Small in the heat, transparent in the bloom: copepod morphological responses in the California upwelling ecosystem

ABSTRACT Copepods are central to marine food webs as they link primary producers to higher trophic levels. This study used in situ images collected with an Underwater Vision Profiler during four cruises (2008, 2012, 2014 and 2016) in the California Current Ecosystem to investigate how copepod morphologies relate to environmental conditions. Consistent with Bergmann’s rule, our results indicate that smaller copepods were associated with warmer environments. Copepods with more complex body shapes, owing to extended appendages, were observed in waters with higher fluorescence and diatom concentrations. Finally, more transparent copepods were found in shallower waters with higher fluorescence, potentially suggesting predatory avoidance of darker copepods found deeper in the water column. These findings support the power of imaging-based functional trait-based approaches to link zooplankton morphological variability with environmental gradients, enhancing our understanding of zooplankton dynamics in productive upwelling systems.