Biodiversity Nepal
For the Future Generation

Why are our Lakes Turning Green?

Eutrophication leads to algal bloom, hypoxia and ultimately a dead zone.

Soil-net

If you have visited Phewa Lake or Beeshajari Lake, have you ever noticed a green layer of algae on the surface? They may look like harmless aquatic species at first, but deep down these protists are sucking the life out of these lakes.

The carpet of algae forms in the water due to a process called eutrophication. It occurs when any water body accumulates excessive amounts of phosphates and nitrates. Though these nutrients are naturally available, their concentration in water is spurred by two main reasons: agricultural runoff and sewage discharge.

In the last few decades, human interventions have emitted nearly twice the amount of nitrogen and thrice the amount of phosphorus compared to natural sources. Rapid urbanization, population growth and increased water pollution have added extra nutrients into our water bodies. Likewise, anthropogenic activities like the discharge of sewage, industrial effluents, and agricultural runoff have significantly increased eutrophication.

If available in excess, phosphorus and nitrogen stimulate the disproportionate and rapid growth of algae and other phytoplanktons. This phenomenon, also known as an algal bloom, forms a green layered blanket on the water surface and prevents the sunlight penetration in the benthic ecosystem.

Once these algae begin to die, more problem arises for the aquatic species. Decomposing bacteria use a large amount of oxygen to break down the algae. In fact, they almost use all the aquatic oxygen available. Though some mobile organisms can migrate, most of the slow-moving animals and plants suffocate in this these hypoxic i.e. oxygen-depleted area. As a result, the region can no longer sustain any species and is declared as a “dead zone”.

Fish dead in a lake due to eutrophication (Source: EPA)

Currently, there are more than 400 dead zones worldwide. For the past 50 years, the numbers have increased dramatically with 169 added between 1960 to 2007 only. The largest and most prominent dead zone lies in the Arabian Sea which is almost a 63,200-square mile in area. Moreover, 233 areas around the world are in a vulnerable place of transforming into a dead zone.

Though Nepal currently doesn’t host any dead zones, the recent findings have been alarming. According to a research published in the Journal of the Institute of Engineering (2019), two of the famous lakes of Nepal, Begnas and Rupa Lake, were indicated as having mild organic pollution. The CO2 and Phosphate concentration in these lakes were much higher than WHO guidelines with an indicated rapid eutrophication. 

Previously, a 2018 study had concluded similar results from Jagadishpur Reservoir, Kapilvastu. The investigation indicated a high level of nitrogen, rapid eutrophication and low transparency in the lake. Another limnological study conducted in Besshazari Lake concluded the same findings. 

Eutrophication in Beeshajari Lake, Chitwan (Source: Tunza-Eco Generation)

Eutrophication and restoration of aquatic communities is a complex issue and will require equal efforts from policymakers, scientists and citizens. To prevent it from occurring, regulation of the nutrients level entering the water bodies is extremely necessary. This can be achieved by controlling sewage and agricultural runoff from the point source. Similarly, water purifying systems can be installed in the industry to filter excess pollutants. In addition, planting vegetation along the water sources can also absorb extra nutrients.

Protecting our freshwater resources is one of the most pressing environmental concern. They are homes to plenty of our aquatic and terrestrial biodiversity. In addition, human health and wealth are a direct part of the ecosystem. Any delay in the conservation programs are likely to be more complicated in future as the temperature rises and pollution further degrades our water. If we don’t prevent our lakes from turning green now, the chances are it may never be alive again!

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