|Title||CCE IV: El Nino-related zooplankton variability in the southern California Current System|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Lilly L.E, Ohman MD|
|Journal||Deep-Sea Research Part I-Oceanographic Research Papers|
|Type of Article||Article|
|Keywords||biological consequences; california current system; central oregon coast; climate; El Nino; enso; enso variability; equatorial pacific-ocean; events; evolution; mesozooplankton; northern california; oceanography; Primary productivity; warm|
We analyzed seven El Nino 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 Nino 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 Nino 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 Nino years compared to the surrounding years; the exception was the mild event of 2003. Principal Component Analysis revealed coherent species-level responses to El Nino 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 Nino. By grouping El Ninos 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 Ninos, although we had low statistical power for these comparisons. Several species showed consistent biomass changes across La Nina events as well, generally opposite in sign to El Nino responses, but overall community composition showed minimal change during La Nina. Carbon biomass and community composition returned to pre-Nino levels within 1-2 years following almost every event, suggesting high resilience of southern CCS zooplankton to El Nino perturbations to date.
Our goal in quantifying mesozooplankton variability during El Niño was to identify patterns of change that could suggest possible mechanisms affecting biomass and community composition. El Niño may affect mesozooplankton in several ways: changes in advection can produce species influxes from different regions; oceanic and atmospheric forcing can alter in situ physical and biological conditions, including temperature, thermocline and nutricline depths, and food sources (phytoplankton and microzooplankton); and altered species interactions may occur via predation, parasitism or competition for food (Ohman et al., 2017). Evidence for increased poleward and onshore advection during past El Niño events suggests that this is frequently an important forcing mechanism of species transport. In our study of the southern CCS, presence of offshore and southern euphausiid species suggest some component of advective forcing: their high-magnitude but transient increases during El Niño events, with no time-lag to indicate local population growth or reproduction, suggests a direct physical forcing mechanism such as advection. The near-absence of subtropical species off Southern California during the 2003 and 2010 El Niños, in conjunction with a lack of enhanced poleward or onshore flow, further supports this interpretation.