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. They also aid in improving water quality by filtering sediments and nutrients from surface water. Wetlands play a major role in removing dissolved nutrients such as nitrogen and to some extent heavy metals (Ramachandra et al., 2002). Hence, they are often described as “Kidneys of the landscape”.  Wetlands encompass many different habitats including wetlands, marshes, swamps, flood plains, bogs, shallow ponds, littoral zones of larger water bodies and peatlands. All these share the fundamental feature of complex interactions among basic components such as soil, water, flora and fauna.

Wetlands are ecologically important in relation to stability and biodiversity in a region and also in terms of energy and material flow. Wetlands are “lands transitional between terrestrial and aquatic ecosystems where the water table is usually at or near the surface or the land is covered by shallow water” (Mitch and Gosselink, 1986). Hydrological conditions of a wetland modify or change chemical and physical properties such as nutrient availability, degree of substrate anoxia, soil salinity, sediment properties and pH, which in turn, influence the biotic integrity (Gosselink and Turner, 1978).  Wetlands retain water during dry periods, thus keeping the water table high and stable. During floods they diminish floods intensity and biotic components trap suspended solids and attached nutrients. A healthy wetland retains a natural flow of water, minimising flooding in the catchment. Wetlands receive water deposited as groundwater, during dry seasons.  Thus, a healthy wetland does the function of water recharge and discharge effectively, while meeting the human needs. However, humans have altered the natural flow regime of wetlands either by altering the natural drains, changing the land cover drastically or letting the untreated sewage in urban areas in recent times. The removal of such wetland systems or letting untreated sewage has caused the deterioration of water quality and ecological degradation in catchment (Prasad et al., 2003).

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. They have 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 waterbodies in and closer to urban centres have been targeted. The water crisis, frequent flooding in urban areas has necessitated understanding the role of wetlands, and the need for integrated approaches to maintain the ecological balance, while meeting the demands of the growing population.

Rising water demand has exacerbated the impacts. Societies need to adopt improved strategies for integrated wetland management to ensure the quantity and quality of water is maintained for the ecosystem functions. In this regard, Ramsar Convention’s Agenda 21 recommends the work towards better understanding of these threatened ecosystems through basic research, awareness and education, ecosystem and species conservation.

Effective assessment tools are needed for consistent evaluation of the condition with stressors of wetland resources for solving problems. This entails inventorying and regular monitoring of wetlands. Physical and chemical monitoring of water quality has been practiced for a long time. Standard techniques are used for measuring light penetration, turbidity, conductivity, dissolved oxygen, biological oxygen demand and nutrients like phosphates, nitrates, nitrites, ammonia, and so on (Chapman, 1992). These measurements even though provide us simple values, but don’t provide overall health and condition of the ecosystem enabling both preventive as well as restorative measures. Many environmental factors vary on different spatial and temporal scales in complex ecosystems such as wetlands.  These variables range from climate, landuse, and geomorphology of a watershed (eg, Richards et al., 1996) to the physical, chemical and biological characteristics. In this context, monitoring involving biological communities of an ecosystem would help in assessing, as they can integrate and reflect the effects of chemical and physical disturbances that occur in short duration as well as over extended period of time.

Monitoring using organisms, to assess the ecosystem’s condition is referred as biological monitoring or biomonitoring.  Biological indicators based on organisms living from one day to several years provide an integrated assessment of environmental conditions in streams, rivers and wetlands that are spatially and temporally variable. An ideal biomonitoring should be useful for both long and short term monitoring. Current conditions may be linked to the past conditions very effectively, if the same biomonitors are used for both short and long-term monitoring (Dixit et al., 1992). Biomonitoring consists of groups of species, each group with well defined habitats, so that they may reflect changes in a variety of habitats. Biological indicators are important parts of environment assessment because protection and management of these organisms are the objectives of most programs. Aquatic communities (like algae, fish, riparian vegetation, macro-invertebrates), integrate and reflect the effects of chemical and physical disturbances.  A biota that undergoes change from dominance to gradual disappearance of a species is of ecological significance. The primary aim here is to detect changes in abundance, structure and diversity of a target species assemblage as compared to the reference condition.  Bio-indicators include organisms that are:

• close to the transfer of nutrient and energy in the food web;
• wider range of distribution;
• simple life-cycle stages, and identifiable to the species or even the morphotype level;
• sensitive to fine changes in the environment with a range of tolerance; and
• preference to environmental variables, so a change in the environment is reflected by a shift in species dominance.

Now, biological monitoring has begun to address the question of biological integrity of wetlands influenced by various anthropogenic land use activities.

Numerous methods have been developed in biomonitoring for an assessment of the integrity of aquatic systems. Most are based on the attributes of whole assemblages of organisms such as fish, algae or invertebrates. A variety of assemblages have been used in biological assessments ranging from macrophytes (Galatowitsch et al., 1999, Gernes and Helgen, 1999) macroinvertebrates (Kerans and Karr, 1994 and Barbour et al., 1996); amphibians (Micacchion, 2004); fish (Schulz et al., 1999); birds (O’Connell et al., 1998) and diatoms (Fore and Grafe, 2002).

Diatoms under Class Bacillariophyceae comprise a ubiquitous, photosynthetic and distinctive group of unicellular algae. Diatoms are made up of siliceous cell wall consisting of two valves; epivalve and hypovalve which fit together like a petri dish together known as frustules. In between two valves series of bands are present known as girdle bands. During cell division the new frustules are formed from the inside of the cell. The outer or older is the epivalve and inner or newly formed one is hypovalve forms one daughter cell where as outer or older hypovalve acts as epivalve and newly formed valve will become hypovalve. This forms another daughter cell. During this process cell size goes on decreasing. The original size is attained by undergoing sexual reproduction by auxospore formation.

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).  Diatoms respond directly and 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 climatic, 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 indicators of both pollution increases and habitat restoration success. Frustules are preserved in sediments and record habitat history. Diatoms collection and methods are ease and low cost. Samples can be archived easily for long periods of time for future analysis and long term records.

Diatoms occur in all types of environment where ever moisture is present. 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, 1986, 1990; Cox, 1991). However, there is no information  available on diatoms as indicator species of wetlands in India.  The present study assesses six major wetlands in an urban ecosystem using diatoms as bioindicators.