MEPS 589:33-44 (2018)  -  DOI: https://doi.org/10.3354/meps12450

ABSTRACT: Greenhouse gas emissions, such as carbon dioxide (CO₂), lead to enhanced atmospheric and surface ocean temperatures. At the same time, CO₂ equilibrates between the atmosphere and the surface ocean, resulting in lower seawater pH. The changes in physical and chemical properties of the ocean potentially affect marine primary producers in various ways. A number of researches have addressed the effects of ocean acidification on marine phytoplankton. However, phytoplankton responses to combined effects are still poorly understood. Here, we chose the cosmopolitan chain-forming diatom Asterionellopsis glacialis to assess the combined effect of ocean acidification and carbonation (~420 to 2800 µatm) and water motion on its physiological rates. At current CO₂ levels, we observed an increase in growth rates of A. glacialis accompanied by a prevalence of longer chains (>6 cells) under enhanced water motion. However, at increasing CO₂ levels (up to ~2800 µatm) and decreasing pH values, enhanced water motion significantly decreased growth rates, chain length and organic matter production of A. glacialis. Thus, our study suggests that even though A. glacialis benefited from enhanced water motion at present CO₂ concentration, at higher CO₂ levels, the more unstable environment magnified the stress caused by acidification. If in the future the ocean surface layer will be more frequently exposed to storm and wind events, then phytoplankton communities might be more sensitive to lower pH, with potential consequences for community composition and productivity.

KEY WORDS: Phytoplankton · Climate change · Growth rate · Ocean acidification · CO₂ · Water motion