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 SDSLAcorrelated 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.

We analyzed impacts of the 2014–2015 Pacific Warm Anomaly and 2015–2016 El Niño on physical and biogeochemical variables at two southern California Current System moorings (CCE2, nearshore upwelling off Point Conception; CCE1, offshore California Current). Nitrate and Chl‐a fluorescence were <1 μM and <1 Standardized Fluorescence Unit, respectively, at CCE2 for the entire durations of the Warm Anomaly and El Niño, the two longest periods of such low values in our time series. Negative nitrate and Chl‐a anomalies at CCE2 were interrupted briefly by upwelling conditions in spring 2015. Near‐surface temperature anomalies appeared simultaneously at both moorings in spring 2014, indicating region‐wide onset of Warm Anomaly temperatures, although sustained negative nitrate and Chl‐a anomalies only occurred offshore at CCE1 during El Niño (summer 2015 to spring 2016). Warm Anomaly temperature changes were expressed more strongly in near‐surface (<40 m) than subsurface (75 m) waters at both moorings, while El Niño produced comparable temperature anomalies at near‐surface and subsurface depths. Nearshore Ωaragonite at 76 m showed notably fewer undersaturation events during both warm periods, suggesting an environment more conducive to calcifying organisms. Planktonic calcifying molluscs (pteropods and heteropods) increased markedly in springs 2014 and 2016 and remained modestly elevated in spring 2015. Moorings provide high‐frequency measurements essential for resolving the onset timing of anomalous conditions and frequency and duration of short‐term (days‐to‐weeks) perturbations (reduced nitrate and aragonite undersaturation events) that can affect marine organisms.

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