In reply to Alfie Hughes:
There has been a major investment in the sewage works here, which seems to have genuinely improved the situation, I've swum in the area you're talking about a few times this year with no ill effects, and there is no sign of any unsuitable discharge. And no algal blooms for at least 10 years (??). Llyn Padarn is now a designated bathing lake, so I think it's monitored much more carefully. Some information here - hopefully a good news story in the midst of all the bad? https://naturalresources.wales/about-us/news-and-events/statements/llyn-pad...
I have no doubt there is still too much pollution getting in, but it's been improving. I think the main threat these days is all the sun cream which must be getting into the lake on a busy summer's day!