Why is there such a big difference if people empty sewage into lakes and rivers considering that all sorts of aquatic life are obviously already doing the exact same thing? Is it a matter of scale, or is it something unique about human sewage?
When it comes to fecal matter, the breakdown products- ammonia and associated nitrogenous compounds are highly toxic to aquatic life, but ecosystems have evolved together- animals don’t live in high enough densities to poison themselves with their own waste in a normal lake; if they did, they’d have wiped themselves out already. There’s not enough food, for a start, and predators, oxygen levels and other factors will all prevent a naturally occurring system from choking on its own waste; the lake as a whole forms a complex network of carbon, nitrogen and other cycles, where, in the normal course of things, levels stay relatively constant.
But when something comes along and adds a whole load of human waste, especially all at once, it’s just too concentrated. There’s not enough bacteria to break it down into compounds usable by plants, not enough plants to take it up anyway, and the oxygen levels can drop as certain microorganisms flourish, which kills the few animals which could survive the toxins. When it comes to more contained systems, like lakes, even a smaller quantity can build up over time with the same effect. You wind up with a lake full of algal sludge and not much else.
But… when it comes to human sewage, that’s not even everything. There’s all sorts of stuff people put down drains- like drain cleaner, for example, and medicines- which are both highly toxic to aquatic life and don’t break down easily. Even in rivers, where the water is mixing and moving, you can get toxic dead spots if you pour enough crap in there.
Human sewage carries pathogens that are evolved to infect humans (e.g. the fecal-oral route of transmission). Other animals can also carry human-adapted pathogens, hence why you should never drink water from a pond or stream without treatment or filtration, but generally speaking wild animals do not create anything like the concentration of sewage that a municipal waste system dumps on a constant basis, and the biosystem is evolved to break down the natural amount of waste that is produced, notwithstanding all of the other non-sewage waste people flush down the toilet or drain such as wet wipes, chlorine bleach, roughage, soaps and detergents, et cetera that interfere with normal microbial breakdown of organic waste.
Stranger
One guy living beside a lake builds a dock with an outhouse on the end it’s no big deal. 500,000 people do the same thing on the same lake, it’s a big deal.