Course:EOSC270/2022/Group 2 - Ocean Acidification Limiting Shell Development on Shellfish

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What is ocean acidification?

Figure 1: It presents the relationship between increased atmospheric CO2 concentration, seawater pCO2 concentration, and decreased pH of seawater. The sample is from a coastal water area in Hawaii (The Ocean Portal Team, 2019).

How does ocean acidification occur?

Ocean acidification refers to the gradual acidification of seawater due to the absorption of excess carbon dioxide (CO2) from the atmosphere by the oceans. The composition of the world's oceans is gradually changing as CO2 emissions in the atmosphere increase[1]. The sea has always acted as a giant 'sponge', absorbing CO2 from the atmosphere. Due to the buffering nature of seawater, ocean acidification has received significantly less attention than global warming, and the impact of acidification on marine ecosystems has not received sufficient attention [2]. Not until 2003, the term 'ocean acidification' first appeared as an academic term in the British journal Nature [3].

Do human activities lead to ocean acidification?

Figure 2: Sampling Maps of NOAA’s Ocean Acidification Research. Data in the figure 1 are from the locations near Hawaii islands. (NOAA, 2019)

Since the industrial revolution, around 25-33% of the CO2 released by human activity has been absorbed by the oceans, leading to a 30% increase in the concentration of hydrogen ions in surface seawater and a 0.1 drop in pH over nearly 200 years[4]. This increase in acidity has altered the chemical balance of seawater, putting at significant risk all kinds of marine life and even ecosystems that depend on the stability of the abiotic environment[5][4]. In the absence of significant anthropogenic intervention, changes in the atmospheric content of CO2 are influenced by a variety of natural factors, which ultimately affect global climate change. However, the beginning of the industrial age marked the beginning of human activities that had an incalculable impact on atmospheric CO2 levels[3]. However, beginning with the industrial revolution, humans developed and used fossil fuels such as coal, oil, and gas, emitting over 500 billion tonnes of CO2 by the beginning of the 21st century, which has led to a year-on-year increase in the atmosphere's carbon content[6].

Figure 3: The global ocean's alkalinity. The warmer color indicates a higher alkalinity, which means stronger resistance to ocean acidification (Pappas, 2015).

Is ocean acidification a pervasive problem?

The seawater surface is permanently exposed to the atmosphere and constantly interacts with it. As a result, the entire ocean is under threat of ocean acidification as atmospheric CO2 levels increase. Atmospheric CO2 influenced by sea breezes first dissolves into the ocean's surface layer at depths of several hundred meters and gradually spreads to all seafloor corners over the following centuries[5]. However, the degree of ocean acidification is not uniform. For example, coastal areas tend to be impacted due to the human activities nearby, such as the excess land uses, eutrophication, agricultural and industrial wastes[7]. In contrast, pelagic oceans are comparatively less affected by human activity, leading to more alkaline conditions than coastal areas, creating more buffer zones to counteract ocean acidification. In other words, pelagic oceans are more resistant to ocean acidification, while many coastal areas are more susceptible[7].

How does this problem impact marine ecosystems?

How does ocean acidification affect global ecosystems?

The uptake of carbon dioxide by surface waters causes a direct shift in the inorganic carbon equilibrium towards increased CO2 and reduced carbonate ion (CO32-) concentration [8]. Carbonate ions are the principal component of the precipitation of calcium carbonate (CaCO3) by calcifying organisms. There are unique ecosystems displaying very high biodiversity that depend on stable calcification conditions to sustain such a rich state. For example, the rate of skeletal growth in coral reefs is crucial for competing for light and space and repairing damage caused by humans and natural events. Rates of skeletal growth depend on calcification, and therefore the concentration of carbonate ions which is decreasing due to ocean acidification [9]. Restrictions on coral growth lead to smaller three-dimensional coral structures which reduces biodiversity and the population capacity of coral reefs.

Are there unique characteristics of this habitat that make it vulnerable?

Coral reefs are among the most complex and biodiverse ecosystems on Earth, alongside being of exceeding economic importance. Because the coral structures are integral to the function of the ecosystem, if their growth is impeded it has great consequences for the organisms that live there. Furthermore, there is noticeably slower productivity for coral’s symbiotic dinoflagellates following effects of ocean acidification [10].

