Assessing the contribution of euphausiids’ fecal pellets to greenhouse gas inventories in the Eastern South Pacific

The Eastern South Pacific (ESP) is a highly heterogeneous environment, characterized by a productivity gradient that goes from eutrophic waters in the coastal zone to ultraoligotrophic waters in the center of the South Pacific Gyre. Its coastal region is a significant source of CO2, CH4, and N2O to the atmosphere, mainly due to upwelled subsurface waters high in greenhouse gases (GHGs), where microbial processes produce these gases through the remineralization of organic matter. Few studies have assessed the contribution that microbial processes within particles, especially those rich in organic matter such as fecal pellets, could make to GHG inventories. We aimed to evaluate whether krill fecal pellets generated in the surface layer contribute to GHG inventories (CO2, N2O, and CH4) in the mixed layer. Experiments were conducted at four stations along a zonal transect near 27° S on board the CIMAR 26 Oceanic Island Expedition (June-July, 2022). The estimated contribution of a krill assemblage was less than 1% of the excess/deficit of each GHG, which, due to experimental limitations that introduced uncertainties, rather than providing absolute values for contribution or consumption, our results should be viewed as a potential range, indicating that krill fecal pellets can act as both sources and sinks of GHGs. Still, this contribution also depends on fecal pellet production rates and the abundance of krill assemblages, suggesting that large krill aggregations can contribute to changes in local GHG concentrations. This study represents the first effort to simultaneously quantify CO2, N2O, and CH4 production in zooplankton fecal pellets and assess their contribution to GHG inventories in the ESP, providing a baseline for quantifying future changes in GHG production in upwelling ecosystems and the open ocean under a climate change scenario.