The Danube Delta: The Achilles Heel of Danube River–Danube Delta–Black Sea Region Fish Diversity under a Black Sea Impact Scenario Due to Sea Level Rise—A Prospective Review

Abstract

The Danube Delta is one of Earth’s biodiversity hotspots and includes many endemic, rare, and important species of both major conservation and economic value. This unique complex of ecosystems also plays a key role for Danube River and Black Sea fish fauna through its role as a natural safe buffer, shelter, feeding, reproduction, and smooth transitional area for a large number of fish species. Climate change is inducing a progressive sea level rise in the Black Sea, a fact that is expected to impact the delta’s key complex and dynamic habitats, biocoenoses, and associated biota, and last but not least the key taxonomic group, namely, fish. Around one-third of the fish species of this delta will be greatly affected, sometimes negatively, by this climate change scenario, another one-third to a lesser extent, and the final one-third not at all. The ecological positive feedback of fish can stimulate environmental change and is expected to be responsible for changes within Danube Delta ecosystems, and also for the near Danube River and Black Sea diverse matrix of aquatic and semi-aquatic ecosystems. Sea level rise in the Black Sea is considered to have been one of the main stress factors of the Danube Delta fish fauna in the past, and is likely to be the case in the future. In this spatio-temporal dynamic context, for the fish species under threat and risk, in situ-adapted management measures are highly required. The current work brings for the first time such a prospective knowledge about the potential impact on Danube River–Danube Delta–Black Sea coast fish diversity in the potential climate change–sea level rise scenario.

1. Introduction and Background

Europe’s deltas, for example, those of the Danube, Ebro, and Rhone, began to be studied through an eco-economic lens at the beginning of the 20th century [1,2]. Surely, each one is unique, and every long-term human impact on them produces a distinct combination of forces and counterforces that should be assessed and monitored at the most accurate level of ecological identification and the description of trends. Finally, optimum integrated management plans based on reliable data need to be proposed.

The Danube River Basin is one of the major global river basins due to its geographical, ecological, biodiversity, and human historical importance. The natural attractions of the Danube River Basin for human societies throughout history have led to the development of both extensive and intensive, complex, and long-term human impacts in spite of some progress through modern ecological and “green” policies [3].

The related abiotic and biotic elements of wetland lotic systems are defined by high composite structures, functions, and ecological states, as well as spatial and temporal variation, both natural and human-induced, especially in the Anthropocene [4,5]. These characteristics also describe the Danube Basin in the 21st Century [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. In this watershed context, the fresh, brackish, and saline interfaces of transitional waters in the Danube Delta constitute exceptionally highly complex and dynamic ecosystems, but they also correspondingly face direct and indirect human pressures.

The Danube River–Danube Delta–Black Sea geo-ecosystem (Figure 1), with its many ecosystems, provides a smooth transition zone from the fresh water of the Danube River to the salt water of the Black Sea, and is a specific complex of unique ecosystems [27].

The complex geo-ecosystem of the Danube River–Danube Delta–Black Sea, shaped by a unique long-term mix of built-in biotopic and biocoenosis elements of both Danube River- and Black Sea-related influences and counter-influences in time and space [28,29], represents a significant example of how varied human pressures have cumulatively damaged a highly dynamic, rich, and complex “ichthyosystem”, formed of three sub-ichthyosystems (related to sea, delta, and river habitats), which evolved interdependently, sustaining the flourishing of a high number of important fish species [27].

This complex of three sub-ichthyosystems, among which the key role of the Danube Delta is prominent, has exhibited a significant level of flexibility, resilience, and adaptation over geological time, with a positive effect on Danube River and Black Sea fish fauna diversity [2]. These ichthyosystems have become much more sensitive to the overexploitation and degradation of natural resources and services, environmental perturbations, and changes that include decreases in ecological diversity during the last century of human impacts [2]. The Danube River Delta is an ecological point of transition between the Danube River basin hinterland, with all the synergic effects of its basin tributaries, and the Black Sea and further afield, and it has a natural strategic position and functions as a carbon trap, a buffer for storm surges, and a toxin filtration system; perhaps above all, it is a place for fish feeding, refuge, reproduction, and nursery development, and it provides a habitat buffer [27]. Problems related to the quality and biodiversity of lotic habitats in the entire Danube River Basin due to human impacts [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,30] are also ultimately directly and indirectly reflected in the habitat quality and biodiversity of the Danube Delta. Despite these problems, the Danube Delta remains the second largest and best preserved of the European deltas, and was inscribed in 1991 in the World Heritage List, under Natural Criteria VII [31], as a unique landscape formed by the dynamic paleo-climatic and near-present climatic conditions [32,33,34,35]. This astonishing biogeographical node occupies a central position between the Central European, Palearctic, Ponto-Turanic, Central-Asian, Mediterranean, Sub-Mediterranean, European, West-Asian, and Euro-Siberian geographical elements, is characterized by exceptional biodiversity, and plays a major role in biogeographical connectivity, providing a point of transition and stepping stones to surrounding large geographical areas, with a key influence on fish in particular [36,37].

Fish are a global economic food resource for wildlife and people, and play a key role in the functioning of aquatic ecosystems. For huge numbers of people, fish are the primary protein source, and for even greater numbers, fish make up a significant portion of their diet [38,39]. Climate change is already altering aquatic ecosystems, with significant implications for wild capture fisheries. Across the globe, the incomes, food security, and livelihoods of aquatic resource-dependent communities are already at risk, as climate change threatens food security by endangering fish [40].

Fish are still a key element of aquatic ecosystems and a key component of ecosystem services, both quantitatively and qualitatively, through the high number of taxa present and their variety of ecological needs and adaptations, in spite of their alarming extinction risk and the increasing human pressure on freshwater ecosystems [41,42,43]; they are thus of high value for humankind [28,44,45].

In this paper we focus on the fish of the Danube Delta because their economic and ecological importance is significant on a global scale. Due to the fact that fishing is one of the oldest human activities, while at the same time knowledge of fish species and their environments has existed since the dawn of human existence, this accumulated knowledge makes fish an ideal taxonomic group for long-term comparative ecological studies of the ecosystems in which they live [46,47,48].

The value of Danube Delta fish fauna reflects the biogeographically and ecologically rich connections of this area [47,49], and, as with the majority of the significant larger wetlands of the world [50], large populations of endangered or rare fish species contribute to the great value of this area in terms of biodiversity [47,49].

Climate change, and its effects on fish resources, can be viewed as an inadvertent global experiment [51,52]. The hydrological processes that determine the status of fish habitats largely govern habitat features [53,54,55], and these are modified by human-induced changes, of which climate is a significant element [56]. Consequently, the changed ecological characteristics influence the environmental drivers and therefore impact a variety of fish species [57]. However, one shortcoming of fish response studies in relation to altered climate is that they relatively rarely consider ecosystem complexes such as deltas to be at risk due to potential sea level rises.

While limited-scale actions are underway, such as water level forecasting [58], it is unclear if we are able to face this new threat in regional to global correlated working scenarios for Earth’s key ecosystems. Most deltas lie only 5 m above mean sea level, while 24 major deltas are subsiding, some by as much as 10 cm a year, due to compaction, up-stream sediment retention behind dams, and water and mineral exploitation. Recent studies suggest that the proportion of the world’s deltas that are vulnerable to flooding could increase by 50%, and 85% of deltas have faced severe flooding, causing the temporary submergence of 260,000 km²; half of the world’s largest deltas will slowly become vulnerable to sea level rise [59]. Can global sea level rise cause the Danube Delta—with an average altitude of 0.52 m, with 20% of the territory below sea level, and with more than half not exceeding 1 m in altitude [60] —to be vulnerable and in peril due to salinity intrusion, and even partially or permanently flood it due to the sea level rise of the Black Sea? Can such a delta, with predominantly negative relief [28], and transporting to the sea less and less sediment due to human impacts [61], transform itself or even vanish due to the intrusion of sea water? Are the Danube Delta branches—Chilia (104 km), Sulina (71 km), and Sfântu Gheorghe (112 km) [62,63,64,65] —long enough to avoid being flooded? What will happen regarding Danube Basin and Black Sea fish if the Danube Delta is impacted even more by the direct or indirect negative effects of human activities, or even worse, reshaped or destroyed? Are society’s conscious and unconscious perceptions and attitudes towards delta flood risk in general good enough [66,67,68,69], or well-enough defined, in the case of the Danube Delta [70]?

In this context, efforts are required to match fish management and protection in this area, and to “build a raft” of knowledge for saving these important fish communities.

In the context of desperately needed environmental ethics [71], and based on the authors’ data and on specific reliable available scientific information, the present work aims to highlight potential sea level changes in the Black Sea within the current climate change regime, and link it to significant potential changes in Danube Delta fish fauna.

This work provides an overview of the potential impacts on Danube Delta–Danube River–Black Sea coast fish diversity in a climate change–sea level rise scenario.

