Conspecific aggregations mitigate the effects of ocean acidification on calcification of the coral Pocillopora verrucosa

In densely populated communities, such as coral reefs, organisms can modify the physical and chemical environment for neighbouring individuals. We tested the hypothesis that colony density (12 colonies each placed ∼0.5 cm apart versus ∼8 cm apart) can modulate the physiological response (measured through rates of calcification, photosynthesis and respiration in the light and dark) of the coral Pocillopora verrucosa to partial pressure of CO₂ (P_CO2 ) treatments (∼400 μatm and ∼1200 μatm) by altering the seawater flow regimes experienced by colonies placed in aggregations within a flume at a single flow speed. While light calcification decreased 20% under elevated versus ambient P_CO2  for colonies in low-density aggregations, light calcification of high-density aggregations increased 23% at elevated versus ambient P_CO2 As a result, densely aggregated corals maintained calcification rates over 24 h that were comparable to those maintained under ambient P_CO2 , despite a 45% decrease in dark calcification at elevated versus ambient P_CO2 Additionally, densely aggregated corals experienced reduced flow speeds and higher seawater retention times between colonies owing to the formation of eddies. These results support recent indications that neighbouring organisms, such as the conspecific coral colonies in the present example, can create small-scale refugia from the negative effects of ocean acidification.