Development and Field Testing of a Spectral Radiometer for Measurements in Extremely Low-Light Marine Environments

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Low-light conditions in the ocean have critical significance in the lives of marine animals due to various light-dependent physiological mechanisms and behavioral patterns associated with animal vision, communication, reproduction, predation, antipredator tactics, and vertical migration within the water column. Therefore, low-light environments have multiple implications for oceanic habitats, water quality, and commercial fisheries production. Measurements of extremely low-light marine environments including twilight (sunset and sunrise) conditions in the surface ocean layer as well as very dim solar daylight within deeper layers extending from about 200 to 1000 m depth are not possible with existing conventional instruments. As a result, such measurements have been very scarce and greatly limited in scope. The instrument to be developed in this project, called LARS (Low-light Aquatic Radiometer System), will have an unparalleled capability to accurately measure extremely low light, which will pave the way to improved characterization of deep-sea light and twilight conditions in the ocean beyond oversimplifying historical descriptions. This instrument will aid in expanding research of deep-sea habitats to protect the biodiversity and integrity of these largest ecosystems by volume on Earth, an important goal recognized by international community of scientists and policy-makers.The proposed development of LARS will be based on state-of-the-art highly sensitive photon counting technology which will allow measurements of extremely low levels of underwater spectral irradiance of 10 - 20 photons/(cm2 s nm) within the visible spectral range. This will enable measurements of dim daylight at ocean mesopelagic depths (~200-1000 m) not only of blue light but also, for the first time, potentially biologically significant levels of green and red light generated locally at depth by inelastic radiative processes such as Raman scattering by water molecules. In addition, LARS will provide the capability to measure both the downward and upward irradiances in low-light marine environments, which has never been done before. Field demonstrative data will be collected to comprehensively characterize the deep-sea light field and provide first experimental evidence for the role of inelastic processes and approach to the asymptotic light regime at depth. The measurements of poorly characterized twilight and moonlight conditions in the ocean surface layer will be also demonstrated. The broader utility of LARS will be demonstrated by data analysis in support of the study of specific biological questions related to low-light marine habitats.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.