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Article

Hydrological and Water Quality Implications of Water Hyacinth: A Case Study of Lake Tana, Ethiopian Highlands

by
Alemu B. Mengesha
1,
Temesgen Enku
2,
Assefa M. Melesse
3 and
Minychl G. Dersseh
2,4,*
1
Amhara National Regional State Bureau of Agriculture, Bahir Dar P.O. Box 437, Ethiopia
2
Faculty of Civil and Water Resources Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Bahir Dar P.O. Box 26, Ethiopia
3
Department of Earth & Environment, Florida International University, Miami, FL 33199, USA
4
Blue Nile Water Research Institute, Bahir Dar University, Bahir Dar P.O. Box 26, Ethiopia
*
Author to whom correspondence should be addressed.
Water 2026, 18(10), 1247; https://doi.org/10.3390/w18101247
Submission received: 8 April 2026 / Revised: 14 May 2026 / Accepted: 18 May 2026 / Published: 21 May 2026
(This article belongs to the Section Water Quality and Contamination)
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Abstract

Water hyacinth (Eichhornia crassipes) is a widespread invasive plant in tropical and subtropical regions, creating serious ecological and hydrological problems. Beyond disrupting aquatic ecosystems, it increases unaccounted water loss and alters key physicochemical properties. This study evaluated the evapotranspiration of water hyacinth and its influence on water quality in Lake Tana, Ethiopia’s largest freshwater lake. Two artificial ponds (one control and one covered with water hyacinth), each measuring 1 m × 1 m × 0.94 m, were monitored over three months to quantify water loss. In parallel, water samples were collected from the lake at 0.5 m depth along 2 km intervals, comparing hyacinth infested and open-water sites. The results showed clear differences between conditions. Dissolved oxygen was significantly lower in hyacinth-covered areas (6.65 ± 0.44 mg/L) than in open water (7.93 ± 0.42 mg/L). Similarly, pH decreased under hyacinth cover (5.53 ± 0.53) compared to non-infested sites (6.53 ± 0.40). In contrast, water temperature increased in infested areas (23.70 ± 0.42 °C) relative to open water (22.08 ± 0.33 °C). Total dissolved solids were slightly but significantly lower in hyacinth-covered water. Evapotranspiration from water hyacinth was about 1.6 times higher than evaporation from open water, with an estimated monthly loss of 0.28 m3 per square meter. When scaled to lake conditions, this corresponds to approximately 0.78 to 7.01 million m3 of water loss per month, though actual values may vary due to environmental factors. Overall, water hyacinth substantially affects both water quantity and quality, highlighting its importance for lake management and sustainable water use.

