Diel Dynamics of Zooplankton Fecal Pellet Flux Revealed by Integrated Optical Observations and Modeling

The contribution of sinking fecal pellets to the biological carbon pump depends on pellet properties, producer abundance, and the depth and timing of pellet production, which can be modulated by diel vertical migration. We examined diel variability in zooplankton fecal pellet flux in the subarctic Northeast Pacific using two image-based tools: the Underwater Vision Profiler (UVP5) and upward-facing cameras (GelCam) on a surface-tethered sediment trap array. Three fecal pellet types were classified across both platforms, enabling complementary estimates of pellet abundance, modeled carbon content, and flux. Daily composites revealed distinct diel patterns in three zooplankton types (crustacea, salps, and appendicularia) and their associated pellet flux. A simple model linking zooplankton vertical migration and pellet production, sinking, and attenuation reproduced the observed temporal variability for the migrators. A compelling diel pattern in appendicularian pellets warrants further investigation. Together, these results highlight how diel behavior and physiology impact vertical carbon transport.