The high complexity of this delta, the adjacent influential biogeographical areas, the natural processes, and the human impacts involved make it difficult to simplify the main aspects of the story while acknowledging that outside elements can have a crucial role in influencing the identified potential direction, complexity, and speed of trends.

Together with the main aims of this work, secondary objectives form an additional part of the foundation of this approach in an attempt to at least identify other synergic causes and interests; for example, some of the related cultural and socio-economic aspects, which can complicate and speed up the diverse negative effects on this delta.

2. Results and Analysis

2.1. Global Sea Level Rise Elements: Short History, Present Situation, and Trends

Sea level dynamics, inland movement, and the re-establishment of mutual relationships with inland near areas are based on sea level functions related to both the volume of water of the global oceanic system and the vertical shift of the land surface [72]. Rising and falling seas represent one of the endless rhythms of the Earth, the background track that has been playing during the 4 billion years of life on the planet [65]. Sea level changes have occurred over a wide range of time and space [73]; for example, during the last glacial maximum, sea levels were more than 100 m below current levels but rose at the end of the ice age due to land-based ice mass melting [74]. Sea level rise accelerated during this period [75]. What is different now is that we are interfering more with this rhythm by heating up the planet and melting the vast planetary ice sheet [76,77,78], inducing global changes such as sea level rise at rates of up to 0.06 m per decade since the 1950s and up to 3.2 mm per year since the early 1990s; every subsequent decade has experienced increased rates of sea level rise [79]. The sea level rose substantially from 1900 to 2018.

Sea level rise between 1995 and 2020 was caused primarily by two factors related to global warming: the added water from melting ice sheets and glaciers, and the expansion of seawater as it warms.

Sea level rise is one of the most tangible effects of climate change [80], which has been known as a slow but relentless major global issue and a direct threat to low-lying beaches and coasts since the 1980s [81,82,83,84]. Extreme sea levels may change as a result of both changes in the mean sea level rise [85], but also due to changes in wind velocity, atmospheric pressure, sea current circulation, differences in water density, rates of thermal expansion, salinity, atmospheric storminess, etc. [73,86,87,88,89]. All over the world, coastal adaptation to sea level rise has been considered during the last few decades [90,91], building on the large experience of coastal adaptation to extremes; despite this experience, great uncertainties about the success or failure of adaptation to sea level rise remain [82].

All recent observations and scientific publications have presented the possibility of a high and accelerated sea level rise later this century, mainly due to ice sheet instability and retreat in Antarctica. Under a high warming scenario, this may result in a sea level in 2100 that is up to 2 m higher than at present and 5 m higher in 2150 [92]. More significant sea level rise trends are expected to impact the coastal ecosystems as long as there is enough water locked in land-based ice to raise sea levels by 80 m if it all melted [74,93], contributing to the need for scientific investigations, assessments, monitoring, and projections for in situ potential uncertainties of the projected extreme scenarios of major impacts.

2.2. Black Sea Level Fluctuation Elements: Dynamic History and Future Rise Potential Trends

The Black Sea area has had a very complex dynamic throughout its geological history, with highly variable environmental conditions and extremely varied living organisms; we can note briefly the presence and dynamics of the Tethys Sea, the Sarmatic Basin, the Maeotic Basin, the regional brackish Pontic large lake, and the later Pontic and Caspian basins, all in continuous interrelations with the adjacent sea and land masses [94].

The Black Sea is the most isolated internal sea in the Atlantic Ocean system (413.488 km² without the Azov Sea, maximum depth of 2258 m, average depth of 1253 m, and a volume of 547,000 km³). It is a marginal inland sea of the Mediterranean, lying between Europe and Asia, filled with salty water, and supplied with freshwater by the Danube, Dnieper, and Don rivers [95,96,97,98,99,100,101,102,103]. The Bosporus, Dardanelles, and Gibraltar straits connecting the Black Sea to the Mediterranean Sea, Atlantic Ocean, and the World Ocean give these water bodies permanent connection and interdependency [104,105,106,107,108,109,110,111,112].

The water exchanges through straits are the main cause of long-term periodic Black Sea level oscillations, while the secondary causes are accidental, seasonal, or multiannual, such as liquid and solid discharge from rivers, rainfall over the sea, sea surface evaporation, the global Black Sea freshwater budget, the spatial distribution of river discharges, atmospheric pressure, waves, tsunami waves induced by strong earthquakes, steric effects, synoptic alluvial discharges, salinity, temperature, present-day tectonics, etc. [96,100,101,102,103].

The variation in the Black Sea follows and will follow global ocean water ascending and descending patterns [96,97,98,99,100,101,102,103]. Today, this phenomenon is largely influenced by the present day rise of the world ocean level, which triggers flooding in low coastal land and adversely influences coastal morphological processes. Therefore, determining the causes and analyzing the processes that produce the sea level changes are imperative for the rational management of coastal and near-coastal natural ecosystems and resources [96,97,98,100,101,102,103], including those related to the Danube Delta area.

Any significant climate change scenario can induce sea level change with numerous and complex consequences for both local and regional ecosystems.

The authors define the water level as a surface that coincides, to a first approximation, with the multiannual mean global ocean level, considered as the surface of a geoid. This level is not fixed and is constantly changing under the action of various forces. The Black Sea level is affected by factors leading to changes in the volume of water.

Changes in the level of the Black Sea show that there are temporal scales of sea level fluctuation spectra within periods ranging between a few minutes to thousands of years. Thus, there are long-term sea level oscillations on a geological scale of thousands and millions of years, triggered by world ocean water mass variations, as well as slow epeirogenic movements. These oscillations, which reach heights of tens of meters in amplitude, can be accelerated due to human-induced climate change [96,97,98,100,101,102,103].

The oscillations with long periods in the Quaternary, Upper Pleistocene, and Holocene in the Black Sea were characterized by repeated transgressions and regressions. During the ice age, there was a lowering of the Black Sea level by about 110 m. The Black Sea basin was brought close to the current situation at the end of Pleistocene [96,97,98,100,101,102,103].

The results of large-scale geological research distinguish a continuous sea level oscillation up to the present: a minimum regression between 20 and 16 thousand years ago, with the sea level at −60 m; an intermediate phase between 16 and 6 thousand years ago, related to the Holocene melting, characterized by a sea level rise up to −3.5 to −4 m; a maximum in the last 6000 years, characterized by a transgression, when the present-day sea level was reached; an interval from 18 to 8 thousand years BC in which the Black sea level rose steadily at a rate of ca. 9–10 mm/year, during which time the sea level rose from −85 to −25 m, and the sea level continued to rise at a lower annual rate, and at certain periods the sea level exceeded by about 2 m the present-day sea level; the new transgression, which took place about 2–3000, when the sea level reached the value of +3–4 m above the present level; the Phanagorian regression, which took place between 2500 and 1000, when the sea level fell to −3 m compared to the present one; and the present transgression, which began in the 20th century. In the last 6000 years, the sea level oscillations had a 3000-year period, and the Black Sea level deviated from the present by −5 to +2 m at a rate of 3–4 mm/year. As for the event that passed into legend as Noah’s Flood, it arose 7600 thousand years ago as a result of a terrible earthquake that allowed the Marmara waters to penetrate the Black Sea, which then rose by more than 100 m, flooding large areas [96,97,98,100,101,102,103].

2.3. Danube Delta Habitats’ Historical Exposure and Potential Trends and Risks to Potential Black Sea Level Rise

The Danube Delta is sited largely in south-eastern Romania and to a lesser extent in south-eastern Ukraine. As a biosphere reserve, this wetland is also the largest protected zone (5800 km²) and one of the key natural/semi-natural areas in Europe. One of Europe’s youngest and most complex mixtures of water and land, this delta is a fluvial–maritime floodplain of the lower Danube, where its diverse geomorphology, soils, and hydrological conditions favor the proliferation of a large number of aquatic, semi-aquatic, and terrestrial habitats and biocoenoses [104,108,112].

The unique features of the Danube Delta were formed via thousands of years of interactions between the Black Sea and the Danube River. The deltaic conditions were established during the Quaternary, when the Danube started flowing into the Black Sea. The Quaternary changes of the sea level influenced the Danube Delta’s evolution. The Würmian regressions of the Black Sea, when the sea level lowered to −130 to −150 m, brought about the intense erosion of much of the older Quaternary delta deposits. The current Danube Delta’s structure was formed mainly during the Upper Pleistocene and especially during the Holocene, with the Letea–Caraorman initial split phase; the first delta formation; the development of the new distributary Sulina and its deltaic lobe; the Cosna–Sinoie little secondary delta formation; the Chilia/Kiliya and St. George II deltas lobes; etc. [64,109,110,111].

