Contrasts in the Sensitivity of Community Calcification to Temporal Saturation State Variability Within Temperate and Tropical Marine Environments

Ongoing emissions of carbon dioxide (CO₂) and invasion of part of this CO₂ into the oceans are projected to lower the calcium carbonate saturation state. As a result, the ability of many marine organisms to calcify may be compromised, with significant impacts on ocean ecosystems throughout the 21^st Century. In laboratory manipulations, calcifying organisms have exhibited reduced calcification under elevated pCO₂ conditions. Consequently, in situ observations of the sensitivity of calcifying communities to natural saturation state variability are increasingly valued as they incorporate complex species interactions, and capture the carbonate chemistry conditions to which communities are acclimatized. Using intensive seawater sampling techniques we assess the community level sensitivity of calcification rates to natural temporal variability in the aragonite saturation state (Ω_arag) at both a tropical coral reef and temperate intertidal study site. Both sites experiences large daily variation in Ω_arag during low tide due to photosynthesis, respiration, and the time at which the sites are isolated from the open ocean. On hourly timescales, we find that community level rates of calcification have only a weak dependence on variability in Ω_arag at the tropical study site. At the temperate study site, although limited Ω_arag sensitivity is observed during the day, nighttime community calcification rates are found to be strongly influenced by variability in Ω_arag, with greater dissolution rates at lower Ω_arag levels. If the short-term sensitivity of community calcification to Ω_arag described here is representative of the long-term sensitivity of marine ecosystems to ocean acidification, then one would expect temperate intertidal calcifying communities to be more vulnerable than tropical coral reef calcifying communities. In particular, reductions in net community calcification, in the temperate intertidal zone may be predominately due to the nocturnal impact of ocean acidification.