What organisms does it impact?

The issue of ocean acidification affects any marine organism that precipitate calcium carbonate from the sea water. Calcifying organisms in the ocean include molluscs, foraminifera, coccolithophores, crustaceans, echinoderms such as sea urchins, and corals. More specifically, mussel shell development has been shown to be restricted by lower pH, even to the extent of increased mortality.

Figure 4: Mytilus galloprovincialis. Mean shell lengths of normocapnic (filled) and hypercapnic (hollow) mussels (Michaelidis, 2005,[11])

Figure 4 shows the mean shell length of mussels under normocapnic conditions (existing pH of the ocean) and the mean shell length of mussels under hypercapnic conditions (reduced pH to 7.3) [11]. It demonstrates that the range of fully grown shells for mussels under normocapnia is from 24 to 29mm, whereas for hypercapnic mussels the range is from 17 to 22mm [11]. Not only are fully grown shells smaller but the rate of growth is also significantly lower for mussels in hypercapnia.

How and why does it impact this organism/s?

For calcifying shellfish such as scallops and oysters, a lower pH in ocean waters is associated with a lower carbonate concentration which these organisms are reliant on for constructing their shells. The increasing acidity means shells are thinner and development is slower which results in higher death rates. Models show that future ocean acidification is like to have a varying affect across taxonomic groups. Corals, coccolithophores, and mollusks show the greatest reductions in calcification, whereas a significant effect is not detected for the calcification of echinoderms or crustaceans [12].

What is the extent of the problem?

What are the measurable ecosystem changes that have occurred?

An ecosystem is "a biological system composed of all the organisms found in a particular physical environment, interacting with it and each other"[13]. In the marine ecosystem, ocean acidification may have significant effects on the survival of shellfish, with decreased survival, development and growth observed in hard clams, bay scallops and eastern oysters exposed to such environmental conditions[14][15].

As shellfish are keystone species which serve as a source of "habitat, refuge and foraging ground" for other animals, the limiting of their shell development due to ocean acidification may have pervasive trickle-down effects on other species in the ecosystem. For instance, oysters are a group of aggregating species which live attached to hard substrata, creating big 3-dimensional structures called bivalve beds, that serve as unique biogenic habitats for many animals, and contribute to the maintenance of their life cycles[16]. Bivalve beds are a "hotspot for diversity", with mussel beds on rocky shores and sedimental coasts hosting more diverse communities than the surrounding rock or tidal flats[16]. Hence, the loss of bivalve beds due to decreased shellfish survival associated with ocean acidification may result in ecosystem impacts such as the restructuring of ocean habitats, and changes in the available niches for marine species[17].

What is the present status compared to the past?

In the 21st century, reported effects of ocean acidification in marine animals include changes in cellular metabolism, organism physiology, sensory perception and community, biogeochemical and ecosystem-level dynamics[18]. In addition, ocean acidification has been found to increase primary producer biomass and decrease taxonomic diversity, with the latter likely catalyzing functional consequences in marine wildlife, although its specific effect on ecosystem function has yet to be studied at a greater depth[18]. Ocean acidification has also contributed to the spatiotemporal homogenization of community structure associated with altered competitive interactions, such as for food or space[18].

[19]Figure 5: Infographic depicting the impacts of anthropogenic ocean acidification, which affects the livelihoods of benthic shellfish, and in turn negatively impacts humans, by catalyzing a fall in fishermen's hauls and a rise in consumer seafood prices

What is the prognosis for the future if we continue on our current trajectory?

As for human impacts, many human economies rely on shellfish as a substantial source of food, with the supply of scallops, clams and oysters in the United States being valued at $400 million annually[20]. A study conducted off USA's west coast attempted to measure the impact of ocean acidification on fisheries by modeling it as the greater mortality of shelled organisms on the seafloor, which was observed to catalyze a 20-80% decrease in the biomass of three key species of fisheries' fishes[21][15]. If significant steps are not taken to halt ocean acidification, the United States Environmental Protection Agency projected that a dwindling supply of shellfish may lead to losses of an estimated $480 million/year by the end of the century[20].

