Hemeroby reveals the dynamics of vegetation cover following the destruction of the Kakhovka Reservoir

Study area

The Dnipro Hydroelectric Power Plant (HPP) near Zaporizhzhia was the first hydroelectric power station constructed on a river. Upon its completion in the 1930s, it was the largest hydroelectric power plant in Europe, with a capacity of 560 megawatts (MW). In August 1941, the retreating Red Army destroyed the dam; however, the plant was rebuilt after World War II. The second hydroelectric dam to be constructed was the Kakhovka Dam, which was completed in 1956. This dam is located downstream of the Dnipro HPP and approximately 60 kilometres upstream of Kherson. The Kakhovka Dam created the largest reservoir on the Dnipro River, significantly surpassing the reservoir created by the Dnipro HPP, with a volume of 18.2 cubic kilometres (km³) of water at a normal retaining level of 16.0 m (m) above mean sea level (MSL). In addition to generating hydropower, the primary purpose of the Kakhovka reservoir was to provide reliable irrigation and water supply to southern Ukraine. In the 1980s, the Zaporizhzhia Nuclear Power Plant (NPP), the most powerful in Europe, was constructed on the south bank of the Kakhovka reservoir near Enerhodar (Gleick, Vyshnevskyi & Shevchuk, 2023). The Kakhovka reservoir belonged to the Lower Dnipro sub-basin, which covers an area of 82,625 km², accounting for approximately 28% of the total area of the Dnipro basin and 13.7% of the total area of Ukraine. The Dnipro River flows through this sub-basin for a total length of about 440 km, traversing the Dnipro, Zaporizhzhia, and Kherson regions of Ukraine (Iaroshevych et al., 2021). Kakhovka HPP was under the occupation of the Russian Federation. The Kakhovka dam and hydroelectric power plants were destroyed due to a massive explosion of the Kakhovka dam on 6 June 2023. The water level in the Dnipro River decreased by 8 m after the dam was destroyed, according to the Nikopol hydrological gauging station (Gleick, Vyshnevskyi & Shevchuk, 2023).

Khortytsia is the largest island on the Dnipro River, near Zaporizhzhia, downstream of the Dnipro hydroelectric power station (Fig. 1). The island extends from northwest to southeast, measuring 12 kilometres in length and 1.6 to 2.7 kilometres in width, encompassing an area of 2,359 hectares before the destruction of the Kakhovka dam. From a geological perspective, Khortytsia Island belongs to the Ukrainian crystalline shield, a Precambrian geological formation underlying much of central and southern Ukraine. The island’s geological foundation consists of a sloped granite plateau, which is covered by a layer of younger sedimentary rocks of varying thickness (Liashenko et al., 2022). The granite base rises 20 to 30 m above the river’s water surface in the island’s northern region. The elevation ranges from four to 15 m in the central area, while it lies below the water level in the southern (floodplain) section. The island’s territory can be categorised into three parts based on the predominant landforms. The rocky section is situated in the north, the upland area occupies the central part, and the lowland region is found at the southern edge, where floodplain ecosystems develop. Before the Kakhovka Dam’s destruction, the reservoir’s water level near Khortytsia Island was 15 m above sea level. The island’s northern part features a plateau that rises 58 to 62 m above sea level, culminating in granite cliffs. The central section of the island is the highest, with elevations reaching 75 to 77 m above sea level. The shoreline in this area is significantly indented due to an extensive system of gullies. In the southern part of the island, the elevation ranges from 17 to 35 m above sea level, and numerous floodplain lakes of varying sizes and depths are present. The flora of Khortytsia Island comprises 1,128 species of higher vascular plants, including 847 species of native flora and 281 cultivated species (Okhrimenko & Shelegeda, 2016).