1. Introduction

Freshwater resources are fundamental to ecosystem functioning and human well-being, supporting agriculture, industry, energy production, and domestic water supply [1,2,3,4]. The ecological integrity of aquatic systems relies on balanced nutrient cycling and stable physicochemical conditions in water and sediments; however, increasing surface water pollution, particularly from agricultural runoff, has become a major and persistent driver of freshwater ecosystem degradation worldwide [5,6,7,8]. Natural lakes, which play a crucial role in climate regulation, biodiversity conservation, and livelihood support, are increasingly exposed to multiple stressors driven by human activities and biological invasions. Expanding agricultural practices, urbanization, and watershed modification have intensified nutrient loading and sediment inflow, while invasive species further disrupt ecological balance and ecosystem functioning. Together, these pressures degrade water quality, alter habitat structure, and reduce the resilience of lake ecosystems, ultimately threatening the services they provide to surrounding communities and regional environments [9,10,11].
Ethiopia possesses substantial freshwater resources, including lakes, rivers, wetlands, and reservoirs; however, these systems are increasingly threatened by invasive floating aquatic weeds that rapidly colonize water surfaces. By reducing light penetration, modifying temperature regimes, and constraining oxygen exchange, these species disrupt ecological balance and threaten long-term freshwater sustainability [12,13].
Water hyacinth (Eichhornia crassipes) is one of the most problematic invasive aquatic weeds in tropical and subtropical regions. Native to the Amazon Basin, it was first documented in the early nineteenth century and has since spread extensively across Africa, Asia, and the Middle East through human activities and favorable environmental conditions [14,15]. Its rapid growth, free-floating nature, and ability to form dense surface mats make it highly persistent and difficult to control. Recent studies, show that its expansion in African freshwater systems is being accelerated by nutrient enrichment, hydrological variability, and climate-related changes that enhance biomass production and seasonal regrowth [16]. As a result, it continues to cause severe ecological and socioeconomic impacts, including disruption of navigation, reduced irrigation efficiency, impairment of hydropower operations, decline in fisheries productivity, loss of aquatic biodiversity, deterioration of water quality, and increased public health risks through the creation of stagnant water conditions favorable for disease vectors [16,17,18].
Water hyacinth significantly alters the physical, chemical, and biological characteristics of aquatic ecosystems. Dense infestations reduce dissolved oxygen through restricted air–water exchange and enhanced organic matter decomposition, increase water temperature by forming insulating surface mats, and disrupt nutrient cycling processes, thereby degrading water quality and threatening aquatic biodiversity [16,19,20,21]. Recent studies further confirm that these impacts are strongly linked to eutrophication processes driven by agricultural runoff and hydrological variability, which intensify oxygen depletion and alter ecosystem metabolism in invaded waters [13,16]. In addition, the plant interferes with water flow and sediment transport, disrupts aquatic habitats, and promotes stagnant conditions favorable for disease vector proliferation. Because of its rapid vegetative reproduction, high ecological plasticity, and strong competitive ability, water hyacinth often dominates invaded water bodies, leading to persistent ecological imbalance and long-term management challenges [13,16,22].
One of the most critical yet often underestimated impacts of water hyacinth is its contribution to increased water loss through evapotranspiration. Transpiration refers to the loss of water vapor from plants, mainly through stomata and, to a lesser extent, through the cuticle. In floating aquatic macrophytes such as water hyacinth (Eichhornia crassipes), transpiration rates per unit area can exceed open-water evaporation, leading to substantial additional water loss from invaded water bodies [23,24]. Consequently, lakes and reservoirs infested with dense mats of water hyacinth may experience higher total water losses than those estimated from evaporation alone, particularly under warm and high-radiation conditions [25]. Beyond water loss, large infestations also modify local microclimate and energy balance, further enhancing evapotranspiration fluxes at the water–plant interface [26]. Given the high financial and logistical costs associated with mechanical or chemical removal, accurate estimation of these additional water losses is essential for effective water resource planning and sustainable management of invaded aquatic ecosystems [27].
In Ethiopia, water hyacinth was first reported in the 1950s around the Rift Valley, particularly near Aba-Samuel Dam [28]. Since its introduction, the species has expanded rapidly across major river basins, including the Awash, Abay (Blue Nile), Baro-Akobo, and Rift Valley lake systems, where its spread has been facilitated by hydrological connectivity and nutrient enrichment [29]. Recent studies further confirm that the invasion has intensified in both spatial extent and ecological impact over the past decade [30,31]. The infestation continues to pose serious challenges to water transport, irrigation schemes, hydropower operations, fisheries productivity, and aquatic biodiversity conservation, while also contributing to increased flooding risk and associated public health concerns in affected communities [32,33].
Lake Tana, the largest freshwater lake in Ethiopia and the headwater of the Blue Nile, has recently been severely affected by water hyacinth infestation, which was first formally reported in September 2011 [17]. Although the precise introduction pathway remains uncertain, the rapid and persistent expansion of the weed along the littoral zones indicates favorable ecological conditions such as nutrient enrichment, shallow water habitats, and limited natural control [33]. Recent assessments show that approximately 5000 ha of the shoreline accounting for more than 30% of the lake’s 385 km perimeter is now covered by dense mats of water hyacinth [8,31]. This extensive invasion continues to pose significant risks to fisheries productivity, lake navigation, hydropower operations, and the overall ecological integrity of Lake Tana [32].
Lake Tana plays a central role in regional and national development, functioning as a key socio-ecological and hydrological system in the Upper Blue Nile Basin. As Ethiopia’s largest freshwater lake and the primary source of the Blue Nile, it provides essential ecosystem services that support the livelihoods of millions of people in Bahir Dar and surrounding lakeshore communities [34]. The lake supplies water for domestic consumption, livestock, irrigation, fisheries, tourism, and small-scale industries, thereby directly linking ecosystem health to socio-economic stability [35]. In addition, Lake Tana sustains extensive wetland ecosystems that regulate nutrient cycling, sediment retention, and biodiversity conservation, including endemic fish species of high ecological and economic importance [32,35,36,37]. Recent studies also highlight its increasing strategic importance in the Nile Basin, as it contributes significantly to downstream water availability for agriculture and hydropower generation in Sudan and Egypt, making its sustainable management a transboundary priority [35]. However, growing anthropogenic pressures and ecological disturbances threaten these services, emphasizing the need for integrated basin-scale management to maintain its long-term functionality.
The uncontrolled proliferation and subsequent decomposition of water hyacinth biomass can intensify environmental degradation by promoting oxygen depletion, generating anaerobic conditions, and releasing greenhouse gases such as methane and hydrogen sulfide during decay processes. These processes further deteriorate water quality and alter biogeochemical cycling within affected aquatic systems [32,33]. When combined with rising external nutrient loading, land-use change, and increasing human pressure within the catchment, the expansion of water hyacinth significantly amplifies ecosystem stress and water loss through enhanced evapotranspiration. In Lake Tana, such dynamics represent a growing threat to long-term water security, ecological integrity, and socio-economic sustainability [33]. Given the increasing demand for freshwater resources and the strategic importance of Lake Tana within the Upper Blue Nile Basin, there is an urgent need for integrated quantification of water hyacinth impacts on both water quality and basin-scale water balance to inform evidence-based management, control, and restoration strategies.
Therefore, the general objective of this study was to evaluate the impacts of water hyacinth on the physical water quality of Lake Tana and to quantify water losses associated with water hyacinth evapotranspiration using constructed experimental ponds. The specific objectives were to: (1) assess the effects of water hyacinth on selected physical water quality parameters in Lake Tana; (2) estimate the evapotranspiration rate of water hyacinth using controlled pond experiments; and (3) compare evapotranspiration losses from water hyacinth-covered surfaces with those from open water surfaces.

2. Materials and Methods

2.1. Study Area

Lake Tana is the largest freshwater lake in Ethiopia, located in the northwestern highlands at an elevation of approximately 1786 m above sea level. The lake covers an area of about 3000 km2, with an average depth of 9 m and a maximum depth of roughly 14 m. Geographically, Lake Tana lies between latitudes 11°00′00″ N and 12°00′00″ N and longitudes 37°00′00″ E and 38°00′00″ E.
The water level of Lake Tana is regulated by natural inflows and controlled outflows, primarily through discharge to the Abay (Blue Nile) River and water withdrawals associated with hydropower generation and irrigation schemes, notably the Tana–Beles project. The lake receives most of its inflow from surrounding rivers, particularly during the rainy season. The major tributaries feeding the lake include the Gilgel Abay, Gumara, Rib, and Megech rivers, which collectively play a crucial role in maintaining the lake’s hydrological balance.
The climate of the Lake Tana basin is characterized by four distinct seasons: the main rainy season (July–September), the post-rainy season (October–November), the dry season (December–April), and the pre-rainy season (May–June). The area receives an average annual rainfall of approximately 1355.74 mm. Climatically, the basin exhibits characteristics of a semi-arid tropical highland environment, with pronounced diurnal temperature variations, where daytime temperatures can reach up to 30 °C, while nighttime temperatures may drop to as low as 6 °C.
Lake Tana and its surrounding watershed (Figure 1) are of significant ecological, economic, and strategic importance. The sub-catchment has been identified as a development growth corridor by both the Amhara National Regional State and the Federal Government of Ethiopia. Recognizing its global ecological value, UNESCO has designated Lake Tana as a Biosphere Reserve. However, the lake is increasingly exposed to environmental pressures, as it receives substantial agricultural runoff, urban storm water, and domestic wastewater from its catchment. These inputs are largely drained from Bahir Dar, the regional capital city, which has an estimated population of approximately 243,300 people [8].

