The relationship between drift rates, benthos density, and food level for select taxa of stream invcrtebratcs was studied in an artificial stream system to dctermine if drift is independent of or dependent on population density, and if food level affects drift rates. Two taxa were introduced in a range of densities at a single level of periphyton food, and the drift from each density was measured. Four taxa were introduced in a range of densities at low and high food levels, and the drift from each density at each food level was measured.The drift of all taxa, examined by simple linear regression analysis, at each food level was judged a density independent response to increasing population density. The drift of three taxa at a given density was significantly greater at the lower food level: it is suggested that increased activity in searching for food at the lower food level resulted in increased drift.

Hypothetical populations of aquatic insects were periodically sampled, using a computer model, to estimate production by the instantaneous‐growth, removal‐summation, and Hynes/Hamilton methods. These estimates were compared with the production values calculated from the daily growth of all individuals in the populations. The removal‐summation method yielded the most accurate estimates for a variety of growth curves and sampling regimes and appeared to be the least sensitive to violations of the growth assumptions of the method.

A method is presented for extrapolating laboratory toxicity data to aquatic ecosystem effects such as decreased productivity or reduction in game fish biomass. The extrapolation requires translating laboratory data into changes in the parameters of an ecosystem model, the Standard WAter COlumn Model (SWACOM). The translation is effected through knowledge of toxicological modes of action. The uncertainties associated with both laboratory measurements and extrapolations are explicitly retained, and risk estimates are given in the form of probabilities that an effect could occur. The approach is illustrated by scenarios in which effects of toxic substances are distributed across different trophic levels. Each scenario affects population interactions in different ways and alters both the level and the nature of the risks to ecosystem processes. Particular attention is paid to analyzing the interaction between toxicity and the uncertainties associated with extrapolation.

This study was undertaken to determine what effects a spawning run of coho salmon would have on the benthos of the Platte River, Benzie County, Michigan. It was hypothesized that the physical disruption of the substrate, by spawning salmon, would reduce the density of benthos by dislodging the organisms and causing their downstream displacement. Estimates of density of benthic invertebrates were made in two control sections (salmon excluded) and in one experimental section (salmon having free access) in May and August 1967 prior to coho spawning run, and in December 1967 and May 1968 after the run. Coho spawning activity in the fall of 1967 disturbed the bottom materials. The densities of 12 taxa, as well as the total number and total weight of organisms per square foot, decreased significantly in samples collected in December after disruption. The decrease in total nmnber of organisms per square foot in the experimental section as compared to the controls was 270.5; the decrease in weight in the experimental section was 1.97 grams. This is a percent decrease in the experimental section relative to the controls of 66% for total number and 78% for total weight. In May 1968, the nmnber of organisms per square foot was still significantly lower in the experimental section (57.4 per square foot), but weight of organisms per square foot was not. The percent decrease in the experimental section relative to the controls for total number was 39%. Three of twelve taxa, whose density was reduced in the fall of 1967, were still lower in May 1968. Complete recovery of benthos was not observed in the 5-month period from December to May.

Larvae of Tipula spp., a detritus-feeding aquatic insect collected from an unpolluted spring-fed stream, were analyzed for 30 elements before and after gut evacuation to determine the contribution of gut contents to whole-body concentrations. Chromium and Al concentrations were inversely related to body size, suggesting surface contamination. All elements analyzed except Na, Cr, Hg, Zn, and Se were significantly lower (P < 0.01) in concentration after gut evacuation (18% decrease for K to approximately 70% for V, Mn, Hf, and some of the rare earths). Ratios of variances of mean concentrations in organisms with guts filled to those with guts evacuated indicate that gut contamination is a major source of variation in measured whole-body concentrations of many elements. Calculated percentage of element body burden associated with gut contents ranged from < 1% for Na to 89% for Zr, with a mean for all elements analyzed of 57 ± 8% (± 2 SE). Calculated trophic transfer factors (TTF = element concentration in larvae with guts evacuated/concentration in leaf detritus) for elements which are obtained primarily through the food chain were significantly > 1 only for Zn; TTFs for all other elements were [Formula: see text].