We calculated net calcification rates with the ammonia-corrected alkalinity anomaly technique (Gazeau et al  2015), divided by incubation duration and mussel dry tissue mass raised by a factor of 0.72 (see related dataset). The alkalinity anomaly technique builds on the observation that precipitation of CaCO3 results in an equivalent reduction in seawater [CO32-] (or reduction of [HCO3-] followed by an increase in [H+]) which contributes two equivalents of total alkalinity—simultaneous production of ammonia is the major metabolic process in mussels that can obscure this and its signal must be removed (Gazeau et al  2015).

Following the incubation, we dissected each mussel and dried it at 60 °C for at least 24 hours to obtain the dry tissue mass (excluding byssal threads) and dry shell mass of each individual mussel. We conducted additional incubations (n=87, between 3 and 9 per experiment day) without mussels throughout the trials as experimental blanks to determine background changes in alkalinity (Figure S7). We excluded from our analysis any experimental days where background alkalinity changes exceeded 5 µmol kg-1. The mean of the absolute values of alkalinity change during the incubations of these experimental blanks was 1.3 ± 1.2 µmol kg-1 (n = 72). 

We performed all computations with R statistical software, version 4.1.0. We performed carbonate system calculations using the package seacarb, using equilibrium constants from Lueker et al. We computed linear mixed models using the lmer function in in the lmertest package in R and focused on assessing likely candidate parameters as fixed factors, and mussel collection date as a random intercept to account for natural seasonal differences between cohorts. Conditional R2 was calculated with the package MuMIn. We determined parameters for non-linear fits employed to model dissolution rates by minimizing the sum of squares of model residuals using the optim function. Colors for plots were chosen from color palettes in the cmocean package in R.