Freshwater Diatoms as Proxies of Assessing Environmental Changes in Reservoirs of Sri Lanka

Proxy indicators are used worldwide to understand the changes in the environment we live in. Among them, diatoms are well known for their species specific responses to environmental variables. Therefore this study was carried out to determine whether reservoir diatoms can be used to understand the changes in the environment. Six reservoirs selected for the study included, Bathalagoda, Bowatenne, Kurunegala and Nalanda, representing the Intermediate Zone and Kandy and Dambarawa reservoirs representing the Wet Zone of Sri Lanka. For environmental measurements of six study reservoirs, both onsite measurements and laboratory analysis were carried out at monthly intervals to understand the changes of pH, temperature, conductivity, dissolved oxygen, nitrate and phosphate for the study period (February-July 2013). For species data, plankton and sediment samples were collected using plankton net, a dipnet and a gravity corer. Relative abundance of present day diatoms and subfossil remains of diatoms were used to understand whether diatoms are preserved in sediments and whether they are sensitive to measured environmental variables. Twelve diatom species were identified from six reservoirs during the study period. Aulacoseira granulata was the most abundant diatom in all six sites. Bathalagoda, Dambarawa and Nalanda reservoirs supported a high diversity of diatoms while the lowest diversity was observed in Kandy Lake. Although a monthly variation of relative abundance of species was detected in different reservoirs, sediment analysis showed that they are well preserved at reservoir bottoms (Pearson correlation values obtained between the present day diatoms and subfossil diatoms Kurunegala-0.996, Bathalagoda-0.994, Nalanda-0.983, Bowatenne0.999, Damabarawa–0.841 and Kandy –1.000). According to the statistical analysis we found those species specific responses to measured environmental variables and the species data helps to discriminate study reservoirs showing the importance of diatoms in tropical reservoirs for environmental predictions.