Testing the Field of Dreams Hypothesis: functional responses to urbanization and restoration in stream ecosystems

Abstract

As catchments become increasingly urban, the streams that drain them become increasingly degraded. Urban streams are typically characterized by high-magnitude storm flows, homogeneous habitats, disconnected riparian zones, and elevated nitrogen concentrations. To reverse the degradation of urban water quality, watershed managers and regulators are increasingly turning to stream restoration approaches. By reshaping the channel and reconnecting the surface waters with their riparian zone, practitioners intend to enhance the natural nutrient retention capacity of the restored stream ecosystem. Despite the exponential growth in stream restoration projects and expenditures, there has been no evaluation to date of the efficacy of urban stream restoration projects in enhancing nitrogen retention or in altering the underlying ecosystem metabolism that controls instream nitrogen consumption. In this study, we compared ecosystem metabolism and nitrate uptake kinetics in four stream restoration projects within urban watersheds to ecosystem functions measured in four unrestored urban stream segments and four streams draining minimally impacted forested watersheds in central North Carolina, U.S.A. All 12 sites were surveyed in June through August of 2006 and again in January through March of 2007. We anticipated that urban streams would have enhanced rates of ecosystem metabolism and nitrate uptake relative to forested streams due to the increases in nutrient loads and temperature associated with urbanization, and we predicted that restored streams would have further enhanced rates for these ecosystem functions by virtue of their increased habitat heterogeneity and water residence times. Contrary to our predictions we found that stream metabolism did not differ between stream types in either season and that nitrate uptake kinetics were not different between stream types in the winter. During the summer, restored stream reaches had substantially higher rates of nitrate uptake than unrestored or forested stream reaches; however, we found that variation in stream temperature and canopy cover explained 80% of the variation across streams in nitrate uptake. Because the riparian trees are removed during the first stage of natural channel design projects, the restored streams in this study had significantly less canopy cover and higher summer temperatures than the urban and forested streams with which they were compared.

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].…”

“…The notably high rates in Wilson Creek were attributed to rapid algal growth causing a transient spike in whole system nutrient demand which leveled off 35 days after restoration [80]. An 8-year chronosequence of five restored urban streams in North Carolina, USA found that P uptake was greater in newly restored sites; this finding was primarily attributed to assimilation by algal biofilms [78]. Coarser substrates (e.g., cobbles, rocks, and boulders) can serve as a more stable surface for biofilms because they are less likely to be disturbed by turbulence during high flow events than finer substrates like sand and silt [92].…”

Use of Ecohydraulic-Based Mesohabitat Classification and Fish Species Traits for Stream Restoration Design

“…This linear design thinking tends to rely on the "build it and they will come" premise for ecological considerations [25,26]. The NCD approach assumes that the selected geomorphic reference provides the physical habitat template needed to enhance the restored reach's biological integrity, but this may not always be the case.…”