Indication of Long-Term Changes of Algae Communities in a Hydrologically Transformed Estuary Sasyk, Black Sea, Ukraine

: Forty years ago, the transformation of the estuary of the Black Sea to a freshwater reservoir was started by its connection with the Danube River through the Danube–Sasyk Canal. Today, the inﬂow of the Danube water into the Sasyk is the main component of the water balance, aﬀecting the internal and external water exchange, and it is, in general, responsible for the quality of the aquatic environment. In addition, the channel is a migration route for hydrobionts and the main source of invasion. We describe the changes in the composition of algal communities in Sasyk, which was converted from an estuary to a reservoir during three stages: estuary-lake in 1967–1977 (stage I), the forming of the reservoir from 1980–1990 (stage II), and the reservoir from 2013–2019 (stage III). Average salinity decreased from 7790 to 491 mg L −1 chlorides, and the species richness of algae increased from 259 to 289 taxa during the Sasyk transformation. Analysis of the algal communities revealed that salinity could have an eﬀect on their composition. The species list for the Sasyk , including 586 taxa of algae and cyanobacteria ( 613 infraspeciﬁc taxa ), is presented here for the ﬁrst time. Rare marine and freshwater taxa present in the algal community at the modern stage of the reservoir and their transformation are discussed. An ecological analysis of the diﬀerent stages of the Sasyk transformation from an estuary into a reservoir was conducted. The increase in species richness for stages II and III along with an increase in desalination was noted. To keep the current ecological status of the reservoir and its hydrological stability, the role of the Danube River should be considered. Bioindicators, statistical analysis of species, and environmental variables’ relationships conﬁrm salinity as a major regulating factor that can be observed by way of long -term monitoring.


Introduction
Humans engage in the transformation of nearly all of the earth's systems, including its atmosphere, hydrosphere, lithosphere, and biosphere [1]. They have extensively and intentionally modified their environments for tens of millennia, and some of these modifications have had a fundamental impact on the existence of many species [2]. At present, An attempt to create a freshwater reservoir instead of a brackish estuary led to a change not only to the hydrological regime of the reservoir but also to its morphometric characteristics that allow the existence of the created reservoir for a long time in future. Until 1979, Sasyk was a Black Sea estuary; however, due to its periodic connection with the sea and proximity to the Danube Lakes, it was often called a "lake" or "estuary-lake" [12] (Figure 1a). For this study, the described hydrological characteristics allow us to differentiate this phase as stage I of the existence of the Sasyk (Table 1). The Sasyk was shallow and brackish at the stage I. The main factor influencing the salinity of the estuary-lake Sasyk was the frequency of the seawater exchange. The seasonality of mixing river water with the salts of the bottom sediments and with the groundwater as well as frequent changes to its size and depth were investigated.
At the next stage II, the transformation of the Sasyk into a reservoir was based on creating channels (Figure 1b), which made changes to the shoreline and regulated the freshwater inflow. That period was characterized as heterogeneous due to the stabilization of natural processes after the anthropogenic regulation of the reservoir.
After 2000, the "reservoir" period began (stage III; Figure 1c), and the nature of the anthropogenic impact on the reservoir changed. The withdrawal of water for irrigation stopped, and the inflow of the Danube River's freshwater through the Danube-Sasyk Canal became more regulated. At the same time, stricter restrictions began to be applied to part of the water area due to its inclusion in nature.
The comparison of water salinity in the Sasyk during stage I and stage III demonstrates completely opposing changes: before the transformation, the rivers exhibited reduced salinity in the northern part of the reservoir; and now, especially in spring, they mineralize it. Most of the freshwater is in the area near the confluence of the Danube-Sasyk Canal. However, other channels that were needed originally to provide irrigation in the region are no longer used.
