Heat Waves Alter Macrophyte-Derived Detrital Nutrients Release under Future Climate Warming Scenarios

In addition to a rise in global air and water mean temperatures, extreme climate events such as heat waves are increasing in frequency, intensity, and duration in many regions of the globe. Developing a mechanistic understanding of the impacts of heat waves on key ecosystem processes and how they differ from just an increase in mean temperatures is therefore of utmost importance for adaptive management against effects of global change. However, little is known about the impact of extreme events on freshwater ecosystem processes, particularly the decomposition of macrophyte detritus. We performed a mesocosm experiment to evaluate the impact of warming and heat waves on macrophyte detrital decomposition, applied as a fixed increment (+4 °C) above ambient and a fluctuating treatment with similar energy input, ranging from 0 to 6 °C above ambient (i.e., simulating heat waves). We showed that both warming and heat waves significantly accelerate dry mass loss of the detritus and carbon (C) release but found no significant differences between the two heated treatments on the effects on detritus dry mass loss and C release amount. This suggests that moderate warming indirectly enhanced macrophyte detritus dry mass loss and C release mainly by the amount of energy input rather than by the way in which warming was provided (i.e., by a fixed increment or in heat waves). However, we found significantly different amounts of nitrogen (N) and phosphorus (P) released between the two warming treatments, and there was an asymmetric response of N and P release patterns to the two warming treatments, possibly due to species-specific responses of decomposers to short-term temperature fluctuations and litter quality. Our results conclude that future climate scenarios can significantly accelerate organic matter decomposition and C, N, and P release from decaying macrophytes, and more importantly, there are asymmetric alterations in macrophyte-derived detrital N and P release dynamic. Therefore, future climate change scenarios could lead to alterations in N/P ratios in the water column via macrophyte decomposition processes and ultimately affect the structure and function of aquatic ecosystems, especially in the plankton community.