Ontogenetic δ15N Trends and Multidecadal Variability in Shells of the Bivalve Mollusk, Arctica islandica

Abstract

Bulk stable nitrogen isotope values of the carbonate-bound organic matrix in bivalve shells (δ15NCBOM) are increasingly used to assess past food web dynamics, track anthropogenic nitrogen pollution and reconstruct hydrographic changes. However, it remains unresolved if the δ15NCBOM values are also affected by directed ontogenetic trends which can bias ecological and environmental interpretations. This very aspect is tested here with modern and fossil specimens of the long-lived ocean quahog, Arctica islandica, collected from different sites and water depths in the NE Atlantic Ocean. As demonstrated, δ15NCBOM values from the long chronologies show a general decrease through lifetime by −0.006‰ per year. The most likely reason for the observed δ15NCBOM decline is a change in the type of proteins synthesized at different stages of life, i.e., a gradual shift from proteins rich in strongly fractionating, trophic amino acids during youth toward proteins rich in source amino acids during adulthood. Aside from this ontogenetic trend, distinct seasonal to multidecadal δ15NCBOM variations (ca. 50 to 60 years; up to 2.90‰) were identified. Presumably, the latter were governed by fluctuations in nutrient supply mediated by the Atlantic Multidecadal Variation (AMV) and Atlantic Meridional Overturning Circulation (AMOC) combined with changes in nitrate utilization by photoautotrophs and associated Rayleigh fractionation processes. Findings underline the outstanding potential of bivalve shells in studies of trophic ecology, oceanography and pollution, but also highlight the need for compound-specific isotope analyses.

Organic Phases in Bivalve (Arctica Islandica) Shells: Their Bulk and Amino Acid Nitrogen Stable Isotope Compositions

“…Due to the limited number of shell powders, a triplicate run within each sub‐aliquot sample was not always possible. Given that the average value was calculated from multiple measurements of the same sample, we consider the variance of sub‐aliquots by propagating the respective error following Schöne and Huang (2021). The nitrogen concentration was computed based on various amounts of L‐glutamic acid that was also processed in the IRMS.…”

Nitrogen Isotope Sclerochronology—Insights Into Coastal Environmental Conditions and Pinna nobilis Ecology

“…Over the past decade, methods for the analysis of nitrogen stable isotopes in carbonate-bound organic matter (δ 15 N CBOM ) using direct combustion of carbonates have been developed and optimized enabling the investigation of δ 15 N CBOM in bivalve shells from freshwater, estuarine, and marine habitats (e.g., Versteegh et al, 2011;Darrow et al, 2017;Gillikin et al, 2017;Graniero et al, 2021;Kukolich and Dettman, 2021;Schöne and Huang, 2021). Nitrogen isotopes measured in carbonate-bound organic matter are a function of δ 15 N values of dissolved nitrogen in the ambient water as well as the physiology and diet of the bivalve (e.g., Gillikin et al, 2017;Whitney et al, 2019;Das et al, 2021;Graniero et al, 2021).…”

“…In many previous studies, δ 15 N CBOM was measured in marine bivalve species including Mercenaria sp. (O'Donnell et al, 2003), Mercenaria mercenaria (Carmichael et al, 2008;Oczkowski et al, 2016), Ruditapes philippinarum (Watanabe et al, 2009), Mytilus edulis (Versteegh et al, 2011), Crassostrea virginica (Kovacs et al, 2010;Oczkowski et al, 2016;Black et al, 2017;Darrow et al, 2017), Pecten maximus (Gillikin et al, 2017), Rangia cuneata (Graniero et al, 2021), Spisula solidissima (Das et al, 2021), and Arctica islandica (Whitney et al, 2019;Schöne and Huang, 2021). An increase in δ 15 N CBOM has been related to nutrient enrichment reflecting anthropogenic perturbation (e.g., Carmichael et al, 2008;Kovacs et al, 2010;Oczkowski et al, 2016).…”

“…An increase in δ 15 N CBOM has been related to nutrient enrichment reflecting anthropogenic perturbation (e.g., Carmichael et al, 2008;Kovacs et al, 2010;Oczkowski et al, 2016). As yet, only a limited number of bivalves were sampled at high-resolution to generate δ 15 N CBOM time-series, e.g., Pecten maximus (Gillikin et al, 2017), Spisula solidissima (Das et al, 2021), and Arctica islandica (Schöne and Huang, 2021). Due to its size (>1 m, Zavodnik et al, 1991) and very fast shell growth rate (Richardson et al, 2004), Pinna nobilis presents an interesting taxon for highresolution δ 15 N CBOM research.…”

“…Due to its size (>1 m, Zavodnik et al, 1991) and very fast shell growth rate (Richardson et al, 2004), Pinna nobilis presents an interesting taxon for highresolution δ 15 N CBOM research. High-resolution δ 15 N CBOM can provide an important insight into temporal variation in bivalve diet, physiology (including ontogenetic changes), as well as background nitrogen isotopic signatures (Gillikin et al, 2017;Das et al, 2021;Schöne and Huang, 2021).…”