Anthropogenic transformation of the Sasyk indirectly affected other hydro-ecosystems. For example, the creation of a dike along the sand spit between the sea and the Sasyk led to the intensification of the coastal strip from the Black Sea seaside during 1980-2020. [10].  1979-1999 (1979-1985: an active transformation; 1986-1994: irrigation; 1995-1999: stabilization) 2000-now

Environmental Data Collection
The environmental variables were collated for three periods of the Sasyk's existence: lake-estuary, formation of a reservoir, and reservoir (Table 1). Hydrological parameters (water exchange period, evaporation, inflow through the channel, and inflow of water from the sea) were gathered from literature sources or calculated by authors of published data [11][12][13][14][15] and are presented in Table 2. Hydrophysical parameters were measured during our periodical field studies in 2013-2019, with some additional information from literature sources [11][12][13][14][15] and the monitoring data of the State Water Agency and the Danube Hydrometeorological Observatory (work with unpublished data in archives). Hydrochemical parameters were also measured during our periodical field studies in 2013-2019 and also supplemented by some published data [13,[16][17][18][19] (Table 2).

Algae Floristic Data Collection
To evaluate changes in the algal composition of the three stages of the existence of the Sasyk, the phytoplankton and phytobenthos data were analyzed. A species composition at stage I of the reservoir existence was made with the help of analysis of historical datasets that included period of studies in 1967-1987, presented in [11]. For stages I and II of the Sasyk's existence, it is known that the phytoplankton and phytobenthos were sampled at 24 sites of the water area in 1978-1987. Therefore, 17 deep-water sites for phytoplankton sampling and 7 coastal sites for phytobenthos sampling were chosen [11].
To characterize the modern period of the Sasyk's existence, namely stage III, phytoplankton analysis conducted in 2013 and 2014 [19,20] was combined with the phytobenthos analysis conducted in 2018 [8]. For analysis of phytoplankton species composition, the sampling sites were evenly distributed around the Sasyk's surface area. The number of sites was 17, including those near to the Danube-Sasyk Canal. For phytobenthos investigations, sampling included seven sites that were mostly located along the shoreline of the water body to evenly cover all of its banks. The obtained lists of species for the modern period of the Sasyk's existence have already been published [8,19,20], and detailed methodological explanations of the sampling strategy and methods used for the collection and analysis of the algal material can be found in these works. In the present study, we focus on the analysis of published data for both the historical and modern periods of the Sasyk's existence.
The general list of species mentioned in the references and found by us was brought into line with the modern taxonomic system, validated by using the AlgaeBase system [21], and divided by presence-absence data into three periods.

Bioindicators and Statistical Analysis
To determine the environmental factors affecting the diversity of phytoplankton in the Sasyk, a few studies were performed. Bioindicator analyses were performed according to [22], based on species-specific ecological preferences of the revealed algae and cyano-bacteria [23][24][25], with the combination of species with similar ecological properties to indicator groups reflecting the response of aquatic ecosystems to habitat, pH, oxygenation, water mass dynamics, salinity, trophic state, and the class of organic pollution. The analysis of bioindicators included several steps. First, the species composition was identified in each stage of the Sasyk. The available information on the autecology of the identified species was then added. Then, the number of indicators in each environmental group for each period was calculated. Finally, the dynamics of the percentage composition of indicators for each indicated environmental variable were expressed in histograms and tabular data and reflected the changes in the composition of the communities due to the changes in the environment during the three periods of the study.
Network analysis in JASP (significant only) using the botnet package in statistic R of [26] following the comparison of their distribution was performed. The statistical analysis of the species and their environmental variables' relationships was performed using the CANOCO Program 4.5 [27].
To visually present the degree of dissimilarity of the algal community structure between the stages, we built a dendrogram using the unweighted pair groups method with arithmetic mean cluster analysis (UPGMA) in the Biodiversity Pro 2.0 program based on the Bray-Curtis similarity matrix [28] calculated from species presence-absence data in the three lists that correspond to the three stages of the Sasyk's existence. The same type of analysis was performed for the species present in each stage, the number of species in indicator groups, and the current available environmental data for the Sasyk.

