High-resolution Hydrodynamic Measurements and Simulations of an Amazon Floodplain Lake

Abstract:

Shallow, warm water lakes often undergo daily cycles of stratification and mixing with important ecological consequences. As part of an investigation of metabolism and gas exchange in Amazon floodplain lakes, we deployed sensors to measure thermal structure and adapted a 3-dimensional hydrodynamic model (AEM3D) to link physical and biological processes. At our study site, Lake Janauacá, water level fluctuated up to 13 m, and water temperatures varied from 28 to 37 ^oC. Vertical temperature profiles were obtained with RBR solo^® thermistors (accuracy of 0.002 ^oC, recording at 0.1 Hz) deployed on taut-line moorings in the main lake (53 km²) and in a wind-protected bay (0.2 km²). The field data consists of 20 periods of 1.6 to 7 days spanning 2 years. Analysis of the data shows that buoyancy flux induced by diurnal heating reached about 2.0 x 10^-7 m²s^-3. With light winds, stratification developed in the top 0.5 m of the water column in the morning with near-surface buoyancy frequency reaching 60 to 120 cycles per hour. Nocturnal mixing can reach as deep as 6 m, to which the contribution by convection exceeds 90% when winds are light and approximately 50% on the few occasions when wind speed reached ~5 m s^-1. To be able to capture the stratification, horizontal resolution of the AEM3D simulations was 100 m and 5 m at the main lake and the bay, respectively, and 0.1 m in the vertical in both. The simulations of diurnal stratification and nocturnal mixing at both sites during low, rising, high and falling water levels matched the field data well. The model reproduced the structure of the near-surface water and internal waves.