Sensing volume and bandwidth characteristics of a multi-function sensor for eddy correlation measurements of benthic flux

Eddy Correlation (EC) is a method for measuring the transport of substances between bottom sediments and their overlying waters (benthic fluxes). Based on rapid, simultaneous, and co-located measurements of water velocity and chemical concentration, it has been successfully used in various freshwater and marine habitats to measure benthic fluxes of dissolved oxygen. However, extension to other substances of interest has been limited, as EC requires chemical sensors that meet demanding requirements of speed, resolution, and sensitivity. New chemical sensors are needed to expand the range of fluxes that can be measured by EC; in addition, new sensors can enable further exploration of certain aspects of the technique, such as the impact of sensor bandwidth and sensing volume on measured fluxes. FACT (Fluorescence and Conductivity / Temperature) is a prototype multi-function sensor designed for use in EC applications, comprising an optical fiber spectrofluorometer combined with a conductivity cell and thermistor. It enables EC measurements of benthic fluxes of fluorescing compounds (e.g. fluorescent dissolved organic matter), as well as heat and salinity, which can be used as tracers for submarine groundwater discharge. The ability of FACT to measure fluorescence, temperature, and conductivity simultaneously and in approximately the same sensing volume was demonstrated in tests in a flume. All three sensors were able to detect similar features from a combined tracer of warm, salty, fluorescent dye injected in front of their sensing volumes. Differences in the signals, combined with video imagery, demonstrate the importance of sensing volume size, shape, and location on sensor performance, with likely consequences for eddy flux measurement in natural waters.