4.1. Physico-chemical parameters
The pH of Ketar River varied from 7.84 - 8.11 indicating the alkaline nature of the river water, The pH values of the present study are within the range of desirable levels of pH (6.5-8.5) set by WHO [47] for optimal growth of aquatic organisms. The slight increase in pH observed along the river course (Table 2) may be associated with the deposition of sediment, which is known to contribute to an increase in pH values [48].
Temperature is a factor of great importance for an aquatic ecosystem, as it affects the organisms, as well as the physical and chemical characteristics of water [49]. The present surface water temperatures are cooler than those reported by Degefu et al. [50] (23.53 - 25.65 ºC) for Awash River. The lower level of surface water temperature of the present study might be due to the shading effect of macrophytes found along the banks of Ketar River, a condition, which was shown to impact river water temperature by, Lin and Herold [51] and Willis et al. [52]. The recorded mean levels of EC (202 to 239 µS/cm), which is a function of the amount of total dissolved salts [53], are lower than the permissible limit set by WHO [47] for drinking water. Koning and Roos [54] reported that the average EC value of typical, unpolluted rivers is approximately 350 mS/cm. Thus, the present result, which is less than 350 mS/cm indicates that the river water is suitable for direct domestic use. Compared to the levels of EC reported by Degefu et al. [50] for Awash River (327.67 - 492.87 μS/cm), the present results for Ketar River indicate its much lower level of EC. This suggests that the river receives a low amount of dissolved inorganic substances in ionized form from its surface watershed [55].
The TSS content of water depends on the number of suspended particles, soil and silt, which are directly related to the turbidity of water. The present average values of TSS (231.1 - 303.5 mg/L) are much higher than the permissible limit (150 mg/L) set by WHO [47] for drinking water. These high TSS values could be attributed to surface runoff and disposal of domestic sewage. Although the recorded TSS levels showed no significant differences among the study sites, the slightly higher values recorded at site 3 seem to have resulted from surface runoff from nearby agricultural lands. According to Akan et al. [56], river water with TSS values greater than 100 mg/L but less than 220 mg/L is classified as medium wastewater. Thus, the overall mean TSS value for Ketar River is 267.3 mg/L, which warrants its classification as high wastewater.
Dissolved Oxygen is the most important parameter related to the sustainability of aquatic life. The lowest level of the range of concentrations of DO recorded in this study (5.25 – 6.3 mg/L) occurred at sites 2 and 3, which receive agricultural runoff and animal wastes from nearby livestock holding operations. The high mean concentrations of DO recorded at the lower sites (sites 4 – 6) could be due to the self purification of the water along the course of the river. The absence of statistically significant difference in the DO levels among sites at 95% confidence level (Table 2) might be that the river flowing down its course creates turbulence, which favors the dissolution of atmospheric oxygen [57]. The mean values of the present study varied within a narrower range compared with those reported previously by Degefu et al. [50] and Eliku and Leta [58] for Awash River (3.62 – 7.58 mg/L). At all sites, the concentrations of DO were above the minimum required (4mg/L) for the survival of the biological components of aquatic ecosystems [59]. According to USEPA [60], the values of DO within the range of 5−14.6 mg/L indicate a healthy water body. Furthermore, the measured values of DO of all sampling sites are within the range of desirable levels (>5 mg/L) recommended by WHO [47] for the survival of aquatic life.
The mean concentrations (mg L-1) of nitrite (0.11 - 0.18), nitrate (0.21 - 0.28) and ammonia (0.64 - 0.7) varied within narrow ranges (Table 2). The values of nitrate are less than those reported by Degefu et al. [50] for Awash River, while those of ammonia are much higher than the levels documented by Degefu et al. [50]. In the present study, the concentration of both nitrate and ammonia were much lower than the maximum permissible limit set by the Ethiopian EPA [61]. Agricultural practices within the catchment taking place in the vicinity of the river seem to have resulted in the high concentrations of nitrate-nitrogen [62]. The increasing trend of ammonia levels (0.64 - 0.7 mg/L) from upstream to downstream of the river may have been associated with the differences in the level of application of fertilizers.
Means of the concentrations (mg L-1) of TP (0.43 - 0.66) and SRP (0.06 - 0.13) measured in Ketar River, which did not show significant differences among sampling sites, were noticeably low despite the occurrence of agricultural practices that involve the application of fertilizers within the basin, The maximum concentrations of TP and SRP recorded at site 1 could be associated with the application of phosphate-containing fertilizers in agricultural activities carried out in the vicinity of Ketar River [62]. SRP values recorded for the river water are agreed with the value reported for Elala River (0.03 ± 0.001 to 0.14 ± 0.008 mg/L) by Gebreyohannes et al. [63] while, much lower than the value reported for the Awash River (49 – 56 mg/L) by Degefu et al. [50]. In contrast, Awash River originates from the central highland area of Ethiopia and receives effluents from different sources while crossing extensive areas of agricultural farms as well as various industries [58]. Thus, the lower levels of such physico-chemical parameters as SRP and TP in Ketar River relative to those of Awash River are not surprising.
