Thondamanaru Lagoon is a brackish water lagoon in the northern province of Sri Lanka, with an area of approximately 7800 hectares (30 sq miles) and an average depth of 1.5 meters. In 1953, the Sri Lanka Irrigation Department established a barrage with sluice gates 400 metres from the mouth of the lagoon. The barrage was installed primarily to prevent sea water from entering the lagoon. By collecting the rain water and lowering salinity, the barrage transformed the lagoon's brackish water into freshwater.
Architects of the barrage anticipated that collecting freshwater will slowly reduce the accumulated salt in the land around the lagoon, making the land less saline and suitable for crop production. Collecting freshwater for irrigation is also considered a bonus.
The plan worked, and from 1963 to 1969, salinity levels in the lagoon and in groundwater of nearby wells declined. However, because of changes in water salinity levels, the lagoon's fauna and flora also changed. Before the barrage was installed, the average annual fish catch in Thondamanaru lagoon was 150 tonnes; after the barrage was created, fish catch plummeted to a measly 35 tonnes per year. In the period 1968 to 1978, 32 of the 47 identified fish species living in the lagoon disappeared. Arguably, the pros and cons of the lagoon's planned change from saltwater to freshwater was debatable to begin with; but further scientific study had to wait because of civil unrest in Jaffna in the last 30 years.
Now that peace has returned in the Jaffna region, a team of scientists from the Department of Zoology, University of Jaffna picked up the Thondamanaru lagoon issue in 2014. With the help of a small grant from Mangroves for the Future, they conducted a baseline study to assess the environment and socio-economic situation of the lagoon.
The findings of the study presented new ideas for future land use in Thondamanaru Lagoon, and drew the attention of provincial and district authorities. The research findings and management recommendations were presented to senior officials of the Jaffna Government in May 2015, to an audience which included the District Secretary of Jaffna, Mr N. Vethanayahan. The Divisional Secretaries of Point Pedro, Karaveddy and Kopay (representing divisions around the Thondamanaru Lagoon) were also present. Two civil engineers who were originally part of the Thondamanaru barrage project, and a highly respected national scientist, Prof K Chitravadivelu, were also among the audience.
The Thondamanaru Lagoon study recommended a two pronged approach for future land use around Thondamanaru Lagoon. First, it proposed letting sea water enter the lagoon to bring back the natural ecological balance to reverse the negative trend in lagoon fishery. Second, the study proposed to prepare available land for cultivation by creating sand bunds, or protective dikes to prevent salt water intrusion.
The results of this study is expected to be useful to relevant government agencies for years to come.
A background about this project can also be seen on the MFF Grants pages, "Baseline analysis of Development opportunities for Thondamanaru Lagoon".
A view from the mouth of Thondamanaru Lagoon, Sri Lanka (c) ... , Sri Lanka © IUCN, 2014