Vertical distribution of feeding and particle‐selective transport of 137Cs in lake sediments by lumbriculid oligochaetes

Abstract

Lumbriculid oligochaetes are an important member of the conveyor belt deposit‐feeding species responsible for advective reworking of profundal sediments in the Great Lakes. These threadlike worms ingest sediments over a range of depths (0–5 cm) and deposit gut contents at the sediment surface. This action results in a depth‐dependent particle advection rate which can be related to the distribution of their feeding. In this study, two replicate microcosms of rectangular cross section (3×5 cm) containing evenly spaced (1 cm) thin layers (0.5 mm) of sediment labeled with gamma‐emitting 137Cs were inoculated with the lumbriculid Stylodrilus heringianus at a density of ∼7.9×104 m−2 (10°C). The subsequent amount of tracer lost from each layer as well as the downward movement of the layers were determined by peiodically scanning the length of the cells with a well‐collimated gamma detector system. During the course of the 600‐hour experiment, 137Cs activities decreased only in layers between 0 and 6 cm depth. These decreases were accompanied by the appearance of a new layer at the sediment surface. Between 80 and 90% of the activity lost from the underlying layers appeared in the emergent surface peak, thus indicating that subsurface defecation was minor aspect of conveyor belt transport. Decreases in 137Cs activity within layers were consistent with first‐order kinetics with a maximum loss rate of about 0.08% hour−1. The vertical distribution of the rate constant was very nearly Gaussian with the maximum at 3.3 cm and a spread (SD) of 1.4 cm. The rate of layer burial decreased monotonically but nonlinearly with sediment depth from an extrapolated surface value of about 1.8×10−2 mm h−1. A model based on conservation of mass was developed which relates the tracer feeding distribution to the velocity distribution assuming that the distribution of mass removed through feeding was proportional to the amount of tracer removed. The model takes rigorous account of depth‐dependent porosity and provides an excellent description of the velocity distributions if the transfer of 137Cs is taken to be 1.67 times the mass transfer rate. This factor, termed the feeding selectivity factor, is attributed to the selection of fine particles by worms and is important in models describing trace and contaminant diagenesis in sediments reworked by conveyor belt species.