The Chilia/Kiliya, which is the youngest and relatively most dynamic part of the Danube Delta, is only 250–350 years old. On the Bowra map (1770), it is shown without islands, and only a small mouth is indicated at the Pereprav (against Vilkovo) branch. On post-1800 maps, islands of the secondary delta appear. A feature of the front of the Chilia/Kiliya arm of the Danube is the ongoing formation of new land. In the last 25–30 years, the Dalnyaya, Novaya Zemlya, and Ptichya spits were formed here. The sediment runoff pushes the edge of the delta into the sea by several tens of meters every year [105,106].

This situation determines the difference in the structure of the biota. In particular, using the example of the Danube Delta and the Lena River Delta, it has been shown that the main factors determining the nature of the grouping are the flow speed, geographical location, and genesis of the delta’s water bodies. The depth, nature of the soil, etc., are of secondary importance [113,114,115]. When studying the delta of the Lena River, four species complexes were found: a channel complex of the main river, a complex of “older” slow-flowing eastern channels; a complex of “young” fast-flowing northwestern straits; and a lake complex of species. Regarding the structure of the dominance–diversity of zoobenthos groups, a trend of increasing species richness and decreasing degree of dominance of individual species from northeast to west was observed [113]. Similarly, the channel complex of species, the eastern complex of “young” fast-flowing channels of the Ukrainian part of the delta, the western complex of “older”, slow-flowing channels of the Romanian part of the delta, and the complex of lakes were distinguished in the Danube Delta [114]. In the dominance–diversity structure of the zoobenthos group, there is also a trend of increasing species richness and decreasing degree of dominance of individual species from east to west. If in the delta of the Lena River the Ber effect and the geological structure of the riverbeds determine the higher speed of the flow of the northwestern straits, then for the Danube, at first glance, such regularity in the “mirror” repetition contradicts the direction of Coriolis force. In this case, the explanation lies in the different volume of solid waste. The Danube Delta is characterized by huge volumes of suspended sediment, the deposition of which in the older Romanian part of the delta led to siltation and the inhibition of flow, which, in turn, forced the Danube to develop a gentler eastern bank [115].

Biogeographically, the Danube Delta is characterized by European, Eurasian, Pontic, Sarmatic, Irano-Turanian, Balkan, and Mediterranean biodiversity elements [113,115].

All these sea level rises will affect the state of the complex coastal and delta ecosystems. These processes will continue alongside global climate change, and their impact will depend on the synergic effects of factors controlling the deltaic and littoral processes, as well as on the delta plain and shoreline elevation, which can reveal possible environmental changes and flooding risk areas induced by climate changes and sea water rise. The marine delta plain is very low-lying, with shallow marine, euryhaline, and freshwater lakes, marshy areas, and low beach ridges that also create accumulative littoral bodies. The interdistributary and old inter-beach–ridge depressions lie a few centimeters above and sometimes below sea level, while the beach ridges are a few tens of centimeters to over one meter above sea level, with a few elevated, rather remote zones of the marine delta plain slightly above the above-mentioned accumulative bodies. Very rare islands such as small dune areas reach altitudes of over 11 m [29,64,97,98,99,106,116,117].

To all of these elements should be added the negative effects of anthropogenic stress factors, which have reduced the resilience and ecological status of local ecosystems on the Danube River and its tributaries: pollution; habitat fragmentation; mineral, aquatic, and biological resource over-exploitation; invasive species; etc. [118].

In the fluvial delta plain, features such as fluvial natural levees and some old lacustrine spits remain, with altitudes between a few tens of centimeters to over 400 cm above sea level [110,119]. Interdistributary depressions are described by numerous lakes and swamps of diverse size and depth; the water inflow/outflow into/from these depressions is almost continuous, and is realized through numerous natural channels and crevasses, artificial canals, and, during high waters, by the overflowing of natural levees [64,109].

In accordance with generally accepted present models, the most important modifications in the climate are the northward shift of climate zones, the lengthening of summer at the expense of other seasons, the changes of winter cyclonic patterns, etc. [110,120].

The models show that an increase in the mean temperature by 1.5 °C in these conditions will determine a decline of at least 10% in the river flow, and that this combined with a decrease in water energy caused by the rising of the base level will substantially lower the freshwater input into the sea [110,120].

Lower, but more unpredictable, precipitation will reduce the groundwater recharge and refresh, and will create an imbalance between fresh versus marine water. Storage in the reservoirs will fall as an effect of decreased river flow and increased evapo-transpiration [110,120].

In response to the forecast of the 2020–2030 period, during which the sea level will rise by 20–30 cm, the regression of beaches will, obviously, continue all along the north-western and western Black Sea coast. Despite a not-yet-critical amount of sea level rise, the impact on the shore zone will be strong enough because of the cumulative effect of the sea level rise, the wind set-up, the shortage of beach feeding by decreased river-borne sediment input (especially of the Danube River), and of course the human pressure on the coastal area [110,120].

The modification of the base energy level will reduce the water and the sediment discharge of the Danube River. Although a very rough climate change model of the impacts on the Danube River water and sediment discharge within the delta shows a decrease in Danube River water discharge and an influence of the salinity of the Black Sea, particularly in the present situation when the general sea level rises uninterruptedly, this encompasses a greater supply of saline Mediterranean water by the Bosphorus strait bottom current, and a decrease in the thickness of the less saline superficial layer of the Black Sea [99].

At the mouth zone of the Danube River tributaries, the penetration of the salt wedge deeper upstream into their course will generate a major disturbance in the transfer processes of bed-load to the mouth bar, and further to the littoral area. The reduced sediment input will induce a higher deficit in the sedimentary budget of the littoral area [109,120].

As regards the deltaic shore, a rise of sea level by 20–30 cm corresponds to an equivalent water rise of at least 3–4 hydro-degrees. This means that a large area of the delta would be flooded, and greater flood risks would be incurred over the entire delta territory [99].

The deltaic coast will be reshaped by marine processes, but in the more exposed Gârla Împuţita–Câsla Vădanei, Ciotic-Perişor, and Portiţa-Periboina areas, conditions will exist that will transform the intradeltaic depressions or lagoons into bays. Such risks are higher in the Gârla Împuţita–Câsla Vădanei, which links to the Roşu–Puiu–Lumina interdistributary depression, the Ciotic–Perişor section, and to a lesser extent the Portiţa–Periboina (corresponding to the lagoon complex Razim–Sinoie), where the beach barrier is protected by a set-back line of embankments, limiting losses of beach material by overwashing [99].

All of these synergic processes and phenomena are occurring presently, and in this lowland deltaic environment, land subsidence due to the compaction of unconsolidated sediments leads to an increase in flooding risk as floodplains subside. Consequently, the Danube Delta will be affected in a relatively short to average historical period of time. It should be mentioned that this category of change has also happened in the past, although perhaps over much longer time periods than the present one. It is clear that in the forecast trends of climate change, this 20–30 cm sea level rise scenario is only a first, minimal step of unfortunately longer and more accentuated expected phases of Black Sea level rise.

2.4. Potential General Consequences in Changing Danube Delta Microhabitats and Habitats Due to the Negative Potential Effects of Forecast Black Sea Level Rise

The first degree of importance of the Danube Delta’s exceptional value is based on the unique and complex matrix of habitats, with a mix of freshwater, eurihaline and salt water bodies, floodplains, and sand dunes [120,121,122].

About 80% of the Delta’s territory consists of areas that are permanently covered by water, 3% of the area is naturally dry, and the remaining 17% is temporarily flooded [123].

The main foundations of the area’s habitat diversity are the specific hydrological regime characteristics, which encompass fluctuations of water levels, water circulation in the inner Delta, and its residence time in the various water bodies. The duration, height, time period in the course of a year, and frequency of floods all play major roles in the area’s complex of habitats, and influence their distribution along ecological gradients and their biota. In some areas, the natural ecological gradients are overlain with human-induced changes that modify habitat conditions and alter biotic communities [115,116,120].

The complex framework and interrelated dynamic of salt, eurihaline, freshwater, nutrients, suspended solids, soils/sediments, pollutants, etc., generate and maintain the delta’s specific habitats and their accompanying biotal features, many of them being included in Annex I of the EU Habitats Directive or the annexes of the Bird Directive [122,124,125]. We see typical floodplain habitats along the main and secondary larger branches, smaller streams, characteristic lake habitats with surrounding reed vegetation, aquatic macrophyte communities in the open area, silting-up processes with growing reed beds and developing reeds, as well as sedge-peat deposits and lakes connected among them with first, second, or third category branches, small and narrow channels, and flood depressions, sometimes isolated within the reeds [122]. The water courses are bordered by natural river bank levees, with gallery-like floodplain forests on and above mean water level. The marine zone of the delta, and the transition area between the marine and fluvial, are characterized by beach barriers and fan-like sand dune complexes, with a multifaceted mosaic of different, specific habitats, adjacent to the sea and along the coast, where saline areas are present. In fact, this mosaic of habitats always changes, not only in the aspect but also in the way of being linked or more or less isolated, and the mode of functioning, the ecotone characteristics being more present here in space and time that in many other numerous complexes of ecosystems. The number and variety of plant species reveal the very high diversity of habitat and microhabitat conditions, which also characterizes almost all the main invertebrates and vertebrate taxa [120,126,127], with fish being one of the most diverse and abundant groups of vertebrates present [46,47,49].

