title: Carbon and nitrogen isotopes from top predator amino acids reveal rapidly shifting ocean biochemistry in the outer California Current.
abstract: Climatic variation alters biochemical and ecological processes, but it is difficult both to quantify the magnitude of such changes, and to differentiate long-term shifts from inter-annual variability. Here, we simultaneously quantify decade-scale isotopic variability at the lowest and highest trophic positions in the offshore California Current System (CCS) by measuring δ15N and δ13C values of amino acids in a top predator, the sperm whale (Physeter macrocephalus). Using a time series of skin tissue samples as a biological archive, isotopic records from individual amino acids (AAs) can reveal the proximate factors driving a temporal decline we observed in bulk isotope values (a decline of ≥1 ‰) by decoupling changes in primary producer isotope values from those linked to the trophic position of this toothed whale. A continuous decline in baseline (i.e., primary producer) δ15N and δ13C values was observed from 1993 to 2005 (a decrease of ∼4‰ for δ15N source-AAs and 3‰ for δ13C essential-AAs), while the trophic position of whales was variable over time and it did not exhibit directional trends. The baseline δ15N and δ13C shifts suggest rapid ongoing changes in the carbon and nitrogen biogeochemical cycling in the offshore CCS, potentially occurring at faster rates than long-term shifts observed elsewhere in the Pacific. While the mechanisms forcing these biogeochemical shifts remain to be determined, our data suggest possible links to natural climate variability, and also corresponding shifts in surface nutrient availability. Our study demonstrates that isotopic analysis of individual amino acids from a top marine mammal predator can be a powerful new approach to reconstructing temporal variation in both biochemical cycling and trophic structure.
authors.affiliation: Ocean Sciences Department, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Rocio I
authors.initials: RI
authors.lastname: Ruiz-Cooley
authors.affiliation: Earth and Planetary Sciences Department, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Paul L
authors.initials: PL
authors.lastname: Koch
authors.affiliation: Southwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, La Jolla, California, United States of America.
authors.forename: Paul C
authors.initials: PC
authors.lastname: Fiedler
authors.affiliation: Ocean Sciences Department, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: McCarthy
mesh_terms: D000596:Amino Acids / Q000032:analysis*; D000818:Animals; D002247:Carbon Isotopes / Q000032:analysis*; D004784:Environmental Monitoring / Q000379:methods / Q000706:statistics & numerical data*; D020387:Food Chain; D009587:Nitrogen Isotopes / Q000032:analysis*; D010140:Pacific Ocean; D019601:Pacific States; D012044:Regression Analysis; D012623:Seawater / Q000737:chemistry*; D012867:Skin / Q000737:chemistry; D050661:Sperm Whale / Q000378:metabolism*; D014872:Water Movements
chemical_list: D000596:Amino Acids; D002247:Carbon Isotopes; D009587:Nitrogen Isotopes