Spring ammonium maximum in the mid-depth layer of Lake Biwa, Japan

Abstract

The spatio-temporal variation in ammonium-nitrogen (NH4-N) profiles was investigated in the North Basin of Lake Biwa in 2002, 2004, and 2005 and in the Uso River in 2004  –  2005. The in-lake concentration of NH4-N was low throughout the water column, except between late April and mid-June, when a distinct maximum developed between 10 and 25  m depth, in and below the thermocline. This layer extended offshore from both east and west boundaries to the center of the lake. By comparing water temperature estimates from the Uso River with water temperatures measured in the lake, we hypothesize that cooler water intrudes into the lake during the night at depths between 8 and 28  m. The intrusions are likely more pronounced from mid to late May when the NH4-N concentration in the Uso River is highest. This suggests that riverine input is an important factor in the formation of the mid-depth ammonium maximum in Lake Biwa. These findings underscore the importance of fluvial discharge and lateral transport of dissolved nutrients in temperate lakes.