Restoring stream processes in a degraded landscape: The case of in-stream carbon

Background/Question/Methods

Agricultural land use is a widespread negative anthropogenic effect on stream ecosystems. There has been much restoration effort, for example riparian replanting, but this effort has neglected stream processes in favor of plant and animal diversity. Dissolved organic carbon (DOC) exerts a strong influence on many stream ecosystem processes, particularly microbial respiration. Despite this, there has been little management or monitoring of stream ecosystems for DOC. Stream DOC quantity and composition is regulated by hydrological, chemical, and biological processes occurring both across the terrestrial-aquatic ecotone and within stream ecosystems. Therefore, DOC may be a potential indicator of catchment health, but the effects of agriculture on DOC quantity (concentration, load) and dissolved organic matter (DOM) quality (source, composition, lability) are variable.

We measured the effects of agriculture on DOC quantity and quality on ten streams across an agricultural-remnant vegetation land-use gradient in south-eastern Australia, and assessed the potential for DOC as an indicator of catchment condition. Baseflow and event-based sampling of DOC concentration was conducted monthly for at least 12 mo per stream. This work was combined with seasonal characterisation of DOM composition using spectroscopic methods, and experimental determination of the proportion of DOC bioavailable to the microbial community.

Results/Conclusions

There was a positive relationship between mean DOC concentration (range 3-10 mg L^-1) and percentage catchment agricultural cover, driven by loss of tree cover and differences in soil drainage characteristics. DOC export (mean 911 kg C km^-2 y^-1) was controlled by water yield and rainfall, although higher catchment tree cover reduced annual export. A strong negative relationship (R²=0.68, P < 0.001) between catchment tree cover and fluorescence index (indicates DOM source; i.e. terrestrial or aquatic), showed that agricultural catchments had more DOM from in-stream microbial activity. DOC quality (indicated by molecular-weight and % DOC bioavailable) was not related to catchment landuse.

DOC properties across the land-use gradient demonstrated DOC is a good indicator of processes occurring across the terrestrial-aquatic ecotone. However, catchment land-use differences were not related to DOC lability, suggesting DOC quantity or quality may be an unreliable indicator of agricultural effects on in-stream processes. Ecological restoration along the riparian zone would be expected to change DOM quantity and quality towards the pre-agricultural condition as terrestrial inputs and in-stream processes are restored. As organic matter underpins many stream functions, restoration and conservation of stream process should explicitly focus on managing for, and monitoring, organic matter quantity and quality.