lithospheric crust is generated by seafloor spreading at tectonic plate boundaries. Some stretch for tens of thousands of kilometres through the oceans, but others can be quite short e.g., in back arc basins. They host a range of habitats including active and inactive hydrothermal vents, peripheral areas, rocky slopes and soft sediments, all connected to the pelagic environment above. Active vents harbour exceptionally productive ecosystems fuelled by chemosynthesis and are colonized by endemic species. Seafloor massive sulfide deposits are formed over long periods of time at hydrothermal vents and may be rich in metals including copper, zinc, lead, gold, and silver. Recent interest by the mining industry in these minerals has stimulated scientific projects to better understand these poorly-known environments, including their biodiversity, functioning and connectivity. New knowledge is needed to inform management plans and ensure the protection of the marine environment.

International audience; The unique ecosystems and biodiversity associated with mid-ocean ridge (MOR) hydrothermal vent systems contrast sharply with surrounding deep-sea habitats, however both may be increasingly threatened by anthropogenic activity (e.g., mining activities at massive sulphide deposits). Climate change can alter the deep-sea through increased bottom temperatures, loss of oxygen, and modifications to deep water circulation. Despite the potential of these profound impacts, the mechanisms enabling these systems and their ecosystems to persist, function and respond to oceanic, crustal, and anthropogenic forces remain poorly understood. This is due primarily to technological challenges and difficulties in accessing, observing and monitoring the deep-sea. In this context, the development of deep-sea observatories in the 2000s focused on understanding the coupling between sub-surface flow and oceanic and crustal conditions, and how they influence biological processes. Deep-sea observatories provide long-term, multidisciplinary time-series data comprising repeated observations and sampling at temporal resolutions from seconds to decades, through a combination of cabled, wireless, remotely controlled, and autonomous measurement systems. The three existing vent observatories are located on the Juan de Fuca and Mid-Atlantic Ridges (Ocean Observing Initiative, Ocean Networks Canada and the European Multidisciplinary Seafloor and water column Observatory). These observatories promote stewardship by defining effective environmental monitoring including characterizing biological and environmental baseline states, discriminating changes from natural variations versus those from anthropogenic activities, and assessing degradation, resilience and recovery after disturbance. This highlights the potential of observatories as valuable tools for environmental impact assessment (EIA) in the context of climate change and other anthropogenic activities, primarily ocean mining. This paper provides a synthesis on scientific advancements enabled by the three observatories this last decade, and recommendations to support future studies through international collaboration and coordination. The proposed recommendations include: i) establishing common global scientific questions and identification of Essential Ocean Variables (EOVs) specific to MORs, ii) guidance towards the effective use of observatories to support and inform policies that can impact society, iii) strategies for observatory infrastructure development that will help standardize sensors, data formats and capabilities, and iv) future technology needs and common sampling approaches to answer today’s most urgent and timely questions.

Sampling the Peltospira smaragdina gastropod assemblage at the Y3 site in the Lucky Strike vent field (37°17.51′N; 32°16.66′W, Mid-Atlantic Ridge).

Sampling the Peltospira smaragdina gastropod assemblage at the Les Ruches site in the Snake Pit vent field (23°22.06'N; 44°56.99'W, Mid-Atlantic Ridge).

Sampling the Lepetodrilus azoricus gastropod assemblage at the MG3 site in the Menez Gwen vent field (37°50.47’N; 31°33.37’W, Mid-Atlantic Ridge).

Societal concerns over the potential impacts of recent global change have prompted renewed interest in the long-term ecological monitoring of large ecosystems. The deep sea is the largest ecosystem on the planet, the least accessible, and perhaps the least understood. Nevertheless, deep-sea data collected over the last few decades are now being synthesised with a view to both measuring global change and predicting the future impacts of further rises in atmospheric carbon dioxide concentrations. For many years, it was assumed by many that the deep sea is a stable habitat, buffered from short-term changes in the atmosphere or upper ocean. However, recent studies suggest that deep-seafloor ecosystems may respond relatively quickly to seasonal, inter-annual and decadal-scale shifts in upper-ocean variables. In this review, we assess the evidence for these long-term (i.e. inter-annual to decadal-scale) changes both in biologically driven, sedimented, deep-sea ecosystems (e.g. abyssal plains) and in chemosynthetic ecosystems that are partially geologically driven, such as hydrothermal vents and cold seeps. We have identified 11 deep-sea sedimented ecosystems for which published analyses of long-term biological data exist. At three of these, we have found evidence for a progressive trend that could be potentially linked to recent climate change, although the evidence is not conclusive. At the other sites, we have concluded that the changes were either not significant, or were stochastically variable without being clearly linked to climate change or climate variability indices. For chemosynthetic ecosystems, we have identified 14 sites for which there are some published long-term data. Data for temporal changes at chemosynthetic ecosystems are scarce, with few sites being subjected to repeated visits. However, the limited evidence from hydrothermal vents suggests that at fast-spreading centres such as the East Pacific Rise, vent communities are impacted on decadal scales by stochastic events such as volcanic eruptions, with associated fauna showing complex patterns of community succession. For the slow-spreading centres such as the Mid-Atlantic Ridge, vent sites appear to be stable over the time periods measured, with no discernable long-term trend. At cold seeps, inferences based on spatial studies in the Gulf of Mexico, and data on organism longevity, suggest that these sites are stable over many hundreds of years. However, at the Haakon Mosby mud volcano, a large, well-studied seep in the Barents Sea, periodic mud slides associated with gas and fluid venting may disrupt benthic communities, leading to successional sequences over time. For chemosynthetic ecosystems of biogenic origin (e.g. whale-falls), it is likely that the longevity of the habitat depends mainly on the size of the carcass and the ecological setting, with large remains persisting as a distinct seafloor habitat for up to 100 years. Studies of shallow-water analogs of deep-sea ecosystems such as marine caves may also yield insights into temporal processes. Although it is obvious from the geological record that past climate change has impacted deep-sea faunas, the evidence that recent climate change or climate variability has altered deep-sea benthic communities is extremely limited. This mainly reflects the lack of remote sensing of this vast seafloor habitat. Current and future advances in deep-ocean benthic science involve new remote observing technologies that combine a high temporal resolution (e.g. cabled observatories) with spatial capabilities (e.g. autonomous vehicles undertaking image surveys of the seabed).

Ce document présente les activités de valorisation de l‘infrastructure EMSO-Açores et de la série de campagnes MoMARSAT entre 2010-2020. La série de campagnes MoMARSAT assure la maintenance annuelle de l'observatoire EMSO-Açores sur le champ hydrothermal Lucky Strike (https://doi.org/10.18142/130). Cet observatoire fond de mer est opéré depuis 2010 et vise à acquérir des séries temporelles ≥10 ans sur les processus hydrothermaux, tectoniques, volcaniques et les écosystèmes d'un site hydrothermal actif de la dorsale médio-Atlantique. Il fait partie du réseau européen EMSO (European Multidisciplinary Seafloor and water column Observatory-ERIC - http://emso.eu/), soutenu en France par l'Infrastructure de Recherche (MESR) EMSO-FR dont le pilotage est assuré par une collaboration Ifremer-CNRS.