By 2019, more than 60% of the world's marine ecosystems have been degraded, mangroves in coastal areas have decreased by 30% to 50% in 50 years, and coral reef areas have been reduced by 20%, a phenomenon closely related to the emission of blue carbon from the oceans.
Many people may feel unfamiliar with the concept of "blue carbon". The original international definition of "blue carbon" is relatively vague: "More than half (55%) of all living organisms in the world are captured by marine organisms, and this part of the carbon is not on land, which is called blue carbon". Some foreign experts believe that the blue carbon system in a broad sense also includes the freshwater part, that is, the carbon dioxide fixed by all aquatic organisms can be called blue carbon.
A reservoir of blue carbon in the ocean
The fixation and release of blue carbon has an important impact on global climate change. As we all know, the rise in global climate temperature and sea level rise are caused by a large number of carbon emissions, and the source of carbon is mainly "black carbon" formed by dust particles, "brown carbon" formed by fossil fuels, "green carbon" composed of terrestrial ecosystems and "blue carbon" formed by marine ecosystems.
<h1 class="pgc-h-arrow-right" > so what is a blue carbon ecosystem? What threat is it facing? </h1>
Global research on the ocean carbon cycle has been around for more than 170 years, and oceanographers have long discovered the large amounts of organic carbon stored in phytoplankton and seagrass before noticing the role of large land plants in global carbon sinks.
Blue carbon is an important part of the ocean carbon cycle, and coastal ecosystems play an important role in the blue carbon cycle. Mangrove forests, salt marshes, and seagrass beds are the most typical representatives of blue carbon ecosystems that can capture and store most of the carbon buried in marine sediments. While the combined size of these three ecosystems is less than 0.5% of the total area of the global seabed, the global blue carbon captured and stored is as high as 71%.
Mangroves, salt marshes, seagrass beds
The global coverage of blue carbon ecosystems is 509,170 km², of which seagrass bed area accounts for 63% of the area of 319,000 km², mangrove forest area accounts for 27% of the area of 139,170 km², and salt marsh area accounts for the smallest area, only 10%, of which is 51,000 km². These blue-carbon ecosystems store 11.5 billion tonnes of blue carbon, of which mangroves store up to 6.5 billion tonnes, seagrass beds at 3 billion tonnes and salt marshes at 2 billion tonnes.
The rates at which the three ecosystems fix carbon dioxide are also different: seaweed beds are the fastest, fixing 26 million tonnes of carbon dioxide per year (49%); mangroves can fix 16 million tonnes (30%); and salt marsh sites are fixed 11 million tonnes per year (21%).
But in recent decades, 33 percent of the world's blue carbon ecosystems have disappeared and are still decaying at a rate of 0.5 to 3 percent per year, with direct economic losses of $42 billion. Marine biologists estimate that at the current rate, nearly 100 percent of mangroves and 30 to 40 percent of salt marshes and seaweed beds will disappear from the earth over the next 100 years.
<h1 class="pgc-h-arrow-right" > what are the serious consequences of increasing blue carbon emissions? </h1>
1. The most obvious is the greenhouse effect caused by carbon emissions.
There are global calls to reduce carbon emissions from industry, agriculture, deforestation, mining, oil spills, overfishing, urbanization and more, but the increase in blue emissions has been rarely noticed.
In fact, due to the development of tourism, industry and agriculture, coastal ecosystems have been devastated, and blue carbon emissions are also increasing dramatically. Statistics from 2012 show that global annual blue carbon emissions were 58.7 million tonnes, of which mangrove emissions were 33.5 million tonnes (57%), seagrass beds were 14.7 million tonnes (25%), and saline marshes were 10.5 million tonnes (18%).
2. Another consequence is the acidification of seawater.
Carbon dioxide is released into seawater, which will combine with water molecules to form carbonic acid, and further dissociate into bicarbonate ions and carbonate ions, and the pH of seawater will be reduced. pH is a comprehensive indicator of the aquatic environment, and its changes will directly affect the normal growth of marine life and have a more direct impact on aquaculture.
In general, freshwater fish are more adaptable to pH (6.5-9.5) than marine fish (7.0-8.7) because marine fish live in oceans where pH has always been stable, while freshwater fish live for a long time in freshwater with a large pH (in summer, photosynthesis of phytoplankton can raise the pH of water bodies from 6.5-7.0 in the morning to 9.5-10.0 at noon).
Marine fish and freshwater fish
Shellfish such as oysters, scallops, clams, and mud snails also have different adaptations to pH. The optimal pH for gulf scallops is 8, purple blood clams are 8.2-8.5, and bay mud snails are 8.4.
Therefore, the rise in the pH of seawater will pose a certain threat to the entire marine aquaculture system, some of the more "squeamish" breeding species are easy to cause production reduction due to growth discomfort, and the development of green aquaculture can effectively alleviate the environmental pressure caused by the release of blue carbon.
What are the models of green aquaculture > <h1 class="pgc-h-arrow-right"? Does it help reduce blue carbon emissions? </h1>
The purpose of ecological breeding is to greatly reduce environmental pollution while increasing fish production, and the two most typical cases are rice field fish farming (aquaponics, fish and livestock symbiosis) and three-dimensional polyculture of different nutritional levels.
Rice paddy fish farming
The rice field fish farming model is a combination of aquaculture and traditional agriculture, which has a history of thousands of years in china. The grass flower fish cultivated in the rice fields of Guangxi is a famous local specialty, although it is not large, but the flavor is unique, the taste is very good, and it is a rare natural aquatic product.
In the rice field fish farming model, rice is the main crop, and the set of fish, shrimp and crabs and turtles and turtles can increase the added value, and some economic varieties such as crayfish, river crabs, Chinese turtles, etc. create economic value that even far exceeds rice.
