I. Overview
Seagrass is the only angiosperm known to live entirely in seawater, and a large area of plant communities consisting of one or more seagrasses is called a seagrass bed.
As the most productive ecological community with vegetation biomass per unit area in the world, the seagrass bed is higher than that of tropical rainforests, known as "submarine grasslands" or "submarine forests", which can absorb a large amount of carbon dioxide in air and seawater and bury it in marine sediments, making it one of the most primary productive biomes. The annual carbon sequestration is about 500~1000 g/m2, which is an important contributor to the global blue carbon sink. Seagrass beds, coral reefs and mangroves are internationally recognized as the three typical offshore ecosystems.
2. Distribution
The world's seagrass beds are distributed in tropical to temperate shallow and sheltered coastal waters in 159 countries, with more than 70 species of seagrass species covering an area of 300,000 square kilometers (equivalent to the size of Italy), but these account for less than 0.2% of the total ocean area.
The total area of existing seagrass beds on the mainland is about 23,062 hectares, and there are 22 species of seagrass in 4 families and 10 genera, accounting for about 30% of the number of seagrass species in the world. It is mainly distributed in the Yellow Sea bohai sea area seagrass bed and the South China Sea sea grass bed.
The distribution area of seagrass bed in the South China Sea includes coastal areas such as Hainan, Guangxi, Guangdong and Fujian, and the most widely distributed is salt-loving grass;
The distribution area of seagrass bed in the Yellow Bohai Sea includes coastal areas such as Shandong, Hebei, Tianjin and Liaoning, and the most widely distributed is eel grass (alias: macrophyllum).
These seagrasses can not only reproduce sexually, which helps them generate new gene combinations and genetic diversity. It is also possible to reproduce more individuals connected to each other by asexual reproduction, extending their rhizomes, that is, the underground stems of roots and shoots.
Asexual reproduction of seagrass
3. Ecological function
(1) Carbon sequestration. In 2009, the concept of "blue carbon" was first proposed in the Blue Carbon Sink: Assessment Report on the Role of Carbon Sequestration in Healthy Oceans, jointly published by the United Nations Environment Programme (UNEP), the Food and Agriculture Organization of the United Nations (FAO) and the Intergovernmental Oceanographic Commission of UNESCO-IOC, which uses marine organisms to absorb carbon dioxide from the atmosphere and fix it in the ocean.
About 55% of the carbon captured by natural ecosystems worldwide through photosynthesis is captured by marine organisms and stored in marine ecosystems. The oceans store 20 times more carbon than on land and 50 times that of the atmosphere. Moreover, compared to the "green carbon" of terrestrial ecosystems, "blue carbon" is stored for a longer time, reaching an average of hundreds of years, and some can even reach thousands of years, playing a good role in the global carbon cycle.
Seagrass bed ecosystems are an important part of "blue carbon" and exist mainly in both biomass and sediment. The world's seagrass grows in less than 0.2% of the total ocean area, but the annual carbon sequestration accounts for 10% to 15% of the world's total oceans. Studies have shown that the organic carbon storage of the global seagrass bed ecosystem can reach 1.99 billion tons, and the storage of carbon in the bottom of the seagrass bed is as high as 400 million to 800 million tons.
Biomass, in addition to the seagrass plant itself, all kinds of attachment organisms and large algae on it, which can fix carbon in the organism through photosynthesis, contributes 20% to 60% to carbon sequestration in the seagrass bed. Become an important part of the carbon pool in the seagrass bed ecosystem.
Sediments, large amounts of fibers and lignin-like substances (roots and rhizomes) produced by seagrass beds can form layers of seagrass clastics of several meters or even more than ten meters, and can be preserved intact for thousands of years, and can also sequester carbon.
(2) Seagrass beds can purify and regulate water quality, can absorb a large number of nitrogen, phosphorus and other nutrient salts in water bodies, regulate the level of eutrophication in water bodies, and play an important role in the nitrogen and phosphorus cycles of water bodies. Participates in the ocean carbon cycle through photosynthesis and respiration, regulating dissolved oxygen and pH levels in water bodies.
When the light and nutrient salts are sufficient, large seaweeds can release phase chromotic compounds into the environment, which has obvious biochemical inhibition effects on red tide microalgae, preventing the formation of marine red tide, stabilizing seabed materials, and purifying water bodies. At the same time, the humus at the bottom of the seagrass bed can also provide a good habitat and breeding place for benthic and deep-sea animals and plants, such as fish, shrimp, crabs and other marine organisms.
(3) Seagrass is a rhizome plant that grows on the silt or sandy sediments of the coastal coast, and can grasp the soil, and when the tide passes through the seagrass bed, it can slow down the current and settle the sediment carried by the current on the coast. Weaken the impact of waves, reduce sand erosion, play a role in consolidating and protecting the bottom of the seabed and coastline, reduce the wave energy of seawater, and slow down the impact of waves.
(iv) The leaves of seagrass and the epiphytes that grow on its surface can provide food sources for other organisms. Crustaceans and snails feed on seagrass or epiphytes, and small fish prey on other zooplankton, which constantly maintain the cycle of seagrass and promote symbiotic development and reproduction.
