Turbulent eddies and their implications for phytoplankton within the euphotic zone of Lake Biwa, Japan

Abstract

Profiles of temperature-gradient microstructure were obtained from Lake Biwa, Japan, as part of BITEX'93. These profiles were used to characterize the turbulence and extent of vertical circulations of phytoplankton in the euphotic zone of this mesotrophic lake. Overturning scales and the percentage of the upper mixed layer that was mixing were strongly dependent upon meteorological forcing. Maximum overturns were on scales of 1 to 10 m in the North Basin and 1 m to the depth of the lake in the shallower South Basin when hourly wind speeds persistently exceeded 4 m.sec-1. Within the South Basin, lateral intrusions of cold water sometimes reduced the depth of penetration of turbulence due to heat loss at the air-water interface. In such cases, nutrient fluxes from waters near the sediment-water interface would be reduced. From late morning to mid-afternoon on calm days, phytoplankton would circulate over at most 10% of the euphotic zone in the North Basin, 20% in the South. During high winds, or when solar irradiance was low, phytoplankton were circulated in overturns of varying sizes with some equal to or exceeding the depth of the euphotic zone.