The peak of blue algae bloom is approaching, so stop adding nutrients to the water indiscriminately!

Summer is coming, and the peak period of blue algae blooms in inland shallow lakes is about to come. Don't think that this thing is not as fierce as earthquakes and tsunamis, or as scary as venomous snakes and beasts. It can be very harmful!

01

What is an algal bloom ? What is a cyanobacterial bloom?

Algal bloom, also known as "lake indigo", refers to the phenomenon of large-scale algae and cyanobacteria blooms caused by eutrophication of water bodies, causing the water body to change color . Global climate change has exacerbated the occurrence of this problem. Algal blooms are mainly caused by the massive reproduction of algae and cyanobacteria such as green algae and diatoms in freshwater or marine environments [1][2]. Among them, cyanobacterial blooms are the most common. Although they are called "cyanobacteria", the colors of cyanobacteria actually include green, yellow, brown, red and even pink [2].

The left and right pictures show the blue algae bloom phenomenon in two different regions. The left picture was taken in Kenya, and the right picture was taken in Taihu Lake in China. (Image source: [2])

Cyanobacteria blooms are often caused by outbreaks of the following cyanobacteria: Dolichospermum, Planktothrix, Microcystis, Cylindrospermopsis, Nodularia, and Trichodesmium.

a. Longispora b. Hyphae c. Microcystis d. Pseudocyprodinium e. Nodularia f. Trichoderma (Image source: [2])

In eutrophic water bodies, cyanobacteria blooms usually manifest as a large number of cyanobacteria reproducing in a short period of time, forming a "floating film" on the water surface. This phenomenon will occur periodically before the eutrophication of the water body is effectively controlled. The rapid outbreak of cyanobacteria blooms is mainly achieved through two methods: vegetative reproduction and spore reproduction . Cyanobacteria also have other reproduction modes such as parasexual reproduction [1].

02

The mechanism of cyanobacterial bloom

Eutrophication refers to the accumulation of nutrients in water bodies due to the application of phosphorus fertilizers, nitrogen fertilizers, etc., and is considered to be the main cause of algal blooms (i.e., the massive reproduction of algae or cyanobacteria). It is worth noting that the enrichment of nutrients is cumulative, so its impact may be underestimated in environmental monitoring [3].

In addition to nutrients, which are the dominant factor, temperature, ocean acidification, interactions between organisms (such as zooplankton grazing), and water dynamics are also considered important environmental and biological factors that affect the outbreak of algal blooms [4] [5] [6]. To comprehensively study and monitor the formation of algal blooms and their impact on the ecosystem, it is also necessary to track the changes in each trophic level and the evolution of the entire ecosystem over a long period of time [3].

Based on the research on the factors affecting the outbreak of cyanobacterial blooms, scholars have proposed a variety of hypotheses on their occurrence mechanism and carried out relevant verification, such as: total nitrogen to total phosphorus ratio hypothesis, water temperature increase hypothesis, trace element hypothesis and zooplankton grazing hypothesis [1].

Among them, the total nitrogen to total phosphorus ratio hypothesis and the low light intensity hypothesis have been widely discussed and empirically studied. The former believes that when the total nitrogen to total phosphorus ratio in the water body is about 15 , algae in the ocean and freshwater will explode in large numbers , which has been verified in the laboratory using Aphanizomenon [7]. Moreover, although the suitable ratio varies for different algae, it usually does not exceed 29:1 [8]. The latter proposes that because cyanobacterial cells contain a variety of light-harvesting pigments, they can survive at different water depths, have a high growth efficiency, and are prone to forming algal blooms [9][10].

Previous studies have found that the growth of some algae is more related to eutrophication, while others are more affected by rising water temperatures [11]. Therefore, more research is needed to explore whether the above hypothesis is only applicable to specific waters or specific cyanobacteria species, or whether the cyanobacterial bloom is driven by multiple mechanisms.

Corresponding to the hypothesis of rising water temperatures, climate change has been widely proven to affect the reproduction and survival of marine life, including phytoplankton. In other words, climate change is very likely to lead to an increase in the frequency and intensity of algal blooms, especially harmful algal blooms [2][12][13].

Pollution caused by blue algae blooms (copyright image from the library, reprinting may cause copyright disputes)

03

The harm of algae blooms

Cyanobacteria blooms release toxins and form toxic harmful algal blooms (HABs), which not only threaten the normal function of the ecosystem, but also affect water quality (causing the water to emit foul odors) and fisheries (causing water hypoxia and leading to mass fish deaths), and have a negative impact on human health [2][14].

