Technical Note: Constraining stable carbon isotope values of microphytobenthos (C₃ photosynthesis) in the Arctic for application to food web studies

Abstract. Microphytobenthos (MPB) tends to be omitted as a possible carbon source to higher trophic level consumers in high latitude marine food web models that use stable isotopes. Here, we used previously published relationships relating the concentration of aqueous carbon dioxide ([CO₂]aq), the stable carbon isotopic composition of dissolved inorganic carbon (DIC) (δ¹³C_DIC), and algal growth rates (μ) to estimate the stable carbon isotope composition of MPB-derived total organic carbon (TOC) (δ¹³C_p) and fatty acid (FA) biomarkers (δ¹³C_FA). We measured [CO₂]aq and δ¹³C_DIC values from bottom water at sampling locations in the Beaufort and Chukchi Seas (n = 18), which ranged from 17 to 72 mmol kg^–1 and −0.1 to 1.4 ‰ (0.8 ± 0.4‰, mean ±1 s.d.), respectively. We combined these field measurements with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to determine δ¹³C_p and δ¹³C_FA values. Theδ¹³C_p and δ¹³C_FA values for a mixed eukaryotic algal community were estimated to be −23.6 ± 0.4‰ and −30.6 ± 0.4‰, respectively. These values were similar to our estimates for Phaeodactylum tricornutum (δ¹³C_p = −23.9 ± 0.4‰, δ¹³C_FA = −30.9 ± 0.4‰), a pennate diatom likely to be a dominant MPB taxon. Taxon-specific differences were observed between a centric diatom (Porosira glacialis, δ¹³C_p = −20.0 ± 1.6‰), a marine haptophyte (Emiliana huxleyi, δ¹³C_p = −22.7 ± 0.5‰), and a cyanobacterium (Synechococcus sp., δ¹³C_p = −16.2 ± 0.4‰) at μ = 0.1 d⁻¹. δ¹³C_p and δ¹³C_FA values increased by ≃ 2.5‰ for the mixed algal consortium and for P. tricornutum when growth rates were increased from 0.1 to 1.4 d⁻¹. We compared our estimates of δ¹³C_p and δ¹³C_FA values for MPB with previous measurements of δ¹³C_TOC and δ¹³C_FA values for other carbon sources in the Arctic, including ice-derived, terrestrial, and pelagic organic matter. We found that MPB values were significantly distinct from terrestrial and ice-derived carbon sources. However, MPB values overlapped with pelagic sources, which may result in MPB being overlooked as a significant source of carbon in the marine food web.