The likely effects of sea level rise on the delta can be determined rationally [121]; the denial of reality is never a wise option, and this event should be prevented and mitigated based on integrated research approaches to highlight potential management measures.

As a significant factor, the historical factual reality should be stressed, namely, that humans have taken mostly reasonable advantage of the natural resources and services that thrive within this area, with its high diversity of habitats and biocoenoses [104,120].

In the 20th century planetary context in which dramatic various human impacts became significant, it is predictable that, in addition, both marine and fluvial processes influenced by climate modifications will induce further changes in the Danube Delta, their significance being correlated with the magnitude and speed of Black Sea level rise in the future, jeopardizing the opportunities to protect and sustain this natural region.

2.5. Potential Specific Negative Ecologic Consequences of Altering and Changing Danube Delta Habitats for Fish Fauna Species Presence/Absence, Distribution, and Status

The highly complex Danube River–Danube Delta–Black Sea geoecosystem has nurtured a dynamic, rich, and unique ichthyofauna. The extinctions of fish species in ancient times were related to evolutionary processes, while in recent times (since ca. 155 years ago), fish species extinctions have been related more to direct anthropogenic impacts [128].

The complex and balanced Danube Delta fish species fauna structure, with 139 species, representing over 70% of Romania’s (238.397 km²) ichthyofauna, includes three classes, 20 Orders, and 45 families; 44.8% are freshwater species, 14.8% are migratory freshwater/marine and/or brackish water species, and 40.4% are marine species [129]. These reveal the role of the structural and functional connectivity of the Danube Delta for the Danube River on the one hand and for the North West Black Sea on the other. The key role of the Danube Delta is evidenced by the fact that 57.26% of the Danube River–Danube Delta–Black Sea fish species require two or even three characteristic areas of those habitats, which provide conditions for shelter, reproduction, feeding, wintering, migration, colonization, recolonization, etc. The role of this delta as a complex and dynamic matrix of habitats offers the chance for Danube River and Black Sea fish to recover from an ecological point of view when one or both of them are negatively affected by external factors. All three of these ichthyosubsystems are part of the Lower Danube River–Danube Delta–North Western Black Sea ichthyosystem, which evolved in time and space interdependently, and which facilitated their flexibility and adaptation in an integrated way [2].

In the last century, compared to the data of Antipa and Bănărescu [46,47,104,130,131,132,133,134,135,136,137,138], the habitat heterogeneity, fish native diversity, and stock abundance of fish species of economic and conservation value of the Danube River–Danube Delta–Black Sea have notably decreased [49,55,139,140,141,142]. Currently, there is no significant indication that this trend will slow or cease in the near future, because the fish fauna and their habitats have over the long term been significantly altered, qualitatively and quantitatively, by a variety of human interventions, and new threats are imminent.

The questions that this review poses are as follows: Given the actual relative accelerated climate change events, will a too-rapid sea level rise in the Black Sea be the next trigger for complex synergic effects on habitats and fish fauna? Which of the present fish species already affected by different stressors will be harmed in the future? This review also suggests some potential risks for the fish species of the studied area.

2.5.1. Potential Specific Ecologic Negative Consequences of Altering and Changing Danube Delta Habitats for the First Category of Fish Species under Threat (Fish Species Presence/Absence, Distribution, and Status)

The first fish species category at risk under the scenario of relatively fast rising sea levels in the Black Sea, based on ecological characteristics and needs, include some species that live only in the Danube Delta, others for which optimum specific habitats are located in the Danube Delta, and those not present or with significantly less presence upstream and/or downstream of the Danube River in the Black Sea. These species are Carassius carassius (Linnaeus, 1758), Petroleuciscus borysthenicus (Kessler, 1859), Rutilus frisii (Nordmann, 1840), Cobitis tanaitica (Băcescu and Mayer, 1969), Cobitis megaspila (Nalbant, 1993), Pungitius platygaster (Kessler, 1895), Bentophiloides brauneri (Beling and Iljin, 1927), and Knipowitschia cameliae (Nalbant and Oţel, 1995). Among them, only P. borysthenicus has increased in abundance in the last century; Carassius c. and Pungitius p. have an accentuated decreasing trend in abundance; and the situation of R. frisii, Cobitis tanaitica, C. megaspila, Bentophiloides b., and Knipowitschia c. is worse, their presence nowadays being considered questionable or locally extinct [47,49,130,131,139,140,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166].

Carassius carassius (Linnaeus, 1758) (Actinopteri, Cypriniformes, Cyprinidae, Cyprininae), Crucian carp, a freshwater, brackish fish species, is a key part of the EU species conservation strategy (e.g., the Habitat Directive, the Water Framework Directive, and the EU Biodiversity Strategy for 2030) [130,131]. Adults are found in still, shallow ponds, lakes rich in vegetation, and the slow-moving parts of large rivers, and are usually restricted to densely vegetated backwaters and oxbows. Reproduction is in May–July in shallow water with dense vegetation. In spite of the fact that this species was considered resilient to diverse threats (anthropogenic and natural) [139,140], there is an accentuated gradual and continuing trend of population decrease in this species [47,143,144,145,146], mainly through numerous threats of anthropogenic origin [147,148,149]. Due to the fact that the causes of crucian carp decline across its range include this species’ ecological needs related to habitat characteristics linked to the consequent disappearance of floodplain lakes and backwaters [150,151,152], the flooding of the Danube Delta by the sea could drastically affect the Crucian carp, which were already in accelerated decline by 1960 in the studied area and elsewhere [153]. Here, it should be noted that a main problem for Carassius carassius is hybridization with Carassius gibelio (Prussian carp). In the potential context described, Carassius gibelio will have additional advantages in contrast to Carassius carassius, such as access to the closed water bodies present in the northern/Ukrainian part of the Danube Delta that are protected from spring floods by dams and where Carassius carassius can still live now [154].

Petroleuciscus borysthenicus (Kessler, 1859), (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), Dnieper chub, is a freshwater, brackish fish species, which prefers shallow areas with slow to no current, inhabiting lowland rivers, limans (lagoons or estuarine features of a type prominent on Black Sea coasts), lakes, deltas, and backwaters [155,156]. This fish prefers to stay in warm water, with temperatures up to 30–32 °C, on sand, sand-muddy, or muddy bottoms, and in shallow places with a slow current along the banks, in backwaters, in small lakes, and in similar calm water sites. It can tolerate only slightly brackish water and low oxygen concentrations [153,157,158]. Sharp declines in some populations have been registered due to habitat changes [159]; from this perspective, the possible flooding of the Danube Delta would obviously affect this fish species.

Rutilus frisii (Nordmann, 1840) (Actinopteri, Cypriniformes, Leuciscidae, Leucisninae), Black Sea roach, is a freshwater to brackish fish species [47,160] protected under the Bern Convention [49]. It has been registered periodically in the north/Ukrainian Chilia/Kiliya part of the Danube Delta, and only once [161] in other areas of the Danube Delta, and we can regard it as a species coming from the north-east, possibly from the Nistru River delta, and which in some particular conditions also entered into the Razim area. With such a restricted area of distribution and questionable long-term presence, any increases in sea level may induce its extinction due to the destruction of its present habitat in the studied area.

Cobitis tanaitica (Băcescu and Mayer, 1969) (Actinopteri, Cypriniformes, Cobitidae), spined loach, is a freshwater fish distributed in the Danube Delta, and any increase in the level of the Black Sea may induce its extinction due to the destruction of its present habitat.

Cobitis megaspila (Nalbant, 1993) (Actinopteri, Cypriniformes, Cobitidae), Danubian spined loach, is a lenitic or slow-flowing freshwater species of rather muddy substrates [49,153,162]. With a restricted area of distribution in the Danube Delta, any increased level of the sea may induce its extinction due to the destruction of its habitat.

Pungitius platygaster (Kessler, 1895) (Actinopteri, Performes/Gasterosteoidei, Gasterosteide), Ukrainian stickleback, is a fish species found in the study area mainly in freshwater, which appeared in some littoral lakes only after they became brackish/freshwater [47]; in other geographical areas, it also inhabits marine habitats [47,159,163]. Any rise in sea level may affect its populations in the Danube Delta due to the pressure of habitat changes.

Benthophiloides brauneri (Beling and Iljin, 1927) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), beardless tadpole goby, is a freshwater, brackish fish species, rare in the studied Lower Danube area, and sensitive to changes in habitat quality [47,153,164,165]. Increases in the level of the sea may affect the quality of its habitats and change their characteristics to affect the population of this species in the delta.

