ASSESSMENT OF BENTHIC FLUX OF DISSOLVED ORGANIC CARBON IN ESTUARIES USING THE EDDY-CORRELATION TECHNIQUE

ASSESSMENT OF BENTHIC FLUX OF DISSOLVED ORGANIC CARBON IN ESTUARIES USING THE EDDY-CORRELATION TECHNIQUE

By: Michael P. Swett

Thesis Advisor: Dr. Aria Amirbahman

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Civil Engineering)

May, 2010

Dissolved organic carbon (DOC), which typically originates from the breakdown of organic matter, is a water quality concern in estuarine environments, as DOC facilitates transport of metals and organics in sediments and creates toxic byproducts during drinking water treatment.  Benthic DOC flux, or the rate of exchange of DOC across the sediment-water interface (SWI), can vary with changing environmental conditions, including wetland restoration and rising sea levels.  Therefore, it is important to quantify flux of DOC across the SWI.  Existing DOC flux measurement techniques, such as measurement from porewater extraction, are intrusive to the sediment environment and underestimate flux by only capturing certain flux contributions.

In this research, methods for estimating DOC flux using the eddy correlation 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 flux measurement, is a non-intrusive, field-based method that relies on measurement of turbulent fluctuations of properties such as fluid velocity, contaminant concentration, and temperature.  The methods employed here utilized vertical velocity measured with an acoustic Doppler velocimeter (ADV) and DOC concentrations approximated with a chromophoric dissolved organic matter (CDOM) fluorometer.  Two statistical methods were investigated for separation of turbulent fluctuations from mean flow in the velocity and concentration data, and another mathematical method was used to analyze flux contribution in the frequency series, as an alternative to traditional time series analyses.

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 fluxes calculated from porewater extraction, a technique that is thought to underestimate flux, as turbulent fluctuations dominate 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 complex environments such as the ones studied here.  In addition, frequency analysis shows that DOC flux estimates can be compromised by high-frequency noise caused by particle contamination of the CDOM fluorometer measurements.

Keywords: dissolved organic carbon; eddy correlation; benthic flux; sediment-water interface; acoustic Doppler velocimeter; CDOM fluorometer; turbulence