A threat to ecosystems, health and economy

Algal blooms, though less widely discussed than other environmental concerns, are rapidly emerging as a serious threat to Nepal’s aquatic ecology, public health and economic sustainability. While pollution debates often center on waste management and air quality, nutrient pollution—which drives algal blooms—demands urgent attention.

Algal blooms are dense clusters of algae—ranging from single-celled to complex multicellular organisms—that form on the surface of lakes, ponds, reservoirs and slow-moving rivers due to excess nutrients, mainly phosphorus and nitrogen. This process, called eutrophication, is often caused by human activities such as untreated wastewater, sewage discharge, agricultural runoff and fertilizer use. The resulting uncontrolled algal blooms have severe ecological, health and socioeconomic consequences.

The science behind

Algal blooms arise due to a range of factors including eutrophication, climate change, the introduction of invasive species and nutrient-rich conditions from aquaculture. Agricultural runoff, particularly from chemical fertilizers and manure, is a key driver of eutrophication, while invasive species can exacerbate bloom frequency. Rising global temperatures and altered weather patterns linked to global warming and climate change create environments conducive to algal proliferation. Aquaculture practices further contribute by introducing high nutrient levels promoting algal growth.

Though algae play fundamental roles in aquatic ecosystems by producing oxygen and forming the base of several food chains, excessive nutrients—mainly nitrogen and phosphorus—cause algal overgrowth and ecological imbalance. This blocks sunlight, reduces oxygen and releases toxins hazardous to aquatic life, disrupting food chains and posing serious public health risks. Cyanobacteria, or blue-green algae, are particularly menacing due to their production of cyanotoxins, such as those from the genus Microcystis, which threaten both humans and animals. Climate change exacerbates this risk, as warmer, stagnant waters promote algal growth. Decreased water flow due to drought, water extraction and altered waterways further supports algal blooms by causing thermal stratification.

As algae die and decompose, they release nutrients back into the water, fueling further blooms. Algal toxins, like those from Alexandrium, can accumulate in seafood and cause paralytic shellfish poisoning, while cyanobacterial toxins compromise potable freshwater sources, recreational waters and fisheries. Human exposure occurs through direct contact, ingestion of contaminated fish or inhalation of toxin-laden droplets, leading to serious health concerns.

Challenges for Nepal

Algal blooms are critically impacting several prominent lakes and reservoirs in Nepal, each holding significant ecological, cultural and economic value. These water bodies are increasingly vulnerable due to rising nutrient levels from human activities and environmental changes. 

Located in the Pokhara valley, Phewa lake is a major tourist destination facing algal blooms due to nutrient pollution. A 2020 study by Kathmandu University’s Environmental Science and Engineering Department classified it as eutrophic, with a Trophic State Index (TSI) of 59, indicating high biological productivity. The Center for Research for Environment, Energy & Water reported that during the monsoon, Phewa, Begnas and Rupa lakes were mesotrophic (TSI: 40-50), suggesting moderate nutrient levels. Despite usually clear water, these lakes are at increased risk of anoxia during summer. The nutrient influx from agricultural runoff and waste accelerates eutrophication, threatening biodiversity, aesthetic value and ecotourism.

The Kulekhani reservoir, situated in Makwanpur district and crucial for hydroelectricity generation, is also facing eutrophication. The same 2020 study reported a TSI of 62, indicating its eutrophic status. The reservoir’s deteriorating water quality due to nutrient pollution poses risks to aquatic life, ecotourism and public health.

Maipokhari, a mid-hill wetland in Ilam district significant for its religious value, is experiencing elevated nutrient levels, particularly phosphates and nitrates, leading to algal blooms. The dissolved oxygen levels fall below Nepal’s water quality guidelines, signaling poor conditions for aquatic life. Increased chemical oxygen demand further promotes the growth of algae and aquatic weeds.

The Jagadishpur Tal, an artificial reservoir in Kapilvastu, Lumbini province, built for irrigation and fed by the Vanganga river, is rich in biodiversity but faces algal bloom risks due to its highly productive nature. A 2008 study by Tribhuvan University's Environmental Science Department revealed the reservoir to be hypereutrophic, with excessive macrophyte growth and elevated levels of phosphate (>387 µg/L) and nitrogen (>200 µg/L), injuring its ecological health.

The Beeshazari lake, located in Chitwan National Park's buffer zone, has experienced significant algal blooms. A 2012 study by Niraula et al. found high concentrations of nitrogen (>1324 µg/L) and phosphorus (>218 µg/L), supporting the growth of aquatic macrophytes like Lersia hexandra, Trapa bispinosa and Eichhornia crassipes. These blooms decrease the lake’s aesthetic value, reduce open water surface and lower oxygen levels, threatening biodiversity and ecotourism.