Knipowitschia cameliae (Nalbant and Oţel, 1995) (Actinopteri, Gobiiformes, Gobiidae, Gobionellinae), Danube Delta dwarf goby, has been found in the Danube Delta area only in brackish sea–delta littoral ponds [166]. For this endemic, critically endangered species [166], with such a restricted area of distribution [153], any increase in the level of the sea may induce its extinction due to the diminishing of its characteristic habitat quality, and even the destruction of its present suitable habitat.

It is clear for this first category of risk that if the Danube Delta habitats were to be affected by sea level rise, the species would be at high risk due to habitat loss, distribution and populations, even possible leading to local or total extinction.

2.5.2. Potential Specific Ecologic Negative Consequences of Altering and Changing Danube Delta Habitats for the Second Category of Fish Species under Threat (Fish Species Presence/Absence, Distribution, and Status)

The second category of risk related to fish species, based on their ecological features and needs in a Black Sea level rise scenario, include the fish living in the Danube Delta and Upstream Danube River, and not or only accidentally present or rare in the near Black Sea. These species are Acipenser güldenstaedti (Brandt and Ratzeburg, 1833), Acipenser stellatus (Linnaeus, 1758), Acipenser ruthenus (Linnaeus, 1758), Huso huso (Linnaeus, 1758), Esox lucius (Linnaeus 1758), Abramis ballerus (Linnaeus, 1758), Abramis sapa (Pallas 1814), Vimba vimba (Linnaeus, 1758), Alburnus alburnus (Linnaeus, 1758), Barbus barbus (Linnaeus, 1758), Leuciscus aspius (Linnaeus, 1758), Cyprinus carpio (Linnaeus, 1758), Chondrostoma nasus (Linnaeus, 1758), Hypophthalmichthys molitrix (Valenciennes, 1844), Hypophthalmichthys nobilis (Richardson, 1845), Ctenopharyngodon idella (Valenciennes, 1844), Pseudorasbora parva (Temminck and Schlegel, 1846), Romanogobio albipinnatus vladykovi (Fang 1943), Romanogobio antipai (Bănărescu 1953), Leucaspius delineatus (Heckel, 1843), Squalius cephalus (Linnaeus, 1758), Leuciscus idus (Linnaeus, 1758), Rutilus rutilus (Linnaeus, 1758), Scardinius erythrophthalmus (Linnaeus, 1758), Pelecus cultratus (Linnaeus, 1758), Rhodeus amarus (Bloch, 1782), Tinca tinca (Linnaeus, 1758), Cobitis elongatoides (Băcescu and Mayer, 1969), Sabanejewia bulgarica (Drensky, 1928), Misgurnus fossilis (Linnaeus, 1758), Silurus glanis Linnaeus, 1758, Ameiurus melas (Rafinesque 1820), Ameiurus nebulosus (Lesueur 1819), Lota lota (Linnaeus, 1758), Sander volgensis (Gmelin, 1788); Gymnocephalus baloni (Holcik and Hensel 1974), Gymnocephalus cernua (Linnaeus, 1758), Gymnocephalus schraetser (Linnaeus, 1758), Percarina demidoffi (Nordmenn 1840), Zingel streber (Siebold, 1863), Zingel zingel (Linnaeus, 1766), Benthophilus stellatus (Sauvage, 1874), Knipowitschia caucasica (Berg, 1916), Babka gymnotrachelus (Kessler, 1857), Ponticola kessleri (Günther, 1861), Lepomis gibbosus (Linnaeus, 1758), and Umbra krameri (Walbaun, 1792). Among them, only Esox lucius, Scardinius erythrophtalmus, Rutilus rutilus, Alburnus alburnus, Hypophthalmichthys molitrix, Hypophthalmichthys nobilis, Ctenopharyngodon idella, Pseudorasbora parva, Pelecus cultratus, and Lepomis gibbosus have seen an increase in distribution and abundance, and some of them are invasive in the upper Danube lately; Acipenser güldenstaedti, Acipenser stellatus, Acipenser ruthenus, Huso huso, Abramis ballerus, Abramis sapa, Vimba vimba, Leuciscus aspius, Cyprinus carpio, Romanogobio albipinnatus vladykovi, Romanogobio antipai, Squalius cephalus, Leuciscus idus, Tinca tinca, Lota lota, Gasterosteus aculeatus, and Zingel streber have shown a falling trend in distribution and abundance; and the situation of Barbus barbus, Chondrostoma nasus, Leucaspius delineatus, Cobitis elongatoides, Sabanejewia bulgarica, Sander volgensis, Gymnocephalus baloni, Gymnocephalus schraetser, Benthophilus stellatus, Babka gymnotrachelus, and Ponticola kessleri is worse, with their questionable presence, determined as locally extinct, or there being no data available [47,49,148,153,159,167,168,169,170,171,172,173,174,175,176].

Acipenser güldenstaedti (Brandt and Ratzeburg, 1833) (Chondrostei, Acipenseriformes, Acipenseridae, Acipenserinae), Danube sturgeon, is a marine, freshwater, brackish species [167] that has shown an accentuated decreasing trend in abundance in recent decades [49]. In the case of the delta being flooded by the sea, the shrinking or disappearance of this salt–freshwater–salt transitional zone during the migration period will be an important issue for the species, given that this species is Critically Endangered, being under the protection of the Habitats Directive, CITES, CMS, IUCN Red List, etc.

Acipenser stellatus (Pallas, 1771) (Chondrostei, Acipenseriformes, Acipenseridae, Acipenserinae), Starry sturgeon, is a marine, freshwater, brackish species [167] with a high decreasing trend in abundance in recent decades [49]. In a case of the delta flooding by the sea, the shrinking or disappearance of this salt–freshwater–salt transitional zone during the migration period will be an important issue for the species, given that this species is Critically Endangered, being under the protection of the Bern Convention, Habitats Directive, CITES, CMS, IUCN Red List, etc.

Acipenser ruthenus (Linnaeus, 1758) (Actinopteri, Acipenseriformes, Acipenseridae, Acipenserinae), sterlet sturgeon, is a vulnerable [168], freshwater, brackish, demersal species [168,169], protected under the Bern Convention, Habitats Directive, CITES and CMS, with an accentuated deterioration and shrinking area of distribution in the lower Danube basin [153]. The decrease in Danube Delta habitats due to a rise in the level of the sea may induce a continuous decrease in its population size and distribution.

Huso huso (Linnaeus, 1758) (Chondrostei, Acipenseriformes, Acipenseridae, Acipenserinae), beluga sturgeon, is a marine, freshwater, brackish species [167] that has shown an accentuated decreasing trend in abundance in recent decades [49]. In the case of the delta being flooded by the sea, the shrinking or disappearance of this salt–freshwater–salt transitional zone during the migration period will be an important issue for the species given that this species is Vulnerable, being under the protection of the Bern Convention, Habitats Directive, CITES, CMS, IUCN Red List, etc.

Esox lucius (Linnaeus, 1758) (Esociformes, Esocidae), pike, is a freshwater to brackish species [169], with a decreasing trend of biomass seen in recent decades [49]. In a case of the Danube Delta flooding by the Black Sea, only the local delta population would suffer, which would not be a problem at the regional distribution level for this species (a common fish in the Danube Basin’s large and medium rivers and lakes, and even some other smaller water bodies) [47].

Abramis ballerus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), blue bream, is a freshwater, brackish species [169], protected under the Bern Convention, accidentally found in the Danube Delta [49] where specific habitat disappearance due to sea level rise would have a relatively low impact on this species’ area of distribution.

Abramis sapa (Pallas, 1814) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), white-eye bream, is a freshwater, brackish species [170] protected under the Bern Convention. It has shown a decreasing trend in its area of distribution [49], and the disappearance of its Danube Delta habitats could have a significant impact on its range.

Vimba vimba (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), vimba bream, is a freshwater, brackish species [168] protected under the Bern Convention. With a decreasing trend in the Danube Delta area [49], the diminishing or disappearance of the delta᾿s habitats could further negatively influence its range in the Danubian area.

Alburnus alburnus (Linnaeus, 1758) (Cypriniformes, Leuciscidae, Leuciscinae), bleak, is a freshwater, brackish species [167]. Due to the fact that it is an abundant species in the Danube Basin, and in general over its distribution range [47,49], in the event of the Danube Delta being flooded by the Black Sea, only the local delta population would be affected, and flooding would not a problem for this species as a whole.

Barbus barbus (Linnaeus, 1758) (Cypriniformes, Cyprinidae, Barbinae), barbel, is a freshwater species [167]. Due to the fact that it is an abundant species in large rivers of the Danube Basin and generally over its distribution range [47,49], with a sporadic presence in the Danube Delta, the flooding of this area by the Black Sea would affect only the local delta population, which is not a problem for this species in general.

