RESULTS AND DISCUSSION

Water Quality Analysis

Varthur  Wetland: The overall water quality parameters measured are listed in Table 1. pH was recorded as neutral to slightly alkaline with lowest and highest at VTI (7.1) in October and VTI (8.5) in November respectively. Electric conductivity and total dissolved solids values were consistent with a narrow range of 823 to 948 mgL^-1 and 636 to 730 mgL^-1 respectively. Hypoxic and even anoxic condition due to low dissolved oxygen was observed at VTI site (1.22 mgL^-1) and at VTO site as well with a range of 1.63 -7.15 mgL^-1. This attributed to the presence of water hyacinth covering the water surface with heavy domestic organic load and decomposition of organic matter. This condition is also reflected in elevated concentrations of BOD and COD with exceeding permissible limits at all sampling sites across months (Table 1). Total hardness (236-420 mgL^-1), alkalinity (55-440 mgL^-1) and chlorides (119.28-153.36 mgL^-1) were recorded very high due to sewage inflow.

Table 1: Variation in physical and chemical parameters across months at Varthur and Yellamma Wetland

Yellamma Wetland: pH was recorded as neutral to slightly alkaline with lowest at YMO (7.20) in the month of October and highest at YMI (8.90) in the month of September. Electric conductivity and total dissolved solid values show a significant range. In September, YMO showed a less EC value of 863 µScm^-1 and Yellamma inlet showed high value of 1120 µScm^-1 owing to high ionic concentrations inflow from industrial wastes. Dissolved oxygen content varied in both inlet and outlet ranging from 0 to 5.04 mgL^-1. DO was less than measurable amount in the month of October in YMO and September in YMI reasoning to high organic load. In the month of August DO of 4.22 mgL^-1 in YMI and 5.04 mgL^-1 in YMO was observed. The discharge of sewage containing organic material from the nearby factories contributed to this situation. This condition was also reflected in elevated concentrations of BOD and COD with exceeding permissible limits at all sampling sites across months (Table 1). In the month of October no sampling could be done in Yellamma inlet due to blockage on account of immersion of idols (Ganesha).

Nelakondoddi Wetland: pH ranged from 7.94 at NKO site (Sep) to 8.60 at both the sites (Nov) indicating slightly neutral to alkaline nature of water and within the permissible limits (Table 2). EC, TDS and salinity ranged from 480 to 687 µScm^-1, 295 to 468 ppm and 220 to 278 ppm respectively indicating low mineralization in this Wetland. However, slight gradation was observed in September due to monsoon climate. DO at all sampling sites was within the permissible limit and ranged from 6.5 mgL^-1 at NKI to 11.05 mgL^-1 at NKO. The higher DO recorded during monsoon and post monsoon seasons (i.e., Oct and Nov) may be due to the impact of rain water resulting in aeration ( Ayoade et al., 2006). A huge variation in BOD (5.42 to 16.26 mgL^-1 ) and COD (13.33 to 32 mgL^-1 ) was studied across months, the highest value of BOD being in the November month (18.44 mgL^-1 at NKI) and COD  being highest at both sites in August month (Table 2).

Table 2: Variation in physical and chemical parameters across months at Nelakondoddi and Vaderahalli Wetland

Vaderahalli Wetland: The pH in both sites indicates slightly alkaline ranged from 8.20 to 9.11 (Table 2). Water temperature varied depending on the time of sampling with a range of 24 to 29.5 0C. EC, TDS and salinity ranged from 541 to 711 µScm^-1, 390 to 564 ppm and 218 to 351 ppm respectively indicating low mineralization in this Wetland. However, slight gradation was also observed in September due to monsoon climate. DO at all sampling sites was within the permissible limit and ranged from 5.854 mgL^-1 at VHI to 10.73 mgL^-1 at VHO except in October where the DO was observed to be very low. A huge variation in BOD (2.03mgL^-1 to 20.34 mgL^-1) and COD (8 mgL^-1 to 32 mgL^-1) was studied across months being within the permissible limits, the highest value of BOD and COD being in the August month. (Refer Table 2).

Water Quality across Wetlands

The level of pollution status and spatial distribution of Wetlands from urbanized area is well reflected by water quality. Across Wetlands, pH was recorded as slightly alkaline with minimum of 7.6 at Varthur inlet and maximum of 8.75 at Vaderahalli inlet. EC, turbidity and TDS at Varthur and Yellamallappa chetty was in extremely high concentrations due to high cation concentrations. EC was more than the permissible limit at Yellamallappa chetty inlet (1101.50 µScm^-1) and high turbidity of 94.9mgL^-1 in Varthur inlet and high TDS of 857.5 was observed in Yellamallappa chetty inlet. These parameters were low in Vaderahalli inlet with 6.18 µScm^-1 of EC, turbidity of 13.81 NTU and total dissolved solids of 366.50 mgL^-1. These parameters show marked seasonal variations (Awasthi and Tiwari, 2004). As in figure 2 and figure 3, BOD and COD values reflected high pollution at Varthur, Yellamallappa chetty and Nelakondoddi sampling sites but contradictory values were observed in Nelakondoddi and Vaderahalli with a range of 8.959 mgL-1 to 12.97 mgL^-1. The study by Atobatele et al., 2008 shows pH, conductivity, temperature and dissolved oxygen as important parameters contributing to the annual variability of Wetland water. Dissolved oxygen concentration was found very less in all sampling sites of Varthur Wetland and Yellamallappa chetty Wetlands compared to other two Wetlands, which is quite evident by heavy organic load and macrophyte cover and hence reduces redox potential of the system.

