2.1. Study Area

This study was conducted at two human-made wetlands, locally known as Washa and Borale. These wetlands are located in Kebele 01, Debre Birhan Regio-Politan Town (DBRPT), within the North Shewa Zone of Central Ethiopia (Figure 1). Both Washa and Borale lie outside the center of the town, at distances of 10 and 15 km, respectively. Their respective elevations are 2763 and 2832 m above sea level (m.a.s.l.). Washa’s geographic coordinates are 9°39′45″ N, 39°32′45″ E, while Borale’s are 9°39′62″ N, 39°32′36″ E. DBRPT experiences a range of temperatures, with an average annual minimum of 2.3°C and a maximum of 22°C. The average annual rainfall for the area is 906 mm [19]. These human-made wetlands serve a vital role for the local community. They provide essential water resources, as well as habitat for fish, birds, and various micro- and macroaquatic organisms. Observations within the wetlands’ catchments revealed diverse activities, including farming, grazing, small woodlot plantations, settlements, and water extraction for irrigation. These human activities placed stress on the two wetlands, particularly Borale, causing ecological damage and water scarcity. During the long dry seasons (December to May), the water levels in the wetlands dropped significantly, leading to a serious water shortage for local people, who rely on it specifically for irrigation and cattle watering, besides for others.

2.2. Study Site Selection

The Washa and Borale sites were selected due to their offering socioeconomic services to the localities and the commencement of conversion of the upper catchments and wetland parts at the downstream into agricultural and grazing lands, which in turn lead to reservoirs’ water reduction and degradation.

2.3. Sampling Design, Vegetative Data Collection, and Identification Procedures

To observe the changes of plants and hydrological and ecosystem landscape in the wetlands during the dry and wet seasons, and determine the sampling design, a reconnaissance survey was carried out for 2 weeks in May and September 2019. Accordingly, a systematic sampling design was employed for laying out the quadrats in both sites. For this, reservoir boundaries were firstly delineated with the maximum extent of flooding or the edge of depressions. Since almost all plants of the peripheries were herbaceous, a 1 × 2 m quadrat size [20] was fixed for maximizing to get more species. Then, the quadrats were laid out in the peripheries of the reservoirs. The plant sampling was made using 50-m intervals between two consecutive quadrats. Yet, where the periphery sites were wide enough, adjacent quadrats were aligned parallel to one another, 50 m apart. Hence, 40 total quadrats (20 per site) were laid out in the study area.

The specimens and percent cover of all vascular plant species encountered in each quadrat of the study area were collected and estimated, respectively. Accordingly, the specimens were collected at the end of the rainy and dry seasons. The first round plant specimens’ collection was made in October 2020 at the end of the rainy season, and the second round in February 2021 during the dry season. The aerial ground cover-abundance value of each herbaceous species was determined in percent (%) using ocular estimation and immediately converted into the Braun–Blanquet (BB) scales ranged from 1 to 9 as modified by van der Maarel [21]: 1 for 1–3 individuals; 2 for some individuals (0.5%–1.5%); 3 for some individuals (1.5%–3%); 4 for some individuals (3%–5%); 5 for some individuals (5%–12.5%); 6 for some individuals (12.5%–25%); 7 for some individuals (25%–50%); 8 for some individuals (50%–75%); and 9 for some individuals (75%–100%). Accordingly, the whole specimens collected, dried, and pressed were taken to Debre Berhan University for identification using the flora volumes of Ethiopia and Eritrea [22–30]. However, for further identification and approval being assisted by expert, mounted specimens, and microscope, the whole specimens were taken to the National Herbarium (ETH) of Addis Ababa University. Additionally, wetland indicator species status [obligate wetland (OW), facultative wetland (FW), facultative (F), facultative upland (FU), and obligate upland (OU) plants], perennial, annual, and native/non-native species were determined being assisted by experts and published documents including the flora volumes of Ethiopia and Eritrea [22–32]. Finally, the voucher specimens were placed there.

