Twenty-six key research questions in urban stream ecology: an assessment of the state of the science Author(s): Seth J. Wenger, Allison H. Roy, C. Rhett Jackson, Emily S. Bernhardt, Timothy L. Carter, Solange Filoso, Catherine A. Gibson, W. Cully Hession, Sujay S. Kaushal, Eugenia Martí, Judy L. Meyer, Margaret A. Palmer, Michael J. Paul, Alison H. Purcell, Alonso Ramírez, Amy D. Rosemond, Kate A. Schofield, Elizabeth B. Sudduth, and Christopher J. Walsh Source: Journal of the North American Benthological Society, 28(4):1080-1098. Published By: The Society for Freshwater Science https://doi.org/10.1899/08-186.1 URL: http://www.bioone.org/doi/full/10.1899/08-186.1

BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder.

BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. J. N. Am. Benthol. Soc., 2009, 28(4):1080–1098 ’ 2009 by The North American Benthological Society DOI: 10.1899/08-186.1 Published online: 27 October 2009

Twenty-six key research questions in urban stream ecology: an assessment of the state of the science

Seth J. Wenger1,17, Allison H. Roy2,18, C. Rhett Jackson3,19, Emily S. Bernhardt4,20, Timothy L. Carter1,21, Solange Filoso5,22, Catherine A. Gibson6,23, W. Cully Hession7,24, Sujay S. Kaushal5,25, Eugenia Martı´8,26, Judy L. Meyer9,27, Margaret A. Palmer5,28, Michael J. Paul10,29, Alison H. Purcell11,30, Alonso Ramı´rez12,31, Amy D. Rosemond13,32, Kate A. Schofield14,33, Elizabeth B. Sudduth15,34, 16,35 AND Christopher J. Walsh 1 River Basin Center, University of Georgia, 110 Riverbend Road, Athens, Georgia 30602 USA 2 Office of Research and Development, US Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268 USA 3 Warnell School of Forestry and Natural Resources, University of Georgia, Athens, Georgia 30602 USA 4 Department of Biology, Phytotron Box 90338, Duke University, Durham, North Carolina 27708 USA 5 University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, 1 Williams St., P.O. Box 38, Solomon, Maryland 20688 USA 6 Department of Environmental Studies, Skidmore College, 815 North Broadway, Saratoga Springs, New York 12866 USA 7 Department of Biological Systems Engineering, 304 Seitz Hall, Virginia Polytechnical Institute and State University, Blacksburg, Virginia 24061 USA 8 Limnology Group, Centre d’Estudis Avanc¸ats de Blanes (CSIC), Acce´s a la Cala Sant Francesc 14, 17300 Blanes, Girona, Spain 9 Odum School of Ecology, University of Georgia (emeritus), 498 Shoreland Dr., Lopez Island, Washington 98261 USA 10 Tetra Tech, Inc., 400 Red Brook Blvd., Suite 200, Owings Mills, Maryland 21117 USA 11 Department of Environmental and Natural Resources Sciences, Humboldt State University, 1 Harpst St., Arcata, California 95521 USA 12 Institute for Tropical Ecosystem Studies, University of Puerto Rico, P.O. Box 21910, San Juan, Puerto Rico 00931 USA 13 Odum School of Ecology, University of Georgia, Athens, Georgia 30602 USA 14 National Center for Environmental Assessment, US Environmental Protection Agency, 2400 Pennsylvania Ave. NW, Washington, DC 20460 USA 15 Department of Biology, Duke University, Durham, North Carolina 27708 USA 16 Department of Resource Management and Geography, The University of Melbourne, Parkville, Victoria 3010, Australia

17 E-mail addresses: [email protected] 18 [email protected] 27 [email protected] 19 [email protected] 28 [email protected] 20 [email protected] 29 [email protected] 21 [email protected] 30 [email protected] 22 [email protected] 31 [email protected] 23 [email protected] 32 [email protected] 24 [email protected] 33 [email protected] 25 [email protected] 34 [email protected] 26 [email protected] 35 [email protected] 1080 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1081

Abstract. Urban streams have been the focus of much research in recent years, but many questions about the mechanisms driving the urban stream syndrome remain unanswered. Identification of key research questions is an important step toward effective, efficient management of urban streams to meet societal goals. We developed a list of priority research questions by: 1) soliciting input from interested scientists via a listserv and online survey, 2) holding an open discussion on the questions at the Second Symposium on Urbanization and Stream Ecology, and 3) reviewing the literature in the preparation of this paper. We present the resulting list of 26 questions in the context of a review and summary of the present understanding of urban effects on streams. The key questions address major gaps in our understanding of ecosystem structure and function responses (e.g., what are the sublethal impacts of urbanization on biota?), characteristics of urban stream stressors (e.g., can we identify clusters of covarying stressors?), and management strategies (e.g., what are appropriate indicators of ecosystem structure and function to use as management targets?). The identified research needs highlight our limited understanding of mechanisms driving the urban stream syndrome and the variability in characteristics of the effects of urbanization across different biogeoclimatic conditions, stages of development, government policies, and cultural norms. We discuss how to proceed with appropriate management activities given our current incomplete understanding of the urban stream syndrome.

