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.
Highlights
The stable nitrogen isotope composition of organic materials serves as a powerful tool in trophic ecology, hydrography and pollution studies (e.g., Fry, 1988; Hobson and Welch, 1992; Ohkouchi et al, 2015; Doherty et al, 2021)
The modern bivalve from NE Iceland (ICE12-05-12 A-L) and the historical specimen from the Irish Sea (M071868-A1L) attained a lifespan of merely 33 years, whereas the two subfossil A. islandica specimens lived for 92 (Fladen Ground, WH241-604-BoxLD11R) and 162 years (E Farn Deeps, WH241-677-BoxB-D1R) based on annual increment counts (Figure 2 and Table 3). δ15NCBOM chronologies of the shells from NE Iceland, the Fladen Ground and the E Farn Deeps showed a general and statistically significant decrease through ontogeny which were estimated with linear regression models (Table 3)
It should be noted that the increase in δ15NCBOM which is coincidental with early ontogeny of A. islandica fits into the multidecadal cycle throughout both records (See further)
Summary
The stable nitrogen isotope composition of organic materials serves as a powerful tool in trophic ecology, hydrography and pollution studies (e.g., Fry, 1988; Hobson and Welch, 1992; Ohkouchi et al, 2015; Doherty et al, 2021). According to recent sclerochronological studies, shells of bivalve mollusks serve as suitable δ15N archives, because they provide high-resolution (annually and seasonally), temporally well-constrained records of past ecological and environmental changes (Gillikin et al, 2017; Whitney et al, 2019; Das et al, 2021) Both bulk δ15N values of the periostracum (δ15NP) and the carbonate-bound organic matrix of their shells (δ15NCBOM) typically correlate strongly with δ15N values of contemporaneous soft tissues (LeBlanc, 1989; O’Donnell et al, 2003; Carmichael et al, 2008; Delong and Thorp, 2009; Watanabe et al, 2009; Kovacs et al, 2010; Versteegh et al, 2011; Graniero et al, 2016, 2021; Darrow et al, 2017), the adductor muscles (Gillikin et al, 2017), and with the diet of the bivalves (δ15NDiet) (O’Donnell et al, 2003; Carmichael et al, 2004; Graniero et al, 2021). Since the different amino acids exhibit vastly different nitrogen isotope fractionation patterns (McClelland and Montoya, 2002; Chikaraishi et al, 2007), a compositional change would inevitably be associated with a shift in bulk δ15N values of shell organics and periostracum
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