All things ocean: ocean acidification, blue carbon, fish carbon and the UN High Seas Treaty


Before going to the heart of the article, I would like to remind readers of two amazing fun facts about our ocean and its ecosystems:

Therefore, understanding and facilitating this regulating role of the ocean is essential in the fight against man-made climate change and biodiversity loss. 

Let’s dive in!

What is ocean acidification? What are its consequences? 

You probably heard about ocean acidification before, often described as climate change’s “equally evil twin.” The term “acidification” can cause confusion for many people. Indeed, it doesn’t mean that the ocean’s water will taste sour like lemon juice. The changes are more subtle and deadly

Let’s start with an important fact: The ocean is one of the planet’s most important carbon sinks, absorbing around 30% of global C02 emissions since the beginning of the industrial era, which is equal to the amount of C02 emissions absorbed by forests (terrestrial carbon sink). 

ocean as an important carbon sink
C02 emissions since the beginning of the industrial era and where did they go (ocean sink, land sink and in our atmosphere) (Source)

Yet, this massive absorption of C02 is not without consequences. As CO2 gas dissolves rapidly in seawater, this sets off a chemical reaction as follows: 

The chemical reaction of ocean acidification
The chemical reaction of ocean acidification (Source)

As you can see, the chemical reaction releases free hydrogen ions. As the number of hydrogen ions increases, the pH of the ocean decreases and becomes less alkaline and more acidic

A high concentration of hydrogen ions could consume carbonate ions, which are important building blocks of structures such as sea shells and coral skeletons. Decreases in carbonate ions can hinder the building and maintaining shells and other calcium carbonate structures in calcifying organisms such as oysters, clams, sea urchins, crabs, shrimps, corals, coccolithophores and calcareous plankton.

A mollusc shell dissolves under acidic conditions - ocean acidification
A mollusc shell dissolves under acidic conditions (Source)

“So what?”, you ask. There will be fewer oysters and clams to eat and fewer coral reefs to look at, yet we humans can definitely live without them. But ocean acidification can lead to disastrous consequences on the planet’s climate and biodiversity if we look at the bigger picture. 

Take calcareous plankton and coccolithophores (single-celled organisms that are a part of the phytoplankton) for example. They are also one of the major carbon sinks. Throughout their lives, they soak up carbon dioxide, and when they die, they sink through the water to form “marine snow”, carrying that carbon with them to be buried at the depth of the ocean. 

While they may be tiny, phytoplankton contributes “at least 50% of all oxygen and capture an estimated 37 billion tonnes (40%) of all CO2 produced”. That’s the same amount captured by 1.7 trillion trees or four Amazon rainforests. 

Marine snow cooling the planet with dead plankton for millions of years
Marine snow cooling the planet with dead plankton for millions of years. (Source: Atlas Obscura)

Considering fish, which doesn’t have shells, yet can still suffer from ocean acidification. Any small change in pH can make a huge difference in survival. For us humans, a drop in blood pH of 0.2-0.3 can cause seizures, comas, and even death. For a fish, the change in the pH of the fish’s blood can cause acidosis, making its body burn extra energy to get rid of the excess acid “out of its blood through its gills, kidneys and kidneys and intestines”. This slows the fish’s growth and causes more difficulty in reproducing or escaping predators. More acidic water could also affect the minds and behaviour of certain fish species, making it harder to navigate or escape danger and/or affecting their hearing and balance. 

Much more research will need to be done in order to paint a much more complete picture of how ocean acidification affects marine life. Yet, one can make a guess: a shift in fish species and others could cause major impacts on the food chain and human fisheries, whereas fish and other seafood products provide vital nutrients for more than three billion people around the globe. 

What is blue carbon? What is fish carbon?

We have talked about how the ocean as a whole and phytoplankton are important carbon sinks. Many other incredible, living forms of carbon sink come from our blue oceans. 

One of them is the “blue carbon”, referring to tidal marshes, mangroves and seagrasses that play an important role in carbon sequestration. Even though these habitats occupy only about 0,2% of the global ocean, they account for 14% of carbon sequestration by the global ocean. 

Mangroves, for example, store up to five times as much organic carbon as tropical upland forests, pound for pound. (Indeed, this statistic should not be used to pit one carbon sink against others. They are all necessary). 

