66 million years ago an event that altered the course of evolution and natural history occurred. The Chicxulub Asteroid, which was 10km in diameter and struck just below the Yucatán Peninsula, decimated 75% of all plant and animal life on earth. Creatures on land and in the water were affected by both the fiery impact and sequential “nuclear snow” that blocked the sunlight from touching the surface of the earth for several months. Although all life was affected, there is a significant difference in the percent of species wiped out in the ocean versus freshwater ecosystems. Marine ecosystems lost around 50% of their species while freshwater lost a comparatively low 10%-22% of all species. This was a mysterious issue that troubled scientists over the years.
Robertson et. al have finally tackled it with an analysis that seems to resolve the issue in a study recently published in the Journal of Geophysical Research: Biogeosciences. The study begins by outlining three main stresses due to the aftermath of the impact that would affect these two ecosystems. The first is starvation brought about by the collapse of the food chain. This was a bottom-up effect brought on by the sun being blocked which prevented phytoplankton from photosynthesizing. The collapse occurred when these phytoplankton, the near base of the food chain, were unable to sustain themselves and died off in mass.
The second obstacle brought about by the impact was reduced dissolved oxygen in the water. This was also a factor of highly reduced photosynthetic activity because oxygen is the main byproduct of the process. An additional reason there was reduced oxygen was the sudden microorganism blooms resulting from increased decomposing organic material in the water. These microorganisms would have used up the majority of the oxygen to help sustain their increased metabolic rates while feeding. The final impact was a reduced water temperature because the sun was unable to reach and warm surface waters.
So how did freshwater organisms outlive their marine counterparts through these harsh conditions? Robertson et. al proposes that the freshwater organisms were able to survive at higher rates for several reasons. The first is that higher renewal rates of organic material from increased and richer land runoff provided an essential food source to freshwater species where marine species had to go without. Similarly, increased and rapid flow of freshwater environments helped to more efficiently aerate these ecosystems providing greater access to oxygen. Finally, freshwater species were already adapted to cope with cold spells because of seasonal temperature variation. They could deal with these conditions by having a greater aptitude for dormancy as well as being more physiologically fit for the reduced temperatures. These environmental and physiological factors allowed freshwater organisms to outlive marine organisms through the severe and trying time after the Chicxulub impact.
Read the full study, published July 11 2013, in the Journal of Geophysical Research: Biogeosciences
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