Biological mixing responses to sublethal concentrations of DDT in sediments by Heteromastus filiformis using a 137Cs marker layer technique

Abstract

Sediment mixing by benthic macroinvertebrates is an important process affecting the fate of sediment-bound and dissolved contaminants in marine environments. A non-invasive, state- of-the-art radiotracer technique was used to study sediment mixing by Heteromastus filiformis (Capitellidae), a common marine head-down deposit feeder, exposed to several sub-lethal concen- trations of DDT (0, 5, 10 and 20 µg g -1 ; control, Treatments 1, 2 and 3). Several horizontal sub- millimeter layers of 137 Cs-labeled clay were deposited approximately every 2 cm in each of 3 repli- cate sediment columns per treatment; 4 polychaetes were then introduced to each column and the γ activity of each column was measured vertically using an automated scan detector. Nonlinear least- square fits were applied to obtain parameterized values that were used to determine the mixing rates of each 137 Cs layer over time. A simple diffusion model was used to calculate biological diffusion coef- ficients (Db) for H. filiformis. Overall mixing rates increased towards the surface. Control and Treat- ment 1 had higher Db values at the surface compared to Treatments 2 and 3. The Db depth profiles were similar in the control and Treatments 1 and 2, with mixing occurring at the sediment water interface and a subsurface maximum at 10 to 12 cm below this interface. This pattern was not clear in Treatment 3, where Db had the lowest values and decreased with depth. Bioturbation besides mix- ing of solids also changed the water content throughout the sediment column. Porosity profiles at the end of the experiments increased by 10 to 20% at 10 to 12 cm depths compared to above and below this horizon. The DDT depth concentration profiles decreased towards the surface as a result of the mixing by the benthic macroinvertebrates, clearly indicating removal/uptake by the organism. The feeding rate constant (γb, % h -1 ) in the control showed a maximum at 7 to 12 cm. However, the γb in the treatments was essentially constant with depth. For all treatments and the control, the burial rate (Wb) (downward movement of radiolabeled layers) decreased with depth. The surface layers were buried faster (ANOVA, p < 0.05) in the control than in sediments containing DDT. A sensitivity ana- lysis comparing burial rate, Db, γb (surface only) and worm weights showed that worm weights and burial rate have the highest fractional rate changes per µg g -1 DDT.