Long‐term irreversible changes in a lake ecosystem affected by the <scp>I</scp> ndian <scp>O</scp> cean <scp>T</scp> sunami

This study focused on ecosystem responses to the environmental perturbations caused by the 2004 Indian Ocean Tsunami in a small lake that was a freshwater body in 1996, prior to the tsunami. The physicochemical and biological characteristics of Kokilamedu Lake (KKM) revealed drastic changes, compared with pretsunami conditions. Monthly average observations on water quality indicated the electrical conductivity of the water increased steeply to 17.41 mS cm−1 in 2009, from the lowest pretsunami value of 1.83 mS cm−1 (range of 1.83–5.25 mS cm−1). Simultaneously, the nitrate + nitrite (NO3 + NO2) values increased significantly from 0.49 μmol L−1 in 1996 to 74.47 μmol L−1 in 2006. Silicate (SiO4-Si) exhibited a dramatic increase in concentration, from an average of 64.87 μmol L−1 in the pretsunami period to 309.71 μmol L−1 the post-tsunami period (2009–2010). Inorganic phosphate had increased to a maximum of 9.59 μmol L−1 from a pretsunami maximum of 1.09 μmol L−1. The chlorophyll-α concentrations did not respond to the increased nutrient stoichiometry of the lake. There was a decreased chlorophyll-α concentration under post-tsunami conditions. The recent infilling of the lake with sediment during the tsunami, associated with wind-driven resuspension reduced the light penetration. There was a significant improvement in dissolved oxygen concentrations (2006–2010 average of 8.27 mg L−1) in the lake, however, compared with the pretsunami values (1994–1995 average of 5.94 mg L−1). The algal component is now dominated by blue-green algae, while green algae had dominated in the pretsunami period. Pre- and post-tsunami observations from a control site did not exhibit such dramatic shifts from the 1995 and 1996 conditions, whereas a shift was apparent in the case of KKM. Certain marine fishes have adapted to this altered ecosystem. These marine species encountered (Elops machnata, Cociella punctata, Sphyraena jello, Platycephalus indicus, Glossogobius giuris) might have been recruited during the intrusion of the tsunami waves.