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We investigate impacts of Freshwater Salinization Syndrome (FSS) on mobilization of salts, nutrients, and metals in urban streams and stormwater BMPs by analyzing original data on concentrations and fluxes of salts, nutrients, and metals from 7 urban watersheds in the Mid-Atlantic U.S. and synthesizing literature data. We also explore future critical research needs through a survey of practitioners and scientists. Our original data show: (1) sharp pulses in concentrations of salt ions and metals in urban streams directly following both road salt events and stream restoration construction (

Freshwater salinization is an emerging global problem impacting safe drinking water, ecosystem health and biodiversity, infrastructure corrosion, and food production. Freshwater salinization originates from diverse anthropogenic and geologic sources including road salts, human-accelerated weathering, sewage, urban construction, fertilizer, mine drainage, resource extraction, water softeners, saltwater intrusion, and evaporative concentration of ions due to hydrologic alterations and climate change. The complex interrelationships between salt ions and chemical, biological, and geologic parameters and consequences on the natural, social, and built environment are called Freshwater Salinization Syndrome (FSS). Here, we provide a comprehensive overview of salinization issues (past, present, and future), and we investigate drivers and solutions. We analyze the expanding global magnitude and scope of FSS including its discovery in humid regions, connections to human-accelerated weathering and mobilization of ‘chemical cocktails.’ We also present data illustrating: (1) increasing trends in salt ion concentrations in some of the world’s major freshwaters, including critical drinking water supplies; (2) decreasing trends in nutrient concentrations in rivers due to regulations but increasing trends in salinization, which have been due to lack of adequate management and regulations; (3) regional trends in atmospheric deposition of salt ions and storage of salt ions in soils and groundwater, and (4) applications of specific conductance as a proxy for tracking sources and concentrations of groups of elements in freshwaters. We prioritize FSS research needs related to better understanding: (1) effects of saltwater intrusion on ecosystem processes, (2) potential health risks from groundwater contamination of home wells, (3) potential risks to clean and safe drinking water sources, (4) economic and safety impacts of infrastructure corrosion, (5) alteration of biodiversity and ecosystem functions, and (6) application of high-frequency sensors in state-of-the art monitoring and management. We evaluate management solutions using a watershed approach spanning air, land, and water to explore variations in sources, fate and transport of different salt ions (e.g. monitoring of atmospheric deposition of ions, stormwater management, groundwater remediation, and managing road runoff). We also identify tradeoffs in management approaches such as unanticipated retention and release of chemical cocktails from urban stormwater management best management practices (BMPs) and unintended consequences of alternative deicers on water quality. Overall, we show that FSS has direct and indirect effects on mobilization of diverse chemical cocktails of ions, metals, nutrients, organics, and radionuclides in freshwaters with mounting impacts. Our comprehensive review suggests what could happen if FSS were not managed into the future and evaluates strategies for reducing increasing risks to clean and safe drinking water, human health, costly infrastructure, biodiversity, and critical ecosystem services.

The Earth's population will become more than 80% urban during this century. This threshold is often regarded as sufficient justification for pursuing urban ecology. However, pursuit has primarily focused on building empirical richness, and urban ecology theory is rarely discussed. The Baltimore Ecosystem Study (BES) has been grounded in theory since its inception and its two decades of data collection have stimulated progress toward comprehensive urban theory. Emerging urban ecology theory integrates biology, physical sciences, social sciences, and urban design, probes interdisciplinary frontiers while being founded on textbook disciplinary theories, and accommodates surprising empirical results. Theoretical growth in urban ecology has relied on refined frameworks, increased disciplinary scope, and longevity of interdisciplinary interactions. We describe the theories used by BES initially, and trace ongoing theoretical development that increasingly reflects the hybrid biological–physical–social nature of the Baltimore ecosystem. The specific mix of theories used in Baltimore likely will require modification when applied to other urban areas, but the developmental process, and the key results, will continue to benefit other urban social–ecological research projects.

Understanding transport mechanisms and temporal patterns in the context of metal concentrations in urban streams is important for developing best management practices and restoration strategies to improve water quality. In some cases, in-situ sensors can be used to estimate unknown concentrations of trace metals or to interpolate between sampling events. Continuous sensor data from the United States Geological Survey were analyzed to determine statistically significant relationships between lead, copper, zinc, cadmium, and mercury with turbidity, specific conductance, dissolved oxygen, and discharge for the Hickey Run, Watts Branch, and Rock Creek watersheds in the Washington, D.C. region. We observed a significant negative linear relationship between concentrations of Cu and dissolved oxygen at Rock Creek (p &lt, 0.05). Sometimes, turbidity had significant positive linear relationships with Pb and Hg concentrations. There were negative or positive linear relationships between Pb, Cd, Zn, and Hg and specific conductance. There also appeared to be relationships between watershed areal fluxes of Pb, Cu, Zn, and Cd in streams with turbidity. Watershed monitoring approaches using continuous sensor data have the potential to characterize the frequency, magnitude, and composition of pulses in concentrations and loads of trace metals, which could improve the management and restoration of urban streams.

Urbanized watersheds in colder climates experience episodic salinization due to anthropogenic salt inputs and runoff from impervious surfaces. Episodic salinization can be manifested as a ‘pulse’ in concentrations and fluxes of salt ions lasting from hours to days after snowstorms in response to road salting. Episodic salinization contributes to freshwater salinization syndrome, characterized by cascading mobilization of chemicals and shifting acid–base status. We conducted laboratory experiments and analyzed high-frequency sensor data to investigate the water quality impacts of freshwater salinization syndrome and episodic salinization across 12 watersheds draining two major metropolitan regions along the U.S. East Coast. Sediments from 12 watersheds spanning land use gradients across two metropolitan regions, Baltimore, Maryland and Washington DC, were incubated across a range of replicated salinity treatments (0–10 g/L sodium chloride). There were statistically significant linear increasing trends in calcium and potassium concentrations with experimental salinization across all 12 sites and in magnesium concentrations at 11 of 12 sites (p

