New model to assessing nutrient assimilative capacity in plant-dominated lakes: Considering ecological effects of hydrological changes

• A process-based model was developed to assess nutrient assimilative capacity.
• The model considered hydrological changes and the effects on ecological processes.
• Assimilative capacity in plant-growing season accounts for over 90% of annual total.
• Nutrient assimilative capacity is previously overestimated in plant-dominated lakes.

Assessing nutrient assimilative capacity of water body is an essential component of the Total Maximum Daily Load program. Nutrient assimilative capacity of lakes is affected by hydrological changes through physical effects on the dilution capacity of water body and ecological effects on nutrient removal processes. Previous research has focused on the effects of hydrological changes on dilution capacity, but rarely considered the ecological effects on nutrient assimilative capacity. In this study, a process-based model considering both ecological effects and dilution effects of hydrological changes is proposed for assessing nitrogen and phosphorus assimilative capacities of plant-dominated lakes. Plant uptake and biological denitrification are two crucial nutrient removal processes. Water depths in a lake affect plant growing area. Lake submerged condition affects nutrient accumulation capacity of plants and the intensity of biological denitrification. The model considered dynamic variations of lake water depths and submerged condition, and the associated effects on nutrient removal by plant nutrient uptake and biological denitrification in lake systems. It is a discrete 0D model depending on a small amount of data and has satisfactory simulation accuracy. We chose the largest freshwater lake in northern China (Baiyangdian Lake) as a case. The annual total assimilative capacity in the lake was 1536 t for nitrogen and 157 t for phosphorus. Assimilative capacity during plant-growing season accounted for over 90% of the annual total capacity. The results determined by the new model were less than half of that from previous model, demonstrating the significance of considering ecological effects of hydrological changes. The new assessment model offers a useful tool for directing pollutant emission control and eutrophication prevention in global plant-dominated lakes.