Ocean acidification enhances primary productivity and nocturnal carbonate dissolution in intertidal rock pools

Abstract. Human CO₂ emissions are modifying ocean carbonate chemistry, causing ocean acidification, and likely already impacting marine ecosystems. In particular, there is concern that coastal, benthic calcifying organisms will be negatively affected by ocean acidification, a hypothesis largely supported by laboratory studies. The inter-relationships between carbonate chemistry and marine calcifying communities in situ are complex and natural mesocosms such as tidal pools can provide useful community-level insights. In this study, we manipulated the carbonate chemistry of intertidal pools to investigate the influence of future ocean acidification on net community production (NCP) and calcification (NCC) at emersion. Adding CO₂ at the start of the tidal emersion to simulate future acidification (+1500 μatm pCO₂, target pH: 7.5) modified net production and calcification rates in the pools. By day, pools were fertilized by the increased CO₂ (+20 % increase in NCP, from 10 to 12 mmol O₂ m⁻² hr⁻¹), while there was no measurable impact on NCC. During the night, pools experienced net community dissolution (NCC < 0), even in present-day conditions, when waters were supersaturated with regards to aragonite. Adding CO₂ in the pools increased nocturnal dissolution rates by 40 % (from −0.7 to −1.0 mmol CaCO₃ m⁻² hr⁻¹) with no consistent impact on night community respiration. Our results suggest that ocean acidification is likely to alter temperate intertidal community metabolism on sub-daily timescales, enhancing both diurnal community production and nocturnal calcium carbonate dissolution.