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I took my first lake swim recently and found my lake has a fair amount algae, which bothered me more than I expected it too. After some limited research, I found someone saying that it's Eurasian Watermilfoil (though I'm not sure that's correct), here's a picture:

Nasty algae

For an idea of volume, you can see the video here (skip to 41s [probably turn the volume off too]):

Video of windsurfing with a good view of the lake's plantlife (YouTube video in French)

Are there any dangers of swimming with this stuff(assuming volume and proximity to surface are the same as in the video)? Specifically my concerns are:

  1. Getting caught up in it
  2. Something living in it taking a dislike to me

What (if any) are the dangers, and what can i do to mitigate them?

Lake is Lac Miribel Jonage, Lyon, France if that helps- ignore if it makes the question too specific

  • I concur with the answer below that you photo shows a plant, not an alga. Further, things that are large enough for you to see are possibly the least likely to hurt you when swimming. It's the microscopic things you need to look out for, like E. Coli infections (from a bacteria common in goose scat) and swimmer's itch (from an aquatic parasite). Thorough washing after you swim will help with both. Finally, avoid swallowing any lake water. – cobaltduck Jun 8 '17 at 15:02
  • Hi tom! Did you take this picture? It looks a lot like this which I found at a website about Lac de Miribel. The article's in French and I can't read it, but I think it defines it as the right name, but an aquatic plant rather than an algae. That concurs with a number of American sources. – Sue Jun 8 '17 at 21:27
  • If that's the case, maybe we should split this into two questions, one to identify what's in the picture and discuss its potential problems, and the other asking the dangers of swimming with algae. There are risks associated with both, but they're different. Swimming in water with Algae can be very dangerous, as @wigwam said. Large invasive aquatic plants are very bad for water and fish, but are more of a nuisance to people. They make water murky, dark, hard to see for swimming. They can also entangle boat motors. The ecosystem, and leave no trace principles, are also negatively affected. – Sue Jun 8 '17 at 21:43
  • @Sue Europe is a natural habitat for that plant, so it's not an invasive species in France. – Carey Gregory Jun 8 '17 at 23:03
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Those are just aquatic plants, not algae, and you'll find them in any healthy lake anywhere in the world. They pose no danger to you. No, you won't get tangled in them and drown. People swim through stuff like that all the time. And I really can't imagine what might be lurking in a lake in France that should concern you.

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    French people? Buh Dum Tish – Draken Jun 8 '17 at 16:14
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    While the OP appears to be in France, milfoil is an invasive species in many areas of North America, where it can grow out of control into thick clumps and mats. While you'd have to be pretty careless, it can get bad enough to be a concern. The saving grace is anywhere it's that thick is going to be nasty and unappealing to swim in anyway. North American Lakes with milfoil are not considered healthy. – whatsisname Jun 9 '17 at 3:50
  • @whatsisname Whether they're considered "healthy" or not by ecological standards is a fairly moot point with regard to this question. There's nothing about their presence that indicates the water is unsafe or polluted, and to become a hazard to swimmers they would have to be so thick that the water would be largely unswimmable anyway. – Carey Gregory Jun 9 '17 at 13:26
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You can indeed get sick from swimming in algae, if the algae is a bloom. Not all algae in lakes is harmful - in fact, it shows that the lake's ecosystem is alive. But too much algae can be dangerous, as it deprives the water of oxygen, and introduces harmful bacteria.

Here is a warning from New York State:

Harmful Algal Blooms (HABs) FAQs

And another from Indiana:

Swimming, Boating and Harmful Algal Blooms (HABs)

Michigan:

Harmful Algal Blooms in the Great Lakes

Washington:

Cyanobacteria

...and your neck of the woods, the European Environmental Agency:

Blue-green algae - check the water before you swim

I agree that the photo you shared is of plants, not algae, so not harmful. But if you really meant algae, and not the plants listed, you should pay attention to these links. (Well, you should pay attention to the links anyway...)

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All of the lakes in my city are infested with milfoil. In small amounts it is not a hazard. It is a tough, scratchy plant and I have gotten (brief) rashes on my arms after swimming through thick patches. The other hazard is that it drags on your limbs. If you remain calm it is fatiguing. If you panic it could be quite dangerous.

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