No, you don't have to change any ecosystems to build a NPP. No idea why you would think that in the first place really.

To be clear, I'm in favor of nuclear, but people attack it saying it does change the local ecosystem (heating up water for cooling and pumping the warm water into rivers, and of course the nuclear waste).

Here we just had someone say that hydro is fine because it only changes the local ecosystem so I jumped on that line of reasoning. I would argue with you that nuclear changes the local ecosystem way, way less than a dam does and so it's even better.