Assessing seasonal nitrogen export to large tropical lakes

Area of Science:

• Environmental Science
• Hydrology
• Limnology

Background:

• Increasing nitrogen (N) export to lakes from rivers drives eutrophication, particularly in tropical regions.
• Seasonality of nutrient loading in tropical areas is not fully understood, hindering effective management.
• Dissolved inorganic nitrogen (DIN) inputs to tropical lakes require detailed seasonal and source-specific analysis.

Purpose of the Study:

• To develop and apply a seasonal model for quantifying dissolved inorganic nitrogen (DIN) inputs to tropical lakes.
• To understand the seasonality and sources of DIN from sub-basins and rivers.
• To account for diffuse N sources, including open defecation, in riverine inputs.

Main Methods:

• Integrated existing approaches into a seasonal model incorporating human activities, meteorology, and hydrology.
• Applied the model to sub-basins of Lake Tana, Ethiopia, a representative tropical lake.
• Calibrated seasonality parameters and validated model outputs against measured nitrogen loads.

Main Results:

• The model accurately predicted nitrogen loads, distinguishing four seasons: rainy, post-rainy, dry, and pre-rainy.
• Annual DIN export to Lake Tana was approximately 9 kton in 2017, with highest loads during the rainy season and lowest during the dry season.
• Diffuse agricultural sources, primarily animal manure, were the dominant contributors to DIN in rivers across all seasons.

Conclusions:

• The seasonal sub-basin and river model effectively identifies main nutrient sources and informs water quality management options.
• Targeted nutrient management, such as aligning application levels with crop needs, can mitigate N pollution.
• The model serves as a valuable tool for analyzing future trends and can be adapted for other large tropical lakes facing eutrophication challenges.