Ocean acidification may cause biodiversity loss, alter ecosystems and impact food security, yet uncertainty over ecological responses to ocean acidification remains considerable. Most work on the impact of ocean acidification on foraminifera has been short-term laboratory experiments on single species. To expand this, benthic foraminiferal assemblages were examined across shallow water CO2 gradients in the Gulf of California, off the islands of Ischia and Vulcano in Italy and off Papua New Guinea. Living assemblages from the Gulf of California did not appear to show a response across a pH range of 7.55 – 7.88, although the species assemblage was impoverished in all locations and the dead assemblage was less diverse at the lowest pH sites where there was evidence of post mortem dissolution. At Vulcano, the small macroalga, Padina pavonica, did not protect calcareous foraminifera from the adverse effects of ocean acidification. Calcareous taxa disappeared from the assemblage and were replaced by agglutinated foraminifera as mean pH reduced from 8.19 to 7.71. Settlement of benthic foraminifera onto artificial collectors off Vulcano was adversely affected in the acidified water, with few species as pCO2 increased and evidence of post-mortem dissolution. The foraminiferal tests, collected off Papua New Guinea, had lower δ11B as mean pH decreased from 7.99 – 7.82 for small (250 – 500 µm) Amphistegina lessonii, but not for A. lessonii or Calcarina spengleri >500 µm. In the larger foraminifera, photosynthetic activity by symbionts may begin to dominate the boron isotopic signature. Overall, the responses of foraminiferal assemblages to ocean acidification are complex, but there was an overall reduction in species diversity in infaunal, epifaunal and epiphytic assemblages as pCO2 increased. This raises serious concerns for the survival of shallow water calcareous benthic foraminifera as the oceans continue to acidify, with implications for benthic ecosystems and inorganic carbon cycling.

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