Consideration of coastal carbonate chemistry in understanding biological calcification

Correlations between aragonite saturation state (Omega(Ar)) and calcification have been identified in many laboratory manipulation experiments aiming to assess biological responses to ocean acidification (OA). These relationships have been used with projections of Omega(Ar) under continued OA to evaluate potential impacts on marine calcifiers. Recent work suggests, however, that calcification in some species may be controlled by the ratio of bicarbonate to hydrogen ion, or the substrate-to-inhibitor ratio (SIR), rather than Omega(Ar).SIR and Omega(Ar) are not always positively correlated in the natural environment, which means that OAr can be a poor indicator of the calcifying environment when Omega(Ar)->1. Highly variable carbonate chemistry in the coastal zone challenges our ability to monitor fluctuations in OAr, SIR, and the Omega(Ar)-SIR relationship making it difficult to assess biological OA exposures and vulnerability. Careful consideration of natural variability throughout ocean environments is required to accurately determine the influence of OA on biological calcification.