Frederick M. Soster

Suspended sediment sources and transport distances in the Yellowstone River basin

The activity of fallout radionuclides ( 7 Be, 1 3 7 Cs, and 2 1 0 Pb) was measured on upland and floodplain soils and on suspended sediments to quantify sources of fine sediment and to estimate sediment transport distances in stream channels in the Yellowstone River basin. Samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of Soda Butte Creek to Billings, Montana, a 423-km-long reach of channel. The inventory of radionuclides in soil increases with precipitation and is highest in the headwaters. The activity of radionuclides in suspended sediment decreases downstream, and more activity is observed earlier than later in the flood hydrograph. The radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion. Fine suspended sediment has an intermediate radionuclide signature that is quantified in terms of the relative contribution of these two sources of fine sediment. At sites high in the drainage, soils contribute 50% to the suspended load and this value decreases to 11%-26% downstream. Fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source. Transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream. These estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure. This study of a large watershed confirms earlier results from smaller basins and suggests that transport distances increase with basin size.

The Effects of Size-Selective Feeding by Oligochaetes on the Physical Properties of River Sediments

ABSTRACT Tubificid oligochaetes selectively ingest silt- to clay-sized particles at depth within the substratum, transport them vertically upward through their gut, and deposit them as feces at the sediment-water interface. These activities form three distinct sedimentary layers. The sediment-water interface becomes covered with sand-sized fecal pellets. A silt-clay layer forms directly below this. The third layer is a sandy concentrate that represents the zone of tubificid feeding. The upper, pelletized layer is enriched in water content and organic carbon. The high water content of this layer, its irregular surface, and the low density of the constituent pellets destabilize the sediment surface and increase its erodability. In addition, the coarse-to-fine layered structure of the deposits fo ms distinctive biogenic graded bedding that is a potentially useful indicator of low current velocities and low rates of inorganic sediment accumulation in ancient fluvial environments.

The effect of tubificid oligochaetes on the uptake of zinc by Lake Erie sediments

A laboratory experiment was conducted to determine the effect of tubificid worms on the flux of zinc into lake sediments. Forty-six cores of Lake Erie sediment, with and without (control) tubificid worm populations, were exposed to aquarium water with a zinc concentration of about 5 mg 1−1 for 139 days. Pore water and exchangeable particulate zinc concentrations in the top 12 cm of sediment were periodically determined in pairs of cores — one with worms and one without worms — at 1 cm depth increments. After 139 days, pore water zinc concentrations in sediments with and without worms were nearly identical in the 0–1 cm interval (4.1 and 4.3 mg 1−1 respectively), but were significantly greater in the sediments with worms in the 1–2 cm (4.4 vs. 0.3 mg1−1) and the 2–3 cm (1.3 vs. 0.3 mg 1−1) intervals. Exchangeable particulate zinc concentrations in the 0–1, 1–2, and 2–3 cm intervals in sediments with worms were 612.3, 750.7, and 191.5 µg g−1 dry sediment respectively, whereas in sediments without worms, concentrations were 375.4, 5.9, and 3.2 µg g−1 dry sediment. The increased flux of zinc into tubificid-inhabited sediments was caused by the ‘conveyor belt’ feeding activity of the worms, which continuously exposed sedimentary particles to the overlying water. Movement of zinc into sediments with worms was dominated by adsorption and by particle movement, whereas movement of zinc into control sediments was by adsorption at the sediment-water interface and diffusion. The increased concentration of zinc in tubificid-inhabited sediments has important implications with respect to the trophic transfer of zinc through the aquatic food chain.

Predation by Coelotanypus (Diptera: Chironomidae) on Laboratory Populations of Tubificid Oligochaetes

Laboratory experiments were conducted with sediments and organisms collected from Lake Eries western basin to examine the effect of Coelotanypus (Chironomidae: Diptera) predation on tubificid oligochaetes. Populations of predator and prey organisms were kept together in replicate laboratory microcosms at densities ranging from 2,500–12,500 predators/m2 and 12,500–62,500 prey/m2 for 15- to 30-day periods. Control populations of predators and prey were maintained separately for comparative purposes. A separate selection experiment showed that larvae preferred sediments containing tubificids or previously settled larvae over sediments that contained neither. Mortality in worm populations kept with predatory Coelotanypus larvae averaged 56.9% (S.D. = 3.9%) and was significantly higher (p < 0.001, t-test) than the 9.7% (S.D. = 3.1%)average mortality in control populations. Simulated effects of chironomid burrowing and burrow irrigation did not cause increased worm mortality. An average predation rate of 0.11 (S.D. = 0.04) prey·predator−1·day−1 was calculated from the data. Based on average Coelotanypus and tubificid densities in western Lake Erie and assuming that natural feeding rates are comparable to those in the laboratory, Coelotanypus may be capable of reducing tubificid populations by about 20% over a 2-month period during the late summer. Natural abundance data indicate that tubificid population declines are associated with periods of peak Coelotanypus density.