The Ghodaghodi lake, an oxbow lake in Kailali district of far-western region, has phosphorus levels exceeding natural ranges and WHO guidelines, indicating hypertrophic conditions and significant algal blooms. Elevated nutrient levels, mainly from agricultural runoff and human encroachment, threaten the lake’s ecological balance and biodiversity.

A key economic driver in the Tarai region, however, fish ponds are experiencing rising red algal blooms, primarily caused by Euglena sanguinea, as reported by Mandal and team from Agriculture and Forest University in 2016 and 2018. The excessive use of fertilizers by farmers to boost fish production creates eutrophic conditions that promote the growth of harmful phytoplankton, including euglenophytes. These red blooms, common in shallow, nutrient-rich waters, threaten pond ecosystems, fish health, public health and the local economy.

Socioe-conomic impact

Nepal’s lakes and reservoirs are vital for ecological balance, biodiversity, and ecotourism. Algal blooms, driven by nutrient influx from rapid urbanization, agriculture, excessive use of pesticides and fertilizers, deforestation and improper waste disposal, pose severe ecological, health and socioeconomic menaces. Without intervention, eutrophication could irreversibly damage aquatic ecosystems and the livelihoods they support.

Economically, tourism—crucial for the local and national economy—is especially vulnerable. Degraded water quality from blooms diminishes the aesthetic and recreational values, deterring tourists and reducing revenue. Additionally, foul-smelling lakes deter boating and fishing, further impacting ecotourism. The fishing industry suffers as well as algal blooms create ‘dead zones’ with low oxygen, leading to fish kills and reduced fish stocks, which threaten fishermen’s livelihoods. Contaminated fish are unsafe for consumption, decreasing market demand and harming the industry.

Socially, communities that rely on water bodies for drinking, irrigation, aquaculture and fishing face economic and health risks. Cyanobacterial toxins can contaminate drinking water supplies, leading to severe health consequences. Despite the gravity of the issue, community and local government involvement in managing and mitigating algal blooms remains minimal. Raising public awareness and encouraging local participation in water management and conservation is crucial.

The path forward

Global efforts to combat algal blooms offer valuable lessons. In 2022, the US Environmental Protection Agency’s One Water approach highlighted the need for multisectoral collaboration among agriculture, industry and government to manage nutrient pollution. It emphasizes reducing nutrient inputs through sustainable farming, enhanced wastewater treatment and stringent regulation, showcasing the need for multi-stakeholder cooperation in mitigating water pollution.

A promising approach from Japan involves using algicidal bacteria associated with seagrass beds to inhibit toxic Chattonella antiqua—fish-killing algae. Dr Inaba’s research highlights this eco-friendly solution that avoids harsh chemicals and underscores the importance of restoring seagrass beds, serve as natural habitats for these beneficial bacteria and maintain coastal ecosystems.

In the USA, biological control using parasitic organisms, such as Amoebophyra, targets toxic algae species-Alexandrium but requires careful evaluation to avoid long-term ecosystem disruption. 

Bioremediation with algicidal bacteria like Thalassospira has shown promise in lysing harmful algae such as Karenia mikimotoi, Proteobacterium, Pelagibaca abyssi inhibits toxic dinoflagellates like Pyrodinium bahamense, while Pseudomonas fluorescens affects winter-blooming diatoms, Stephanodiscus hantzschi, though results vary. These findings highlight the need for extensive field testing of biological controls to validate the efficacy in natural ecosystems. Introducing invasive species or large-scale bacterial dispersals raises biosafety concerns, as bacteria like Vibrio, Shigella, and Alcaligenes pose risks to health and the environment. Recent nanoparticle research shows potential through photocatalysis, but toxicity concerns necessitate careful evaluation.

To mitigate rising algal blooms, a multipronged strategy is essential, including reducing agricultural runoff, preventing waste dumping, and enforcing regulations to limit nutrient pollution. Effective early detection, monitoring and research are crucial for assessing algal species, bloom development, and specific impacts, allowing for timely management. Increasing community awareness and involvement, along with empowering surrounding communities, youth volunteers and local governments, is vital for effective water management and conserving natural resources, which will enhance ecosystem resilience.

In conclusion, algal blooms pose a significant empirical menace to Nepal’s aquatic ecosystems health and economic future. However, through the effective implementation of preventative measures, regulatory enforcement and bioremediation techniques, we can restore the ecological balance of these lakes and reservoirs. Preserving these much-needed freshwater bodies is crucial not only for environmental sustainability but also for ensuring the country's long-term economic prosperity.

The authors are Nepal-based researchers in the frontiers of biological sciences