What primary ecological disaster results from significant ozone layer depletion due to supernova radiation?

Mass mortality in phytoplankton leading to global food web collapse.

The critical long-term danger posed by X-rays and gamma rays from a nearby supernova is not the direct radiation dose, but the chemical destruction of stratospheric ozone ($ ext{O}_3$). When ozone depletion reaches significant levels, such as 50% or more within a 100 light-year range event, the amount of biologically damaging ultraviolet (UV) radiation reaching the surface drastically increases. Phytoplankton, which form the absolute base of nearly all oceanic food webs and are vital oxygen producers, are extremely sensitive to UV radiation. A massive die-off at this foundational level precipitates a cascading global ecological collapse across both marine and terrestrial ecosystems over a period spanning years to decades following the initial radiation event.