Environmental Variables in Long-Term Changes
The studies conducted between the 1960s and the 1970s describe the Sasyk as a water body partially connected with the Black Sea and the Dzhantei Estuary at stage I ( Figure  1a). The average period of water exchange during these years of observation of the reservoir was 505 days ( Table 1). The water salinity during this described stage was similar to the marine salinity both quantitatively (on average 14-15 g dm −3 ) and by ion composition (Table 2).
At stage II, the period of water exchange was at its shortest and amounted to just 163 days during the early 1980s. That is, the water in the reservoir was artificially renewed three times more often than naturally in stage II (Figure 1b). The first decade of the reservoir's existence was designated by the phrase "forming a reservoir", and this phase is named stage II of the Sasyk's existence.
The water exchange in stage Ш (Figure 1c) became less intensive with an average period of 312 days, which is less than was occurring immediately after the creation of the reservoir but more intensive than at stage I of the Sasyk's existence.
In recent times, water salinity and its distribution to the water area have changed but have not reached the level of being "freshwater". After being actively used in the 1980s, during the 1990s, the Danube-Sasyk Canal was almost non-functioning.
Modern investigations during the period from 2013-2019 at various observation posts for the aquatic zone of the reservoir revealed that the water salinity in the Sasyk ranged from 0.30 to 2.7 g dm −3 . According to surveillance monitoring carried out during different seasons, the content of the dissolved oxygen in the water ranged from 6.14 to 13.31 mgО2 dm −3 (59.3-101.4% saturation), and at the near coastal station, it was 4.1-12.7 mgО2 dm −3 (51.3-161% saturation). The average annual water temperature in the reservoir was 10.6-13.2 °C. In summer, the water could warm up to 25-30 °C. By pH values, the water in the Sasyk was weakly alkaline (8-8.11). In the northern part of the reservoir, in different years of the average annual values of the ammonium concentration range was 0.059-0.284 mg dm −3 ; for nitrites nitrogen, 0.018-0.049 mg dm −3 ; for nitrates nitrogen, 0.467-1.59 mg dm −3 ; and for orthophosphates, 0.1-0.22 mg dm −3 [12].

Algae Composition Characteristics of the Stages of the Sasyk's Existence
The total composition of algae during the three stages of the Sasyk's existence is represented by 586 species (613 with infraspecific taxa or ssp.). However, each stage of the existence of the water body has its own species composition. At stage I, the number of species was 253 (259 ssp.) and at stage II, 281 (295 ssp.) [11]. At stage III, 278 species (287 ssp.) were found [8,19,20] (Appendix A, Table A1). Thus, we can analyze the created database of phytoplankton and phytobenthos species for tracking changes through a 40-year period covering the three stages of the Sasyk's existence from being an estuary to becoming a reservoir.
The statistically calculated dissimilarity between algal communities during the different stages of the Sasyk's existence is well illustrated in the graph (Figure 2). The algal composition at stage I was only 45% similar to that of stage II. The similarity between stage III and stages I and II is quite low at 23%. The comparison of algal species composition at all periods of the existence of the Sasyk allows us to assume that nowadays, its flora is considerably richer than during its natural stage.
Interestingly, the modern stage III is more similar to stage II and less similar to stage I. In the first case, it has a 31.5% similarity and in the second, 23%. This suggests that in the first decade after the transformation of the water body, more than 50% of the algae composition changed, and during stage III, almost 70% of the species changed over a forty-year period. With this, one can assume that long-term and significant hydromorphological changes to the water body affected the species composition of algae.  Table A1) between different stages of the Sasyk's existence. This figure was built based on the Bray-Curtis similarity index.
The species composition at the studied stages was mostly formed by Bacillariophyta, Chlorophyta, Miozoa, Cyanobacteria, Euglenozoa, and Charophyta ( Figure 3). Some other groups, namely Ochrophyta (Xanthophyceae), Cryptista, and Ochrophyta (Chrysophyceae), were represented by a few species. The number of Bacillariophyta species markedly increased at stage III in comparison to stages I and II. The number of Chlorophyta species increased after stage I and was stable over forty years during stages II and III. The amount of Miozoa present sufficiently decreased. At stage II, the number of Cyanobacteria species increased, but in the modern period/stage III, it is at almost the same level as it was during stage I. The peak of Euglenozoa reached its maximum during stage II, and nowadays, it is even smaller than during stage I. As for the Charophyta species, their number during stage I was low, with its maximum being during stage II and with some decrease at stage III.
