Cyanobacterial blooms are predicted to increase in aquatic systems worldwide due to climate change and eutrophication, and are expected to exert diverse effects on food webs and ecosystem processes. While cyanobacteria are typically found in high nutrient lakes, they are also increasingly reported from low nutrient lakes. Many studies have demonstrated that cyanobacteria have inhibitory effects on other plankton; however, most of this work was conducted in high nutrient conditions. Does trophic state mediate the effect of cyanobacterial blooms on freshwater ecosystems, and if so, how? We examined the effect of Gloeotrichia echinulata, a colonial cyanobacterium increasing in oligotrophic lakes in the northeastern U.S., on plankton food webs and nutrients. We hypothesized that G. echinulata, which is also found in high nutrient lakes, may be able to alter aquatic ecosystems because it fixes nitrogen and transports large quantities of phosphorus from the sediment to water column. We also predicted that zooplankton would intensify any negative effects of G. echinulata on other phytoplankton by grazing down small algae. To test these hypotheses, we manipulated G. echinulata presence, trophic state, and zooplankton biomass in experimental ponds.
Results/Conclusions
The effects of G. echinulata on our focal response variables were controlled by trophic state in this experiment. In low-nutrient treatments, phytoplankton were stimulated by G. echinulata because G. echinulata significantly increased nitrogen and phosphorus concentrations. At high nutrient concentrations, G. echinulata significantly decreased nutrients and other phytoplankton, potentially by creating scums that decreased light availability. Further, we observed interactions between trophic state and zooplankton biomass. As predicted, increasing zooplankton biomass intensified G. echinulata’s inhibitory effect on phytoplankton at high nutrient concentrations; however, increasing zooplankton intensified G. echinulata’s facilitative effect on phytoplankton at low nutrient concentrations. We found that zooplankton exerted significantly more damage, likely by grazing, on G. echinulata colonies at low nutrient concentrations than at high nutrient concentrations, which may have increased G. echinulata’s nutrient leakage and consequently its stimulatory effects. In sum, G. echinulata significantly altered plankton food webs and nutrient concentrations in both low and high nutrient systems, but trophic state determined if the effect was stimulatory or inhibitory. As both cyanobacterial blooms and nutrient pollution are increasing worldwide, it is important that we understand how these two stressors interact to affect freshwater ecosystems.