The composition of algal species and pigments and the structural and functional characteristics of the algal community were investigated in an acid stream of southwestern Spain, the Río Tinto. The algal community had low diversity and showed few seasonal differences. It was mainly made up of Klebsormidium flaccidum Kütz. (Silva, Mattox & Blackwell) that produced long greenish or purplish filaments, Pinnularia acoricola Hust. (producing brown patches) and Euglena mutabilis Schmitz. The algal filaments made up a consistent biofilm that also included fungal hyphae, iron bacterial sheaths, diatoms, and mineral particles. HPLC analyses on Río Tinto samples showed that undegraded chl accounted for 67% of the total chl in the filamentous patches but were a minority in the brown patch (2.6%). The brown patch had a concentration of carotenoids eight times lower than that observed in the green patch. When chl concentrations were weighted for the proportion of the different patches on the streambed, undegraded chl a accounted for 89.2 mg chl a·m−2 of stream surface area (5.4 g C·m−2). This high algal biomass was supported by relatively high nutrient concentrations and by a high phosphatase activity (Vmax = 137.7 nmol methylumbelliferyl substrate·cm−2·h−1, Km = 0.0045 μM). The remarkable algal biomass in Río Tinto potentially contributed to the bacterial–fungal community and to the macroinvertebrate community and emphasizes the role that the algae may have in the organic matter cycling and energy flow in extreme systems dominated by heterotrophic microorganisms.

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Fifteen European rivers and streams belonging to watersheds in Sweden, the Netherlands, and Spain respectively, were sampled by allowing periphyton to colonize submerged glass substrata. Their zinc tolerances were quantified in short-term laboratory tests, where inhibition of photosynthesis in microalgae and thymidine incorporation in bacterial DNA was measured, and expressed as EC50 values. The variability in zinc tolerances was high reaching 1.5-2.5 orders of magnitude, ranging from 25-8145 microM for photosynthesis and 15-467 microM for thymidine assays. Based on the observed variability, uncertainty factors were estimated for the extrapolation of zinc toxicity data from river to river, both regionally and interregionally. Under the assumption to protect 95% of the observed communities the regional uncertainty factors were 1.7-4.3 and the interregional 2.4-8.6. The sampling sites were characterized in terms of biotope physiography, water chemistry, periphyton biomass, trace element content, and species composition. Multivariate analysis of the data using PLS (Projection to Latent Structure), was used to generate hypotheses about the relation between periphyton zinc tolerance and the 123 so-called predictor variables. Zinc contamination, phosphate, nitrogen nutrients, pH, calcium, bicarbonate, dissolved organic carbon, and various diatom species are important predictors for zinc tolerance in the entire data set representing all 15 river stretches. Regional models suggested that very different factors determined the zinc tolerance in the Swedish and Dutch periphyton. The results are interpreted in terms of Pollution-Induced Community Tolerance (PICT) and the bioavailability of zinc.