2.2. Methodology

2.2.1. Data Collection and Experimental Setup

This study used a combined field observation and controlled experimental approach to investigate the effects of water hyacinth on water quality and water loss in Lake Tana (Figure 2). The rationale for integrating both approaches was to simultaneously capture the natural variability of the lake system and isolate the specific influence of water hyacinth under controlled conditions. This dual framework addresses a common limitation in earlier studies, where field observations alone are influenced by multiple interacting environmental drivers, while model-based approaches often introduce uncertainty due to generalized assumptions and limited site calibration [38,39].
Field measurements were conducted in April 2021 to characterize the in situ physicochemical conditions of the lake. The measured variables—temperature, pH, dissolved oxygen (DO), and total dissolved solids (TDS)—were selected because they are widely recognized as key indicators of aquatic ecosystem response to macrophytes invasion and changes in energy and nutrient dynamics [13]. All parameters were measured directly in the field using a calibrated multi-parameter probe to ensure real-time data acquisition and high measurement fidelity, avoiding distortions associated with sample transport and laboratory delay [40].
Sampling locations were selected to represent both water hyacinth-infested and non-infested conditions, enabling direct comparative analysis of ecological differences. This targeted contrast-based sampling strategy improves detection sensitivity compared to random sampling designs, which may obscure localized impacts of invasive species [41].
To complement field observations, a controlled pond experiment was conducted from March to May 2021 to quantify water loss attributable specifically to water hyacinth evapotranspiration. This approach was necessary because widely used evapotranspiration estimation methods often rely on parameterized models that require extensive meteorological inputs and may produce uncertainty when applied without local calibration [38]. Controlled experiments, by contrast, allow direct measurement of water loss under standardized environmental conditions, improving causal attribution [39].
Two identical ponds were constructed with dimensions of 1 m × 1 m and 0.94 m depth, ensuring uniform storage conditions. Both ponds were lined with impermeable material to eliminate seepage losses. One pond served as an open-water control, while the second contained water hyacinth to simulate natural infestation conditions. A small amount of nutrient enrichment was applied to the experimental pond to support uniform plant growth and development of a dense floating mat, consistent with observed field conditions in invaded aquatic systems [13].
Both ponds were covered with mesh to prevent external disturbances. Water levels were recorded daily using fixed staff gauges, with measurements consistently taken at midnight to minimize diurnal variability in temperature, wind speed, and evaporation demand. Such standardized temporal control improves measurement consistency and reduces short-term climatic noise [41].
Rainfall events were explicitly accounted for by measuring water levels before and after precipitation, allowing separation of rainfall inputs from evaporation and evapotranspiration losses. This correction is essential in small-scale hydrological experiments, where unadjusted precipitation can significantly bias water balance estimates [38]. All water additions used to restore reference levels were carefully recorded to maintain mass balance integrity.
Overall, this integrated methodology provides a more robust framework than studies relying solely on field observation or model-based estimation by combining direct measurement, experimental control, and comparative analysis. However, a key limitation is that the small-scale pond system does not fully represent lake-scale hydrodynamic processes, including wind-driven mixing, wave action, and spatial energy exchange, which are known to influence evaporation dynamics in natural systems.

2.2.2. Sampling and Measurement of Physicochemical Parameters

This study employed an in situ field-based monitoring strategy to assess physicochemical water quality conditions in Lake Tana (Figure 3). The primary motivation for using direct field measurements was to ensure that recorded values represent real-time environmental conditions, thereby avoiding biases associated with sample transport, storage, and delayed laboratory analysis. Such an approach is particularly important in dynamic aquatic environments, where sensitive parameters such as dissolved oxygen and temperature can change rapidly after sampling [13].
Physicochemical variables—including temperature, pH, dissolved oxygen (DO), and total dissolved solids (TDS)—were measured using a calibrated portable multi-parameter probe. Sampling was conducted at approximately 0.5 m depth, representing near-surface (epilimnetic) conditions where interactions among atmospheric forcing, floating vegetation, and upper water column processes are most active. This depth is widely adopted in limnological studies because it effectively captures zones most responsive to ecological disturbances such as macrophytes infestation [41].
Sampling locations were distributed at approximately 2 km intervals along the lakeshore to capture spatial variability in near shore conditions. From these, four representative sites were selected and classified into two categories: water hyacinth-infested (WH) and non-infested (non-WH) sites. This contrast-based sampling design was intentionally used to enhance the detection of vegetation-driven differences by directly comparing impacted and reference conditions under similar environmental settings. Compared with random sampling, this approach increases analytical sensitivity in detecting localized ecological effects [40].
At each site, measurements were taken directly in the field, and probe readings were recorded immediately after immersion to minimize errors associated with oxygen exchange, temperature equilibration, and handling effects. This rapid in situ measurement approach improves data reliability and ensures that observed variations reflect actual environmental conditions rather than methodological artifacts [38].
Although this approach provides high temporal accuracy and strong comparability between sites, several limitations should be acknowledged. The relatively small number of sampling locations (four sites) limits spatial representativeness and may not fully capture the strong heterogeneity of water quality conditions across the lake. In addition, the fixed 0.5 m sampling depth provides only near-surface information and does not represent vertical stratification, which can be significant in deeper zones where thermal and oxygen gradients develop. The contrast-based design (WH Vs non-WH), while effective for isolating vegetation effects, may introduce site-selection bias and reduce the generalizability of findings across the entire lake system. Furthermore, although portable multi-parameter probes provide rapid measurements, they remain susceptible to minor uncertainties arising from calibration drift and short-term environmental fluctuations. Finally, the limited temporal coverage of sampling does not capture seasonal variability, particularly differences between wet and dry periods, which strongly influence physicochemical dynamics in tropical lake ecosystems.

