Print PageCyanobacteria are generally thought to have strong negative effects on aquatic food webs due to their scums, toxins, poor food quality for zooplankton grazers, and post-bloom anoxia. In oligotrophic lakes, however, cyanobacteria may stimulate plankton food webs by increasing nutrient availability. Gloeotrichia echinulata is a large colonial cyanobacterium that is increasing in nutrient-limited lakes across the northeastern United States. We hypothesize that G. echinulata has the potential to substantially affect ecosystem functioning in oligotrophic systems because it both fixes nitrogen and translocates large quantities of stored phosphorus from the sediments to the water column annually during recruitment. We examined the effects of G. echinulata on nutrient concentrations and plankton by adding rinsed colonies to nutrient-limited ~100 L in situ mesocosms during the summer of 2008 and ~1000 L experimental ponds in the summer of 2010. In 2010, we also manipulated background nutrient concentrations in order to evaluate whether the effects of G. echinulata on plankton food webs differ between nutrient-limited versus eutrophic systems.
Results/Conclusions
G. echinulata blooms substantially affected food webs in nutrient-limited mesocosms. In both experiments, G. echinulata blooms significantly increased water column total nitrogen and phosphorus concentrations in comparison to no-bloom controls. In the 2008 experiment, in the absence of cladoceran grazers, G. echinulata blooms altered phytoplankton community structure by significantly increasing taxon richness and the biomass of small-sized phytoplankton. In the 2010 nutrient-limited experimental ponds, G. echinulata also increased small-sized phytoplankton biomass in comparison to no-bloom controls, but with a surprising twist: G. echinulata’s stimulation of phytoplankton was positively related to zooplankton biomass. Increased zooplankton biomass led to increased grazing of G. echinulata colonies, which may have increased the rate of G. echinulata’s nutrient leakage to other phytoplankton, thereby intensifying G. echinulata’s stimulatory effect. In contrast to G. echinulata’s positive effects on phytoplankton in the nutrient-limited ponds, G. echinulata blooms in eutrophic ponds had a negative effect on small-sized phytoplankton. Our findings indicate that in nutrient-limited systems, G. echinulata may subsidize plankton food webs through nutrient leakage, and thus could accelerate eutrophication. Furthermore, our data demonstrate the importance of trophic state in determining the effect of cyanobacterial blooms, thereby highlighting the context-dependency of G. echinulata’s effect on freshwater food webs.
