Relationships between nontaxonomic aufwuchs community structure and functions were examined over a variety of environmental or physiological conditions in model streams to ascertain any direct relationships or covariance between indices of structure (dry weight, ash free dry weight, adenosine triphosphate and chlorophyll a) and function (primary productivity and assimilatory sulfate reduction). In eleven experiments, individual streams were treated with potential"enrichments" of sucrose, dextrose, ammonium nitrate, potassium dihydrogen phosphate, and toxicants including copper sulfate, potassium dichromate and chlorine, in order to elicit responses by the aufwuchs communities that accumulated on artificial substrates. The carbon-14 method was adapted and refined using a chamber for measurements of primary productivity in lotic systems. Assimilatory (sulfur-35) sulfate reduction was investigated as a method for estimating net microbial production rates. The ability of indices or ratios of structural and functional values to reduce variability and identify perturbed model streams relative to untreated streams was evaluated. In laboratory studies, assimilatory sulfate reduction closely followed growth rates in batch cultures of algae (Stigeoclonium sp., Navicula pelliculosa and Oscillatoria tenuis), bacteria (Pseudomonas flurescens, Enterobacter aerogenes and Bacillus subtilis) and fungi (Penicillium clavigerum, Mucor hiemalis and Coprinus patouillardii). Linear uptake kinetics of sulfur-35 sulfate during a six hour incubation period indicated that the process was active or energy mediated. Patterns of algal sulfate assimilation closely paralleled those for primary productivity. Bacterial and fungal sulfate assimilation rates were generally lower in the light than in the dark although the differences were not statistically significant. Observed rates of assimilatory sulfate reduction by cultures of algae, bacteria and fungi were affected by concentrations of organic carbon (glucose) in the media; at concentrations of glucose greater than 5 mg l⁻¹, dark sulfate uptake by algae was 77-98% of light uptake rates, and at glucose concentrations greater than 50 mg l⁻¹, bacterial and fungal sulfate assimilation rates did not increase proportionately with increasing glucose concentrations. The concentration of organic sulfur as as cysteine and methionine, that caused a decrease in the rates of sulfate assimilation, was between 10 and 100 S l⁻¹ for both the cultures and mixed populations in microcosms. No extracellular sulfur-containing products were found to be excreted by mixed populations in microcosms, and no"luxury" consumption of sulfur was observed. The use of assimilatory sulfate reduction rates to estimate net microbial productivity in aerobic systems was supported by the results from investigations with the cultures and microcosms. A reduction in accumulated matter (as indicated by dry weight, ash-free dry weight and ATP) and chlorophyll a on artificial substrates occurred in toxicant (copper, dichromate and chlorine) treated streams. Photosynthesis (carbon fixation) was most sensitive to the copper treatment and assimilatory sulfate reduction was affected most often by dichromate. The disaccharide, sucrose, significantly increased accumulated matter on substrates, but dextrose did not elicit similar responses. Phosphate and ammonium nitrate additions did not cause significant increases in aufwuchs production and were not considered to be limiting in this system. Net microbial productivity of aufwuchs and primary productivity were estimated by assimilatory sulfate reduction and carbon-14 fixation, respectively, with heterotrophic productivity being the difference. Overall, the reduced carbon compounds (sugar) and the toxicants resulted in stream aufwuchs communities dominated by heterotrophs. Indices used for condensation of information in the model stream experiments were generally unsuccessful in discerning perturbations. From all of the model stream experiments, measurements of functional parameters or indices calculated from functional measurements were significantly less variable than structural measurements or structurally based indices The perturbations or treatments did not reduce variability in the nontaxonomic structural and functional aspects measured. Direct correlations were found between: 1) dry weight and ash-free dry weight, 2) ash-free dry weight and assimilatory sulfate reduction, 3) ash-free dry weight and chlorophyll a, 4) chlorophyll a and assimilatory sulfate reduction, 5) chlorophyll a and primary productivity, 6) primary productivity and assimilatory sulfate reduction, and 7) and primary productivity and net microbial productivity. Field investigations of aufwuchs communities in the vicinity of a fossil-fueled power plant on the New River, Virginia, were inconclusive Variability in measurements overshadowed any differences in nontaxonomic structural and functional aspects that might have been attributed to an intermittently chlorinated thermal effluent. / Ph. D.