Este artículo contiene 25 páginas, 9 figuras., In the Anthropocene, watershed chemical transport is increasingly dominated by novel combinations of elements, which are hydrologically linked together as ‘chemical cocktails.’ Chemical cocktails are novel because human activities greatly enhance elemental concentrations and their probability for biogeochemical interactions and shared transport along hydrologic flowpaths. A new chemical cocktail approach advances our ability to: trace contaminant mixtures in watersheds, develop chemical proxies with high-resolution sensor data, and manage multiple water quality problems. We explore the following questions: (1) Can we classify elemental transport in watersheds as chemical cocktails using a new approach? (2) What is the role of climate and land use in enhancing the formation and transport of chemical cocktails in watersheds? To address these questions, we first analyze trends in concentrations of carbon, nutrients, metals, and salts in fresh waters over 100 years. Next, we explore how climate and land use enhance the probability of formation of chemical cocktails of carbon, nutrients, metals, and salts. Ultimately, we classify transport of chemical cocktails based on solubility, mobility, reactivity, and dominant phases: (1) sieved chemical cocktails (e.g., particulate forms of nutrients, metals and organic matter); (2) filtered chemical cocktails (e.g., dissolved organic matter and associated metal complexes); (3) chromatographic chemical cocktails (e.g., ions eluted from soil exchange sites); and (4) reactive chemical cocktails (e.g., limiting nutrients and redox sensitive elements). Typically, contaminants are regulated and managed one element at a time, even though combinations of elements interact to influence many water quality problems such as toxicity to life, eutrophication, infrastructure corrosion, and water treatment. A chemical cocktail approach significantly expands evaluations of water quality signatures and impacts beyond single elements to mixtures. High-frequency sensor data (pH, specific conductance, turbidity, etc.) can serve as proxies for chemical cocktails and improve real-time analyses of water quality violations, identify regulatory needs, and track water quality recovery following storms and extreme climate events. Ultimately, a watershed chemical cocktail approach is necessary for effectively co-managing groups of contaminants and provides a more holistic approach for studying, monitoring, and managing water quality in the Anthropocene., This work was funded by USDA (award # 2016-67019-25280) and NSF-EPSCoR (#1641157) for supporting collaborations at the AGU Chapman Conference on Extreme Climate Events. Significant funding for data collection/analyses in this paper was provided by NSF EAR1521224, NSF CBET1058502, NSF Coastal SEES1426844, NSF DEB-0423476 and DEB-1027188, NSF RI EPSCoR NEWRnet Grant No. IIA-1330406, EPA ORD, Chesapeake Bay Trust, and Multi-state Regional Hatch Project S-1063.

Although many climate predictions suggest that the frequency and intensity of large storm events might increase in the coming decades, few studies document the full impact of such events along their path. Here, we synthesize information on the impact of Hurricane Irene (formed August 21 2011) and Tropical Storm Lee (formed August 30, 2011) on erosion and sediment transport, lake metabolism, riparian hydrology and biogeochemistry, and stream water quality, from North Carolina to Maine. In almost all cases, these storms generated unprecedented changes in water quality (concentrations, loads), from tenfold increases in DOC and 100-fold increases in POC in Maryland, to 100-fold increases in TSS concentrations in Pennsylvania. Overbank flooding and up to 200-year streamflow events were recorded in New York and Vermont. In many cases, particulate loads (e.g. POC, PP, TSS) occurring during Irene and Lee represented more than 30% of the annual load. The dominance of particulate exports over solutes during Irene and Lee is consistent with the mobilization of normally immobile sediment pools, and massive erosion as reported at many locations across the Northeastern US. Several studies reported long lasting (> 1 year) effects of Irene and Lee on cyanobacterial blooms, erosion, or stream suspended sediment concentrations. However, this review also highlighted the lack of a consistent strategy in terms of methods, and measured water quality parameters. This strongly hinders our ability to fully assess the large-scale impact of such events on our environment, and ultimately their impact on our economy and society.

Human-dominated land uses can increase transport of major ions in streams due to the combination of human-accelerated weathering and anthropogenic salts. Calcium, magnesium, sodium, alkalinity, and hardness significantly increased in the drinking water supply for Baltimore, Maryland over almost 50 years (p

Streams and rivers are significant sources of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) globally, and watershed management can alter greenhouse gas (GHG) emissions from streams. We hypothesized that urban infrastructure significantly alters downstream water quality and contributes to variability in GHG saturation and emissions. We measured gas saturation and estimated emission rates in headwaters of two urban stream networks (Red Run and Dead Run) of the Baltimore Ecosystem Study Long-Term Ecological Research project. We identified four combinations of stormwater and sanitary infrastructure present in these watersheds, including: (1) stream burial, (2) inline stormwater wetlands, (3) riparian/floodplain preservation, and (4) septic systems. We selected two first-order catchments in each of these categories and measured GHG concentrations, emissions, and dissolved inorganic and organic carbon (DIC and DOC) and nutrient concentrations biweekly for 1 year. From a water quality perspective, the DOC : NO3− ratio of streamwater was significantly different across infrastructure categories. Multiple linear regressions including DOC : NO3− and other variables (dissolved oxygen, DO; total dissolved nitrogen, TDN; and temperature) explained much of the statistical variation in nitrous oxide (N2O, r2 = 0.78), carbon dioxide (CO2, r2 = 0.78), and methane (CH4, r2 = 0.50) saturation in stream water. We measured N2O saturation ratios, which were among the highest reported in the literature for streams, ranging from 1.1 to 47 across all sites and dates. N2O saturation ratios were highest in streams draining watersheds with septic systems and strongly correlated with TDN. The CO2 saturation ratio was highly correlated with the N2O saturation ratio across all sites and dates, and the CO2 saturation ratio ranged from 1.1 to 73. CH4 was always supersaturated, with saturation ratios ranging from 3.0 to 2157. Longitudinal surveys extending form headwaters to third-order outlets of Red Run and Dead Run took place in spring and fall. Linear regressions of these data yielded significant negative relationships between each gas with increasing watershed size as well as consistent relationships between solutes (TDN or DOC, and DOC : TDN ratio) and gas saturation. Despite a decline in gas saturation between the headwaters and stream outlet, streams remained saturated with GHGs throughout the drainage network, suggesting that urban streams are continuous sources of CO2, CH4, and N2O. Our results suggest that infrastructure decisions can have significant effects on downstream water quality and greenhouse gases, and watershed management strategies may need to consider coupled impacts on urban water and air quality.

