The functional diversity of phytoplankton depends on the type of ecosystem and the seasonal period in areas affected by residues from the biggest Brazilian mining disaster

1. Nine years ago, the southeastern region of Brazil experienced the largest mining disaster (in the Lower Doce River Basin), which impacted several natural ecosystems. The relationships between the functional diversity of phytoplankton and environmental conditions provide evidence of this community's responses to anthropogenic impacts. Thus, we aim to identify the main factors influencing the functional diversity of phytoplankton considering three types of ecosystems and two distinct seasonal periods. It is expected that functional diversity and the distribution of functional traits will be influenced by light availability, nutrient and metal concentrations, and grazing pressure. 2. Sampling was conducted between October 2018 and September 2021 in the Doce River and in shallow lakes and deep lakes of the floodplain. Functional diversity indices were measured based on seven functional traits, including volume, silica demand, heterocyst presence, mixotrophy capacity, aerotopes presence, flagela presence, and life form. 3. The RLQ models highlighted functional traits that can be used as indicators of environmental variation in the region. Deep lakes exhibited higher functional diversity due to lower eutrophication conditions, metal contamination, and greater light availability. In the Doce River, the increase in functional diversity during the dry season suggested an attenuation of anthropogenic pressures, considering the improvement in environmental conditions. 4. Our results showed a dependence of the functional diversity of phytoplankton on the type of environment and that these environments experience distinct effects according to seasonality. Additionally, we demonstrated that resource availability, lower metal contamination, and reduced grazing pressure sustained greater functional diversity of phytoplankton. 5. We emphasize the importance of considering functional aspects in phytoplankton as a tool in biomonitoring areas strongly impacted by anthropogenic activities.