**1. Introduction and Background**

Water, the fundamental factor of initiation, persistence, and evolution of life on Earth is ever-present; it influences everything and is a key to comprehending the universe in general [1,2], including the biodiversity structure, distribution, and ecological state [3–11]. In this framework, the relevance of small water bodies for biodiversity and ecosystem services is not negligible [12,13].

Climate change is one of the most known crises to all-encompassing environmental, economic, social, and human health conditions [14–17] modern human have ever challenged [18]. The simulations conducted using global climate models uncover that the most important factors that induce this planetary phenomena are natural (variations in solar radiation, volcanic activity, and aerosol concentrations) and anthropogenic (fluctuation in the content of the atmosphere due to human actions); only the amassing effect of the

two factors can provide a reason for the transformations noticed in the world average temperature in the last century and a half [19,20], despite the fact that this is only a small part of the extent of fluctuations in climatic parameters, even including the planet ocean level [21].

It is accepted that inland water conditions are closely connected with weather and atmospheric temperature fluctuations, so climate change may have forceful direct and indirect effects on freshwater biota [22].

The most recent IPCC Climate Report, "Code Red for Humanity", emphasizes undeniable proofs accompanying the reality that warming has sped up in recent decades; the planet warming is affecting all regions on Earth, and additional heating is anticipated for the next century. Many of the modifications becoming irreversible climate impacts will certainly exacerbate [23]. Another effect of climate change is the alteration of hydrologic cycles; with rising intensity and frequency of extreme events such as droughts, this scenario could influence freshwater biota, generating changes in phenology, life cycles, and dispersion areas, and even the extinction of sensitive species [24]. Accelerated climate change is estimated to influence the biodiversity of huge areas forcefully, with changes in the presence and distribution of numerous species, the decline of the taxonomic richness, and the vanishing of entire ecosystems [25]. There is much proof that global warming is threatening the biodiversity of our planet, including fish [26–32], a significant taxonomic group under permanent high global human-impact threats and risks [33–45].

In this global warming scenario, freshwater ecosystems are highly vulnerable and their communities could experience significant impacts. Some new research highlights that freshwater biodiversity has declined quicker than both marine and terrestrial [46].

Due to the fact that in the above-mentioned recent United Nations report that the researched Carpathian area will be characterized by heat waves and severe drought periods [46], and the freshwater biota is under a accentuated risk in general, the aim of this study was to provide information about the state of the local fish species refuge habitats in the context of climate change and human impact effects.

Climate change forecasts for Europe are not an exception and it is clear that air temperatures will rise because of the impact of human activities on the atmosphere [47–49]. The predictions are also clear that the contemporary precipitation regime will be modified and altered and will vary from one area to another beyond the normal known seasonal patterns, with drought episodes becoming accentuated [50], and severe and even extreme climatic crisis are anticipated [51].

The climate system heating is unquestionable due to the relatively uninterrupted long-term warming trend since the mid-20th century, which may be correlated with anthropogenic influence [52–54].

Drought is an effect-dependent phenomenon [55], and considerable human impact is at least partly culpable for the harshness of the contemporary frequent and persistent drought episodes [56] generating a very intricate hydro-climatic risk affecting the natural and anthropogenic systems [57]. More than that, the heat wave magnitude is projected to rise everywhere in future [58].

Climate change-associated issues are some of the most contentious scientific issues of the present day. At this time of the climate change situation, the temperature increases all around [54], even in unanticipated ranges on Earth [59–63], and drought, decreasing altered flows on streams, which is a significant driver for aquatic ecosystems' ecological status [64], aquatic biodiversity [65], and even their potential economic use [66,67] appear and remains even in what are considered "safe" geographical areas. From this viewpoint, the Carpathian Basin was considered, bringing their synergic effects together with other human impact types that could also have consequences on functional traits of aquatic assemblages in terms of the abundance and distribution of their species [68–75].

Carpathian Basin climate change trends exceed the Earth warming rate since the 1950s of the last century through rising temperature and precipitation and drier spring, summer, and autumn seasons [76–79]. For example, in the last century, a 0.8 ◦C increase in surface temperature and a 60–80 mm decrease in precipitation were registered [80].

In the last decades, the Carpathian Basin has experienced very persistent and accentuated droughts, a trend connected with climate change, inducing remarkable drying in this region, particularly in the summer, in the chiefly exposed south-eastern sectors that have had environmental and socio-economic effects [81].

In the general climate change circumstances, the Carpathian Basin and its valuable biosphere are considered to be very sensitive. Drought is one of the important climate-related detrimental natural phenomena, and it has been appearing with increasing frequency, severity, and extent in the last decades [82,83].

The causes of fast expansion of the drought in the South-Eastern Carpathian basin are: the rise of the yearly average temperature by 0.3 ◦C, the rise in number of tropical days (>30 ◦C), the diminishing of winter days (<0 ◦C), the diminishing of precipitation, and the lessening of runoff. As results: reduced inflows to aquatic ecosystems, decreased stream-flows in a most of basins, reduced recharge of groundwater, high repetitiveness and period of the drying up of lotic systems (principally those smaller than 500 km2), and the stream flow drought which has arisen more often since 2000 [84,85].

Climate changes disturb ecosystems and cause potential threats and risks regarding their natural products and services; in these circumstances, human society should anticipate and adapt in time to these major global challenges [60].

The ephemeral and small lotic systems are the most abundant and hydrologically dynamic of all freshwater ecosystems, existing across most of our planet, including rivers in alpine and hilly zones as well as temperate regions [86–88].