Understanding how land use and climate variability interact could amplify riverine nitrogen exports is needed to develop effective watershed management strategies in coastal China, a region that is experiencing the most rapid urbanization globally. We investigated the interactive effects of land use and climate variability on riverine nitrogen exports. We analyzed long-term historical data (1961–2014) in two river reaches of the Jiulong River Watershed and spatial data (2010–2014) across a land use gradient spanning 17 monitored headwater streams. Results showed that annual riverine nitrate-N (NO3−-N) and ammonia-N (NH4+-N) exports have significantly increased from 1961 to 2014 coinciding with urban expansion. Nitrate-N exports were correlated with river flow during 1961–1980, while they were not correlated with river flow during 1981–2002 when fertilizer use rapidly increased in this region. Nitrate-N export was tightly correlated with river flow during 2010–2014 as fertilizer use decreased and there was conversion of agricultural lands to urban lands. Annual NO3−-N and NH4+-N exports for 17 headwater streams across land use showed significant inter-annual variability (2–10 times) during 2010–2014 across dry and wet years. There was greater inter-annual variability in N exports at the urban/agricultural sites than at the forest sites. Urban watersheds had the highest runoff and nitrate-N export during the flood season. Runoff was the “master variable” controlling nitrate export in urban watersheds across time and space. Given rapid urbanization, the interaction between climate variability and land use change can amplify river nitrogen loads and coastal water quality issues in the developing world unless more effective management of the watershed and associated riverine system is applied.

Urbanization has altered the fate and transport of anthropogenic nitrogen (N) in rivers and estuaries globally. This study evaluates the capacity of an urbanizing river–estuarine continuum to transform N inputs from the world's largest advanced (e.g., phosphorus and biological N removal) wastewater treatment facility. Effluent samples and surface water were collected monthly along the Potomac River estuary from Washington D.C. to the Chesapeake Bay over a distance of 150 km. In conjunction with box model mass balances, nitrate stable isotopes and mixing models were used to trace the fate of urban wastewater nitrate. Nitrate concentrations and δ15N-NO3− values were higher down-estuary from the Blue Plains wastewater outfall in Washington D.C. (2.25 ± 0.62 mg L−1 and 25.7 ± 2.9 ‰, respectively) compared to upper-estuary concentrations (1.0 ± 0.2 mg L−1 and 9.3 ± 1.4 ‰, respectively). Nitrate concentration then decreased rapidly within 30 km down-estuary (to 0.8 ± 0.2 mg L−1), corresponding to an increase in organic nitrogen and dissolved organic carbon, suggesting biotic uptake and organic transformation. TN loads declined down-estuary (from an annual average of 48 000 ± 5000 kg day−1 at the sewage treatment plant outfall to 23 000 ± 13 000 kg day−1 at the estuary mouth), with the greatest percentage decrease during summer and fall. Annually, there was a 70 ± 31 % loss in wastewater NO3− along the estuary, and 28 ± 6 % of urban wastewater TN inputs were exported to the Chesapeake Bay, with the greatest contribution of wastewater TN loads during the spring. Our results suggest that biological transformations along the urban river–estuary continuum can significantly transform wastewater N inputs from major cities globally, and more work is necessary to evaluate the potential of organic nitrogen and carbon to contribute to eutrophication and hypoxia.

Rivers transport and transform significant quantities of carbon to coastal zones globally. Urbanization and climate change impact the transport and transformation of carbon by altering hydrology, water temperatures, and in-stream metabolism rates. Changes in exports, sources, and metabolism of carbon influence ecosystem processes, food webs, and greenhouse gases. We characterized exports, sources, and metabolism of carbon in four urban watersheds using a combination of discrete stream chemistry measurements and continuous water-quality sensors. Over three years, watershed DOC exports in the Baltimore-Washington D.C. metropolitan area ranged from 9 to 23 kg ha−1 year−1. DIC exports ranged from 19 to 59 kg ha−1 year−1. Daily contributions from in-stream metabolism varied between −65 and 90 % of DIC export depending on stream size and streamflow conditions. Negative contributions from metabolism occurred on days when streams were autotrophic. All streams were heterotrophic during 60 to 87 % of each year, but showed significant peaks in autotrophy during spring and summer. Differences in the timing and magnitude of peaks in springtime net ecosystem productivity were likely driven by varying light availability across streams of different sizes and riparian shading. CO2 was consistently over-saturated with respect to the atmosphere on all sampling dates and was 0.25–2.9 mg C L−1. Exports, sources, and metabolism of DOC and DIC showed strong predictable patterns across streamflow. Thus, we present a new conceptual model for predicting carbon transport and transformation across changing streamflow and light availability (with impacts on sources and fluxes of DOC, DIC, and CO2). Overall, our results and conceptual model suggest that urbanization accelerates the transition of streams from transporters to transformers of carbon across streamflow, with implications for timing and magnitude of CO2 fluxes, river alkalinization, and oxygen demand in downstream waters.

Dissolved organic matter (DOM) provides nutrients and energy subsidies for harmful algal blooms in Maryland's Coastal Bays (MCBs, USA). The composition, sources and dynamics of DOM in MCBs are not well known. In this study, dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) were monitored monthly over a period of one year (July 2011 to June 2012) at 13 stations in the MCBs. Absorption and fluorescence spectroscopy were used to characterize DOM composition and track terrestrial inputs versus aquatic sources of DOM. Results show that DOC and TDN concentrations in the MCBs (187–501 μM and 19.4–40.6 μM) were comparable to eutrophic coastal waters in the United States but much higher than the mid-Atlantic Bight. This suggests that the MCBs are a DOM source for the coastal ocean. Spatially, nearshore sites had relatively higher DOC and TDN concentrations and terrestrially-derived aromatic DOM (indicated by high humification index) than the bays that were directly connected to the Atlantic Ocean. Seasonally, DOC from the main body of the MCBs (Chincoteague and Assawoman Bays) displayed a pronounced seasonal pattern with the highest values occurring in summer. Protein-like DOM from algal/bacterial inputs (indicated by high biological freshness index or fluorescence index) was also highest in summer but then decreased gradually until April. DOC concentrations at the nearshore site were highest in later spring, probably due to terrestrial inputs. Conversely, TDN concentrations in smaller bays with faster flushing rates (Newport, Sinepuxent and Isle of Wight Bays) were highest in April, probably due to watershed nitrogen inputs. This spring TDN peak was not apparent in the main body of the MCBs, which have longer flushing times (indicating stronger biological nitrogen uptake). This study suggests that DOM in eutrophic, shallow coastal bays such as the MCBs consists of a large fraction of protein-like components from local algal/microbial sources during summer. This enhanced primary productivity is likely fueled by watershed nitrogen inputs from agricultural and urban land use during spring. Further insights regarding spatiotemporal variations and controls of DOM abundance, composition and sources in the MCBs can be helpful in guiding and prioritizing coastal restoration efforts for reducing eutrophication and water quality degradation.

