Disruption of Aquatic Ecosystem Biodiversity by Intense Pollution—A Study on Zooplankton from the Tietê River Basin (São Paulo, Brazil)

The Tietê River, heavily polluted by the largest Brazilian city (São Paulo), has significant ecological and socioeconomic importance. The effects of water-quality degradation on zooplankton diversity (taxonomic and functional) and limnological variables were evaluated through a comparison of the Tietê River’s main channel, one of its marginal lagoons and a low-impacted tributary. Samplings covered both a rainy and a dry season. Environmental conditions are distinctive, with the main river and lagoon classified as hypereutrophic and the tributary as oligo-mesotrophic. The zooplankton, an essential community for aquatic ecosystem functioning, also exhibited remarkable spatial variation. Richness varied between six (lagoon/dry) and 43 (tributary/rainy). There was a much higher abundance in the lagoon (mean = 6.5 × 105), followed by the Tietê River (mean = 4.0 × 104) and tributary (mean = 2.5 × 103), and a higher taxonomic diversity (Shannon mean = 2.98) and functional richness (mean = 0.66) in the tributary, contrasting with the intermediate values in the Tietê River (means of 1.7 and 0.31, respectively) and lower in the lagoon (1.49 and 0.01). Zooplankton from the Tietê River and the lagoon positively correlated with pH, total solids, chlorophyll and phosphorus. Negative pollution’s effects on the zooplankton community were intensified by the lagoon’s lentic hydrodynamics. The low-impacted tributary seems to act as a refuge for the regional zooplankton biodiversity, taxonomically and functionally, which is restricted to highly tolerant species in the main river.