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Role of phytoplankton diversity and nutrient release potentials of dominant macrophytes in trophic evaluation of Lake Anasagar, an urban wetland of Ajmer, was assessed. Out of 120 phytoplanktons, Chlorella vulgaris, Chlorococcum infusionum, Pediastrum tetras, Stiqeoclonium, species of Navicula, Euglena, Microcystis aeruginosa and Oscillatoria were recorded as indicators of nutrient-rich conditions. Trophic State Indices showed that even comparatively unpolluted zone of the lake is weak eutrophic and may switch over to a high level of eutrophication estimated for the site influenced by sewage-wate disposal. Besides human activities, macrophyte species i.e. Azolla pinnata, Trapa bispinosa as floating and Potamogeton crispus, Vallisneria spiralis as submerged, release nutrients (N, P, Ca and Mg) in lake waters and form a natural culture medium for phytoplanktons. It was estimated that comparatively more amount of nitrogen and phosphorus added to the nutrient pool is released by Azolla and Trapa respectively. Floating species contribute much to the nutrient-enrichment than submerged macrophytes. It has been emphasised that alongwith algal diversity and characteristic vegetation dynamics, values of decomposition constants (k), nutrient losses and biomass may be utilised for trophic evaluations of shallow urban wetlands in semi-arid regions.

Key words: Phytoplankton, Diversity, Macrophytes, Eutrophication, Nitrogen, Phosphorus, Decomposition, Trophic evaluation.