Growth dynamics and primary production of eichhormia crassipes (mart.) solms in Lake Naivasha, Kenya

Abstract

Eichhornia crassipes (Mart.) Solms is an aquatic weed. It has caused havoc in fresh water all over the world. It is a native of tropical South America. It grows between 38 degrees north and 38 degrees South. It was reported to have invaded Lake Naivasha in 1988.

Several parameters of E. crassipes growth were studied. Percentage plant cover, area covered, plant density and frequency were assessed along the shores of Lake Naivasha. Percentage cover showed significant (P<0.05) seasonal variation with mean of 69.0 5.21% during the rainy and dry seasons respectively. The percentage and area covered were found to be influenced by lake level. Plants per unit are 34.35.35 and 33.06.32 during the rainy and dry seasons respectively. The frequency ranged between 70.010.53 and 72.98.32% during the rainy seasons respectively. However, E. crassipes was conspicuously absent from the rocky shores along the Hippo point. The total are of lake Naivasha that was infested by the plant ranged between 0.460.019 (dry season) and 0.6470.029 km2 (rainy Season).

A comparative study of relative growth rate (RGR), doubling time (DT) and primary productivity of E. crrasssipes in the floating macrophyte zone and under the papyrus was carried out in Lake Naivasha. The study showed that E. crassipes had a growth period corresponding to the rainy season (high lake level and nutrient concentration).

Plants growing in the floating macrophyte zone had a RGR of 5.470.51% and doubled every 13.7 1.51 days during the rainy season while those under the papyrus had a RGR of 1.280.39% and doubled in 27.378.63 days during the same period. The leaves of plants growing under papyrus had a RGR of 1.370.50% and doubled in 21.22 2.35 days while the leaves of those plants growing under papyrus had a RGR of 1.360.20% and doubled every 66.9515.0 days.

During the dry season DT was significantly (P<0.05) longer and RGR was low in both habitats. Plants growing in the floating macrophyte zone had a RGR of 1.19 0.33% and doubled in 40.458.65 days. Leaves for plants growing in the floating aquatic macrophyte zone had a RGR of 1.890.18% and doubled every 39.53.72 days while the leaves of those plants growing under the papyrus leaves had a RGR of 0.560.33% and doubled in 77.0632.18 days.

Distribution of dry matter among organs of E. crassipes growing in the floating macrophyte zone, under the papyrus and on the mud was carried out. For plants growing in the floating macrophyte zone, petiole accounted for 38%, blades 19% while roots accounted for 43% of the total dry weight. For plants growing under the papyrus, petioles contributed 37%, blades 21% while roots contributed 41%.

During the rainy seasons, primary productivity in the floating macrophyte zone was 11.091.09 gm-2 day-1 while under the papyrus, a production rate of 4.420.60gm-2 day-1 was recorded. Maximum seasonal biomass of 739.65173.42gm-1 was recorded after the short post rain flowering period. Minimum seasonal biomas of 179.2738.79gm-1 was recorded at the end of the dry period. Another peak though smaller than the maximum biomass was recorded after the pre-rain flowering season. During the dry season primary production rates fell to 6.080.84gm-2 day-1 and 3.350.56gm-2 day-1 in the floating macrophyte zone and under the papyrus respectively.

Nutrient availability and other radiation were certainly the most important variables regulating the growth of E. crassipes although temperature and relative humidity did play an essential role. Within the floating macrophyte zone nutrient availability played a significant role in regulating the growth and population size of E. crassipes. Under the papyrus light was an explicit factor regulating growth. However the role of nutrients in plants growing under the papyrus could not be ascertained with confidence because competition for nutrient with papyrus was not assessed.

The growth period in Lake Naivasha corresponds to a time when nutrient levels in the water were high especially after the drawdown. Nutrient concentrations were 6.30 and 0.346mg1-1 for total nitrogen and phosphorus respectively during the growth period. During this period, relative humidity and temperature were moderate. These together with nutrient levels were certainly the factors responsible for the observed growth period in both the floating macrophyte zone and under the papyrus. However, The relative growth rate (RGR) and primary productivity were low while doubling time (DT) was longer under the papyrus compared to the macrophyte zone. This may be attributed to the low light in intensity and temperature observed under the papyrus.

Apart from environmental factors, growth characteristics influenced the growth of E. crassipes. The plants growing under the papyrus had lower leaf are ratio (LAR) compared to those in the floating macrophyte.