Seasonal behaviour of thermal stratification and trophic status in a sub-tropical Palustrine water body

Thermal and chemical stratification of water bodies has a strong bearing upon the water quality since it regulates mixing of nutrients/pollutants and affects the aquatic life in it. The present study was undertaken in a shallow pond ecosystem to study thermal stratification and trophic status over a period of 4months (January-April, 2015) to understand the seasonal variations associated with. Chemical stratification with respect to dissolved oxygen was also observed along the depth. The pond was weakly to strongly thermally stratified throughout the study. The redox potential of water varied from -246mV (February 15) to 137mV (March 15) indicating dominantly reducing conditions. The dissolved oxygen profile with depth confirmed that reducing conditions prevailed in the hypolimnion. The surface epilimnion represented higher values of Trophic State Index for total phosphate (TSI-TP) and relatively lower values of total chlorophyll (TSI-Chl). The value of buoyancy frequency (N) and Richardson number (R) indicated complete and strong mixing in surface epilimnion due to the temperature, wind velocity, and current speed during the study with relatively stronger stratification and lower surface mixing during early summer (March-April). The higher value of TSI-TP compared to TSI-Chl indicated that zooplankton grazing or limitation of nitrogen plays a role in limiting eutrophication and algal growth. Although the wind velocity may be higher during summer, stronger thermal stratification can be formed due to temperature difference between upper layer and lower layer resulting in lower mixing of surface epilimnion, which may be a cause of concern if dissolved phosphate levels are high.