Exploring B/Ca as a pH proxy in bivalves: relationships between <I>Mytilus californianus</I> B/Ca and environmental data from the northeast Pacific

Abstract

Abstract.A distinct gap in our ability to understand changes in coastal biology that may be associated with recent ocean acidification is the paucity of directly measured ocean environmental parameters at coastal sites in recent decades. Thus, many researchers have turned to sclerochronological reconstructions of water chemistry to document the historical seawater environment. In this study, we explore the relationships between B/Ca and pH to test the feasibility of B/Ca measured on the ion probe as a pH proxy in the California mussel, Mytilus californianus. Heterogeneity in a range of ion microprobe standards is assessed, leading to reproducible B/Ca ratios at the 5 % level. The B/Ca data exhibit large excursions during winter months, which are particularly pronounced during the severe winters of 2004-2005 and 2005-2006. Furthermore, B/Ca ratios are offset in different parts of the skeleton that calcified at the same time. We compare the M. californianus B/Ca record to directly measured environmental data during mussel growth from the period of 1999-2009 to examine whether seawater chemistry or temperature plays a role in controlling shell B/Ca. A suite of growth rate models based on measured temperature are compared to the B/Ca data to optimise the potential fit of B/Ca to pH. Despite sampling conditions that were well-suited to testing a pH control on B/Ca, including a close proximity to an environmental record, a distinct change in pH at the sampling locale, and a growth model designed to optimise the correlations between seawater pH and shell B/Ca, we do not see a strong correlations between pH and shell B/Ca (maximum coefficient of determination, r 2 , of 0.207). Instead, our data indicate a strong biological control on B/Ca as observed in some other carbonate-forming organisms.

Investigating controls of shell growth features in a foundation bivalve species: seasonal trends and decadal changes in the California mussel

“…While all Mytilus congeners precipitate both calcite and aragonite in two distinct layers in their shell (Taylor et al, 1969), M. californianus is the only Mytilus species known to precipitate a secondary layer of calcite. A few previous M. californianus studies (e.g., McCoy et al 2011McCoy et al , 2018Pfister et al 2011Pfister et al , 2016 noted the presence of a secondary inner layer of calcite, as initially described by Dodd (1964) and corroborated by the specimens analyzed here, but M. californianus shell mineralogy is often described or assumed to be bi-layered, with 370 an outer calcite layer and an inner aragonite layer only.…”

“…We chose to examine 10 year-long windows of time at daily resolution for three reasons: (1) to gauge intra-annual and interannual environmental variability at the study area, (2) to account for the decadal scale variability of PDO, and (3) to examine environmental conditions over the typical lifespan of intertidal M. californianus. While the full lifespan of M. californianus is unknown, individuals have been known to live up to 11 years (McCoy et al, 2011;Pfister et al, 2011) and even hypothesized to be capable of surviving 50-100 years in 190 undisturbed settings (Suchanek, 1981), although this has not been tested or documented in the literature. We chose ocean records spanning a decade over the years that these shells were collected to provide reasonable environmental context for shell growth patterns for individuals of various and unknown ages.…”

“…The dark-light bands in M. californianus are more comparable to those found in Crassostrea virginica, which are described as alternating dark and light increments visible in cross-section (Kirby et al, 1998;Andrus and Crowe, 2000;Surge et al, 2001;Zimmt et al, 2019; Table 1). Dark-light 380 bands have been suggested to represent alternating periods of shell deposition during aerobic and anaerobic respiration, with light bands forming during aerobiosis and dark bands forming during anaerobiosis (Lutz and Rhoads, 1977;McCoy et al, 2011). Under optimal conditions, such as during immersion or moderate to warm temperatures, M. californianus gapes and respires aerobically, and during sub-optimal conditions, such as during aerial exposure and/or extreme temperatures, M. californianus closes its valves and respires anaerobically (Bayne et al, 1976;Connor and Gracey, 2011;Connor et al, 385 2016).…”

“…During anaerobiosis, glucose and aspartate ferment, resulting in the production of alanine and succinate, which is then converted to propionate if anaerobic conditions persist for several days (Connor and Gracey, 2011). Succinate is an acidic end-product of anaerobiosis, which may contribute to the production of an organic-rich dark band in the shell's inner calcite layer (McCoy et al, 2011). While it has been suggested that M. californianus partially dissolves its own shell during https://doi.org/10.5194/bg-2021-219 Preprint.…”

“…CC BY 4.0 License. anaerobiosis in order to neutralize the acidic end-products of its own metabolism (McCoy et al, 2011), the mechanism that 390 produces dark bands in M. californianus has not been identified; dark bands could be dissolution bands, organic-rich bands, or simply the visual expression of slow calcite biomineralization during sub-optimal growing conditions.…”