We analyzed spatial distributions of 10 euphausiid species in the southern California Current System across seven El Niño events (1951–2018) and the 2014–15 Warm Anomaly to determine variations in habitat utilization and reproduction during Eastern Pacific (EP) and Central Pacific (CP) Niños. Our goal was to characterize the main forcing mechanisms by which El Niño events influence these dominant species. Our findings suggest cool-water euphausiids respond predominantly to changing in situ habitat conditions during El Niño, while subtropical species require initial advection to increase in the southern CCS. Cool-water coastally-associated species (Euphausia pacifica, Thysanoessa spinifera) compress shoreward and retract poleward to upwelling waters during EP Niños, likely in response to offshore warming. A subtropical coastal species (Nyctiphanes simplex) extends poleward nearshore during EP Niños, suggesting anomalous advection, but increases only moderately and variably off southern California during CP Niños. A Tropical Pacific-Baja California species (Euphausia eximia) only appears off southern California in spring during El Niño years (EP, some CP), suggesting direct advection and low tolerance for cooler, fresher conditions. Subtropical offshore species (Euphausia gibboides, Euphausia recurva, Stylocheiron affine, Euphausia hemigibba) expand shoreward during most Niños (strongest during 2014–15 Warm Anomaly) and show moderate in situ post-event persistence, suggesting combined influence of advection and temporarily favorable habitat nearshore. Regionwide temperate species (Nematoscelis difficilis, Thysanoessa gregaria) contract only moderately shoreward during some Niños. Predictions of Year 2100 distributions using generalized additive models suggest future non-Niño conditions and CP Niños will produce regionwide in situ increases in subtropical species and moderate poleward and onshore expansions, while EP Niños will produce continued nearshore habitat compression and reduced abundance of coastal species. Understanding zooplankton spatial responses to El Niño can help predict community compositional shifts under other ocean changes (e.g., long-term trends, basin-wide warm anomalies).

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.

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.