Norway has a huge number of lakes, some of them are popular swimming spots and may even have small piers but the majority are just regular lakes in the forest. How can I know if one of these lakes is safe and allowed to bathe in? Are there dangers that a non-Norwegian might not realise e.g. a dangerous algae or similar?
You can swim in pretty much every lake in Norway. There are some lakes that are used for drinking water, but these will have signs saying that swimming is illegal.
- Any dangerous bacteria that can make you sick: Nope
If you're not in an urban areas of close to pasture you can actually drink the water straight from the rivers (or even from the lakes if there is some circulation)
Some lakes and rivers (especially in northern Norway) have high concentration of lead due to military activity in the area. But don't worry, you won't stumble upon those lakes by chance. And there will be large signs saying it's polluted.
Blue-green algae might be an exception, but there are no records of people being ill from exposure to it in Norway. FHI (Norwegian health institute) states that the water is safe if you can see the bottom at 1 meter depth.
Strong currents: Not really, but there will always be currents close to rivers or water falls
Common plants that entangle you: Nope.
Industrial pollution: possibly a few places, but generally no. There are lake close to old factories, where the factories used to dump waste straight into the lake, but that doesn't happen anymore. Some lakes might still be a little bit polluted, but not enough to make swimming unsafe. You might not want to drink the water though. I would probably not swim in small lakes in very urban areas (such as Lille Lungegårdsvann in Bergen), but I've seen someone doing on a dare.
I don't know a single lake in rural areas I wouldn't be comfortable swimming in, and I've lived in many different places.