Kenneth T. Belt

Nitrogen Fluxes and Retention in Urban Watershed Ecosystems

AbstractAlthough the watershed approach has long been used to study whole-ecosystem function, it has seldom been applied to study human-dominated systems, especially those dominated by urban and suburban land uses. Here we present 3 years of data on nitrogen (N) losses from one completely forested, one agricultural, and six urban/suburban watersheds, and input–output N budgets for suburban, forested, and agricultural watersheds. The work is a product of the Baltimore Ecosystem Study, a long-term study of urban and suburban ecosystems, and a component of the US National Science Foundation’s long-term ecological research (LTER) network. As expected, urban and suburban watersheds had much higher N losses than did the completely forested watershed, with N yields ranging from 2.9 to 7.9 kg N ha−1 y−1 in the urban and suburban watersheds compared with less than 1 kg N ha−1 y−1 in the completely forested watershed. Yields from urban and suburban watersheds were lower than those from an agricultural watershed (13–19.8 kg N ha−1 y−1). Retention of N in the suburban watershed was surprisingly high, 75% of inputs, which were dominated by home lawn fertilizer (14.4 kg N ha−1 y−1) and atmospheric deposition (11.2 kg N ha−1 y−1). Detailed analysis of mechanisms of N retention, which must occur in the significant amounts of pervious surface present in urban and suburban watersheds, and which include storage in soils and vegetation and gaseous loss, is clearly warranted.

Down by the riverside: urban riparian ecology

Riparian areas are hotspots of interactions between plants, soil, water, microbes, and people. While urban land use change has been shown to have dramatic effects on watershed hydrology, there has been surprisingly little analysis of its effects on riparian areas. Here we examine the ecology of urban riparian zones, focusing on work done in the Baltimore Ecosystem Study, a component of the US National Science Foundation’s Long Term Ecological Research network. Research in the Baltimore study has addressed how changes in hydrology associated with urbanization create riparian “hydrologic drought” by lowering water tables, which in turn alters soil, vegetation, and microbial processes. We analyze the nature of past and current human interactions with riparian ecosystems, and review other urban ecosystem studies to show how our observations mirror those in other cities.

Rising stream and river temperatures in the United States

Water temperatures are increasing in many streams and rivers throughout the US. We analyzed historical records from 40 sites and found that 20 major streams and rivers have shown statistically significant, long-term warming. Annual mean water temperatures increased by 0.009–0.077°C yr−1, and rates of warming were most rapid in, but not confined to, urbanizing areas. Long-term increases in stream water temperatures were typically correlated with increases in air temperatures. If stream temperatures were to continue to increase at current rates, due to global warming and urbanization, this could have important effects on eutrophication, ecosystem processes such as biological productivity and stream metabolism, contaminant toxicity, and loss of aquatic biodiversity.

The urban watershed continuum: evolving spatial and temporal dimensions

Urban ecosystems are constantly evolving, and they are expected to change in both space and time with active management or degradation. An urban watershed continuum framework recognizes a continuum of engineered and natural hydrologic flowpaths that expands hydrologic networks in ways that are seldom considered. It recognizes that the nature of hydrologic connectivity influences downstream fluxes and transformations of carbon, contaminants, energy, and nutrients across 4 space and time dimensions. Specifically, it proposes that (1) first order streams are largely replaced by urban infrastructure (e.g. storm drains, ditches, gutters, pipes) longitudinally and laterally within watersheds, (2) there is extensive longitudinal and lateral modification of organic carbon and nutrient retention in engineered headwaters (3) there are longitudinal downstream pulses in material and energy exports that are amplified by interactive land-use and hydrologic variability, (4) there are vertical interactions between leaky pipes and ground water that influence stream solute transport, (5) the urban watershed continuum is a transformer and transporter of materials and energy based on hydrologic residence times, and (6) temporally, there is an evolution of biogeochemical cycles and ecosystem functions as land use and urban infrastructure change over time. We provide examples from the Baltimore Ecosystem Study Long-Term Ecological (LTER) site along 4 spatiotemporal dimensions. Long-term monitoring indicates that engineered headwaters increase downstream subsidies of nitrate, phosphate, sulfate, carbon, and metals compared with undeveloped headwaters. There are increased longitudinal transformations of carbon and nitrogen from suburban headwaters to more urbanized receiving waters. Hydrologic connectivity along the vertical dimension between ground water and leaky pipes from Baltimore’s aging infrastructure elevates stream solute concentrations. Across time, there has been increased headwater stream burial, evolving stormwater management, and long-term salinization of Baltimore’s drinking water supply. Overall, an urban watershed continuum framework proposes testable hypotheses of how transport/transformation of materials and energy vary along a continuum of engineered and natural hydrologic flowpaths in space and time. Given interest in transitioning from sanitary to sustainable cities, it is necessary to recognize the evolving relationship between infrastructure and ecosystem function along the urban watershed continuum.

