Mass transport to streambed biofilms

This study developed a mass transfer equation for the movement of biodegradable materials from the water column of shallow streams to the biofilm-colonized surfaces of sand-free gravel and cobble streambeds. The equation was developed from a series of well-controlled, batch-biodegradation tests performed in an artificial stream with streambeds between 1.5 to 2 rock layers thick. The batch tests determined the sensitivity of substrate flux into the streambed biofilms to short-term changes in water velocity. Experimental results indicated that substrate flux into the cobble-streambed biofilms was more sensitive than flux into the gravel-streambed biofilms to short-term changes in water velocity, and that the cobble streambed had faster removal rates. Rates of substrate removal by both streambeds were much more sensitive to short-term changes in water velocity than would be predicted by previous mass transport models, which only addressed the transport of materials from the water column to the external (exposed) surface of the streambed. This greater than expected sensitivity to changes in water velocity suggests that interstitial biofilms play an important role in determining substrate removal rates in streams with sand-free gravel and cobble streambeds.