Figure 2: Variation in water quality across sampling sites [For sampling sites and its codes refer annexure I] (a) pH (b) Electric conductivity (c) Biological oxygen demand (d) Chemical oxygen demand

Figure 3: Percentage relative abundance of species across months [A-August, S-September, O-October, N-November] (a) VarthurSiddapura (b) Varthur Fishing (c) Yallamma Outlet (d) Yallamma Inlet (e) Vaderahalli Outlet (f) Vaderahalli Inlet (g) Nelakondoddi Outlet.

Diatom Distribution

58 species belonging to 29 genera has been recorded and are listed in annexure 1. The dominant taxa were Achnanthidium sp., Gomphonema. parvulum (Kutzing var. parvulum f. parvulum) Gomphonema sp., Nitzschia palea (Kutzing) W.Smith, Nitzschia umbonata (Ehrenberg) Lange-Bertalot, C. meneghiniana Kutzing, Cymbella sp. and Fragilaria sp. Most of the species occurred in polluted regions are recorded as cosmopolitan (Taylor et al., 2007). The diatom community structure shows a strong correlation with various environmental variables (Soininen et al., 2004). The species such as G. parvulum, C. meneghiniana, N. palea and N. umbonata are tolerant to high electrolyte and organic rich condition (Karthick et al., 2009) which inhabited Varthur and Yellamallappa chetty Wetlands. This clearly signifies that both these Wetlands are polluted and eutrophic in condition. Nelakondoddi and Vaderahalli show low electric conductivity, BOD and COD values and were dominated by Achnanthidium sp., Gomphonema sp. and Cymbella sp. These species were recorded as inhabiting in moderate pollution.

Temporal variation and diatom distribution across Wetlands

The monthly variation in water quality was reflected by diatom community composition. G. parvulum and N. palea were dominated in all months at Varthur outlet while N. linearis was recorded as abundant in October at Varthur inlet notifying the pollution level. C. meneghiniana and N. palea was dominant across months at both sampling sites in Yellamallappa chetty followed by G. parvulum in October at Yellamallappa chetty outlet. Diatom species such as Achnanthidium sp, Gomphonema sp and C. kappi (Cholnoky) Cholnoky being dominant at Vaderahallli Wetland resembled a different community structure than former Wetlands. Ecological significance of Achnanthidium sp. needs to be studied as it shows a wide range of occurrence, from oilgotrophic to slightly mesotrophic condition.

Temporal variation is a significant factor responsible for changes in diatom distribution and its abundance (Sivaci et al., 2008). In Nelakondoddi outlet (NKO), N.palea, which was dominant in the month of August, was replaced by C. kappi and Mastogloia smithi Thwaites in September. However, Achnanthidium sp. dominated in October followed by Achnanthidium sp. together with Navicula sp. in November. C.kappi was dominant in September which was followed by N. amphibia Grunow f.amphibia and Achnanthidium sp. reflecting moderate trophic status. The eutrophic status and electrolyte rich was significant in November with the dominance of Fragillaria. biceps (Kutzing) Lange-Bertalot and N. linearis (Agardh) W Smith.

Relationship between dominant taxa and Water Quality

CCA triplot explained 65.43% of the variability in the diatom and environmental data with 45.92% in axis 1 and 19.51% in axis 2 (Figure 4; Table 3).  Monte Carlo permutation test (n=1000) showed that both axes were statistically significant (p<0.01). The ordination of sampling sites was based on the species composition and their relationship with environmental and land-cover variables. The axis 1 represented an urban to rural gradient, where rural sampling sites were ordinated towards the right side and urban sites were on the left side. The sampling sites on the right side were Vaderahalli and Nelakondoddi sites while clustered on the left side were Varthur sampling sites. Axis 2 represented Nelakondoddi and Vaderahalli sites and dominance of ACHD on the right side of the axis. Axis 1 was significantly negatively correlated with variables such as EC, TDS, Turbidity, P, K and % built up and taxa such as NUMB, GPAR and NPAL Likewise, a significant positive correlation of axis 1 was observed with DO, pH and % vegetation along with dominance of CKAP and GGRA. There was no significant correlation of BOD, COD, sodium and chlorides with both axes.

Figure 4. Canonical correspondence analysis (CCA) plot explaining impact of land use/ land cover on species distribution.

Table 3. Correlation coefficients between selected environmental variables and the first two CCA axes (Significant correlation p<0.01).

