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Stratigraphic and dynamic effects of sediment reworking by Great Lakes zoobenthos
Authors:Robbins  J A
Institution:(1) National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory, 2300 Washtenaw Avenue, 48104 Ann Arbor, MI, USA
Abstract:Recent sediments of the North American Great Lakes are inhabited by numerous species of macrobenthos which alter the physical and chemical properties of sediments and modify interface transport characteristics. Distributions of such radionuclides as cesium-137, lead-210, and isotopes of plutonium exhibit a zone of constant activity extending down from the sediment-water interface from 1 to 15 cm. Recent studies have confirmed that radiometrically determined mixed depths are consistent with the vertical distribution of oligochaete worms and the amphipod,Pontoporeia hoyi. Generally, 90% of the benthos are contained within the radiometrically defined mixed zone. Where comparisons are possible, rates of sediment reworking by ‘conveyor belt’ species are comparable to or exceed sedimentation rates. Systematic variations in the mixed depth occur within depositional basins with greatest depths tending to be associated with least consolidated, organically rich materials. A quantitative steady-state mixing model accounts satisfactorily for observed radioactivity and heavy metal profiles. Bioturbation appears to be an important process, limiting the resolution with which historical records of particle-associated contaminants may be reconstructed from sediment cores. As bioturbation serves to maintain contact of contaminated sediments with overlying water, this time may also characterize the long-term lake recovery for contaminants removed by burial. As the time varies with location, a mean for an entire lake is not well known, but is on the order of 20 years for Lake Huron. Contribution No. 300 of the Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan.
Keywords:zoobenthos  Great Lakes  sediments  radionuclides
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