Seasonal changes in water quality and biomass of water hyacinth in lake naivasha, Kenya

Lake Naivasha faces the challenge of nutrient enrichment and persistent floating aquatic macrophytes. The study aimed to determine seasonal changes in water quality and biomass of water hyacinth in Lake Naivasha. The changes in water hyacinth phytomass and values of physico-chemical parameters were measured fortnightly from September 2011 to January 2012 in three sites within the lake; Malewa River inlet, off wastewater treatment plant (WWT) - both with water hyacinth and an open water site. Sites with hyacinth mats had significantly low dissolved oxygen concentrations, low pH values, high concentration of total phosphorous (TP) and turbidity. TP increased seven fold while NH4-N and NO3-N increased at least thrice during the rainy season (November and December 2011) in Malewa and WWT sites. The highest hyacinth phytomass density (98 plants/m2) was recorded at the Malewa site in January. Live biomass of hyacinth changed form 13.2 tons/ha in September to 33.6 and 25 tons/ha at the WWT and Malewa sites respectively in January. Senesced biomass increased during the dry period (September and October 2011). In conclusion, increase in water hyacinth growth coincided with the period of significantly high nutrient concentrations in the lake during and soon after the rainy season.