The following takes rice field crab farming as an example to briefly explain its economic benefits (the following cases do not include the economic benefits of rice).
In 2018, the scale of rice field crab farming in Shaocun Flower Field Planting Cooperative in Hebei Province was 3.3 hectares, and the cost of input per mu was 299 yuan, of which the cost of crab seedlings accounted for 184 yuan, and other expenses totaled 115 yuan, with a total investment of 14800 yuan.
At harvest time, the average yield of river crabs per mu of rice field was 12.5 kg, and the market price of river crabs at that time was 120 yuan per kilogram, so the benefit of river crabs was 74250 yuan.
After deducting the cost, the net income from the cultivation of river crabs is 59450 yuan, and the average net income per mu is 1201 yuan.
In this ecosystem, the input of fertilizers and pesticides is greatly reduced, and the resulting pollution-free rice and aquatic products are more popular with consumers. In addition, fish manure feeds rice, while rice paddies provide benthic, periphytic and planktonic food for fish. There are also many aquatic weeds in the rice fields, which can also be used by fish. Therefore, rice field fish farming effectively reduces the emission of organic carbon within the system through the recycling of substances.
Three-dimensional polyculture
Three-dimensional polyculture is also known as multi-trophic level of aquaculture (IMTA), three-dimensional polyculture and rice field fish farming have many similarities: the seawater fish, shellfish and seaweed in the same sea area, fish manure can improve the fertility of seawater and the density of phytoplankton, the abundant nutrient salts in the water body can promote the growth of seaweed, and phytoplankton can be directly filtered by shellfish, which not only reduces the pollution of organic waste, but also improves the yield of aquatic products. The seawater three-dimensional polyculture model is widely used in more than 40 countries and regions around the world, including China, Canada, Chile, Japan, the United States and many developed european countries.
<h1 class="pgc-h-arrow-right" >what is the relationship between aquaculture and the fixation of blue carbon? </h1>
As mentioned above, a powerful measure to promote blue carbon fixation is to restore ecosystems in seagrass beds, mangroves and salt marshes, so what is the relationship between aquaculture and these ecosystems?
1. Aquaculture and mangrove restoration
Mangroves have a wide range of ecological services, but the area of mangroves showed a rapid decline in the 1980s and 1990s, mainly due to the cultivation of shrimp. According to incomplete statistics, the area of mangroves caused by shrimp farming has decreased by about 1.89 million hectares, and farmers in Bangladesh, India, Brazil, Malaysia, Mexico, Indonesia and other countries have excavated shrimp ponds in areas where mangroves grow, only to pursue economic benefits and ignore the ecological value of mangroves.
The widely proposed international approach is to transfer shrimp farming from mangroves to offshore waters, which will help reduce blue carbon emissions. Mangroves are currently fixing blue carbon at a rate of 1.15-1.39 tonnes/ha per year, and if the location of shrimp ponds shifts and restores 25% (470 000 ha) of the global area of deforested mangroves, then another 540,000-6.5 million tonnes of blue carbon can be absorbed per year, which is a very significant figure.
2. Aquaculture and restoration of seagrass beds
Affected by human activities, in the past 20 years, the world has lost 33,000 square kilometers of seagrass beds, and the rate of decay of seagrass beds is currently accelerating dramatically. Bottom trawling fishing methods have a devastating impact on seagrass beds, as trawling destroys the young shoots and rhizomes of seaweed. This method of underwater scavenging is simply a disaster for benthic flora and fauna.
▲▲The damage of bottom trawling to the marine substrate
The restoration of seaweed beds is facilitated by artificial cultivation of kelp in marine areas. Large algae such as kelp create barriers to trawling and thus help reduce the ecological impact of anthropogenic fishing on seagrass beds. Globally, seagrass beds fix blue carbon at an annual rate of 0.54-0.83 tonnes/ha, and if aquaculture could similarly restore 25% of the seagrass bed loss area (820 000 ha), an additional 0.44-68 000 tonnes of blue carbon could be fixed per year.
3. Shellfish culture and fixation of salt marsh blue carbon
Bivalve shellfish are recognized as the only aquaculture species that are completely environmentally friendly, and they can purify and improve water quality by filtering particulate matter, microorganisms and phytoplankton. During culture, shellfish can remove up to 54% of feed pellets and nutrient residues. According to data released by the Food and Agriculture Organization of the United Nations (FAO), global mollusk production in 2014 was 16.1 million tonnes, of which the carbon dioxide sequestration rate per tonne of mussel and oyster shell was 0.22 tonnes and 0.44 tonnes, respectively. According to this ratio, 16.1 million tons of molluscs can contain a total of 979,300 tons of blue carbon.
A common species of double-hulled shellfish cultured in China
Therefore, in aquaculture, increasing the culture of shellfish can increase the global sequestration of blue carbon. For every 25% increase in shellfish culture production (4.03 million tonnes), an additional 0.24-48 000 tonnes of blue carbon can be fixed per year.
List of contributions of green aquaculture to blue carbon fixation
<h1 class="pgc-h-arrow-right" > summary</h1>
The development of green aquaculture is an effective way to reduce blue carbon emissions. The transfer of shrimp culture from mangroves to offshore can reduce mangrove losses and thus reverse blue carbon emissions; These aquaculture measures will play a positive and effective role in delaying global warming and rising sea levels.
Green aquaculture contributes to the mitigation of global climate change, which is a common benefit of both humans and nature.
What do you want to say about the fixation and emission of blue carbon? Feel free to leave a message in the comments section to discuss!
#Aquaculture #Blue Carbon #Rice Paddy Fish Farming # #生态环境 #