Freshwater -- including rivers, aquifers, lakes and wetlands -- covers less than one percent of Earth's surface but hosts more than 10 percent of known species, including half of fish and one-third of vertebrates.
This diversity supports the livelihoods of billions of people and provides a bulwark against climate change but is under "substantial stress", says the study published in the peer-reviewed academic journal Nature.
A new assessment covering more than 23,000 freshwater species found that 24 percent overall were threatened with extinction, with variations among the groups studied.
Some 30 percent of decapods -- such as shrimps, crabs and crayfish -- were at risk, compared with 26 percent of fish, 23 percent of tetrapods including frogs and reptiles, and 16 percent of odonates like dragonflys.
Since 1500, some 89 freshwater species have reportedly become extinct, with another 178 suspected of meeting the same fate.
These figures are likely to be an underestimate, the authors wrote, because so little is known about certain species.
There "is urgency to act quickly to address threats to prevent further species declines and losses", they wrote.
Pollution, dams and water extraction, land-use changes and farming, invasive species and disease, climate change and extreme weather were the primary threats to freshwater species.
The decline of freshwater sources occurs "generally out of sight and out of mind, despite the importance" of these critical habitats and climate regulators.
Some 35 percent of wetlands like marshes, swamps and pools was lost between 1970 and 2015, a rate three times faster than forests, the study said.
Around one-third of rivers over 1,000 kilometres long (620 miles) are no longer free-flowing over their full length, it added.
"Until recently, the freshwater realm has not been given the same priority as the terrestrial and marine realms in global environmental governance," the authors wrote.
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