2.4. Field Checklist and Human Disturbance Gradient Score (HDGS) Protocol

The severity of ecosystem condition of wetlands could be estimated using various assessment methods of ecological disturbances. Thus, the field checklist and HDGS protocol of Gernes and Helgen [33] were employed (Appendix A). Based on this, from December 2020 to May 2021, frequent ecological evaluations were carried out during the dry and wet seasons. Hence, the reservoir/wetland type (hydroperiods), geometric setting (flood plain, riverine, and lacustrine), hydrological modification (due to stormwater inputs, filling, culvert, drainage, and ditch inlet and outlet), land-use patterns around the reservoirs, habitat assessment (presence or absence of fauna and flora such as birds, fish, mammals, and dominant macrophytes), and major threats (free grazing, water abstraction, infrastructure, and investment activities) to the reservoirs were assessed to rate the ecological state (natural, agricultural, grazing, or urban impaired) of the reservoirs using a field checklist.

Similarly, the physical buffer landscape disturbance (within 50-m radius) and further landscape disturbance up to 500-m radius from the edges of the reservoir were made (Appendix A). That means the extent of vegetation existence/removal, hydrological modifications, and habitat alterations were evaluated and rated following the protocol. Furthermore, chemical pollutants such as nitrate and phosphate, and the absence or presence of biological data like fish and birds were assessed. For this purpose, data collection from each referenced, mid-, and high-impacted sampling sites of each reservoir, one bottle water sample (1L), was taken for water chemical analysis, besides in situ measures of dissolved oxygen and electric conductivity.

Additionally, field visits and fish nets were made for determining the presence of birds and capturing fish, respectively, since they are considered as good indicators of relatively healthy ecological status. During the field visits, different kinds of birds were observed within the reservoirs and their peripheries. However, gillnets were used for fishing and positioned in the late afternoon (8:00 pm) and inspected after two (10:00 pm) being assisted by local fish catchers, but captured a very few. The dead fish was also found in the periphery of Washa Reservoir. Informal interviews of local experts and individuals were also made about the presence of fish in both reservoirs.

2.5. Data Analysis

3.1. Plant Species Composition

Totally, 74 plant species, belonging to 26 families and 57 genera, were identified from both Washa and Borale Wetlands (Appendix B). The Asteraceae, followed by Poaceae (grasses), were the most abundant, with the plants typically found in upland (drier) areas or in ecologically disturbed aquatic systems. Additionally, families like Cyperaceae (sedges), Fabaceae (legumes), Onagraceae (evening primroses), Polygonaceae (buckwheats), and Scrophulariaceae (figworts) were important for the wetlands’ character (Figure 2). However, the remaining 16 families, each with only one genus and one to two species (Supporting file 2), were less common in terms of their distribution and diversity. Interestingly, 14 families were found in both wetlands, while three and nine families were unique to Washa and Borale, respectively, indicating greater family diversity in Borale.

Examining plant habits, the majority (91.89%) were herbs, followed by shrubs (6.75%) and a minimal presence of climbers (1.35%) (Figure 3). This dominance of herbaceous plants aligns with the typical characteristics of aquatic ecosystems, which favor these low-growing species.

3.2. Diversity, SSC, and Dominant Species of the Study Sites

The gamma diversity (richness) of the overall study area, drawn from 40 total plots, was 74. The species richness distribution in 40 plots of the study area was ranged from eight species (in each plot of 20 and 21) to 20 species (in each plot of 7 and 12). The beta diversity (average richness) via pooling the whole sampling sites (40 plots) of the study area was 14.55. Similarly, the richness of Washa and Borale sites was 51 and 64 species, respectively. The beta diversity through pooling richness of the 20 plots of Washa Wetland and 20 plots of Borale wetland was 11.25 and 13.55, respectively, with their significance difference at p value < 0.05 (p = 0.025) (Table 1). As also presented in Table 1, the Shannon diversity (H′) of Washa and Borale sites was 2.32 and 2.53, respectively, with a significant difference at p value < 0.05. Similarly, the evenness (J) values of Washa and Borale sites were 0.93 and 0.96, respectively, and had a significant difference at p value < 0.05. Moreover, the SSC for the plant species found in Washa and Borale wetlands was 0.71 or 71% (Table 1).