Key words: urbanization, impervious surface, stressors, management, ecosystem function, community structure.

Catchment urbanization sets into motion a cascade stream management objectives. Our intention is to of changes to stream ecosystems. These changes, identify research areas that will yield insights useful collectively termed the urban stream syndrome for achieving efficient, effective management of urban (Meyer et al. 2005a, Walsh et al. 2005b), have been stream stressors. the subject of an expanding field of scientific research We define urban in the broadest possible sense to that has contributed to successful characterization of include the entire landscape developed for residen- urban streams in many parts of the world (see reviews tial, commercial, industrial, and transportation pur- by Paul and Meyer 2001, Walsh et al. 2005b). poses, including cities, towns, villages, suburbs, and Nevertheless, considerable gaps in our understanding exurban sprawl that has a density of .1 residential remain. Some areas, such as urban stream processes unit/2 ha. We use this encompassing definition and sublethal effects on biotic assemblages, are only because even very low-density development can have partially understood (Martı´ et al. 2004, Smith et al. measurable negative effects on aquatic ecosystems. 2009), and the work of characterizing the variability of We focus on impervious cover (total imperviousness the urban stream syndrome under differing govern- [TI]) as an indicator of urban intensity because, in the ment policies, cultural norms, and biogeoclimatic absence of deliberate management, TI is highly conditions has only just begun. Perhaps the biggest correlated with stream degradation (Leopold 1968, gap in our knowledge concerns the relative impor- Schueler 1994, Booth and Jackson 1997). This rela- tance of different mechanisms driving the urban tionship is captured by the Impervious Cover Model stream syndrome and the interactions among them. (ICM; Center for Watershed Protection 2003), which Our ability to make inferences about the relative takes the form of a wedge-shaped distribution of importance of different stressors is often confounded ecological condition. Streams with low TI can vary because the process of urbanization changes multiple widely in condition, from minimally altered to stressors at the same time (Paul and Meyer 2001, degraded, but as TI increases, the best attainable Walsh et al. 2005b). condition declines until only degraded streams are Our objective is to advance the pace of research into observed. these difficult problems by identifying some of the The strong relationship between TI and stream most critical unanswered questions in urban stream ecosystem indicators is driven by the fact that TI ecology. We are motivated in part by an interest in affects streams via multiple mechanistic pathways developing a body of information useful to urban and is closely correlated with other stressors (Brabec stream managers. The current lack of mechanistic et al. 2002, Walsh et al. 2005b). Elevated TI causes an understanding makes difficult the identification of the increase in contaminant-laden stormwater runoff that specific stressors of concern under different condi- alters stream hydrology at the same time that it alters tions and the most efficient solutions for meeting water chemistry (Fig. 1). This effect is often exacer- 1082 S. J. WENGER ET AL. [Volume 28

FIG. 1. Conceptual model of urban impacts on streams. Arrows show selected major pathways, but many important pathways are omitted for readability. For example, piped/filled channels affect virtually all other stressors but most of these linkages are not shown. All water-quality variables (grouped within dashed lines in center) are treated as a group for purposes of linkages to instream stressor sources; all stream ecosystem variables (grouped within dashed lines at right) are likewise treated as a group. Pathways among the stream ecosystem variables are not shown for readability. EI = effective impervious, mgt. = management, regs. = regulations.

bated by routing runoff to streams through storm- driving the urban stream syndrome in some regions water drainage pipes. For this reason, such directly of the world. These various stressors and their connected impervious cover (termed effective imper- relationships to sources and responses are shown viousness [EI]) is often a better predictor of stream graphically in Fig. 1. Only selected major causal ecological condition than is TI (e.g., Wang et al. 2001, pathways are shown and much detail, including the Hatt et al. 2004; see also review by Brabec et al. 2002). complexity of urban stream food webs and functional High TI and EI tend to be correlated with sewerage relationships (grouped in a box at the right of the infrastructure, stream piping, and reductions of diagram), is omitted. More detailed diagrams for riparian vegetation, all of which can further degrade individual stressors were developed for the US water and habitat quality. On the other hand, some Environmental Protection Agency (EPA) Causal urban stressors, such as septic systems and point- Analysis/Diagnosis Decision Information System source discharges, with weaker correlations with (CADDIS) program (www.epa.gov/caddis). We have impervious cover might be equally important in adopted symbols and terms consistent with CADDIS 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1083

