Effects of selected pollutants on grazer utilization of Aufwuchs

The trophic level impact of structural changes in Aufwuchs communities resulting from low levels of stress can be assessed through analyses of the nutritive value of the microfloral community in conjunction with measurements of grazer consumption rates and assimilation efficiencies. Artificial streams dosed with either intermittent chlorination (20 minute doses 3 times per day) or continuous treatments of copper (0.05 ppm) or dextrose (1 or 2 ppm) were used to obtain Aufwuchs communities from stressed environments. Aufwuchs communities were also sampled from the New River within and around a chlorinated-thermal, power plant discharge. Food quality analyses included quantification of organic carbon, protein, carbohydrate, and caloric content through gravimetric, Coomassie blue staining, anthrone staining, and wet chemical oxidation procedures, respectively. Snail (Pleuroceridae:Anculosa) utilization of Aufwuchs from the variously perturbed environments was assessed through laboratory feeding studies employing radiolabeled (sulfur-35) Aufwuchs to determine snail feeding rates and assimilation efficiencies.

The heterotrophically dominated Aufwuchs communities developing under the dextrose enrichment contained more protein, carbohydrate, and calories compared to reference communities. These increases were attributed to the copious extracellular mucilage associated with the bacteria. There was no significant change in snail consumption of Aufwuchs from the dextrose enriched streams, although this community was consistently assimilated with greater efficiency relative to reference communities. When Aufwuchs developing under the chlorine or copper treatments became dominated by blue-green algae, the protein content of these communities increased as a result of the algal proteinatious sheath. Carbohydrate content was generally less than reference values for Aufwuchs developing under chlorine or copper stress. Aufwuchs from these treated streams were consumed to a less extent than reference communities and assimilated with 2 to 12% less efficiency. These results agree with literature reports that blue-green algae are a less preferred food for aquatic grazers and are assimilated with less efficiency. Aufwuchs sampled from habitats influenced by the power plant discharge were of less nutritive value compared to samples from uninfluenced stations. Snails consumed more of the reference communities and assimilated them with greater efficiency compared to Aufwuchs influenced by the power plant discharge.

Both the autotrophic and heterotrophic components of the Aufwuchs community were labeled with sulfur-35, allowing sensitive measurements of the feeding parameters. Snail feeding studies demonstrated that low levels of stress may not alter grazer consumption of Aufwuchs until substantial changes in algal composition occur. Snail assimilation efficiency was generally a sensitive parameter in assessing trophic level effects of changes in Aufwuchs communities resulting from pollutant impacts. Food quality analyses provided useful information regarding changes in Aufwuchs community structure. A lack of correlation among Aufwuchs food quality parameters and snail utilization measurements reflected the opportunistic feeding strategy of the grazer.