Ring around the pool

[ZeroCool]

I've got a black ring around my pool at the waterline or slightly above it. I've tried scrubbing it with with the pool brush but no luck. The below pictures were taken last year and it's gotten worse since then. Can someone give me some ideas on how to remove it and what's causing it?

Thanks!

 

[mknauss]

Can you take a chlorine puck and hold it on it to see if it lightens or removes it? Is it in the 'wet' or 'dry' zone?