The importance of resource pulse timing to the response of stream ecosystems

Background/Question/Methods

Resource subsides moving across habitat boundaries can have profound consequences in recipient communities and ecosystems. Accumulating evidence suggests that inputs of resource subsidies are temporally variable (i.e., pulsed). Theory has predicted that seasonal timing of the pulsed subsidies can be a critical component that determines the intensity of the cascading effects of the subsidies. However, the most empirical studies of subsidy effects have been done over very short-time scales, and less is known about the importance of subsidy timing in natural systems. We conducted a large-scale field experiment to test if the seasonal timing of the terrestrial invertebrate input into streams affects consumer responses to the subsidy, which may have cascading consequences for whole communities and ecosystem functions.

Results/Conclusions

We found cutthroat trout responded strongly to the invertebrate pulse in the early period (June to August), but responded less to the pulse during the late period (August to October). While these consumer responses seemed not to alter the trophic cascade strongly in either period, we found 10% reduction of the leaf-break down rate in pulsed treatment reaches during the early pulse. In addition to the timing effects on trophic interactions, we found evidence of enhanced nutrient recycling by cutthroat trout (e.g., >100% increase of NH₃ excretion) in pulsed treatment reaches especially during the early period, which increased NH₃ in stream water and might change periphyton stoichiometry. Given the temporal changes in water discharge and temperature, our results suggest the temporal context of physical environment would be important in determining the effects of subsidy timing on the stream ecosystems.