I was trying to figure out the early geological/biological history of the world I'm creating, and I was wondering about the development of early life. Basically, I've envisioned a world where Eukaryotic cells are the most complex form of life, but at the same time, a global catastrophe comparable to the Siberian Traps has occurred. Having researched that, I've come across the fact of water stagnation across entire oceans due to lack of water currents in the wake of climate change. Although I've decided for this point in time to be a handy moment for the evolution of photosynthesis, it won't do much good if all life's already died out by this point. So, basically...
In the event of ocean stagnation, where could Eukaryotic life be capable of surviving?
A quick note, in spite of the alien world, for the sake of expedience, the Eukaryotes possess Earth-like biochemistry and use oxygen for respiration.
Any answers would be greatly appreciated.