Descriptions of plant communities

The field study was conducted in the southern part of Khortytsia Island, covering a total area of approximately 5,300 m². The research focused on two distinct landscape zones that were most affected by the destruction of the Kakhovka Reservoir: (1) floodplain lakes located within the interior of the island, and (2) the external shoreline along the Dnipro River, where the water level had dropped drastically. These zones were selected due to their pronounced hydrological alteration and ecological sensitivity. A total of 135 phytosociological relevés were recorded. Sampling plots were positioned to capture the variability of vegetation types across both former aquatic and transitionally terrestrial habitats. The selection of plots was stratified based on visible topographic and hydrological heterogeneity. Fieldwork was carried out between 10 and 21 August 2024, a period corresponding to the peak seasonal development of most invasive and ruderal plant species typical for this region. While certain early spring ephemeral species (e.g., geophytes) may not be detected during this period, summer sampling is generally sufficient to assess the dominant community structure and hemeroby levels in floodplain systems. Instantaneous soil moisture values can vary over the course of the summer; however, in this study we used phytoindication-based indices, which reflect the long-term soil moisture regime as interpreted from the ecological preferences of the recorded species. The Braun-Blanquet approach to floristic classification was employed (Braun-Blanquet, 1964; Westhoff & Van Der Maarel, 1978). The standard size of plots was four × four m. The entire database of geobotanical relevés was processed and categorised into smaller groups based on their floristic differences using the two-way indicator species analysis (TWINSPAN) method (Hill, 1979) with the TWINSPAN for Windows 2.3 software (Hill & Šmilauer, 2005). The cut levels for “pseudospecies” were set at 0%, 2%, 5%, 10%, and 20%. Diagnostic species were identified within syntaxa using the fidelity index (Willner, Tichý & Chytrý, 2009). Pearson’s phi coefficient was used to determine the association between species and vegetation types (Chytrý et al., 2002). The phi coefficient was adjusted to account for differences in the number of sites among the plant community groups (Tichy & Chytry, 2006). The primary occurrence data from vegetation surveys conducted on Khortytsia Island and adjacent areas after the Kakhovka Reservoir disaster have been published in the Global Biodiversity Information Facility (GBIF) and are openly available at https://doi.org/10.15468/rewgwn (Zhukov et al., 2025).

Phytoindication evaluation of ecological regimes

The calculation of environmental parameters was conducted using phytoindication scales. The scale developed by Didukh (2011) enables ordination analysis based on 12 factors: soil humidity (Hd), soil moisture variability (fH), soil aeration (Ae), soil nitrogen content (Nt), soil acidity (Rc), salt regime (Sl), carbonate content (Ca), temperature regime (Tm), ombroregime (Om), climate continentality (Kn), cryoregime (Cr), and light intensity (Lc).

The plant ecological groups are categorised into 23 gradations based on their relationship to the humidity regime (Didukh, 2011). The moisture regime scores can be converted into phytoindication estimates of the productive moisture stock within the one-meter soil layer, as outlined by Maslikova (2018a) and Molozhon et al. (2023): ${\displaystyle W=18.65exp\left(0.15H\right),}$

where W represents the content of productive moisture in the one-meter layer of soil (measured in millimetres), and H indicates a score of the moisture regime. A productive moisture content of less than 60 mm in a one-meter layer is classified as very low; a content ranging from 60 to 90 mm is classified as low; a content between 90 and 130 mm is deemed satisfactory; a content ranging from 130 to 160 mm is considered good; and a content exceeding 160 mm is classified as very good for agricultural plants (Vadunina & Korchagina, 1986).

The plant ecological groups were categorised into 12 gradations based on their relationship to the soil moisture variability. These scores can be converted into the coefficient of irregularity of soil moisture (ω), which ranges from 0 (indicating the lowest level of contrast in moisture conditions, such as consistently moist or consistently dry habitats) to 0.5 (indicating the highest level of contrast in moisture conditions, where nearly complete water immersion is followed by drought). The indicator scores of soil moisture variability can be converted to the moisture irregularity coefficient as follows (Maslikova, 2018b): (1)${\displaystyle \omega =0.042fH-0.032,}$ where ω is the coefficient of irregularity of soil moisture, and fH is an indicator score of soil moisture variability.

The plant ecological groups were categorised into 15 gradations based on their relationship to the aeration regime. These scores can be converted into the percentage of air-filled porosity relative to the total porosity volume as follows (Maslikova, 2018b): ${\displaystyle P=\frac{100-A{e}^4}{100\ast \left(A{e}^4+1700\right)},}$

where P represents the air-filled porosity percentage, which is the percentage of the total volume of pore space in the soil, and Ae denotes the score of the aeration regime.

The plant ecological groups are categorised into 15 gradations based on their tolerance to soil acidity. These scores can be converted to the pH of the soil aqueous solution as follows (Maslikova, 2018c): (2)${\displaystyle \mathrm{pH}=2.26\ln \left(Rc\right)+1.88,}$ where pH is defined as the negative logarithm of the concentration of hydrogen ions in the soil solution, while Rc represents the acidity score.