During stage II, the cases of cyanobacterial blooms were noted quite often and were formed by Aphanizomenon flos-aquae and Microcystis aeruginosa. Furthermore, some common diatom algae should be noted for this period: Aulacoseira granulata, A. italica, Cyclotella meneghiniana, Stephanodiscus hantzschii, and S. subtilis. As also for the green algae, Coelastrum microporum and Mucidosphaerium pulchellum were noted.
The common species found among the Sasyk plankton during the investigation at the modern/stage III were the following: Aphanizomenon flos-aquae, Aphanocapsa   The following species of diatoms identified from benthos that have rarely been observed so far in Ukrainian waters were noted during stage III: Amphora copulata, A. inariensis, Cymbella neocistula, C. compacta, Fallacia clepsidroides, Gomphonema utae, Navicula recens, Nitzschia filiformis var. conferta, N. archibaldii, Paraplaconeis minor, and Rhaphoneis amphiceros (Appendix A, Figure A3).
Our investigations revealed that the following new species were found in the Ukrainian territory [7]: Fragilaria microvaucheriae, F. nevadensis, Mastogloia pseudosmithii, Navicula germainii, N. cf. vandamii, Nitzschia angustata var. minuta, N. sociabilis, and Planothidium lacustre (Appendix A, Figure A4). Some additional photos of diatoms from the modern period were not included during the preparation of the paper about phytobenthos [8] and are presented in this work (Appendix A, Figures A1-A4).

Ecological Assessment Based on Bioindicators for the Different Stages of the Sasyk's Existence
The congeneric coverage of all phyla of algae during the three stages of the existence of the Sasyk (stage I, stage II, and stage III) (  During the studied stages, the plankto-benthic (P-B) taxa prevailed, even considering that the species composition was different. However, the hydrological transformation of the Sasyk during stage II revealed the changes that were reflected in the floristic composition. Habitat preferences indicated the increased proportion of planktonic (P) taxa and decline of the benthic (B) taxa during stage II. Now, at stage III, the proportion has become similar to that at stage I ( Figure 4b).
The pH indicators during different stages of the Sasyk's hydrological transformations were represented by acidophiles (acf), alkaliphiles (alf), alkalibiontes (alb), and indifferents (ind) [29]. Comparisons of their proportion between the stages revealed that at stage I, the water contained proportionally greater amounts of alkalibiontes, indicating more alkaline conditions compared to stages II and III. Acidification was observed at stage II, but at stage III, the pH of the reservoir almost returned to the values of stage I (Figure 5a).
Water mass dynamics and oxygen regime revealed prevailing low-streaming (standing-streaming) water (st-str) taxa during all periods of the hydrological transformations of the Sasyk. The percentage of indicators of standing water with low oxygenation de-creased during stage II. In stage III, the situation has become more stable, and the proportion of indicators for streaming water (str) with a high amount of oxygen has increased ( Figure 5b).
As for the salinity changes during the different stages of the hydrological transformations of the Sasyk, oligohalobe-indifferent (i) taxa were prevailing, with the number of mesohalobes (mh) and halophiles (hl) significantly changed [30]. As for the oligohalobehalophob (hb) taxa that live exclusively in freshwater, their proportion was at its lowest during stage I (Figure 5c).
To understand the trophic changes during the three stages of the Sasyk Reservoir's existence, trophic status indicators [23,31,32] were analyzed. Mesotraphentic (m) and eutraphentic (e) taxa prevailed during all the stages, which characterizes the water body as having mesotrophic and eutrophic status. Additionally, the percentage of the increase in oligotraphentic (ot) and oligo-mesotraphentic (o-m) taxa can be noticed at stage III in comparison to previous stages ( Figure 5d).