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.

The structure, function, and services of urban ecosystems evolve over time scales from seconds to centuries as Earth’s population grows, infrastructure ages, and sociopolitical values alter them. In order to systematically study changes over time, the concept of “urban evolution” was proposed. It allows urban planning, management, and restoration to move beyond reactive management to predictive management based on past observations of consistent patterns. Here, we define and review a glossary of core concepts for studying urban evolution, which includes the mechanisms of urban selective pressure and urban adaptation. Urban selective pressure is an environmental or societal driver contributing to urban adaptation. Urban adaptation is the sequential process by which an urban structure, function, or services becomes more fitted to its changing environment or human choices. The role of water is vital to driving urban evolution as demonstrated by historical changes in drainage, sewage flows, hydrologic pulses, and long-term chemistry. In the current paper, we show how hydrologic traits evolve across successive generations of urban ecosystems via shifts in selective pressures and adaptations over time. We explore multiple empirical examples including evolving: (1) urban drainage from stream burial to stormwater management; (2) sewage flows and water quality in response to wastewater treatment; (3) amplification of hydrologic pulses due to the interaction between urbanization and climate variability; and (4) salinization and alkalinization of fresh water due to human inputs and accelerated weathering. Finally, we propose a new conceptual model for the evolution of urban waters from the Industrial Revolution to the present day based on empirical trends and historical information. Ultimately, we propose that water itself is a critical driver of urban evolution that forces urban adaptation, which transforms the structure, function, and services of urban landscapes, waterways, and civilizations over time.

Fluvial sediment transport impacts fisheries, marine ecosystems, and human health. In the upper Chesapeake Bay, river-induced sediment plumes are generally known as either a monotonic spatial shape or a turbidity maximum. Little is known about plume evolution in response to variation in streamflow and extreme discharge of sediment. Here we propose a typology of sediment plumes in the upper Chesapeake Bay using a 17 year time series of satellite-derived suspended sediment concentration. On the basis of estimated fluvial and wind contributions, we define an intermittent/wind-dominated type and a continuous type, the latter of which is further divided into four subtypes based on spatial features of plumes, which we refer to as Injection, Transport, Temporary Turbidity-Maximum, and Persistent Turbidity-Maximum. The four continuous types exhibit a consistent sequence of evolution within 1 week to 1 month following flood events. We also identify a "shift" in typology with increased frequency of Turbidity-Maximum types before and after Hurricane Ivan (2004), which implies that extreme events have longer-lasting effects upon estuarine suspended sediment than previously considered. These results can serve as a diagnostic tool to better predict distribution and impacts of estuarine suspended sediment in response to changes in climate and land use.

This overview and synthesis paper focuses on the evolution of urban biogeochemical cycles across time. We synthesize empirical data and review existing literature, including papers in this special issue, and we propose the concept of “urban evolution.” The built environment often changes quickly in response to human activities, thus contributing to an urban evolution that affects structure, function, and ecosystem services of human settlements over time. Depending upon management, these changes can result in rapid losses of ecosystem functions/services or progress towards restoration. We explore urban evolution through empirical examples such as: (1) land development and nitrogen inputs within a metropolitan region over half a century; (2) watershed drainage by different forms of stormwater management over decades; (3) human-accelerated weathering in urbanized watersheds over decades; and (4) global salinization of freshwater across urbanizing landscapes over a century. We also synthesize concepts relevant to studying urban evolution of infrastructure and ecosystems including: (1) urban watersheds have challenged our whole notion of the “watershed approach” due to complex hydrologic boundaries and flow paths over time; (2) the urban hydrologic cycle evolves due to changing infrastructure and human water use over time; (3) the importance of extending research beyond individual sites using an urban watershed approach over space and time; (4) salinization as a universal tracer of watershed urbanization over time; (5) human-accelerated weathering of concrete and construction materials contributing to an “urban karst” over time; (6) human alteration of the carbon cycle in urban watersheds over time; and (7) detecting distinct biogeochemical signatures across cities globally over time. Our synthesis and this special issue suggest that urban biogeochemical cycles have exerted a major influence on the elemental composition of the Earth’s surface from local to global scales. A new global research agenda is needed to track the evolution of urban biogeochemical cycles as land development proceeds and infrastructure/management changes so we can better evaluate potential losses in ecosystem services, set realistic watershed and river restoration goals, and formulate effective environmental policy for Earth’s growing urban population.

Leaf litter plays a critical role in regulating ecological functions in headwater forest streams, whereas the effects of leaves on water quality in urbanized streams are not fully understood. This study examined the potential importance of leaf litter for the release and transformations of organic carbon and nutrients in urban streams, and compared the effects with other types of natural organic substrates (periphyton and stream sediment). Nutrients and organic carbon were leached from senescent leaves of 6 tree species in the laboratory with deionized water, and maximal releases, leaching rate constants, composition and bioavailability of the leached dissolved organic carbon (DOC) were determined. Stream substrates (leaf debris, rocks with periphyton, and sediment) were seasonally collected from urban and forest reference streams of the NSF Baltimore Long-term Ecological Research Site and incubated with overlying stream water to estimate areal fluxes of DOC and nitrogen. Leaf litter leaching showed large ranges in maximal releases of DOC (7.0–131 mg g−1), dissolved organic nitrogen (DON; 0.07–1.39 mg g−1) and total dissolved phosphorus (TDP; 0.14–0.70 mg g−1) among tree species. DOC leaching rate constants, carbon to nitrogen ratios, and DOC bioavailability were all correlated with organic matter quality indicated by fluorescence spectroscopy. Results from substrate incubation experiments showed far higher DOC and DON release and nitrate retention with leaf debris than with sediment, or rocks with periphyton. DOC release from leaf debris was positively correlated with stream nitrate retention at residential and urban sites, with the highest values observed during the fall and lowest during the summer. This study suggests the potential importance of leaf litter quantity and quality on fostering DOC and nutrient release and transformations in urban streams. It also suggests that species-specific impacts of leaves should be considered in riparian buffer and stream restoration strategies.