2.2.3. Data Analysis

Field and experimental data were systematically organized, screened, and prepared for statistical analysis. Prior to analysis, datasets were checked for completeness and consistency, and their distributional properties were examined to assess the suitability of parametric statistical tests. This step is important in environmental studies because physicochemical variables often exhibit non-normal distributions due to spatial heterogeneity and natural variability in aquatic systems [41].
Field data were collected using a judgmental (purposive) sampling strategy, focusing on locations representing both water hyacinth-infested and non-infested conditions. This approach was selected to enhance the detection of ecological contrasts associated with invasive macrophytes presence. Compared to random sampling, which may dilute localized environmental signals, targeted sampling improves sensitivity when the objective is to evaluate specific ecological drivers [13,40].
Following data screening, one-way analysis of variance (ANOVA) was applied to assess spatial differences in physicochemical parameters across sampling sites. This test was used to determine whether mean values of temperature, pH, and dissolved oxygen (DO), and total dissolved solids (TDS) differed significantly between water hyacinth-infested and non-infested areas of Lake Tana. One-way ANOVA is widely applied in environmental studies for group comparison when assumptions of normality and homogeneity of variance are satisfied [38].
Similarly, one-way ANOVA was used to compare water loss between the experimental pond containing water hyacinth and the control pond with an open water surface. This approach enabled statistical evaluation of evapotranspiration-related differences under controlled conditions, strengthening causal interpretation by minimizing external environmental variability [39].
Statistical significance was evaluated at conventional confidence levels (p < 0.05), ensuring that observed differences between groups and treatments reflected meaningful ecological and hydrological effects rather than random variation.
However, several limitations should be acknowledged. The use of one-way ANOVA assumes normality and homogeneity of variance, which may not always be fully satisfied in heterogeneous environmental datasets, potentially affecting the robustness of statistical inference. In addition, the judgmental sampling design, while effective for capturing contrast between infested and non-infested sites, introduces potential selection bias and limits the broader generalization of results across the entire lake system. The reliance on mean-based comparisons may also obscure finer-scale spatial patterns and nonlinear relationships that are common in aquatic ecosystems. Furthermore, the limited number of sampling sites and experimental replicates reduces statistical power and may increase sensitivity to local variability.

3. Results

This study presents the results of the evapotranspiration (ET) rate of water hyacinth and open-water evaporation measured in constructed ponds, as well as the impacts of water hyacinth infestation on selected physicochemical water quality parameters of Lake Tana. The assessed water quality parameters included temperature, dissolved oxygen (DO), total dissolved solids (TDS), and pH.

3.1. Physical Water Properties

The formation of dense water hyacinth mats, resulting from the interlocking growth of individual plants, has noticeably altered water quality conditions in Lake Tana. Water quality parameters—physical, chemical, and biological—play a critical role in maintaining ecological balance and supporting aquatic life [42]. Changes in these parameters can therefore have significant ecological consequences.
To evaluate the influence of water hyacinth invasion on the physicochemical characteristics of Lake Tana, water samples were collected and analyzed from two distinct site categories: areas infested with water hyacinth and areas free from water hyacinth. A total of four representative sampling sites were selected and assessed across both categories. Samples were collected from the lake surface at approximately 0.5 m depth and at 2 km intervals along the shoreline. The summary of the measured physicochemical parameters at the different sampling sites is presented in Table 1.
Water Temperature: Water temperature in areas infested with water hyacinth ranged from 23 to 24 °C, whereas non-infested areas showed slightly lower values between 22 and 23 °C. The mean temperature in hyacinth-covered areas (23.7 ± 0.42 °C) was significantly higher than in areas without water hyacinth (22.08 ± 0.33 °C, p = 0.01). This increase is likely due to the thick mats of water hyacinth, which act as an insulating layer, limiting heat exchange between the lake surface and the atmosphere. Additionally, decomposition of organic matter within the mats can generate heat, further raising water temperature. These findings suggest that dense water hyacinth mats can meaningfully influence thermal conditions in Lake Tana.
Dissolved Oxygen (DO): Dissolved oxygen concentrations were generally lower in water hyacinth-infested areas, ranging from 2.4 to 7 mg L−1, with a mean of 6.65 ± 0.44 mg L−1. In contrast, areas without water hyacinth had higher DO levels (7.93 ± 0.43 mg L−1). One-way ANOVA indicated that the difference between infested and non-infested sites was statistically significant (p = 0.01). The reduced oxygen levels in infested areas may result from the dense mats limiting gas exchange at the water surface and from the respiration of epiphytic and decomposing organisms associated with water hyacinth. Lower DO concentrations in infested areas can adversely affect aquatic life, reducing biodiversity and overall ecosystem health.
Total Dissolved Solids (TDS): TDS values were slightly lower in water hyacinth-infested areas, ranging from 89.4 to 91.8 mg L−1 (mean 90.6 ± 10.62 mg L−1), compared to 91.5–92.5 mg L−1 (mean 92.1 ± 5.35 mg L−1) in non-infested areas. The difference was significant (p = 0.05). Dense water hyacinth mats can trap suspended particles, organic matter, and phytoplankton, which likely explains the lower TDS in infested areas. Reduced TDS and turbidity may influence zooplankton communities, which play a critical role in transferring energy from primary producers to higher trophic levels in the lake.
pH: Water hyacinth-infested areas exhibited lower pH values, ranging from 4.8 to 6 (mean 5.53 ± 0.53), whereas non-infested areas had higher pH values between 6.2 and 7.1 (mean 6.53 ± 0.43). ANOVA results confirmed that pH was significantly lower in hyacinth-covered areas compared to non-infested sites (p = 0.023). The decrease in pH may be associated with organic matter decomposition and changes in nutrient cycling under dense mats of water hyacinth.
Overall Observations: The results indicate that water hyacinth infestation significantly alters the physicochemical characteristics of Lake Tana. Areas covered by dense water hyacinth mats showed lower dissolved oxygen, reduced TDS, higher water temperature, and lower pH compared to open-water areas. One-way ANOVA confirmed that these differences were statistically significant (p < 0.05). The findings underscore the substantial impact of water hyacinth on lake water quality, suggesting that high-density infestations can reduce nutrient availability and degrade habitat conditions for aquatic organisms (Figure 4).

