THE INFLUENCE OF ZOOBENTHOS ON FINE-GRAINED PARTICLE REWORKING AND BENTHIC SOLUTE TRANSPORT IN GREAT LAKES SEDIMENTS

The effects of freshwater benthic macroinvertebrates on the reworking of sediments and the transport of solutes across the sediment-water interface were studied in laboratory microcosms using gamma-ray spectroscopy techniques. The objectives of the study were to systematically determine the particle reworking modes and rates for five dominant Laurentian Great Lakes taxa, to determine whether any relationship exists between benthic fluxes and sediment reworking, and to determine the usefulness of a dual radiotracer system in studying simultaneously the major physical components of sediment and solute transport.;Natural sieved sediment in rectangular plexiglass cells, stored in a temperature regulated aquarium, was labeled with Cs-137, which binds strongly to sediment particles (K(,D) = 2000). Other sediment cells were labeled with Cs-137 and with Na-22, which is weakly coupled to sediments (K(,D) = 0.5). Vertical distributions of these gamma-emitting isotopes were determined at daily to weekly intervals for approximately three months using a well-collimated Na-I detector. Both Na-22 (an indicator of solute movement) and Cs-137 (an indicator of particle transport) moved downward into the sediments in response to organism activity in the cells.;In three replicate cells containing the annelid Stylodrilus heringianus (6.7 x 10('4) individuals m('-2)) a layer of Cs-137 placed at the sediment surface was buried at an initial rate of 0.12 (+OR-) 0.005 cm day('-1) (11.6 (+OR-) 0.2(DEGREES)C) although the overall Cs-137 burial rate was best described by an exponential decay function. The downward movement of Na-22, characterized as a simple eddy-diffusional process, implied an average diffusion coefficient, D(,b), of 1.33 (+OR-) 0.07 x 10('-5)cm('2)sec('-1). Subsequent analysis of Stylodrilus feeding distribution measured in two replicate cells with ten sediment layers labeled with Cs-137 at 1 cm intervals revealed decreasing burial velocity with increasing depth as well as uniform feeding between 0 and 4.9 cm sediment depth.;In two replicate cells with the amphipod Pontoporeia hoyi (4.7 x 10('3) individuals m('-2)), D(,b) was 1.43 (+OR-) 0.25 x 10('-5)cm('2)sec('-1) (10.4 (+OR-) 0.2(DEGREES)C). The downward smearing of Cs-137 labeled sediment particles by amphipods is well characterized as a non-advective eddy diffusive process with an eddy diffusion coefficient, K(,B), of 1.24 (+OR-) 0.17 x 10('-7)cm('2)sec('-1). In control cells with no animals, K(,B) = 0.0 while D(,B) = 0.95 (+OR-) 0.02 x 10('-5)cm('2)sec('-1). In a special control cell with worm tubes but no live worms, there was no enhancement in the rate of solute transport. In other cells with the tubiculous chironomid Chironomus tentans (4.0 x 10('3) organisms m('-2)), and the bivalves Pisidium sp. (4.0 x 10('3) organisms m('-2)) and Sphaerium sp. (2.7 x 10('3) organisms m('-2)), no significant movement of labeled particles occurred.;Thus, the presence of either Stylodrilus or Pontoporeia markedly alters the distribution of sediment particles near the sediment-water interface and enhances the transport of solutes into sediments (('(TURN))50% enhancement). Moreover, Stylodrilus is shown to feed uniformly within the uppermost 4 to 5 cm of sediment and its enhancement of solute transport is primarily due to its dynamic interactions with sediments rather than to alterations in the sediment matrix.