Nearly two decades ago, autonomous bio-optical platforms emerged as a promising response to one of oceanography's most persistent challenges: the chronic undersampling of the ocean interior. Early deployments of optical sensors on profiling floats demonstrated the value of sustained observations of the water column. Over time, these pioneering efforts laid the foundations of the global Biogeochemical-Argo (BGC-Argo) program, in which bio-optical measurements, including chlorophyll fluorescence, particle backscattering, and radiometry, have become essential components of a sustained global observing system.
As BGC-Argo has expanded, the role of bio-optical observations has also broadened. Initially developed to characterize the ocean interior, they now provide an indispensable complement to satellite ocean-colour observations. Vertical profiles reveal the processes hidden beneath the surface, improve and validate satellite retrievals, support the development of merged 3D biogeochemical products, and provide new constraints for machine-learning approaches and numerical models. Rather than operating independently, autonomous platforms and satellites are progressively becoming two complementary elements of a single, integrated observing system.
The next step is already underway. The deployment of new optical sensors (including hyperspectral radiometers, imaging systems, and advanced particle measurements) together with the new generation of ocean-colour satellite missions, will considerably strengthen these synergies. By linking surface observations to the interior ocean with unprecedented detail, these complementary technologies will open new opportunities for ecosystem monitoring, carbon-cycle studies, and climate applications.