Although cyanobacteria can absorb carbon dioxide and release oxygen, they consume a lot of oxygen during the degradation process by microorganisms, which may cause the death of fish and benthic invertebrates [15]. In addition, the cyclic peptides and alkaloid toxins synthesized by cyanobacteria can cause damage to the liver, digestive system and nervous system of birds and mammals including humans [16][17].

For example, Taihu Lake, which covers an area of ​​about 2,400 square kilometers, is often completely covered by cyanobacteria blooms in the spring, which has serious adverse effects on water quality, fisheries and tourism. In 2007, a toxic bloom dominated by Microcystis even threatened the drinking water safety of Wuxi residents [2]. Similar situations have also occurred in the Great Lakes of North America, which have long been plagued by cyanobacteria blooms. In 2014, an outbreak of cyanobacteria also threatened the drinking water safety of residents in Ohio, USA [18].

The bloom caused by tiny blue algae is so harmful. Is there any good way to treat it?

04

Treatment methods for blue algae blooms

There are many ways to control algal blooms, including reducing eutrophication levels, using hydrodynamic and chemical methods, and biological control . These methods have been proven to be effective in the management of some lakes, but there is still no guarantee that they are suitable for the management of all cyanobacterial blooms [2].

▶Reduce the eutrophication level of water bodies

Controlling the nutrients that flow into water bodies from the source and reducing the degree of eutrophication is generally considered a fundamental solution to the outbreak of blue-green algae blooms. For example, specific measures include prohibiting the addition of phosphates to detergents and reducing the use of phosphate fertilizers . However, this process often takes years or even decades to take effect [19][20].

▶Hydrodynamic method

Hydrodynamic methods refer to artificially increasing water flow, such as increasing the amount of water, to destroy the buoyancy advantage of cyanobacteria that relies on air bladders, so that their position is replaced by green algae and diatoms, thereby alleviating algal blooms. Although this method is effective, it is usually costly [21].

▶Chemical method

Although chemical methods can quickly remove algal blooms, they are unlikely to be effective in the long term, and most chemicals are toxic to other aquatic organisms, so they are generally not recommended [22].

▶Biological control

Although biological control seems feasible, there are many challenges in its practical application. For example, although viruses or fungi can quickly kill cyanobacteria, they are unlikely to work for a long time [23][24][25]. The effectiveness of filter-feeding mollusks such as mussels in controlling cyanobacterial blooms has yet to be verified [2][13]. And artificially regulating the population structure of zooplankton and fish to suppress algal blooms can only achieve long-term effects when the eutrophication level of the water body is reduced at the same time or when human intervention is sustained [26][27][28].

summary

Cyanobacterial blooms are a phenomenon caused by multiple factors, including eutrophication and climate change. They occurred extensively in many lakes and reservoirs in China in the 20 years from the late 20th century to the early 21st century. Although this transformation process of aquatic ecosystems remains to be further studied, we have verified through field surveys and indoor controlled experiments that excessive nitrogen and phosphorus nutrients and high temperature environments are the main factors that induce the rapid proliferation of cyanobacteria [1].

Therefore, controlling the nitrogen and phosphorus content in the water is considered to be the fundamental countermeasure for controlling cyanobacteria blooms. However, for large and medium-sized lakes, the implementation of this strategy requires a lot of manpower and material resources, and the effect period is long. Based on the experience of previous research, combined with China's actual situation and traditional fishery knowledge, Researcher Xie Ping has been tirelessly carrying out innovative practices and finally gained valuable experience. He successfully completed the prevention and control of cyanobacteria blooms, achieved a key breakthrough in the comprehensive management of cyanobacteria blooms in China, and successfully prevented the outbreak of cyanobacteria blooms in Wuhan East Lake in 22 years [29].

Thanks to the unremitting efforts of many scientific researchers, a feasible and iterative methodology has gradually been formed for the complex system engineering of repairing and maintaining aquatic systems. With the scientific governance concept combined with the increased efforts in domestic pollution control, the increasingly perfect laws and regulations, and the gradual improvement of public environmental awareness, harmful blue algae blooms and other water quality problems will eventually be solved.

References

Author: Li Weichi, Master of School of Life Sciences, Peking University

Reviewer: Zhong Jin, Researcher, Institute of Microbiology, Chinese Academy of Sciences

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.