Do channel restoration structures promote ammonium uptake and improve macroinvertebrate-based water quality classification in urban streams?

Comparisons of ammonium uptake parameters in restored and unrestored urban streams suggest that sufficient light penetration to areas where hard substrates have been installed should be an important management consideration to enhance biofilm accumulation and subsequent ammonium removal to the streambed. We studied ammonium uptake parameters and macroinvertebrate communities in 3 types of restoration structures (riffle, cross vane, and step pool) in restored streams and in unrestored urban streams in Greensboro, North Carolina, USA, where urbanization has led to high nutrient concentrations, degraded channel conditions, and low biotic diversity in streams. Restored streams had a significantly higher percentage of large substrates (boulder, cobble, and gravel) and less canopy cover compared to unrestored streams (P = 0.029; t-test), providing substrates and sufficient light penetration for biofilm growth. Benthic chlorophyll a was higher in restored compared to unrestored streams. Significantly shorter ammonium uptake length (P = 0.02) was observed in restored compared to unrestored sites. This effect was probably related to greater biofilm development and therefore more assimilation sites for removal of ammonium from the stream water. Differences in uptake velocity (P < 0.07) and areal uptake (P < 0.06) were not significant. Despite the shorter ammonium uptake length in restored streams, there was little improvement in measures of macroinvertebrate-based water quality classifications between restored and unrestored streams (P = 0.545). Because this study was completed 2 years postrestoration, continued, longer-term monitoring of restored streams is needed for full evaluation of the effects of the restoration approaches used in these streams.