DYNAMIC DISSOLVED ORGANIC MATTER RESPONSE TO INCREASES OF ENERGY AVAILABILITY IN FORESTED HEADWATER STREAMS

Forested headwater streams receive inputs of dissolved organic matter (DOM) that can be transformed by biota as they are transported downstream. Biotic uptake of ambient DOM is difficult to assess through direct experimentation, as the fate of a specific organic compound added to a stream is unlikely to mirror that of the ambient DOM pool. Here we examined the dynamics of DOM by using a hybrid approach that combines uptake metrics of a simple organic compound with a direct assessment of the effects of adding this presumably labile organic carbon source (acetate) on the ambient DOM pool. We proposed that adding acetate could result in three different types of responses in the ambient DOM pool: inert, production and priming. Our results provide evidence that each of these scenarios can occur in small streams, but among all our study sites the overall response of ambient dissolved organic (DOC) to added acetate was production of additional DOC, while the dissolved organic nitrogen (DON) pool was unresponsive to addition of acetate (inert response)