Each girl has at least 1 power throughout the show, at first Amy has all powers, because she is the only mermaid (Except Terra who no one knew about yet). But after Brenna becomes a mermaid, Amy loses half of her powers, and they become Brenna's powers. Then when Kelsey becomes a Mermaid, Amy and Brenna both loose some of their powers, and Kelsey gets them. So in the end, Amy has water powers, Brenna has heat/fire/electricity powers, and Kelsey has wind/freezing powers. It is also revealed that Terra has some sort of earth power, along with the fact that she is a siren. Even Blaire has the power to breathe underwater, because her mom was a mermaid.