For the cyanobacterial bloom incident in East Lake, I believe everyone has heard about it. The last serious bloom incident occurred in 2015, when citizens found hundreds of dead fish in the waters of The East Lake's Scenery Village, each with a size of about 2 to 3 kilograms, mainly white carp. Previously, in 2007, 30 tons of silver carp died overnight in Wuhan's East Lake, also due to bloom outbreaks and lack of oxygen in the water.
In recent years, the water quality treatment of East Lake has achieved initial results, all indicators have improved significantly, and the frequency of bloom outbreaks has decreased significantly. From November 2019 to September 2020, the water quality of East Lake has been ranked among the class III waters, and the comprehensive score has reached 96.77 points for the first time, which is considered to be the best level in the past 40 years.
East Lake, Wuhan
The latest monitoring results in 2021 also show that the water quality of Tuanhu Lake, Houhu Lake and Tangling Lake from January to June is Class III water, and the water quality of Guo Zheng Lake is slightly inferior, which is Class IV water. However, compared with the 1980s, today's East Lake can be said to have made a "leap forward" change.
<h1 class="pgc-h-arrow-right" data-track="38" > Wuhan East Lake</h1>
Wuhan East Lake is located in the east of Wuchang District, Wuhan City, through the Chu River and Shahu Lake (opened in 2011), the water area of 33 square kilometers, the average water depth of 2.4 meters, is a semi-enclosed shallow lake. Many lakes and bays divide the East Lake into 10 different sub-lakes such as Lingjiao Lake, Guo Zheng Lake, Tangling Lake, Yujia Lake, Miao Lake, Swan Lake, Shauji Lake, Xiaotan Lake, Hou Lake, Tuan Lake, etc., and the pollution degree and pollution indicators of each sub-lake are different.
It is worth mentioning that the Institute of Hydrobiology of the Chinese Academy of Sciences is located on the shore of the East Lake, and the Research Institute has taken the East Lake as the research object and monitored the water quality of the East Lake for more than 60 years. Literature shows that before the 1960s, the water quality of East Lake was still very good, with transparency of more than 2 meters, low nitrogen and phosphorus content, and poor nutrition.
But in the 1970s, the water quality deteriorated sharply, and a thick layer of oil-green "algae film" began to appear on the surface of the lake. Since then, for many consecutive years, the scope of the bloom has gradually expanded, and it has once shown a trend of loss of control. In the 1980s, the ecological deterioration of East Lake reached its peak, and the water quality was in an inferior class V state for a long time, which was no longer suitable for bathing and entertainment. Almost every summer (especially in August), Shuihua "arrived as promised", and the smell of the lake was fishy and the scenery was greatly disturbed, which caused complaints and uneasiness among Wuhan citizens.
In August 2006, cyanobacteria erupted in Guanqiao Lake, Fengguang Village
A survey article published in the Journal of Hydrobiology pointed out that the deterioration of the water quality of East Lake is closely related to the growth of the local population and economic development. According to the census data, there were only 1,018,300 people in Wuhan in 1949, which increased to 8,048,100 in 2000 and further increased to 9,785,400 in 2010. At the same time, Wuhan Iron and Steel Company, Wuhan Heavy Machine Tool Factory and other chemical enterprises have been built and put into operation, and the number has increased year by year, distributed in the form of dots around the lake area, which has aggravated chemical pollution.
According to statistics, the total amount of sewage discharged to the East Lake from various types of drainage outlets is 300,000 tons every year, which is the direct cause of the decline in the water quality of the East Lake. Summer is the high incidence of cyanobacterial blooms in the East Lake, ichthyosa, leukophyllus, microcystis are common harmful algae species, aeruginosa microcysticus is one of the "extremists", to aquatic vascular plants, fish, crustaceans brought round after round of survival challenges.
<h1 class="pgc-h-arrow-right" data-track="65" > water bloom disappeared for more than ten years, silver carp control water is the key? </h1>
As mentioned earlier, the 1980s were the worst period for the water quality of East Lake, but what is confusing is that the water bloom of East Lake did not break out as scheduled in the summer of 1985, and it did not happen again for more than 10 consecutive years!
The water bloom of East Lake seems to evaporate in the world, and the strange disappears, triggering fierce discussions from all walks of life, and experts and scholars are also divergent. Some experts believe that East Lake has become a "sewage pond", and even harmful cyanobacteria cannot survive normally. However, according to the Surface Water Environmental Quality Standard (GB 3838-2002), the researchers tested the various water quality indicators in the lake area and found that the above views were not valid.
The people who really unveiled this mystery were Liu Jiankang and Xie Pingping scholars at the Institute of Hydrobiology of the Chinese Academy of Sciences at that time. It turns out that behind the disappearance of the water, the real "heroes" are silver carp and bighead carp in the water. As early as the 1970s, silver carp fry were stocked in the East Lake, but the number was very small, and the annual output was only more than 100 tons, which did not attract people's attention.
