Dataset: Field measurements of periostracum cover of mussels (Mytilus californianus) from focal population at Marshall Gulch Beach, CA in July and August 2022

To provide ecological context for the laboratory dissolution experiments, we also measured the percent cover of periostracum of living California Mytilus californianus mussels in the field, focusing on mussels inhabiting different microhabitats within the intertidal zone. We sampled mussels across two tidal heights and three levels of sun exposure on 17 July and 18 August 2022 from the same population (Marshall Gulch Beach, California) where we had collected mussels for the laboratory trials (see the related BCO-DMO datasets). We sampled from the topmost layer of mussels within a 20 cm x 20 cm quadrat positioned within the center of each of 8 mussel beds at each tidal height (characterized as ‘high’ or low’, -0.3 – +0 m sea level height (SLH) and  +0.3 - +1 m SLH), haphazardly choosing the first 20 adult individuals within a pre-selected size range (n = 113 mussels total, 54 mm + 4 mm SD). We characterized sun exposure based on the cardinal direction of the mussel bed orientation, which included mussels oriented 200° – 240° SW (termed ‘intense’), 30° – 70° NE (termed ‘average’), and 300° – 340° NW (termed ‘shade’). We photographed both valves of each mussel, ensuring a scale bar was visible in the field of view for use in ImageJ, and averaged the coverage of periostracum across the two valves as the metric of percent cover for a given individual.