Introduction :

Diatoms constitute a fundamental link between primary (autotrophic) and secondary (heterotrophic) production and form a vital component of aquatic ecosystems. Cellular characteristics such as a siliceous wall (frustule), possession of unique photosynthetic pigments and specific storage products (oil and chrysolaminarin) make them unique amongst the algae. Two groups of freshwaters diatoms are  i) centric diatoms, which are in general, circular in shape and adapted to live in the water column as part of the phytoplankton and ii) pennate diatoms that live in benthic habitats but are often temporarily re-suspended in the water column.

Aquatic ecosystem monitoring has been carried out in India based on either chemical or biological analyses. The chemical approach is useful in order to determine levels of nutrients, metals, pesticides, radioactive substances, etc. while the biological approach aids in assessing the overall effect of the chemical input on organisms. The chemical constituents of a waterbody exhibit enormous fluctuation as certain compounds are broken down, absorbed or complexed over stretches of river which may vary greatly in length; other compounds are added in variable quantities from permanent or intermittent sources. However, both chemical and biological features are influenced by climatic factors, especially those affecting current speed and the associated dilution/concentration of nutrients. Due to these factors, the interpretation of the 'chemical status' of a stretch of river becomes difficult except in crude terms. In this context, biological monitoring relies on assessment of the changes in the biota to establish the relationships between the biological features and the overall chemical status and the response of the individual species to specific nutrients, pollutants, etc.