In this paper we report the results of impact of hypolimnetic aeration on phytoplankton assemblages in a subtropical lake of India. The study was conducted for a period of two years (2017)(2018), and the data obtained were compared with those of unaerated time. The aeration in the hypolimnion was done by a high speed aerator. The aeration appeared to change the phytoplankton assemblage significantly. It reduced the number of species and the standing crop of phytoplankton. Most importantly, due to aeration the Cyanophyeae was completely eliminated while the Chlorophyceae and Bacillariophyceae were increased in number. In general, the light weight species of phytoplankton disappeared or reduced in number as a result of aeration. The Shannon-Weiner diversity index as well as concentration of dominance was also found to be reduced. The share of Chlorophyceae and Bacillariophyceae were increased due to reduced pH and release of competition from Cyanophyceae during aeration.

The effect of hypolimnetic aeration on the water quality of Lake Naukuchiyatal , Central Himalaya was studied during 2017-2018 with the objective of determining the changes in physico-chemical variables of the lake due to aeration. It was noticed that aeration destroyed the thermal stratification and changed the anoxic hypolimnion into oxic zone. The pattern of clinograde distribution of oxygen during pre – aeration, was altered during aeration. The dissolved oxygen was distributed throughout the water column. As a result of aeration, the secchi disc transparency was increased considerably while the hydrogen ion concentration declined significantly. The BOD and the concentration of free carbon dioxide got reduced as a result of aeration. The concentrations of phosphate - phosphorus and nitrate - nitrogen were increased while the concentration of ammonium nitrogen and nitrite nitrogen were dropped. The N:P ratio was also reduced during aeration as compared to unaerated time.

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This paper focuses on the long-term changes on phytoplankton community of a subtropical lake of Himalaya, Lake Khurpatal. The study was carried out during 2016-2018. The data obtained on phytoplankton assemblages were compared with the data of 1980s. The results showed that all attributes of phytoplankton assemblages were changed in last 35 years. The species richness declined by 27.2% while the community abundance increased by 21% in last 35 years. The most important changes occurred in the group composition of phytoplankton: the Cyanophyceae which was almost absent during 1980s appeared in good quantity during the present study; the concentration of Bacillariophycae increased by 8.5 % while the Cholorophyceae did not change significantly. The Shannon-Weiner diversity index during the present study oscillated from 0.8 to 1.07 while the concentration of dominance varied from 0.12 to 0.25. The high concentration of blue greens during current year suggested that the concentration of nitrogen will further increase in the lake as the bluegreens are capable of fixing the atmospheric nitrogen. Thus, even if the entry of nitrogen into the lake from the catchment area is checked the concentration of nitrogen is likely to increase in the future. It will further increase the trophic status of the lake.

This research was designed to obtain information on long term changes in water quality of Lake Khurpatal, (Kumaun Himalaya, India). The study was conducted during 2016-17 and 2017-18. The results of the study were compared with the studies carried out about 35 years back. It was noticed that water transparency and concentration of dissolved oxygen in the lake were lowered by 37.5% and 32.2%, respectively in the last 35 years. The water pH was reduced from 8 to 6.7 in the same period of time. The concentration of silica was increased by 26%, however, the lake water was within the set limit of freshwater with regards to salinity. The concentration of PO 4 -P was increased by 26 times while the ammonium -nitrogen and nitratenitrogen increased by 23% and 33%, respectively in last 35 years. The studied parameters of water quality suggested that lake has turned to eutrophic state in this duration.