the upper layers of the lake had a great deal of life, but the bottom was anoxic. because the water was not much disturbed, little oxygen got to the lower levels. this prevented many epifaunal (bottom-living) and infaunal (burrowing) species from living there. that is why dead bodies were so little damaged. overturn of the lake layers (caused by seasonal variations) lowered oxygen content near the surface and led to a periodic "die-off" of aquatic species. combined with a relatively low rate of deposition (0.1 mm/yr), this was a prime environment for the preservation of fauna and flora.