A Multi-Function Sensor for Eddy Correlation Measurements of Benthic Flux

Eddy Correlation (EC) is a technique that can be used to measure the transport of substances in aquatic ecosystems between bottom sediments and the overlying water (i.e. benthic fluxes). Based on high-speed, simultaneous, and co-located velocity and concentration measurements, EC has been successfully used in a variety of freshwater and marine settings to determine benthic fluxes of dissolved oxygen. Application to a larger range of compounds is limited, however, by the lack of suitable chemical sensors. Here, we describe FACT, a novel, high-speed, multi-function sensor created to expand the range of benthic fluxes that can be measured with EC. An optical fiber spectrofluorometer with a proximally located conductivity cell and thermistor, FACT enables benthic flux measurements of fluorescing compounds, such as fluorescent dissolved organic matter, as well as of heat and salinity which can be used as tracers for submarine groundwater discharge. The high bandwidth and open-beam geometry of the fluorescence sensor are particularly beneficial for EC measurements. FACT was integrated with a velocity sensor into a full EC system capable of simultaneous benthic flux measurements of fluorescing compounds, heat, and salinity. Tested in a laboratory tank, fluxes measured by all three sensors were found to track each other as well as compare favorably with expected values. The ability to measure fluxes of multiple substances both extends the applicability of EC to a wider range of natural sites, and can provide insight into issues of sensing volume and time responses as they affect the application of EC to natural waters.