Restoring urban infrastructure and managing the nitrogen cycle represent emerging challenges for urban water quality. We investigated whether stormwater control measures (SCMs), a form of green infrastructure, integrated into restored and degraded urban stream networks can influence watershed nitrogen loads. We hypothesized that hydrologically connected floodplains and SCMs are “hot spots” for nitrogen removal through denitrification because they have ample organic carbon, low dissolved oxygen levels, and extended hydrologic residence times. We tested this hypothesis by comparing nitrogen retention metrics in two urban stream networks (one restored and one urban degraded) that each contain SCMs, and a forested reference watershed at the Baltimore Long-Term Ecological Research site. We used an urban watershed continuum approach which included sampling over both space and time with a combination of: (1) longitudinal reach-scale mass balances of nitrogen and carbon conducted over 2 years during baseflow and storms (n = 24 sampling dates × 15 stream reaches = 360) and (2) 15N push–pull tracer experiments to measure in situ denitrification in SCMs and floodplain features (n = 72). The SCMs consisted of inline wetlands installed below a storm drain outfall at one urban site (restored Spring Branch) and a wetland/wet pond configured in an oxbow design to receive water during high flow events at another highly urbanized site (Gwynns Run). The SCMs significantly decreased total dissolved nitrogen (TDN) concentrations at both sites and significantly increased dissolved organic carbon concentrations at one site. At Spring Branch, TDN retention estimated by mass balance (g/day) was ~150 times higher within the stream network than the SCMs. There were no significant differences between mean in situ denitrification rates between SCMs and hydrologically connected floodplains. Longitudinal N budgets along the stream network showed that hydrologically connected floodplains were important sites for watershed nitrogen retention due to groundwater–surface water interactions. Overall, our results indicate that hydrologic variability can influence nitrogen source/sink dynamics along engineered stream networks. Our analysis also suggests that some major predictors for watershed N retention were: (1) streamwater and groundwater flux through stream restoration or stormwater management controls, (2) hydrologic residence times, and (3) surface area of hydrologically connected features.

Nonpoint source pollution from agriculture and urbanization is increasing globally at the same time climate extremes have increased in frequency and intensity. We review >200 studies of hydrologic and gaseous fluxes and show how the interaction between land use and climate variability alters magnitude and frequency of carbon, nutrient, and greenhouse gas pulses in watersheds. Agricultural and urban watersheds respond similarly to climate variability due to headwater alteration and loss of ecosystem services to buffer runoff and temperature changes. Organic carbon concentrations/exports increase and organic carbon quality changes with runoff. Nitrogen and phosphorus exports increase during floods (sometimes by an order of magnitude) and decrease during droughts. Relationships between annual runoff and nitrogen and phosphorus exports differ across land use. CH4 and N2O pulses in riparian zones/floodplains predominantly increase with: flooding, warming, low oxygen, nutrient enrichment, and organic carbon. CH4, N2O, and CO2 pulses in streams/rivers increase due to similar factors but effects of floods are less known compared to base flow/droughts. Emerging questions include: (1) What factors influence lag times of contaminant pulses in response to extreme events? (2) What drives resistance/resilience to hydrologic and gaseous pulses? We conclude with eight recommendations for managing watershed pulses in response to interactive effects of land use and climate change.

Although wetlands are known to be sinks for nitrogen (N) and phosphorus (P), their function in urban watersheds remains unclear. We analyzed water and nitrate (NO3−) and phosphate (PO43−) dynamics during precipitation events in two oxbow wetlands that were created during geomorphic stream restoration in Baltimore County, Maryland that varied in the nature and extent of connectivity to the adjacent stream. Oxbow 1 (Ox1) received 1.6-4.2% and Oxbow 2 (Ox2) received 4.2-7.4% of cumulative streamflow during storm events from subsurface seepage (Ox1) and surface flow (Ox2). The retention time of incoming stormwater ranged from 0.2 to 6.7 days in Ox1 and 1.8 to 4.3 days in Ox2. Retention rates in the wetlands ranged from 0.25 to 2.74 g N/m2/day in Ox1 and 0.29 to 1.94 g N/m2/day in Ox2. Percent retention of the NO3−-N load that entered the wetlands during the storm events ranged from 64 to 87% and 23 to 26%, in Ox1 and Ox2, respectively. During all four storm events, Ox1 and Ox2 were a small net source of dissolved PO43− to the adjacent stream (i.e., more P exited than entered the wetland), releasing P at a rate of 0.23-20.83 mg P/m2/day and 3.43-24.84 mg P/m2/day, respectively. N and P removal efficiency of the oxbows were regulated by hydrologic connectivity, hydraulic loading, and retention time. Incidental oxbow wetlands have potential to receive urban stream and storm flow and to be significant N sinks, but they may be sources of P in urban watersheds.

An urban watershed continuum framework hypothesizes that there are coupled changes in (1) carbon and nitrogen cycling, (2) groundwater-surface water interactions, and (3) ecosystem metabolism along broader hydrologic flowpaths. It expands our understanding of urban streams beyond a reach scale. We evaluated this framework by analyzing longitudinal patterns in: C and N concentrations and mass balances, groundwater-surface interactions, and stream metabolism and carbon quality from headwaters to larger order streams. 52 monitoring sites were sampled seasonally and monthly along the Gwynns Falls watershed, which drains 170 km2 of the Baltimore Long-Term Ecological Research site. Regarding our first hypothesis of coupled C and N cycles, there were significant inverse linear relationships between nitrate and dissolved organic carbon (DOC) and nitrogen longitudinally (P

