Pollen

You wouldn't believe this is the surface of one of the cleanest lakes in Finland, but it is indeed.

The water in this particular spot (at our summer cottage) is normally bright clear and clean, but as the pollen season is at its worst, the whole lake surface is covered all over with this ...stuff.

But, luckily this is the situation only when it's dead calm, so the pollen mixes with the water quite easily when the wind blows and the waves wallow.

It still might give you some symptoms if you're susceptible - I personally get reallllly itchy when swimming in cold water saturated with pollen.