The spreading patterns and environmental implications of water hycinth(eichhornia crassipes) infestation in lake Naivasha, Kenya

Abstract

Water hyacinth tfichhornia crassipes) is an invasive aquatic macrophyte associated with major inagxir-‘e economic and ecological impacts in the Lake Naivasha region since the plant’s inz\‘L:1:'ic-2 into the lake in 1986. This study sought to examine the extent and status of water infestations in Lake Naivasha and its environmental implications. The study ':'_\;cC'lesiled that water hyacinth had significantly affected water quality, benthic biodiversity zc; the general utility of Lake Naivasha. Landsat satellite images, processed and analyzed using s hybrid unsupervised image classification approach, were used to assess water hyacinth extent xii distribution in Lake Naivasha for the years 1995, 2000 and 2003. The inferred extent of water hyacinth coverage for the years considered were 74, 86 and 102 ha respectively. This niicated that water hyacinth has continued to spread in the lake despite the presence of some control measures. A sample of 60 riparian community members consisting of 45 fishennen and 15 farmers was interviewed to assess the challenges they are facing as a result of the continued presence of water hyacinth. A structured interview of 31 key informants and discussions with groups of people involved in the management of Lake Naivasha was also conducted. Data was amlyzed using both descriptive and inferential statistics. The result showed that water hyacinth infesaxion in Lake Naivasha has affected the socioeconomic status of the communities on it There was for example a decrease of total fish production fi'om 513 to 110 tons per ymr ta 22% decrease). In addition there was a 30% decline in water supply to Naivasha .\hmicipality (from 10,000 m3 to 7,000 ma) in the year 2000. Field observations of the locations of water hyacinth, the control measures in place and visual estimation of water hyacinth were dam. Field measurements were done to detennine the impact of water hyacinth on water quality and the diversity of aquatic macroinvertebrates and zooplanktons. Two sampling sites were selected (one under water hyacinth and another at shore line without water hyacinth) to compare the results of the measurements. At each of these habitats 10 sampling areas were randomly selected. Water quality variables from the two habitats were compared by means of one-way analysis of variance (AN OVA). For species diversity, Shannon-Weiner Diversity Index (H’) and Simpson Diversity Index (D) were applied to both habitats of the 10 sampling sites for the sampling dates (between October 2003 and November 2004). The sample analysis showed that free carbon dioxide was significantly higher (P<0.05) in water hyacinth infested areas (26.-1511.02 mgL") than in open water (l2.86il.92 mgL'1). Dissolved oxygen was significantly lower (P<0.05) in the infested areas (l.96:h0.7l mgL‘1) when compared with open water t5.98:0.85 mgL'1). Similarly pH was significantly lower (P<0.05) in Water hyacinth infested area 6.9I:0.0-3| than in open water (7.71:k0.05). Although the temperature was higher in the infested areas (27.5:0.60OC) than open water (26.7i0.520C) the difference was not significant (P>0.05). The Shannon-Weiner Diversity Index (H’) of macroinvertebrates was significantly lower (P< 0.005) under water hyacinth mats (I-I’=0.56) than in water hyacinth-free zones (H’=0.34). Similarly, the Simpson Diversity Index (D) of zooplankton was found to be significantly lower (P< 0.005) under water hyacinth mats (D=0.50) than in water hyacinth-free zones (D=0.79). Fish species diversity was found to be higher at the areas with water hyacinth (H’= 0.94) than in the open water (H’= 0.69) at 5% level. Water hyacinth appeared to have reduced the abundance and diversity of the macroinvertebrates and zooplankton in the lake. It can therefore be concluded that the presence of water hyacinth was found to have affected the ecology of Lake Naivasha and therefore it’s utility. Effective control of water hyacinth in Lake Naivasha is important, in order to prevent both ecological and economic loss due to loss of biodiversity