Assessment of Benthic Flux of Dissolved Organic Carbon in Estuaries Using the Eddy Correlation Technique

Dissolved organic carbon (DOC) is a water quality concern in estuarine environments, as DOC facilitates mobilization of metals and organics in sediments and leads to toxic disinfection byproducts during water treatment. Export of DOC from sediments can vary with changing environmental conditions, including wetland restoration and rising sea levels. Therefore, it is important to quantify flux of DOC across the sediment-water interface (SWI). Existing DOC flux measurement techniques, such as equilibrium dialysis, porewater extraction, and benthic chamber measurement, are intrusive to the sediment environment and underestimate flux by only capturing certain flux contributions. In this research, methods for estimating benthic DOC flux using the eddy correlation technique (also known as the eddy covariance technique) were developed and implemented at three estuarine mudflats and one freshwater wetland throughout Maine and New Hampshire. The eddy correlation technique, first developed for use in atmospheric sciences and later applied to aquatic O2 and groundwater flux measurement, is a non-intrusive, in situ method based on measurement of turbulent fluctuations of properties such as fluid velocity, solute concentration, and temperature. The methods employed here utilized vertical velocity vectors obtained with an acoustic Doppler velocimeter (ADV) and DOC concentrations approximated with a chromophoric dissolved organic matter (CDOM) fluorometer. Both linear regression and moving average techniques were investigated for isolation of turbulent fluctuations in the velocity and concentration data, and spectral analysis was used to analyze flux contribution in the frequency series. DOC flux values obtained using eddy correlation were compared with results from porewater extraction. Eddy flux values were typically an order of magnitude higher than the diffusive fluxes calculated from porewater gradients, which are thought to underestimate flux, as turbulent eddy diffusion dominates vertical transport in these aquatic systems. Reasonable flux estimates are a function of adequate trend removal to separate turbulent fluctuations from mean flows and wave-induced fluctuations. This can be difficult in heterogeneous environments such as the ones studied here. In addition, spectral analysis shows that DOC flux estimates can be compromised by high-frequency noise caused by particle attenuation of the CDOM fluorometer measurements