This risk of negative impact may be even more significant for communities who have relied directly and heavily on the ecosystem services provided by the sea, such as Native American tribes in the United States[20]. For these indigenous groups, the harmful impact of ocean acidification poses an insidious threat to their livelihoods, food supply and the preservation of their cultural heritage[20].

With regards to ecosystem impacts, ocean acidification may result in an overall reduction in species richness and diversity, and biodiversity loss of species from both the upper and lower trophic levels of the marine ecosystem.

Given the impact, what are the solutions?

Global Solutions

As mentioned previously, the warming of Earth is increasing ocean acidification by adding in more CO2 to be absorbed into the ocean. The increase of CO2 thus increases the acidity within the ocean. Majority of solutions done today are with reducing atmospheric CO2 to lower the input getting into the ocean.

Human activities greatly influence global warming, and solutions to reduce numbers of greenhouse gasses include different energy sources. Examples of these sources would be wind, wave, tidal, solar and geothermal power: the consequences of these sources include a minimal increase of thermal heat [22]. However, as of 2022 technology present to fully reduce the amount of carbon dioxide used are not ready to be used around the world [23]. Suggestions of integrating secluded energy systems from the atmosphere reservoir have been mentioned as well [23]. However, this solution could lead to other environmental problems of where society would put the carbon dioxide instead, as well as cost issues.

An organization that’s actively trying to reduce ocean acidification is the International Alliance to Combat Ocean Acidification, also known as the OA Alliance [24]. Founded in 2016 British Columbia, Washington, Oregon and California joined together to raise awareness and advance scientific knowledge [24]. The OA Alliance has strategies to lower carbon emissions and local land-base pollution. In 2021, 12 countries formed together to become the Executive Committee for the OA Alliance [24]. With more countries, there’s more coordination present to reduce ocean acidification. The OA Alliance’s goal of protecting the environment and coastal communities form climate-ocean impact starts by reducing carbon emissions and local pollution [24]. Gaining more attention by conducting more research and gaining public awareness around the world leads to a better understanding of the ecosystems that we live in to preserve them.           

Figure 6: The progress of negative impact technologies within CleanBC's action plan to 2030 [25]. Currently in 2021, we are currently in the 'Emergent' phase where research on different sustainable technologies is being done. In 2030, CleanBC hopes to be in the 'early development' stage where these new technologies are started to being used.

Local Solutions

British Columbia’s action plan towards ocean acidification is more focused on the rise of greenhouse gases (atmospheric CO2) in general. CleanBC is an action plan leading all the way up to 2030, with different phases of where BC would be with implementing new methods to reduce carbon amounts [25]. As of 2022, we are still during Phase 1, where lots of research and monitoring are being done in different areas such as kelp forests, eelgrass beds and estuaries [25]. With gaining more knowledge on the ecosystems, scientists can develop new methods to reduce CO2 within oceans and test them in different environments. This leads to less chances of having other causes come within implemented methods.


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  11. 11.0 11.1 11.2 Michaelidis, B.; Ouzounis, C.; Paleras, A.; Pörtner, H. O. (2005). "Effects of long-term moderate hypercapnia on acid–base balance and growth rate in marine mussels Mytilus galloprovincialis". Marine Ecology Progress Series. 293: 109–118.CS1 maint: multiple names: authors list (link)
  12. KRISTY J. KROEKER, REBECCA L. KORDAS, RYAN CRIM, IRIS E. HENDRIKS, LAURA RAMAJO, GERALD S. SINGH, CARLOS M. DUARTE, JEAN-PIERRE GATTUSO (2013). "Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming". Global Change Biology. 19: 1884–1896. line feed character in |title= at position 52 (help)CS1 maint: multiple names: authors list (link)
  13. "A review of the ecosystem concept — Towards coherent ecosystem design". Technological Forecasting and Social Change – via Elsevier Science Direct.
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  22. Rashid, M. H. (2016). Electric renewable energy systems Academic Press.
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  25. 25.0 25.1 25.2 We can rise to meet the challenge of climate change. Clean BC. (n.d.). Retrieved February 28, 2022, from