Leuciscus aspius (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae, asp), is a freshwater, brackish species protected by the Bern Convention, Habitats Directive, IUCN Red List. [167]. It is one of the common raptor fish species in the Danube River and its deltas [49]; sea level rise would affect only the local populations.

Cyprinus carpio (Linnaeus, 1758) (Actinopteri, Cypriniformes, Cyprinidae, Cyprininae) [171], common carp, is a freshwater, brackish species [167] that is relatively abundant but has lately shown a trend of relative general reduction in the study area; sea level rise would affect only the local populations of the delta.

Chondrostoma nasus (Linnaeus, 1758) (Cypriniformes, Leuciscidae, Leuciscinae), common nase, is a freshwater species [167]. Due to it being quite widespread in the large rivers of the Danube Basin, and in general within its distribution range [47,49], with a rare presence in the Danube Delta, this area’s flooding by the Black Sea would eliminate only the local delta population, which is not a problem for this species as a whole.

Hypophthalmichthys molitrix (Valenciennes, 1844) (Cypriniformes, Xenocyprididae), silver carp, is a freshwater, brackish species [167]. Due to its presence in the majority of large rivers of the Danube Basin [49], with a rare presence in the Danube Delta, this area’s flooding by the Black Sea would significantly affect only the local delta population, which is not a problem for this species in general.

Hypophthalmichthys nobilis (Richardson, 1845) (Cypriniformes, Xenocyprididae), bighead carp, is a freshwater, brackish species [167]. It is fairly rare in the Danube Basin, including in the delta; if the delta populations were to be affected, given that its natural reproduction was not proven here until recently [49], a regional impact on the Danube River can be considered, along with the elimination of the delta populations.

Ctenopharyngodon idella (Valenciennes, 1844) (Cypriniformes, Xenocyprididae), grass carp, is a freshwater, brackish species [167]. It is relatively rare in the Danube Basin, including in the Danube Delta, and if the delta’s populations were to be affected, in the context in which its natural reproduction was not proven here until recently [49], a regional impact on the Danube River can be considered, as well as the elimination of the delta populations.

Pseudorasbora parva (Temminck and Schlegel, 1846) (Cypriniformes, Gobionidae), topmouth gudgeon, is a freshwater, brackish, benthopelagic species [172] that is rather abundant in the Danube Basin [49], including the Danube Delta. Its populations would be considerably affected by sea flooding, but would not impact the species as a whole.

Romanogobio albipinnatus vladykovi (Fang, 1943) (Actinopteri, Cypriniformes, Gobionidae), white-finned gudgeon, is a freshwater species [159,173] protected under the Bern Convention and Habitats Directive. In comparison with other areas of its geographical range, this species is frequent and abundant in the Danube Delta [49]; the diminution or disappearance of delta habitats could negatively influence this species.

Romanogobio antipai (Bănărescu 1953) (Actinopteri, Cypriniformes, Gobionidae) [174], Danube Delta gudgeon, is a freshwater species [159] protected under the Bern Convention, Habitats Directive, IUCN Red List, etc. Its presence in the Danube Delta is very scarce, with an accentuated decreasing trend in recent years [49], and any habitat modification induced by sea level rise could eliminate this species from the Danube Delta area, to which it is endemic.

Leucaspius delineatus (Heckel, 1843), (Actinopteri, Cypriniformes, Leuciscidae), sunbleak, is a freshwater, brackish species [167], protected under the Bern Convention. It is present in the numerous stagnant lowland and hilly water bodies, is frequent in some parts of the Danube Delta, and any habitat modification induced by sea level rise could eliminate this species from the Danube Delta area.

Squalius cephalus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae), chub, a freshwater, brackish species [169], occurs in low numbers in the Danube Delta [49]; any drastic habitat changes following sea level rise could exclude the species from this area.

Leuciscus idus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), orfe, is a freshwater, brackish species [175]. Its presence in the Danube Delta is sporadic [49], and habitat modification induced by sea level rise could eliminate this species from the Danube Delta area.

Rutilus rutilus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), roach, is a freshwater, brackish species [169], and is one of the most common species in the Danube Delta [49]; the disappearance of delta habitats would eliminate a high biomass of this fish.

Tinca tinca (Linnaeus, 1758) (Actinopteri, Cypriniformes, Tincidae), tench, has been found in the lower Danube Basin in recent years [176], and any habitat modification induced by sea level rise could affect the distribution of this species in the Danube Delta.

Lota lota (Linnaeus, 1758) (Actinopteri, Gadiformes, Lotidae), burbot, is a freshwater, brackish species [159]. Its presence has decreased in the lower Danube Basin in recent years [49], and any habitat modification induced by sea level rise could affect this species’ distribution in the Danube Delta.

Gasterosteus aculeatus (Linnaeus, 1758) (Actinopteri, Perciformes/Gasterosteoidei, Gasterosteidae) is a marine, freshwater, brackish species [169]. In the past, this fish was known all along the Black Sea coast in the Danube Delta area, but it is now known only in the freshwater and brackish water habitats of the Delta [49]; sea level rise could affect this species’ distribution and abundance in the study area.

Zingel streber (Siebold, 1863) (Actinopteri, Perciformes/Percoidei, Percidae, Luciopercinae), streber, is a freshwater species [170] very rarely found in the Danube [49]; sea level rise could affect the distribution and abundance of this species in the study area.

Knipowitschia caucasica (Berg, 1916) (Actinopteri, Gobiiformes, Gobiidae, Gobionellinae), Caucasian dwarf goby, is a marine, freshwater, brackish fish [172]. It has vanished in the lower Danube River area, and its relatively high frequency and abundance in the Danube Delta [49] could be significantly affected by any sea level rise.

Scardinius erythrophthalmus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), rudd, is a freshwater, brackish species [169] common in the Danube Delta [167]; the loss of delta habitats would eliminate a high biomass of this fish.

If the specific Danube Delta habitats were to be significantly affected by sea level rise, this second category of fish species would be at a low/medium ecological risk of decreased population abundance and dispersion.

2.5.3. Potential Specific Ecologic Negative Consequences of Altering and Changing Danube Delta Habitats for the Third Category of Fish Species under Threat (Fish Species Presence/Absence, Distribution, and Status)

The third category of risk of fish species, based on their ecological characteristics and needs, in a relatively fast Black Sea level rise scenario, includes fish living in the Danube Delta and Black Sea and not or only accidentally in the upstream Danube River. These species are Syngnathus schmidti (Popov, 1927), Syngnathus tenuirostris (Rathke, 1837), Syngnathus variegatus (Pallas, 1814), Syngnathus typhle (Linnaeus, 1758), Mesogobius batrachocephalus (Pallas, 1814), Ponticola eurycephalus (Kessler, 1874), Ponticola syrman (Nordmann, 1840), Pomatoschistus marmoratus (Risso, 1810), Zosterisessor ophiocephalus (Pallas, 1814), Aidablennius sphinx (Valenciennes, 1836), Parablennius sanguinolentus (Pallas, 1814), Parablennius tentacularis (Brűnnich, 1768), Callionymus risso (Lesueur, 1814), Chelon auratus (Risso, 1810), Chelon saliens (Risso, 1810), Planiliza haematocheilus (Temminck and Schlegel, 1845), Mugil cephalus (Linnaeus, 1758), Pomatomus saltatrix (Linnaeus, 1766), Sciaena umbra (Linnaeus 1758), Umbrina cirrosa (Linnaeus, 1758), Scomber scombrus (Linnaeus, 1758), Atherina boyeri (Risso, 1810), Salmo labrax (Pallas, 1814), Gasterosteus aculeatus (Linnaeus, 1758), Trachurus mediterraneus (Aleev, 1956), Mullus barbatus (Essipov, 1927), Dicentrarchus labrax (Linnaeus, 1758), A. boyeri (Risso, 1810), Atherina hepsetus (Linnaeus, 1758), Platichthys flesus luscus (Pallas, 1814), and Anguilla anguilla (Linnaeus, 1758). Among them, only Parablennius sanguinolentus, P. tentacularis, Ponticola syrman, and Planiliza haematocheilus have seen increases in abundance and distribution in the last century; Ponticola eurycephalus, Pomatoschistus marmoratus, Zosterisessor ophiocephalus, Aidablennius sphinx, Callionymus risso, Chelon auratus, Chelon saliens, Mugil cephalus, Pomatomus saltatrix, Scomber scombrus, Trachurus mediterraneus, Mullus barbatus, A. boyeri, Salmo labrax, and Platichthys flesus luscus have experienced a decreasing trend of abundance and distribution; and the situation for Syngnathus schmidti, Syngnathus tenuirostris, S. variegates, S. typhle, Mesogobius batrachocephalus, Sciaena umbra, Umbrina cirrosa, Dicentrarchus labrax, A. hepsetus, Anguilla anguilla is worse, their presence being considered as questionable or locally extinct at present.

