Freshwater microalgae as bioindicators: A review of trait-based advances, ecosystem insights, and biomonitoring innovations

Freshwater microalgae, comprising both phytoplankton and benthic algae, are sensitive and versatile bioindicators for diagnosing water quality and ecological integrity across aquatic systems. This review synthesizes post-2017 (2017–2025) advances in freshwater microalgal biomonitoring, emphasizing the shift from taxonomy-centered indices toward trait-based, molecular, and integrative approaches. We evaluate how functional traitsstrengthen mechanistic interpretation and improve cross-regional comparability of algal metrics. We then review community responses to major stressors (eutrophication, organic and chemical pollution, hydrological and hydromorphological alteration, and climate-related extremes) and connect these responses to ecosystem functions such as primary production, nutrient cycling, water clarity, and resilience. Recent innovations on including DNA metabarcoding, remote sensing, automated imaging, and artificial-intelligence-assisted predictive modelling are transforming algal assessment from descriptive status indicators to actionable, early-warning tools. Finally, we propose a concise decision framework linking stressors, diagnostic algal traits, and management actions, and we document our literature-search strategy and inclusion criteria to support transparency and reproducibility. Under escalating pressures on freshwater ecosystems, microalgae remain indispensable for monitoring and environmental management.