Experimentalcyanobacterialbloomscanincreasethebiomass,richness,diversity,andstabilityoftheeukaryoticphytoplanktoncommunity Cyanobacterialbloomsareincreasinggloballyinmanyfreshwaterlakes,botheutrophicandoligotrophic.Freshwatercyanobacteriagenerallyhavenegativeeffectsoneukaryoticphytoplanktonineutrophiclakesbecauseoftheirabilitytoformdensesurfacescumsthatreducelightavailability.However,lessisknownabouttheeffectsofcyanobacteriaonotherphytoplanktoninoligotrophiclakes.WeconductedseveralexperimentsmanipulatingthedensityofGloeotrichiaechinulata,alargecolonialcyanobacteriumthatisincreasinglyobservedinlow-nutrientlakesinthenortheasternUnitedStatesandCanada.Inlow-nutrientmesocosms,Gloeotrichiaincreasedthebiomass,richness,anddiversityofotherphytoplankton,likelybecauseitincreasednitrogenandphosphorusconcentrationsinthemesocosms.Interestingly,wealsoobservedthatGloeotrichiastabilizedthecompositionoftheeukaryoticphytoplanktoncommunity:theno-Gloeotrichiacontroltreatmentexhibitedgreatermultivariatedispersionovertimethananyofthe+Gloeotrichiatreatments.AsthedensityofGloeotrichiaincreased,itscontributiontothevarianceofthephytoplanktoncommunityalsoincreased.NetworkanalysisshowedthattheincreasinginfluenceofGloeotrichiacoincidedwithincreasingcomplexityofthephytoplanktoncommunity.However,inhigh-nutrientmesocosms,Gloeotrichiainhibitedotherphytoplankton,suggestingthatGloeotrichia’sfacilitationeffectsmaybecontext-dependent.Ascyanobacterialbloomscontinuetoincreaseinmanylakes,itiscriticallyimportantthatwedeterminehowtheeukaryoticphytoplanktoncommunitieswillrespond.