Experimental blooms of the cyanobacterium Gloeotrichia echinulata increase phytoplankton biomass, richness and diversity in an oligotrophic lake

Cyanobacterial blooms are increasing in lakes, both eutrophic and oligotrophic, in many parts of the world. Freshwater cyanobacteria generally have negative effects on eukaryotic phytoplankton in eutrophic systems because of their ability to form dense surface aggregations (scums) that reduce light availability. However, less is known about the effects of cyanobacteria on other phytoplankton in oligotrophic lakes. Because Gloeotrichia echinulata, a large colonial cyanobacterium, has been increasingly observed in low-nutrient lakes in the northeastern USA and Canada, we investigated its effects on phytoplankton biomass and community structure. In field and laboratory experiments, high densities of Gloeotrichia had significant positive effects on the biomass of small phytoplankton (<30 mu m, typically considered edible to zooplankton) relative to no-Gloeotrichia controls. Interestingly, Gloeotrichia also increased phytoplankton taxa richness and Shannon diversity, primarily by stimulating the richness and biovolume of Bacillariophyta (diatoms) and Chlorophyta (green algae). Our laboratory experiment further suggests that at high densities, Gloeotrichia may have stimulated the other phytoplankton by leaking nitrogen and phosphorus. Thus, this study suggests that continued increases in Gloeotrichia in low-nutrient lakes are likely to increase phytoplankton biomass and alter community structure in these systems.