SOLUTE TRANSPORT IN SEDIMENTS BY A LARGE FRESH-WATER OLIGOCHAETE, BRANCHIURA-SOWERBYI

Laboratory experiments using the radionuclide Na-22 as a solute tracer were conducted in microcosms containing the freshwater tubificid olig ochaete, Branchiura sowerbyi, to determine the exchange of solutes bet ween sediments and overlying water. Three different mathematical model s of solute transport in sediments are applied to the data to evaluate which modeled processes best quantifies solute exchange by B. sowerby i and how that exchange is affected by worm density. An enhanced diffu sion model, in which the solute diffusion coefficient is higher in the bioturbated zone than in unmixed sediments best described the data. A t population densities of 4000 and 8000/m(2), the effective diffusivit y of Na-22, D-e, is 1.87 and 4.78 times that in the absence of the wor ms, respectively (91.69 and 234.9 cm(2)/yr as compared to 49.17 cm(2)/ yr in the control). A cylindrical burrow model does not describe the t ransport processes well, presumably because B. sowerbyi does not activ ely irrigate its burrows and abandoned burrows do not remain open for fluid exchange with the overlying water. A nonlocal exchange model wit h a constant exchange coefficient within the mixing layer also was fou nd to be inadequate to describe solute transport. A depth-dependent ex change coefficient might yield better agreement with the data but woul d still be inferior to the enhanced diffusion model. The nonlocal exch ange model and the cylindrical burrow model yield similar results as p redicted by Boudreau (J. Mar. Res. 1984, 42, 731-735).