Differential zooplankton trophic responses to environmental perturbations in the California Current Ecosystem

¹⁵N isotopic compositions of zooplankton species in the California Current Ecosystem (CCE) vary over interannual and decadal scales of climate variability, but the extent to which those changes are driven by variations in source N to phytoplankton versus altered dietary composition and trophic position (TP) of zooplankton is poorly resolved.  Here, we use field samples collected during a large natural environmental perturbation, the 1998-1999 alternation between El Niño and La Niña states, to test the capacity of large dominant zooplankton in the CCE (Euphausia pacifica; adult females and copepodite C5s of Calanus pacificus) to alter their TPs in response to environmental variability.  Compound Specific Isotope Analysis of Amino Acids (CSIA-AA) was used to distinguish trophic changes from variations in source N.  Linear mixed-effect (LME) models were also developed to utilize data from all AAs, providing greater statistical power than the typical CSIA-AA approach of using only one trophic and one source AA.  We confirm significant isotopic baseline enrichment of 2-5 ‰ during the 1998 El Niño.  With a three-level LME model, we also demonstrate a significantly elevated TP for E. pacifica during 1998, implying increased carnivory equivalent to 1/3^rd of a trophic step.  Neither adults nor C5s of C. pacificus showed altered TPs between years.