All the described is supplemented by the analysis of the organic pollution by which the water was assigned to the corresponding quality classes. In Figure 5e, the assessment of water quality classes is presented in the EU colors' code system; we observed that during the studied stages of the Sasyk's existence, the water body was of class III water quality. However, at stages II and III, we observed that the number of indicators of the class II water quality increased, and those of class IV decreased. Our assessment yielded the same results in both assessment systems of organic pollution: Sládeček (as index saprobity S) based on all taxa and Watanabe for diatom species only (Appendix A, Table A1).
The analysis of indicators of nitrogen-uptake metabolism revealed the presence of all groups of autrotrophy-heterotrophy: (ats), (ate), (hne), and (hce). The clear trend of the increasing percentage of nitrogen-autotrophic taxa tolerating very small concentrations of organically bound nitrogen (ats) and decrease in the indicators tolerating very small concentrations of organically bound nitrogen (ate) can be noted. The increase in obligately nitrogen-heterotrophic taxa (hce), needing continuously elevated concentrations of organically bound nitrogen, is also noticeable in Figure 5f. : st, taxa that prefer standing water with low amount of oxygen; str, taxa that live in streaming water that is highly saturated by oxygen; st-str, taxa living in standing-streaming water with a medium amount of oxygen. Salinity (c): hb, oligohalobes-halophobes; I, oligohalobes-indifferents; hl, halophiles; mh, mesohalobes. Trophic state (d): ot, oligotraphentic; o-m, oligo-mesotraphentic; m, mesotraphentic; me, meso-eutraphentic; e, eutraphentic; o-e, oligo-to eutraphentic (hypereutraphentic); he, hypereutraphentic. Class 1-5 of organic pollution indicators according to species-specific index saprobity S (e). Nitrogen uptake metabolism (f): ats, nitrogen-autotrophic taxa, tolerating very small concentrations of organically bound nitrogen; ate, nitrogen-autotrophic taxa, tolerating elevated concentrations of organically bound nitrogen; hne, facultatively nitrogen-heterotrophic taxa, needing periodically elevated concentrations of organically bound nitrogen; hce, obligately nitrogen-heterotrophic taxa, needing continuously elevated concentrations of organically bound nitrogen.

Species-Environment Relationships
An analysis of the available ecological characteristics of the Sasyk as well as the dynamics of bioindicators revealed the importance of identifying the main operating factors determining the composition and prospects for the response of communities in changed conditions. Cluster analysis of the species composition alone ( Figure 2) showed similarities between the communities for stages I and II, while the composition at stage III was significantly different. For the next analysis, we selected data on species representation (Appendix A, Table A1), as for the first analysis: the number of species in the phyla, the composition of bioindicators, and the main environmental characteristics for the three stages of the Sasyk's existence (Appendix A, Table A2). Thus, the complete database of available data on the environment and communities in the three stages of the Sasyk's existence were analyzed. Figure 6 shows that the inclusion of the analysis of environmental indicators and their indicators significantly changed the pattern of similarity in the same type of analysis. The indicators of stages II and III seem the most similar when the desalination of the Sasyk began. This allowed us to progress to the next step in the analysis. Correlation relationships based on the same database as in the previous analysis, which were identified in the JASP program, appear as a significant correlation of data for stages II and III ( Figure 7). Thereby, the Sasyk's desalination was revealed as having a significant role in the algal assemblages.  Table A2) between the different stages of the Sasyk's existence. Group 1 includes data of stage I with high salinity, and group 2 includes data of stage II and III, when the Sasyk had a dam and was enriched by waters from the Danube-Sasyk Canal that formed a reservoir with low salinity. Bold lines show largest similarity on type of analysis; "huge" correlation > 0.5. The next stage of the analysis included the calculation of the relationships between the environmental parameters (Tables 1 and 2) and the number of species in phyla (Appendix A, Table A2) for each stage of the Sasyk's existence. The RDA tri-plot ( Figure 8) demonstrates the division of the environmental indicators into three clusters. Chlorides and the connection with the Black Sea favor the development of Miozoa species at stage 1 (cluster 1). Opposite cluster 2 combines the diversity of phyla near the inflow of the water from the Danube-Sasyk Canal during stage II, and cluster 3 includes species of the diatom phylum, for which the increase in diversity is associated with desalination and great water evaporation at stage III. Thus, the RDA analysis sharply emphasizes the influence of hydrological conditions on the composition and the flourishing of algae and cyanobacteria species at various stages during the Sasyk's existence, with the desalination factor being critical to the growth of community diversity.

