UC San Diego

We analyzed seven El Niño events (springs 1958–59, 1983, 1992–93, 1998, 2003, 2010, and 2016) and the 2014–15 Pacific Warm Anomaly (spring 2015) for their impacts on zooplankton biomass and community composition in the southern sector of the California Current System (CCS). Although total mesozooplankton carbon biomass was only modestly affected during El Niño springs, community composition changed substantially. Carbon biomass of five major zooplankton taxa correlated negatively with San Diego sea level anomaly (SDSLA), a regional metric of El Niño physical impacts in the CCS. Additional taxa were negatively related to SDSLA via a time-lagged response reflected in an autoregressive-1 (AR-1) model. All five SDSLA-correlated taxa decreased in carbon biomass during most El Niño years compared to the surrounding years; the exception was the mild event of 2003. Principal Component Analysis revealed coherent species-level responses to El Niño within the euphausiids, copepods, and hyperiid amphipods. Percent similarity index (PSI) comparisons showed pronounced changes in the compositions of euphausiid and, to a lesser extent, calanoid copepod communities during El Niño. By grouping El Niños into Eastern Pacific (EP) versus Central Pacific (CP) events based on their expressions along the equator, we found that CCS zooplankton assemblages showed a tendency toward greater changes in species composition during EP than CP El Niños, although we had low statistical power for these comparisons. Several species showed consistent biomass changes across La Niña events as well, generally opposite in sign to El Niño responses, but overall community composition showed minimal change during La Niña. Carbon biomass and community composition returned to pre-Niño levels within 1–2 years following almost every event, suggesting high resilience of southern CCS zooplankton to El Niño perturbations to date.