The Growth Rate of Water Hyacinth (Eichhornia crassipes (Mart.) Solms) in Rawapening Lake, Central Java

Rawapening Lake is one of Indonesia’s national priority lakes that is experiencing environmental problems which are urgently required to be solved due to its functions. The decline in the environmental quality of Rawapening Lake includes sedimentation, water pollution and excess of nutrients, especially Phosphorus (P) and Nitrogen (N) into the lake that induced uncontrolled growth of aquatic plants, one of which is water hyacinth (Eichhornia crassipes (Mart.) Solms). Many activities had been done to reduce the covering of water hyacinth in Rawapening Lake that tends to increase by the time, but no significant result has been achieved. Therefore, this research was conducted in order to study the growth rate of water hyacinth with mesocosm in Rawapening Lake as a baseline to develop suitable management. There were three different sites, namely: Site I in the floating net cage area (FNCA), Rowoboni Village, Site II in the natural area of Bejalen Village which is far from the aquaculture sites, and Site III in the upper reaches of the Tuntang river, Asinan Village. The research was performed in November-December 2019 with the measurements of growth rate, addition number clump and water hyacinth covering every week. The experiment was conducted in the 1 x 1 meter mesocosm, with three replication in every site. In every mesocosm water hyacinth with similar initial weight of 160 grams and number of leaves 6-7 strands were grown in the mesocosm. On day 7 (H7) the average wet weight of water hyacinth increased by 201%. In the fourth week (H28) the average wet weight of water hyacinth increased by 788% compared to the initial weight when planted. The highest relative growth rate (RGR) value of water hyacinth was at site III (7.26%/ day), followed by Site I (7.03%/day), and Site II (6.40%/day), respectively. The doubling time (DT) value of water hyacinth at the site I was 9.9 day, site II – 10.8 day, and site III – 9.6 day. One clump of water hyacinth weighing 160 grams was able to cover 1 m2 of mesocosm within 21 days. On the basis of these results, to manage water hyacinth blooms one has to consider its growth rate.