Discussion
Changes in the algal composition reflect the forty-year hydrological transformation of the water body. In the modern stage of the Sasyk Reservoir's existence (stage III), the algal composition is at its most different when comparing it to the natural stage (stage I), which is quite reasonable. Stage II is more similar to the natural (stage I), mainly because of the short time of existence of the transformed water ecosystem. This may explain the unstable flora and reflect the hydrological transformation of the water body. This was a short time to have allowed for a hugely differentiating species composition. After that period, great changes to the hydrological regime occurred, which indeed changed almost half of the species composition (stage III).
The obtained results on the dynamics of the indicator species of the pH regime during the noted periods testify to the changes in their proportion. The increased number of alkaliphiles (alf taxa) living at pH > 7 indicate medium-alkaline waters, thereby corresponding to the existing data characterizing the modern period of the Sasyk's existence [8,19].
According to hydro-chemical data [11], the oxygen saturation of the water was sufficient at all stages of research [16,17]. Further, the stabilizing of the hydrological regime nowadays (stage III) in comparison to stage II is similar to the stable stage I and revealed the same percentage quantities of standing water (st), streaming water (str), and lowstreaming (standing-streaming) water (st-str) taxa. Interestingly, the amount of standing water (st) taxa increased during stage II along with a decreasing amount of streaming water taxa (str) compared to the stage I. Considering the intensity of the water exchange during the reservoir-formation stage, one would have expected an increase in the number of the streaming water taxa (str); however, this was not the case. This could be explained firstly by the emergence of the active restructuring of the ecosystem and secondly by the large volumes of water mass that flowed through the reservoir. Therefore, it is important to consider not only the general period of water exchange but also the hydrodynamic processes, namely mixing. In this case, water masses did not have time to "replace" and saturate oxygen throughout the water area. Due to this, shallow-water stagnant zones arose, the presence of which could cause an increase in indicators of standing waters (st). Reducing the intensity of water inflow from the Danube-Sasyk Canal during the reservoir stage in this modern time has led to water masses replacing a larger area of water, therefore supporting mixing processes and thereby aeration. This is also reflected by reduced standing waters (st) and the significantly greater amount of streaming water (str) taxa. The ecosystem of the reservoir has become more stable, so the species found and consolidated in areas better met the characteristics of their ecological niches.
Nowadays, the identified dominant complex of the plankton algal species is typical for freshwater; however, the presence of mesahalobes is still registered in the benthos. This can be explained by their storage or conservation in a thick layer of silt at the bottom of the reservoir as well as possible sources of groundwater saturated with chlorides. In turn, the presence of mesahalobes and (mh) and halophiles (hl) in algal composition for the current period can be explained by the presence of shallow waters that were periodically dry as coastal zones, where salts from the bottom soils continued to be present. The probability of such a process is described in ref. [33] but still has not been sufficiently studied. Furthermore, it is worth mentioning that according to our studies of the modern period, the mh in phytoplankton were found only in a few places in the south-eastern part of the reservoir [19]. Although the water salinity did not meet the requirements for mh existence mesohalobes (mh) and halophiles (hl) in phytobenthos were found in almost every sample [8]. This once again confirms the above-mentioned assumption and emphasizes the importance of simultaneous studies of the hydro-chemical parameters and bioindication results. The presence of such species may be an indicator of a long-term effect of the salinity factor that may not be noticed in a one-off study of hydrochemical samples.
The amount of oligotraphentic (ot) and oligo-mesotraphentic (o-m) taxa reveals that the trophic state at the modern period of the reservoir is slightly improving compared to its natural state (stage I).
