I have a question I hope someone can answer. My Administrative and Operating Manager is about to go on Maternity leave, and I have been trained to take over pay roll and payables while she is gone. By law am I supposed to get compensated for this?

The quick answer is "no."

If you are non-exempt (eligible for overtime) AND these are additional duties on top of what you already do AND you have to work longer in order to get both jobs done, then you would be getting extra compensation, possibly even overtime.

Other than that, governments don't care if your duties change significantly.