Beaches are closed in Australia because of red algae blooms. Some amazing pictures of it here.
It is blooms like this and other toxic algae that will become more frequent as increasing rains and floods due to global warming runs off nutrients to the oceans. They stimulate growth of algae that deplete oxygen from the water, which causes types that produce toxins to dominate.
Our oceans, due to warming and acidification, are quickly turning toxic (the acidification is already hindering organism growth and cell division). In the usual relativistic minimization of risk style of reasoning one might say ‘Its only the oceans, we live on land’. But that doesn’t work if the oceans are capable of producing gasses. While it provides 50% of the oxygen we breath. We are going to be affected by dying toxic oceans, in fact we have a very good idea of how that would work as it happened before.
The red algae are not the hyrdogen sulfide producing algae desrcribed in the video, but they will become dominant if we don’t cool Earth (like we heated it) and actively move against ocean degradation and anoxia. Green algae, washed up on the beach, are so toxic they can kill animals and people coming to close. May still be caused by fertilizer runoff now (even though that may mean a large part of the coastal Mexican gulf becomes toxic), but the reason for the toxicity is anoxia, and our oceans are losing oxygen by the day. We wrote about it here several times and there are even solutions to the whole problem, but work on them must start ASAP, no time to lose.