Possible relation between enrichment of δ¹⁵N in the top predator Zalophus californianus and the expansion of the oxygen minimum zone

Stable isotopes of nitrogen were analyzed from the fur of nursing California sea lion (CSL) pups (that depend nutritionally on their mother’s milk for nourishment) collected in the thirteen rookeries from the Gulf of California from 1999 to 2014. Additionally, analyses were obtained from vibrissae of adult females at Los Islotes, a sea lion rookery in Bahía de La Paz with the longest record of CSL pup isotope information. The data show that consistent ¹⁵N enrichment occurred at all sea lion rookeries from the Gulf of California. Scat samples covering this time period showed no trend that might explain the δ¹⁵N enrichment as a result of shifts in feeding behavior. The enrichment pattern is paralleled by similar shifts in the enrichment of settling particulate organic matter (POM) recovered from sediment traps in the Bahía de La Paz, as well as published changes in POM δ¹⁵N in sediment cores from a number of coastal basins along the Pacific coast since 1990. The data are best explained by progressive transfer to surface waters of fractionated ¹⁵N nitrogen associated with the expansion of the Oxygen Minimum Zone in the Eastern Tropical Pacific, which is most intense along the coast of Mexico. In the Gulf of California, all levels of the food web are enriched in δ¹⁵N values with respect to similar counterparts outside of the region. We propose the nitrogen isotope composition in sea lions as an effective and easy method to monitor the regional expansion of the OMZ in the Eastern Tropical Pacific.