Tidal Flat and Blue Carbon

The moon’s gravitational pull generates a high tide and a low tide in the sea. Tidal flats indicate level muddy surface bordering an estuary, alternately submerged and exposed to the air by changing tidal levels. Tidal flats are mainly formed by the accumulation of sediments on the shoreline with a gentle slope and with a great difference between the two tides. A constant flow of tides allows a tidal flat to be habitat to various organisms as it is rich in oxygen and contains a lot of organic matter.

In the past, tidal flats were considered useless muddy lands by the sea, or mere lands for fishermen to gather shellfish. So, in some parts of the world, such as East Asia and Southeast Asia, tidal flats have been reclaimed for aquaculture, agriculture, and industrial development. In particular, around the Yellow Sea region of East Asia, more than 65% of tidal flats had been destroyed from the early 1950s to the late 2000s. It is estimated that up to 16% of the world tidal flats have disappeared since the mid-1980s.

Tidal Flats, the Kidney of the Sea

Nowadays, however, various functions and values of tidal flats have been discovered, and the importance of the environment has been raised, leading to a new perspective and interest in the tidal flats.

Tidal flats have the nickname, the “kidney of the sea.” It is because they purify the pollutants that are flowing from the sea and the land just as the kidneys filter waste in our body. In tidal flats, salt plants (coastal glossy plantain, reeds, etc.), benthic organisms, diatoms, and microorganisms actively absorb and decompose pollutants. Not only these organisms but also all the organisms living in tidal flats are directly or indirectly involved in the purification process. Recently, tidal flats have also been recognized as a key habitat for migratory birds, crabs, mollusks, and fish, contributing to ecosystems and human communities. The more stable the tidal flats’ ecosystem is and bigger the biodiversity is, the greater their purification capacity is.

Microbes in 1 km2 of tidal flat absorb and decompose 2.17 tons of pollutants per day, which is equivalent to the capacity of one city sewage treatment plant that treats organic matter. Also, 500 lugworms have the capability to purify 2 kg pollutants that is equivalent to the amount of daily excretion of a person.

What Is Blue Carbon?

Earth’s carbon is largely classified into three types: black carbon, green carbon, and blue carbon. Black carbon refers to the carbon formed through the incomplete combustion of fossil fuels such as coal and petroleum or trees. Green carbon refers to the carbon absorbed by terrestrial ecosystems such as tropical rainforests and coniferous forests. Blue carbon is the carbon captured by the coastal ecosystems.

Blue carbon was first mentioned in a 2009 UN report: “Blue carbon: The role of healthy oceans in binding carbon.” Its outstanding carbon storage ability is drawing tremendous attention now when the global warming has come to the fore as a serious problem. The UN and the International Union for Conservation of Nature [IUCN] have reported that marine ecosystems absorb greenhouse gases up to 50 times faster than terrestrial ecosystems. Also, international research institutes and organizations have suggested commercialization plans for blue carbon at the 2010 United Nations Climate Change Conference [COP16].

Tidal flats store carbon in the soil accumulated by the tides, and it can be several meters deep. The carbon sequestration rates by tidal marshes are about two to four times greater than those observed in mature tropical forests. Marine plankton, which inhabit tidal flats, absorb carbon and produce oxygen through photosynthesis, and the oxygen they produce in marine ecosystems accounts for 80% of the earth. Also, mangroves, seaweed, and seagrass absorb carbon from the air and water for photosynthesis. This carbon is stored in leaves, branches, and roots, and makes a very efficient “carbon sink.” However, if these regions are lost, the carbon that have been stored there will be released into the atmosphere, accelerating global warming.

Marine Ecosystem Is Disappearing

Currently, coastal ecosystems are being lost due to the increase in population surrounding the coastline. What is alarming is that almost a third of the world’s underwater seagrass forests have disappeared during the last century; and that mangroves have been lost at a rate of 2% a year for half a century; and that nearly 50% of the natural extent of global coastal wetlands have been lost since the 19th century. This shows that the marine ecosystem is of the world’s most vulnerable environments. The global tidal flat ecosystem continues to be negatively affected by environmental deterioration caused by coastal development, by decreased sediments from rivers by dam development, and by increased coastal erosion and sea level rise due to global warming. According to a recent analysis, the annual loss of the three major blue carbon ecosystems (mangroves, seagrasses, and tidal and salt marshes) causes carbon emission at a level similar to that of the United Kingdom’s annual CO2 emission (the world’s ninth-largest country in terms of emission).

Efforts to Protect the Tidal Flat

Despite the rate of loss, these “big three” blue carbon ecosystems still account for more than half of all carbon storage in ocean sediments. They cover almost 50 million hectares on all the continents apart from Antarctica. They could absorb almost 1.4 billion tonnes of CO2 by 2050. Scientists say that ocean-based climate actions could account for a fifth of the annual cuts in greenhouse gas emissions that the world needs by 2050 in order to limit the rise in global temperatures to 1.5°C.

But increasing protection for blue carbon ecosystems would prevent over 1 billion tonnes of carbon from entering the atmosphere by 2050, according to the High Level Panel for a Sustainable Ocean Economy, a group of world leaders that was joined by U.S. President Joe Biden at the COP26 climate summit.

The World Economic Forum’s 2021 report on Nature and Net Zero said that natural climate solutions, like restoring blue carbon ecosystems, were fundamental to tackling climate change and could deliver around one third of global emission reductions needed by 2030.

Healthy blue carbon ecosystem also provides habitat for marine organisms, supports fish stocks and food security, maintains livelihood with coastal communities, and filters and purifies sea water with marine and reef systems. It also protects the coastline from erosion and storm waves.

Efforts to protect and restore nature as well as coping with climate change such as reducing carbon emissions and converting to eco-friendly energy are very important for promoting sustainable development. Also, natural climate solutions can play an important role in securing the recovery from the COVID-19 pandemic by providing equitable livelihoods for the world’s most disadvantaged people.