3.2. Impact of Water Hyacinth on Evaporation

The evaporation rates from the experimental ponds revealed a clear influence of water hyacinth on water loss. While evaporation from the hyacinth-covered ponds was generally comparable to that of the open-water control pond, some differences were observed due to the black plastic lining, which increased heat absorption and may have slightly enhanced evaporation.
In March, the water depth in the pond containing water hyacinth decreased from 0.94 m to below 0.80 m, whereas the control pond without hyacinth dropped from 0.94 m to 0.84 m. In April and May, the effect became more pronounced: the hyacinth-covered pond’s water depth fell from 0.94 m to below 0.56 m, while the control pond decreased from 0.94 m to 0.69 m.
These results indicate that water hyacinth mats can significantly increase water loss through evapotranspiration compared to open water surfaces. The dense vegetative cover enhances water loss by both transpiration through the plant tissues and by altering the microclimate at the water surface, emphasizing the hydrological impact of water hyacinth infestations on lake and reservoir water balances (Table 2).
Water Loss and Evapotranspiration: The experiment revealed that water hyacinth significantly increased water loss compared to open-water surfaces. In March, the average evapotranspiration (ET) rate from the hyacinth-covered pond was 10.6 mm/day, compared to 7.2 mm/day from the control pond, indicating that ET under water hyacinth was approximately 1.5 times higher than open-water evaporation (E0).
In April, the ET rate from the hyacinth pond averaged 8.5 mm/day, while open-water evaporation was 5.00 mm/day. Under these conditions, the ET of water hyacinth was 1.7 times greater than E0. In May, the ET rate of water hyacinth was 8.6 mm/day, compared with 5.70 mm/day from the open-water pond, representing a 1.5-fold increase relative to E0.
Over the three-month experimental period, the mean ET rate of water hyacinth was 9.22 mm/day, while the mean evaporation from the open water surface was 5.95 mm/day. On average, water loss from hyacinth-covered surfaces was 1.6 times greater than from open-water surfaces. These results demonstrate that dense mats of water hyacinth can substantially enhance water loss through evapotranspiration, emphasizing their significant impact on the water balance of lakes and reservoirs (Figure 5).

4. Discussion

This study explored how water hyacinth affects the main physicochemical properties of the Lake Tana ecosystem, focusing on differences in water quality between infested and non-infested conditions. It also measured the evapotranspiration rate of water hyacinth through controlled pond experiments, allowing a clearer understanding of its role in water loss. Taken together, the findings highlight both the ecological effects of water hyacinth on lake water quality and its contribution to increased surface water depletion through evapotranspiration.

