Water publications review (34)

The natural world dynamically responds to changing climate conditions. Scientists from Poland have studied the thermal tolerance of fish in the rivers of the southern Baltic, identifying species at risk due to progressive warming. Meanwhile, Danish ecologists conducted long-term experiments analyzing the impact of nitrogen on lake eutrophication, providing new arguments for water quality management.

Invasive plant and animal species are increasingly affecting rivers worldwide—an analysis covering 17 countries reveals their significant impact on aquatic ecosystems and suggests the need to modify biomonitoring methods. On a global scale, hydrological changes are becoming more apparent, with scientists confirming an increase in multi-year droughts that threaten agriculture and nature. Recent discoveries about polar bear fur reveal how its unique sebum composition prevents ice from sticking, potentially aiding in the development of eco-friendly anti-icing agents.

1. Radtke G., Bernaś R., (2025). Temperature tolerance of European fish species based on thermal maxima in southern Baltic Sea-basin streams. Ecological Indicators 170, 113107

Looking out the window in the middle of winter, it’s hard to deny climate change. Statistically and over the long term, it is warmer than before, and we shouldn’t be surprised that nature also has to adapt. Two ichthyologists from the Department of Migratory Fish at the Inland Fisheries Institute examined how the thermal tolerance of fish and lampreys inhabiting Poland’s northern rivers (southern Baltic Sea basin) is changing. Using data on temperature maxima in rivers, they determined the upper thermal tolerance limits for 22 species and compared them with literature data on tolerance and lethal temperatures defining their thermal niches.

For all analyzed species, the upper limit of physiological endurance (lethal temperature) was significantly higher than the tolerance threshold. For most cold- and cool-water species, the upper thermal tolerance limits matched or were similar to field observations. For the remaining species, mainly Cypriniformes, tolerance thresholds (obtained from laboratory experiments) were significantly higher than observed field temperatures. The most extreme cold-water stenotherm was the alpine bullhead (Cottus poecilopus). Based on the results, the authors conclude that species whose upper tolerance limit is below 28°C live at the thermal edge of their range in the studied region, meaning progressive warming will reduce their habitat. Species whose tolerance thresholds exceed 30°C have a thermal reserve and are not directly threatened by warming trends.

2. Jeppesen E., He H., Søndergaard M. et al. (2025). Experimental evidence of the role of nitrogen for eutrophication in shallow lakes: A long-term climate effect mesocosm study. The Innovation, 100756

The role of nitrogen in water degradation is well known—after all, that’s why we have the European Nitrates Directive and the national nitrogen program. Surprisingly, however, there are not many studies on the role of this biogenic element in lake eutrophication, especially experimental ones. A study by a team of Danish ecologists fills this gap. They analyzed water quality changes in 24 mesocosms reflecting the conditions of flow-through lakes in Denmark over five years. Their system has been in operation since 2003, with two nutrient content variants (high and low) under three climate scenarios (baseline, warming A2 IPCC, and extreme warming A2+50%)

For one year, in high-nutrient-load mesocosms, external nitrogen supply was halted while phosphorus input remained unchanged. The authors monitored fluctuations in key environmental variables and system metabolism for two years before nitrogen reduction, during the reduction year, and for two years after its resumption. The experiment showed a strong impact of supply cut-off on nitrogen forms and the N:P ratio, as well as on algal biomass (chlorophyll a), production, and respiration, with an overall weak ecosystem response to IPCC temperature scenarios.

The experiment is somewhat reminiscent of a groundbreaking study conducted in the 1970s by Schindler and co-authors on Lake 226 in Canada’s ELA, which examined phosphorus’s role in eutrophication. Like that study, the findings have significant implications for reservoir management, highlighting the importance of reducing external nitrogen inputs for shallow lake recovery.

