INTERACTIVE EFFECTS OF NUTRIENT AVAILABILITY AND LIGHT LEVELS ON THE PERIPHYTON COMPOSITION OF A LARGE OLIGOTROPHIC LAKE

We investigated the effects of nitrogen, phosphorus, and light on peri phyton in a clear oligotrophic lake. Experimental treatments consisted of three levels each of nitrogen, phosphorus, and light in a three-wa y factorial design. Nitrogen and phosphorus concentrations were manipu lated using nutrient-diffusing clay substrates; light levels were mani pulated with shade cloth. Periphyton biovolume and cell densities incr eased significantly with both nitrogen and phosphorus enrichment but w ere not affected by shading. Phosphorus enrichment increased periphyto n accumulation only when nitrogen levels were high, indicating a signi ficant interactive effect between nitrogen and phosphorus. Differences in species composition among treatments were mostly due to changes in the diatoms Nitzschia palea and Rhopalodia gibba. Ambient and phospho rus-enriched substrates were dominated by R. gibba, which contains a n itrogen-fixing endosymbiont, and blue-green algae. Nitrogen enrichment and nitrogen and phosphorus in combination resulted in an assemblage dominated by N. palea. Unshaded and highly shaded substrates were domi nated by N. palea and Achnanthes minutissima, but the medium-shade sub strates were dominated by R. gibba.