Scallop shells as geochemical archives of phytoplankton‐related ecological processes in a temperate coastal ecosystem

Abstract

Phytoplankton dynamics in coastal ecosystems is increasingly altered by land‐based human activities. Yet, this global vision conceals major disparities, among sites and through time. As conventional monitoring time series are quite sparse and relatively short, biological records of environmental variability appear as relevant tools to gain insights into phytoplankton dynamics over larger temporal and spatial scales. Here, we present results of an interdisciplinary project dealing with chemical information archived in shells of Pecten maximus (Bivalvia; Pectinidae), known to form daily growth striae on its shell surface. Several individuals were collected in the Bay of Brest (France) in 2011 and 2012, and analyzed for the molybdenum and lithium concentrations in their soft tissues and the element‐to‐calcium ratios (Mo : Ca and Li : Ca) in their calcitic striae. All shells revealed high synchrony and reproducibility in their Mo : Ca and Li : Ca profiles, characterized with a major peak at the end of May and in mid‐June 2011, respectively. Detailed analysis of physical, chemical, and biological variables measured in seawater during an extensive 9‐month environmental survey enabled a meticulous description of phytoplankton dynamics in 2011 and its impact on shell geochemistry. Main findings strongly suggest that (1) the timing of Mo : Ca peaks reflects the occurrence of silicon limitation and diatom aggregation periods, (2) the height of these peaks relates to the amplitude of the first spring diatom bloom, and (3) Li : Ca serves as a proxy for the temporal dynamics of diatom biovolume and of biogenic silica recycling at the sediment–water interface.

Ultradian rhythms in shell composition of photosymbiotic and non-photosymbiotic mollusks

“…Note that water depth in the Bay of Brest varies significantly due to the macrotidal regime with a mean tidal range of 2.8 -5.9m with extreme ranges up to 7.2m (Guillaume-Olivier et al, 2021; Service Hydrographique et Océanographique de la Marine; 2022). Collected specimens contained at least one full year of growth based on the visibility of one winter growth line on the outside of the shell (age class 1; see Thébault et al, 2022 ;Fig 1F and S1). Specimens were frozen at -20°C immediately after collection.…”

“…Shells were superficially cleaned using a plastic brush and adhering sediment was removed by ultrasonication in deionized water. The flat, left valves were used for elemental and sclerochronological analysis following previous studies on P. maximus (Thébault et al, 2022;Fröhlich et al, 2022).…”

Different life strategies of the three commercially exploited scallop species living under the same environmental conditions

“…Daily growth rates obtained in this study contribute to the understanding of the growth dynamics of commercially important scallops. According to a recent study by Thebault et al (2021), daily shell growth rates of Pecten maximus ranged from 0.05 mm day -1 in late February representing the beginning of the main growing season, to ca. 0.25 mm day -1 during the summer months.…”

“…Unlike the great scallop, P. maximus, which was the focus of many previous studies (e.g., Laing, 2002;Chauvaud et al, 2005;Andersen et al, 2011;Chauvaud et al, 2012;Lavaud et al, 2014;Götze et al, 2020;Fröhlich et al, 2022;Thebault et al, 2022), scientific research on P. jacobaeus is relatively limited (e.g., Mattei and Pellizzato, 1996;Peharda et al, 2003;Katsanevakis, 2005;Margušand Teskeredzǐc, 2005;Peharda et al, 2019). Based on genetic studies, some authors suggested that these two species could represent a single species or races/subspecies (Wilding et al, 1999;Saavedra and Peña, 2004;Saavedra and Peña, 2005;Morvezen et al, 2016;Vendrami et al, 2019) as their phylogenetic status is not completely clear.…”

Uptake of barium, molybdenum, and lithium and incorporation into scallop shells: Refining proxies for primary production dynamics

“…However, the underlying mechanisms leading to the formation of Mo/Ca shell and Li/Ca shell peaks differed notably from those of Ba/Ca shell peaks. Enhanced dietary uptake of Mo by P. maximus (Barats et al 2010; Tabouret et al 2012) seems to be related to the ingestion of small Mo‐enriched aggregates of phytoplankton cells (Thébault et al 2022) and specific dinoflagellates with a high load of cellular Mo (Fröhlich et al 2022 a ). Yet, it remains unresolved if these mechanisms provide sufficient quantities of Mo that can explain the formation and magnitude of Mo/Ca shell peaks.…”