Harmful algal blooms (HABs) are a problem in drinking water supply reservoirs because they can produce unpleasant tastes and odours in the water. Some cyanobacterial HABs produce cyanotoxins which can cause illness or death in livestock and humans. Environmentally, HABs can clog fish and invertebrate gills, and shade aquatic vegetation. Decomposing blooms can lower oxygen levels, causing fish kills, and mobilising nutrients from the sediments. There is therefore considerable interest in methods to control such blooms.
Limiting the runoff of nutrients helps to reduce HABs, but is difficult to achieve while maintaining crop fertility. Traditional methods for controlling algae, such as algicides, flocculants and adsorbents, are often toxic, and are too expensive to use in large waterbodies. A more environmentally-friendly approach is to use macrophytes (large aquatic plants) to control the blooms. Macrophytes can inhibit algal growth by competing with them for nutrients, or by shading them and reducing their rate of photosynthesis. Some macrophytes can also release chemicals known as allelochemicals, which can inhibit algal growth.
This research aimed to identify species of macrophytes that can produce allelochemicals which can inhibit growth of two common species of cyanobacteria, Microcystis aeruginosa and Dolichospermum circinale. For each plant species, exudates and/or crude plant extracts were mixed with nutrients and cyanobacterial culture in wells of 24-well microplates. From the growth curves of the cyanobacteria, the lag phase, maximum growth rate (µmax) and carrying capacity (K) were determined and compared with controls.
The macrophytes Ludwigia peploides and Ceratophyllum demersum have been shown to affect cyanobacterial growth, predominantly by increasing the length of the lag phase before the exponential growth phase. Facilitating the growth of these plants in drinking water reservoirs may be used as a management tool to help control cyanobacterial HABs.