We propose Energy Watermark Algorithm (EWA), an adaptive algorithm to reduce power consumption in Internet Protocol (IP)-over-Wavelength Division Multiplexing (WDM) networks. Our solution wisely adapts the set of powered on line cards to the actual traffic demand. In particular, EWA trades between network power consumption and Quality of Service (QoS) by changing the level of overprovisioning of the lightpaths. We evaluate the performance of EWA over a set of realistic scenarios. Results show that EWA adapts the power consumption following the traffic variation, while limiting the amount of rerouted traffic. Moreover, we show that a proper setting of the input parameters allows to eliminate overload in the network.