Temporal and environmental conditions regulating biomass dynamics in water hyacinth and its effects on water quality in Lake Naivasha, Kenya

Abstract

The influence of the water hyacinth mats on the physico-chemical characteristics of the underlying water is important in understanding the potential impacts on the aquatic organisms and human uses. The objective of the study was to determine the temporal and environmental conditions regulating biomass dynamics in water hyacinth and its effects on water quality in Lake Naivasha, Kenya. Field measurement and experimental set up were used to generate research data. Field research was carried out in three selected sites in Lake Naivasha; off Sewage, Malewa River inlet (both with hyacinth cover) and an open water site without water hyacinth cover. Field measurements and water sample collection was carried out once in a fortnight for five months (September 2011 to January 2012). Physico-chemical variables were measured in situ. The water samples from the lake and experimental buckets were analyzed for N, P and total suspended solids (TSS). The density and dry weights of live and senesced E. crassipes per square metre was determined using a 0.5×0.5M quadrat in triplicates per site. In all the statistical analyses, 95% level of significance was used as the critical point (P= <0.05). The water column below water hyacinth mats had three times lower dissolved oxygen concentration as compared to the open water, with values below 1mg/l recorded in some cases. In sites with water hyacinth, water transparency was two times lower with twofold higher total phosphorous concentration compared to the open water. Temporal variation of hyacinth biomass was observed in the lake. The number of juvenile plants increased fourfold within a month during the rainy season. By the end of the rains (two months after the onset), the juvenile population had reached thirteen fold increase. The results from the experiment indicated that juvenile plants are capable of taking up to 2.5 times more P and N per unit area than the mature ones. The development of a new plant in 9 days in 3.20 mg P/L and 25.60 mg N/L of nutrient enrichment was the fastest multiplication rate. The highest rates of change in plant biomass recorded were 1.48 and 3.69 Kg/m2/week (fresh weight) for the young and mature hyacinth respectively grown in 3.20 mg P/L and 25.60 mg N/L of nutrient enrichment. This was achieved between the 33rd and 47th day. In conclusion, increase in N and P concentrations in the water column resulted to increase in water hyacinth growth which in turn influences the water quality both positively through nutrient uptake and negatively by organic matter rain-down from decaying biomass. Management strategies that limit nutrient input into the lake should be enforced.