Mesoscale and submesoscale variability of organic matter in island-induced eddies across their life cycle: implications for the biological carbon pump

Abstract

The Canary Islands region exhibits intense mesoscale activity, with eddies generated south the islands by perturbations of the Canary Current. After detachment, these eddies drift southwestward along the Canary Eddy Corridor (CEC), exporting their properties into the open ocean. While mesoscale dynamics in the region are well documented, associated submesoscale structures and their role in the production, transport and remineralization of organic matter remain poorly understood. We investigated meso- and submesoscale variability of dissolved organic carbon (DOC), suspended particulate organic carbon (POCsus), and the chromophoric (CDOM) and fluorescent (FDOM) fractions of dissolved organic matter (DOM), at high spatial resolution across four eddies sampled in 2022. In summer, we studied three newly formed or developing eddies: the cyclonic Garajonay eddy, trapped between La Gomera and El Hierro Islands and sustaining an intense phytoplankton bloom and a developing eddy pair south of Gran Canaria (cyclonic Nublo) and Tenerife (anticyclonic Anaga). In autumn, we sampled Bentayga, a five- month-old anticyclonic intrathermocline eddy drifting along the CEC. Cyclonic eddies showed decreasing DOC, CDOM and protein-like FDOM with age, whereas anticyclonic eddies accumulated humic-like FDOM. At the submesoscale, DOC decreased above the pycnocline and humic-like FDOM below it across the Nublo-Anaga front, while Bentayga exhibited pronounced intra-eddy submesoscale variability in DOC, CDOM, and humic-like FDOM. The residuals of the multiple regression with the thermohaline variables of apparent oxygen utilization (ΔAOU), ΔDOC, ΔPOC, ΔCDOM and ΔFDOM, are independent of water mass mixing and physical motions, and therefore retain the biogeochemical variability associated with local processes. Significant correlations among the residuals of DOC/AOU, CDOM/AOU and FDOM/AOU highlight the central role of DOM in local remineralization, which accounts for up to 68 ± 15% of mesopelagic oxygen demand, resulting in the accumulation of humic-like substances and substantial consumption of chromophoric and protein-like materials. Furthermore, the combined residual relationships of DOC/AOU and POCsus/AOU indicate that up to 88% of mesopelagic oxygen demand in the CEC is driven by local remineralization of DOC and POCsus, suggesting only a minor contribution from sinking POM. Our results demonstrate that meso- and submesoscale processes strongly regulate organic matter transport, remineralization and oxygen consumption in the CEC.