Sampling collection details

We gathered naturally settled, adult California mussels (M. californianus between 30 and 80 mm in maximum shell length) by hand from the mid-intertidal zone of Carmet Beach, along the northern California coast. We cleaned mussels of all epibionts and external byssal threads, then transported them in buckets (< 0.5 hr transit time) to Bodega Marine Laboratory, where we acclimated individuals for seven days in flow-through seawater tables prior to subsequent experiments.

Experiment details

To account for calcification scaling allometrically with animal size, we determined the relationship between mussel tissue mass and calcification rate by incubating 26 different mussels spanning 24 mm to 67 mm in shell length and 0.07 g to 2.55 g of dry tissue in ambient, unmanipulated seawater (Supplementary Figure S3 in Romano de Orte et al. (2021), F1,24 = 61.76, R2 = 0.71, p <0.001). This procedure yielded a scaling exponent of 0.72 and we therefore divided calcification rates of the experimental mussels by g0.72.

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 the Related Datasets section of this metadata page for this incubation data). 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. 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).