Employing amino acid δ13C analysis to quantify environmental change in a Late Holocene nearshore ecosystem

Kelp forests are among the most productive but vulnerable marine ecosystems on earth. These regions serve as biodiversity hotspots and CO₂ sinks, but are also highly sensitive to human impacts such as overexploitation and shifting ocean chemistry. Understanding their dynamics is crucial for effective conservation and management. Here, we employ δ¹³C analysis of individual amino acids from two top marine consumers, sea otters (Enhydra lutris) and sheephead (Semicossyphus pulcher) to evaluate the importance of kelp forests in the late Holocene (~3500 ybp – present). Since only primary producers and microbes synthesize essential amino acids (AA_ESS), consumers typically directly route them into tissues and thus AA_ESS are minimally altered as they move up food chains. Moreover, different producers (e.g., phytoplankton and macroalgae) in nearshore marine ecosystems have highly distinct δ¹³C values and so the δ¹³C values of AA_ESS in top consumers can provide a ‘fingerprint’ of the dominant producers in the local foodweb. We analyzed bone collagen from late Holocene sea otters and sheephead from two islands (San Nicolas and San Miguel) off the coast of southern California. We also characterized the baseline amino acid δ¹³C profiles for modern producer groups: kelp (Laminaria and Nereocystis), green algae (Ulva) and red algae (Neorhodomela). We used mixing models to quantify the contribution of each algal group to ancient sea otter and sheephead AA_ESS δ¹³C values. As expected, kelps had significantly higher δ¹³C values than red and green algae for all amino acids measured. We found remarkable consistency between ancient sea otters and sheephead, both between individuals and among sites. Both top consumers were predominantly feeding in ecosystems driven by kelp production; in some instances, an estimated 99% of consumer essential amino acids were derived from kelp. These findings suggest that at these sites kelp forests may have been more extensive in the late Holocene than they are today. Our study demonstrates the utility of amino acid δ¹³C analysis in investigating historical ecological problems that hold relevance for modern conservation biology.