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After ten years of field surveys on various water bodies ranging from stagnant water ponds, pools and man-made lakes to flowing waters such as rivers, streams and canals, there is a clear evidence of four problematic weeds in Malaysia. These species are Eichhornia crassipes, Salvinia molesta, Lemna perpusilla, and Pistia stratiotes. Among these weeds, E. crassipes and S. molesta are widely distributed througout Malaysia. E. crassipes generally dominates canals and rivers although, recently, this species has spread to man-made lakes. The favourable tropical climate and conducive environmental factors help to trigger the massive growth of these weeds. The high nutrient concentrations, notably phosphate which has a soluble reactive concentration greater than 0.1 mg 1–1, initiate a high productivity. Manual control methods are generally used and several herbicides including 2,4-D and glyphosate are frequently employed to eradicate these weeds.

The simplest definition of a weed is a plant that is a nuisance. Thus, an aquatic weed is an aquatic plant which interferes with the use of water, or in some other way constitutes a nuisance to man or hazard to human welfare. However, in contrast with single purpose systems such as an agricultural crop in which weeds are readily identified as such, water bodies frequently have more than one use, and assessment of the weediness of a plant may be confused when it interferes with one use. such as navigation, while promoting another, such as fish production (Mitchell, 1974, 1978). The value judgements made in these situations are often one-sided and based on incomplete understanding of the nature of the system, the role of the relationships involved in these situations. Sometimes, as in the case of Eichhornia crassipes in the Nile (see section 7.2.2) or Salvinia molesta on Lake Kariba (see section 7.2.5), there is general agreement that the plants in question are weeds, though it should be noted that even the most detrimental plant invasion can have some benefits and that adjustments to problems that are intransigent or too expensive to control are always possible.

Subang reservoir was originally oligotrophic. Due to continuous heavy nutrient loading, especially of N, P, K and organic matter from a nearby polluted auxiliary riverine intake source, the reservoir becomes progressively eutrophic. An increase in the total ionic content and a decrease in NO3 occurs, while PO4 and NH3 content fluctuates. A marked successional change in the phytoplankton composition occurs favouring blooms of undesirable algal species viz. Microcystis aeruginosa, and in later stages of the floating macrophyte, Salvinia molesta. On stoppage of the auxiliary intake water, the reservoir gradually becomes soft and acidic. A decrease in the PO4 and NH3 content occurs, while NO3 fluctuates. A somewhat reverse succession al change occurs favouring Staurastrum spp., more characteristic of Malaysian soft, acidic waters.

Euhydrophytes, i.e. floating-leaved rooted, and submerged plants (see Chapter 1). rarely attract the same attention or have the same environmental impact in African waterways as emergent and surface-floating species. The problems with papyrus swamps in East Africa and the Sudd in the Sudan, and the water-weed menaces of Pistia stratiotes, Eichhornia crassipes and Salvinia molesta come immediately to mind (see Chapter 7). However, euhydrophytes are not without importance as they provide breeding and feeding grounds for fish, and valuable niches for many invertebrates (Chapter 5). The submerged plants, through their photosynthetic activity, release oxygen into the water for subsequent use by the aquatic fauna. On the detrimental side, they encourage the spread of water-borne diseases by supplying suitable habitats for invertebrates such as snails, mosquitoes, and Simulium larvae, all of which are intermediate hosts for some of the more serious parasites affecting man. Additionally, euhydrophytes can grow profusely, thereby impairing the free How of water in canals and irrigation schemes, impeding navigation and restricting fishing.