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Zooplankton grazing of Gloeotrichia echinulata and associated life history consequences


Interactions between cyanobacteria and zooplankton influence both the production of upper trophic level biomass and the occurrence of cyanobacterial blooms in lakes. Gloeotrichia echinulata, a large colonial diazotrophic cyanobacterium, is causing nuisance blooms in oligotrophic and mesotrophic lakes in northern New England (USA). We sought to determine whether crustacean zooplankton are capable of grazing G. echinulata, and if so, which species could be responsible for the observed grazing and what the life history consequences of G. echinulata consumption might be. In three in situ mesocosm experiments, there was a positive relationship between crustacean zooplankton density and the number of damaged G. echinulata colonies. In laboratory incubations, Daphnia pulex, Holopedium gibberum, Ceriodaphnia quadrangula and Bosmina longirostris increased the proportion of damaged G. echinulata colonies; C. quadrangula damaged the greatest proportion of colonies. Additionally, survivorship of Daphnia pulex feeding exclusively on G. echinulata was higher than that of D. pulex fed no food, but lower than that of D. pulex fed nutrient-rich Cryptomonas erosa; however, only one D. pulex fed G. echinulata reproduced. Our studies therefore demonstrate that G. echinulata is damaged by exposure to crustacean zooplankton; we believe this damage may be evidence of active grazing because G. echinulata can be used as a supplemental food source, at least by D. pulex. Further study is required to determine the full life history consequences and food web implications of G. echinulata consumption by zooplankton.