Study of Nutrients (Nitrogen and Phosphorous) in Phewa Lake Sediments

Introduction

Lakes are sensitive to increase in nutrient concentration. Normally, urban lakes are more prone to nutrient loading and eutrophication caused by watershed development. The lake sediment acts as a sink as well as source of nutrients for lake water. The nutrients concentration in the bed sediments has substantial influence on the trophic status of lake. Thus, regular monitoring of nutrient concentration in the lake sediment is essential.

Tropic structure of Phewa lake has changed from oligotrophic to mesoeutrophic and then to eutrophic from 1973 to 1986 and till now (Heyojoo & Takhacche, 2014; Shrestha & Janauer, 2001). Hydrological and sediment transport dynamics in Phewa lake watershed is being altered by modification in natural landscape. Accumulation of sediments from tributaries, creeks, and the sewage inlet from hotels in Phewa Lake has reduced both effective depth and surface area of the lake, which eventually reduces the effective life of the lake (Balla & Sthapit, 1998; Gurung et al., 2007). This shows lakes of Nepal are at risk.

Nitrogen and Phosphorous are essential nutrients for growth of plants but excess amount of those nutrients will result in depletion of dissolved oxygen (DO). If the amount of DO decreases resulting less or no concentration of it in water, then lake is considered as dead lake. Also, it is said that the concentration of nutrients in lake is normally 10 times the concentration in water (Havens et al., 2001). In lake, nutrients are flushed out through outlet as well as from sinking in sediments. Therefore, sediments are the source of internal loading of nutrients into water. For this reason, it is necessary to study nutrients of water along with sediments to prevent the lake from being dead lake.

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