The decrease in indicators of class IV and increase in the number of indicators of class II reveals the improvement of the ecological status during the modern period of the existence of the water body (stage III). Moreover, the increase in the number of ats along with decrease in other indicator groups of autrotrophy-heterotrophy also testifies to the improvement of trophic status of the investigated water body.
The influence of factors that created a mosaic distribution of biota over the reservoir was reported [8,19,20]. The originality of the flora is reflected in the high variety of rare species and occurrence of species that are new to Ukraine. As the aquatic territory of the Sasyk Reservoir belongs to European Ramsar Sites and is under European protection [7], another important part of our investigation of the territory was to accurately identify some species of algae that are rare for Ukraine. The Sasyk Reservoir ecosystem nowadays is quite open for penetration by new species entering from the Danube River. Moreover, the Sasyk's hydrological regime is specific and linked with the unstable condition of the ecosystem after transformation, where the different salinity levels of water masses meet each other due to the rivers outflow, underground waters, and freshwater from the Danube-Sasyk Canal. A high variability of species composition in this water body was impacted by waters with different salinity that form the conditions inherent for transitional zones [34][35][36][37]. The importance of the salinity ranges and the forming of a transitional zone along the seaside water bodies has been explored for the Ukrainian territory and has proven to have a significant effect on the composition of algae [38]. The provided investigation of a rapidly changed coastal zone is of high importance due to the reflection of the changes induced not only by physical but also climatic and anthropogenic patterns [8,10]. Floristic and faunistic study in the Danube Delta area can give a chance to assess the differences in the nature conservation over some parts of delta and adjusted areas in Romania and Ukraine [39]. The list of the Sasyk's algae and cyanobacteria, compiled here for the first time from published and original data, presents a summary of species and their indicator properties that can be used in water quality monitoring. This type of coastal waters has always been used and transformed as a source of water and still retains its importance in the face of future climate change [40][41][42]. For these purposes, new methods can be used in the monitoring and assessment of long-term changes [43,44].
The importance of this conducted research is also explained by the fact that this area is one of the main migration routes for birds. The territory serves as a nature reservation for habitats of migratory birds as well as the fish population of the delta [45]. This was the reason for creation of the Danube Biosphere Reserve in recent times (Figure 1c).
Longitudinal studies of salinity regime changes were assessed with a focus on the dynamics of the structural composition of the algae in the Kuyalnik Estuary [46,47]. Following the reduction of salinity in the waters of the Kuyalnik Estuary, the mesohalobic taxa prevailed along with euhalobic, and during the drying period, the algal composition reflects the changes in the salinity regime, where euhalobes or even polyhalobes are dominant. However, the example of the Kuyalnik is a direct opposite to the Sasyk Reservoir. In the Sasyk, the process of the reduction of salinity prevails, whereas in the Kuyalnik, the process of salinization is observed. The poor management of the Velykyi Kuyal'nyk River has resulted in a decrease in the runoff and an increase in the volume of evaporation, which has led to a catastrophic shallowing and salinization [48][49][50]. The range of salinity in the Kuyalnik is unstable and during different periods has ranged from 5.4 to 399.9 g dm −3 [51,52]. The impact of the increasing salinity to the algae diversity and abundance was revealed for continental water bodies in Ukraine [53]. This problem is also relevant for the Tuzlovsky Estuaries, where catastrophic shallowing is being observed due to insufficient water inflow and a lack of water supply. The solution for such problems is to create not only hydrological transformations that result in the phytoplankton diversity and their decreasing abundance but also with increasing freshwater runoffs and connections with water bodies such as rivers and water from the sea, etc.
