Three treatments of zinc (0.05, 0.5, 1.0 mg Zn 1-l) 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 I-' 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 1-l treatment could be identified by a filamentous green alga in fall. The 1.0 mg Zn 1-l treatment was dominated 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 1-i. Total biovolume-density of all taxa responded slower than individual taxa in spring and failed to distinguish between Zn treatments in summer and fall. Zinc bound to periphyton was much better than total Zn in water for identifying Zn treatments. Zinc treatments as low as 0.05 mg Zn 1-i changed algal species composition despite 0.047 mg Zn 1-i being the Criterion of the US Environmental Protection Agency for the 24-h average of total recoverable Zn.

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Attached‐algal communities were employed to test the U.S. Environmental Protection Agency's (EPA) guidelines for zinc (Zn) and pH. The EPA criterion for Zn is 0.047 mg/L, and the EPA effluent standard generally requires that discharge water fall between pH 6 and 9. 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/L Zn, or (c) the combination of pH 6 or 9 and 0.05 mg/L Zn. Stream mesocosms were continuously supplied with natural water from the New River, Virginia, USA. Established (12‐d colonization) communities on artificial substrates were sampled on days 0, 5, 10, 20, and 30 after treatment began on 9 July 1985. Total algal abundance increased in pH 6 treatment. Treatments of pH 6 and 0.05 mg/L Zn reduced abundance of species characteristic of control mesocosms. These species were replaced by species characteristic of 0.05 mg/L Zn and pH 6 mesocosms. These changes occurred even though these levels are considered “safe” by EPA. Community composition did not differ significantly between ambient (pH 8.5) and pH 9 treatments. Although further research is needed, one might speculate that this change in community composition may have potential ecosystem effects by reducing food quality from diatoms to green algae that may inhibit higher level consumers.

Elevated aluminum (Al) concentrations are often associated with acid-stressed aquatic ecosystems, so it has been unclear whether acidic water or elevated Al is more responsible in changing community composition. Experiments were done to investigate effects of acidification and increased Al on the abundance of benthic algae in artificial streams supplied with natural water and nominal treatments of (a) pH 4.8, (b) 500 /lg 1-1 Al, or (c) the mixture of pH 4.8 and 500 jig 1-1 Al compared to a control without added Al or acid. These treatments are referred to as 'Acid', 'Al-only', 'Acid + Al', and the 'control', respectively. In the Acid treatment the abundance of two diatoms, two green algae, dry weight biomass, and chlorophyll a decreased; one diatom and one filamentous blue-green alga increased. In the Al-only treatment, densities of two diatoms, one green alga, one blue-green alga, dry weight biomass, and chlorophyll a increased. In the Acid + Al treatment, abundances of one green alga, two blue-green algae, and concentrations of chlorophyll a decreased below the levels observed in the Acid treatment. Acid and Al concentrations were altered by each other and by chemical and biological processes in the stream system. Species of diatoms, green algae, and blue-green algae responded individually to treatments and mixtures of acid and Al. Shifts in the abundance of species may change food web relationships for higher-level consumers, and algae may be useful biomonitors of ecological stress.

Communities of freshwater benthic algae were exposed to water acidified daily to pH 4.8 alone and in mixtures with 50, 100, or 500 μg L−1 Al. Daily acidification to nominal pH 4.8 in acidified treatments exposed algae to a range of pHs that led to a variety of Al species of different toxicity. Calcium concentrations in stream water were higher in acidified and Al treatments on day 1, and dissolved Al in stream water was higher in the highest Al treatment on days 1, 7, and 28; otherwise acid and Al treatments did not significantly alter Al and Ca in the exposure water. Calcium bioaccumulation by periphyton was lower in acidified and Al treatments on all sampling days. Concentrations of Mg in water and periphyton did not differ between treatments. The artificial stream system generated alkalinity to buffer acidified water; the amount of generated alkalinity returned to approximately the same levels for the first week, but the amount of alkalinity generated during the second week appeared to decline, and acid‐only and acid‐Al mixtures diverged in their ability to generate alkalinity during the third and fourth weeks. Aluminum in acidified water inhibited abundance of diatoms and green and blue‐green algae more than the effects of acid stress alone. The green filamentous alga Mougeotia showed a slight (but not statistically significant) increase in abundance in the acid‐only treatment. The middle‐Al treatment (nominally 100 μg L−1 Al and acid) generated more alkalinity and had higher abundance of some algal taxa on days 14 and 28, even though measured Al concentrations in water and periphyton fell between low‐ and high‐Al‐treatment levels. Community‐level tests, combining a taxonomic analysis of algal population abundance with chemical analysis of water and bioaccumulation, provide valuable insight to assess anthropogenic stress.