Phytoplankton primary production, chlorophyll‐aand nutrient concentrations in the water column of mountainous Lake Phewa, Nepal

Abstract

The seasonal patterns of phytoplankton primary production, chlorophyll-a concentration, cell number and several other limnological variables in Lake Phewa, located in the active monsoon zone in Central Himalaya, Nepal, were studied for a year beginning in April 2001. During the study period, the gross primary production and chlorophyll-a concentrations were relatively low during the monsoon season. The phytoplankton cell number, represented by 24 genera, also fluctuated seasonally, but tended to increase in the pre- and post-monsoon period. These results suggest that the monsoon plays a crucial role in the primary production and phytoplankton dynamics for Lake Phewa. Among the phytoplankton species, Microcystis aeruginosa, a representative species for eutrophic lakes, was the dominant phytoplankton. At the same time, however, it is clear that the lake is not yet heavily eutrophic. The present study suggests that the exchange of lake water during the monsoon season contributes to maintaining the health of the lake against further degradation. Nevertheless, the silt carried in the monsoon rain run-off from the lake's catchment area suggests increasingly serious degradation problems for this small mountainous lake.