Solar ultraviolet radiation (UVR) can reduce the photosynthesis and growth of benthic diatom communities in shallow freshwater. Nevertheless, greater amounts of algae accumulate in UVR-exposed habitats than in UVR-protected environments. Near-ultraviolet (UVA wavelengths of 320 to 400 nanometers) and mid-ultraviolet (UVB, wavelengths of 280 to 320 nanometers) radiation also inhibit algal consumers (Diptera: Chironomidae). Larval chironomids are more sensitive to UVB than sympatric algae. Differential sensitivity to UVB between algae and herbivores contributes to counterintuitive increases in algae in habitats exposed to UVB. These mesocosm experiments illustrate that predictions of the response of entire ecosystems to elevated UVB cannot be made on single trophic-level assessments.

Growth rates, accumulation dynamics, and species succession of periphytic diatom communities were examined in the presence and absence of natural ultraviolet (UV) radiation using a series of outdoor, continuous‐flow experimental flumes located on the South Thompson River, British Columbia. In a short‐term experiment (2–3 wk), log‐phase growth rates of naturally seeded diatom communities comprised of Tabellaria fenestrata (Lyngb.) Kütz., T. flocculosa (Roth) Kütz., Fragilaria crotonesis Kitton, and F. vaucheriae (Ehr.) Peter. exposed to 90% ambient photosynthetically active radiation (PAR) + UV were 30–40% lower than growth rates under 90% PAR alone. UV inhibition of growth rate was independent of the degree of P limitation within the range of relative specific growth rates (μ:μmax‐P) of 0.5–1.0. In a long‐term trial, inhibition of attached diatom accumulation under 90% PAR + UV during the first 2–3 wk was corroborated. Reduction of full sunlight to 50% PAR + UV prevented the initial inhibition phase. The initial inihibitory effect of 90% PAR + UV on algal accumulation was reversed after 3–4 wk, and by 5 wk total diatom abundance (chlorophyll a, cell numbers and cell biovolumes) in communities exposed to PAR + UV were 2–4‐old greater than in communities protected from UV. Under 90% PAR + UV and 50% PAR + UV, a succession to stalked diatom genera (Cymbella and Gomphoneis) occurred. Species succession under UV radiation doubled the mean cell size of the diatom communities. The shift from inhibition to a long‐term increase in the autotrophic community under PAR + UV compared to PAR alone provides further evidence against the use of short‐term incubation experiments to define the long‐term implications of increases in UVB. These results suggest that the ecological effects of present‐day levels of UVB and UVB:UVA ratios on autotrophic communities are not well understood and might be mediated through complex trophic level interactions.

Three long-term phosphate enrichment experiments were conducted at the Experimental Troughs Apparatus (EXTRA), South Thompson River British Columbia to determine the relationship between external orthophosphate (PO43−) concentration and peak areal biomass (PB) of periphytic diatom communities. Levels of PO43− which saturated PB were two orders of magnitude greater than those required to saturate specific growth rates in thin film periphyton communities of similar taxonomic composition. With PO43− additions between 0.1 to 1.0 μg P∙L−1, PB responded in a hyperbolic fashion, initially increasing rapidly, then showing signs of saturation PB continued to increase in a slow, linear manner above 1.0 μg P∙L−1. Maximum PB (PBmax) was calculated to occur at ca. 28 μg P∙L−1. At higher PO43− concentrations (> 30–50 μg P∙L−1) PB was no longer P limited. Below the saturation point, PB was approximated by a log–linear function of PO43−.

Current velocity significantly affected the biomass and shoot density of aquatic macrophytes in two slow-flowing rivers in western Canada. Studies of aquatic macrophyte communities at three sites on the Bow River, Alberta, Canada, between 1982 and 1985 showed that biomass decreased with increasing current velocity within the weed bed over the range 0.01-1 m/s; at current speeds in excess of 1 m/s, aquatic macrophytes were rare. Transplant experiments in which Potamogeton pectinatus was grown in pails containing three sediments differing in texture at three sites with different current velocities also demonstrated that biomass and shoot density were affected by both the direct effects of current velocity on plant shoots and its indirect effects on sediment nutrient concentrations. These results indicate that current velocity is an important factor regulating aquatic macrophyte biomass in flowing waters and suggest that even a relatively modest increase in current velocity within weed beds reduces the abundance of submerged aquatic plants.

Periphyton growth rates and relative degrees of phosphorus deficiency were compared with onsite, continuous-flow troughs in three parts of the Thompson River system. Soluble reactive phosphorus (SRP) in the lower Thompson, North Thompson, and South Thompson Rivers averaged 3.4, 1.1, and 0.7 pg liter-'. Several physiological and chemical composition parameters ranked the degree of P deficiency in the rivers in the same sequence as did SRP. Among these were alkaline phosphatase activity, V,,, for 32P043-uptake, and cellular N:organic P, Chl a:ATP, C:ATP, and C:organic P. Specific growth rates (EL) estimated by biomass accrual and by 14C0, uptake usually, but not always, indicated higher p with greater availability of P. However, relative specific growth rates (p:p,,.J consistently reflected the influence of P limitation. As assessed from N:organic P and by application of the Droop and Goldman-Carpenter equations, p:prnax was 0.8-0.9 at the lower Thompson, 0.3-0.6 at the North Thompson, and 0.0-0.3 at the South Thompson sites. Hence, periphyton growth rates in the lower Thompson River were near the maximum set by temperature and light at ambient SRP of only 3-4 pg liter-l. Evidence of P-limited growth rates in the South Thompson and North Thompson Rivers was found at temperatures approaching 0°C.