Yes, alk can decline rapidly in certain tanks. I think you shutting down your kalk reactor had a lot to do with this.
A solid number is hard to give for declining alkalinity, but in your case it is certainly possible.
You could dose some baking soda to get your alk back up while waiting for your kalk reactor to go back on line.
I'm not sure why you think your reactor was adding phosphate to your tank.
