Importance of spatial heterogeneity of nutrient loading on the ecological status of lake Tana, Ethiopia

Understanding spatial variability of nutrient loading and transport in large lakes, and its effect on the eutrophication status are often lacking and hinder effective and sustainable management of lakes. Yet knowledge on the extent of spatial variability of eutrophication in large shallow tropical lakes hinders sustainable management. Understanding how eutrophication varies across lakes helps spatially targeted nutrient load mitigation strategies. This modelling study assesses how spatial heterogeneity in nutrient loads may drive spatial variability in in-lake eutrophication effects for Lake Tana, a large tropical lake. We applied a novel method of coupling a 2D application of the flow model Duflow with the ecosystem model PCLake+ (a zero-dimensional food-web model). We defined different impact zones of major tributaries with a tracer model and simulated ecological processes and food web relations for each impact zone. Furthermore, we defined critical nitrogen and phosphorus loads for each impact zone and the whole lake. Subsequently, we analysed the spatial variability of phytoplankton and aquatic vegetation dynamics among the impact zones, as well as the differences in critical loadings. The model results indicate different ecological conditions in the impact zones and the importance of spatial heterogeneity of eutrophication reflected in different critical loading. The model shows that the north impact zone and northwest impact zone are mainly vegetation-dominated, and southeast impact zone and southwest impact zone are phytoplankton-dominated. Simulated chlorophyll-a concentrations ranged from 0 to 59 µg l⁻¹ and the coverage of the vegetation ranged from 1 to 127 g d.w.m⁻², aligning with the range of observed values in the lake. We demonstrate that this model approach can help to identify spatial heterogeneity in hydrology and eutrophication, as such lakes cannot be regarded as completely mixed systems. Our new approach demonstrates how spatially variable nutrient loading correspond to variability in eutrophication effects in a large shallow tropical lake, and will aid in setting more spatially-targeted lake management strategies.