Macrophytes of Lake Victoria and succession after invasion of Water Hyacinth

Item Type Proceedings Paper

Authors Omondi, R.; Kusewa, Mwende

Citation Odada, Eric & Olago, Daniel O. & Ochola, Washington & Ntiba, Micheni & Wandiga, Shem & Gichuki, Nathan & Oyieke, Hilda (Ed.) Proceedings of the 11th World Lakes Conference: vol. 2, 2006. p. 600-602.

Download date 25/09/2021 04:12:16

Link to Item http://hdl.handle.net/1834/1496

Macrophytes of Lake Victoria and succession after invasion of Water Hyacinth

Reuben Omondi1 and Mwende Kusewa2

1Kenya Marine and Fisheries Research Institute, P. O. Box 1881, Kisumu, Kenya Email:[email protected] 2 Kenya Agricultural Research Institute, P. O. Box 680, Kisumu, Kenya E-mail: [email protected]

the water. Emergent macrophytes dominate the Abstract shoreline flora while the middle and lower littoral zones supports stands of floating-leafed The distribution of Lake Victoria macrophytes is described. macrophytes. Macrophytes usually show a Succession of macrophytes in the lake became more dramatic and dynamic after invasion of water hyacinth. succession of zones between the dry land and The weed pushes and smothers other free-floating water, each zone with a dominating species. macrophytes like Pistia stratiotes. It then provides Variation in the latter depends on the duration of the substrates for the emergent , which later flooding and may also be affected by ecological reduces its population by competition for light and succession - where a plant community alters nutrients. The noxious weed is believed to have led to environmental conditions in a way that makes the extinction of Azolla nilotica in the lake. habitat less favourable for its own survival but more favourable for the development of a different Key words: Lake Victoria, Macrophytes, zonation, community. The present study was carried out to succession determine the distribution, zonation and succession patterns of macrophytes in Lake Victoria. Introduction Materials and methods Macrophytes are higher that grow in ecosystems whose formation has been dominated The Kenyan portion of Lake Victoria can by water and whose processes and characteristics conveniently be divided into Nyanza Gulf and open are largely controlled by water. Factors that waters. The Nyanza Gulf receives water from five influence the establishment of macrophytes include: rivers viz. Nyando, Sondu, Awach, Kibos and Oluch depth, topography, type of substrate, exposure to while the open waters receive waters from Rivers currents and/or wind and water turbidity. The Yala, Nzoia and Kuja. Monitoring and surveillance of distribution of macrophytes is often related to their macrophytes in the lake was done monthly using a mode of attachment (Sculthorpe, 1976). In Lake canoe with an outboard engine. Macrophyte species Victoria, the numerous numbers of haplochromines occurring in the various sites were recorded and the before the introduction of Lates niloticus hindered depths at which they occurred. Specimens were the establishment of macrophytes in the inshore collected for herbarium preparation and further areas by constantly disturbing the substrate (Witte et identification. Satellite image analysis was used to al., 1992a). The distribution, permanency and quality estimate the macrophyte population densities within of the water bodies available for their occupation the lake especially water hyacinth. govern the distribution and ecology of these plants. The most variable environmental factors of basic Results and discussion ecological importance for aquatic plants are the length of the period during which water is present, whether the water is still or moving, the availability of A total of 20 families and 28 species common plant nutrients and the quality and quantity of light macrophytes were recorded from the different sites penetrating into the water. in the lake (Table 1).

Macrophytes can be subdivided into four groups on More species of macrophytes were found at the river the basis of their water requirements and habitats mouths and sheltered bays. This was probably so (Plate 1). Submerged macrophytes are those that because these areas have muddy substrates are completely covered with water. These have suitable for attachment and the plant nutrients from leaves that tend to be thin and finely divided adapted the rivers enriched the environment. Comparison of for exchange of nutrients with water. Floating leafed open water station macrophytes and gulf station macrophytes are those that are rooted but have ones further shows that the distribution of floating leaves while free floating are those that float macrophytes, especially emergents, is related to the on the water surface. The last group is the emergent type of substrate. Stations within the gulf were macrophytes. These are rooted plants with their dominated by Cyperus papyrus while those in the principal photosynthetic surfaces projecting above open waters (Yala and Kuja/Migori) were dominated by Phragmites australis. Sediment deposition from 600

the many rivers in the gulf seemed to favour papyrus.

