INFLUENCE OF NATURAL ULTRAVIOLET-RADIATION ON LOTIC PERIPHYTIC DIATOM COMMUNITY GROWTH, BIOMASS ACCRUAL, AND SPECIES COMPOSITION - SHORT-TERM VERSUS LONG-TERM EFFECTS

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.) Kutz., T. flocculosa (Roth) Kutz., 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 (mu:mu(max-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 inhibitory 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-fold 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.