Biogeochemical functions must be considered individually when evaluating stream restoration outcomes: Evidence from Fawn River (IN)
Stream restoration is an often-used tool to improve impaired watersheds and offset functional losses elsewhere, but considerable doubt remains concerning its effectiveness in restoring important biogeochemical functions such as nutrient retention and organic matter processing. These functions are rarely monitored in restored streams and are often inferred from structural observations rather than directly measured. In addition, processes are not considered individually and are instead aggregated into a single index measuring “biogeochemical integrity.” This study investigated aerobic metabolism and nutrient cycling in Fawn River (IN), where fine sediments and macrophytes dating from a 1998 catastrophic event were removed to reestablish hydrologic exchange with groundwater and facilitate recolonization of natural biotic communities. To evaluate the biogeochemical response of Fawn River to restoration, we conducted a series of assessments over 4 sampling periods between June and November of 2013, in an unrestored reach and a reach that had been restored for 10-12 months. Aerobic metabolism estimations from dissolved oxygen monitoring and nutrient uptake measurements from short-term releases of nitrogen (N) and phosphorus (P) were made in conjunction with analyses of organic matter carbon-to-nitrogen ratio (C:N), oxygen demand, and denitrification enzyme activity in benthic sediments from each reach.
Primary production and ecosystem respiration were lower in the restored reach across all sampling events, indicating that restoration decreases rates of organic matter production and decomposition. Sediment analyses reveal that organic matter C:N is significantly higher in the restored reach and negatively correlated with oxygen demand, suggesting that the primary factor driving the observed differences in ecosystem respiration is organic matter bioavailability. No discernable patterns in N or P retention were detected during the study period, suggesting that these changes may not emerge as fully or as quickly. But denitrification enzyme activity was found to be lower in the sediments of the restored reach and negatively correlated with organic matter C:N, suggesting that denitrification may be affected through the same mechanism as ecosystem respiration. We conclude that aerobic metabolism and denitrification are the biogeochemical processes most affected by the restoration approach employed at Fawn River and that these changes appear to be driven by the effects of restoration on sediment organic matter bioavailability. Furthermore, we believe that these results highlight the importance of considering biogeochemical functions individually when evaluating restoration outcomes as they may emerge to differing extents and over different timescales.