The proposed instrument for measuring PSD in underway systems has wide applicability in the field of ocean optics and ocean biology and biogeochemistry. Existing methods commonly used in oceanography are time-consuming and expensive, thus the proposed system will be an attractive option for particle size measurement and advancing the state of the art in ocean color and biogeochemistry. NASA scientists and NASA-funded researchers--especially those working on developing phytoplankton functional group algorithms and/or increasingly-complex biogeochemical models--are currently hindered by a lack of ground truth PSD data. Given the current push within NASA programs in preparation for launch of the PACE ocean color mission and EXPORTS field study, development of this system is very timely for advancement of Carbon Cycle and Ecosystems research. Similar to the NASA Applications, the target market for the proposed instrument is a broad. Government agency-funded (including NSF, EPA, NOAA) researchers routinely use turbidity, pigment analysis, and cell counting for water quality monitoring and science applications. Adding the ability to make measurements of PSD at higher space-time resolution would be highly significant. One potential application with major societal relevance is monitoring of changes in PSD as a tool for detecting harmful algal blooms.