4.2. Diversity and distribution of macrophytes
In this study, 16 species of macrophytes belonging to 14 families were identified. From the identified species of macrophytes, Cyperaceae and Poaceae shared 2 species each, while other families were represented by a single species. A comparable result with the present study, fourteen macrophyte species belonged to nine families were reported in Lake Ziway (which feed by Ketar River) by Tamire and Mengistou [21]. The present study was higher than the result reported in other Rivers, Viz.,9 species in the main channel of the Upper Paraná River reported by Souza et al. [64] and 13 species in the Lepenci River by Bytyqi et al., [65], while was much lower than 31 species reported in mid cross River by Uneke & Ekuma [66]. The difference in limnology and water level fluctuations of the Rivers water could be the reason for the variation in the macrophytes assemblages between these rivers [67]. In addition, the sampling effort and frequency might be a reason for the variation. However, no submerged macrophytes were recorded in Ketar River. Nurminen [68] noted that water transparency or turbidity, a fluctuation in water level and dominance of free floating macrophytes are among limiting factors of the diversity and abundance of submerged macrophytes.
Among the identified macrophyte species, 11 of them were emergent, while 3 were rooted with floating leaves (Nymphoides peltata and Nymphaea lotus) and 2 free floating (Pistia stratiotes and Azolla nilotica). Ketar River is highly dominated by emergent macrophytes in terms of species diversity (compared with floating and submerged macrophytes), which could be due to their high tolerance of water-level fluctuation [5] and water current [69]. Among the emergent species Ludwigia stolonifera, Echinochloa stagnina and Persicaria senegalensis were dominant and shared a relative frequency of 10%, 12.07% and 12.76%, relative density of 9.13%, 4.93% and 3.94%, respectively. However, emergent species did not show dominancy in abundance compared with free floating species.
On the contrary, free floating macrophytes shared the highest abundance. The free floating species of macrophytes identified in this study; A. nilotica (at all sites) and Pistia stratiotes (at site 3) shared highest abundance and were the dominant macrophytes throughout the sampling periods with the relative frequency of 7.24% and density of 40.91%, and 7.93% and 26.54%, respectively. A. nilotica was presented at all the study sites, while Pistia stratiotes presented at site 3 only. In contrast to the other macrophytes, A. nilotica has the ability to fix nitrogen from the air [70] which could create favorable condition for it to compete with other macrophytes. P. stratiotes was also the dominant in abundance next to A. nilotica in the River. Temperature is one of the most important factors determining growth rates of free floating macrophytes and P. stratiotes can grow very quickly in tropical conditions [71]. The optimum temperature growth of Azolla ranges from 18 and 28°C [72]. Thus, in addition to a nutrient concentration in Ketar River, the temperature might be created favorable conditions for these dominant species (A. nilotica and P. stratiotes) to flourish and out-compete the other species. As Sadeghi et al. [73, 74] reported, the presence of macrophytes communities also provides a good opportunity for the flourish of free floating macrophytes by breaking wind speed and water velocity.
Site 3, the site where faced minimal human impact was contributed the higher species diversity (12) and Shannon Diversity Index (1.44) than other sites. During the study period, the site was dominantly covered with P. stratiotes and A. nilotica. However, in addition to the higher taxa richness at site 3, the site exhibited a higher evenness value than sites 1 and 2. Research conducted by Larbi et al. [75] concurred that the site away from human impact is rich in diversity. So, species richness at site 3 might be related to the minimal human impact at the site. However, species richness among sites did not show significant differences among each other which might be related to the homogeneity among the sites [76], which also agreed with the recorded physicochemical parameters of the present study.
4.3. Environmental drivers of Macrophyte Abundance
Redundancy analysis (RDA) indicated that all the environmental parameters studied in this study had strong correlations and were important predictors of macrophyte species distribution in Ketar River. A number of reports indicate that the distribution, abundance and diversity of macrophytes have an association with various environmental factors such as temperature [4, 71], water turbidity [67], nutrient enrichment [77, 78, 13], pH and DO [79] and conductivity [80] which is also shared with the present study. In this study, macrophytes including A. nilotica, N. peltata and L. stolonifera and P. stratiotes were strongly associated with pH, temperature, conductivity, dissolved oxygen, total phosphorous and total suspended solids. Ammonium, total phosphorous and nitrate were determined the distribution of most macrophytes such as A. nilotica, E. stagnina, I. aquatic, L. stolonifera and N. peltata. Frankouich et al. [81] also confirmed the association of the distribution and growth of aquatic macrophytes with nutrient rich.
A. nilotica and P. stratiotes are the worst invasive floating macrophytes and have the ability to invade new habitats within a short period of time under a favorable environment. I observed that at the late dry season (before set on the wet season) the river was loaded with nutrients that could be encouraging the infestations of these macrophytes. In contrast to P. stratiotes, A. nilotica can exist even under low nutrient conditions by fixing nitrogen from the air. The occurrence of A. nilotica in the River seems not to be affected by differences in the nutrient condition among sites, and its ability to colonize these varied sites indicates its potential to adapt to diverse trophic conditions. Additionally, the presence of an emergent macrophyte provides a good opportunity for Azolla to grow widely by breaking wind speed and water current.
Ketar River is the main tributary of Lake Ziway. Research conducted by and Tamire and Mengistou [21] indicates that Lake Ziway is dominated by emergent macrophytes. But, the littoral area of the Lake has been affected by anthropogenic activities; such as irrigation developments and abstraction of water for floriculture, and as result the water level of Lake Ziway has been declining [82, 83]. The above authors [82, 83] also reported that due to high evaporation, the lake showed a net loss of 74 million m3 volume of water annually. The emergence of some invasive species of macrophyte such as P. stratiotes and A. nilotica including water hyacinth (Eichhornia crassipes) in Lake Ziway further makes worse the condition which calls for serious intervention in the catchment.