3. Feio M. J., da Silva J. P., Hughes R. M. et al. (2025). The impacts of alien species on river bioassessment. Journal of Environmental Management, 374, 123874

There is ample evidence that the pressure of invasive alien species on native ecosystems has significantly increased in recent years. However, it turns out that this issue is not sufficiently addressed in biomonitoring and river assessment systems worldwide. Scientists from 17 countries analyzed the impact of alien taxa on the richness and abundance of native species, as well as the prevalence and effectiveness of their inclusion in river assessment methods across Europe, Africa, Asia, North America, Central and South America, and Oceania.

Among the analyzed countries, the highest number of alien species was recorded for fish (1,726), followed by macrophytes (925), macroinvertebrates (556), and diatoms (7). Their abundance was linked to various stress factors and pressures causing river degradation, significantly impacting native community composition. The distinction between alien and native species was applied only in some indicators, mainly those related to fish and macrophytes, even though including or excluding alien species in biotic indices yielded significantly different results.

Based on their findings, the authors propose: 1) incorporating alien species indicators into biological quality assessment systems; 2) expanding research on alien taxa among small organisms (e.g., diatoms, small benthic invertebrates); 3) excluding sites with confirmed biological invasions from reference site selection; 4) omitting alien species from total richness and taxonomic diversity calculations; 5) identifying alien organisms to the species level in biomonitoring programs; 6) eliminating legislation and management tools that protect alien species; 7) promoting behaviors that prevent biological invasions.

Chen L., Brun P., Buri P. et al. (2025). Global increase in the occurrence and impact of multiyear droughts. Science 387, 278-284

In recent years, much has been said about the increasing threat of drought—its frequency and duration are both rising. But can we quantify this phenomenon on a global scale? A team of Swiss scientists took on this challenge, cataloging persistent multiyear droughts (MYDs), also known as megadroughts, that occurred worldwide over nearly 40 years (1980-2018)

The researchers identified over 13,000 events lasting at least two consecutive years. Five of the ten most severe MYDs occurred between 2007 and 2018. The longest affected the eastern Congo Basin, lasting nearly a decade from 2010 to 2018, peaking in 2014 with an area of nearly 1.5 million km². The extent of MYD-affected areas is increasing at an average rate of 16,700 km² per year for the ten largest MYDs and 49,300 km² per year for the 500 largest MYDs.

Using the normalized vegetation vitality index (kNDVI), Chen and co-authors also assessed drought impacts on vegetation. Moderate grasslands showed the most significant declines in vegetation vitality due to MYDs, while boreal and tropical forests responded relatively weakly. The study confirms that megadroughts are becoming more frequent and severe globally, highlighting regions where vegetation is highly vulnerable to such events. The effects of MYDs are most pronounced in Mongolia, southeastern Australia, and the western United States.

5. Carolan J., Jakubiec M., Xavier N.F. Jr. et al. (2025). Anti-icing properties of polar bear fur. Sci. Adv. 11, eads7321

Do you know why a polar bear’s fur doesn’t freeze after diving? “Because it’s not cold enough,” you might say—but that’s incorrect, as the Arctic still experiences freezing temperatures. The fur of a polar bear has low ice adhesion due to the presence of sebum (hair oil). Analysis of its composition revealed cholesterol, diacylglycerols, anteiso-methyl-branched fatty acids, but unexpectedly, no squalene—one of the main components of the lipid mantle of human skin and other aquatic animals’ sebum. The authors demonstrate that the exceptional anti-icing properties of polar bear fur result from its unique sebum composition, thanks to high amounts of cholesterol, DGs, and long-chain fatty acids. This study presents exciting prospects for developing environmentally friendly anti-icing lubricants.

[1] https://doi.org/10.1016/j.ecolind.2025.113107
[2] https://doi.org/10.1016/j.xinn.2024.100756
[3] https://doi.org/10.1016/j.jenvman.2024.123874
[4] DOI: 10.1126/science.ado4245
[5] DOI: 10.1126/sciadv.ads7321