Urbanization has resulted in the extensive burial and channelization of headwater streams, yet little is known about the impacts of stream burial on ecosystem functions critical for reducing downstream nitrogen (N) and carbon (C) exports. In order to characterize the biogeochemical effects of stream burial on N and C, we measured NO3 − uptake (using 15N-NO3 − isotope tracer releases) and gross primary productivity (GPP) and ecosystem respiration (ER) (using whole stream metabolism measurements). Experiments were carried out during four seasons, in three paired buried and open stream reaches, within the Baltimore Ecosystem Study Long-term Ecological Research site. Stream burial increased NO3 − uptake lengths by a factor of 7.5 (p

Streams and rivers are major exporters of C and other dissolved materials from watersheds to coastal waters. In streams and rivers, substantial amounts of terrigenous organic C is metabolized and degassed as CO 2 to the atmosphere. A long-term evaluation of CO 2 dynamics in streams is essential for understanding factors controlling CO 2 dynamics in streams in response to changes in climate and land-use. Long-term changes in the partial pressure of CO 2 ( p CO 2 ) were computed in the Anacostia River and the lower Potomac River in the Chesapeake Bay watershed. Long-term estimates were made using routine monitoring data of pH, total alkalinity, and dissolved nutrients from 1985 to 2006 at 14 stations. Longitudinal variability in p CO 2 dynamics was also investigated along these rivers downstream of the urban Washington D.C. metropolitan area. Both rivers were supersaturated with CO 2 with respect to atmospheric CO 2 levels (392 μatm) and the highly urbanized Anacostia waters (202–9694 μatm) were more supersaturated than the Potomac waters (557–3800 μatm). Long-term variability in p CO 2 values may be due to changes in river metabolism and organic matter and nutrient loadings. Both rivers exchange significant amounts of CO 2 with the atmosphere (i.e., Anacostia at 0.2–72 mmol m −2 d −1 and Potomac at 0.12–24 mmol m −2 d −1 ), implying that waterways receiving organic matter and nutrient subsidies from urbanized landscapes have the potential to increase river metabolism and atmospheric CO 2 fluxes along the freshwater–estuarine continuum.

Rising water temperatures due to climate and land use change can accelerate biogeochemical fluxes from sediments to streams. We investigated impacts of increased streamwater temperatures on sediment fluxes of dissolved organic carbon (DOC), nitrate, soluble reactive phosphorus (SRP) and sulfate. Experiments were conducted at 8 long-term monitoring sites across land use (forest, agricultural, suburban, and urban) at the Baltimore Ecosystem Study Long-Term Ecological Research (LTER) site in the Chesapeake Bay watershed. Over 20 yr of routine water temperature data showed substantial variation across seasons and years. Lab incubations of sediment and overlying water were conducted at 4 temperatures (4 °C, 15 °C, 25 °C, and 35 °C) for 48 h. Results indicated: (1) warming significantly increased sediment DOC fluxes to overlying water across land use but decreased DOC quality via increases in the humic-like to protein-like fractions, (2) warming consistently increased SRP fluxes from sediments to overlying water across land use, (3) warming increased sulfate fluxes from sediments to overlying water at rural/suburban sites but decreased sulfate fluxes at some urban sites likely due to sulfate reduction, and (4) nitrate fluxes showed an increasing trend with temperature at some forest and urban sites but with larger variability than SRP. Sediment fluxes of nitrate, SRP and sulfate were strongly related to watershed urbanization and organic matter content. Using relationships of sediment fluxes with temperature, we estimate a 5 °C warming would increase mean sediment fluxes of SRP, DOC and nitrate-N across streams by 0.27–1.37 g m−2 yr−1, 0.03–0.14 kg m−2 yr−1, and 0.001–0.06 kg m−2 yr−1. Understanding warming impacts on coupled biogeochemical cycles in streams (e.g., organic matter mineralization, P sorption, nitrification, denitrification, and sulfate reduction) is critical for forecasting shifts in carbon and nutrient loads in response to interactive impacts of climate and land use change.

Stratigraphic boundaries are ideally defined by distinct lithological, geochemical, and palaeobiological signatures, to which a chronological framework can be applied. We present a range of observations that illustrate how the Holocene–Anthropocene transition meets these criteria in its expression in sediments from remote arctic and alpine lakes, removed from direct, catchment-scale, anthropogenic influences. In glaciated lake basins, the retreat of glaciers commonly leads to lithological successions from proglacial clastic sedimentation to non-glacial organic deposition. Sediments from the majority of lakes record marked depletions in the nitrogen stable isotopic composition of sediment organic matter, reflecting anthropogenic influences on the global nitrogen cycle. In all cases, siliceous microfossil assemblages (diatoms and chrysophytes) change markedly and directionally, with regional nuances. These stratigraphic fingerprints begin to appear in the sediment record after AD 1850, but accelerate in pulses between AD 1950 and 1970 and again after AD 1980. Our review indicates that recent environmental changes associated with humankind's dominance of key global biogeochemical cycles are sufficiently pervasive to be imprinted on the sediment record of remote lakes. Moreover, these changes are of sufficient magnitude to conclude that the Holocene has effectively ended, and that the concept of Anthropocene more aptly describes current planetary dynamics. The synthesis of these observations pertains directly to ongoing discussions concerning the eventual formalization of a new stratigraphic boundary.

Stream burial—the routing of streams through culverts, pipes, and concrete lined channels, or simply paving them over—is common during urbanization, and disproportionately affects small, headwater streams. Burial undermines the physical and chemical processes governing life in streams, with consequences for water quality and quantity that may amplify from headwaters to downstream receiving waters. Knowledge of the extent of stream burial is critical for understanding cumulative impacts to stream networks, and for future decision-making allowing for urban development while protecting ecosystem function. We predicted stream burial across the urbanizing Potomac River Basin (USA) for each 10-m stream segment in the basin from medium-resolution impervious cover data and training observations obtained from high-resolution aerial photography in a GIS. Results were analyzed across a range in spatial aggregation, including counties and independent cities, small watersheds, and regular spatial grids. Stream burial was generally correlated with total impervious surface area (ISA), with areas exhibiting ISA above 30% often subject to elevated ratios of stream burial. Recurring patterns in burial predictions related to catchment area and topographic slope were also detected. We discuss these results in the context of physiographic constraints on stream location and urban development, including implications for environmental management of aquatic resources.

