Krill defecation at depth reduces carbon flux attenuation in the Weddell Sea euphotic zone

Abstract. The Weddell Sea, Southern Ocean, is a highly productive location of deep-water formation and a globally important site of carbon sequestration. Here, the biological carbon pump is dominated by particulate processes (e.g. zooplankton faecal pellets and phytoplankton detritus). However, climate driven changes in sea ice have the potential to disrupt these processes, highlighting a need for contemporary observations. This study quantified the flux of particulate organic carbon (POC) and nitrogen (PON) across three depths (50, 100 ,150 m) at five locations (including shelf, off shelf, ice covered and ice-free environments) in the western Weddell Sea using a drifting sediment trap. POC and PON fluxes were greater on shelf than off-shelf, likely reflecting increased nutrient supply and productivity on shelf. No strong patterns between sea ice and ice-free stations were present, likely because the ice pack was constantly shifting, with most sites influenced by sea ice. The POC flux remained stable or increased with depth at most stations, ranging from 42.5–364.1 mg C m-2 day-1 (mean of 123.2 mg C m-2 day-1). Krill faecal pellets represented 98 % of all pellets, which contributed an estimated 17–99 % (median of 48 %) of the POC flux. The faecal pellet flux peaked at 100 m across the shelf, suggesting krill defecating at depth during daily migrations effectively counteracted attenuation in the upper ocean. Our findings emphasise the importance of zooplankton mediated processes in determining the particle flux and the benefits of resolving the vertical flux at a resolution which incorporates their ecology. It is unclear how changing sea ice dynamics will impact zooplankton, so a process-driven understanding of biogeochemical fluxes is integral for predicting the future of carbon cycling in the Southern Ocean.