Ecological preference of dominant taxa

Figure 5 illustrates the occurrence of dominant taxa at differing water quality. The dominant taxa G. parvulum (GPAR), C. meneghiniana (CMEN), Achnanthidium sp. (ACHD) and N. palea (NPAL) at varying pH and EC show the dominance of particular taxa at respective pH and EC optima . G. parvulum was persistent across months and abundant at pH ranging from 7.6 to 8 and was less towards alkaline pH. The electric conductivity more than 850 µScm^-1 attributed to G. parvulum optima while sampling sites less than 700 µScm^-1 comprised a different composition with G parvulum as less in abundance. C. meneghiniana was recorded to be more dominant at pH of 7.7 to 7.9 and as the EC increases (>900 µScm^-1). This range of pH and EC limits the distribution of G.parvulum and C. meneghiniana to extremely eutrophic water condition. The sensitivity and tolerance of diatoms to such specific environmental factors attributed towards the species- specific ecological characterization (Sabater et al., 2007).

Figure 5: Distribution and autecology of dominant taxa across pH and Electric conductivity

Figure 6: Land use in the catchments of (a) Nelakondoddi, (b) Vaderahalli, (c) Varthur and (d) Yellammawetlands.

Achnanthidium sp. was present at all sampling sites whilst, the abundance was optimum at pH 8.1 to 8.2 and at EC 600 to 650 µScm^-1 and later decreased at elevated EC concentration. N. palea was present at all sampling sites and revealed a wide range of optima though was less abundant at alkaline pH. N. palea was also abundant at its optima of EC i.e., more than 850 µScm^-1. Low EC concentration (<800 µScm^-1) was limiting the distribution of N. palea. Thus, in consideration with observed species autecological values the sampling sites with profuse Achnanthidium sp. can be classified as oligo to slightly eutrophic at the same time as, the sampling sites with N.palea can be classified as in eutrophic status and extremely polluted. However, many studies have investigated autecological status of indicator species (Taylor et al., 2007; Álvarez-Blanco et al., 2010), very less study contributes to species optima of Nitzschia sp., Gomphonema sp., and Achnanthidium sp. and further none of the study come from Asia region. However, ecological optima of N. palea can be classified as eutrophic status.  Performing the ecological optima for few more taxa that commonly occur in wetlands of Bangalore can lead to developing specific diatom indices for bioassessment practices.

LULC analysis in wetlands catchment

Table 4 lists LULC statistics which indicates that catchment of Yellamalappa chetty  wetland was dominated by vegetation cover (42.90%) followed by built-up (51.68%) and water (1.92%). The catchment of Varthur wetland has 55.16% built up and 45.85% vegetation. Varthur wetland receives 595 million liters per day (MLD) of sewage and other waste materials stressing the wetland’s functional ability. Water quality and diatom community structure analysis in Yellamma and Varthur wetlands highlight that these wetlands are under stress due to the anthropogenic activities. Wetlands and streams are linked intimately to their catchments and have implications regarding conservation and restoration (management) endeavors (Stendera and Johnson, 2006). Sustained flow of higher quantum of sewage inflow into Varthur and Yellamma wetlands has increased the nutrient levels, evident from the profuse growth of macrophytes.  Chattopadyay et al., (2005) also report of the similar scenario of urban landuse with poor water quality throughout the year. The increased amount of organic concentration and degradation in water quality is mainly due to increasing urbanization (built up) at Yellamma and Varthur regions (Chandrasekhar et al., 2003). In contrast to this situation, vegetation in Vaderahalli catchment (61.21%) and Nelakondoddi catchment (65.98%) is higher compared to the built up land (35.96% and 31.48% respectively). Water body accounts to 2.82% in Vaderahalli catchment was and 2.61% in Nelakondoddi. This analysis also shows that the influence of anthropogenic activity was less in these two wetlands. Majority of the area is under vegetation (with less human interventions) and thus less chances of contamination of water compared to the wetlands situated in urban region. LULC changes influence varying diatom community composition (Soininen et al., 2004, Weijter et al., 2009). Yallamallappa chetty and Varthur Wetlands are having high percent of built-up with high sewage and industrial inflow into the Wetland. Diatom community comprised of pollution tolerant species reflecting trophic status. The high percent of vegetation (including forest) cover at Nelakondoddi and Vaderahalli Wetland comprised species, which inhabit oligo to slightly mesotrophic conditions.

Table 4: Land use/ Land cover classification of selected 4 Wetlands of Bangalore

Pandey & Verma, (2008) study illustrates that the catchment integrity is significant in determining ecosystem properties of freshwater Wetlands. Li et al., (2010) focused on rapid landscape change and regional environmental dynamics in the Lianyungang bay area from 2000 to 2006 based on remote sensing data indicating that the area has a widespread urban–rural interface with rapid land-use changes, urban expansion and wetland degradation. Rapid increase in urban built-up land has led to large-scale salt wetlands degradation. Allan et al., (1997) highlight that in streams, habitat structure and organic matter inputs are determined primarily by local conditions such as vegetative cover at a site, whereas nutrient supply, sediment delivery, hydrology and channel characteristics are influenced by regional conditions, including landscape features and land use/cover at some distance upstream and lateral to stream sites. Understanding the effects of changes in land use and land cover (LULC) is important for maintaining a desired level of water quality and also for restoring water quality in affected areas (Gove et al., 2001).