Why can mangroves be that effective when it comes to carbon sequestration? There are a couple of explanations:

  • First, carbon sequestration is “enhanced in deltaic environments because rivers continuously deposit sediment in mangrove soils.” And, this leads to “higher burial rates of carbon, both from the trees themselves and from carbon carried by the rivers”. 
  • Second, high productivity and slow soil decomposition rates significantly increase mangroves’ ability to capture and store organic carbon, particularly in their soils. 
How tidal marshes, seagrasses and mangroves sequester and store CO2
How tidal marshes, seagrasses and mangroves sequester and store CO2. However, these fragile habitats need stricter protection in order to survive the impact of human activities. For example, shrimp farming has caused 30% of mangrove deforestation and coastal land use change in Southeast Asia. (Photo Source)

The other form of living carbon sink is “fish carbon”, referring to carbon interactions of “all marine vertebrates”: turtles, sea birds, mammals such as whales and dolphins, and fish such as sharks, tuna and sardines. 

Let’s take the biggest mammals on Earth – whales – for example. During their long life of around 60 years, each great whale sequesters 33 tonnes of CO2 on average. In comparison, a tree absorbs no more than 48 pounds of C02 per year. (The ratio is therefore 1 whale = 1000 trees).

How do whales do that? It is through 2 mechanisms called the “Whale pump” and the “Whale fall”. 

  1. Whales eat krill in the depth of the ocean…
  2. then go to the surface of the ocean to breathe, about to create the “Whale pump”
  3. when at the surface, whales do gigantic poos, which release nutrients that phytoplankton need to survive
  4. through photosynthesis, phytoplankton absorbs around 40% of carbon from the atmosphere (reminder: this is the equivalent of 4 Amazon rainforests) and releases 50% of the planet’s oxygen 
  5. when a whale dies, their carcass sinks to the ocean floor. This is called the “Whale fall”
  6. the carcass stores the carbon for thousands of years in the depth of the ocean, preventing the carbon from being released back into the atmosphere. The carcass also provides food and a home for more than 200 species. 

A carcass of a sperm whale
A carcass of a juvenile sperm whale near San Jorge Island, Portugal. The whale fall represents death and the beginning of life at the same time. (Source)

Many other marine creatures play the same role as the great whales. This graphic resumes 8 different fish carbon mechanisms across different species and habitats. 

What is the UN High Seas Treaty? What is the COP15 biodiversity agreement? What does it all mean for the ocean? 

Alright, so far we have learnt many things about the ocean: how it sustains and protects life on Earth and how it suffers from this job (i.e being a major carbon sink and absorbing C02 way past its point of capacity, which leads to ocean acidification and then the destruction of marine life). 

The next thing we will learn is how humans can protect the ocean back, after many devastating consequences caused by our own actions. 

I have good news and bad news for you. 

The good news is: 

  • These parts of the ocean currently have very little or non-existent protection against pollution, overfishing and habitat destruction. This treaty is materialised through the capacity to “create marine protected areas through decisions of a conference of the parties to the treaty” or through forcing companies that plan commercial activities to “carry out environmental impact assessments.”
  • This can help protect the increasingly threatened marine species, among which one in ten are at risk of extinction. 
The IUCN Red list of threatened marine species
The IUCN Red list of threatened marine species (Source)
  • In December 2022, the United Nations Biodiversity Conference, known as COP15 Biodiversity, reached an agreement to adopt the Kunming-Montreal Global Biodiversity Framework (GBF) from 188 governments. GBF features important targets to protect biodiversity (both on land and sea) to achieve by 2030, including:
    • Effective conservation and management of at least 30% of the world’s land, coastal areas and oceans. Currently, only 17% of land and 8% of marine areas are under protection
    • Restoration of 30% of terrestrial and marine ecosystems
    • Mobilizing at least $200 billion per year from public and private sources for biodiversity-related funding
    • Raising international financial flows from developed to developing countries to at least US$ 30 billion per year

(One of) the bad news is: 

  • The UN High Seas Treaty exempted deep-sea mining from stricter environmental rules. Deep-sea mining has become a high-potential source as demand for rare minerals increases. 
  • By May 2022, there are 31 exploration contracts to explore deep-sea minerals with a total area of 1,5 million km2 of the international seabed (roughly the size of Mongolia). These contracts are issued by the International Seabed Authority (ISA), which regulates activities in the seabed beyond national jurisdiction. So far, these contracts are just for exploration purposes, but ISA is developing regulations to transition exploitation
  • One thing is certain, we don’t know enough about the seafloor ecosystems to make sure that mining won’t cause irreversible harm.

The fight to protect the ocean and marine life, and to a greater extent – life on Earth – is an uphill battle. The first thing one can help to join forces is to stay informed, for informed citizens are building blocks of a functioning democratic society that takes care of life and future generations. 

*** Bonus: 

Our ocean is such a unique living ecosystem if we look at the scale of our Solar System. You can watch this one-hour documentary from the BBC Earth Lab to marvel at the incredible odds of the existence of our life-supporting planet!

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