Recent studies have shown that stormwater control measures (SCMs) are less effective at retaining phosphorus (P) than nitrogen. We compared P retention between two urban/suburban SCMs and their adjacent free-flowing stream reaches at the Baltimore Long-Term Ecological Study (LTER) site, and examined changes in P retention in SCMs across flow conditions. Results show that, when compared with free-flowing stream reaches, the SCMs had significantly lower dissolved oxygen (%DO) and higher P concentrations, as well as lower mean areal retention rates and retention efficiencies of particulate P (PP). In all the SCMs, concentrations of total dissolved phosphorus (TDP) consistently exhibited inverse correlations with %DO that was lower during summer base flows. Particulate phosphorus (PP) concentrations peaked during spring high flow period in both streams and in-line pond/SCMs, but they were also higher during summer base flows in suburban/urban SCMs. Meanwhile, PP areal retention rates and retention efficiencies of the SCMs changed from positive (indicating retention) during high flows to negative (indicating release) during low flows, while such changes across flow were not observed in free-flowing stream reaches. We attribute the changing roles of SCMs from a PP sink to a PP source to changes in SCM hydrologic mass balances, physical sedimentation and biogeochemical mobilization across flows. This study demonstrates that in suburban/urban SCMs, P retained during high flow events can be released during low flows. Cultivation of macrophytes and/or frequent sediment dredging may provide potential solutions to retaining both P and nitrogen in urban SCMs.

O decades, there has been increased salinization of fresh water in many regions of the world. Salinization was previously thought to be an environmental problem restricted primarily to arid regions. However, it is now recognized as a global environmental concern impacting humid regions due to human inputs from road deicers, sewage inputs, and water softeners (Figure 1). In both arid and humid regions, there can be “natural” variations in geologic sources of salts due to chemical weathering. Recently, it has been suggested that human activities have contributed to accelerated weathering of geologic sources and additional inputs of salinization. Thus, both human inputs and accelerated weathering are contributing to the increased salinization of fresh water. Increased salinization can impact drinking water through leaching of contaminants in soils, sediments, and water infrastructure. For example, corrosion of water infrastructure influenced by road salt has recently been linked to lead contamination in drinking water in Flint, Michigan. Deicers can also contain impurities and toxic anticaking agents. Despite these and other concerns regarding drinking water contamination, salt concentrations are not currently regulated by the U.S. Environmental Protection Agency. Given widespread increasing salinization trends, federal regulation of salt concentrations could lead to better management aimed at reducing impacts to drinking water and aging infrastructure. In northern regions, salinization of fresh water is significantly related to increasing impervious surface cover in watersheds. In addition to geologic sources and other human inputs, salinization increases as deicer is applied to impervious surfaces to promote safe transportation. During winter months, salt concentrations in urban waters can spike up to approximately 25% the salinity of seawater. It can take weeks and months for these salt pulses to diminish, and salt concentrations can remain chronically elevated during summer months. Interestingly, long-term baseline concentrations of salts have increased in fresh water, even during seasons when deicer is not applied. These widespread trends are due to the accumulation of salts stored in soils and groundwater. Thus, even if road salt applications ceased, many freshwater ecosystems would still remain chronically salinized for decades. Increased salinization of fresh water now represents a chronic environmental problem. In watersheds, salinization of fresh water enhances mobilization of contaminants from soils and sediments. It is well-known that sodium from road salts can displace toxic metals and base cations from ion exchange sites on soils and sediments. For example, toxic metals such as copper, lead, and zinc are released from soil exchange sites near streams following applications of road salt. In response to salinization, organic carbon can be released from changes in soil structure, microbial decomposition, and desorption. Salinization enhances leaching of organic carbon from soils and sediments, which can increase disinfection byproducts in fresh water. In addition, salinization enhances leaching of organic nitrogen, ammonium, and phosphorus from ion exchange sites on soils. Mobilization of nitrogen and phosphorus to drinking water supplies can further contribute to eutrophication. Overall, increased salinization of fresh water enhances mobilization of previously

Organic carbon is important in regulating ecosystem function, and its source and abundance may be altered by urbanization. We investigated shifts in organic carbon quantity and quality associated with urbanization and ecosystem restoration, and its potential effects on denitrification at the riparian–stream interface. Field measurements of streamwater chemistry, organic carbon characterization, and laboratory-based denitrification experiments were completed at two forested, two restored, and two unrestored urban streams at the Baltimore Long-Term Ecological Research site, Maryland, USA. Dissolved organic carbon (DOC) and nitrate loads increased with runoff according to a power-law function that varied across sites. Stable isotopes and molar C:N ratios suggested that stream particulate organic matter (POM) was a mixture of periphyton, leaves, and grass that varied across site types. Stable-isotope signatures and lipid biomarker analyses of sediments showed that terrestrial organic carbon sources in streams...

Legacy effects of past land use and disturbance are increasingly recognized, yet consistent definitions of and criteria for defining them do not exist. To address this gap in biological- and ecosystem-assessment frameworks, we propose a general metric for evaluating potential legacy effects, which are computed by normalizing altered system function persistence with duration of disturbance. We also propose two distinct legacy-effect categories: signal effects from lags in transport and structural effects from physical landscape changes. Using flux records for water, sediment, nitrogen, and carbon from long-term study sites in the eastern United States from 1500 to 2000, we identify gaps in our understanding of legacy effects and reveal that changes in basin sediment dynamics precede instrumented records. These sediment dynamics are not generally incorporated into interpretations of contemporary records, although their potential legacy effects are substantial. The identification of legacy effects may prove to be a fundamental component of landscape management and effective conservation and restoration practice.

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.

