Long-term monitoring reveals unprecedented stability of a vent mussel assemblage on the Mid-Atlantic Ridge

Understanding scales and drivers of ecological variability is essential to a full understanding of ecosystem functioning. At remote deep-sea hydrothermal vents, infra-annual dynamics remain poorly described. This study aims to characterise the factors that drive the dynamics of a vent faunal assemblage dominated by Bathymodiolus azoricus mussels from infra-daily to monthly time steps. We analysed a 7-year time series of images and environmental data collected at a depth of 1695 m at the base of the active Eiffel Tower edifice in the Lucky Strike vent field (Mid-Atlantic Ridge). Using images acquired by the TEMPO ecological module connected to the EMSO-Azores observatory, we assessed the dynamics of key species inhabiting the faunal assemblage in relation to changes in environmental conditions monitored daily. Our results show that habitat conditions were generally stable over the 7-year period, with small-scale variability related to tidal periodicity and local temperature anomalies. Likewise, the mussel and zoanthid assemblages exhibited remarkable stability. Changes in fluid exposure and substratum instability induced decimetrescale movements of the mussel assemblage. Microbial mats displayed infra-annual changes characterised by aperiodic growth and decline. Their development patterns could not be entirely attributed to environmental conditions, because other factors, including biotic interactions, appeared to be involved. The crab population preferentially occupied the mussel habitat, but no predation was observed. Scales of variation and driving factors were compared with those governing intertidal zones. The outcomes question the assumption that vent fauna experience extreme and highly variable conditions. On the MAR, mussel assemblages appear to experience relatively stable and mild environmental conditions compared with their coastal counterparts.