Why freshwater life survived the asteroid impact that wiped out most dinosaurs

Researchers cite several environmental, adaptive advantages for freshwater ecosystems

Credit: Image credit: Flickr/Cliff1066

Nearly 66 million years ago, an asteroid measuring more than 6 miles in diameter crashed into the Earth near the Yucatan Peninsula in Mexico. The resulting havoc -- monster tsunamis, shock waves, earthquakes, volcanic eruptions, super-heated air, uncontrollable fires and an atmosphere shrouded in dust and particulates -- soon led to the extinction of non-avian land-based dinosaurs and about 50% of ocean life. Curiously, species within inland freshwater ecosystems were able to avoid the same fate as the future stars of many dinosaur movies. And as EarthSky reports, scientists think they may know why. In a paper published last week in the Journal of Geophysical Research, a team led by Douglas Robertson of the University of Colorado offered some theories. Among other things, underwater species mostly would have been unaffected by the scorching heat produced by the asteroid collision with Earth. Of course, heat was an initial byproduct of the asteroid event (which was neither the first nor only in our planet's history). More damaging over time was the resulting impact winter; the shroud of ash and dust covering the Earth caused temperatures to drop and hampered the process of oxygen-producing photosynthesis. Yet the freshwater ecosystems survived those as well. So what's up with that? Here's what the researchers say in the paper's abstract:

Inland waters, although probably subject to high mortality, showed lower proportionate extinction than marine environments probably because of the greater potential among the freshwater taxa for dormancy, the greater efficiency of reaeration by rapid flow to offset oxygen demand, abundant thermal refugia fed by groundwater at moderate temperatures, and preadaptation of freshwater taxa to a great degree of environmental variability.

OK, then! Now EarthSky.org will translate into English: "Freshwater organisms, often accustomed to annual freeze-thaw cycles and periodic hypoxia, would have held up better to the impact winter conditions. Fast-flowing river water could have reoxygenated inland waterways. Furthermore, groundwater seeps could have kept freshwater ecosystems warm and supplied with organic matter." In addition, as EarthSky points out, many freshwater species whose existence relies in part on periods of dormancy (not hibernation) could have waited out the impact winter, especially if it were on the short side of the six months to two years estimated by scientists. Bottom line: Freshwater organisms largely were insulated by their environment from the worst effects of the asteroid event that obliterated most other life elsewhere on Earth. Now read this:

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