A modelling approach to determine systematic nitrogen transformations in a tropical reservoir

The systematic nitrogen transformations were investigated in a tropical freshwater system using a three-dimensional Hydro-Eutrophication model, which simulated hydrodynamics, phytoplankton processes and nutrients cycling in Delft3D. The nitrification and denitrification processes were explicitly considered as the functions of water temperature and oxygen conditions in the model. The validated model was able to capture the thermal differences in vertical direction, reproduce the characteristics of nutrients cycles and primary productions across years with the forcing functions from the area-specific meteorological conditions, discharges and input loads. The modelled nitrification and denitrification process rates were verified by the lab process-oriented measurements. The validated model preformed as a monitor, supplying the knowledge that sediment played an important role in nitrogen cycling, producing twice of the total external nitrogen load to water column. The water column bioactive nitrogen was controlled dominantly by phytoplankton assimilation, which had a nitrogen uptake rate of fifteen fold the external nitrogen loads. The bioactive nitrogen turnover rate by algal growth in this tropical water system was lower than the conditions in the other eutrophic lakes. Fifty-eight percent of the total external nitrogen load was retained via denitrification and burial processes in the Upper Peirce Reservoir. The discovery of the systematic nitrogen transformations in tropical freshwater systems is proposed to be incorporated into future conceptual models of global nitrogen cycles. (C) 2016 Elsevier B.V. All rights reserved.