Eco Bites

Potamogeton in Rietvlei

A problem for recreational users of Rietvlei and the members of the Milnerton Aquactic Club is the excessive growth of Potamogeton pectinatus, the water grass also known as sago pondweed, or fennel-leaved pondweed. It has always been present in Rietvlei, but the last few years have seen a bloom in growth around the edges of North and South Lake. There are about 20 Potamogeton species internationally, most of which occur only in the northern hemisphere; Potamogeton pectinatus is the only one found throughout the world. It grows in wetlands, lakes, rivers, canals, ditches and ponds from sea level up to nearly 5 000m above sea level; it is also the only Potamogeton species found in brackish water. It has been recorded growing in waters 6-7m deep, but it is dependent on light penetration and in turbid waters will rarely grow deeper than 2-3m. Potamogeton stands are important feeding and rearing habitats for waterfowl, fish and many other organisms. It is not only a source of food, but also provides a protected habitat for many aquatic animals. In many parts of the world migratory waterfowl have established their migration pathways via water bodies dominated by Potamogeton spp. In Rietvlei and many other wetlands in South Africa it is a major source of food for coot (bleshoender) and several other waterfowl species, as well as many fish species - they not only eat the plant itself, but also feed on the large populations of microfauna living on Potamogeton stems and leaves. Another positive function of Potamogeton is that it protects shorelines from wave erosion. The plants form wide stands close to shore and is a very effective buffer against waves. This can be seen quite clearly in Rietvlei when the southeaster is blowing strongly; the Potamogeton stands along the northern shores of North Lake very effectively dampen the waves and minimise erosion. However, there is also a downside to Potamogeton. Because it grows so prolifically, it can clog irrigation canals and is a nuisance to recreational users – it interferes with fishing lines and winds around powerboat propellers. It is also unpleasant and can even be dangerous for swimmers to venture into Potamogeton stands as they can get tangled up in the plants and get into difficulties. Control of Potamogeton Numerous research projects into control methods have been carried out throughout the world, including the use of herbicides and other chemicals. These herbicides range from aromatic solvents, copper sulphate solutions, diquat, paraquat, 2,4-D butoxethanol ester pellets, and numerous others. While many affected the growth of Potamogeton temporarily, none stopped sprouting and the plants were not exterminated. Many also have deleterious effects on aquatic life and are not recommended for use in open waters. (Please note that the use of aquatic herbicides requires approval from various authorities). Mechanical methods of removal have also been tried; these include using large rakes, cables, and mechanical harvesters. While such methods are successful in clearing artificial canal systems and in opening channels through Potamogeton stands, they are not permanent and must be repeated at regular intervals to keep growth at bay. Biological control methods have been studied in various parts of the world; a burrowing chrysomelid beetle causes great damage to Potamogeton, but has not permanently eradicated the plant in study areas. Grass carp was introduced into the United States to control Potamogeton and other nuisance aquatic plants, but it has exterminated virtually all aquatic plants in water bodies to which it had been introduced and caused a complete collapse of the ecosystem – leading to the disappearance of other fish and bird life in such systems. Today the transportation of grass carp world-wide is strictly controlled, South Africa has strict legislation in place to prevent importation of grass carp (several years ago grass carp were imported by several fish farmers, but these populations are under strict surveillance to prevent escape into natural systems). The problem recreational users have with Potamogeton in Rietvlei will not be easily solved. People using Rietvlei will have to accept that the plant is here to stay and that they must live with it. The only remedy is to clear channels in the Potamogeton stands to allow boats access to the shore, it can also be cleared in areas where anglers require access to the water. However such clearing will have to be repeated at regular intervals to ensure that these channels stay open. Compiled by Niel van Wyk from various sources.