Calcification patterns of a foundation species reveal resilience to environmental change in southern California over the twentieth century

Abstract Oceanographic changes due to carbon dioxide emissions have complex and varied impacts on marine calcifying taxa. Changes in calcification can unfold over decades or longer, requiring a longer-term understanding of biomineralization patterns than laboratory-based studies can provide. Here, we investigate the calcification history of the marine mussel Mytilus californianus to determine how this ecologically critical foundation species has responded to past and ongoing environmental changes in the southern end of its range within the California Current System (CCS) between 27.72° N and 34.47° N. Calcification metrics based on shell morphology, growth banding, and mineralogical structure in 130 museum-archived M. californianus specimens collected decadally from 1913 to 2010 revealed unexpected resilience in southern M. californianus shells after previous studies found calcification vulnerability in this species farther north. Shell growth data were paired with temperature datasets and regional pH records. Despite increasingly warmer and more variable conditions in the southern CCS, M. californianus shell features remained quantitatively unchanged in this region since 1913, although we present preliminary microstructural data that align with previous evidence of increased crystallographic disorder in this species. Based on overall consistencies in calcification metrics, M. californianus appears more resistant to environmental changes at the southern end of its range.