Effects of a marine heatwave on adult body length of three numerically dominant krill species in the California Current Ecosystem

Abstract Krill are an abundant and globally distributed forage taxon in marine ecosystems, including the California Current Ecosystem (CCE). The role of krill trophodynamics depends on both abundance size (biomass), but impact extreme climate events body is poorly understood. Using samples collected from 2011 to 2018, we tested hypotheses that adult length three species (Euphausia pacifica, Thysanoessa spinifera, Nematoscelis difficilis) declined during 2014–2016 Northeast Pacific heatwave/El Niño event due elevated seawater temperatures, reduced upwelling, low primary productivity. Hierarchical mixed-effects modelling showed mean E. pacifica T. spinifera N. difficilis increased 2015. These trends differed by sex reverted a pre-heatwave state 2016. Temperature, food availability (chlorophyll-a content) did not explain decreased 2015, environmental drivers varied regionally across all years. This study documents major heatwave (MHW) one world's upwelling systems indicates how pelagic ecosystems may respond increasingly frequent MHWs.