Grazing impacts on phytoplankton in South American water ecosystems: a synthesis.

The role of grazing as a controlling factor of phytoplankton has an extensive debate in the literature. In this article, five mechanisms that have been explored as potential controlling factors of grazing influence on phytoplankton in South America are discussed and compared with other latitudinal works. The temperature impact on zooplankton is not conclusive, with planktivorous fish appearing as the main controlling factor of zooplankton size ranges. Fish grazing effects on phytoplankton look despicable, but the impact of exotic filter-feeding fish remains controversial. Microphagous rotifers and Copepoda nauplii affect phytoplankton by selective size grazing, while large Cladocera and Copepoda adults can control phytoplankton when they reach high densities in the absence of fish. Both groups mainly feed on small sizes, with microzooplankton having a higher impact on very small phytoplankton. Some contradictory evidence arises for large colonial and filamentous algae. Exotic invasive filter-feeding bivalves are selective grazers in experimental approximations. Corbicula fluminea feeds on smaller particles, does not have taxonomic preferences, and has lower densities in nature than Limnoperna fortunei. Their effect on nature is not fully documented. In sum, several aspects still need deep scrutiny to fully understand the role of grazing on phytoplankton in South America. [ABSTRACT FROM AUTHOR]