We investigated the effects of nitrogen, phosphorus, and light on periphyton in a clear oligotrophic lake. Experimental treatments consisted of three levels each of nitrogen, phosphorus, and light in a three-way factorial design. Nitrogen and phosphorus concentrations were manipulated using nutrient-diffusing clay substrates; light levels were manipulated with shade cloth. Periphyton biovolume and cell densities increased significantly with both nitrogen and phosphorus enrichment but were not affected by shading. Phosphorus enrichment increased periphyton accumulation only when nitrogen levels were high, indicating a significant 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 phosphorus-enriched substrates were dominated by R. gibba, which contains a nitrogen-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 dominated by N. palea and Achnanthes minutissima, but the medium-shade substrates were dominated by R. gibba.
Long-term nitrogen and phosphorus enrichment alters vegetation species composition and reduces carbon storage in upland soil
