Regionalization of oceanic waters based on satellite bio-optical properties in the central and southern Gulf of Mexico

Abstract

The absorption or scattering of photosynthetically active radiation (PAR) in the ocean by its components is crucial in determining the extent of the euphotic zone. This stratum is the ocean's uppermost layer, where light availability governs primary productivity in marine ecosystems. This study aims to bio-optically categorize Gulf of Mexico oceanic waters based on regional differences in water transparency and the euphotic zone depth using remotely sensed diffuse attenuation coefficient data (Kd490) validated with in situ observations (KdPAR) from cruises conducted between May 2016 and June 2019. An overestimation of about 15 % of remotely sensed depths was found, which was significantly reduced using in-water records. A 22-year climatological analysis on the calibrated euphotic zone depths derived from satellite-based Kd identified three optical regions in the Gulf of Mexico (I, IA, and IB). Deeper euphotic zones were observed in Region I (in central waters), while Region IB (in the Bay of Campeche) had shallower euphotic zones. The differences in PAR light penetration were linked to variations in chlorophyll-a, which were modulated by mesoscale circulation and river discharges. Our findings support the extent of the euphotic zone down to 0.1 % of incident surface light, especially in Region I, where the large anticyclonic eddies detached from the Loop Current have the most influence in the oceanic waters of the Gulf of Mexico. This study contributes to understanding important optical variables that can be used to improve models for estimating primary productivity and carbon fluxes in oligotrophic surface layers.