Researchers from the Council for Scientific and Industrial Research (CSIR) have found that some alien invasive water plants choke estuaries, tossing them off balance – literally.
The study was led by Aadila Omarjee, senior researcher, at the CSIR.
South Africa’s estuaries are home to a unique mix of plant and animal life and provide vital ecosystems services. However, Omarjee says estuaries on South Africa’s subtropical east coast are especially vulnerable to invasive water plants. Many of these estuaries naturally close off from the sea during dry seasons, leaving them more exposed to pollution-driven plant growth.
Pollution, often from agricultural and urban runoff as well as the lack or failure of sewerage systems, leads to eutrophication – a process in which excess nutrients supercharge plant and algae growth.
While plants may seem like a good thing, too much of the wrong kind can cause serious problems.
The wrong kind are water lettuce (Pistia stratiotes), a floating invasive alien plant, and filamentous algae (Rhizoclonium riparium), a cosmopolitan species that reaches nuisance levels when it grows excessively.
By simulating real estuary conditions, the researchers measured how these plants affected pH levels during the day (when photosynthesis happens) versus at night (when respiration takes place).
Ormarjee said “we simulated natural estuarine conditions specifically to see how aquatic plants affect daytime and nighttime pH levels. In other words, our question was: How does the pH of the water that the animals live in change on a daily basis?”
Omarjee’s pH study forms part of the CSIR’s extensive research on estuaries to assess their health, identify threats and develop strategies for sustainable management and restoration.
The results demonstrated how aquatic plants can absorb carbon dioxide and hydrogen carbonate from the water during daytime photosynthesis, thereby raising the pH (lowering acidity). At night, however, they release the carbon dioxide back into the water, lowering the pH again (increasing acidity).
Although natural pH fluctuations in estuaries are normal and important for biological and chemical processes, an excess of these plants can lead to extreme day-night shifts in pH or long-term acidification. This, in turn, can increase the toxicity of metals to animals and plants living in the water.
In the case of subtropical east coast estuaries, the submerged algae caused the biggest pH swings between day and night, while water lettuce made the estuary more acidic over time. This happens because submerged algae absorb carbon dioxide from the water during the day, raising the pH, and release it at night, making the water more acidic. In contrast, floating water lettuce releases its photosynthesis byproducts into the air. However, over time, water lettuce also contributes to increased acidity in the water.
“This balance between photosynthesis and respiration drives daily pH changes.”
Omarjee highlights that this study is the first to show how South Africa’s estuaries – especially smaller, temporarily closed ones – are particularly vulnerable to these disruptions. Most previous research has focused on larger, permanently open estuaries in the northern hemisphere, which behave differently.
“South Africa’s estuaries provide essential ecosystem services and support rich biodiversity, so understanding threats like eutrophication is critical,” says Omarjee.
The study was funded by a CSIR Parliamentary Grant and CSIR Young Researchers Establishment Fund.
Picture of Water Lettuce