Eutrophication and Algal bloom

The top few meters of any water body of contain planktonic algae. They fix carbon dioxide into organic molecules that can be consumed by other organisms. They release oxygen (0₂) as a by-product of their photosynthesis. Though they are part of normal and beneficial flora, occasionally they cause disturbances such as turbidity, discolouration and unpleasant odour and taste. Seasonal changes in nutrients, light, and temperature cause fluctuations in algal populations; periodic increases in numbers of planktonic algae are called algal blooms. Algae thrive in high concentrations of organic materials that exist in sewage or industrial wastes. When algae die, the decomposition of the large numbers of cells associated with an algal bloom depletes the level of dissolved oxygen in the water.

Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem. There is increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce use of water body.

Biodegradable detergents containing phosphates have a major role in eutrophication. Influx of nitrogen and phosphate rich matter into lakes and streams result in overabundance of nutrients or eutrophication. Algae and cyanobacteria get their energy from sunlight and their carbon from carbon dioxide dissolved in water. In most waters, only nitrogen and phosphorus supplies are the limiting factor for algal growth. When there is inefficient waste treatment, these nutrients enter water from domestic, farm, and industrial wastes and they cause algal blooms. Many cyanobacteria can fix nitrogen from the atmosphere, so these photosynthesizing organisms require only traces of phosphorus to initiate blooms.

Process

Eutrophication is characterised by a significant increase of algae due to the greater availability of one or more growth factors necessary for photosynthesis, such as sunlight, carbon dioxide and nutrients (nitrogen and phosphorus). This results in blooms of algae or cyanobacteria, which occupy the surface layer of water body and can interfere with oxygen diffusion. These algae and cyanobacteria supply oxygen in the beginning which can be utilized by other life forms in water. However, they eventually die and are degraded by bacteria. During the degradation process, the oxygen in the water is used up, killing the fish. Undegraded remnants of organic matter settle to the bottom and add to the organic load of the lake. To destroy all the dead algae, an excessive consumption of oxygen is required, in some cases almost total, by microorganisms. This affects the growth of other life forms in water. The absence of oxygen reduces biodiversity causing even the death of animal and plant species.

An anoxic (oxygen-free) environment is created on the lake bottom, with anaerobic organisms carrying out degradation of the biomass. The microorganisms, decomposing the organic substance in the absence of oxygen, produce compounds that are toxic, such as ammonia and hydrogen sulphide (H₂S). The anaerobic conditions promote the growth of bacteria such as Clostridium botulinum which produces toxins deadly to aquatic organisms, birds and mammals. 

Most algal blooms are not harmful, but some produce toxins. These are known as Harmful Algal Blooms (HABs). Some algal blooms when died or eaten, release neuro & hepatotoxins which can kill aquatic organism & pose a threat to humans. E.g. Shellfish poisoning. Gonyaulux and other dinoflagellates produce toxins which are posionous to fish. Toxigenic blue green algae like Anabaena, Microcystis, Nostoc etc produce toxins that kill fish or contaminate drinking wtaer supplies. HAB adversely affect commercial and recreational fishing, tourism, creating a significant impact on local economies and the livelihood of coastal residents

Thus, the two most acute phenomena of eutrophication are hypoxia (or lack of oxygen) in the deep part of the lake and algal blooms that produce harmful toxins, processes that can destroy aquatic life. Algal blooms are unsightly and can have unpleasant smells. Infected waters increase the costs of water treatment. It is a threat for fishing industries, Increased vegetation may impede water flow and the movement of boats The water may become unsuitable for drinking even after treatment.

                                                          

          Causes

Eutrophication is a serious environmental problem since it results in a deterioration of water quality. All water bodies are subject to a natural and slow eutrophication process, which in recent times is accelerated due to the presence of man and his activities (cultural eutrophication). The cultural eutrophication process consists of a continuous increase in the contribution of nutrients, mainly nitrogen and phosphorus (organic load) until it exceeds the self-purification capacity of the water body, causing structural changes in the waters.

The factors contributing are:

Use of fertilisers: Agricultural practices and the use of fertilisers in the soil contribute to the accumulation of nutrients. When these nutrients reach high concentration levels and the ground is no longer able to assimilate them, they are carried by rain into rivers and groundwater that flow into lakes or seas.

Discharge of waste water into water bodies: In various parts of the world, and particularly in developing countries, waste water is discharged directly into water bodies such as rivers, lakes and seas. The result of this is the release of a high quantity of nutrients which stimulates the disproportionate growth of algae.

Reduction of self-purification capacity: Over the years, lakes accumulate large quantities of solid material transported by the water (sediments). These sediments are such as to able to absorb large amounts of nutrients and pollutants. Consequently, the accumulation of sediments starts to fill the basin and this leads to a further deterioration of water quality, increasing the processes connected with eutrophication.

          Control

The main control mechanism of the eutrophication is based on prevention techniques.

The first step is to limit the flow of nutrients into waters. Proper disposal of urban and agricultural wastes and reduction of phosphorous in detergents is important. Excessive application of fertilizers to crops should be avoided.

Advanced treatments to remove the nutrients that are introduced into water bodies from the water, particularly nitrogen and phosphorus by precipitation methods, microbial activities such as denitrification, adsorption with activated carbon, etc. can be effective.

Algicides may be used to control/kill algae. Biological control may be attempted using bacterial predators or viruses.

Further Reading

https://www.pmfias.com/water-pollution-biological-oxygen-demand/