Interactive effects of nutrient availability and light levels on the periphyton composition of a large oligotrophic lake

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 N levels were high, indicating a significant interactive effect between N and P. 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-fixed endosymbiont, and blue-green aglae. Nitrogen enrichment and N and P 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. -from Authors