A few days ago, as I was reading local online news from back home, my attention was captured by an article raising a huge concern over the colour change of waters of Lake Kivu.
A very interesting illustration of a fisherman wondering what colour should be the fish, if the lake’s water has turned into green.
I reflected back in middle school when our teachers had us memorise the properties of water: colourless, insipid, tasteless… wait I forgot the last two.
Kivu is one of the African Great Lakes – a series of lakes in the eastern part of Africa’s Rift Valley, and is home to the world’s 10th largest inland island. It is a beautiful place—and if you have been there, there is no place in Africa that you can compare to the beauty of its beach and sensual breezes that blow you on a sunny day.
When you use satellite images, a big portion of the surrounding land is dominated by farming activities.
In a natural environment, a lake like Kivu that originated from volcanic activity and whose bed is made of sand and igneous rock, it shouldn’t look green. Other factors should be taken into consideration and it should be an alarm of possible bio chemical processes undergoing at the lake’s bottom.
The green colour is a result of possible vegetation developing in Lake Kivu’s bed. An easy experiment can be made: if you take cabbage leaves or any green leaf, put it into a transparent jar and fill it with water and let it sit under the sun for a couple days.
On the second day, you will realise that water in the jar startedturning into green due to leaves deposition.
So, what is the possible process that made Lake Kivu turn into green? Obviously, Rubavu residents didn’t dump cabbage or green leaves into the whole lake.
These past six months, the Rwandan Meteorological Agency has recorded the highest rain events ever known in this decade.
For instance, the rain intensity for these previous four months was extremely high and resulted into flooding where massive land/ soils were transported through erosion process.
Rubavu residents suffered from flooding that enormously impacted houses, crops and human lives.
The transport of soils from land to the lake results also into soil nutrient loss. The common nutrient that is behind Kivu’s green colour is phosphorus, one of major components that compose NPK, a fertilizer distributed to local farmers to improve crop yields, and also present in cow manure.
When phosphorus gets transported into the lake, it causes algal blooms (we commonly see them on dead trees or rocks). Algae plants are green and their deposition under the lake followed by sun reflection might be the reason behind the Lake’s green color.
Is it a bad indicator to have green water? Yes, scientists have proven that the development of algae leads to the depletion of oxygen levels in water. Once there is no oxygen, it becomes harder and even impossible for fish to survive, and we know how Kivu plays an important role in feeding us with sardine fish known as “Isambaza”.
If the algae population continues to increase, we might completely run out of sardines and also compromise the quality of the only country’s large lake. Would there be any effect to methane gas deposited in Lake Kivu? Well, I prefer to answer this question next time.
Collective efforts should be taken to conserve agricultural land in order to avoid soil/nutrient loss caused by erosion. Farmers near the shore should be aware of proper amount of fertilizer to use and how to manage their farm fields in a way that reduces erosion risks.
The writer is a Rwandan pursuing his master’s degree in environmental sciences and policy at the University of Wisconsin, Green Bay, USA.