Unusually low euphausiid biomass during the warm years of 2014–2016 in the transition zone of the California Current

Euphausiids represent a significant part of the total zooplankton biomass and are a key link in the trophic web of the California Current System (CCS). During 2014–2016, the northeast Pacific experienced warming caused by two successive events, the Blob and the 2015–2016 El Niño, which affected the whole ecosystem. Here we present evidence for a strong impact of the 2014–2016 warm events on euphausiids in the Baja California region, namely two species (Euphausia pacifica and Nematoscelis difficilis) that are dominant in the CCS transition zone. Both species decreased approximately 95% in relation to the previous years (1999–2010). A similar decrease was observed for E. pacifica during the 1997–1998 El Niño, but the decrease in N. difficilis was higher in 2014–2016 and unprecedented during the 1997–2016 period. Biomass of the coastal species Nyctiphanes simplex showed negative anomalies during the Blob but not during the 2015–2016 El Niño. The tropical species Euphausia eximia and Euphausia recurva increased in biomass during the Blob in 2014 relative to the previous years, and even more during the 2015–2016 El Niño. The strong increase of E. eximia, E. recurva, and N. simplex during El Niño may have been due to upwelling activity in winter 2015–2016. Canonical correspondence analysis (CCA) was performed to assess the relation between the euphausiid community and environmental variables. The most influential variables were temperature and chlorophyll, and to a lesser extent salinity. The subarctic E. pacifica showed covariance with chlorophyll while a set of tropical species did with thermohaline conditions. However, the variance accounted for was low (20% taking the first three axis). N. difficilis did not show significant scores in the CCA suggesting that other variables such as availability of prey could be important for this carnivore species.