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In laboratory and field experiments I investigated the influence of competitive interactions on biomass development of planktonic and periphytic algae. In the laboratory periphytic algae, covering the sediment surface, reduced the outflow of labeled phosphorus ( 33 PO 4 3‒ ) from sediment to water. The reduced outflow caused a decrease in phosphorus content as well as in biomass of planktonic algae. An enclosure field experiment showed that the biomass of periphyton decreased as the density of planktonic algae increased. These results suggest that periphytic algae have a competitive advantage with respect to nutrients released from sediments, whereas phytoplankton are competitively superior with respect to light. The imbalance in competitive ability between the two life‐forms predicts a decrease in periphytic and an increase in planktonic algal biomass with increasing trophic state of lakes. This prediction was corroborated with a field study of Swedish lakes. My investigations suggest that competitive interactions influence the balance between planktonic and periphytic primary production in lakes.
Lars‐Anders Hansson (Fri,) studied this question.