Variation in surface turbulence and the gas transfer velocity over a tidal cycle in a macro-tidal estuary

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Abstract:

The gradient flux technique, which measures the gas transfer velocity (k), and new observational techniques that probe turbulence in the aqueous surface boundary layers were conducted over a tidal cycle in the Plum Island Sound, Massachusetts. Efforts were aimed at testing new methods in an estuarine system and to determine if turbulence created by tidal velocity can be responsible for the short-term variabitity in k. Measurements were made during a low wind day, at a site with tidal excursions of 2.7 m and a range in tidal velocity of nearly 1 m s(-1). Estimates of k using the gradient flux technique were made simultaneously with the Controlled Flux Technique (CFT), infrared imagery, and high-resolution turbulence measurements, which measure the surface renewal rate, turbulent scales, and the turbulent dissipation rate, respectively. All measurements were conducted from a small mobile catamaran that minimizes air- and water-side flow distortions. Infrared imagery showed considerable variability in the turbulent scales that affect air-water gas exchange. These measurements were consistent with variation in the surface renewal rate (range 0.02 to 2 s(-1)), the turbulent dissipation rate (range 10(-7) to 10(-5) W kg(-1)), and k (range 2.2 to 12.0 cm hr(-1)). During this low wind day, all variables were shown to correlate with tidal speed. Taken collectively our results indicate the promise of these methods for determining short-term variability in gas transfer and near surface turbulence in estuaries and demonstrate that turbulent transport associated with tidal velocity is a potentially important factor with respect to gas exchange in coastal systems.