Competition between a planktonic diatom and a dinoflagellate during enclosure experiments in a mountain lake

Lake Tovel (Trentino, Brenta Dolomites, NE Italy) was renowned for red blooms due to the dinoflagellate Glenodinium sanguineum. These blooms suddenly ceased in 1964. Responses of the summer phytoplankton communities to nutrient enrichments, analysed experimentally in enclosures, identified phosphorus as the factor limiting both G. sanguineum and a dominant Fragilaria tenera. Competitive interactions suggested that the dinoflagellates were favoured by higher temperatures, whereas the diatoms were limited by silica reduction. The ratio between the biovolumes of the two algae was an important parameter for predicting which species would benefit from the phosphorus additions and outcompete the other one under the experimental condition.