Four experiments were performed to test the feasibility of using taxonomic composition and abundance of attached algae to identify treatments of zinc (Zn) alone and in combination with treatments of phosphate, snail grazing, and pH. In the experiment presented in chapter 2, three treatments of zinc (0.05, 0.5, 1.0 mg Zn•l⁻¹) and a control could be identified by different algal communities in outdoor, flow-through, stream mesocosms. Established communities were continuously exposed to Zn, and samples were collected on days 0, 2, 5, 10, 20, and 30 after treatment began. Experiments were conducted in spring, summer, and fall 1984. Control stream mesocosms could be identified by diatoms in all seasons. The 0.05 mg Zn•l⁻¹ treatment could be identified by certain diatom taxa being more abundant than in the control in all seasons and by a filamentous green·alga in summer and fall. The 0.5 mg Zn•l⁻¹ treatment could be identified a filamentous green alga in fall. The 1.0 mg Zn•l⁻¹ treatment could be identified by unicellular green-algae in all seasons and by a filamentous blue-green alga in summer. A similarity index (SIMI) indicated that Zn stressed samples generally became less similar to control samples as Zn concentration increased from 0.05 to 1.0 mg Zn•l⁻¹. Total biovolume·density of all taxa responded more slowly than did individual taxa in spring and failed to distinguish between Zn treatments in summer and fall. Zn bound to periphyton (microbial community on solid substrates) was more reliable than total Zn in water for identifying Zn treatments. Zn treatments as low as 0.05 mg Zn•l⁻¹ changed algal species composition. This conflicts with the criterion (0.047 mg Zn•l⁻¹) of the U. S. Environmental Protection Agency for the 24-hour average of total recoverable Zn. ln the experiment presented in chapter 3, individual and combined effects of phosphate (P) and zinc (Zn) on the abundance of dominant algae and protozoa in a community were observed. Nutrient·diffusing artificial substrates were colonized in Douglas Lake, Michigan, and then placed in laboratory microcosms containing one of five Zn treatments (control, 0.1, 1.0, 3.0, and 10.0 mg Zn•l⁻¹). After one week of exposure in the laboratory the substrates were scraped and algal and ciliated protozoan abundances determined. Ten of thirteen algae and five of eight ciliated protozoa responded to experimental treatments. Some algae (diatoms and green algae) and ciliated protozoa were stimulated by high P, some stimulated by intermediate P, and some inhibited by high P. One alga and four protozoa responded positively to Zn. Two algae and three protozoa responded to a significant interaction between P and Zn so that abundances were from 3 to 19 times higher than the added effects of individual P and Zn treatments. Total algal abundance was increased by high P and total protozoan abundance was increased by intermediate P but at control levels for high P. The number of protozoan species was increased by P. Total algal abundance was increased by · combinations of Zn P and the number of protozoan species was decreased by Zn P. Altered abundance by combinations of Zn and P had not been demonstrated for a community of algae and protozoa previously. Although concentrations of Zn were initially above the level considered safe by the U.S. Environmental Protection Agency, many factors may prevent Zn stress. In the experiment presented in chapter 4, effects of 0.5 mg Zn•l⁻¹ and snail grazing (400 snails m⁻²) on density of dominant algal taxa were examined using established (12-day colonization) periphyton communities in flow-through stream mesocosms with four treatments (Zn, snails, Zn and snails, control) for 30 days. Grazing and Zn similarly reduced the abundance of 5 of 10 dominant taxa during the first 10 days of treatment. Temperature may play a very important role in determining the effect of snail grazing on attached algal communities. Cold temperatures (< 15 C) may have inhibited snail grazing to the extent that abundance of four taxa increased to levels found in non·snail treatments. However, one diatom was more than twice as abundant in snail treatment over non-snail treatment -- apparently stimulated by the presence of snails during cold conditions; and two diaoms remained at low abundance in snail treatment despite rapid growth in non-snail treatment -- apparently inhibited or selected as a food source by snails during cold con- ditions. No algal taxa replaced the diatoms inhibited by 0.5 mg Zn•l⁻¹ in this October-November, 1984, experiment by day 10. This is in contrast to an experiment performed one month earlier, in September-October, in which a community characteristic of this treatment developed by day 5. Testing individual and combined variables that affect attached algal communities will enhance understanding of population dynamics in algal ecology and pollutant assessment. In the experiment presented in chapter 5, attached-algal communities were employed to test the US Environmental Protection Agency’s (USEPA) guidelines for zinc (Zn) and pH. The experiment was designed to determine whether algal community composition and abundance would be altered by (a) ph 6 or 9, (b) 0.05 mg Zn•l⁻¹, or (c) the combination of ph 6 or 9 and 0.05 mg Zn•l⁻¹. Stream mesocosms were continuously supplied with natural water from the New River, VA, USA. Established (12-day colonization) communities on artificial substrates were sampled on days 0, 5, 10, 20, and 30 after treatment began on 9 July 1985. Zinc and pH treatments changed algal community composition from diatoms and a filamentous blue-green alga to different diatom taxa, green algae, or a coccoid blue-green alga. Total algal abundance was moderately increased by pH 6 treatment. Treatments of pH 6 and 0.05 mg Zn•l⁻¹ significantly altered attached-algal community composition even though these levels are considered "safe' by the USEPA. The pH 9 treatment did not significantly alter community composition, most likely because ambient pH was near this level. / Ph. D. / incomplete_metadata