Harmful algal blooms (HABs) constitute one of the most pressing water quality challenges confronting aquatic ecosystems globally, with nitrogen pollution serving as a fundamental driver of bloom development and persistence. This comprehensive review examines the multifaceted relationships between nitrogen availability and toxic algal bloom formation across freshwater and marine environments. Anthropogenic nitrogen inputs from agricultural runoff, wastewater discharge, and atmospheric deposition have fundamentally altered nutrient dynamics in water bodies worldwide, creating conditions that systematically favor bloom-forming species over diverse phytoplankton communities. The chemical form of nitrogen, whether nitrate, ammonium, or organic compounds differentially influences algal community composition, with various taxa exhibiting distinct preferences and competitive advantages under specific nitrogen regimes. Temporal patterns of nitrogen delivery, from pulsed agricultural runoff to chronic wastewater discharge, shape seasonal bloom dynamics and species succession patterns. Interactions between nitrogen and phosphorus create complex co-limitation scenarios that determine which organisms dominate under given environmental conditions, while nitrogen availability directly influences cellular toxin production in cyanobacteria, dinoflagellates, and diatoms through metabolic regulatory pathways. Climate change intensifies these nitrogen-driven processes through warming, altered precipitation patterns, and strengthened water column stratification. Management strategies encompassing agricultural best practices, wetland restoration, and advanced wastewater treatment show promise for bloom mitigation, though implementation faces substantial economic, political, and technical challenges. The slow recovery of eutrophic systems due to legacy sediment nutrients demands sustained commitment to load reduction over decadal timescales.
Ugbe et al. (2026) studied this question.