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Harmful algal blooms (HABs) are common in freshwater ecosystems and promote poor water quality. Toxins released by cyanobacteria affect animals and humans. Eutrophication is assumed to be one of the causes for the increase of HABs. Alternatively, bivalves can directly control phytoplankton biomass through filtration and nutrients excretion. However, these interaction remains less known about different bivalve species. Diplodon parallelopipedon (native to South America) is capable of consuming a wide range of particles by filter feeding and release nutrients available for phytoplankton. The aim of this study was to understand the role of the native bivalve D. parallelopipedon in phytoplankton dynamics. First, we compared grazing of 2 different food sources: (i) laboratory culture of Cryptomonas spp. (C) and (ii) wild populations of Microcystis aeruginosa complex (MAC). Both food sources were mixed to create a gradient of relative abundance expressed in % of MAC:C treatments: (1) 100 MAC, (2) 75:25, (3) 50:50, (4) 25:75, and (5) 100 C. Second, we studied nitrogen and phosphorus excretion rates displayed by D. parallelopipedon. Our results supported the hypothesis that D. parallelopipedon might favor the occurrence of MAC blooms by three mechanisms. On one hand, not being able to consume it when it is forming scums. Moreover, D. parallelopipedon actively consumed potential competitors. Finally, Diplodon potentially boosts MAC growth by nutrient releasing. [ABSTRACT FROM AUTHOR]