The metabolic signature of small headwater streams: Natural variability and the response to forestry

A strong connection to the landscape means that small headwater stream metabolism is easily disturbed by land management practices such as forestry. A current landscape ecology framework, the hierarchical patch dynamics paradigm, was applied to examine the heterogeneity of small headwater stream metabolism and how it is affected by anthropogenic disturbance. Metabolism was examined at several scales to elucidate specific metabolic components and processes, including microbial community structure, bacterial carbon productivity, cellulose decomposition potential, algal accumulation, and ecosystem respiration and productivity. Firstly, in-stream metabolic patch dynamics was examined throughout a calendar year. Small headwater streams displayed very low metabolic rates dominated by heterotrophic processes. Predictable spatial and temporal patterns in population-level, community-level, and whole system-level metabolisms were observed, shaped by temperature, hydrology, and the physical and chemical properties of patches. Secondly, an inundation experiment was conducted to examine metabolic patch dynamics in a small headwater landscape. Inundation resulted in increased metabolic response and a change in the metabolic community profile of the terrestrial patch. It is suggested that regular, above-bank flows are likely to transfer the complexities of in-stream patch dynamics into the terrestrial environment, exemplifying the strong connection between headwater streams and the surrounding landscape. Thirdly, the impact of forestry on stream metabolism was examined by a survey of small headwater streams 2-5 years after logging. Forestry stimulated autotrophy and lead to increased metabolic rates. Methods that incorporate temporal variability, such as cellulose decomposition potential, provided a strong assessment of forestry impact and are recommended as robust indicators of disturbance. Fourthly, the potential recovery of stream metabolism from the impacts of forestry was examined by a survey of small headwater streams 2-15 years after logging. Only autotrophic processes displayed recovery over time, with a sustained change in heterotrophic form and function. It is suggested that, together, these surveys demonstrate a lack of resistance and resilience to forestry disturbance in the key metabolic processes of small headwater streams. Finally, through the description of metabolic patterns a metabolic 'signature' of small headwater streams is presented. The metabolic signature provides a characterisation of small headwater stream metabolism and contributes to our understanding of small headwater stream ecology.