4.1. Physical Water Quality Parameters

Water quality is inherently dynamic, and even moderately polluted waters can still support certain ecological functions depending on ecosystem resilience and balance among physical, chemical, and biological components. The overall health and biodiversity of lake systems are closely linked to the integrity of these interacting components, while natural drivers such as hydrological cycles, seasonal variability, and climatic conditions further shape lake water characteristics [43]. In this study, all physical water quality parameters were measured in situ using a multiparameter probe to ensure accurate representation of field conditions with minimal disturbance.
Temperature: The results indicated that water hyacinth-infested areas consistently exhibited higher water temperatures compared to non-infested sites. The mean temperature recorded in infested areas was 23.70 °C, whereas non-infested areas averaged 22.58 °C, and this difference was statistically significant (p ≤ 0.01). This pattern is consistent with recent studies from tropical freshwater systems, which report that dense floating vegetation mats reduce surface heat exchange and restrict wind-induced mixing, leading to localized warming of surface waters [12,13,44].
Similar findings have been reported in Lake Tana and other African lakes where water hyacinth expansion has been associated with altered thermal regimes due to shading effects combined with reduced evaporative cooling and turbulence [45]. Elevated water temperature under hyacinth cover is ecologically important because it reduces dissolved oxygen solubility and can intensify physiological stress on aquatic organisms, particularly sensitive fish and invertebrate species [12]. Recent comparative studies also emphasize that even small temperature increases in tropical systems can significantly shift oxygen dynamics and metabolic rates, further compounding the impacts of eutrophication and organic matter decomposition under dense mats [13].
Total Dissolved Solids (TDS): Total dissolved solids showed a slight but statistically significant reduction in water hyacinth-infested areas (90.6 mg/L) compared to non-infested sites (92.1 mg/L) (p = 0.05). Although the difference is relatively small, it reflects the influence of hyacinth mats on particulate and dissolved matter distribution within the water column.
Recent studies similarly report that dense Eichhornia crassipes mats modify sediment dynamics through their extensive fibrous root systems, which act as physical filters that trap suspended particles, organic debris, and phytoplankton [13,15,45]. This process often leads to reduced suspended solids in open water beneath or adjacent to infestation zones, while simultaneously promoting the accumulation of organic material within the mat structure itself. Comparable observations from Lake Tana and other invaded tropical lakes indicate that this filtration effect can alter nutrient distribution and influence local water clarity patterns [44].
However, the recent literature also highlights that this apparent reduction in TDS or suspended load does not necessarily reflect improved water quality. Instead, it often represents a redistribution of particulate matter, where trapped organic material undergoes decomposition within the dense mats, potentially increasing localized nutrient recycling and altering biogeochemical processes [13]. Similar mechanisms have been documented in East African lake systems, where hyacinth infestation has been linked to shifts in primary productivity and nutrient limitation in submerged vegetation zones [45].
The reduced light penetration resulting from surface mat coverage further compounds these effects. Shading limits photosynthetic activity in submerged macrophytes, thereby suppressing primary production and altering habitat structure for aquatic organisms [15]. In agreement with [42], such shading effects can disrupt aquatic food webs by reducing benthic vegetation growth, diminishing habitat complexity, and ultimately affecting fish productivity and biodiversity.
Overall, the findings of this study align with recent research showing that water hyacinth exerts complex, indirect control over sediment dynamics and ecosystem functioning rather than simply altering dissolved solid concentrations in a linear manner.
Dissolved oxygen (DO): Represents the concentration of oxygen available in water in dissolved form and is a key indicator of aquatic ecosystem health. It is essential for the survival of fish and other aerobic organisms and is continuously replenished through atmospheric diffusion and photosynthetic activity by algae and aquatic macrophytes [43]. Because DO is highly sensitive to physical cover, organic loading, and biological activity, it is often one of the earliest parameters to respond to ecosystem disturbance.
In this study, DO concentrations were consistently lower in water hyacinth-infested areas compared to non-infested sites. The mean DO recorded in infested waters was 6.65 mg/L, while non-infested areas showed a higher average of 7.93 mg/L, and this difference was statistically significant (p = 0.01). This pattern aligns closely with recent findings from tropical and sub-Saharan African lakes, where water hyacinth invasion has been repeatedly associated with oxygen depletion due to restricted air–water gas exchange and enhanced organic matter accumulation beneath dense mats [12,13,45].
The observed reduction in DO can be explained by multiple interacting mechanisms. First, the dense floating canopy physically limits oxygen diffusion from the atmosphere into the water column. Second, shading from the mats suppresses photosynthetic oxygen production by submerged plants and phytoplankton, thereby reducing internal oxygen generation within the system. Third, trapped organic matter within the root network (Figure 6) undergoes decomposition, increasing biological oxygen demand and accelerating oxygen consumption processes. Similar mechanisms have been reported in Lake Victoria and other invaded wetlands, where prolonged hyacinth coverage resulted in persistent hypoxic or near-anoxic conditions in affected zones [46].
Recent studies further emphasize that such oxygen depletion can restructure aquatic communities by favoring species with aerial respiration or tolerance to low-oxygen environments, while excluding sensitive fish species and reducing overall biodiversity [13,45]. In agreement with [47], anaerobic decomposition processes beneath dense mats also contribute to carbon dioxide accumulation and further deterioration of water quality conditions.
Overall, the findings confirm that water hyacinth significantly reduces dissolved oxygen availability, thereby disrupting ecological balance and potentially reducing fish productivity. These results are consistent with recent regional studies and highlight the need for effective management strategies to restore oxygen dynamics and maintain aquatic ecosystem stability.
pH: pH is a fundamental indicator of water chemistry that reflects the acidity or alkalinity of a system on a logarithmic scale from 0 to 14. It plays a central role in regulating chemical reactions, nutrient availability, and the physiological performance of aquatic organisms. In this study, water hyacinth had a clear influence on pH dynamics in Lake Tana, with infested areas recording a significantly lower mean pH (5.53) compared to non-infested sites (6.53) (p = 0.023).
This shift toward more acidic conditions is consistent with recent studies showing that dense water hyacinth mats alter carbon and nutrient cycling within invaded aquatic systems. The accumulation and subsequent decomposition of organic matter trapped beneath the mats increase carbon dioxide release and organic acid formation, which collectively contribute to reduced pH levels [13,45]. Similar acidification patterns have been reported in other African lake systems, where prolonged macrophytes infestation modifies local biogeochemical processes and shifts water chemistry away from baseline conditions [12].
Although water hyacinth is known to tolerate a relatively wide pH range, its optimal growth typically occurs under near-neutral conditions [48]. The observed mean pH of 5.53 in infested zones suggests that, while still within the species’ survival threshold, the system has undergone notable chemical alteration. Importantly, even adjacent non-infested areas exhibited a reduced pH of 6.53, indicating that the influence of hyacinth mats extends beyond their immediate coverage and affects broader lake chemistry. This spatial spillover effect has also been observed in Lake Victoria and other invaded wetlands, where macrophytes expansion contributes to system-wide shifts in acidity and buffering capacity [13].
Comparatively, historical records for Lake Tana report a more alkaline condition, with average pH values around 8.2 prior to extensive invasion [30]. The contrast between historical and current measurements strongly suggests that water hyacinth proliferation has contributed to progressive acidification in affected areas of the lake. Such changes are ecologically significant because reduced pH can alter nutrient solubility, affect microbial activity, and disrupt reproductive and metabolic processes of aquatic organisms [42].
Overall, these findings align with the recent literature indicating that water hyacinth not only modifies physical habitat structure but also drives measurable chemical shifts in freshwater ecosystems, including localized acidification and reduced buffering stability.

