tl;dr:
in Brittany, seaweed appears in huge amounts as nitrates pollute the sea, courtesy of the pork industry.
as it rots, hydrogen sulfide is produced.
hydrogen sulfide stinks. but more hydrogen sulfide stinks less, as it starts to numb your olfactory sense. then it kills.
edit: apparently you wont see lethal concentrations way above the weed, but break through the crust and you can get a lethal whiff
serious question: how does that jive with Nigel Farage's party of morons becoming ever stronger?