The plant ecological groups are categorised into 19 gradations based on their relationship to the overall salt regime. These gradations can be converted into water-soluble salt content as follows (Maslikova, 2018c): ${\displaystyle S=2^{0.6Sl+1},}$

where S represents the salt content in the soil solution, measured in micrograms per litre (µg/l), and Sl denotes the score of the soil salinity regime.

The plant ecological groups were categorised into 13 gradations based on their response to the carbonate content of the soil. The scores can be converted to CaO + MgO content as follows (Maslikova, Zhukov & Kovalenko, 2019): ${\displaystyle \mathrm{CaO}+\mathrm{MgO}=14\frac{C{a}^{4.5}}{C{a}^{4.5}+45000},}$

where CaO+MgO is the carbonate content, expressed in terms of calcium and magnesium oxides (%). Ca serves as a score for the carbonate content.

The plant ecological groups are categorized into 11 gradations based on their relationship to soil nitrogen content. The scores can be converted to soil nitrogen content as follows (Maslikova, Zhukov & Kovalenko, 2019): ${\displaystyle N=5\frac{N{t}^{3.7}}{N{t}^{3.7}+345},}$

where N represents the soil’s nitrogen content, measured in grams per kilogram (g/kg), and Nt is a score that indicates the soil’s trophic regime.

The plant ecological groups are categorised into 17 gradations based on their relationship to the thermal regime. These scores can be translated into a radiation balance as follows (Molozhon et al., 2023): (3)${\displaystyle RB=0.21Tm,}$ where RB is the radiation balance measured in gJ m–² year⁻¹, while Tm represents the thermal regime score, one must note that if the radiation balance is expressed in Kcal cm⁻² year⁻¹, one can multiply the score by 5.

The plant ecological groups are classified into 23 categories based on their relationship to atmospheric humidity regimes. Ombroclimate scores can be interpreted as the difference in the amount of precipitation before evaporation from open water surfaces, measured annually in terms of average days (Molozhon et al., 2023): (4)${\displaystyle Hum=0.54Om-7,}$ where Hum represents the difference between the average daily precipitation and the evaporation from the open water surface over the same period, measured in millimetres (mm). Om denotes the climate humidity score.

The plant ecological groups are categorized into 17 gradations based on their relationship to continentality. The continentality scores can be converted to the Ivanov continentality scale (Ivanov, 1959) as follows (Molozhon et al., 2023): (5)${\displaystyle SKn=10Kn+41,}$ where SKn represents the Ivanov continentality scale, Kn denotes the score of the continentality regime.

The plant ecological groups are divided into 15 gradations based on their relationship to the cryoclimate. The cryoclimate scores can be converted to reflect the average temperature of the coldest month of the year as follows (Molozhon et al., 2023): (6)${\displaystyle Temp=3.83Cr-38.17,}$ where Temp represents the average temperature of the coldest month of the year, measured in degrees Celsius (^∘C), while Cr is a score that indicates the cryoclimate.

The relationship between the measured light and the photoinduction assessment of light levels is as follows: (7)${\displaystyle \text{log\_Lighiting}=0.22\ast L\text{-value},}$ where log_Lighiting is the decimal logarithm of the relative light level, L-value is the phytoindication assessment of the light level according to the Ellenberg scale.

Geomorphological calculations

The configuration of the water bodies was reconstructed using detailed satellite imagery from Bing Maps (https://www.bing.com/maps) and further refined through field surveys. The depth map of the Dnipro riverbed was derived from Navionics SonarChart™ data (http://www.navionics.com). A digital elevation model (DEM) of the surface area was created based on data obtained from the Advanced Land Observation Satellite (ALOS) (https://www.eorc.jaxa.jp/ALOS/en/index_e.htm). The spatial resolution of this model is 12.5 m. The morphometric characteristics of the reservoirs were calculated using the lakemorpho (Hollister & Stachelek, 2017).

Catastrophic dynamics of morphometric parameters of floodplain water bodies

In the period preceding the disaster, the natural decline in water levels that occurred after the regular spring flood and continued into the summer led to a significant reduction in the area of floodplain water bodies on Khortytsia Island (Fig. 2). This decline also affected the shoreline length, surface area, and volume of the floodplain water bodies, though the extent of reduction varied across parameters (Table 1). Following the destruction of the Kakhovka Dam, the spring values of shoreline length, surface area, and water volume decreased by 76.8%, 85.6%, and 79.9%, respectively, compared to the corresponding spring measurements before the disaster. During the summer period, these reductions were 79.2%, 88.4%, and 79.3%, respectively. The seasonal (spring-to-summer) decrease in shoreline length after the disaster was 23.1%, which exceeded the pre-disaster seasonal decrease of 14.4%. Similarly, the summer reduction in surface area reached 41.2% after the disaster, compared to 26.7% before. In contrast, the seasonal change in water volume remained relatively stable, at 7.0% post-disaster versus 9.3% prior to the event.