Silver carp is active in the upper middle and upper layers of water, feeds on plankton, and is the most common filter-feeding fish in China. Interestingly, in the silver carp recipe, harmful cyanobacteria account for a fairly high proportion! Later studies found that only when the number of silver carp increased to a certain extent could the proliferation of harmful algae be inhibited. Through a series of experiments, Xie Ping scholars finally determined that the stocking density of silver carp is controlled at 40 to 50g/m³, as long as this biomass is reached, it can effectively "subdue" cyanobacteria!
Speaking of this, everyone should have understood that the bizarre disappearance of the water bloom in the East Lake is actually directly related to the increase in silver carp in the lake. From 1971 to 1995, the production of catches in East Lake increased year by year, gradually increasing from 180 tonnes to 1840 tonnes. In the process, the biomass of silver carp continued to increase, and reached a critical value in 1985, successfully preventing the occurrence of blooms.
According to the data, in 1985, the fishery output of East Lake was 1015 tons, and the proportion of silver carp was more than 90%, and the converted biomass was about 41.4g/m³, just more than 40g/m³!
At this point, the mystery of the disappearance of the water bloom has been completely solved, and the silver carp is the biggest "unknown hero". It should be noted that if the actual reserves of silver carp in the lake fall below the threshold, that is, to the level of the 1970s, then microcystic algae blooms may also make a comeback.
What's the real problem? From a deeper level, the reason why algae have abnormal proliferation is essentially that the total nitrogen, total phosphorus and other nutrients are too high, and the water quality is too fertile. As long as there is sufficient nutrient salt and suitable light, the reproduction of algae will continue until the occurrence of "inverted algae" and decay and death. Silver carp only reduces the density of harmful algae through filter feeding, so that it does not multiply on a large scale, but it does not eliminate the real bane: nutrients such as nitrates and phosphates.
In August 2012, cyanobacterial blooms broke out in the waters of Guanqiao Lake in Wuhan's East Lake
In a sense, it can also be said that silver carp treats water as "curing the symptoms but not the root causes". Therefore, there are two ideas for the treatment of "water and fertilizer disease" in the East Lake: the first is to continue to stock the fry of silver carp and increase the intensity of breeding and stocking; the second is to reduce nitrogen and phosphorus emissions, or to plant aquatic grasses on a large scale in the lake area to strengthen the absorption of nitrogen and phosphorus nutrient salts and reduce the fertility of water bodies.
<h1 class="pgc-h-arrow-right" data-track="66" > grass to control water</h1>
The average water depth of Wuhan East Lake is 2.4 meters, which belongs to the shallow water lake, and the lighting conditions at the bottom of the lake are good, which is conducive to the cultivation of aquatic vascular plants. The roots and stems and leaves of these aquatic plants can directly absorb the nutrient salts in the water, reduce eutrophication pollution, and play a role in improving water quality. Taking Singua as an example, under the eutrophic conditions, the total phosphorus of the experimental group decreased by 87.5% in 18 days, and the total nitrogen decreased by 73.57% at 30 days, and the water purification effect was very satisfactory.
Since April 2020, the aquatic grass of East Lake has gradually converged into pieces, forming a large-scale "underwater forest". In just 3 months, the scientific research team cultivated a total of 5 kinds of submerged plants in the four major areas of East Lake, with a total of 460,000 bushes.
As of April 2021, the "underwater forest" of Yangling Lake and Xiaotan Lake has begun to take shape. In the Goose Tsui Demonstration Area of Guo Zheng Lake, the area of submerged plants has reached 26,000 square meters, and the water grass at the bottom of the water and the fish and shrimp that pass through it can be clearly seen from the lake, and the transparency has been improved. It is estimated that by the end of this year, the area of "underwater forest" will be expanded from the initial 100,000 square meters to 1.3 million square meters, an increase of 1,200%.
Aquatic weed planning area / image source Wuhan hotline
According to the ecological governance plan of East Lake, by 2022, the ecological restoration plan of the four areas of the lake A, B, C and D will be fully implemented, and the planting area of submerged plants will reach 3.3 million square meters - at that time, 1/10 of the total lake area will be covered by "underwater forest"!
Why "underwater forest" and not "water forest"? This has to start with another "problem" in the East Lake - silt.
Silt is a major "stubborn disease" of the East Lake, its thickness can reach 40 centimeters, and the local lake bed even exceeds 60 centimeters. These silts enrich various pollutants and can cause harm to benthic animals through food chain action. In order to remove the silt at the bottom of the lake, the Wuhan Municipal Government has invested a lot of manpower and material resources, and the cumulative dredging volume has exceeded 3.4 million cubic meters.
Dredgers dredge
There is another way to deal with this silt: grow submerged plants. Studies have shown that the vegetation of East Lake such as snapdragon algae, bitter grass and pineapple can survive underwater depth of 1.0 to 1.9 meters, and can absorb harmful substances in silt through flocculation precipitation and nutrient adsorption, and achieve in situ "ecological bottoming".
Submerged plants have a competitive and inhibitory effect on harmful algae, coupled with the filter feeding of algae by silver carp and bighead carp, the two prongs work together to control the density of microalgae within a suitable range. In the past 40 years, from fish to grass, the governance of East Lake has explored a practical and feasible path, which provides valuable experience for the governance of shallow lakes such as Taihu Lake and Chaohu Lake, and is worth vigorously promoting.
The future of East Lake, we witness together!