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.