Harmful Algal Blooms (HABs) are growing environmental concerns as they impact water quality, ecosystem health, and economy. There is a need to understand the underlying mechanisms, implment predictive models and develop effective mitigation strategies. This session aims to bring together research efforts that interpret the mechanisms driving HABs, innovative monitoring techniques, prediction models and effective mitigation strategies. Example foci are: (1) ecological mechanisms on the biogeochemical and physical drivers of HAB occurrence, development and decline in freshwaters and marine/coatstal waterbodies; (2) advances in monitoring HABs (e.g., remote sensing, in-situ and/or citizen science); (3) modeling the occurrence and intensity of HABs using statistical, process-based and machine learning models; (4) impacts of nonstaionary environment (e.g., climate and land cover change and ocean acidification) on HABs; (5) public health and economic impacts of HABs; (6) mitigation, decision-making and management approaches to prevent HABs, including nutrient load reduction and biological controls.