If all the fresh water were at least 25% deuterium, would humans be able to survive?
Would there be significant other detrimental side effects to human physiology as a result?
Heavy water proves toxic to human cells at levels ~20% of a human's body weight (although other sources give 25%). At lower levels, however, it's harmless. Given that ~2/3 of a human's body weight is water, you'd have to have much higher ratios of D2O to H2O than you have in this scenario.
Even at levels from 15%-20% body weight, heavy water may produce no adverse effects. In this world, therefore, it seems like humans would be fine.
Raise heavy water levels, though, and things could get really bad. Death comes to mind at very high concentrations.
This is a problem on Venus the solar wind unslowed by a decent magnetic field has blown a lot of the standard hydrogen away. Eucaryote Mitosis stops at about 20% heavy water so we would have to extract the heavy water before using the light water for agriculture or culinary purposes.
Considering a sizable chunk of our ecosystem including most of our food is eucaryotic there my be some discomfort.
conclusion: a tiny fraction of humanity survives in a very different world. Domed farms with sprinklers because the rain is too heavy. There would onlt be intentional macro-organisms like a moon base. Vastly smaller population and nobody living apart from the main groups. A silent landscape of dead trees and slimy water from the prokaryote explosion caused by all the resources freed up by the mass die off.
It depends on the rate of uptake and retention of the heavy portion in the human body, if the human metabolism for some reason has a biochemical propensity to take up heavy water (DHO) preferential to standard H2O then an environment with concentrations much lower than the proposed 25% could be deadly since DHO can be toxic at high levels and it would be taken in at a higher rate. On the other hand if the bodily uptake of heavy water is lower than that for normal water much higher levels of heavy water would be required to poison someone.
Humans are about 70% water to get to lethal 20-25% of total body weight you need to saturate someone to between 30 and 35% of their total bodily water with heavy water to kill them and between 20 and 30% to start seeing ill effects. So assuming that heavy water is taken up in the same way as regular water and retained without prejudice then the environment described could be rather harmful but wouldn't be lethal. From my classwork on forams I know that most living organisms actually retain H2O preferentially to DHO since it "fits" better into biochemical structures so you probably need slightly higher levels than the pure math above would indicate.
That doesn't however address three things 1. the fact that you are going to see at lot of super-heavy D2O with two deuterium because I don't know how toxic or otherwise that stuff is, 2. the fact that there will be an ongoing build up of Hydrogen-2 in living tissue with progressively deleterious effects and 3. the presence of tritium, radioactive Hydrogen-3, which may or may not be a factor but depending on the mechanism of accumulation high levels might be expected. Tritium is a beta emitter that decays to Helium-3, a process that really messes with life-systems that it is integrated into at the time. Tritium uptake is even lower than that for Deuterium but it's also going to be a progressive issue especially since the half-life is a relatively short 12.32 years well within the human span.