Climate Crisis: The Effects of Ocean Acidification On Marine Ecosystems

by Linda Monney

Increased carbon dioxide (CO₂) concentration in the atmosphere affects the balance of dissolved carbon dioxide in the world’s oceans, which are a significant carbon sink [1,2]. In the past few decades, more than 525 billion tons of additional atmospheric CO₂ has dissolved in the ocean, currently 22 million tons per day [1].  When additional CO₂ dissolves in the seawater, equilibrium reactions acidify the ocean: first, carbonic acid is formed by the reaction of dissolved CO₂ and water; second, carbonic acid breaks down or ionizes, increasing the free hydrogen ion concentration in the seawater; and third, the carbonate ion concentration is reduced, removing an important component for calcification of marine organisms [1]. 

Scientists estimate that since the industrial era, seawater’s pH has decreased from 8.19 to 8.05, which represents a 30% increase in acidity [4]. Ocean acidity is expected to rise as the oceans absorb elevated amounts of carbon dioxide. Based on current emissions, future carbon dioxide levels could be nearly 150% higher by the end of this century, conditions that haven’t occurred in over 20 million years [5]. 

The effect of ocean acidification on marine ecosystems

The climate crisis severely impacts coastal and marine ecosystems [6]. The effects of ocean acidification combined with melting ice, oxygen depletion, and coastal erosion threaten the survival of many marine species [1]. The rise in the acidity of seawater of 30% has already affected marine creatures [1].

Many organisms rely on calcium carbonate minerals for their skeletons and shells [7]. Oceans are supersaturated with calcium carbonate minerals in areas where most life now congregates, which means calcifying organisms have abundant building blocks to construct their skeletons and shells [5]. As a result of ocean acidification, some parts of the ocean are undersaturated with these minerals, which affects shell production and the maintenance of organisms such as oysters, corals, and plankton [5]. 

Ocean acidification affects shellfish

Oyster larva aquaculture and natural ecosystems have been damaged along the West Coast by ocean acidification [5]. Anthropogenic CO₂ contributes to seasonal undersaturation on the West Coast during what are called upwelling events [5]. Despite recent evidence that low pH might be a factor in oyster reproductive failure, researchers need to disentangle the potential effects of acidification from other risk factors, such as pathogen increases, episodic freshwater inflows, and low oxygen levels [5]. 

Summary 

In recent years, the ocean science community focused much attention on ocean acidification. Without sustained global efforts to monitor ocean acidification, the severity of impacts will be uncertain it currently needs to be seen to identify exactly how ocean acidification impacts the marine food chain and ecosystems [5]. An urgent and substantial reduction in anthropogenic CO₂ emissions is the only way to reduce ocean acidification and widespread extinctions [2].  

Reference List

[1] Ocean Acidification, Smithsonian Ocean, Available at: https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification [Accessed 23 Jul. 2022].
[2] Turley, C. and Findlay, H.S, 2016, Ocean Acidification, ScienceDirect, Doi:10.1016/b978-0-444-63524-2.00018-x.
[3] How Rising Carbon Dioxide Threatens Shell Builders, Available at: https://www.chesapeakequarterly.net/V11N1/side1/ [Accessed 3 Mar. 2023].
[4] Rafferty, J.P, 2016, Ocean acidification | biochemistry, Encyclopædia Britannica, Available at: https://www.britannica.com/science/ocean-acidification [Accessed 25 Jul. 2022].
[5] What is Ocean Acidification?, Noaa.gov, Available at: https://www.pmel.noaa.gov/co2/story/What+is+Ocean+Acidification%3F.
[6] McNeil, B.I. and Matear, R.J, 2006, Projected climate change impact on oceanic acidification, Carbon Balance and Management, 1(1), Doi:10.1186/1750-0680-1-2.
[7] Ocean acidification, AdaptNSW, Available at: https://www.climatechange.environment.nsw.gov.au/ocean-acidification [Accessed 28 Jul. 2022].