Most cyano will produce (fix) nitrogen from N2 rather than consume it. I've found some good articles related to this topic, but I haven't had a chance to read through them more thoroughly. A preliminary scan of these articles suggests iron may be more limitting for cyano than other bacteria/algae. So, I could be on to something, but I'm not sure.Unfortunately I will be away most of the day, but hopefully I'll get caught up on my reading early next week.
In the meantime, if you are intrested in more detailed info try searching google scholar with keywords phophate, iron, limitation (hint: particularly look at the cyanobacterium Trichodesmium)