UC San Diego

The zooplankton community of the California Current System (CCS) can change substantially during El Niño events, but few studies have definitively identified the forcing mechanisms underlying these changes. Physical expressions of El Niño can vary in both the equatorial Pacific and CCS, producing different combinations of remote and local forcing on zooplankton. This thesis considers the relative impacts of two dominant mechanisms, anomalous advection and altered in situ population growth, on CCS zooplankton populations across multiple El Niños of the last seven decades. This thesis uses the California Cooperative Oceanic Fisheries Investigations (CalCOFI) spring zooplankton timeseries (1951-2018) to analyze zooplankton community variability in the southern CCS during seven El Niño events and the 2014-15 Warm Anomaly. It first determines whether the zooplankton community varies consistently during El Niño and further aligns with physical delineations into Eastern Pacific (EP) and Central Pacific (CP) Niño events. Second, it uses spatial, habitat, and reproductive variability to identify forcing mechanisms that impact ten dominant euphausiid (krill) species. A particle tracking model forced by the California State Estimate (CASE), the regional implementation of a general circulation model, investigates the influence of advection on euphausiids. Biogeochemical fluctuations at two CCS moorings during 2014-16 provide a high-resolution analysis of habitat changes for pelagic molluscs. El Niño events primarily affect certain zooplankton taxa and species proportions rather than total zooplankton biomass. Within the euphausiids, dominant cool-water species undergo nearshore and poleward compression during major EP Niños; reduced larval abundances suggest dominant population forcing by decreased reproduction under unfavorable habitat conditions. In contrast, subtropical coastal and offshore species increase variably in the southern CCS with anomalous advection; prolonged warm conditions may support temporary in situ reproduction. The 2014-15 Warm Anomaly produced zooplankton community changes comparable to moderate CP Niños. The zooplankton community rapidly rebounds to pre-Niño composition within 1-2 years, indicating high resilience to short-term perturbations. Projections of euphausiid distributional changes by Year 2100 suggest non-Niño and CP Niño conditions will enhance subtropical populations, including moderate poleward and onshore expansion. Future EP Niños will continue to induce shoreward compression and decreased abundances of cool-water species, with potentially significant impacts on marine mammals and seabirds that preferentially target those species. Identifying El Niño-related forcing mechanisms on zooplankton sectors will improve future predictions of changes in zooplankton biomass and distributions, with implication for fisheries management and carbon flux estimates.