Relationships between aquatic plants and environmental factors along a steep Himalayan altitudinal gradient

We examined the relationships of aquatic plants with physical–chemical characteristics among 28 lakes within a steep altitudinal gradient ranging from tropical (77 m) to high alpine (4750 m) in the Himalayas of Nepal. Species richness and diversity showed an approximately linear decrease with increasing altitude. The study region exhibits a relatively high proportion of monocotyledonous helophytes, as is typical of aquatic plants on the Indian subcontinent. A canonical correspondence analysis of the entire altitudinal gradient (CCA-1) suggested that the strongest abiotic influences on the distribution of aquatic plants are associated with water temperature, substrate quality, altitude, pH, transparency and conductivity. Two more-restricted CCA analyses examined a shorter altitudinal gradient of 70–1500 m. The CCA-2 analysis (all aquatic plants) and CCA-3 (only euhydrophytes) found that the most important abiotic influences were associated with temperature, lake surface area, suspended solids, bicarbonate and dissolved phosphorus. These results suggest that relatively local influences are different from those that have a regional basis, but that climate-related influences are key along altitudinal gradients. The temperature gradient in the CCA distinguished montane and alpine Arcto-tertiary floristic elements of the highest altitudinal regions from the more widely distributed temperate and tropical species of the lower regions.