Using aquatic plants to enhance raw water quality

NUS scientists are exploring an ecological approach to enhance the water quality at Pandan Reservoir.

Freshwater ecosystems can exist in different conditions or states depending on the balance between competing ecological mechanisms. For freshwater reservoirs, it can exist in two alternate states. One of them is a turbid-water state dominated by phytoplankton (e.g. microscopic algae and cyanobacteria).  The other is a clear-water state dominated by aquatic plants. Aquatic plants can help facilitate a switch to the more desirable clear-water state and inhibit the growth of cyanobacteria, also known as blue-green algae, which has the tendency to form algal scums that are aesthetically unpleasant and costly for water treatment.

A research team led by Prof Darren YEO Chong Jinn, from the Department of Biological Sciences, NUS, together with Dr Simon MITROVIC from University of Technology, Sydney, is partnering PUB, Singapore’s National Water Agency, to carry out a two-year research project on the use of aquatic plants at Pandan Reservoir. The team had identified three species of aquatic plants (water banana (Ludwigia adscendens), knotweed (Persicaria barbata) and tapegrass (Vallisneria spiralis)) which are highly effective in controlling the growth of phytoplankton in our local reservoir setting on a small scale (up to 1,000 litres). They are also working on determining the aquatic plant population density that triggers the switch to a clear-water state.

Prof Hugh TAN Tiang Wah, who is the co-principal investigator said, “Shortlisting species was not easy as the plants had to fulfil many criteria such as being available locally in our reservoirs, speed of growth, ease of maintenance, and so on”.

“A natural assumption would be to select weedy plant species (which are fast-growing) for such a project, but taking into consideration other factors for reservoir management, the challenge was to identify effective but less weedy (i.e. non-invasive with lower maintenance) aquatic plants which may be used in combination with faster growing species that could trigger a switch from cyanobacteria/ phytoplankton dominance,” said Prof Yeo.

The team has commenced the next phase of the project which involves larger scale experiments (~250,000 litres and 100 m² coverage) with the identified aquatic plant species.

Figure 1 shows two aquatic plant species from the small scale experiments which are effective in reducing phytoplankton biomass: (A) water banana (Ludwigia adscendens) and (B) knot grass (Persicaria barbata). [Image credit: Maxine A. D. MOWE]