Influence of benthic macroinvertebrates on mixing of profundal sediments in southeastern Lake Huron1

Vertical distributions of size classed benthic macroinvertebrates were investigated in centimeter-thick sections of twelve cores from each of two stations in southern Lake Huron. Stations were selected on the basis of radioactivity profiles indicating surf&e sediment mixing to 3 and 6 cm. The corresponding occurrence of major taxonomic groups (amphipods and oligochaetes) to 2 cm at the first station and to 4 cm at the second suggests that these organisms play a significant role in sediment redistribution. Comparison of published tubificid defecation rates with sedimentation rates determined radiometrically indicates the extent to which benthic macroinvertebrates can mix surficial sediments. Profiles of natural and artificial radioactivity in recent sediments of the Great Lakes suggest that postdepositional movement of material occurs within the upper 10 cm of sediments (Robbins and Edgington 1975). Robbins and Edgington (1975) and Robbins et al. (1977) have proposed that the observed redistribution of sediment particles and their strongly sorbed radionuclides results from either physical mixing (resuspension) or bioturbation. Larger benthic invertebrates mix both lacustrine and marine sediments (Davis 1967, 1974u,b; Rhoads 1963, 1974). But the vertical distribution of deposit feeders in profundal sediments of the Great Lakes and the fattors which determine their vertical distribution remain largely uninvestigated. We measured the vertical distribution of benthic macroinvertebrates at two stations in Lake Huron (Table l), selected on the basis of differing thicknesses of the mixed layer determined radiometritally, to see whether densities of benthos at these stations were sufficient to ac1 This research was supported by EPA grant R803086 to C. L. Schelske, Great Lakes Research Division, and constitutes part of a M.S. degree by J. R. K. Contribution 230 of the Great Lakes Research Division, University of Michigan. count for the rate of mixing implied by the radioactivity data. We collected our samples with a Benthos gravity corer having a butyrate-plastic liner (area, 35.5 cm’), during August 1975, in an extensive area of fine-grained recent deposits in southeastern Lake Huron (Goderich Basin: Thomas et al. 1973). We lowered the corer without core cutter or retainer at 1 me s-l (less than free-fall velocity) to limit disturbance of the surficial sediments (Flannagan 1970; Milbrink 1973). Our ship did not anchor during sampling but maneuvered on station between casts. On-deck observations of intact chironomid tubes at the mud-water interface within the core liner offered strong evidence of limited sediment disturbance by the corer; however, we must agree with Milbrink (1973) that little can be done to prevent animals from moving in response to the shock of the corer entering the sediments. Our coring procedure smeared a small percentage of sediment and benthos along the inside surface of the tube, especially since no piston was used, and our subsequent handling of the cores during sectioning may have altered the in situ distribution. We believe that these components of the observed vertical distribution are probably not great enough to invalidate our results, however, since we found the greatest densities of organisms within the uppermost 6 cm of sediment (Fig. l), as did Brinkhurst et al. (1969) and Milbrink (1973) using different sampling devices. We determined sample size (twelve cores per station) following Elliott’s (1971) index of precision and using earlier data (S. C. Mozley unpubl.) on populations collected from cores of Lake Michigan sediments. At station 14, which is centered in the northern area of Goderich Basin, the sediments consist of brown, flocculent material in the upper 4 cm, overlying firm,