4.2. Evapotranspiration Rate of Water Hyacinth

Daily evapotranspiration (ET) measurements from the experimental ponds clearly show the influence of water hyacinth when compared with open-water evaporation (E0 pan). Across the study period (March–May), ET from hyacinth-covered ponds consistently exceeded that of the control. Specifically, ET was higher by 31% in March, 40% in April, and 30% in May. Daily ET values over open water ranged from 4.79 to 8.33 mm/day, whereas hyacinth-covered ponds recorded substantially higher rates, ranging from 8.13 to 12.1 mm/day, with an overall mean of 9.22 mm/day.
These findings are consistent with recent studies indicating that floating macrophytes significantly enhance evapotranspiration compared to open-water surfaces due to their large leaf area, active transpiration, and aerodynamic roughness, which promotes vapor exchange [13,44,45]. Similar magnitudes of increase (typically 20–60%) have been reported in tropical lake systems, where dense Eichhornia crassipes cover elevates water loss beyond pan evaporation estimates, particularly during warmer months with high solar radiation.
For comparison, [49] reported average evaporation rates for Lake Tana of approximately 156.6 mm/month (2007) and 156.32 mm/month (2008) during the same March–May period. In the present study, the mean daily evaporation from the open-water pond was 5.95 mm/day, equivalent to 178.54 mm/month, slightly exceeding these historical values. This discrepancy is likely attributable to experimental conditions, particularly the use of black polyethylene lining in the ponds, which increases heat absorption and enhances evaporation rates. Similar methodological effects have been noted in controlled lysimeter and pond experiments, where surface materials can influence thermal properties and evaporation fluxes [50].
Importantly, the magnitude of ET observed in hyacinth-covered ponds highlights the hydrological significance of invasive macrophytes. Recent assessments in Lake Tana and comparable systems have emphasized that widespread hyacinth infestation can lead to substantial cumulative water losses at the lake scale, potentially affecting water balance, reservoir storage, and downstream water availability [42,45].
Overall, the results demonstrate that water hyacinth markedly increases evapotranspiration relative to open water, reinforcing its dual role as both an ecological stressor and a hydrological driver in freshwater systems.
The three-month experiment (Figure 7, Figure 8 and Figure 9) clearly shows a close link between evapotranspiration and the presence of water hyacinth. In general, both hyacinth-infested and open-water ponds experienced a noticeable drop in water levels over time (p < 0.05, α = 0.05). However, the loss was consistently greater in the pond covered with water hyacinth. This suggests that the plant intensifies water loss by increasing both transpiration and evaporation compared to open-water conditions. As the experiment progressed, the gap between the two ponds became more pronounced, indicating that dense hyacinth mats steadily accelerate water depletion.
Across the study period, the overall E/E0 ratio was 1.6, meaning that evapotranspiration from hyacinth-covered water was about 60% higher than from open water. While this is lower than some earlier reports for example, [23] a ratio of 3.7 and [51] reported 3.2—it aligns closely with other findings such as [27,42], both of which reported values around 1.5.
Field observations from Lake Tana show that water hyacinth coverage has varied widely over time, from 278.3 ha in February 2015 to 2504.5 ha in December 2019 [31]. Using the evapotranspiration rate measured in this study, the estimated water loss due to hyacinth is about 0.28 m3 m−2 per month, or roughly 102.2 m3 per year. When scaled up to the lake level, this translates to an estimated loss ranging from 779,240 m3 per month (284,422,600 m3.yr−1) under minimum coverage to 7,012,600 m3 per month (2,559,599,000 m3.yr−1) under maximum coverage. These values are likely conservative, since real lake conditions—such as wind, radiation, and surface reflectivity differ from the controlled experimental ponds.
Overall, the results highlight that water hyacinth can substantially increase water loss from aquatic systems, with clear implications for water availability, lake management, and ecosystem health. If the infestation continues to expand in Lake Tana, total evapotranspiration is likely to rise significantly. In this study, hyacinth-covered ponds lost water at a rate 1.6 times higher than open water, reinforcing the strong influence of this invasive plant on water depletion.

5. Conclusions

This study demonstrates that water hyacinth exerts a substantial influence on both the water quality and hydrological balance of Lake Tana. Key physicochemical parameters differed significantly between hyacinth-infested and non-infested areas (p < 0.05). In particular, dissolved oxygen concentrations were markedly lower in infested zones (6.65 ± 0.44 mg/L) compared to non-infested areas (7.93 ± 0.42 mg/L), indicating that dense hyacinth mats restrict atmospheric oxygen exchange and suppress photosynthetic oxygen production by limiting light penetration. These findings are consistent with recent studies showing that Eichhornia crassipes reduces oxygen availability, alters thermal regimes, and modifies nutrient cycling in tropical freshwater systems [12,13,45].
Beyond water quality impacts, the study confirms that water hyacinth significantly enhances water loss through evapotranspiration. ET from hyacinth-covered ponds was approximately 1.6 times higher than from open-water surfaces, aligning with recent estimates from Lake Tana and similar ecosystems, where increases of 20–60% have been reported [44,45]. When extrapolated to the lake scale, this corresponds to substantial volumetric losses—up to approximately 7.0 × 106 m3 per month under maximum infestation scenarios. Such hydrological impacts highlight the role of water hyacinth not only as an ecological stressor but also as a significant driver of water depletion.
The combined effects of reduced dissolved oxygen, altered pH, and increased water loss have broader ecological consequences. Shading from dense hyacinth mats limits light availability, reducing photosynthesis, primary productivity, and habitat suitability for aquatic organisms. These changes can disrupt food webs, reduce fish production, and ultimately affect local livelihoods dependent on the lake. Similar ecosystem-level impacts have been documented in other African lakes experiencing water hyacinth invasion, emphasizing the regional relevance of these findings [13,45].
Given these impacts, integrated and adaptive management strategies are essential. Mechanical removal, supported by biological control and carefully regulated chemical methods where appropriate, should be prioritized in heavily infested zones. Continuous monitoring of hyacinth coverage and key water quality parameters is critical for early detection and timely intervention. In addition, community engagement, public awareness, and coordinated government action are necessary to prevent further spread and ensure sustainable management.
Future research should look more closely at long-term ecosystem responses, including impacts on biodiversity, fisheries productivity, and basin-scale hydrology, while also evaluating the potential of restoration approaches that use native aquatic vegetation. It is equally important to integrate water hyacinth management into broader water resource planning, considering the vital role of Lake Tana in supporting agriculture, fisheries, hydropower, and local livelihoods. In addition, future studies would benefit from including a wider range of physico-chemical parameters—such as nutrients, alkalinity, hardness, and salinity—to better capture the full scope of water quality changes associated with water hyacinth infestation.
Overall, controlling water hyacinth is not only an ecological priority but also a hydrological necessity to reduce water loss, improve water quality, and safeguard the long-term sustainability of Lake Tana.

