Brown algae, floats to surface of slow moving river during the day to form fragmented scum on surface.
No interactions present.
.. worked with blue green algae - I'm not really convinced that this is anything to do with cyanobacteria. However, I do not have a better idea at the moment
I'm not 100% wedded to it being blue-greens. There are probably green algae that have the same effect. What are your reasons for doubting blue-green algae are involved?
Lat/Lng: 50.6198, -2.4656
OS grid ref: SY671801
Slow moving freshwater.