wrote:
This is relevant since people here have dogs and outdoor cats and
ferals also drink from these water sources

Blue-green algae have been a huge problem in the Baltic Sea for the last
ten years or so. The weather reports start to warn people about them,
and their progress is monitored for a large part pf the year. We always
keep an eye out for it on the island. The cats don't go swimming (except
Nikki used to dive after water voles...), so the threat to them isn't as
big, but if there are a lot of algae, humans had better beware, too.

--
Marina, Miranda and Caliban. In loving memory of Frank and Nikki.