Ocean acidification and harmful algal blooms combine to suppress the growth and survival of North Atlantic bivalve larvae

While harmful algal blooms (HABs) and ocean acidification (OA) are environmental factors that can impair bivalves, the manner in which these 2 stressors may act and interact to impact bivalve larvae is poorly understood. This study exposed larvae of hard clams Mercenaria mercenaria and eastern oysters Crassostrea virginica to a range of p CO 2 levels found in estuaries (400-3000 µatm) and 3 harmful algae, Alexandrium catenella , Dinophysis acuminata , and Margalefidinium polykrikoides , at densities found during HABs (500-7000 cells ml -1 ), with 1 HAB species exposure per experiment. The combined OA and HAB treatment significantly reduced larval survival in all 21 experiments by 91 ± 4.6% (SE) compared to controls and reduced larval sizes in 92% of experiments by 40 ± 3.5%. Cultured M. polykrikoides had a stronger negative effect on larvae than cellular equivalent bloom populations. Densities of D. acuminata ≥750 cells ml -1 reduced larval survival and size (p < 0.01), but the addition of OA to D. acuminata did not suppress survival further. While the combined A. catenella and OA treatment reduced larval growth and survival at all densities (p < 0.01), A. catenella alone did not impact M. mercenaria survival or size at ≤1000 cells ml -1 and did not impact C. virginica at any density. Oyster larvae were less impacted than hard clams by OA (33 vs. 67% of experiments) and by HABs (67 vs. 100% of experiments). Given the very low survival of bivalve larvae when exposed to combined HABs and OA in all experiments (<0.1-5%), bivalve restoration and conservation efforts should seek to avoid regions that experience these co-stressors.