Streamflow distribution of non-point source nitrogen export from urban-rural catchments in the Chesapeake Bay watershed

[1] Nitrogen (N) export from urban and urbanizing watersheds is a major contributor to water quality degradation and eutrophication of receiving water bodies. Methods to reduce N exports using best management practices (BMP) have targeted both source reduction and hydrologic flow path retention. Stream restoration is a BMP targeted to multiple purposes but includes increasing flow path retention to improve water quality. As restorations are typically most effective at lower discharge rates with longer residence times, distribution of N load by stream discharge is a significant influence on catchment nitrogen retention. We explore impacts of urbanization on magnitude and export flow distribution of nitrogen along an urban-rural gradient in a set of catchments studied by the Baltimore Ecosystem Study (BES). We test the hypotheses that N export magnitude increases and cumulative N export shifts to higher, less frequent discharge with catchment urbanization. We find that increasing development in watersheds is associated with shifts in nitrogen export toward higher discharge, while total magnitude of export does not show as strong a trend. Forested reference, low-density suburban, and agricultural catchments export most of the total nitrogen (TN) and nitrate (NO 3 ) loads at relatively low flows. More urbanized sites export TN and NO 3 - at higher and less frequent flows. The greatest annual loads of nitrogen are from less developed agricultural and low-density residential (suburban/exurban) areas; the latter is the most rapidly growing land use in expanding metropolitan areas. A simple statistical model relating export distribution metrics to impervious surface area is then used to extrapolate parameters of the N export distribution across the Gwynns Falls watershed in Baltimore County. This spatial extrapolation has potential applications as a tool for predictive mapping of variations in export distribution and targeting stream channel restoration efforts at the watershed scale.

Effects of Urban Land-Use Change on Biogeochemical Cycles

Urban land-use change, the conversion of agricultural recand natural ecosystems to human settlements, has become an important component of global change. Virtually all of the projected increase in the worlds population is expected to occur in cities so that by the year 2007 more than half of the global population is expected to live in urban areas (United Nations 2004). Yet, urban settlements and surrounding areas are complex ecological systems that have only recently been studied from a rigorous ecological perspective (Pickett et al. 2001).

tir- and stx-positive Escherichia coli in stream waters in a metropolitan area.

ABSTRACT Diarrheagenic Escherichia coli, which may include the enteropathogenic E. coli and the enterohemorrhagic E. coli, are a significant cause of diarrheal disease among infants and children in both developing and developed areas. Disease outbreaks related to freshwater exposure have been documented, but the presence of these organisms in the urban aquatic environment is not well characterized. From April 2002 through April 2004 we conducted weekly surveys of streams in the metropolitan Baltimore, Md., area for the prevalence of potentially pathogenic E. coli by using PCR assays targeting the tir and stx1 and stx2 genes. Coliforms testing positive for the presence of the tir gene were cultured from 653 of 1,218 samples (53%), with a greater prevalence associated with urban, polluted streams than in suburban and forested watershed streams. Polluted urban streams were also more likely to test positive for the presence of one of the stx genes. Sequence analysis of the tir amplicon, as well as the entire tir gene from three isolates, indicated that the pathogenic E. coli present in the stream waters has a high degree of sequence homology with the E. coli O157:H7 serotype. Our data indicate that pathogenic E. coli are continually deposited into a variety of stream habitats and suggest that this organism may be a permanent member of the gastrointestinal microflora of humans and animals in the metropolitan Baltimore area.

Land Use and Climate Variability Amplify Contaminant Pulses

In 2002, the mid- Atlantic region experienced record drought levels. In September 2003, Tropical Storm Isabel produced large amounts of rainfall in the Chesapeake Bay region, and freshwater flow into the Chesapeake Bay was 400% above the long- term monthly average (http://chesapeake.usgs.gov/isabelinfo.html). Record drought conditions followed by a very wet year coincided with pulsed watershed nitrogen exports and one of the most severe zones of hypoxia, or “dead zones,” reported in the Chesapeake Bay. Large pulses of contaminants such as this event may occur more often given evidence of increased variability of precipitation and hydrologic extremes occurring with climate change [Intergovernmental Panel on Climate Change (IPCC), 2007]. Conversion of land to human- dominated uses has increased contaminant loads in streams and rivers and further transformed hydrologic cycles [Vitousek et al., 1997]. Together, land use and climate change may interact in unexpected ways to alter the amplitude, frequency, and duration of contaminant pulses in streams and rivers (i.e., large contaminant loads that are transported over relatively short time scales).

Socioecological revitalization of an urban watershed

Older, economically troubled urban neighborhoods present multiple challenges to environmental quality. Here, we present results from an initiative in Baltimore, Maryland, where water-quality improvements were rooted in a socioecological framework that highlighted the interactions between biogeophysical dynamics and social actors and institutions. This framework led to implementation of best management practices followed by assessment of changes in human perception, behavior, and education programs. Results suggest that such an initiative can improve both water quality (eg reductions in nitrogen and phosphorus runoff) and quality of life (eg increased involvement in outdoor recreation by residents and improvements in student environmental literacy and performance) in urban neighborhoods. However, proposed solutions to the water-quality problems in such neighborhoods have (1) typically emphasized the need for stormwater facilities that are difficult to build and maintain and (2) comprehensively addressed neither the issues related to aging infrastructure and hydrologic complexity nor the benefits derived from linkages between resident perception of environmental improvements and behavior and water-quality outcomes.

Climate Variation Overwhelms Efforts to Reduce Nitrogen Delivery to Coastal Waters

We calculated watershed nitrogen (N) retention (inputs–outputs)/inputs) each year from 1999–2013 for nine sub-watersheds along an urban–rural gradient near Baltimore MD to determine how land use and climate influence watershed N flux. Retention is critical to efforts to control coastal eutrophication through regulatory efforts that mandate reductions in the total maximum daily load (TMDL) of N that specific water bodies can receive. Retention decreased with urbanization as well as with increases in precipitation with retention decreasing from an average of 91% in the forested sub-watershed to 16% in the most urban sub-watershed. Export was 23% higher, and retention was 7% lower in winter (November–April) than during the growing season. Total N delivery to Baltimore Harbor varied almost threefold between wet and dry years, which is significant relative to the total annual export allowed for all non-point sources to the harbor under the TMDL. These results suggest that expectations for TMDLs should consider watershed land use and climate variability, and their potential for change if they are to result in improvements in receiving water quality.