Shells from Panarea vents (Italy): Acidification-driven carbonate loss reshapes benthic communities and the incipient fossil record

Naturally acidified marine systems, such as volcanic vents, provide natural laboratories for examining ecological and taphonomy processes under conditions analogous to future ocean acidification. The Bottaro hydrothermal vent field off Panarea island in the Aeolian Archipelago (Italy) generates a short, meter-scale pH gradient in shallow waters (8–12 m; Goffredo et al., 2014). This study system provides a suitable setting for assessing how carbonate undersaturation affects macrobenthic community structure, shell production, and post-mortem preservation. We compared live-collected and associated dead-collected assemblages across four stations (four replicates per station) along the pH gradient. In total, ~3200 specimens representing 88 species were collected using a uniform sampling methodology. Diversity patterns were broadly concordant between live and dead assemblages. In both live and dead datasets, species richness declined toward lower pH (e.g., for dead-collected specimens, sample-standardized (n = 80) species richness was 20.9 species (CI 19.0-22.8) at the control site, and 11.9 (CI 6.7-11.7) at the vent site). Beta diversity, measured using the Betapart R package (Baselga & Orme, 2012), was primarily driven by species turnover (e.g., live dataset, β-bal = 0.55) rather than nestedness (β-gra = 0.32), consistent with changes in habitat structure, benthic vegetation cover, and pH decrease along the transect.  Indeed, the most acidified station (pH ≤ 7.7) hosted a distinct, acid-tolerant community dominated by Alvania acida. Shell size, based on 50 randomly selected specimens, decreased significantly along the transect (e.g., dead shell median size 5.00 vs 1.75 mm in the control and most impacted site, respectively; Wilcoxon test < 0.01). This trend is consistent with dwarfism and shortened life spans under low-pH conditions. Multivariate taphonomy revealed intensified dissolution and a lack of encrustation at low-pH sites, and this pattern also affected living specimens, suggesting a very rapid rate of taphonomic processes. In addition, shell density and dead-live shell ratios both declined toward the vent indicating an increase in the rate of shell destruction in low-pH conditions. These results imply rapid carbonate loss and extremely short shell residence times, leading to high dead-live fidelity (i.e., high concordance in faunal composition and body size between live and dead assemblages). By integrating ecological and paleoecological evidence, this study illustrates how ocean acidification can reshape nearshore molluscan communities, alter their preservation pathways (hampering the reconstruction of baselines from paleoecological data), and reduce nearshore carbonate storage.

References:

Baselga, A. and Orme, D. 2012 Betapart: an R package for the study of beta diversity, Methods Ecol. Evol., 3, 808–812

Goffredo, S. et al. 2014. Biomineralization control related to population density under ocean acidification. Nature Climate Change 4, 593-597