Introduction

Wetlands are an essential part of human civilization, meeting many crucial needs for life on earth such as drinking water, energy, fodder, biodiversity, flood storage, transport, recreation and climate stabilizers. In recent times humans have distorted the natural flow regime of wetlands in urban area either by altering natural drains, changing land cover drastically or letting sewage into wetland. The removal of wetland systems or letting sewage has caused the deterioration of water quality and ecological degradation in catchment (Prasad et al., 2002). In India, wetlands are distributed in all the biogeographic regions occupying 58.2 million hectares, including areas under wet paddy cultivation (Directory of Indian Wetlands, 1990). They exhibit significant ecological diversity, primarily because of variability in climate, habitat and topography. Today, wetlands are one of the most threatened habitats in India, which has been converted for agriculture, industry or settlements and some are affected by industrial effluents, sewage, household wastes and sedimentation. Due to urbanization and lack of holistic approaches in land management, land and water bodies in and closer to urban area have been targeted. The water crisis, frequent flooding in urban areas necessitates understanding the role of wetlands, and the need for integrated approaches to maintain the ecological balance, while meeting the demands of the growing population. Effective assessment tools are needed for consistent evaluation of the condition with stressors of wetland resources for solving problems. Many environmental factors vary on different spatial and temporal scales in wetlands, which include climate, landuse and geomorphology of a watershed to the physical, chemical and biological characteristics (Richards et al., 1996). In this context, monitoring involving biological communities of an ecosystem would help in assessing, since they integrate and reflect the effects of chemical and physical disturbances that occur in short duration as well as over an extended period of time.

Diatoms are more specific in their preference and tolerance of environmental conditions than most other aquatic biota. Diatoms were the first group of biota used for detecting organic pollution (e.g., the saprobian system by Kolkwitz and Marsson in 1909, cited in Stoermer and Smol, 2001).  They respond directly and are sensitive to many physical, chemical and biological changes such as temperature, nutrient concentration and herbivory. They are sensitive to many habitat conditions and show variability in biomass and species composition. At higher spatial and temporal levels effects of resources and stressors on diatom assemblages can be constrained by climate, geology and land use. Diatoms are readily distinguished to species and subspecies level based on unique morphological features. Diatoms have one of the shortest generation times of all biological indicators.  They reproduce and respond rapidly to environmental change and provide early warning of both pollution increases and habitat restoration success. Diatom frustules are preserved in sediments and record habitat history. Diatoms collection and methods are easy and cost effective. A golden-brown mucilage film on the surface of substrata indicates the presence of benthic diatoms whereas free living in the water column is the planktonic diatoms. Data on diatoms as indicators of water quality reflecting pH, salinity and organic pollution in Europe, America, South Africa and Japan have been available for a long time (e.g. Patrick, 1986; Schoeman, 1973; Round, 1990; Cox, 1991). However, there is no information available on diatoms as indicator species of wetlands in India.  The present study assesses the pollution status of six major wetlands in an urban ecosystem using diatoms as bioindicators.