The news of fish deaths in freshwater bodies such as rivers, lakes, ponds is not new to India. Time and again, we continue to hear reports of mass fish deaths from different parts of the country like the very recent one in Krushnale lake in Panvel, Maharashtra, in Mumbai where hundreds of fish were found dead and floating in the Banganga tank at Walkeshwar or that in a pond in Vishakapatnam, Bengaluru’s Mottanalluru lake, in a pond in Guwahati, or a lake in Kolkata.

Why and how do these events happen

Fish deaths occur due to reduction in oxygen content in the water. One of the important reasons for this is the presence of algal blooms that are formed due to rapid and uncontrolled growth of phytoplankton in freshwater bodies inform Rajendra Prasad and Samendra Prasad (2019) in the paper titled 'Algal blooms and phosphate eutrophication of inland water ecosystems with special reference to India' in the International Journal of Plant and Environment.

Phytoplankton, crucial for freshwater ecosystems

Phytoplankton are microscopic organisms that normally live in both salty and freshwaters and include bacteria and some single-celled plants. The common types of phytoplankton include cyanobacteria, diatoms, organisms such as dinoflagellates, green algae, and one celled plants such as coccolithophores.

Phytoplankton are crucial for the survival of life in the water and for maintaining the ecological balance of freshwater bodies such as rivers, lakes and ponds. They have chlorophyll and use sunlight to produce energy through the process of  photosynthesis. They consume carbon dioxide, and release oxygen. Some phytoplankton also consume other organisms to get additional energy.

Phytoplankton survival and growth depends on the availability of  nutrients such as nitrates, phosphates, silicates and calcium in the water. Some phytoplankton can also  fix nitrogen and can grow in areas where nitrate concentrations are low. They also require trace amounts of iron. Other factors that influence phytoplankton growth include water temperature and salinity, water depth, wind and the kind of predators grazing on them.

Phytoplankton are the foundation of the aquatic food web as they function as primary producers and serve as food for microscopic animal-like zooplankton to large animals such as whales. Thus, small fish and invertebrates graze on phytoplankton and these smaller animals are eaten by bigger ones.

When phytoplankton kill

However, phytoplankton can also lead to disease and death when water bodies get polluted. When availability of nutrients such as nitrogen and phosphate exceeds normal limits in the water bodies, phytoplankton such as algae and cyanobacteria can rapidly grow out of control and bloom when the water is warm and slow moving and there is adequate amount of sunlight and carbon dioxide. This process is referred to as eutrophication.

Algal blooms can occur in freshwater, marine water as well as brackish (a mixture of fresh and salt) water and can appear like foam or scum on the surface of the water and can change the appearance of water to green, blue, brown, red.

Harmful algal blooms (HABs)

Certain species of phytoplankton produce powerful biotoxins leading to harmful algal blooms that can cause damage to animals living in water. After massive blooms, dead phytoplankton sink to the ocean or lake floor. The bacteria that decompose the phytoplankton deplete the oxygen in the water, suffocating animals that live in the water resulting in a dead zone devoid of oxygen.

Algal blooms also limit light penetration, reduce growth and cause death of plants near the shores of the water bodies (littoral zones) while also affecting the ability of organisms living in the water to access food.

Harmful algal blooms (HABs) can have a negative impact on water quality, fisheries and increase public health risks. Within freshwater ecosystems, cyanobacteria are the most important phytoplankton associated with HABs and blooms of toxic cyanobacteria can lead to poisoning of domestic animals, wildlife and even humans. Cyanobacteria blooms have also been found to affect the safety and potability of municipal drinking water systems as well as fish resulting in financial losses and increase in health risks.

Human activities are further increasing the rate and extent of eutrophication due to discharge of nutrients such as nitrogen and phosphorus from industries and discharge of untreated sewage, wastewater, manure and fertiliser runoff from agriculture.

Climate change is also triggering eutrophication. The 2019 Intergovernmental Panel for Climate Change assessment report states that occurrence of HABs and the impending risks to natural and human systems will increase even more with warming and rising CO2 in the 21st century.

Eutrophication and algal blooms in India

Rajendra Prasad and Samendra Prasad (2019) in their paper inform that algal blooms frequently occur in inland waters such as  Udaisagar lake in Rajasthan, Dal Lake in Jammu and Kashmir, Upper Lake (Bhojtal) in Bhopal in Madhya Pradesh, Chilika lake in Odisha and in River Ganges at Varanasi.

Studies in India show that most of the algal blooms are triggered due to excessive amounts of phosphorous in the water sourced from agricultural runoff,  fossil-fuel burning, wastewater and detergents.

Phosphate concentration in rivers in India has been found to be much higher in northwestern rivers such as Ravi, Sutlej, and Beas as compared to that in the eastern, western or southern India.

Fish kill can also happen due to neurotoxins and heavy metals

Other than hypoxia or low oxygen, Rajendra Prasad and Samendra Prasad (2019) in their paper inform that neurotoxins and methanol produced from algal blooms can also be highly toxic to fish. Fish show fatal symptoms such as difficulties in respiration and swimming, wrinkling and fragmentation and reduction in growth and maturity, when exposed to high concentrations of methanol.

Heavy metals can also lead to fish deaths. Industrial effluents from the industries located at Kanpur, Varanasi, and Kolkata include heavy metals, such as cadmium, lead, and mercury, copper, cobalt, and zinc that get deposited in the rivers such as Ganges and Yamuna and lead to mortality among fish.  Organochlorine pesticides and polychlorinated biphenyls (PCBs) that get deposited in rivers in India through agricultural runoff can also kill dolphins and other kinds of fish.

What can be the way out

Measures need to be undertaken to:

•  To reduce phosphate addition to inland waters by reducing phosphate fertiliser addition to agricultural fields and also reducing the flow of industrial effluents, agricultural run offs and household wastes including washings involving detergents to inland waters. Addition of pesticide residues in runoffs from agricultural fields and heavy metals through industrial effluents also need to be reduced.
• Encourage community involvement in saving inland water bodies
• Prevent pollution of water bodies, undertaking research and initiatives to improve water quality and preserve biodiversity
• Ensure that river flows are retained
• Have better functioning regulatory systems in place