The Carpathian Mountains' geographic typical features (i.e., form, orientation, latitude, and altitude) were and are essential elements which played a key driving role concerning the fish species' presence, dispersion, evolution, and their populations' ecological status [89]. In the recent climate change pattern for the hydrological nets, is the drought a new game changer in the Carpathian area? What will be the effect of this type of change on small lotic low-flow systems' habitats and their fish populations, which are already under high stress from human impact? How can we be proactive and identify such risk hotspots and how can we design proper management plans for these threatened lotic systems in order to diminish these climate-induced negative effects?

This research intended to deal with several local case studies of small lotic systems based on which similar areas can benefit by the proposed habitats and fish population risk assessment, monitoring, and management elements. Such a research-targeted area, the socalled "Saxon Villages" area/Southern Transylvania Tableland, in South-East Transylvania (Romania), is one of these types of identified areas where climate change can lead to great pressures both on the water-related habitats and biodiversity.

The research area is located in the arena-like Transylvanian Depression. Encircled by the South-Eastern Carpathians, the middle Târnava Mare River basin sector is located in the central-south part of the Transylvanian Plateau, particularly in the central Târnave Plateau, populated by more than seven million people [90], and its lotic systems are under both historical and modern diverse human-impact negative effects [91–93].

The geological basis is under a deep Neogene bed, formed by the southern segment of the Central Transylvanian massif, composed of crystalline schists, over which were accumulated Miocene, Pliocene, and Quaternary soft structures. The interfluvial areas are topped by dispersed Sarmatian marls-clays, sands, and tuffs. The most frequent is the Pliocene (Pannonian) structure, characterized by marl-clays and sands [94–98].

From the geomorphologic viewpoint, the base of the researched area was the river meadow suspended above the Târnava Mare riverbed and the lower terraces, creating transversely fragmented hilly surfaces. The energy relief is lessened, with a maximum of 100 m, and the regular fragmentation degree is 05–07 km/km2 [94–98].

The climatic regime, connected with the sector of the hilly area with a moderatecontinental climate, is defined by warm summers with rather low precipitation, and

winters with warmer periods. The circulation of the atmosphere is characterized by the high frequency of western and north-western temperate-oceanic air masses, chiefly in summer. Less often, south-west and south Mediterranean air masses appear, and northern Arctic masses less often. The annual average temperatures lie between +8 ◦C and +9 ◦C, with absolute values of +37 ◦C and −32 ◦C. The average July temperatures vary between +18 ◦C and +20 ◦C, and those of January between −3 ◦C and −4 ◦C. The atmospheric precipitation has an annual quantity of 500–700 mm [94–98].

In the study area of the Târnava River basin, the torrential character of the superficial flow has high maximum flows in rainy periods and common minimum low-flows, with rundry during drought [94–98]. In the field activities, in the August months of the 2016–2021 study period, the drought and heat waves kept the general trend of the last decades; moreover, the years 2019 and 2020 were the hottest years since 1961 [99–103].

The studied lotic systems, Dupu¸s (5 km length, 10 km2 basin surface, 0.024 m3/s multiannual flow—2016–2021), Biertan (17 km, 58 km2, 0.124 m3/s), Valchid (16 km, 56 km2, 0.120 m3/s), Laslea (22 km, 111 km2, 0.345 m3/s), Mălâncrav (14 km, 41 km2, 0.100 m3/s), and Fel¸ta/Flores,ti (9.6 km, 17 km2 0.041 m3/s) belong to the southern Târnava Mare Basin. This basin's aquatic and riverine habitats and associated biodiversity are under the influence of the impacts of multiple and diverse human activities [104–113].

The rather new extended-heat summers and warmer winters increased the temperature of rivers, decline snowpack, and the accessibility water and its related resources to riverine human communities. In addition, the present higher human pressure, contrasting with the traditional past environmentally friendly practices of natural resources use, on water resources finally induced a decrease in water quality and quantity.

Recurrently, the accent of drought-connected effects are pushed onto human-centric water resources and agricultural, socioeconomic, and migration aspects due to the related economic losses and social tension [114–117], and avoiding the related and primary triggering features of droughts, namely the meteorological and hydrological elements.

This study addressed some of the ecological dimensions of frequent and prolonged droughts, and their lasting structural effects on some aquatic habitats and their fish communities, which are communities with high relevance for the studied lotic systems' ecological status under climate change/drought seasons' constant pressure.

The human indirect (climate changes/drought) and direct influence (water overuse, water pollution, habitats fragmentation and destruction, scarcity of refuge habitats, wetland and riverine areas mismanagement, stimulating over sedimentation, etc.) are key aspects of understanding many of the drought effects on the researched lotic systems.

Other objectives of this study were to reveal some landscape (geographical, cultural, and natural heritage) values and traditional best-practices loss effects in the new climatic change situation and to identify new threats and the different types of human impact affecting some lotic system fish fauna, based upon specially designed scientific research. In the beginning of the twenty-first century, the human-nature relationship is far from balanced and with significant and variable negative effects on the biodiversity, including the whole Danube Basin [118–133], to which the studied area belongs.

It is very likely that the tendency of climate change in the twenty-first century will be very much alike that of the end of the twentieth century, manifested by rising values of extreme maximum temperatures and heat waves [76–85]; this reiterates the need for applied studies by identification of problems and proposals of integrated management plans for the ecological support systems for human society and its enterprises.

This study had as a main aim to identify and map the lotic refuges in drought periods, and also the sectors where these should be rehabilitated or made. Adjacent monitoring and management elements were proposed for the studied lotic systems' natural processes recovery. It must be highlighted that no such research approach regarding the south-east Carpathians small rivers fish refuge habitats has previously been realized.
