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An endemic distribution means a species occurs only in a specific, limited geographic area, whereas a cosmopolitan distribution means a species is found worldwide across many regions and habitats .

Endemic distributions are more typical for freshwater organisms with low dispersal ability, such as fish and benthic invertebrates, especially in isolated lakes, while cosmopolitan distributions are more common in microorganisms and plankton that can disperse easily via water, wind, or animals .

An oligosaprobic river (SI 1: low level of saprobity) shows:

• none to very slight organic pollution

• clear water

• high oxygen availability

• diverse insect fauna with low individual abundance .

The river continuum concept describes how the physical conditions and energy sources of a river change gradually from headwaters to lower reaches, leading to corresponding changes in biological communities along the river course .

Phytoplankton is chosen to assess nutrient pressure (eutrophication) because its abundance and biomass respond quickly to increased phosphorus and nitrogen inputs, reflected for example by chlorophyll-a concentration. It is mainly used for lakes and other lentic systems, where phytoplankton forms the main primary producer community .

A detailed pathway- and source-oriented analysis (Tier 3 and 4) is needed when high concentrations, increasing trends, or unusual loads of contaminants are detected in rivers that cannot be explained sufficiently by point-source information alone. These approaches are especially required when contaminants show diffuse or ubiquitous emission patterns and sources need to be identified more precisely .

From the source to the mouth, river bed sediments typically change from coarse material (boulders, gravel) to finer sediments (sand, silt).

The main driving factor is the decreasing flow velocity and gradient, which reduces the river’s ability to transport large particles downstream .

Contaminants of emerging concern are substances that are not yet regulated or routinely monitored but are increasingly detected and may cause long-term, subtle ecological effects.

Examples include organic trace pollutants (e.g. pharmaceuticals or hormones) and microplastics .

Effect-based biotechnologies measure the biological effects of contaminant mixtures on organisms or cells instead of individual substances.

They help toxic emission control by detecting unknown or combined effects and providing early warning of ecological risks .

River bed stabilization reduces bed dynamics and the connection to groundwater, leading to lower oxygen supply, sediment clogging, and loss of habitats, which negatively affects benthic organisms and overall river ecology .

eDNA (environmental DNA) is genetic material released by organisms into the environment (e.g. water) and collected from environmental samples without capturing the organisms themselves. In freshwater quality and ecology, it is used for biodiversity monitoring, detection of rare or invasive species, and assessment of community composition .

In oligotrophic lakes, the benthic zone has little organic matter, enough oxygen, and few but well-adapted organisms.

In eutrophic lakes, a lot of organic matter settles on the bottom, causing strong decomposition, low oxygen, and only a few tolerant species to survive .

The main elements of the hydrological regime in lakes are the quantity and dynamics of water flow and the residence time of the water in the lake .

False.

During lake turnover, oxygen is mainly transported from the surface to deeper layers, while nutrients are mainly transported from the bottom to the surface

False.

Composite samples are more representative than grab samples because they integrate variations over time or space, while grab samples only reflect conditions at a single moment and location .

Yes.

Diffuse emissions, especially from agriculture, play an important role in eutrophication because they contribute large amounts of nitrogen and phosphorus to freshwater systems .

An increase in surface water temperature beyond natural ranges can cause stress or death of organisms, reduce oxygen availability, alter nutrient cycles, and make waters more sensitive to chemical pollution .

To assess chemical status under the Water Framework Directive, priority (hazardous) substances are measured and compared to environmental quality standards (EQS), including organic and inorganic pollutants such as metals and pesticides .

“good ecological status”

It is derived by comparing the current biological, physico-chemical, and hydromorphological conditions to type-specific reference (near-natural) conditions, with the worst element determining the final status (“one out – all out” principle) .

Invasive alien species can:

• cause loss of native biodiversity

• alter food webs

• change habitat conditions such as light and oxygen availability.

• They can also lead to ecosystem degradation and high economic costs .

Freshwater chemolithoautotrophic organisms use inorganic energy sources (from chemical reactions) and dissolved inorganic carbon (DIC), mainly CO₂ and bicarbonate, as their carbon source .