ocean acidification, eutrophication and deoxygenation

The ocean absorbs about 23% of our CO2 emissions every year and as a consequence the level of CO2 in the upper layer of the ocean increases by about 2 billion tonnes a year. With a drop in the average pH at the surface from 8.2 to 8.1 what may seem insignificant in terms of pH is actually equivalent to a 30% increase in acidity. Acidification can favour the mass development of toxic algae, a process known as eutrophication. The algal blooms eventually sink and decompose in the water, this process (known as deoxygenation) consumes oxygen and depletes the supply available to healthy marine life. Algal blooms can be found all over the world, so too the resultant dead zones or hypoxic regions, where oxygen levels are too low to support many marine organisms. When water approaches oxygen levels of two parts per million or less anything mobile, like crabs, snails, and fish, will try to move away, making it harder for larger marine animals to find their usual prey. Marine life simply cannot exist in these zones, either it moves on or it dies. With the added element of climate change, hypoxic areas are expected to spread: not only does warm water hold less dissolved oxygen than cool water, it also tends to divide into layers that don’t readily mix, meaning that the oxygen-rich shallow waters don’t mix with the deeper waters, stifling those areas even further.

Acidification effects phytoplankton and zooplankton, at the very base of the marine food chain. While some phytoplankton are stimulated into higher levels of photosynthesis by additional carbon dioxide in the water, this reduces its ability for growth, creating a kind of fast food for other marine species, reducing the quality of the food source for those further up the chain.

Increased acidity also affects the developmental stages of species such as:

• shellfish (scallops, oysters and whelks);

• crustaceans (crabs and lobsters); and

• echinoderms (starfish and sea urchins).

Acidification either kills the young directly or makes them more vulnerable to predators and disease. If they survive the early stages of development their weaker shells are far more susceptible to damage through fishing or storms.

Recognizing the problem, the UN dedicated one of its Sustainable Development Goals to ocean acidification in 2015. The Intergovernmental Oceanographic Commission (IOC) of UNESCO is the custodian agency for SDG Target 14.3, to “Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels". The IOC has established the SDG 14.3.1 Data Portal, a tool for the submission, collection, validation, storage and sharing of ocean acidification data.

The IOC is working with the Global Ocean Acidification Observing Network (GOA-ON), an international network to detect and understand the drivers of ocean acidification and the resulting impacts on marine ecosystems. According to their website, the network is fundamental to providing early warning of the impacts of ocean acidification on natural ecosystems, wild and aquaculture fisheries, coastal protection, tourism and local economies. It provides key input to communities, industry and governments seeking to develop action plans, best practices, and mitigation or adaptation strategies to address ocean acidification impacts.

dive in deeper

BIOACID – Biological Impacts of Ocean Acidification