Marine and freshwater ecosystems are warming, acidifying, and deoxygenating as a consequence of climate change. In parallel, the impacts of harmful algal blooms (HABs) on these ecosystems are intensifying. Many eutrophic habitats that host recurring HABs already experience thermal extremes, low dissolved oxygen, and low pH, making these locations potential sentinel sites for conditions that will become more common in larger-scale systems as climate change accelerates. While studies of the effects of HABs or individual climate change stressors on aquatic organisms have been relatively common, studies assessing their combined impacts have been rare. Those doing so have reported strong species- and strain-specific interactions between HAB species and climate change co-stressors yielding outcomes for aquatic organisms that could not have been predicted based on investigations of these factors individually. This review provides an ecological and physiological framework for considering HABs as a climate change co-stressor and considers the consequences of their combined occurrence for coastal ecosystems. This review also highlights critical gaps in our understanding of HABs as a climate change co-stressor that must be addressed in order to develop management plans that adequately protect fisheries, aquaculture, aquatic ecosystems, and human health. Ultimately, incorporating HAB species into experiments and monitoring programs where the effects of multiple climate change stressors are considered will provide a more ecologically relevant perspective of the structure and function of marine ecosystems in future, climate-altered systems.
