Ending the fertiliser run-off that feeds the Yellow Sea’s green tides may be too challenging, but other measures are showing promise.
This year’s seaweed control work is officially complete!”
In 2023, the smallest amount of seaweed washed up on Qingdao’s beaches since records began in 2007, the local authorities stated on 3 August. That record did not come easy: the government says more than 8,000 people spent two months controlling the seaweed that forms offshore of this east-coast port city.
This is the 17th year in a row that the Yellow Sea has been affected by seaweed blooms. Normally appearing in late spring and lasting for three to four months, they damage the local marine ecosystem and affect shipping and tourism. At its greatest extent, this year’s bloom covered 61,159 square kilometres – one-quarter the size of the United Kingdom.
Current approaches to the blooms include reducing the number of aquaculture rafts on which the seaweed forms, fishing it out from the ocean, and removing it from beaches. Experts say that while these have proved effective, it would be better to reduce the flow into the ocean of the land-based pollution and fertiliser that feed the blooms. To do this requires an integrated land–ocean approach, they add.
Why do green tides form?
The Qingdao blooms are formed of a green algae called Ulva prolifera. It has one main stem with numerous fronds and can reach one metre in length. Highly adaptable, the seaweed can spread quickly in the right conditions, forming green tides.
Ulva prolifera is not toxic itself, but blooms can be hazardous. Massive blankets of algae block sunlight from entering the ocean and deplete oxygen levels, suffocating marine life.
If left to rot at the coast, the seaweed produces large amounts of ammonia and releases sulphides into the seawater, creating a foul stench. This has caused die-offs of farmed sea cucumber and abalone, with huge economic losses to farmers. It gets in the way of shipping as well as swimmers and sunbathers, posing challenges for tourism.
According to the Science Times, a newspaper affiliated with the Chinese Academy of Sciences, Ulva prolifera has lived in the ocean for 2.5 billion years. Until relatively recently, it washed up on Qingdao’s shores in small quantities that did not trigger mass concern. But things have changed due to global warming, which makes the water temperature more conducive to the seaweed’s growth. Another contributing factor is the increase in nutrients entering the water due to human activity.
The latter is explained by the industrialisation and urbanisation of eastern China over the past few decades, which generally leads to increasing amounts of wastewater and agricultural fertiliser reaching the ocean via rivers. This creates conditions in which Ulva prolifera will flourish.
Scientists have been working to identify the source and causes of the algal blooms since they began in 2007. In 2017, researchers from the Institute of Oceanography, Chinese Academy of Sciences and other research bodies declared the source to be seaweed farming rafts off northern Jiangsu, just south of Qingdao. The findings were disputed by some experts in Jiangsu.
The area is a major producer of edible seaweed. Traditionally, the seaweed-farming season starts in October when rafts, nets and ropes are put in the water. Ulva prolifera clings onto these and gradually starts to grow. When the temperature hits 20C in spring, it even starts to crowd out the commercial crop, forcing aquaculture farmers to manually remove it and dump it on beaches. Some seaweed also ends up in the water at the season’s end in April/May, when the rafts are dismantled. It drifts north with the prevailing currents and winds, growing rapidly until it makes landfall in Shandong.
How to tackle green tides?
Currently, physically removing the algae using nets is the most common control method, but this is labour- and resource-intensive.
This year, 3,200 fishers worked solely on removal while living at sea for 50 days starting in June, according to the Qingdao Daily.
Each boat drops and drags a net through the water until it is full of algae, then hauls it back and empties it on deck, before beginning the process again. The “catch” is then packaged up for transfer to another vessel, which takes it to land for processing.
Satellite monitoring and predictive modelling are helping researchers to more accurately gauge where seaweed is drifting, so boats can be dispatched more effectively. This was one of the reasons the amount reaching land this year set a record low, greatly reducing the amount of work needed there. Even so, Qingdao still had 4,800 people involved in removing algae from beaches, cleaning it and transporting it away, at no doubt considerable cost.
Wang Songlin, president and founder of the Qingdao Marine Conservation Society, points out that May to August is both the closed season for fishing in the Yellow Sea, and the key period for keeping seaweed under control. Thus the government’s employment of fishers during their off season – particularly the artisanal fishers who work in these waters – actually benefits both the environment and the local economy.
However, plucking the weed from the water before it makes landfall in Qingdao is merely the last line of defence. Since 2018, the Ministry of Natural Resources has been overseeing a joint effort between Jiangsu and Shandong to control the problem by focusing on Jiangsu’s aquaculture rafts.