Structure of the vegetation cover

The surveys identified 146 plant species. On average, each survey recorded 12.1 ± 4.6 species, ranging from 5 to 28. The projected cover of grass and shrub vegetation was 50.8 ± 23.7%, varying from 3% to 97%. The analysis identified 12 distinct plant associations.

The associations are numbered according to the order in which they were identified by the TWINSPAN procedure (Table S1), reflecting the most significant environmental gradient along which the plant communities are organised. Humidity serves as this gradient, as indicated by the estimates of the humidity regime (Table S2). The associations comprising Leersio-Bidentetum, Chenopodietum rubric, Cyperetum micheliani, and Typhetum angustifoliae are characterized by the highest moisture levels. These associations represent pioneering communities of tall annuals found in waterlogged areas and anthropogenically disturbed, often nitrified and silted ecotopes. They include communities of hygrophilous annuals with ephemeral vegetation in regions experiencing sharp fluctuations in moisture levels and communities of tall aquatic macrophytes, whose lower parts are predominantly submerged in water during the growing season. These communities are distinguished by high projective cover and species richness. Leersio-Bidentetum associations are located in floodplain reservoirs that are relatively distant from the island’s bank (Fig. 3). Chenopodietum rubric associations are typical of the main channels within the island. The Cyperetum micheliani association is found in the most remote water bodies of the island. Typhetum angustifoliae associations occupy transitional positions between the most distant water bodies and those directly connected to the main channel of the Dnipro River.

A lower moisture level was observed in the Salicetum albae and Salici-Populetum associations (Fig. 4). These communities consist of floodplain and riverbank willow and poplar forests and shrubs, which experience varying flood durations on slightly silted sandy and silty soils, with a proximity to fresh groundwater. The Phragmitetum australis is a transitional vegetation type in terms of moisture levels between these groups of associations. This community thrives in eutrophic freshwater and brackish low-flow or closed water bodies, characterised by a neutral to slightly alkaline pH. During the growing season, it undergoes substantial fluctuations in water levels, typically ranging from 25 to 50 cm, and occasionally reaching up to 70 cm. These fluctuations are associated with bottom sediments composed of sandy-silt, silt, silty-peat, and peat. It also exhibits dense plant cover and high species richness. The Salicetum albae association is typically found in dried-up channel beds that connect floodplain reservoirs. The Salici-Populetum associations are close to one another and share ecologically similar environmental conditions. A notable feature of the spatial distribution of Salici-Populetum associations, compared to Salicetum albae associations, is their tendency to be found more frequently in wider or dried lake-type channels that were once characterised by significant water flow. When these two associations occur adjacent, Salici-Populetum associations are likelier to thrive on local micro-uplands or in the upper portions of the thicket belt along the water’s edge before it dries up. Conversely, Salicetum albae associations are commonly found in local micro-depressions or closer to the bottom of a dried-up pond. Ecologically, Salicetum albae associations prefer habitats with higher nutrient content and lower pH levels than those favoured by Salici-Populetum associations. Phragmitetum australis associations are frequently found in desiccated watercourses and streams, particularly in local depressions in the landscape where moisture accumulates.

Other associations represent plant communities that tolerate drier conditions while experiencing greater variability in moisture availability. Notably, attention should be drawn to the Erigeronto-Lactucetum serriolae association, which is characterised by a predominance of more neutral soil pH levels, a higher concentration of soluble salts in the soil, and the highest degree of soil aeration. These plant communities thrive in compacted, nitrified, and loose substrates and are commonly found along clearings in deciduous forests or on previously ploughed river floodplains. The Xanthietum strumarii association is distinguished by its highest cryoclimate score. In contrast, the Bromus tectorum-Corispermum hyssopifolium-associatie association prefers conditions characterised by very low cryoclimate scores. This association is frequently found in anthropogenically disturbed areas, such as roadsides, railway embankments, and dry riverbeds. The Portulacetum oleracei association primarily inhabits steep, sandy banks devoid of water located along the island’s outer edge. The Xanthietum strumarii association occupies similar positions but is typically found in areas with more level terrain, occasionally at the bottoms of dried-up water bodies. The Amarantho retroflexi-Echinochloetum cruris-galli association is commonly located on the sandy banks in the southeastern or southern parts of the island. The Bromus tectorum-Corispermum hyssopifolium-associatie association is most frequently observed in the island’s southeast region.

