Dramatic change in trophic structure in the Southern California Bight in response to the 1997-98 El Niño: 17 years of record from predatorδ15N amino acids

Background/Question/Methods

In marine ecosystems, climate change is altering biochemical cycling, the distribution of species and food web structure, but it is unknown the magnitude of such changes. In recent decades, the highly productive California Current System has experienced increasing frequencies of El Niño-Southern Oscillation (ENSO) events possibly linked to global warming. To understand the effect of the ENSO, in particular, El Niño events, on the food web structure of the Southern California Bight, we quantified a proxy record of variation in nitrogen cycling and trophic structure over 17 years. We analyzed bulk (n = 200) and amino acid compound specific (n = 30) stable isotopes for skin-tissue samples of the common dolphin, Delphinus delphis. We modeled temporal variation in nitrogen isotope values in relation to dolphin age and length, together with oceanographic variables from the same ecosystem and an ENSO index (MEI).  We asked how the El Niño –Southern Oscillation affects both biochemical cycling and trophic structure in the Southern California Bight in recent decades.

Results/Conclusions

Bulk δ¹⁵N values from D. delphis exhibited a strong decline coincident with the 1997-98 Niño, but many factors can influence bulk stable isotopes such as changes in habitat biochemistry, primary production, animal diet and metabolism. Here, amino acid isotope analysis differentiated temporal variation in primary producers’ values (i.e. baseline values) from changes in the dolphins’ trophic position. In 1997-1998, we observed a moderate variability in source amino acids δ¹⁵N, as indicators of algae isotope values, but a strong decline in trophic-AA δ¹⁵N, as indicators of trophic structure. GAM analysis identified that the El Niño oceanographic conditions in 1997-98 such as warm sea surface temperatures, low nutrient availability, and primary productivity likely drove the dramatic change in dolphin trophic status. We argue that this trophic shift was driven by bottom up effects, likely altering trophic interaction at different levels, and reducing food web lengths during the strongest El Niño of the 20^th century.  The results of this study present a novel approach using nitrogen amino acid stable isotopes on cetaceans to understand the effect of ocean climate variation over food web structure in marine ecosystems.