Influence of Restoration Age and Riparian Vegetation on Reach‐Scale Nutrient Retention in Restored Urban Streams

Abstract

In urban watersheds, stormwater inputs largely bypass the buffering capacity of riparian zones through direct inputs of drainage pipes and lowered groundwater tables. However, vegetation near the stream can still influence instream nutrient transformations via maintenance of streambank stability, input of woody debris, modulation of organic matter sources, and temperature regulation. Stream restoration seeks to mimic many of these functions by engineering channel complexity, grading stream banks to reconnect incised channels, and replanting lost riparian vegetation. The goal of this study was to quantify these effects by measuring nitrate and phosphate uptake in five restored streams in Charlotte and Raleigh, North Carolina, with a range of restoration ages. Using nutrient spiraling methods, uptake velocity of nitrate (0.02-3.56 mm/min) and phosphate (0.14-19.1 mm/min) was similar to other urban restored streams and higher than unimpacted forested streams with variability influenced by restoration age and geomorphology. Using a multiple linear regression approach, reach-scale phosphate uptake was greater in newly restored sites, which was attributed to assimilation by algal biofilms, whereas nitrate uptake was highest in older sites potentially due to greater channel stability and establishment of microbial communities. The patterns we observed highlight the influence of riparian vegetation on energy inputs (e.g., heat, organic matter) and thereby on nutrient retention.

The potential of agricultural headwater streams to retain soluble reactive phosphorus

“…In our study, unshaded reaches showed significantly higher chlorophyll-a concentrations and SRP uptake than shaded reaches. Similar observations have been made by Bernot et al (2006), Burrows et al (2013), andMcMillan et al (2014). Sabater et al (2000) found an increase in the biomass of benthic algae together with increased SRP uptake after riparian vegetation removal, emphasizing the role of riparian forests on in-stream processes in small streams.…”

Nutrient Retention in Restored Streams and Rivers: A Global Review and Synthesis

“…Overall, we found highest rates of nutrient uptake in newly restored sites, which suggested the need to evaluate and compare nutrient retention over time (please see section further below). There were high rates of nutrient uptake in urban [30,74,75,77,78,80,81] and agricultural watersheds [29,72,79,91], which are likely linked to positive relationships with nutrient and Chl-a concentrations [76]. There were lower rates of nutrient uptake in forested and acid mine drainage watersheds.…”

“…Some studies injected a single nutrient while others injected multiple nutrients. There were 66 single ammonium injections [59,[72][73][74], 45 single nitrate injections [30,[75][76][77][78], 38 combined ammonium and phosphate experiments [79,80], 59 combined nitrate and phosphate experiments [29,81], and 30 combined ammonium, nitrate and phosphate experiments [82,83]. Based on the type of nutrient injection, the total number of experiments was 134 ammonium, 134 nitrate, and 127 phosphate injections.…”

“…Several studies linked elevated nutrient uptake rates in newly restored streams to increased light availability and coarser substrate composition following construction [74,78,81]. Increased light availability from reduced canopy cover can temporarily increase stream temperature and the abundance of algal biofilms in streams with ample nutrients [78].…”

Urban Evolution: The Role of Water

“…There has been an increase in the number of studies that monitor restoration projects [20,50,[105][106][107][108]. However, there is still a need for more research to identify patterns and trajectories of stream restoration as an urban adaptation in response to watershed impairments and characterize its role in the transition from the Sanitary City to the Sustainable City.…”