The Relationship of Lake Morphometry and Phosphorus Dynamics of a Tropical Highland Lake: Lake Tana, Ethiopia

Abstract

Lakes hold most of the world’s fresh surface water resources. Safeguarding these resources from water quality degradation requires knowledge of the relationship between lake morphometry and water quality. The 3046-km2 Lake Tana in Ethiopia is one of the water resources in which the water quality is decreasing and water hyacinths have invaded. The objective of this study is to understand the interaction between the lake morphometry and water quality and specifically the phosphorus dynamics and their effect on the water hyacinths. A bathymetric survey was conducted in late 2017. Various morphometric parameters were derived, and both these parameters and sediment available phosphorus were regressed with the dissolved phosphorus. The results show that, with a wave base depth that is nearly equal to a maximum depth of 14.8 m, the bottom sediments were continuously suspended in the water column. As a result of the resuspension mixing, we found that the dissolved phosphorus in the water column decreased with lake depth and increased with sediment available phosphorus (R2 = 0.84) in the northern half of the lake. This relationship is not as strong in the south due to a large flow of Gilgel Abay to the outlets. Water hyacinths were found where the lake was shallow and the available phosphorus was elevated. The large reservoir of sediment phosphorus will hamper any remedial efforts in removing the water hyacinths.

Water Balance for a Tropical Lake in the Volcanic Highlands: Lake Tana, Ethiopia

“…Accuracy of the lake levels is of paramount importance for the inflow calculations (Equations (5), (6) and (9ab)). We compared the observed lake levels with those downloaded from Hydroweb [49]. As shown in Figure 6, the lake levels agreed well after subtraction of 1.46 m from the Hydroweb data for 1992-2001 and 1.65 m between 2001 and 2007 from the observed lake levels.…”

“…As shown in Figure 6, the lake levels agreed well after subtraction of 1.46 m from the Hydroweb data for 1992-2001 and 1.65 m between 2001 and 2007 from the observed lake levels. After 2007, the manually observed lake levels were greater than the satellite observed data set from Hydroweb [49] during the rainy phase. Therefore, they are considered inadequate for water balance and it is not shown in Figure 6.…”

“…The storage and area characteristics of the lake are needed for calculating the water balance using the lake-level measurements. The relationship of the lake area and volume with the elevation of the recent bathymetric survey by Kebedew, et al [49] was expressed as third-order polynomial functions with R 2 greater than 0.997.…”

Phosphorus Export From Two Contrasting Rural Watersheds in the (Sub) Humid Ethiopian Highlands

“…In addition, two plot studies were published that report on P export from various land uses by Grum et al (2017) and Mulualem et al (2021). Dersseh et al (2019) and Kebedew et al (2020) measured phosphorus concentration and the degree of eutrophication. Thus, the objective is twofold.…”

Application of Ecohydrology Approach for Mitigation of Freshwater Ecosystems Contamination

“…Kebedew et al [6] in turn, present the distribution of another frequently-monitored pollutant, phosphorus, in the ecosystem of the Tana Lake (the largest freshwater lake in Ethiopia and the third largest in the Nile Basin) in relation to the lake morphometry. This paper, therefore, fits into a cause-effect relationship approach postulated by ecohydrology, as does the paper by Hart et al [7] by demonstrating an insight into the processes regulating pollutant dispersion in a model beaver dam.…”