Detecting mesopelagic-organism using biogeochemical-Argo

During the North Atlantic Aerosols and Ecosystems Study (NAAMES) in the western North Atlantic, float-based profiles of fluorescence by dissolved organic matter (FDOM) and optical backscattering exhibited distinct spikes at ~300 m. Coinciding with optical measurements, ship-based acoustic measurements (at 38 and 120 kHz) identified scattering layers at a similar depth and the presence of zooplankton (largely dominated by Copepods and Euphausiids) was confirmed from images collected by underwater video profiler (UVP). The collocation of spikes in bio-optical properties with mesopelagic organism (including zooplankton and mesopelagic fish) suggests that aggregations of mesopelagic organisms can be detected with float-based bio-optical sensors. These observations open the door to the investigation of such aggregations/layers at global scale across all seasons using the emerging global biogeochemical-Argo array. While the specific organisms generating these spikes are not known as of yet, our results suggest that the floats are not measuring ambient animal concentration but rather diel migrating mesopelagic organisms that have been attracted to the lights of the bio-optical sensors.