Methods and Materials

Study Area

Coimbatore  known popularly as “Manchester of India” is an important industrial city, located in Tamil Nadu (10°55’ - 11°10’ N, and 77°10’ - 76°50’ E) at an average altitude of 470m, ranking 11^th in terms of population (Figure 1).  There are more than 30,000 small, medium and large industries including textile mills and foundries in the city employing about 40% of the population. The growing industrial sector and ensuing immigration of people pose heavy burden on the city infrastructure that did not grow in proportion. The city does not have facilities for treatment of industrial, municipal and domestic wastes. Wetlands and Noyyal River have been used for disposal of wastes of the city.  Natural drainage networks have been converted to storm water drains for letting the sewerage into wetlands without any treatment. In Coimbatore city there are 28 wetlands, mostly fed by the river Noyyal. The river, flowing through the city on its south, originates in the Vellingiri hills in Western Ghats, located on the south-western side of the city. Wetlands in Coimbatore are seasonal and have also been used as dumping yard for garbage and industrial wastes during dry period (Mohan Raj et al., 2000).  During the monsoon, with the inflow of water, this activity leads to contamination of groundwater sources. Six wetlands selected for biomonitoring (Figure 1) are: Vedapatti (VP), Pallapalayam (PP), Sundakamuthur (SM), Perur (PR), Noyyal (NL) and Singanallur (SN).

Figure 1. Coimbatore city map with the sampling points marked. Inset study area marked in India (Maps Courtesy: Google)

Water and Diatom sampling

Water samples were collected from all sampling sites in the sterilised polythene bottles. Physical variables like pH, temperature, electric conductivity, salinity and total dissolved solids were measured on site using EXTECH combo probe. Diatom samples were collected simultaneously during water sampling from 3 habitats such as cobbles (epilithic), aquatic plants (epiphytic) and sediment (episammic) during September 2007. All samples were reserved in 70% ethanol.

In laboratory, samples were processed by KMnO4 + hot HCl method, and slides were prepared using standard methods of Taylor et al., 2005. Diatom communities were then analysed by counting 400 to 450 valves. During enumeration the dimensions of diatom valve characteristics, like length, width and striae densities in 10 µm were measured. Identification of diatoms, to the least possible taxonomic level was carried out using taxonomic guides (Gandhi, 1957 1959a, 1959b, 1961, 1962, 1964, 1967, 1998; Lange-Bertalot, 2001; Krammer, 2002; Taylor, 2007; Karthick et al., 2008). Ecological diversity was calculated for each sample using diversity indices (Magurran, 2004) on PAST version 1.89 (Hammer et al., 2001). Diatom specific indices (Table 1) were calculated from community counts in OMNIDIA version5.3. Canonical correspondence analysis (CCA) using PAST was performed to examine the taxa distribution across sampling sites with reference to environmental variables. CCA mainly focuses on those taxa that vary with measured environmental variables. Only those taxa which are present at least in one sampling site with % relative abundance of 10% are included in the CCA analysis.

Table 1: Diatom Indices used in this study