Herbicide Effects on Phytoplankton Community Structure and Function Under Thermal Conditions Representative of the SSP2-4.5 Climate Scenario

The phytoplankton community, which forms the base of aquatic food webs, plays a central role in ecosystem functioning and accounts for most primary production in aquatic systems, particularly in lentic environments. However, climate change and chemical pollution, especially herbicides, pose increasing threats to its structure and functioning. We hypothesized that herbicide exposure would alter both structural and functional attributes of the phytoplankton community, with primary production responding more strongly than community structure. This experimental study evaluated the effects of two widely used herbicides, atrazine and 2,4-D, applied individually and in combination, on a phytoplankton community maintained at a fixed elevated temperature representative of the Shared Socioeconomic Pathway SSP2-4.5 scenario described in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Because temperature was not experimentally varied, the study was designed to assess herbicide responses under this fixed elevated thermal condition rather than warming effects or herbicide–temperature interactions. Phytoplankton samples collected from an artificial urban reservoir were exposed to six treatments, including a control, comprising different nominal concentrations of atrazine (32 and 64 µg L−1) and 2,4-D (150 and 500 µg L−1), applied alone or in mixture, in a completely randomized design with 10 independent replicates per treatment. Throughout the experiment, all samples were maintained under identical experimental conditions: a constant temperature of 31.8 °C, an irradiance of 140 µmol m−2 s−1, and a 12 h light/12 h dark photoperiod. Response variables included density, taxonomic composition, biomass (chlorophyll a), and primary production (net photosynthesis). The results showed that herbicide treatment altered taxonomic composition and was associated with a marginal response in density but had no significant effects on species richness, diversity, evenness, dominance, or biomass. In contrast, net photosynthesis was the only functional variable significantly affected by herbicide treatments, with post hoc comparisons revealing lower values in the Mixture treatment than in the 2,4-D High treatment. Overall, these findings suggest a decoupling between community structure and phytoplankton functional performance following exposure to atrazine and 2,4-D under a thermal condition representative of the SSP2-4.5 climate scenario, with net photosynthesis appearing more sensitive than conventional structural descriptors.