While considering the dominance of the species based on their total cover values and frequency (F) across 20 sample plots (n = 20) at each location (Borale and Washa), the top 15 species for each site are listed in Table 2. Accordingly, at the Borale site, Geranium dissectum L. was the most dominant species with an average cover value of 5.85 and found in 18 plots. Other dominant species included Eleocharis marginatum Hochst. ex Steud, Raven; Cyperus fischerianus G.W. Schimp. ex A. Rich.; Pennisetum thunbergii Kunth; Andropogon abyssinica Hochst; and Snowdenia polystachya (Fresen.) Pilg. These dominant species were found in most of the plots at Borale, ranging from 6 to 19 plots. Similarly, at the Washa site, G. dissectum was also a dominant species, along with E. marginatum, P. thunbergii, L. stolonifera, C. fischerianus, A. abyssinica, Ludwigia stolonifera (Guill. & Perr.), and Eleusine floccifolia (Forssk.) Spreng. These dominant species were found in a majority of the plots at Washa, ranging from 6 to 19 plots (Table 2). Interestingly, these dominant plant species of the two sites are typically found in upland and tend to thrive in ecologically disturbed aquatic environment.

3.3. Indicators and Characteristic Species of Wetlands

Looking at the wetland indicator species, the Borale site had the highest abundance in FU category (15 species) and OU category (19 species). This trend continued at the Washa site, where FU species (14) and OU species (13) were again the most common. However, the OW category, which typically thrives in healthy aquatic environments, had the fewest species in both locations. Only 4 species were found in Washa and 6 in Borale. This low number suggests a potential ecological disturbance in these two reservoirs. To summarize the findings, the Washa and Borale sites had a relatively low percentage of wetland indicator species (OW + FW)—14 (27.45%) and 19 (29.68%), respectively.

Conversely, upland indicator species (FU + OU) were much more prevalent, with Washa at 27 (52.94%) and Borale at 34 (53.12%) (Table 3).

The Washa site revealed ten characteristic (unique) plant species, including Persicaria decipiens (R.Br.) K.L.Wilson, Cyperus pauper Rochat. exA. Rich., Cyperus brevifolius (Rottb.) Hassk, and Commelina diffusa Burm.f (Table 4). These species are mostly growing in relatively healthy aquatic ecosystems. In contrast, the Borale site contained 23 characteristic species, some of which were Typha latifolia L., Guizotia scabra (Yis.) Chiov., Plectranthus punctatus (Lf) L ′Her., Rumex abyssinicus Jacq, and Sida schimperiana Hochst. ex A. Rich (Table 4). These species, however, are more commonly found in ecologically impaired sites. In total, the study identified 33 characteristic plant species across both sites (Table 4). Notably, 90.9% of these were native species, and 54.54% were perennials. Interestingly, 41 species (or 55.4%) were common to both wetlands (Appendix C, Table 5). While examining the wetland indicator categories of these characteristic species, a surprising trend emerged. Over half (57.57%) were classified as upland species (occurring in drier areas or at the fringe of wetlands). OW species and FW species comprised only 39.39%, suggesting a potential ecological disturbance in both reservoirs.

A total of 41 (55.4%) common plant species were found in both wetlands. These common species were primarily native (92.68%) and annuals (56.1%) (Appendix C). Perennials made up a smaller portion (17 species, 41.46%), and only one species (2.44%) was biannual. An analysis of these shared plants revealed insights into the wetlands’ health. Over half (51.22%) were upland plant indicators (FU + OU), suggesting alterations to the original wetland habitat. In contrast, only a quarter (24.39%) were wetland indicators (OB + FW), typically associated with healthy aquatic ecosystems. Another quarter (24.39%) were classified as F), meaning they can tolerate both wet and dry conditions. The high number of upland plants suggests ecological disturbance in the wetlands. This indicates potential changes in water levels or habitat modifications that have pushed the ecosystems toward a drier state.

