Ocean Acidification

Highlights

• Alexandrium blooms occurred with normoxic conditions and moderate pH (7.5-8.0).
• Alexandrium blooms occurred with moderate pCO₂ (400 – 1,000 µatm) and saturating conditions for aragonite.
• Dinophysis blooms co-occurred nocturnal acidification (8.71 to 13.6 hr d⁻¹) and hypoxia (0.52 to 3.88 hr d⁻¹).
• Dinophysis blooms co-occurred with high pCO₂ (1,000 – 3,500 µatm) and undersaturated aragonite (Ω_ar < 1).
• The co-occurrence of nocturnal hypoxia, acidification, Ω_ar undersaturation, and HABs coupled is a significant threat for marine life.

Abstract

While harmful algal blooms (HABs), hypoxia, and ocean acidification are common occurrences in coastal zones, research investigating the co-occurrence and interactions between these processes has been lacking. Here, we documented the initiation, peak, and demise of eight distinct HAB events caused by Alexandrium catenella and Dinophysis acuminata over a two-year period in two estuaries (Northport Harbor and Cold Spring Harbor, NY, USA). We concurrently characterized the dynamics of carbonate chemistry, including pCO₂ and the saturation state of aragonite (Ω_ar), pH, dissolved oxygen (DO), and general environmental conditions in space and time. HABs occurred in succession and reached high densities with A. catenella blooms exceeding 10⁴ cells L⁻¹ being succeeded by D. acuminata blooms exceeding 10⁶ cells L⁻¹. A. catenella blooms occurred during spring months under generally normoxic conditions with moderate levels of pCO₂ (400 – 1,000 µatm), only brief periods of acidification (pH < 7.5), mostly saturating conditions for aragonite, and an absence of hypoxia. In contrast, D. acuminata blooms, which occurred in summer, consistently co-occurred with bouts of extended nocturnal acidification (8.71 to 13.6 hr d⁻¹) and hypoxia (0.52 to 3.88 hr d⁻¹) coupled with higher levels of pCO₂ (1,000 – 3,500 µatm), and undersaturating conditions for aragonite (Ω_ar < 1). During both blooms, nearshore locations hosted higher cell densities, lower pH and DO, and higher pCO₂ compared to open water regions. The co-occurrence of multiple stressors including nocturnal hypoxia, acidification, Ω_ar undersaturation, and HABs, coupled with strong diel cycling of DO, pH, and pCO₂, especially during D. acuminata blooms, represents a significant and previously unrecognized threat for marine life.

Farrell M., Wallace R. B., Tracy A., Lundstrom A. & Gobler C. J., 2026. Development of hypoxia and acidification during harmful algal blooms: dynamic multi-stressor conditions in NY, USA, estuaries. Harmful Algae: 103141. doi: 10.1016/j.hal.2026.103141. Article.