The Biology of Aquatic Macrophytes

Macrophyte (= large plants) are conspicuous plants that can be seen and identified without the need for microscopic observation. It therefore excludes microscopic algae.

Aquatic macrophytes are defined as those plants that are dependent on free or flowing water i.e. water level is above the substrate (soil) level. Thus aquatic macrophytes are found in rivers, stream, drains, ponds, lakes, swamps and lagoons. It does not include those that need moist soils or occur along the edges of waterways as riparian plants.

Aquatic macrophytes are essential in any healthy freshwater aquatic ecosystem for the following reasons:

  1. Plants are the basis of all food chains. By photosynthesis plants use energy from sunlight to combine carbon dioxide, water and inorganic nutrients into the materials required for their own growth. Plants can be described as autotrophic or self-feeding. Plant tissue in turn provides food for other organisms.

  2. By the same process of photosynthesis, plants produce oxygen which is absorbed by aquatic animals for respiration. Many actively photosynthesising plants are referred to as oxygenators for this reason.

  3. Because plants absorb the nutrients they require from the water, they have the effect of lowering the concentrations of nutrients in that water. This is the basis for the use of aquatic macrophytes in the treatment of sewage and waste water.

  4. Plants provide invertebrates and other fauna with more than just food. They also provide protection shelter, shade, nest sites, substrate.

  5. Plants stabilise the banks of waterways reducing the effect of erosion.

  6. Healthy plants improve the aesthetics of a waterway and increase its value as both an active and a passive recreational resource.

However there can be issues related to macrophytes:

  1. Macrophytes can restrict water flows and impede drainage. This increases the height of water tables and the risk of flooding. Indeed, in the lower Styx water levels drop by 20-30 cm after ‘weed clearance’.

  2. As macrophyte biomass increases oxygen levels can fluctuate dramatically particularly when they decompose. This stresses wildlife and may render the water uninhabitable.