Ponticola eurycephalus (Kessler, 1874) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), mushroom goby, is a marine, brackish species, which in freshwater is found only in the Danube Delta [49,156,169]. Sea level rise could affect these unique freshwater populations by destroying their specific habitats.

Pomatoschistus marmoratus (Risso, 1810) (Actinopteri, Gobiiformes, Gobiidae, Gobiionellinae), marbled goby, is a marine, brackish species [177] that can be found inshore, over sand, and enters mainly brackish and hyper-saline waters [178]; sea level rise would not affect it.

Zosterisessor ophiocephalus (Pallas, 1814) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), grass goby, is a marine, brackish species occasionally recorded in freshwater [165,169,179]; sea level rise would not affect this species within the study area.

Aidablennius sphinx (Valenciennes, 1836) (Actinopteri, Blenniiformes, Bleniidae, Salariinae) is a marine species found in the shallow, rocky, litoral zone [180]; sea level rise would not affect this species within the study area.

Callionymus risso (Lesueur, 1814) (Actinopteri, Callionymiformes, Callionymidae) is a marine species found on sandy bottoms in shallow coastal water [181]; sea level rise would not affect this species.

Chelon auratus (Risso, 1810) (Actinopteri, Mugiliformes, Mugilidae), golden grey mullet, is a marine, freshwater, brackish species [182]. It can be found only sporadically in non-marine habitats, so sea level rise would not affect this local fish population.

Chelon saliens (Risso, 1810) (Actinopteri, Mugiliformes, Mugilidae), leaping mullet, is a marine, brackish species living in coastal waters [182]. In recent years it has been found only in marine habitats [49], so sea level rise would not affect this local fish population.

Mugil cephalus (Linnaeus, 1758) (Actinopteri, Mugiliformes, Mugilidae), flathead grey mullet, is a marine, freshwater, brackish species, the adults of which are found in coastal waters [183,184]. In recent years, due to the reduction in brackish habitats, it has been found only in marine habitats [49], so sea level rise would not affect this local fish population.

Pomatomus saltatrix (Linnaeus, 1766) (Actinopteri, Scombriformes, Pomatomidae), bluefish, is a vulnerable [166], marine, brackish species [169,185]. Sea level rise would not affect this local fish population.

Scomber scombrus (Linnaeus, 1758) (Actinopteri, Scombriformes, Scombridae, Scombrinae), mackerel, is a marine, brackish species [169,186]. Due to the fact that it has not been found in recent years in the delta [49], sea level rise would not affect this fish.

Syngnathus schmidti (Popov, 1928) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), Schmidt’s pipefish, is a brackish species [187,188]. Due to the fact that it has not been found in recent years in the delta [49], sea level rise would not affect it.

Syngnathus tenuirostris (Rathke, 1837) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), narrow-snouted pipefish, is a marine species [188]. Due to the fact that it has been found extremely rarely in recent years in the Danube Delta area (49), sea level rise would not affect this fish.

Syngnathus variegatus (Pallas, 1814) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), thick-snouted pipefish, is a marine species [188]. It has not been found in recent years in the Danube Delta [49], so sea level rise would not affect this fish.

Syngnathus typhle (Linnaeus, 1758) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), broad-nosed pipefish, is a marine, brackish species [189]. It has been found extremely rarely in recent years in the Danube Delta area [49], so sea level rise would not affect this fish.

Mesogobius batrachocephalus (Pallas, 1814) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), toad goby, is a brackish species [177]. Due to the fact that it has not been found in recent years in the Danube Delta [49], sea level rise would not affect this fish.

Umbrina cirrosa (Linnaeus, 1758) (Actinopteri, Eupercaria, Sciaenidae), shi drum, is a vulnerable [174], marine species [178]. Due to the fact that it has not been found in recent years in the Danube Delta [49], sea level rise would not affect this fish.

Atherina boyeri (Risso, 1810) (Actinopteri, Atheriniformes, Atherinidae, Atherininae), sand smelt, is a marine, freshwater, brackish species [178]. Sea level rise would affect this fish locally.

If the specific Danube Delta habitats were to be affected by sea level rise, this third category of species would be at low or no ecological risk of a reduction in population abundance and distribution.

3. Discussion

There are no studies that define tailor-made strategies for easing ecological conditions in the Danube Delta under the latent influence of sea level rise in the Black Sea. This study tries to fill a part of this knowledge gap by describing the in situ risks in relation to a large variety of fish species of conservation and economic importance. In addition, the fish species that would not be affected by this potential event are highlighted.

In the urgent environmental ethics context, based on the authors’ published and unpublished data and on the specific reliable and available scientific information, the present work aims to highlight potential sea level change in the Black Sea within the present climate change scenario and link it with the significant potential change in the Danube Delta fish fauna, as the main objectives of the work.

Before the Second World War, the Danube Delta’s populations of wild fish were significantly greater than they are today [46,47]. High fish abundance requires a set of habitat characteristics that are the direct inverse of those pertaining under the 45 years of the communist regime and the relatively chaotic return in subsequent years to a market economy, with a lack of understanding or ignorance of the sustainable use of resources and development. As a consequence, fish have suffered significant losses and even extirpation.

In this relatively short-term historical context, to date, there have only been a few examples of the successful partial restoration of Danube Delta fish populations in the context in which we have an “overnight” skewed ichthyological structure, with particular species already in steep decline and some even extinct.

On an evolutionary scale, however, fish have withstood epic cataclysms before; indeed, the exceptionally broad stock of genes in fish buffers them against dramatic contractions of population range. In the long geological period of time that the Danube Delta has existed, various ice ages, periods of warming climate, sea transgressions and regressions, etc., have influenced the Delta’s fish fauna evolution. However, after each contraction, the richness of the fish genetic material has allowed populations to opportunistically invade new habitats when these emerge. What makes the contemporary direct and indirect human-influenced delta fish crisis unique and alarming is the effect humans are having on the genome of fish, simultaneously throughout their frequency, abundance, and distribution [2].

In spite of the fact that scientific knowledge cannot provide hard numbers regarding how fast and to what level the sea will rise, the increasing trend is obvious, and certainly it is not a matter of if, but of when and how high.

After years of powerlessness, or of groups of interest watching some fish species populations implode, with some fish species even facing extinction, the maintenance of fish diversity is critical; the more sub-populations survive and thrive, the richer the overall fish species genome will be, and the more adaptable and elastic the populations will remain in the event of the impending crisis.

Fisheries managers will permit “subsistence openings” for limited periods of time, in reduced fishing areas, during which periods local traditional fisherman can catch some fish for their personal consumption. For the species under risk, these fish have to be readily identifiable as subsistence catches and not for sale. Only once the number of restricted fish in the Danube Delta exceeds both the escapement and subsistence goals should authorities allow a “commercial controlled opening”. Furthermore, when a commercial opening takes place, the fisherman can sell what they have caught. If the fisherman crosses the line they should be fined, and if they do it multiple times, they should receive demerits on their fishing license; should this go on too many times, the administration could revoke their license and take their boat.

The potential new upstream wetlands accretion rate is estimated to not be accelerated due to significant upstream dam retention, mineral overexploitation, or the cleaning of the riverbed for the purposes of navigation throughout the Danube Basin. The Danube Delta fish species with conservation as well as direct and indirect economic importance and interest can be preserved if new upstream wetlands are made available simultaneously with the rise in the Black Sea’s level in the present Danube Delta area. However, are we indeed ready to allow all the old wetlands that in the past belonged to the Danube River’s annually flood area to renaturalize from present land agricultural use, and replace at least partially the lost benefits of the potentially disappearing Danube Delta? Of course, in light of the speed of the delta sinking process, the speed at which human adaptation reacts with intelligent and sustainable management measures will matter.

Constant restocking with the fish species of interest so as to counterbalance the careless reversal of nature’s balance by aquaculture is needed, focusing also on caring for genetic diversity. Too often, restocking is conducted without any coherent genetically based selection; many useful genes may have been lost, but the genetic potential remains high, and can be preserved for future use and adaptation. Because fish can produce many thousands of offspring in the course of their lives, once favorable individuals are found, only a few matriarchs and patriarchs could form the basis of a whole new population or even race of highly valuable fish, so high-value adaptable populations could still be created.

The fish populations of the large, composite Danube River, Danube Delta, and Black Sea region have their own distinctive challenges to adapt to, namely, the end to end distance of the Danube River, the complexity of the Danube Delta, and the volume of the Black Sea, to name just few, with different geographical, physico-chemical, biological, and ecological factors, a situation in light of which the loss of the Danube Delta’s biogeographical and ecological transitional role would be incalculable.

