Introduction

Biota, particularly algae (chlorophyta, cyanophyta, bacillariophyta and other algae) of lentic and lotic ecosystems, respond to the environmental changes due to the sustained inflow of pollutants. Among algae, diatoms respond quickly to any subtle changes in the environmental conditions (due to the inflow of pollutants or habitat alterations) and hence they are the most promising tool in biomonitoring (Kelly et al. 1998, Poulíčková et al. 2004, Solak & Àcs 2011). The relationship between diatoms and environmental variables had been studied earlier in Europe, North America, South Africa and East Asia, but such studies expand to diverse corners of the world in this century (van Dam et al. 1994, Blanco et al.2007, Karthick et al., 2011). Understanding cosmopolitan-endemism and the relative preference for pristine or polluted waters form the base for developing diatom based indices for implementing regular diatom based monitoring. Community structure of diatoms are specific to each eco-region and significantly varies from pristine to polluted water bodies and this helps in distinguishing oligotrophic from eutrophic water bodies. For example, Gomphonema stoermeri Kociolek & Kingston, probably endemic to western North America,dominates pristine water bodies in the area (Kociolek & Kingston 1999). In India, Achnanthidium chitrakootense Wojtal et al. is common in Himalayan streams which are electrolyte-rich and contain high amounts of dissolved oxygen (Wojtal et al. 2010), while polluted waters in urban wetland are often dominated with Nitzschia taylorii Alakananda et al. (Alakananda et al. 2011, 2012).

Urban water bodies exhibit high spatio-temporal variations in temperate and tropical regions that influence the distribution pattern and biodiversity of diatoms. The diatom to environment relationship is studied intensely in temperate regions (Cejudo-Figueiras et al. 2010) and it might not be homogenous (with similar species composition) in other continents like Asia (Goulden 2011).  It will be hence an important goal for limnologists and environmental policy makers to understand the eco-region specific diatom distribution and delimit the main limnological variables influencing the species pattern. The influence of physical and chemical variables on diatom distribution in different environmental types needs to be investigated for subsequently developing region specific diatom indices.

Urban wetlands of peninsular India are vital ecosystems, providing services such as water for domestic use, and moderating atmospheric temperature while balancing biodiversity (Ramachandra & Kumar 2008). Previous studies have focused mainly on physical and chemical factors for urban water management, thus excluding other factors such as land–use type in watershed region, and biological organisms and its response to environmental shifts. Humane et al. (2010) and Sharma et al. (2011) focused on the diatom distribution and sediment analysis from various environments in India. Precise ecological aspects, ionic and nutrient related species variations are yet to be explored in classifying water quality classes for prioritizing more polluted wetlands and identifying indicator species for further environmental management in urban wetlands like that of Bangalore region. Bangalore, known as lake city in 1960s, has undergone rapid urbanization with indiscriminate disposal of solid and liquid wastes and unplanned developmental activities. This has amplified pollution levels, which is evident from excess nutrients and organic components affecting the trophic levels thereby declining biodiversity and ecosystem activities. Diatom based monitoring as a surrogate for the conventional water quality parameters is not being practiced in India, especially in wetland systems. Thus, the study of diatom composition and exploring diatom-environment relationship through monitoring is a novel cost-effective approach and has scope for the replication and application at a regional scale in wetland management plans. In this context, the study aims to investigate the diatom community composition along rural–urban gradient reflecting significant environmental variables and classify wetlands based on the key indicator taxa that reflect ecological status of wetlands.