How does consumption of a nutrient stop its production?

It's not stopping the production, but changing the flow.

suppose there's a flow of nutrients of type A from area one to area two. Currently, iron is the chokepoint in nutrient consumption of A, so that A is never completely consumed while going from one to two.

By adding excess iron into area one, or in the middle of the region between this flow from area one to two, you now have the possibility to consume A completely as iron no longer limits A's consumption.

So what happens to area two's consumption of A, if it became more scarce? May be nothing - or may be you now have another choke point of resources that wasn't there previously, leading to a change. If you weren't sure if this change would occur, or dont know, perhaps adding iron to area one is not a good idea, until such outcomes have been studied and acertained first.

I only use my common sense here, but it doesn't stop the production it just prevents the transport from the source to the destination through the area that was previously iron deficient and couldn't use the fully the other nutrient which passed on to another area.