To study effects of altitude, monthly sampling was undertaken from June 1993 to May 1996 in the upper Kodayar reservoir (1312 m ASL), the lower Kodayar (92 m ASL) and Azhakia Pandiapuram (plain) (Tamilnadu, India). Temperature decreased with elevation at a rate of 1 °C per 240 m; the thermal range also decreased with increasing altitude. The number of days and quantity of precipitation decreased from 161 days and 366 cm yr-1 in the upper Kodayar to 92 days and 127 cm yr-1 at the Azhakia Pandiapuram (APP). Transparency decreased from 1.5 m at the upper Kodayar to about 0.5 m at the APP. Dissolved oxygen increased with decreasing altitude but remained high (>7 mg l-1) in all the three systems. Throughout the investigation, the upper Kodayar reservoir (<6.8) and lower Kodayar lake (<7.0) remained acidic, while the APP was always alkaline. Trends for CO2 and alkalinity of the three Kodayar systems were parallel to those for pH. Though widely oscillating across calendar months, nitrate averaged c. 30 μg l-1, while phosphate increased from 8 μg l-1 in the upper Kodayar to 18 μg l-1 at the APP. Wide oscillations in nitrate suggest that it may be limiting productivity more than phosphorus. Chlorophyll-a values were c. 1.9 μg l-1 throughout the year in the upper Kodayar, and between 10 and 20 μg l-1 in the other two ecosystems; values peaked during the dry season, from January to April. Chlorophyll-a concentrations correlated positively with productivity; every μg increase in chlorophyll-a caused 0.15 gC m-3 d-1 more production. The pristine water of the fragile upper Kodayar had the lowest productivity, and poorest diversity and population density. The species richness was assessed using organisms larger than 75 μm filtered by a plankton net. There were five species (Staurastrum spp., Staurodesmus spp., Botryococcus) of phytoplankton, and three species of cladocerans and five species of copepods; population density averaged to 159 l-1 for phytoplankton, 0.3 l-1 for zooplankton. The lower Kodayar proved to be the richest for species diversity; there were 14 species of phytoplankton and six species of cladocerans and six species of copepods. Population density averaged to 203 l-1 for phytoplankton and 0.44 l-1 for zooplankton. The APP displayed moderate species richness; there were seven species of phytoplankton and six species of cladocerans, eight species of copepods and three species of rotifer; but the population density was comparatively higher than the upper and lower Kodayar; it averaged 412 l-1 for phytoplankton and 5.9 l-1 for zooplankton. These values fell within the range of values reported for other tropical and temperate lakes. Staurastrum, Ceriodaphnia cornuta, Tropocyclops, Thermocyclops and Mesocyclops sp. were present in all the aquatic systems and tolerated wide range pH between 6.5 and 8.0. APP was the most productive (4.4 gC m-3 d-1). Productivity holds positive correlation with pH and temperature; for every 1 °C increase in temperature it increased by 0.39 gC m-3 d-1 in these aquatic systems.