Effects of Phosphorus Enrichment and Wave Simulation on Populations of Ulothrix Zonata from Northern Lake Superior

Abstract

The effects of phosphorus enrichment and wave simulation on new (denuded rocks) and established populations of Ulothrix zonata were examined in specially designed stationary and rocking tanks on a bedrock ledge overlooking the western arm of Lake Superior at Castle Danger, Minnesota. Tanks received constant flow of lake water and were exposed to the same insolation and climatic conditions as the lake for a 10-week period during July, August, and September 1972. A rocking tank and a stationary tank were enriched to maintain an average concentration of 0.16 mg/L PO 4–P. Two remaining tanks, receiving only lake water, served as controls. Each week, algal material was harvested from the tanks and analyzed for chlorophyll a, ash-free dry weight, and abundance of U. zonata and associated non-filamentous periphytic algae. Diatoms comprised, on the average, 95% of the total non-filamentous periphytic algae. The results show that phosphorus enrichment has marked effects on established populations of U. zonata in addition to favoring recolonization of U. zonata on bare rock substrates. Statistical analysis of the data revealed that phosphorus enrichment caused a significant increase in Ulothrix biomass and chlorophyll a in the enriched tanks. Wave simulation caused a significant reduction in total numbers of non-filamentous algae associated with Ulothrix in the rocking tanks.