Percent cover of 215 benthic species was quantified using two approaches with scuba diving: i) visual census techniques in Shallow Reef (benthic surveys were performed from 0.5 to 3 m depth) and Reef habitats (benthic surveys at ~10 m depth) dominated by algae and ii) photographic surveys in Caves (benthic surveys at ~3 m depth) and Deep Reef habitats (benthic surveys at ~ 40 m depth) dominated by sessile invertebrate animals. For visual censuses, 12 quadrats (25x25 cm) were haphazardly placed at Vent 1 and Vent 3 and the two corresponding ambient pH sites (n = 3 sites per habitat, n = 36 quadrats for Shallow Reefs and Reefs, respectively). For photographic surveys, 48–54 and 24 photoquadrats (25x25 cm) were taken at Vent 2 and Vent 4 and the two corresponding reference sites with ambient pH for each habitat (n = 3 sites per habitat, n = 144 photographic quadrats for Caves and n = 72 photographic quadrats for Deep Reefs, respectively). Both types of quadrats (visual censuses in the field and photographs in the lab) were divided into a grid of 25 squares (5 cm x 5 cm each). Percentage cover was quantified by counting the number of squares filled in the grid by the species and expressing the final values as relative percentages.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2022) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2024-01-23.