This paper is a synthetic overview of some of the threats, risks, and integrated water management elements in freshwater ecosystems. The paper provides some discussion of human needs and water conservation issues related to freshwater systems: (1) introduction and background; (2) water basics and natural cycles; (3) freshwater roles in human cultures and civilizations; (4) water as a biosphere cornerstone; (5) climate as a hydrospheric ‘game changer’ from the perspective of freshwater; (6) human-induced stressors’ effects on freshwater ecosystem changes (pollution, habitat fragmentation, etc.); (7) freshwater ecosystems’ biological resources in the context of unsustainable exploitation/overexploitation; (8) invasive species, parasites, and diseases in freshwater systems; (9) freshwater ecosystems’ vegetation; (10) the relationship between human warfare and water. All of these issues and more create an extremely complex matrix of stressors that plays a driving role in changing freshwater ecosystems both qualitatively and quantitatively, as well as their capacity to offer sustainable products and services to human societies. Only internationally integrated policies, strategies, assessment, monitoring, management, protection, and conservation initiatives can diminish and hopefully stop the long-term deterioration of Earth’s freshwater resources and their associated secondary resources.

The inheritance of historic human-induced disruption and the fierceness of its impact change aquatic ecosystems. This work reviews some of the main stressors on freshwater ecosystems, focusing on their effects, threats, risks, protection, conservation, and management elements. An overview is provided on the water protection linked to freshwater stressors: solar ultraviolet radiation, thermal pollution, nanoparticles, radioactive pollution, salinization, nutrients, sedimentation, drought, extreme floods, fragmentation, pesticides, war and terrorism, algal blooms, invasive aquatic plants, riparian vegetation, and invasive aquatic fish. Altogether, these stressors build an exceptionally composite background of stressors that are continuously changing freshwater ecosystems and diminishing or even destroying their capability to create and maintain ongoing natural healthy products and essential services to humans. Environmental and human civilization sustainability cannot exist without the proper management of freshwater ecosystems all over the planet; this specific management is impossible if the widespread studied stressors are not deeply understood structurally and functionally. Without considering each of these stressors and their synergisms, the Earth’s freshwater is doomed in terms of both quantitative and qualitative aspects.

In spite of the obvious climate changes effects on the Carpathian Basin hydrographic nets fish fauna, studies on their potential refuge habitats in drought periods are scarce. Multiannual (2016–2021) research of fish in some streams located in the Saxon Villages area during hydrological drought periods identified, mapped, and revealed the refuge aquatic habitats presence, management needs, and importance for fish diversity and abundance for small rivers. The impact of increasing global temperature and other human activities induced hydrologic net and habitats alteration, decreased the refuge habitats needed by freshwater fish, diminished the fish abundance, and influenced the spatial and temporal variation in fish assemblage structure in the studied area. The sites more than one meter in depth in the studied lotic system were inventoried and all 500 m of these lotic systems were also checked to see what species and how many individuals were present, and if there is was difference in their abundance between refuge and non-refuge 500 m sectors. The scarce number of these refuges due to relatively high soil erosion and clogging in those basins and the cumulative effects of other human types of impact induced a high degree of pressure on the fish fauna. Overall, it reduced the role of these lotic systems as a refuge and for reproduction for the fish of downstream Târnava Mare River, into which all of them flow. Management elements were proposed to maintain and improve these refuges’ ecological support capacity.

Two main aspects of the study of diversity can be distinguished: the first is related to the inventory of living organisms, the second is related to the organization of life at the level of biotic communities. Quantitative assessment of diversity is two-components as the richness of elements and their evenness. A model of the ecosystem continuum is proposed. The greatest indicators of diversity should be expected in the middle part of the environmental gradients with temporal stability. Study of producers and consumers in water bodies of Ukraine showed a regular change in their community structure in the gradient of saprobity indices. The decreasing of community diversity estimated by the Shannon index and by species richness was found at both high and low values of the saprobity indices. The fundamental coincidence of the empirical point fields of the Shannon index for the communities of invertebrates and phytoplankton with the field points of the empirical model indicates the universality of the bimodal distribution of diversity indicators in the trophic gradient. It is shown that the estimates by zoobenthos overestimate organic pollution compared with the calculations of the same indicators by phytoplankton.

Ecological assessment of wetland ecosystems of northern Kazakhstan on the basis of hydrochemistry and algal biodiversity: We studied diversity of algae and cyanobacteria in the wetlands of protected natural lakes with salinity ranging from 0.19 up to 32.7 in the arid/semiarid regions of Northern Kazakhstan. In plankton and periphyton of 34 lakes, we found 254 species belonging to 113 genera of 8 algal divisions. The diversity in arid regions is represented by widespread species of diatoms, green algae, and cyanobacteria in similar proportions. Alkaliphiles, among the indicators of acidification, and betamesosaprobionts, among the indicators of saprobity, predominated. The indices of saprobity in lakes varied from 1.47 to 2.7, reflecting low-trophic and low anthropogenically disturbed wetlands. Oligohalobes-indifferents are most common. Highly diverse algal communities were found irrespective of various levels of mineralization. As a consequence of aridization, salinity increase suppressed algal diversity. The mineralization was the most important variable defining the diversity levels, irrespective of the type and location of wetland lakes in the arid regions.