There's been a story kicking around in the news recently about a fight in Hamburg, Germany, where recreational swimmers have supposedly killed fish by peeing in Eichbaum Lake, making fishermen really (get ready) pissed off. Reading this set off my potential-Internet-baloney alarm bells: short articles, coming from a foreign country, based off one English-language report, with a spokesman from the Hamburger Angling Association as the main source. Is there anything to this? Can anything as natural as peeing in a lake kill the fish?
In a word, yes. Whether pee was the murder weapon in this particular whodunnit depends on some factors that aren't clear from these scant reports, like how many people are swimming there and the chemistry of the lake. But urine certainly can kill fish, though it's not actually the urine that does it.
Urine includes plenty of nitrogen, potassium, and phosphorus, nutrients that are helpful for plant growth. In fact, those are the three main ingredients in both industrial fertilizer and natural fertilizers like manure. (The nitrogren in urine isn't N2, the gas that makes up most of the atmosphere—it's N3-, the "fixed" nitrogen that plants need.) If you let your pee sit around for a long while (I don't recommend doing this, for obvious reasons), it will precipitate out an odorless fertilizer called struvite.
So if you add a lot of urine to a lake, you're essentially dumping in a bunch of fertilizer, which cases a bloom of algae. After they've used up the fertilizer, the algae continue using oxygen, and when they die, their decomposition consumes still more, drastically lowering the levels of oxygen in the lake, which is what generally kills fish. The same kind of fish die-off happen when agricultural run-off reaches rivers and lakes—the run-off includes lots of fertilizer, which is basically urine, after all. And Eichbaum's fish may be facing an unusual additional danger: Hamburg University scientists have reportedly found a particular type of algae in the lake that actually releases a toxin.
Ultimately, it seems plausible that some seemingly harmless urine is actually harming the lake's fish. It wouldn't be the first time a large body of water faced the same risk: ecologists warn visitors not to pee when they visit the beautiful Great Barrier Reef (pictured above) for fear that algae blooms will kill the coral.