In MACAN's March 26 webinar, Dr. Chris Gobler discusses current research on OA and HABs interactions and implications for marine ecosystems and organisms. Marine ecosystems are warming, acidifying, and deoxygenating. In parallel, impacts of HABs on coastal zones have increased. Many eutrophic habitats in the mid-Atlantic and northeast experience recurring HABs and coastal ocean acidification (OA), making these locations focal points of these stressors. Although the fields of HABs and OA are well-established individually, the effects of OA on HABs, and HABs on OA, are poorly understood. The effect of co-occurrence of HABs and OA on marine organisms is largely unknown. This project seeks to establish a comprehensive understanding of how three of the most prominent HABs on the US East Coast ( Alexandrium catenella, Dinophysis acuminata, and Margalefidinium polykrikoides) respond to OA, how differing OA scenarios (current and future) will impact the proliferation of these HABs, and how their co-occurrence will impact economically and ecologically important bivalves and fish. The seasonal progression of OA, hypoxia, and HABs is being quantified through continuous monitoring, surface water mapping, and discrete sampling. Effects of OA on the growth and toxicity of these HABs has been quantified in field and lab experiments. The lethal and sub-lethal effects and co-effects of these HABs and OA at levels observed in an ecosystem setting and approximately future OA have been established for multiple life stages of commercially important bivalves indigenous to the entire US east coast (C. virginica, M. mercenaries, A. irradians) and early life stage forage fish (M. beryllina, C. variegatus). We have found that co-occurrence of HABs, hypoxia, and OA vary seasonally with some HABs co-occurring with levels of DO and/or pH known to be lethal to marine organisms. We found reduced larval bivalve growth and survival by up to 90% when HABs and OA co-occur, leading to survival rates <5%. Economic and ecological implications will be discussed.