Trophic state mediates the effects of a large colonial cyanobacterium on phytoplankton dynamics

Although cyanobacterial blooms are typically found in eutrophic lakes, where they are able to exert inhibitory effects on other plankton, they are also reported from oligotrophic and mesotrophic lakes. Here, we explored whether trophic state mediates the effects of Gloeotrichia echinulata blooms in freshwater ecosystems. This taxon is a large, colonial cyanobacterium that may be increasing in low-nutrient lakes in northeastern North America. We manipulated Gloeotrichia presence in mesotrophic and eutrophic mesocosms and measured its effects on phosphorus, nitrogen, phytoplankton growth in two size fractions (< 30 μm, and total fraction), and zooplankton. In mesotrophic mesocosms, Gloeotrichia stimulated the growth of smaller-sized phytoplankton, potentially through significantly higher total nitrogen and phosphorus concentrations than in non-Gloeotrichia controls, although nearly all measured soluble nutrient concentrations were below method detection limits. In contrast, the growth of smaller-sized phytoplankton was inhibited in eutrophic mesocosms, where concentrations of total nitrogen and phosphorus were significantly lower in the presence of Gloeotrichia in comparison to controls. The Gloeotrichia colonies likely inhibited phytoplankton growth in the eutrophic mesocosms by creating scums that decreased light availability, although other mechanisms may be involved. The positive or negative effect of Gloeotrichia did not cascade to higher trophic levels: zooplankton biomass was significantly higher in the eutrophic than mesotrophic mesocosms, but not affected by Gloeotrichia presence. In summary, trophic state determined if the effects of Gloeotrichia on smaller-sized phytoplankton were stimulatory or inhibitory, likely due to several interacting mechanisms.