14:00–14:15 | ABSTRACT 1121
BEYOND THE EUPHOTIC ZONE: A BIO-OPTICAL FRAMEWORK FOR OCEAN VERTICAL PARTITIONING
Conventional partitioning of the ocean water column relies on the euphotic depth (zeu, 1% surface PAR) and a fixed 200m mesopelagic boundary, thresholds that have been historically established rather than biologically motivated. By exploiting the shape of downwelling irradiance profiles at 380nm and 490nm acquired by the global BGC-Argo array, we introduce two bio-optical depths, Pd(380) and Pd(490) to partitioning the ocean. Chlorophyll-a, particulate organic carbon (POC), and apparent oxygen utilization (AOU) distributions are used to corroborate the biological significance of the new identified layers. Globally, Pd(380) exceeds zeu capturing ~70% of the integrated POC profile versus ~45% for zeu. It coincides with the depth of maximum AOU rate, the transition from net autotrophic to net heterotrophic conditions, and a chlorophyll-a concentration of ~0.05 mg m−3, consistent with published estimates of the productive layer base. Furthermore, Pd(490) corresponds to 0.01–0.1% surface PAR, matching reported thresholds for mesopelagic fish occurrence and the effective cessation of photosynthesis. The POC:Chla ratio distribution confirms that Pd(490) falls within a detrital-dominated region. Analysis of AOU suggests that ~75% of remineralization between surface and Pd(380) occurs below zeu, implying overestimates of carbon export of ~10% when zeu is used as the reference surface rather than Pd(380). Together, these depths define three biogeochemically distinct layers: the euphotic zone, the biological transition layer where residual production, respiration and remineralization co-occur, and the upper mesopelagic zone. This novel framework for ocean vertical partitioning, unifies the representation of the upper ocean under a single, bio-optical meaningful definition.
Giovanni La Forgia, CNR ISMAR, [email protected], https://orcid.org/0000-0002-8543-6369
Mengyu Li, CNR ISMAR, [email protected]
Emanuele Organelli, CNR ISMAR, [email protected]
