The consequences could be “problematic”, points the MRC
The recent aquamarine color of the Lower Mekong is possibly a result of the extremely low flow, slow drop in the river sediments, and presence of algae on the sand and bedrock river bottom, the Mekong River Commission (MRC) has said of the phenomenon for the first time after speculation of dam impacts has been locally circulated.
The inter-governmental organisation regulating rules on use of the river said that the phenomenon may also spread to other parts of the River where low flows occur.
The Mekong River at Thailand’s Nakhon Phanom and Lao PDR’s Thakhek has lately taken on a blue-green hue. Based on the MRC’s preliminary analysis that looked at the causes and potential impacts of the phenomenon, the organisation has contributed the likely causes to “many factors” that may have contributed to the occurrence.
It said the consequences could be “problematic”.
The organisation said its analysis has indicated that “the extremely low flows” now being experienced in the Mekong, during one of the worst droughts ever experienced in the region, have changed the water color.
The fine sediments normally found in the fast flowing water and deeper water levels that give the water the brownish color have dropped out, creating clearer water conditions.
This means that when the sunlight hits the river, the water strongly absorbs what is known as the “long-wavelength colors” at the red end of the light spectrum, and makes the river look blue, the analysis notes. This occurs in just a few meters of water, it addes.
The clearer water allows microscopic plants or algae to grow on the sand and bedrock river bottom causing the margins of the river to turn green, notes the analysis.
These algae are normally flushed away by the river current. But due to the exceptionally low river level, this is not happening at Nakhon Phanom and perhaps elsewhere. Instead, the algae are building up, the MRC’s analysis points.
The analysis warns that the conditions could probably be made worse if fertilizers used in agriculture enters the river feeding the growth of algae.
“The blue-green water phenomenon is likely to spread to other stretches of the Mekong River where low flows are encountered,” said the MRC Secretariat’s Chief Environment Management Officer Dr. So Nam, who led the analysis. “The issues of low flows and sedimentation could possibly lead to adverse impacts that have been well-quantified in the MRC Council Study.”
According to the analysis, some of the potential impacts include change in productivity of the river with less foods available for insects and small fishes, and reduced productivity of aquatic biodiversity, including fish due to high water clarity.
This will in turn affect the fish catches and the livelihoods of local communities.
High water clarity also leads to significant algal growth or algal bloom. During the day, this leads to high dissolved oxygen levels, but to very low dissolved oxygen at night. This can affect lives of different fish, according to the analysis.
The analysis also added that if water clarity remains high, the algal bloom can cause thick green mats that will rot and smell badly. It warns that if the conditions get worse, the algae can change from green species (Chlorophyceae) to blue green species (Cyanophyceae), producing toxic substances that can harm animals.
But Mr. Nam said that it is unlikely such conditions will occur in the main river, and be restricted to backwaters. However, he warned that people should be careful watering their animals if the water is very green.
Based on the analysis, the present conditions of blue-green appearance in the Mekong may persist until flows in the river increase at the onset of the next flood season, which usually begins in late May.
The conditions may also “recuperate”, the analysis points, if large volumes of water are released from the storage reservoirs in the Upper Mekong (Lancang) dams and tributary dams to mobilize sediments and give the Mekong its typically brown color.
The MRC’s flow monitoring shows that dry season flows have increased during the past few years due to water release from reservoirs in order to produce electricity.
The analysis does not mention any possible dam impacts on the unusally low flow despite widening speculation over the roles of the dams upstream, including the newly operated Xayaburi Dam.