Using a Lagrangian particle tracking model to evaluate impacts of El Niño-related advection on euphausiids in the southern California Current System

Abstract

We propose a framework to examine the extent to which advection alone can explain spring biomass fluctuations of six euphausiid species in the southern California Current System (CCS) between 2008 and 2017. We modeled population changes as fluxes from defined source boxes using currents from the California State Estimate (CASE), a regional optimization of the MIT general circulation model. This period encompasses a Central Pacific El Niño (2009–10), Warm Anomaly (2014–15) and Eastern Pacific El Niño (2015–16). Total spring populations of all six species show strongest positive correlations with time-lagged flows from the preceding November–December. Three cool-water species (Euphausia pacifica, Nematoscelis difficilis, Thysanoessa spinifera) show opposite patterns for their calyptopis (larval) and adult phases, with strongest positive correlations of calyptopes with February–March flow suggesting periods of in situ reproduction. In contrast, three subtropical species (Nyctiphanes simplex, Euphausia eximia, Euphausia gibboides) show consistent correlation patterns with flow across the adult and calyptopis phases, suggesting advection of entire populations into the region during periods of enhanced favorable flows. We tested whether interannual variations in spring population size were influenced by different winter-origin waters during El Niño compared to non-Niño years. We found that cool-water species associate less strongly and subtropical species associate more strongly with southern-origin waters during anomalous years. Advection alone accounts for varying proportions of species fluctuations, with strongest influences for N. difficilis and T. spinifera. Explicit measurements of growth and mortality rates are needed to further resolve the additional factors influencing euphausiid populations during anomalous events, with implications for future forecasts.