Author Contributions

Conceptualization, A.B.M. and M.G.D.; methodology, A.B.M.; validation, M.G.D., A.M.M. and T.E.; formal analysis, A.B.M.; investigation, A.B.M.; resources, A.B.M.; data curation, A.B.M.; writing—original draft preparation, A.B.M. and M.G.D.; writing—review and editing, M.G.D., A.M.M. and T.E.; visualization, M.G.D.; supervision, M.G.D.; project administration, A.B.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data used in this study are not publicly available due to privacy and ethical considerations. However, they can be shared by the corresponding author upon reasonable request, subject to necessary permissions from the relevant authorities.

Acknowledgments

We would like to express our sincere gratitude to Bahir Dar University, Institute of Technology, for providing support and access to the experimental site. Special thanks go to Bantesew Muluye for his invaluable contribution to the collection of sample data from Lake Tana. Additionally, we extend our appreciation to the Bugna Woreda Agricultural Office for the financial assistance that made this research possible.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Study area overview showing the map of Ethiopia (top left), the Upper Blue Nile Basin (top right), and the Lake Tana basin with sampling and experimental sites.
Figure 1. Study area overview showing the map of Ethiopia (top left), the Upper Blue Nile Basin (top right), and the Lake Tana basin with sampling and experimental sites.
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Figure 2. Evapotranspiration measurement setup at the experimental site: (A) preparation of the experimental pond; (B) supervision by a senior researcher; (C) water hyacinth-infested plot alongside the control pond; and (D) evapotranspiration measurement procedure.
Figure 2. Evapotranspiration measurement setup at the experimental site: (A) preparation of the experimental pond; (B) supervision by a senior researcher; (C) water hyacinth-infested plot alongside the control pond; and (D) evapotranspiration measurement procedure.
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Figure 3. In situ measurements of physical water quality parameters at the surface of Lake Tana in water hyacinth-infested and non-infested areas.
Figure 3. In situ measurements of physical water quality parameters at the surface of Lake Tana in water hyacinth-infested and non-infested areas.
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Figure 4. Comparison of key water quality parameters under water hyacinth infestation and non-infestation conditions.
Figure 4. Comparison of key water quality parameters under water hyacinth infestation and non-infestation conditions.
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Figure 5. Comparison of evapotranspiration rates between ponds with and without water hyacinth infestation.
Figure 5. Comparison of evapotranspiration rates between ponds with and without water hyacinth infestation.
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Figure 6. Dense root system of water hyacinth capturing suspended sediments and organic debris.
Figure 6. Dense root system of water hyacinth capturing suspended sediments and organic debris.
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Figure 7. Temporal variation in daily evapotranspiration showing a comparison between water hyacinth-covered water and open-water evaporation at the experimental station during March.
Figure 7. Temporal variation in daily evapotranspiration showing a comparison between water hyacinth-covered water and open-water evaporation at the experimental station during March.
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Figure 8. Temporal variation in daily evapotranspiration showing a comparison between water hyacinth-covered water and open-water evaporation at the experimental station during April.
Figure 8. Temporal variation in daily evapotranspiration showing a comparison between water hyacinth-covered water and open-water evaporation at the experimental station during April.
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Figure 9. Temporal variation in daily evapotranspiration showing a comparison between water hyacinth-covered water and open-water evaporation at the experimental station during May.
Figure 9. Temporal variation in daily evapotranspiration showing a comparison between water hyacinth-covered water and open-water evaporation at the experimental station during May.
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Table 1. Spatial variation in physicochemical water quality between water hyacinth-infested and non-infested sites in Lake Tana (April 2021). Values are presented as mean ± standard error (n = 4).
Table 1. Spatial variation in physicochemical water quality between water hyacinth-infested and non-infested sites in Lake Tana (April 2021). Values are presented as mean ± standard error (n = 4).
ParametersWith Water HyacinthWithout Water HyacinthProbability LevelLevel of Difference
Temperature (°C)23.70 ± 0.4222.08 ± 0.330.01significant
Dissolved Oxygen (mg L−1)6.65 ± 0.447.93 ± 0.430.01significant
Total Dissolved Solid (mg L−1)90.6 ± 10.6292.10 ± 5.350.05Significant
pH5.53 ± 0.536.53 ± 0.430.023significant
Note: NS = not significant (p > 0.05); values with p ≤ 0.05 are considered statistically significant.
Table 2. Comparison of evapotranspiration rates between ponds with and without water hyacinth.
Table 2. Comparison of evapotranspiration rates between ponds with and without water hyacinth.
NoMonthAverage Water Loss (mm/day)
Without Water HyacinthWith Water HyacinthRation of E/Eo
1March7.210.61.5
2April58.51.7
3May5.78.61.5
4Whole Period69.2
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Mengesha, A.B.; Enku, T.; Melesse, A.M.; Dersseh, M.G. Hydrological and Water Quality Implications of Water Hyacinth: A Case Study of Lake Tana, Ethiopian Highlands. Water 2026, 18, 1247. https://doi.org/10.3390/w18101247

AMA Style

Mengesha AB, Enku T, Melesse AM, Dersseh MG. Hydrological and Water Quality Implications of Water Hyacinth: A Case Study of Lake Tana, Ethiopian Highlands. Water. 2026; 18(10):1247. https://doi.org/10.3390/w18101247

Chicago/Turabian Style

Mengesha, Alemu B., Temesgen Enku, Assefa M. Melesse, and Minychl G. Dersseh. 2026. "Hydrological and Water Quality Implications of Water Hyacinth: A Case Study of Lake Tana, Ethiopian Highlands" Water 18, no. 10: 1247. https://doi.org/10.3390/w18101247

APA Style

Mengesha, A. B., Enku, T., Melesse, A. M., & Dersseh, M. G. (2026). Hydrological and Water Quality Implications of Water Hyacinth: A Case Study of Lake Tana, Ethiopian Highlands. Water, 18(10), 1247. https://doi.org/10.3390/w18101247

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