The effects of vertical position in the intertidal zone on the δ18O and δ13C composition of Mytilus californianus shell carbonate

Interpreting life-history traits, seasonal cycles, and coastal climate from an intertidal mussel species: Insights from 9000 years of synthesized stable isotope data

“…For example, TGB + 1 indicates that one sub-sample was taken at the terminal growth band of the shell, and one additional sub-sample was drilled ~2-3 mm umbo-ward of the TGB, producing a two-sample shell profile used to determine the season or conditions occurring at the end of the individual's life. This commonly used subsampling strategy is based on the estimate of ~2-3 mm of growth per 1-2 months in adult M. californianus shells growing in southern California [25,29,34,74,77].…”

“…Therefore, we used δ 18 O w = -0.32‰ + 0.3‰ for mussel shells aged 9000-8000 BP, -0.32‰ + 0.2‰ for mussel shells aged 8000-7000 BP, and -0.32‰ + 0.1‰ for shells aged 7000-6000 BP (Table 2). While δ 18 O w can vary on a seasonal scale in addition to a millennial scale, Jazwa et al (2020) reported that the standard deviation of δ 18 O w values from water samples collected seasonally at Bechers Pier on Santa Rosa Island from August 2015 to May 2019 was 0.1‰, ranging from a minimum of -0.6‰ to a maximum of -0.2‰ [74]. The low seasonal variance of δ 18 O w would yield a temperature difference that is substantially lower than the seasonal temperature range of ~10˚C, so we used the same modern δ 18 O w value for all data points for which an ice-volume correction was not applicable.…”

“…Ontogenetic trends in short profiles. Since previous studies of other mussel species identified ontogenetic de-coupling between environmental parameters and shell chemistry [100,101], we tested the relationship between ontogenetic stage (shell length) and growing edge δ 18 O shell and δ 13 C shell values in 76 M. californianus shells live-collected on the same day at Santa Rosa Island in August 2017 [74]. These 76 individuals were chosen for this analysis because this was the only dataset to include shell length information for each individual.…”

“…shell and calculated a mean δ 18 O shell and mean δ 13 C shell value for the final 3 mm of growth for each shell [74]. These mean terminal edge values are time-averaged, aiming to represent ~1-3 months of growth depending on the ontogenetic age and growth rate of the shell.…”

“…While seasonality is visually apparent in the long δ 18 O shell profiles (Fig 7), the number of subsamples required to capture seasonal variability depends on the growth rate, and therefore ontogenetic age, of the individual. Typically, ~3 mm is thought to represent 1-2 months of growth, and 8-10 closely spaced subsamples are required to identify an annual cycle [51,74]. Although evenly spaced 2-3 mm increments is a conventional subsampling approach based on field observations of California mussels, we emphasize the need for consistently updated, long-term, and site-specific field studies of M. californianus shell growth rates because this species exhibits highly variable growth rates across sites and through time [29,40,44].…”