Combined Effects of Urbanization and Drought as Modulators of Phytoplankton Structure in Temperate Plain Streams

ABSTRACT Global climate change is intensifying extreme drought events, representing severe threats to aquatic ecosystems. In the Pampas ecoregion of Argentina, prolonged drought intersects with strong anthropogenic pressures such as agriculture and urban effluents, generating complex environmental gradients. We investigated structural and functional responses of phytoplankton assemblages in rural and urban streams of the Samborombón River basin during an extreme drought in which some of the sampling points completely dried. A total of 112 species were identified and classified into six morphologically based functional groups (MBFGs). Urban sites recorded higher diversity (H), evenness (J) and species richness (S), whereas rural sites, particularly R2 and R3, showed markedly higher total phytoplankton density. Differences in both structural attributes (PERMANOVA, p < 0.001) and MBFG composition (PERMANOVA, p = 0.004) between urban and rural streams were statistically significant. Redundancy analysis indicated that environmental variables explained 43.3% of the variation in community attributes and 41.5% of the variation in MBFG distribution. Higher diversity and richness were associated with greater depth, velocity and nitrate and phosphate concentrations, whereas elevated density was linked to higher conductivity, biochemical oxygen demand and ammonium. Functionally, Groups IV and VIa were related to deeper, faster flowing, nutrient‐enriched conditions, whereas Group V was associated with higher conductivity and organic pollution. Overall, the interaction between extreme drought and land use shaped differentiated patterns of phytoplankton community organization, highlighting their sensitivity to combined hydrological and chemical stress.