Local governments in northern Jiangsu have shaken up seaweed aquaculture, shutting down illegal operations and reducing the extent and intensity of the area’s seaweed cultivation. Meanwhile, seaweed raft numbers have been capped, chemical algae killers are being used, aquaculture equipment is being removed from the water earlier, and new, specially coated ropes are in place; these efforts aim to reduce the likelihood of Ulva prolifera appearing and growing on the rafts.
Researchers say that while many chemicals can be used to stop the seaweed cheaply and effectively, this approach is likely to create secondary pollution and other new problems. Scientists are therefore looking into natural substances that might have the same effects, but more research and feasibility studies are needed.
The benefits of the coated ropes are also not yet clear. A Jiangsu industry association has said that over 25% of the ropes used during the city of Nantong’s 2021-2022 seaweed season were coated, but the benefits were not as significant as hoped.
A 2018 study by the Qingdao Ocean Science and Technology National Laboratory suggests that collecting seaweed in the waters of Jiangsu before it grows, combined with adaptations to aquaculture rafts, would more effectively prevent Yellow Sea blooms.
If the sea gives you lemons…
Given that some seaweed can be eaten, how about adding Ulva prolifera to the menu?
According to the Qilu Evening News, one Qingdao restaurant did just that in 2013, drawing in diners after collecting the algae from the rocks. But, the outlet reported, washing the sand off proved time-consuming and experts warned that seaweed can absorb heavy metals wile adrift at sea.
Companies have been trying to commercialise Ulva prolifera, but things have not gone smoothly. According to China Youth Daily, an algae noodle production line established in 2009 had to be shut down after it proved impossible to effectively store the product. A year later, two Fudan University professors successfully converted Ulva prolifera into biofuel, but the laboratory process was never commercialised.
Making use of the algae would offset collection costs. However, the algae “season” only lasts two or three months, which further complicates commercialisation.
Currently, Qingdao dries and compresses its salvaged ocean algae before sending it to one of two processing bases for conversion into organic fertiliser and other products. This year, nine drying lines were added to double output. Seaweed collected from the beach, however, is of less value due to its sand and grit content and is almost all burned in waste-to-power plants.
Even these efforts do not completely fund the collection work. A company official once admitted that their operations wouldn’t be profitable without government investments.
Professor Liu Tao of the Ocean University of China carried out an Ulva prolifera cost–benefit analysis. He found that, even when ignoring the seasonality of supply and the resulting intermittent use of processing facilities, the low-value products produced, such as fodder and fertiliser, didn’t make enough money to offset collection, transportation, processing and production costs. However, he concluded that using the algae as a raw material for drugs and healthcare products would significantly increase returns.
Not just a Chinese problem
Algal blooms occur in many places around the world. The coast of Brittany, in France, has been plagued by them since the 1970s, resulting in significant damage. In the US, the first such bloom was recorded 20 miles off the coast of Long Island, in 1983. According to a New York Times report, bathers complained of bright green water and were “freaking out”. The algae returned the following year, hitting the New Jersey coast. Blooms have also been experienced in Japan, in the bays of Tokyo, Osaka, Hiroshima and Hakata.
Those incidents weren’t necessarily caused by Ulva prolifera, but by other macroalgae of the Ulvaceae family. Speaking in 2011, Ye Naihao, an associate researcher with the Yellow Sea Fisheries Research Institute, said: “All the countries suffering green tides are developed. Economic development means a greater ability to change the coastal environment – and to damage it.”
Addressing the root cause of the problem would mean reducing the flow of nutrients into the oceans, but this could hamper local economies and take decades to achieve. This leaves governments with their hands tied.
Scientists have confirmed that Brittany’s blooms are linked to nitrates from fertiliser and manure being carried into the sea via rivers. According to one report, Brittany is said to feed one out of every three French people: more than 60% of the region’s land is used for agriculture and it is the national leader in pork and egg production.
Since 2010, the French government and local authorities have been spending approximately EUR 1 million (US$ 1.05 million) cleaning up the algae every year. Two official action plans designed to reduce nitrate leakage from agriculture were put in place, bringing levels down from their 1990s peak. But those improvements plateaued in 2014, because the fundamentals of local agricultural practices, such as livestock numbers, have not changed.
In China, Wang Songlin stresses the importance of both controlling land sources of pollution and practising integrated land–ocean management. He proposes keeping nutrient run-off from land and coastal activities within a particular range, one that allows the coastal marine ecosystem to continue operating normally, while also benefitting agriculture and aquaculture:
“Reducing unnecessary fertiliser use is good for soil quality. Meanwhile, increasing take-up rates of fodder, limiting livestock intensity, and tackling agricultural pollution is crucial for the green and high-quality agricultural development the government is advocating.”
Đăng nhận xét