Elemental and isotopic constraints on zooplankton-mediated carbon fluxes in Ryder Bay, Western Antarctic Peninsula

Abstract. Zooplankton faecal pellets are recognised as a key pathway for particulate organic carbon export in the Southern Ocean, yet their elemental and isotopic composition remains poorly characterised, limiting understanding of how zooplankton-mediated processing and particle type shape the chemical signatures of sinking material. Here, we present moored sediment trap observations from Ryder Bay, Western Antarctic Peninsula, quantifying particulate organic carbon and nitrogen fluxes for total sediment trap material and isolated faecal pellets, alongside measurements of POC:PN ratios and the stable isotopic compositions of carbon and nitrogen. Cylinder faecal pellets dominated summer particulate organic carbon flux, reaching up to ~ 100 % of the total flux, which was concentrated in a brief but intense pulse consistent with episodic changes in zooplankton community composition. POC:PN ratios in both the total sediment trap material and cylinder faecal pellets were substantially elevated above Redfield, reaching values of up to 27, well beyond the range previous reported for inshore Western Antarctic Peninsula environments, whereas round pellets retained near-Redfield stoichiometry. This particle-type specific contrast indicates that the elemental composition of exported material is primarily governed by the dominant fluxing particle type rather than directly reflecting system-wide environmental conditions. Stable isotopic signatures are most consistent with post-production modification of organic matter, including preferential nitrogen remineralisation and trophic reworking, including benthic feeding, rather than directly reflecting primary production signals. Together, these findings suggest the organic matter exported across the WAP shelf break may carry a chemically distinct signature, with implications for tracing the contribution of shelf-derived carbon to broader-scale export budgets.