Water quality patterns, trends and variability over 17+ years in Phewa Lake, Nepal

Abstract

Phewa Lake, Nepal's second largest lake, has been the subject of many short-term water quality studies in the past, although the results of such studies have not been consistent. Long-term monitoring data can distinguish typical seasonal dynamics from atypical occurrences. Accordingly, the objective of the present study was to describe typical seasonal patterns and quantify variability, detect long-term trends and provide reliable baseline data for future trend detection. The present study provides 17 years of monthly water quality data (1993–2010) and occasional data to 2018 from the 0.5 to 10 m depth at one sampling site. Phewa Lake experiences a subtropical monsoonal climate, typically being stratified from March to September, although heavy rainfalls disrupted stratification. Bottom-water anoxia, occurring typically between April and June, was associated with increased ammonium concentrations, but not with increased dissolved reactive phosphorus concentrations. The observed nutrient and chlorophyll-a concentrations were generally typical of an oligo- to mesotrophic lake. The water clarity was lowest in the autumn and winter, being associated with high phytoplankton biomass, as well as during the summer, the latter likely attributable to suspended silt in the water. Lake surface waters warmed by 1.1–1.8°C during 1993–2018. Water clarity data suggested slight eutrophication from 1993 to 2018, although no increased nutrient or chlorophyll-a concentrations was detected up to 2010. The water clarity data gave no evidence of a long-term increase in the suspended sediment concentrations. All studied water quality variables exhibited high variability within and between years. The results of the present study provide a baseline for the detection of future water quality trends. The data confirm some previous research findings, while at the same time indicating long-term regular monitoring is needed to accurately characterize Phewa Lake's limnology and to detect long-term trends. Correct trend detection and proper understanding of the lake limnology are essential for proper management of Phewa Lake and other subtropical lakes. The present study represents one of only a few studies that examine water quality and other seasonality variables in subtropical lakes over multiple years.