Principal component analysis of variability in phytoindication-based assessments of environmental factors

Principal component analysis identified four components with eigenvalues exceeding one, in accordance with Kaiser’s criterion (Table 2). Principal component 1 accounted for 32.5% of the variability in the feature space. This component exhibited the highest loadings on the moisture regime indicator scales, allowing it to be meaningfully interpreted as a moisture gradient. More humid conditions are typically associated with more natural plant communities, while more arid conditions are linked to hemeroby communities. Other ecological regimes also change naturally along this moisture gradient. As aridity increases, the variability of moisture, carbonate content of the soil, aeration regime, thermal regime, continental regime, cryoregime, and light regime also increase. Concurrently, with rising moisture content, nutrient levels and ombroclimate indicators generally increase. The lowest scores of principal component 1 were characteristic of near-water plant communities, whereas the highest scores were typical of sandy bank plant communities (Fig. 5).

Principal component 2 accounted for 15.0% of the variability in the traits analysed. This component exhibited the highest loadings on indicator scores related to nutrient content, allowing it to be interpreted as a trophic gradient. Additionally, principal component 2 demonstrated significant loadings on indicators of naturalness and hemeroby within plant communities. An increase in trophicity corresponds with a rise in hemeroby, while a decrease in trophicity is associated with an increase in the naturalness of plant communities. The increase in trophicity is correlated with higher levels of soluble salts and carbonates in the soil, which serve as indicators of the thermoregime, ombroregime, and cryoregime. Conversely, a decrease in trophicity is linked to more significant variability in moisture, pH, and improvements in aeration, lighting, and continentality. The highest scores of principal component 2 were observed in the associations Chenopodietum rubri and Typhetum angustifoliae, while the lowest scores were found in Erigeronto-Lactucetum serriolae and Amarantho retroflexi-Echinochloetum cruris-galli.

Principal component 3 accounted for 13.7% of the variability in the feature space. This component exhibited the highest loading on the indicator score for the lighting regime and was significantly correlated with naturalness and hemeroby. The increase in hemeroby was associated with a rise in the light regime. At the same time, the growth of naturalness was characterised by elevated pH levels, carbonate and dissolved salt content, soil aeration, and indicators of thermal and cryoregime. The highest scores of principal component 3 were observed in the associations Salicetum albae and Bromus tectorum-Corispermum hyssopifolium, whereas the lowest scores were found in Salici-Populetum and Erigeronto-Lactucetum serriolae.

Principal component 4 explained 8.7% of the variability in the feature space. This component was not sensitive to the naturalness and hemeroby of the communities. It exhibited the highest loadings for pH, carbonate, and dissolved salt content in the soil. Therefore, this component can be interpreted as an indicator of the soil’s granulometric composition. The highest scores of this component were observed in the Phragmitetum australis and Bromo tectorum-Corispermetum leptopteri associations, while the lowest scores were found in the Portulacetum oleracea association.

Conclusion

The destruction of the Kakhovka Dam and the subsequent decrease in water levels in the Dnipro River have led to significant alterations in floodplain ecosystems, particularly on Khortytsia Island. The lowered water levels have created conditions conducive to the formation of new habitats, including sandy open areas and degraded floodplain reservoirs. This disaster has triggered ecological succession processes. Pioneer plant communities are capable of rapidly colonizing the newly exposed areas, while other species are disappearing due to changes in soil moisture, chemical composition, and other environmental factors. The changes in plant cover structure can be analysed through the lens of hemeroby. The sensitivity of plant communities to anthropogenic transformations is influenced by environmental conditions. The drainage of the area has induced changes associated with an increase in the hemeroby of plant communities. Increased light availability and elevated soil nutrient levels resulting from the reservoir drainage are promoting the proliferation of more hemerobic plant species. A substantial part of the drained reservoir area is currently dominated by Salicetum albae and Salici-Populetum associations. The Salicetum albae association exhibits a higher level of hemeroby, likely due to greater light availability compared to the Salici-Populetum association. Indicators of hemeroby and naturalness are promising tools for monitoring the dynamics of floodplain ecosystems that have experienced catastrophic anthropogenic changes.