δ15N of Amino Acids in Ancient Cod Bone Collagen Track Shifts in Baseline Nitrogen & Cod Trophic Levels in the Gulf of Maine for the Past 4,400 years

Previous nitrogen isotope studies of bulk proteins extracted from ancient Atlantic cod (Gadus morhua) tissues document a 1-2‰ decrease in δ¹⁵N values over the last couple of centuries (Harris, 2011; Lueders-Dumont et al., 2018). Due to the nature of the nitrogen isotope signal in bulk proteins, this isotopic shift may be attributed to a decrease in trophic level and/or a change in baseline nitrogen in the Gulf of Maine over this time period. Here, we analyze the  δ¹⁵N composition of individual amino acids from ancient cod bone collagen to tease out the relative importance of shifts in trophic level vs baseline nitrogen sources to cod diets through time. Preliminary data indicate that  δ¹⁵N values of phenylalanine (“source” amino acid) extracted from cod bone collagen became more depleted in δ¹⁵N over the last 500 years and into the modern record. These shifts in δ¹⁵N_Phe are in agreement with those found in δ¹⁵N_Phe of deep-sea corals (Sherwood et al., 2011) and bivalves (Whitney et al., 2019) from the Gulf of Maine over the last 100 years. The fact that similar trends are seen in three different species occupying different ecological niches suggests the shift in source nitrogen may reflect broad changes in hydrographic conditions in the Gulf of Maine. More work is needed to corroborate these preliminary findings and is currently underway.