Perturbations and Regime Shift in an Aquatic Ecosystem along with Adaptation of Biota Due to Climatic Extremes

The present study aims to observe the post tsunami changes occurring in a small lake. Physicochemical and biological characteristics of Kokilamedu lake (KKM) revealed drastic changes when compared to the pre-tsunami conditions. The monthly average water quality indicated that salinity was increased sharply (11.31 practical salinity unit [psu] in June 2008) from a lowest pre-tsunami value of 1.19 psu (range, 1.19 to 3.41 psu). Similarly, the NO₃+NO₂ values were increased significantly (0.49 μmol/L in 1996 to 74.47 μmol/L in 2006). The concentration of SiO₄ increased remarkably from an average of 64.87 μmol/L during pre-tsunami conditions to 309.71 μmol/L during post-tsunami conditions (2009–2010). Inorganic phosphate was also increased to a maximum of 9.59 μmol/L from a pre-tsunami maximum of 1.09 μmol/L. Chlorophyll-a values did not respond to the increased nutrient stoichiometry and declined during posttsunami conditions. However, there was a significant improvement in oxygen concentration (2006–2010 average, 8.27 mg/L) of the lake compared to the 1996 values (1994–1995 average, 5.94 mg/L). The algal component is now dominated by the blue-green algae which prevailed over the green algae in the pre-tsunami conditions. Certain marine fishes adapted to this altered ecosystem due to a probable regime shift. Remarkably, out of 20 species, five were marine, nine were brackish, and the remaining six were freshwater species. Preand post-tsunami observations from a control site did not exhibit a remarkable shift during 1994–1996 and 2010, whereas a shift was apparent in the case of KKM.