Compound-Specific δ15N Amino Acid Measurements in Littoral Mussels in the California Upwelling Ecosystem: A New Approach to Generating Baseline δ15N Isoscapes for Coastal Ecosystems

摘要

We explored δ¹⁵N compound-specific amino acid isotope data (CSI-AA) in filter-feeding intertidal mussels (Mytilus californianus) as a new approach to construct integrated isoscapes of coastal primary production. We examined spatial δ¹⁵N gradients in the California Upwelling Ecosystem (CUE), determining bulk δ¹⁵N values of mussel tissue from 28 sites between Port Orford, Oregon and La Jolla, California, and applying CSI-AA at selected sites to decouple trophic effects from isotopic values at the base of the food web. Bulk δ¹⁵N values showed a strong linear trend with latitude, increasing from North to South (from ∼7‰ to ∼12‰, R² = 0.759). In contrast, CSI-AA trophic position estimates showed no correlation with latitude. The δ¹⁵N trend is therefore most consistent with a baseline δ¹⁵N gradient, likely due to the mixing of two source waters: low δ¹⁵N nitrate from the southward flowing surface California Current, and the northward transport of the California Undercurrent (CUC), with¹⁵N-enriched nitrate. This interpretation is strongly supported by a similar linear gradient in δ¹⁵N values of phenylalanine (δ¹⁵NPhe), the best AA proxy for baseline δ¹⁵N values. We hypothesize δ¹⁵NPhe values in intertidal mussels can approximate annual integrated δ¹⁵N values of coastal phytoplankton primary production. We therefore used δ¹⁵NPhe values to generate the first compound-specific nitrogen isoscape for the coastal Northeast Pacific, which indicates a remarkably linear gradient in coastal primary production δ¹⁵N values. We propose that δ¹⁵NPhe isoscapes derived from filter feeders can directly characterize baseline δ¹⁵N values across major biochemical provinces, with potential applications for understanding migratory and feeding patterns of top predators, monitoring effects of climate change, and study of paleo- archives.