The present paper deals with the study of diatoms as indicators of water quality in yercaud lake, Salem district, Tamil Nadu, South India. Forty macrophytes samples and water samples are collected from yercaud lake in pre and post monsoon during 2011-13 and forty two (42) diatom species are recorded from these samples. Among the diatom species, a limited number of them (Amphora ovalis, Bacillaria paxillifer, Cyclotella meneghiniana, Cymbella tumida, Eunotia curvate, Fragilaria intermedia, Gomphonema lanceolatum, G. parvulum, G. undulatum, Nitzschia palea, Synedra ulna) are recorded throughout the year, while the diatom species such as Cyclotella meneghiniana, Gomphonema parvulum, G. undulatum, Navicula mutica, Nitzschia sigma, N. microcephala, N. palea, Pinnularia acrosphaeria, Surirella linearis and Synedra ulna are distributed in different locations of the lake. The physicochemical parameters such as temperature (°C), total dissolved solids (TDS), electrical conductivity (EC) (µs/cm), pH, total alkalinity (TA) (mg/l), salinity (ppm), calcium hardness (CH) (mg/l), magnesium hardness (MH) (mg/l), total hardness (TH) (mg/l) and total solids (TS) (mg/l) are analysed at TWAD board laboratory Salem to understand the water quality. The total diatom density showed significant positive relationship with respect to electrical conductivity (EC) and total dissolved solids (TDS) and significant positive correlation with chemical oxygen demand (COD). The analyses of pH, conductance, alkalinity and hardness levels indicate the moderate quality of water in yercaud lake. The values of DO, BOD and COD levels are very low indicating the absence of major organic pollution. The present study records the abundance of pollution tolerant species during pre-monsoon period due to the merger of sewage in yercaud lake.
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