Plate 1.

Ipomea aquatica L. abyssinica Table 1. Some common macrophytes recorded I. cairica Polygonaceae in Lake Victoria Cyperaceae Polygonum setosulum Cyperus papyrus Pontederiaceae Araceae Lemnaceae C. rotundus Eichhornia crassipes Pistia stratiotes Lemna sp Elegant cyperus Potamogetonaceae Asteraceae Lentibularaceae Gramineae Potamogeton schweinfurthii Enydra fluxuans Utricularia inflexa Phragmites australis P. pectinatus Sphaeranthus spp Malvaceae Vossia cuspidata Trapaceae Azollaceae Hibiscus diversifolius Echinocloa sp Trapa natans Azolla pinnata Najadaceae Hydrocharitaceae Typhaceae Ceratophyllaceae Najas horrida Vallisneria spiralis Typha domingensis Ceratophyllum demersum Compositae lotus Melanthera scandens Onagraceae Convolvulaceae Ludwigia stolonifera

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Succession the above plants in the lake. It is believed that the emergent plant species contributed to the control of Until the invasion of Eichhornia crassipes only small the hyacinth by out-competing it, particularly in the changes were seen in the macrophyte composition littoral zones (Figure 1). These plants, which were during the rainy season. Before water hyacinth not-free floating, used water hyacinth as a substrate invasion, the only common floating macrophyte was to grow upon. This was first observed in the Winam Pistia stratiotes and to a lesser extent Azolla sp. The Gulf late 1999 and early 2000. This cycle is grass, Vossia cuspidata formed a thin layer behind expected to continue so long as the biological P. stratiotes followed by Cyperus, Aeschynomene, control agents exist in the lake. Phragmites and Typha. However, after the invasion of water hyacinth Pistia stratiotes and Azolla sp were Conclusion either smothered or pushed towards the shore. These formed a compact substrate mass which led The invasion of Lake Victoria by water hyacinth led to the increase of Vossia cuspidata and Echinocloa to the initial decrease in the population of emergent sp communities that effectively grew afloat the macrophytes as the hyacinth, which doubles its substrate utilizing the nutrients within. Water population in two weeks under favourable hyacinth also reduced the populations of floating conditions, out-competed them. However, this trend leafed and submerged macrophytes by cutting light was reversed with the reduction of water hyacinth and competition for nutrients. This could have led to population due to biological control. Macrophytes the extinction of A. nilotica that was last seen at the such as Vossia cuspidata and Echinocloa sp quickly mouth of River Nyando in the 1990s. The other increased in population along the shores of the lake. macrophytes whose communities were decimated These, however, cannot survive in the lake once by water hyacinth included N. lotus, Ceratophyllum total decomposition and disintegration of the demersum, Najas horrida and Trapa natans. heterotrophic layer of the floating mats takes place. Pistia stratiotes and Azolla nilotica, which were the only floating macrophytes are now only found in Nutrients Infestation Free-floating, ponds and irrigation canals along the lake shore and floating leafed and not in the main lake. Water hyacinth ‘hot-spot’ areas submerged have been observed in the lake and unless macrophytes environmental degradation around the lake is

Water Shading Free reduced, then the weed is expected persist and hyacinth (in effect habitat dominate these areas. L. Victoria) Emergent macrophytesŠ Recommendations Provides occasionally breaks substrate off as floating islands 1. There is need to control environmental then sinks degradation in the Nyanza gulf catchment. This will prevent eutrophication in the lake, a perfect condition for re-invasion of water Figure 1. Macrophyte succession in Lake Victoria. hyacinth.

The establishment of water hyacinth biological 2. There is need to introduce additional and/or control agents and the subsequent successful alternative biological control agents in the hot- reduction of the hyacinth resulted in the increase of spot areas especially at the river mouths.

References Sculthorpe, C.D. (1976) The Biology of Aquatic vascular plants. Edward Arnold, London. Witte, F., T. Goldschmidt, P. C. Groudswaard, W. Ligtvoet, M. J. P van Oijen & J. H Wannink 1992a. Species extinction and concomitant ecological changes in Lake Victoria. Neth. J. Zool. 42: 214-232.

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