Roughly 65.5 million years ago, a massive asteroid smashed into present-day Chicxulub, Mexico. The impact set fire to Earth's surface. Dust and ash darkened the sky, sending the planet into an "impact winter" that lasted months to years and caused the extinction of nonavian dinosaurs and half of ocean-dwelling species. However, life in inland freshwater ecosystems largely escaped this fate. To try to understand why freshwater organisms held on while ocean life failed, Robertson et al. surveyed relevant research to understand how the mechanisms of extinction would have operated differently in the two environments.
Life in rivers and lakes, as well as the oceans, would have been protected from the initial blast of heat from the asteroid impact. Previous research suggests that the heat would have evaporated the upper half-centimeter (0.2 inches) of water, but that temperatures at depth would have been largely unaffected. In the impact winter that followed, however, a lack of sunlight would have stalled photosynthetic production. Without photosynthesis, the decomposition of existing organic matter could have caused widespread hypoxia. In addition, the veil of dust and ash would have caused temperatures to drop.
Though all of these mechanisms would have operated in both the oceans and inland waterways, the authors propose that biological adaptations and differing physical processes could have made freshwater ecosystems more resilient—an argument supported by fossil evidence previously gathered in Montana's Hell Creek formation.
They hypothesize that 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. And, even in normal conditions, many freshwater organisms have dormant stages, including eggs or adults buried in mud, that allow them to await the return of more clement conditions.
Since the Chicxulub impact winter only lasted from 6 months to 2 years, the authors suggest that these differences could have helped freshwater species hold on until the sky cleared.
Citation, Douglas S. Robertson: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA, William M. Lewis: Department of Ecology and Evolutionary Biology and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA; Peter M. Sheehan: Milwaukee Public Museum, Milwaukee, Wisconsin, USA; Owen B. Toon: Department of Atmospheric and Oceanic Sciences and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA. 'K-Pg extinction patterns in marine and freshwater environments: The impact winter model', Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20086