The vertical migration of Antarctic krill may play a smaller role in oceanic carbon storage than previously believed, according to a year-long study in the Southern Ocean. The findings challenge conventional assumptions about the animal's role in deep ocean carbon sequestration and underscore the need for more nuanced biogeochemical models incorporating ecological complexity. "Antarctic krill play an important role in the biological carbon pump, but without observational data, we risk using inaccurate and misleading assumptions about behaviors that influence carbon export and climate models," write the authors. The biological carbon pump is a critical oceanic process that transfers carbon from surface waters to the deep ocean, where it can remain sequestered for decades to millennia. This natural mechanism plays a vital role in regulating atmospheric carbon dioxide levels and mitigating global climate change. Antarctic krill (Euphausia superba), a cornerstone species in the Southern Ocean ecosystem, are pivotal contributors to this process through two mechanisms: the sinking of carbon-rich fecal pellets and the active transport of carbon via vertical migrations. With the highest estimated biomass of any wild animal species, krill are believed to export millions of tons of carbon annually to the deep ocean. However, the lack of detailed observational data, especially during polar winters, has led to oversimplified assumptions in biogeochemical models about the extent of krill-driven carbon transport.
To refine these estimates, Abigail Smith and colleagues conducted a year-long study in Prydz Bay, East Antarctica, deploying a seafloor lander equipped with video and high-resolution echosounder systems. By integrating krill density observations – both stationary and migrating – with seasonal chlorophyll concentrations into a numerical model, Smith et al. quantified particulate organic carbon (POC) flux from fecal pellet sinking and active migration. They found that sinking krill fecal pellets contributed 9.68 milligrams of carbon per square meter per day (mg C m⁻² day⁻¹) to the POC flux. However, migrating krill contribute much less, accounting for less than 10% (1.28 mg C m⁻² day⁻¹) of the total carbon flux. According to the authors, no more than 25% of krill undertook daily migrations to depths exceeding 200 meters, with this behavior strongly influenced by seasonal variation. Using these observations, Smith et al. show that traditional models, which often assume a uniform 50% krill migration year-round, result in a more than 200% overestimation of carbon export by krill.
