Aspect number one: What is that? "Hey (generic alien/scary film character name), come and look at this...."
It makes you want to double up on gloves!
The surface scum is very densely packed and moves almost as a mass ; its quite hard to describe but its a bit like the skin that forms on custard (but less appetising) or probably more in line, the mouldy skin that forms on a cup of tea when you have left it on your desk for too long (I assure you that I have never done this). I should stress that not all of the tanks are so 'bad' (actually only two, as far as I have observed) and in the one featured above a fight for supremacy has been going on between Euglena sanguinea and Anabaena spirulina.
On a serious note - are there any trends to these blooms? On the surface of it all not really - both of the tanks with the dense scums are nutrient enriched tanks, but one is heated and the other is unheated. Many of the tanks have some reddish tinge to them (confirmed to be E.sanguinea after a peak under the microscope) with varying degrees of scum development; they look more like the 'after' photo below. So, it could be a case of different responses between tanks because of different exposure to mixing from the wind. The clear ability for E.sanguinea to form dense surface blooms means that it can compete with other buoyant species like Anabaena and Microcosytis; seeing as my analysis will be investigating the response of cyanobacteria to different stressors this is definitely one to keep my eye on.