It must be added that local traditional fishing communities have to bear more and more of the direct and indirect risks of rising sea levels, e.g., flood water-borne bacteria from household aseptic systems in the surrounding water (potentially creating a public hazard), the loss of certain economically important fish species and/or a decline in the quality of the fish itself, and changes to the water levels in the delta, which are at just a few tens of centimeters or mere meters, increases in which will cause flooding in spite of any efforts to delay it through expensive critical infrastructure elevation, etc. The local people, balancing the needs of children, elderly parents, economic and medical problems, the adjacent Ukraine–Russia war zone, and life troubles in general, have little time to worry about the future, and local, regional, and when national authorities react positively, it is usually only post factum. The decisions of local people about where and how they will live will certainly drastically change in the pessimistic “no prediction” and “proactive” scenarios.

The Danube Delta and riverine areas must be prepared through the adoption of sustainable measures preparing for higher Black Sea levels in natural, economic, and social contexts, but also to face the impacts the Danube River will have on its delta in more frequent low water level situations.

It is obvious that for different climate change scenarios, risk response strategies should be designed to face the management challenges related to this topic, which should result in vivid debates among interdisciplinary and integrated permanent working teams comprising scientists and practitioners, but this should start intensively as soon as possible, and an ongoing debate process should be permanently upheld and officially sustained.

Following scientific research and debates, rational management decisions should be made by local people in later stages, with bottom-up approaches being essential for the planning and design of risk-mitigating measures.

Even if safety against floods or droughts has recently become a topic of concern for scientists, administrations, and populations, safety measures against the effects of sea level rise may still sound somewhat improbable, or like something that will not affect the current generation. However, they are essential.

The bio-, eco-, and socio-economic consequences for the Danube River’s inland hydrographic web and the Black Sea’s coastal fisheries, as well as river navigation, salt water intrusion, sediment deficit, biodiversity loss, water quality, aquaculture, etc., must be considered. All these potential habitat changes could have devastating effects on coastal habitats further inland, leading to the flooding of delta wetlands, the salt contamination of aquifers and agriculture soils, and finally and most obviously, drastic effects on fish habitats, populations, and species.

Nevertheless, supplementary applied research is necessary to validate the theoretical data so as to raise awareness, and encourage prognoses and proposals, in numerous relevant aspects, such as the qualitative and quantitative capacities of the conservative and economic short- to medium-term adaptations of fish species, and mitigating the effects of sea level rise and subsidence, as well as the likelihood of any new conditions to encourage spread of invasive species.

4. Conclusions

Sea level rise is becoming a hazard and a threat to the Danube Delta, especially because of its potentially accelerating rate. It is likely that we will see a high and accelerating sea level rise in this decade of 20–30 cm, increasing in the following decades up to 2 m, and reaching up to 5 m in the decades after that. We expect sea level rise to impact the Black Sea coastal ecosystems, as there is enough water locked in ice to raise sea levels by 80 m if it all melted.

As regards the deltaic shores, a short-term rise of the sea level by 20–30 cm would induce significant negative effects, such as a greatly extended area of the delta near-shore zone that is flooded, and also greater occasional or permanent flood risks over the entire delta territory. The impact on the littoral zone would be strong because of the cumulative effects of sea level rise, wind intensity, the shortage of beach feeding by decreasing river-borne sediment input (especially of the Danube River), and of course the variable and high anthropogenic pressures exerted on the coast area.

In the short to medium term, it is possible that the present delta area that is below sea level, areas a few or tens of centimeters above sea level, and the few elevated isolated island-like zones of the marine delta plain that reach 11 m will be affected by potential floods of 2–5 m to 80 m.

The low-altitude state of the fluvial delta plain is more serious, as it is typically very low-lying except for certain more elevated features, such as fluvial natural levees and old lacustrine spits, these varying in altitude from a few tens of centimeters to over 4 m above sea level.

The climate change-induced sea level rise will involve a risk of inland hydrographical web and coastal regression processes; the salinization of Danube Delta habitats; and the loss of agricultural land, aquaculture, and protected natural areas, impacting river navigation, tourism, particular localities, specific valuable ecosystems, and habitats. As a consequence, changes of fish community structures will occur, and some of them will even disappear. Further specific holistic and integrated studies are needed to quantify the potential short-, medium-, and long-term bio-, eco-, and socio-economic threats, risks, and impacts.

The present review highlights the fact that the category of fish species that would be at relatively high risk in a Black Sea level rise scenario includes nine fish species: Huso huso (Linnaeus, 1758), Carassius carassius (Linnaeus, 1758), Petroleuciscus borysyhenicus (Kessler, 1859), Rutilus frisii (Nordmann, 1840), Cobitis tanaitica (Băcescu and Mayer, 1969), Cobitis megaspila (Nalbant, 1993), Pungitius platygaster (Kessler, 1895), Bentophiloides brauneri (Beling and Iljin, 1927), and Knipwitschia cameliae (Nalbant and Oţel, 1995). Consequently, in this first category of fish species, if the specific Danube Delta habitats were to be significantly affected by the sea level rise, they would face a high local or regional ecological risk of population diminution, or even local or total extinction.

The category of medium risk to fish species at local or regional levels includes 14 species: Acipenser ruthenus (Linnaeus, 1758), Abramis sapa (Pallas, 1814), Vimba vimba (Linnaeus, 1758), Romanogobio albipinatus vladykovi (Fang, 1943), Romanogobio antipai (Bănărescu, 1953), Squalius cephalus (Linnaeus, 1758), Leuciscus idus (Linnaeus, 1758), Rutilus rutilus (Linnaeus, 1758), Tinca tinca (Linnaeus, 1758), Lota lota (Linnaeus, 1758), Gasterosteus aculeatus (Linnaeus, 1758), Zingel streber (Siebold, 1863), Knipowitschia caucasica (Berg, 1916), and Scardinius erythrophthalmus (Linnaeus, 1758). Consequently, if the specific Danube Delta habitats were to be significantly affected by sea level rise, this second category of species cases would be at low/medium ecological risk of a decrease in population abundance and distribution.

The third category of fish species at significant risk at the local or regional levels includes 16 species. In this third category are included Ponticola eurycephalus (Kessler, 1874), Pomatoschistus marmoratus (Risso, 1810), Zosterisessor ophiocephalus (Pallas, 1814), Aidablennius sphinx (Valenciennes, 1836), Callionymus risso (Lesueur, 1814), Chelon auratus (Risso, 1810), Chelon saliens (Risso, 1810), Mugil cephalus (Linnaeus, 1758), Pomatomus saltatrix (Linnaeus, 1766), Scomber scombrus (Linnaeus, 1758), Syngnathus schmidti (Popov, 1928), Syngnathus tenuirostris (Rathke, 1837), Syngnathus variegatus (Pallas, 1814), Syngnathus typhle (Linnaeus, 1758), Mesogobius batrachocephalus (Pallas, 1814), Umbrina cirrosa (Linnaeus, 1758), and Atherina boyeri (Risso, 1810).

The changes in Danube Delta habitats can be expected to favor more the species that are unimportant from a conservation and economic point of view, and to favor more the alien and invasive species, which will put extra ecological pressure on the native species.

Black Sea level rise is considered one of the major stressors on Danube Delta fish fauna in the past, and is likely to be so in the future. The large degree of uncertainty involved when considering these climate change–sea level rise–ichthyofaunal-related projections, and how they relate to local and regional ecosystems and the safety of human society, serves as a starting point to consider the Danube Delta as a robust transitional foundation for the whole Danube–northwest Black Sea macro region. Any management policy will require shorter- and longer-term actions for different scenarios (i.e., massive losses of habitat characteristic of the delta, delta disappearance, upstream migration of the delta, etc.) and should be based on the constant exploration of adaptive pathways, which help in managing this uncertainty, involving a range of appropriate actions and measures for nature conservation in a sustainable human context.

The current work compiles for the first time prospective knowledge about the potential impact on Danube Delta–Danube River–Black Sea coastal fish diversity in a potential climate change-induced sea level rise scenario.

In summary, while trying to not be too speculative regarding this complex ecosystem-changing situation, we can see that rapid environmental change will have generally negative impacts on most of the species, but not on all of them. Surely, the impact of sea level rise will not be singular, and will unfold in synergy with other natural and anthropogenic stressors. Due to fish structural changes, which will affect interspecific relations, including those with newly arriving alien and invasive fish, different species will take advantage of the newly created situation. Only up-to-date, in situ, and adapted future monitoring programs will be able to register these first intimate changes, which will be the basis for the future establishment of ichthyofaunal structures and relations. These major changes that are currently in progress will certainly bring about very interesting developments and discoveries in terms of regional biological and ecological fish fauna characteristics.

In order to better understand and manage this issue, the following related stressors should be focused on in future, more detailed ecological assessment and monitoring programs: climate changes, Black Sea level rise, the Danube Delta area’s land characteristics and use changes, variations in the qualitative and quantitative phisico-chemical characteristics of water, habitats’ structural and functional characteristic variations, the presence of indigenous and alien invasive species, absence and status, the structural dynamics of local and regional organisms’ associations and communities, etc.