Exploring The Impact Of Stream Restoration On Ecosystem Function Through Changes In Nutrient Spiraling

Abstract

Millions of dollars have been spent on stream restoration and habitat improvement projects in the Lake Superior watershed. The impact of these projects on ecological function of a stream reach are not well measured or understood. This knowledge gap is echoed within restoration work worldwide. Here we provide a test case of one method for closing this knowledge gap. We use physical habitat characteristics to explain the differences seen in stream function between a restored treatment reach and an unrestored control reach on Sargent Creek (Duluth, MN) using ammonia/ammonium nutrient spiraling dynamics as a process-based measure of stream health and function. Nutrient spiraling dynamics describe the level of benthic microbial activity and hyporheic processes within the stream as well as the ability of the stream to increase uptake rates in response to increased nutrient loading. The stream’s “resilience”, the ability to adapt uptake rates, governs nutrient export within a reach, which impacts catchment-scale water quality concerns such as basin eutrophication. Thus, changes in spiraling dynamics have implications for both stream health in situ as well as for the catchment at large. Pairing process-based measures of stream function with physical habitat characteristics allows us to go beyond identifying differences in stream function and start to explain what is causing those differences. By identifying what specific elements of habitat structure drive the processes tied to stream function we can target restoration efforts to produce higher functional lift in our streams. Nutrient dynamics were characterized at each reach through Tracer Additions for Spiraling Curve Characterization analysis (TASCC). Habitat characterization surveys were conducted at each reach using standardized methods from the National Rivers and Streams Assessment and the Minnesota Stream Quantification Tool to enable comparison with existing datasets. Nutrient dynamics were compared between matched control and treatment reaches to evaluate the effect of full-channel realignment on nutrient dynamics. In this case study we find that the restored reach of Sargent Creek has stronger nutrient uptake and retention, higher biological demand for NH4, and is further from biological saturation than the unrestored reach. We were able to explain these differences in uptake behavior through the interaction of habitat characteristics altered by restoration activities (such as reach slope, pool-riffle spacing, grain size distribution, canopy cover, and riparian vegetation assemblage). This provides a strong argument for the use of paired physical habitat surveys with process-based measures of stream function in restoration monitoring and assessment.