Discussion

In the current study, an attempt was made for the first time to analyze the degree of pollution in wetlands using diatom assemblage patterns and assessment of water quality class during post-restoration period. A wide range of species distribution was observed reflecting both clean and pollution status in various wetland types. The relationship between species composition and chemical parameters in different water bodies, their influence on the former has been proved to be indicative of anthropogenic disturbances (Stenger-Kovács et al., 2007). Diatom distribution within Bangalore’s wetlands showed clear differences in five wetland types, and across prior restoration and post-restoration periods. Samples collected prior to restoration and at polluted sites were characterized by pollution resistant taxa of high saprobic status and low oxygen saturation (van Dam et al., 1994). Diatom taxa composition, which was sampled five months after completion of restoration,  showed continued  pollution at Jakkur, Rachenahalli and Kothanur, which were identical to polluted wetland type (as in Table 2), implying that the impact of wetland restoration program is minimal or nil.

Cyclotella meneghiniana in 2008-2009 was replaced with Nitzschia palea in 2011 at Jakkur owing to the persistent human disturbances and inappropriate restoration technique where physical restoration was implemented rather than biological restoration. Pristine water bodies were characterized with pollution sensitive species of genus Achnanthidium and Gomphonema sp. (excluding G. parvulum). The restored wetlands (except Somapura), previously restored and polluted wetlands were composed of >50% taxa that are indicative of their polluted status (excessive contamination with high BOD and COD). This clearly concludes that either the restoration was ineffective or these wetlands continued to receive untreated sewage even after the restoration process. Thus, the treatment of sewage to avoid contamination of surface water has been a dilemma for environment managers (Ramachandra, 2005). The time required by (a solitary or colonial) species to be stabilized/ restructured into a new environment might also vary depending on the environmental conditions in each eco-region. Recovery of aquatic organisms in the restored water body would take time spanning from months to years (Craft and Richardson, 1998; Jüttner et al., 2010). Though we consider the concept of the time period of years to be reasonable enough for the regrowth of original/lost species for reflecting good water status, it has not been observed in any of the wetlands that were restored a decade ago in Bangalore region. Previously restored wetlands such as Ulsoor, Hebbal and Madiwala, showed species composition that characterized high levels of pollution. Diatom species composition reflected trophic levels in water quality based on nutrient load and was similar to that of water quality analyses derived from physical and chemical analyses of water. Thus, diatom based biomonitoring could be used as surrogate as well as supplement for chemical variables. Regular monitoring of wetlands using diatoms help in nutrient management and sewage inflow regulations for better management of different components (water, biodiversity, etc.) of wetlands. Further, the process of restoration must ensure that the restored wetland should provide habitat for all forms of life ranging from microscopic to larger benthic organisms. It has been documented that any disturbance or removal of submerged macrophytes in a shoreline region declines species diversity and if sewage inflow persists, it might impact in endangering the endemic species and causing an increase of invasive species (Carpenter and Waite, 2000).

In this study, pollution tolerant species; Nitzschia palea, Gomphonema parvulum, Nitzschia umbonata and Cyclotella meneghiniana were dominant in polluted sites while pollution sensitive species Cymbella affinis, Achnanthidium minitussimum and Gomphonema sp were dominant in clean/ reference sites (Figure 2). Species distribution was compared among five wetland types through ANOVA and diversity indices. Percentage eutrophic taxa composition was significant (p<0.05) at polluted wetlands (POL), whereas REF sites showed no significant similarity with previously polluted (PVR), post-restoration (POR) and prior restoration sites (PRR), indicating predominant growth of eutrophic taxa in latter groups (Table 1). Though Shannon diversity showed high diversity at POR than REF, it was caused due to the dominance of pollution tolerant taxa (Gomphonema parvulum, Rophalodia gibba, Cyclotella meneghiniana and Nitzschia microcephala). Hesaraghatta (REF)-devoid of any anthropogenic activity (during the study period of 2008-2011) showed less primary productivity- representative of oligotrophic conditions; however, Currently (2012-13), Hesaraghatta is experiencing severe anthropogenic activities including the problems associated with the undefined wetland boundary, insufficient water flow, improper management by local stakeholders. This is evident from the decline in diversity of indigenous fish species and waterfowl within a year. 

Hoskere wetland also showed 20% of eutrophic taxa because of the natural variations like weathering of rocks, nutrient cycle and flood. The main source of rise in conductivity in waters was noticed to be from inflow of untreated effluents (through both domestic and industrial sources) into wetlands in both recently restored and previously restored habitats (Table 2) (Chessman et al., 2007). Further, the restoration measure requires the information on the past history of wetlands, which are available through paleolimnological investigations based on diatom remains settled in deep sediment (Köster et al., 2005). Diatoms in paleolimnological studies provide information on water quality data of several decades representing pre-industrial, pre-eutrophication period, to detect present day water quality change (Norberg et al., 2008; Gell et al., 2009). Restoration of wetlands in the rapidly urbanizing landscape of Bangalore is a major challenge involving ecological and social systems. Methods that were implemented for recovering degraded ecological values have been unsuccessful with only superficial or physical alterations. Restoration regulations, have so far addressed physical structuring such as bund maintenance, fencing and other recreational activities but it has excluded the management of biodiversity and waste disposal (Ramachandra, 2005; Ramachandra and Majumdar, 2009). This study addresses various issues related to degradation of urban wetlands, especially in Bangalore, and the complexities faced in the restoration process (Ramachandra et al., 2002; Ramachandra, 2005). Goals of wetland restoration have to be prioritized based on the scale and nature of threats along with supplementary information on biodiversity prior to the restoration. Further, wetlands management should include biomonitoring using diatoms, sediment analyses, buffer zonation, microhabitat analysis and efficacy of waste management.