Thus, the state of the Sasyk Reservoir compared to the above-mentioned estuaries is relatively better. The issues of water transfer in changing the salinity level and the improvement of water exchange with the help of the Danube-Sasyk Canal are also relevant for the Sasyk Reservoir and nowadays have led to an improved condition of the reservoir. Indeed, the hydrological situation at this moment is better than if there had never been any water input at all. There is a constant flow of water and with an adjustable outflow that is characteristic of the modern stage of the hydrological transformation of the Sasyk Reservoir. Of course, it is possible to intensify water exchange, but due to the size features and hydro-dynamic processes, we can observe the effect of water mass dynamics over the whole Sasyk Reservoir water body [54]. At the same time, the channel remains the main migration corridor for new species of fish and mollusks. (For example, the first finds of Sinanodonta are exactly near the confluence of the Danube-Sasyk Canal [55]). The main part of allochthonous suspended matter also enters through this channel [54]. The sufficient influence of the inflow of suspended matter with the waters of Danube was revealed earlier [40], which in future exploitation of channel could have proper interest. Perhaps, if we could arrange for decantation or preliminary purification of the water that enters the Sasyk, then the quality of water in the reservoir could improve.
Despite the already-implemented hydrological changes that have taken place over the last forty years, the question of further usage for the Sasyk Reservoir remains open. An actively discussed issue is the possibility of restoring the irrigation system in the south of Ukraine and reusing the Sasyk Reservoir as a water reservoir. That would return it to the original transformation purpose. A project for tourism development in the region and the construction of a large complex on a spit between the Sasyk Reservoir and the sea have been suspended, probably for political and economic reasons. From the standpoint of an increase in freshwater demand, the idea of modifying estuaries into freshwater reservoirs may be implemented.
An example of the Sasyk transformation could serve as a showcase for artificially maintained freshwater areas. We believe that such a solution for the water body could also contribute to resolving the most urgent problem of mankind over a freshwater shortage and uneven distribution of freshwater over the planet, mainly concerning developing countries [56]. The described response of the aquatic ecosystem as well as some factors that have an impact on seaside water bodies could be considered for future human interventions in natural estuaries.

Conclusions
The unique Sasyk Reservoir, located on the northwest coast of the Black Sea near the Kiliya Danube Delta, is an unfortunate example of the decrease in salinity in a brackish water reservoir that was analyzed over a forty-year period. The study highlights the significant changes in algal composition due to changes in salinity during three stages of human interventions. Each period of the Sasyk's existence is characterized by specific algal communities, with the similarity of algal composition between periods ranging from 23%-31.5% to 45.8%. The analysis of the algal composition at the modern stage of the Sasyk Reservoir revealed the uniqueness of the conditions that have been formed in this water body. The conducted study fulfilled our understanding of the Ukrainian flora of algae and helped us to find species of algae that are new and thus far rarely observed in Ukraine.
The study was undertaken to analyze the impact of the anthropogenic interventions in the hydrological regime that reflect the ecological changes in the Sasyk Reservoir, with an increase in algae diversity as a result of desalination. The indicator species of habitat preference, water mass dynamics, pH, salinity, trophic state, and water quality class were analyzed. Most of the analyzed indicator characteristics of algae revealed the improvement of the ecological status of the reservoir during this modern time of its existence in comparison to the earlier stages. The salinity of the Sasyk's waters in statistical calculation was revealed as the major impacting factor for the algae and cyanobacteria assemblages. The Sasyk's water masses remain considerably differentiated by the salinity characteristics; therefore, further monitoring of the state of the ecosystem must necessarily consider the study of the distribution of this parameter over the water area of the reservoir. As we can see, the increase in water exchange and the constant maintenance of the Danube water supply in the last decades (stage III) has affected the species composition of algae communities. Therefore, it makes sense to continue supporting the Danube-Sasyk Canal operation. However, this decision depends on the water level in the Danube River and the determination of specialized management bodies to maximize the ecological services of the reservoir and coastal areas. Monitoring the algae and cyanobacteria assemblages can help to assess their changes under the face of the coastal environment and climate changes. For this, carrying out complex hydrobiological, hydrochemical, and hydrological studies is highly important. All the mentioned measures will allow us to determine the anthropogenic component of the influence on the natural processes of the development of the aquatic ecosystem.                        Thalassionema frauenfeldii (Grunow) Tempère et Peragallo