3.4. Ecosystem Condition

The two created wetlands were unlike in their severity of vegetative, habitat, and hydrological changes. Based on the survey made using HDGS format (Appendix A) and field checklist, the HDGS value of each factor for the Washa site was ranged from 3 to 21 and the same thing for the Borale site (Figure 4). For example, the HDGS of landscape disturbance within 500-m radius (Factor 2) was 12 and 18 for Washa and Borale sites, since their landscapes are nearly and all filled with human uses, respectively (Appendix D, Figure 4). However, the buffer landscapes within 50-m radius of the two reservoirs were fair and scored the same vale (12) (Factor 1) of Figure 4. Compared with other factors, hydrological change (Factor 4) and chemical pollution (Factor 5) were high in both wetlands, but higher in landscape disturbance (Factor 2) in Borale site (Figure 4) due to extensive anthropogenic activities such as grazing, farming, urbanization, cattle watering, and water extraction for irrigation, besides the intrusion of cattle, domestic, and urban wastes and other inputs like silts and fertilizers to the reservoirs with floods. Moreover, since fish and birds are found in both reservoirs, they scored the same value of three (Factor 6). Accordingly, the total values of HDGS of Washa and Borale sites were 67 and 80, and belonged to mid- and most impacted wetlands, respectively (Figure 4).

4.1. Plant Species Composition

As the results revealed, 74 different plant species identified from 26 plant families from the study area. This supports the idea that wetlands are crucial for protecting biodiversity [6]. Interestingly, even though the wetlands were disturbed by human activities, this created a variety of microhabitats that provided suitable conditions for many plant species to grow, leading to the high plant diversity observed. Handa et al. [38] and Moges et al. [11] also found that human disturbances in ecosystems can create new microhabitats, favoring the establishment of invaders or upland plant species through replacing native aquatic plants.

Regarding the families, Asteraceae and Poaceae were the most common, making up over a third of the total plant species. This finding aligns with similar studies in Ethiopia by Moges et al. [5] and Uganda by Odull and Byaruhanga [39]. These two dominant families also contained a significant portion of the total plant genera. Lamiaceae, Commelinaceae, and Apiaceae were the next most abundant families. This suggests that these dominant and codominant families are better adapted to disturbed wetlands compared to other plant families. This is likely because the wetlands in our study area were rated as moderately and highly impacted by human activities such as drainage, farming, and pollution.

These disturbances can modify the water flow and overall health of the wetland, creating new microhabitats that favor the growth of both native and non-native plants, including some resistant aquatic species. However, these disturbances can also lead to the decline or disappearance of sensitive plant species. As a result, the remaining 16 plant families only contributed a small portion of the total species and genera found in the wetlands.

When comparing the two study sites, Borale Wetland had a higher number of plant families than Washa Wetland. This difference is likely due to the higher level of disturbance in Borale. This finding is similar to previous research types conducted in southwestern highlands of Ethiopia by Mulatu et al. [20] and Moges et al. [5], who reported that disturbed wetlands in Ethiopia tend to have more plant species than undisturbed ones.

Despite unlike in their extent of their ecological disturbances and number of taxa, the two wetlands had common physiognomies in their climate (moisture and temperature) and were located in the same ecological zone. This resulted in over half of the plant families (∼54%) being common to both sites. Climate and altitude are likely the most important factors determining which plant species can grow in these wetlands.

Finally, this study found that the wetlands primarily consisted of herbaceous plants (∼92%), followed by shrubs (∼7%), and climbers (∼1). This is likely due to the frequent human activities around the wetlands, such as farming and grazing. This is likely due to the frequent human activities around the wetlands, such as farming, grazing, water extraction for irrigation, and settlement, which may have removed larger plants like trees and shrubs. Additionally, wetlands with surface or shallow subsurface water tend to support more herbaceous plants than other plant types. This finding is consistent with previous studies by Mulatu et al. [20] and Moges et al. [5].

4.2. Plant Diversity, Dominant, and Similarity of Plant Species in Wetlands

The scientists used various methods to evaluate the richness and spread of plant life across each wetland. Richness is an apt measure of diversity, and the richness of a range of habitats (of both wetlands) is also termed as gamma diversity, but the average richness is called beta diversity [40]. Yet, the total number of species per wetland site is called alpha diversity [41]. Thus, besides Shannon diversity and evenness, the species richness (S) is the most repeatedly used index [42] for comparing the diversity between sites (wetlands) [43], as discussed as follows.

