So, I just took a nice long shower and had a thought that many of you have probably had. To keep my question simple- why don't lakes, rivers, or the ocean ever need water changes? So obviously my first thought was "well rain brings fresh water to every water source and nearly every river/ocean is connected. But only pure water is evaporated! Everything else stays behind in the water. Now... Just like in aquariums, we have to do water changes to keep our nitrates and everything else in check, but I don't understand why lakes, rivers and oceans (mainly lakes) don't need this done. Are there microorganisms that simply transform nitrates into something else? Or are there just enough aquadic plants and filter feeders in the wild that keep the nitrates low enough to sustain life? If that's not the answer I would be completely baffled but I would love to hear someone's opinion that has had this same thought as me pr someone who has the answer.