Brooke A. Hassett

Stream restoration strategies for reducing river nitrogen loads

Despite decades of work on implementing best management practices to reduce the movement of excess nitrogen (N) to aquatic ecosystems, the amount of N in streams and rivers remains high in many watersheds. Stream restoration has become increasingly popular, yet efforts to quantify N-removal benefits are only just beginning. Natural resource managers are asking scientists to provide advice for reducing the downstream flux of N. Here, we propose a framework for prioritizing restoration sites that involves identifying where potential N loads are large due to sizeable sources and efficient delivery to streams, and when the majority of N is exported. Small streams (1st–3rd order) with considerable loads delivered during low to moderate flows offer the greatest opportunities for N removal. We suggest approaches that increase in-stream carbon availability, contact between the water and benthos, and connections between streams and adjacent terrestrial environments. Because of uncertainties concerning the magnitud...

Effects of urbanization and urban stream restoration on the physical and biological structure of stream ecosystems

Streams, as low-lying points in the landscape, are strongly influenced by the stormwaters, pollutants, and warming that characterize catchment urbanization. River restoration projects are an increasingly popular method for mitigating urban insults. Despite the growing frequency and high expense of urban stream restoration projects, very few projects have been evaluated to determine whether they can successfully enhance habitat structure or support the stream biota characteristic of reference sites. We compared the physical and biological structure of four urban degraded, four urban restored, and four forested streams in the Piedmont region of North Carolina to quantify the ability of reach-scale stream restoration to restore physical and biological structure to urban streams and to examine the assumption that providing habitat is sufficient for biological recovery. To be successful at mitigating urban impacts, the habitat structure and biological communities found in restored streams should be more similar to forested reference sites than to their urban degraded counterparts. For every measured reach- and patch-scale attribute, we found that restored streams were indistinguishable from their degraded urban stream counterparts. Forested streams were shallower, had greater habitat complexity and median sediment size, and contained less-tolerant communities with higher sensitive taxa richness than streams in either urban category. Because heavy machinery is used to regrade and reconfigure restored channels, restored streams had less canopy cover than either forested or urban streams. Channel habitat complexity and watershed impervious surface cover (ISC) were the best predictors of sensitive taxa richness and biotic index at the reach and catchment scale, respectively. Macroinvertebrate communities in restored channels were compositionally similar to the communities in urban degraded channels, and both were dissimilar to communities in forested streams. The macroinvertebrate communities of both restored and urban degraded streams were correlated with environmental variables characteristic of degraded urban systems. Our study suggests that reach-scale restoration is not successfully mitigating for the factors causing physical and biological degradation.

Restoring watersheds project by project: trends in Chesapeake Bay tributary restoration

Restoration of aquatic ecosystems is a high priority regionally and globally, yet only recently have such efforts adopted holistic approaches that include the restoration of streams and rivers flowing to coastal areas. As the largest estuary in the US, the Chesapeake Bay has been the focus of one of the most high-profile restoration programs ever undertaken in North America. While the primary emphasis has been on tidal waters, freshwater tributary clean-up strategies have recently been developed. We have compiled the first comprehensive database of over 4700 existing river and stream restoration projects in the Chesapeake Bay Watershed (CBW) to examine where dollars are being spent, what issues motivate restoration, and what approaches are used. By conservative estimates, in excess of

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

400 million has been invested in restoration projects since 1990. The majority of projects were implemented to restore forest vegetation in riparian areas and improve water quality. Although the CBW has an extremely high density of restoration activities relative to other regions of the US, only 5.4% of the project records indicated that related monitoring of project performance has occurred. To provide cost-effective management solutions, we recommend that a centralized tracking system be developed that includes restoration projects associated with both tidal and non-tidal waterways, along with a substantial increase in investment in the comprehensive monitoring of individual projects following implementation.

Streams in the urban heat island: spatial and temporal variability in temperature

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.

Pulling apart the urbanization axis: patterns of physiochemical degradation and biological response across stream ecosystems

Abstract. Streams draining urban heat islands tend to be hotter than rural and forested streams at baseflow because of warmer urban air and ground temperatures, paved surfaces, and decreased riparian canopy. Urban infrastructure efficiently routes runoff over hot impervious surfaces and through storm drains directly into streams and can lead to rapid, dramatic increases in temperature. Thermal regimes affect habitat quality and biogeochemical processes, and changes can be lethal if temperatures exceed upper tolerance limits of aquatic fauna. In summer 2009, we collected continuous (10-min interval) temperature data in 60 streams spanning a range of development intensity in the Piedmont of North Carolina, USA. The 5 most urbanized streams averaged 21.1°C at baseflow, compared to 19.5°C in the 5 most forested streams. Temperatures in urban streams rose as much as 4°C during a small regional storm, whereas the same storm led to extremely small to no changes in temperature in forested streams. Over a kilometer of stream length, baseflow temperature varied by as much as 10°C in an urban stream and as little as 2°C in a forested stream. We used structural equation modeling to explore how reach- and catchment-scale attributes interact to explain maximum temperatures and magnitudes of storm-flow temperature surges. The best predictive model of baseflow temperatures (R2  =  0.461) included moderately strong pathways directly (extent of development and road density) and indirectly, as mediated by reach-scale factors (canopy closure and stream width), from catchment-scale factors. The strongest influence on storm-flow temperature surges appeared to be % development in the catchment. Reach-scale factors, such as the extent of riparian forest and stream width, had little mitigating influence (R2  =  0.448). Stream temperature is an essential, but overlooked, aspect of the urban stream syndrome and is affected by reach-scale habitat variables, catchment-scale urbanization, and stream thermal regimes.

Synthesizing U.S. River Restoration Efforts

Watershed urbanization introduces a variety of physical, chemical, and thermal stressors to receiving streams and leads to well-documented declines in the diversity of fish and macroinvertebrates. Far less knowledge is available about how these urban stressors affect microbial communities and microbially mediated ecosystem properties. We examined 67 chemical, physical, and biological attributes of streams draining 47 watersheds in the metropolitan area surrounding Raleigh, North Carolina. Watersheds ranged from undeveloped to 99.7% developed watershed area. In contrast to prior investigators, we found no consistent changes in habitat structure, channel dimensions, or bed sediment size distributions along the urbanization gradient. Watershed urbanization led to large and consistent changes in receiving stream chemistry (increases in NO3−, bioavailable and algal-derived dissolved organic C, and the trace metals Pb, Cd, and Zn) and thermal regimes. These chemical and thermal changes were not associated with any consistent shifts in microbial community structure or taxonomic richness, based on terminal-restriction fragment length polymorphism and pyrosequencing methods, despite the fact that these urban stressors were associated with commonly reported declines in macroinvertebrate taxonomic richness and altered macroinvertebrate community composition. Chemical and thermal changes as a function of % developed watershed area also were unrelated to shifts in microbially mediated biogeochemical processes (C mineralization, denitrification potential, and substrate-induced respiration). A broad urbanization gradient sampled in this region suggests that stream ecosystem responses to watershed urbanization can follow diverse trajectories.