4.3. Indicators, Characteristics, and Common Species of Wetlands

Evaluating individual plant species as indicators of the physical environment in wetlands is currently becoming one of the means to show the ecological condition of wetlands. According to Lawton and Gaston [49], a group of wetland indicators that reflect the abiotic or biotic state of an environment provide evidence for environmental change. Accordingly, as revealed in the results, the majority of the plant species identified from the whole study area was upland indicators (21, 51.22%), whereas the wetland indicators were smaller in number (10, 24.39) (Table 5). Even while considering each wetland indicator species (Table 3), many species of Washa (52.94%) and Borale (53.12%) were uplands, indicating the two wetlands’ ecosystem disturbance, particularly the Borale’s one. As also described here, in the characteristic and common species of wetlands, many of them were upland indicators, implying hydrological and ecosystem changes of the studied wetlands.

The characteristic species are species that characterize the specific site or habitat being unique to only that of other sites of the study area. Thus, 10 characteristic species were identified from Washa site, of which Persicaria decipiens, Cyperus pauper, Cyperus brevifolius, and Commelina diffusa were chiefly unique and wetland indicators of Washa, and relatively showed good health condition of the wetland. Contrarily, of the 23 characteristic species identified from Borale Wetland, the plant species including T. latifolia, L. tomentosa, Guizotia scabra, Plectranthus punctatus, Rumex abyssinicus, and Sida schimperiana are considered as good indicators of ecosystem disturbance. Moreover, some of these characteristic plants such as T. latifolia and Emilia leptocephala (Mattf.) C. Jeffrey are indicators of water pollution. As also reported by Moges et al. [5] from Jimma highlands, Boye human-made wetland was highly impaired and invaded by T. latifolia (and Emilia leptocephala). In contrast, from the total characteristic species (33), 90.9% and 54.54% were native and perennials, respectively. This indicates the potentiality of the study sites to be restored, if a good management of the sites is made in the future.

Additionally, 55.4% species were found in both Washa and Borale wetlands, of which 92.68% and 56.1% were native and annual species, respectively, and 24.39% and 51.22% were wetland and upland indicator species, respectively. This also indicates that the majority of indicator species growing in both wetlands were uplands, which might be due to their ecosystem disturbance/changes that could be suitable for growing native upland species invasively (randomly). This finding is in line with Thompson et al. [37]. These all also imply that the invasion of/exotic/alien species in the study sites was very low; however, due to their ecosystem disturbances, many of the aquatic plants were replaced by native and annual upland invaders. Still, those OB, FW, and F characteristic species together represent about 41%, which grow and easily adapt the wetland ecosystems, and which indicate the restoration potential of the wetlands.

4.4. Ecosystem Condition of Wetlands

Finally, based on the assessment made in and around the study sites using the protocol of Gernes and Helgen [33], besides the field checklist, both wetlands, particularly Borale, were impaired due to the anthropogenic activities such as farming, grazing, cattle watering, water extraction for irrigation, and entry of other inputs to the reservoirs with floods. Thus, Borale Wetland was higher disturbed than Washa one. This might be owing to Washa Wetland’s buffer zone, relatively covered with grasses up to 50-m radius, despite farming in between. Additionally, the parts of the catchment of this wetland were nontilled land compared to Borale site. However, owing to free grazing in the catchment including the buffer zone of the Washa site, the water quality was relatively poor than the Borale’s one. Hence, the two study sites were different depending on the extent and severity of their habitat, vegetation, and hydrological alterations; buffer and upper catchment landscapes; and chemical stressors. The analysis of plant indicators also showed that the majority of the two sites’ species were upland indicators due to habitat and hydrological modifications, resulting in the replacement of aquatic plants with upland species. Generally, the changes of the landscape and land size of wetlands are indicators of ecological degradation [50], besides wetland indicator status [31, 36, 51].