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.

The impact of climate change in Serbia, as found throughout the world, is reflected in the rapid changes in drought and heavy rain periods. We show the influence of high rainfall levels on the water quality, composition and seasonal succession of Cyanobacteria in the Šumarice reservoir, Serbia (44° 01'49'' N, 20° 52'28'' E). The changes in physical and chemical parameters of water, Chl-a, Cyanobacteria biomass, abundance of phytoplankton and Cyanobacteria along a temporal gradient are visualized using a self-organizing map (SOM). Data was collected monthly during 2013 and 2014. During 2013, the reservoir was characterized as mesotrophic and eutrophic, with a low level of nutrients (the maximum value of total phosphorus was 0.068 mg L ^–1 and total nitrogen was 0.764 mg L ^–1 in August), while the maximum values of Cyanobacteria biomass (12.7 mg L ^–1) and abundance (1.36 × 10 6 ind L^–1) were in September, when Cyanobacteria represented 21% of the whole phytoplankton. In the following year (2014), after the extreme precipitation (227 L m ^–2 in May and 138 L m ^–2 in July), the concentration of nutrients in the reservoir increased. In September, the maximum value of total phosphorus was 0.310 mg L ^–1, and the maximum value of total nitrogen was 6.828 mg L ^–1. In October, the maximum values of Cyanobacteria biomass (1567.28 mg L ^–1) and abundance (2.36 × 10⁸ ind L ^–1) were recorded when the bloom of Aphanizomenon flosaquae was observed, representing 98 % of the whole phytoplankton. This blooming is strongly positively correlated with the increase of total phosphorus and nitrogen. The extreme and long-term precipitation was significant enough to change the state of the reservoir from eutrophic to hypereutrophic.