Water quality parameters contributing to the invasion of water hyacinth (Eichhornia crassipes) in the Anawilundawa reservoir in Sri Lanka

Eichhornia crassipes has been found to invade the Anawilundawa reservoir located within Ramsar wetland in the northwestern region of Sri Lanka. Present study was carried out to determine the water quality parameters that could contribute to its invasion and identify an effective control measure. During the study period of March-November 2017, mean values for Secchi depth, water depth and nitrate content in areas where E. crassipes was present (7.9 cm, 12.4 cm and 475 µg L^-1 respectively) were found to be significantly lower (p<0.05) than in areas where it was absent (9.0 cm, 14.0 cm and 726 µg L^-1 respectively). In areas where E. crassipes was present, low nitrate content may be due to its absorption and low Secchi depth may be due to trapping of detritus. Principal Component Analysis showed that during the monsoon period, high pH combined with low levels of conductivity, salinity, total dissolved solids and total phosphorus contributed for its invasion. From March to July, coverage of E. crassipes gradually decreased and in August the reservoir was completely dry and E. crassipes was present only as sparsely distributed individual plants rooted to the exposed reservoir bed. In September, with incoming water of inter-monsoonal rains and surface runoff, these plants got uprooted and started floating and by October they were flowering. In June, E. crassipes was confined to an area of about 0.5 ha with a mean depth of 13.3 cm. During this month it can be easily removed manually with a little amount of human labour which can be obtained from the riparian community who use this reservoir for irrigation and domestic purposes. However, if it is to be removed, a suitable indigenous plant species that interact with aquatic avifauna has to be introduced to ensure the continuity of some environmental services provided by this reservoir.