Environmental factors contributing to the invasion of Najas marina L.in Madu Ganga estuary, a Ramsarwetland in Sri Lanka

Najas marina is one of the nine invasive alien species that has been identified in the Ramsar wetland of Madu Ganga estuary in Sri Lanka. This aquatic macrophyte is a major threat to the ecological balance of this wetland ecosystem and also a nuisance to fishing and tourism industry associated with this wetland. Present study was conducted from March to November 2014 to identify the physicochemical parameters that affect the distribution of N. marina in this estuary. None of the physicochemical parameters studied namely depth, Secchi depth, water temperature, total dissolved solids (TDS), salinity, pH, dissolved oxygen (DO), biological oxygen demand (BOD 5 ), nitrate-N content, total dissolved phosphorous (TDP) content and total phosphorous (TP) content were not individually responsible for the invasion of N. marina . However, combination of some of these physicochemical parameters such as high salinity, high TDS, high BOD and high nitrate-N and TDP concentration favors the invasion of N. marina . The entire estuary is under the threat of invasion of N. marina except the area near the mouth of the river that flows in to the estuary due to the high rate of water flow.