Geomorphic spatial heterogeneity affects sediment denitrification, an anaerobic microbial process that results in the loss of nitrogen (N), and other anaerobic microbial processes such as methanogenesis in urban streams. We measured sediment denitrification potential (DEA), net nitrification, methanogenesis, and a suite of ancillary microbial variables in geomorphic stream features (organic debris dams, pools, riffles, and sloughs) in forested, urban degraded and urban restored streams in the Baltimore, MD, USA metropolitan area, between June 2005 and November 2006. DEA was higher in organic debris dams (2783 ± 1405 ng N g −1 h −1 , mean ± standard error) than in pools (505 ± 144), riffles (360 ± 78), and sloughs (270 ± 106), and was higher in forest (1439 ± 613) than in urban degraded (442 ± 98) and restored sites (391 ± 116), but the differences were not statistically significant. DEA was positively related to microbial biomass N (MBN) ( p p = 0.006). DEA and MBN were significantly higher in June 2005 and August 2006 than in November 2006, a temporal pattern that may have been driven by changes in microbial biomass. Methanogenesis was active in all stream geomorphic features across all study sites. Reach scale estimates of nitrification ranged from 157 to 344 mg N m −2 d −1 and were similar to reach scale DEA rates (97–230 mg N m −2 d −1 ), with no significant differences between restored and unrestored reaches. These results suggest that in-stream geomorphic features in urban restored and degraded sites have the potential to function as N sinks by maintaining anaerobic conditions and microbial biomass and activity that stimulate denitrification.

Urban ecological studies, including focus on cities, suburbs, and exurbs, while having deep roots in the early to mid 20th century, have burgeoned in the last several decades. We use the state factor approach to highlight the role of important aspects of climate, substrate, organisms, relief, and time in differen- tiating urban from non-urban areas, and for determining heterogeneity within spatially extensive metropolitan areas. In addition to reviewing key findings relevant to each state factor, we note the emergence of tentative "urban syndromes" concerning soils, streams, wildlife and plants, and homog- enization of certain ecosystem functions, such as soil organic carbon dynamics. We note the utility of the ecosystem approach, the human ecosystem framework, and watersheds as integrative tools to tie information about multiple state factors together. The organismal component of urban complexes includes the social organization of the human population, and we review key modes by which human populations within urban areas are differentiated, and how such differentiation affects environmentally relevant actions. Emerging syntheses in land change science and ecological urban design are also summarized. The multifaceted frameworks and the growing urban knowledge base do however identify some pressing research needs.

Riparian wetlands have been shown to be effective "sinks" for nitrate N (NO3-), minimizing the downstream export of N to streams and coastal water bodies. However, the vast majority of riparian denitrification research has been in agricultural and forested watersheds, with relatively little work on riparian wetland function in urban watersheds. We investigated the variation and magnitude of denitrification in three constructed and two relict oxbow urban wetlands, and in two forested reference wetlands in the Baltimore metropolitan area. Denitrification rates in wetland sediments were measured with a 15N-enriched NO3- "push-pull" groundwater tracer method during the summer and winter of 2008. Mean denitrification rates did not differ among the wetland types and ranged from 147 +/- 29 microg N kg soil(-1) d(-1) in constructed stormwater wetlands to 100 +/- 11 microg N kg soil(-1) d(-1) in relict oxbows to 106 +/- 32 microg N kg soil(-1) d(-1) in forested reference wetlands. High denitrification rates were observed in both summer and winter, suggesting that these wetlands are sinks for NO3- year round. Comparison of denitrification rates with NO3- standing stocks in the wetland water column and stream NO3- loads indicated that mass removal of NO3- in urban wetland sediments by denitrification could be substantial. Our results suggest that urban wetlands have the potential to reduce NO3- in urban landscapes and should be considered as a means to manage N in urban watersheds.

The Humane metropolis is a rubric to summarize and promote environmental and social quality in contemporary urban mosaics. Because cities, suburbs, and exurbs, as spatially extensive and connected socio-ecological systems, exhibit many negative features, the humane metropolis identifies a strategy to combat the ills and instill more positive and sustainable features and processes in urban systems. Because the humane metropolis as a program has arisen primarily from social motivations, there is the opportunity to articulate more explicitly the role that science can play in addressing the humane metropolis program and evaluating its success. A humane metropolis can be summarized as one that 1) protects and restores ecological services in cities and suburbs, 2) promotes physical and mental health and safety of residents, 3) enhances efficiency by conserving energy, matter, water, and time, 4) facilitates equity by being inclusive, as well as socially and environmentally just, and 5) maintains a sense of community and a sense of place. We clarify the nature of science as a contributor to the social program, pointing out the social values motivating science, and the role that scientific knowledge and metaphor play in linking science with the social program of the humane metropolis. We further identify roles that socio-ecological research can play in meeting the goals of the humane metropolis. We use examples of environmental history, watershed function and restoration, and environmental justice research and action from the Baltimore Ecosystem Study, Long-Term Ecological Research program. The humane metropolis as a social program benefits from scientific contributions that 1) expose hidden ecological processes in urban systems, 2) generate knowledge connecting people and institutions to the biophysical environment, 3) contribute to the civic dialog, and 4) bring scientific values to the prioritization and balancing of the goals of the humane metropolis.

Although nitrogen (N) is prevalent in urban stormwater, regulation of this pollutant has occurred only more recently. This paper reviews the concerns over N in urban stormwater, mechanisms and design enhancements for N uptake and denitrification through various stormwater control measures (SCMs), and presents opportunities to integrate this current knowledge into the regulatory framework. A survey of personnel directly involved in various aspects of US state and territory NPDES programs revealed that the top three pollutants of concern were total suspended solids (TSS), pathogens and bacteria, and total phosphorus (TP). Surprisingly, nitrate (NO 3 − ) was of little concern among the survey respondents, with 3.9% giving it the highest level of concern, 2.0% ranking it second, and 6.0% ranking it third. When asked which strategies were currently used in their geographic area for stormwater management, the most common results were wet ponds and dry ponds. At the same time, wet ponds and dry ponds were recognized as less effective practices to manage stormwater. A review of current literature reveals that several alternative SCMs, such as bioretention, filters, and wetlands, show greater promise in their ability to remove N from stormwater than more conventional practices such as dry ponds and wet ponds. Enhanced N removal via denitrification and plant uptake is often observed under the combination of aerobic followed by sustained anoxic conditions, the presence of a carbon source (organic material), and the presence of mature, dense vegetation. Given the lack of concern or awareness of local officials related to N loading from urban stormwater, and variation in the efficacy of various SCMs, it is not surprising that regulators remain focused on conventional dry pond and wet pond control measures. More needs to be done to quantify the impact of urban sources of N on water quality and aquatic ecosystems. In addition, greater focus needs to be placed on the development of design criteria for SCMs, such as bioretention, filters, and constructed wetlands, which show more promise for N removal.

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.