The explorers were surprised when they came across the Ruky River. A tributary of the mighty Congo River, the water of this river is so dark that you literally can’t see your hand in front of your face. “We were amazed by the color of the river,” says ETH Zurich researcher Travis Drake, who published Ruki’s research in the journal “Limnology and oceanography” Together with colleagues from the sustainable agroecosystem group led by Johan Six. from other universities as well.
Comparisons with other major tropical rivers suggest that the Ruqui may be the blackest major blackwater river in the world; probably much darker than the famous Rio Negro in the Amazon.
The reason the water is black is because it contains a large amount of dissolved organic matter and almost no sediment due to the low slope of the river. These carbon-rich substances are mostly washed into the river by rain falling on dead forest vegetation, and organic compounds are leached from decaying plant material. Moreover, during rains, the river floods the forest. It can take weeks for the water to slowly drain, often up to waist level, as it washes away organic matter. “Hands are basically jungle tea,” says Drake.
Peat bogs and virgin tropical forests
It’s not just dark water that’s special. The tributary, which is one kilometer wide and flows into the Congo, is unique in its integrity. The catchment area, which is four times the surface area of Switzerland, is still covered by virgin primary lowland rainforest. There are large peatlands along the river that contain huge amounts of dead, undecomposed plant material, making them important carbon sinks.
Despite its uniqueness and size, the Hand has never been the subject of scientific study before. Although seasonally different water levels have been recorded in the river since the 1930s, there is still no data on its chemical composition. No one has yet determined how much dissolved organic carbon (DOC) is in water and where it comes from.
So in 2019, Drake and his colleagues set up a gauging station near the town of Mbandaka, just above the confluence of the Ruka and Congo rivers, and measured water flow every two weeks and daily water levels throughout the year to determine water levels. annual flow
“Our measurement methods in the field were pretty simple,” says Drake. He says Mbandaki has no permanent electricity supply, only a few diesel generators and almost no infrastructure, not even an electric drill to install a flood level indicator. “So we often had to improvise,” he says with a smile.
What do water samples say about Hands?
Water samples were collected at each discharge measurement and sent to the ETH Zurich laboratory for analysis. Here, the researchers determined the DOC content of the samples and the age of the organic material based on DOC radiocarbon. Among other things, they wanted to know whether the peat along the river was releasing carbon (and decomposing into CO2 at some point).
The researchers chose to study water because it contains basin-wide carbon footprints, which convey information about the origin and use of land. The advantage of water analysis: “We only need to collect samples from one place to get information about a very large area – just like a doctor takes a blood sample to determine the patient’s health status,” says Matti, co-author of the study. Barthel.
Unexplored hydrology
Analysis confirmed the visual impression: “The Ruk is one of the richest DOC river systems in the world,” says Barthel. Its water contains four times more organic carbon compounds than that of the Congo and 1.5 times more than the water of the Rio Negro in the Amazon.
Although the Ruka basin accounts for only one-twentieth of the entire Congo Basin, one-fifth of the dissolved organic carbon in the Congo comes from this flow.
DOC generally occurs as organic acids that increase the acidity of river water. This stimulates the release of carbon dioxide (CO2) as the acids dissolve the carbonates present in the water. “CO2 emissions are relatively high in the Ruka basin, but no different from other tropical rivers,” explains Drake. This is because Ruki is a stagnant and calm river, and it is more difficult for carbon dioxide in the water to escape into the air. “We will see higher emissions in an angry river,” he says.
Carbon isotope analysis also shows that most of the carbon comes from forest vegetation, not peat, Drake said. Researchers find evidence that peatlands release carbon into the water only for a short period of time following heavy runoff, at the end of the rainy season from March to April. It is currently unclear how and why this occurs. “But overall we see very little peat in the river,” says Drake. “And that’s good news because it also means the peatlands are stable.”
He said that because peatlands are now under water almost all year round and therefore not exposed to oxygen, there is no danger of the organic matter they contain escaping. However, companies are showing interest in the natural resources of the Ruka Basin. Changes in land use, such as deforestation, can alter the river’s regime. This can lead to peat bogs drying out and being decomposed by bacteria, releasing large amounts of CO2. “Peatlands in the Congo basin store approximately 29 billion tonnes of carbon,” says Bartel. “It would be better for the climate if they stayed wet.”
An old research project
Johan Six and his group have been studying the carbon cycle of the Congo Basin since 2008. Congo is one of the most important tropical river systems in the world. It removes large amounts of carbon from the sea. Vegetation in a watershed affects the carbon cycle, as does land use. Researchers came across the Ruki River during an expedition to study the carbon cycle (biogeochemistry) of the Congo Basin.
Following the Ruki trip, scientists are now studying other tributaries of the Congo, such as Kasai and Femi, and are slowly solving the carbon puzzle.
Source: Port Altele
