A novel trimodal sensor for eddy correlation measurements of benthic flux in aquatic environments

Abstract

Quantifying chemical fluxes between natural waters and their benthic sediments is a central problem in biogeochemistry, yet it is notoriously challenging. A relatively new method for measuring benthic fluxes, Eddy Correlation (EC) addresses many shortcomings of traditional techniques. Minimally invasive and measured in situ, EC is based on high-speed, simultaneous, and co-located velocity and concentration measurements. It has been successfully used in a range of settings to determine benthic fluxes of dissolved oxygen, using an Acoustic Doppler Velocimeter (ADV) to measure water velocity and an oxygen microelectrode to measure concentration. Widespread application to a larger range of compounds is limited, however, by the lack of chemical sensors that are fast, small, and sensitive enough for EC. To address this need, a novel trimodal sensor has been developed that is capable of high-speed, high-resolution measurements of fluorescence, temperature, and conductivity.

The core of the instrument is an optical fiber spectrofluorometer, which utilizes an LED for low-cost excitation; pair of 1000 [mu]m optical fibers for minimal disruption to velocity measurements; a tunable monochromator to enable a wide range of detection wavelengths; and a custom photon counting detector for maximum sensitivity. It can be used in an EC system to measure benthic fluxes of fluorescing compounds, such as fluorescent dissolved organic material. A fast thermistor and conductivity cell are also located at the tips of the optical fibers, enabling heat and salinity flux measurements that can be used as tracers for submarine groundwater discharge. Additionally, the ability to measure three simultaneous fluxes enables exploration of the potential to use the measured flux of one compound to infer another. Such 'flux tracing' would vastly expand the range of chemicals measurable with EC.

After development and testing of the individual sensors, the ability of the instrument to take three simultaneous, co-located measurements was demonstrated in a flume: under turbulent flow, the three sensors were able to detect similar features from an injection of warm, salty, fluorescent dye. The instrument was then coupled to an ADV for flux measurements, and tested in a specially constructed laboratory tank whereby benthic fluxes were released at known rates from the tank floor. The fluxes measured by all three sensors compared favorably with expected values. In addition, fluxes measured by the three sensors were observed to track each other, demonstrating the viability of flux tracing in settings with co-transported compounds.