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CCE IV: El Nino-related zooplankton variability in the southern California Current System

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.

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