(v) The dead leaves of seagrass will rot on the sediment or be washed onto the beach, which in turn can be provided to a diverse community of decomposers such as bacteria, which obtain nitrogen from decaying substances, continue to grow and develop, and play an important role in the ecosystem.
Fourth, the current situation faced
(1) With the strong development of the global marine economy, human activities such as reclamation, dock construction, ship transportation, mariculture, predatory fishing, and pollution of seawater by land pollution sources, as well as various sudden safety and environmental accidents, have an increasing impact on the seagrass bed.
Globally, according to the United Nations Environment Programme, seagrasses are declining at a rate of about 7% per year. On average, about one football field-sized seagrass area is destroyed every 30 minutes around the world. In the 20th century, about 29% of the world's seagrasses disappeared, such as Denmark losing 95% of its estuaries and bays, and the United Kingdom losing more than 90% of its seagrass.
(ii) Global climate change has also accelerated the decline of seagrass beds, such as rising temperatures, ocean acidification and extreme weather events, which are major challenges and threats to seagrass beds, making it one of the fastest degrading ecosystems in the Earth's biosphere.
In 2010, the seagrass bed in Shark Bay on Australia's west coast was hit by an ocean heat wave 2 to 4°C higher than average, lasting more than 2 months. As a result, the local iconic species, Antarctic root grass, was devastated, with 36% of seagrass destroyed. After suffering from abnormally warming seawater, bacteria undergo a chemical reaction with the intervention of oxygen, and seagrass re-releases the carbon stored in the sediment in the form of carbon dioxide.
According to statistics, in the three years after the incident, the damaged seagrass released about 9 million tons of carbon dioxide, roughly equivalent to the annual carbon dioxide emissions of 800,000 households, or 2 ordinary coal-fired power plants for 1 year, or 1.6 million cars for 1 year.
(iii) The escape and release of carbon buried in the seagrass bed will become a new source of carbon release from climate change, and this carbon dioxide will escape into the seawater, and through hydrolysis and ionization, it will aggravate the acidification of the ocean, thus affecting the stability of the entire ecosystem.
5. Restoration and protection
More and more people are realizing that protecting the current seagrass bed ecosystem can go a long way towards improving the marine ecosystem, maintaining biodiversity and achieving carbon neutrality.
(i) Active restoration measures to reverse the degradation of seagrass ecosystems will help protect their ecological benefits, reduce greenhouse gas emissions and address global climate change. Studies have shown that restoring and maintaining 10,000 acres of seagrass beds is equivalent to fixing the annual carbon emissions of 200,000 cars.
The Seagrass Marine Rescue Team from Swansea University in the United Kingdom formed a team of volunteers, staff and members of the local community to sow 1 million grass seeds in the Dale Bay area off the coast of Pembrokeshire, with the goal of recovering 2 hectares of seagrass.
The Potidonia seagrass grown in Gazi Bay, Kenya, is extremely efficient and can lock in 50% more carbon than regular seagrass pastures. Kenya sells the "carbon credit" model through the seagrass project, obtaining funds for remediation projects.
(2) In recent years, the mainland has continuously expanded the area of coastal "blue carbon" ecosystems through projects such as returning ponds to wetlands and ecological restoration of seagrass beds; Through the implementation of projects such as pollution prevention and control in coastal waters, we will continuously improve the quality of the marine ecological environment and enhance the ability of marine carbon sequestration. Great achievements have also been made in the fields of research, restoration and protection of seagrass beds.
In 2020, the seagrass bed was successfully restored in the sea area of Gaolong Bay, Wenchang City, Hainan Province, with the average survival rate of planted Tailai grass being higher than 56.4%, and the average survival rate of sea calamus being higher than 88.8%, which is the first successful case of ecological restoration of seagrass beds in Hainan Province.
Seagrass community grid layout
In 2015, researchers found a large bed of about 10 square kilometers of seagrass in Caofeidian, Hebei Province, but the growth trend has deteriorated. In order to restore this precious seagrass bed resource, in recent years, about 25 million yuan has been invested in the hollowing out area of the seagrass bed for replanting and conservation, including replanting 1.11 million seagrass plants, sowing 8 million seagrass seeds on the artificial seabed, and transplanting 4.5 million seagrass plants artificially. So far, the area of seagrass bed ecosystems in the region has reached 42.75 square kilometers.
In April 2021, researchers and fishermen in Rongcheng City, Shandong Province, transplanted 9 million laboratory-bred eel seedlings into the sea to increase the number of offshore fishery resources, restore the structure of fishery populations, and help rebuild the seagrass ecosystem in the bay.
Sixth, the outlook
Compared with the trees on land, the seagrass bed is subject to uncertainties such as sea currents, and ecological restoration is relatively difficult, especially in the southern region, small and medium-sized seagrass restoration is more difficult.
On the one hand, it is necessary to establish a seagrass bed protection management system, rationally plan the seagrass bed protection area, and improve management regulations. On the other hand, it is also necessary to popularize the ecological value of seagrass beds, strengthen the publicity of seagrass bed knowledge through multiple channels, improve the attention and protection awareness of the whole society to the seagrass bed, and let people from all walks of life participate in the work of protecting the seagrass bed.
You are welcome to forward it to more friends, let me all join hands to protect our seagrass beds and make our contribution to the realization of "carbon neutrality".