The presence of different species of hydrophytes was investigated in relation to Secchi disk visibility, pH, dissolved oxygen,electrical conductivity, total Kjeldahl nitrogen, total phosphorus and chlorophyll-a concentration in two tropical ponds nearbyKolkata, India, during a three years period (June 1999 to May 2002). The dominant flora in the ponds namely, Alternantheraphiloxeroides, Nymphoides hydrophylla, Lemna aequinoctialis, and Vallisneria spiralis were found to subsist over a wide amplitudeof nutrient levels thereby showing their adaptability to highly eutrophic ecosystems, a common feature of the tropics.However, the presence of some minor species could be associated with a narrow range of specific limnological variables.

The epiphytic diatom communities living on two macroalgae, Chara vulgaris L. and Cladophora glomerata Kütz.) and oneangiosperm Potamogeton densus L., were collected in the Mediterranean river Durance. Each macrophyte species was studied on the basis of three plant replicates, each of which cut into three segments. Although considerable heterogeneity was observed in density of the epiphytic populations between the plant species studied as regards, the composition of these algal populations showed a conspicuous degree of homogeneity within each macrophytic species. These observations were corroborated by the coefficients of variation calculated on the main diatoms and by the non parametric Friedman tests.The qualitative homogeneity of epiphyton on Potamogeton densus foliage made it possible to map the spatial repartition of the main groups of diatoms growing on the upper side of leaves.These data support the hypothesis that the epiphytic algae occupy specific microhabitat niches on their host plants according to a non stochastic pattern of distribution.