Solute transport in sediments by freshwater infaunal bioirrigators

Laboratory experiments using the radionuclide Na-22 as a solute tracer were conducted in microcosms containing the freshwater chironomid larvae Coelotanypus sp. and Chironomus plumosus and the mayfly larvae Hexagenia limbata to determine the exchange of solutes between sediments and overlying water. Three different mathematical models of transient solute transport in bioturbated sediments were applied to the data to evaluate which best quantified solute exchange and to determine how that exchange varied with faunal type and density. Although all three species of larvae constructed burrows and irrigated them, the larger H. limbata frequently abandonded their burrows and vigorously burrowed the sediment. As a result, an enhanced diffusion model, in which the solute diffusion coefficient is higher in the bioturbated zone than in unmixed sediments, best described the data for the larger H. limbata. The enhanced diffusion model generally underestimated solute transport by smaller H. limbata, C. plumosus and Coelotanypus sp. larvae because it did not account for the effects of burrow irrigation. Both the cylindrical burrow model and the nonlocal exchange model with a constant value of the exchange coefficient in the mixed layer described the transport processes well, presumably because the smaller H. limbata, C. plumosus, and Coelatanypus sp. maintained and actively irrigated their burrows. At natural bottom faunal densities, C. plumosus induced the greatest exchange of solutes between sediment and overlying water, followed by the small H. limbata, the large and medium H. limbata, and Coelotanypus. However, H. limbata enhanced the flux by about an order of; magnitude more than did C. plumosus and by about a factor of 20 times that of Coelotanypus on an individual basis.