TABLE 1. Number of individuals attending the 26 questions that we suggest are among the top Symposium on Urbanization and Stream Ecology (SUSE2) research priorities needed to improve our under- by affiliation type. The distribution of affiliations of standing and management of urban streams. members of the urban stream listserv was similar. Although urban stream management was a moti- vation for developing the research questions, the Affiliation type Attendees attendees at SUSE2 and the contributors of research University 75 questions were mostly academics and government US federal government 23 researchers, not watershed managers or planners Consultant 6 (Table 1). Therefore, this list represents the opinion US local or state government 5 of a subset of the research community regarding Nongovernmental organization 3 knowledge gaps that should be filled to develop more Non-US government 3 Private individual 1 informed and effective urban stream management Total 116 policies and strategies. SUSE2 included a panel of watershed managers to help focus discussions, but the primary goal was not to identify research priorities for meeting existing short-term manage- and consider our diagram to be a collapsed version ment goals (although these were considered), but that aggregates all CADDIS diagrams related to urban rather to consider what new research was needed to stressors. In our paper, we used the organizational advance the state of the science in ways that will framework of ecosystem responses, stressors, and better meet the broader objective of healthy urban sources to summarize our current understanding of streams. We intended this project to be international urban stream ecosystems. We identified critical in scope, but most of the authors of this paper and unknown elements, which we list as key research 91% of participants at SUSE2 were from the US questions. Later, we discuss how urban stream (including 7 from Puerto Rico). The remaining SUSE2 management can proceed with our current level of participants were from Australia, Brazil, Canada, understanding. China, New Zealand, and Spain. This geopolitical distribution no doubt introduced some bias into the Methods for identifying key questions findings. We identified key research questions through 3 phases of information gathering: 1) solicitation of Stream Ecosystem Responses to Urbanization questions via a listserv and website survey, 2) Stream community structure responses breakout sessions during the Second Symposium on Urbanization and Stream Ecology (SUSE2; Salt Lake Research has repeatedly demonstrated declines in City, Utah), and 3) drafting of this paper. During assemblage richness, diversity, and biotic integrity of winter 2007 to 2008, we solicited candidate research algae, invertebrates, and fishes with increasing ur- questions from a listserv of .100 ecologists, engi- banization (reviewed in Paul and Meyer 2001, Walsh neers, and environmental scientists. We removed et al. 2005b). The disappearance of sensitive species is redundant questions and combined similar ones to sometimes accompanied by an increase in tolerant produce a list of 47 questions, which were then species, many of which might be nonnative. Fewer resubmitted to listserv members for ranking in a studies have addressed the responses of herpeto- website survey. Twenty-eight people responded to fauna, riparian birds, and other vertebrates to the survey. Based on the results, we cut low-ranked urbanization (Fig. 2), but the limited data suggest questions, combined and split some questions, and reduced abundances of taxa in these groups in urban added a few new ones to produce a revised list of 38 streams (Mattsson and Cooper 2006, Lussier et al. questions. At the SUSE2 meeting held on 22 to 23 May 2006, Miller et al. 2007). Microbial communities in 2008, 116 attendees (Table 1) were divided into 4 urban streams are not well studied and should be a groups to review and refine the proposed questions. focus of future research (question 1; Table 2). The Each group produced a new list of questions, many of general pattern of structural change in response to which were broader in scope than the initial list. This increasing urbanization is fairly consistent across process yielded a total of 19 questions. Our paper is different biogeoclimatic conditions, but responses of based on the list of questions refined by meeting individual taxa might vary in different physiographic participants, complemented by some of the questions regions (Utz et al. 2009). The effects of urbanization from the web survey and additional questions added also might be exacerbated by climate shifts (Webb and during manuscript preparation. The result is a set of King 2009, Nelson et al. 2009), another area in need of 1084 S. J. WENGER ET AL. [Volume 28

observed for different taxa and different geographic locations (Walsh et al. 2005b), we argue against attempting to identify a universal threshold of imperviousness or urbanization at which ecosystem responses become significant. Furthermore, evidence indicates that algae, macroinvertebrate, and fish assemblages are influenced by different urban stress- ors (Brown et al. 2009, Walters et al. 2009). Multiple taxa often are used to assess urban impacts and causal pathways because of the lack of concordance among assemblages and potential differences in response mechanisms (e.g., European Union Water Framework Directive 2000, Walsh and Kunapo 2009). The structural changes described above might be

FIG. 2. Number of papers related to effects of urbaniza- influenced by trophic and other species interactions, tion on biological assemblages published in peer-reviewed but these mechanistic pathways are not well studied journals between 1988 and 2008. Search included all articles in urban streams. For example, both aquatic macro- found in Web of Science using the following search criteria: invertebrate species and invertivore fish species might urban* AND (stream* OR river*) AND algae (alga* OR decline in urban streams, but whether these responses periphyton OR diatom* OR macrophyte*) AND macroin- to stressors are independent or the fish decline is vertebrates (macroinvertebrate* OR invertebrate* OR in- mediated in part by alteration of food resources is not sect*), fishes (fish*), birds (bird*) AND herpetofauna clear. Similarly, loss of sensitive species could result (amphibian* OR salamander* OR reptile*). from loss of suitable habitat conditions, or it could reflect a more subtle shift in the competitive environ- ment that favors generalists. More research on foodweb shifts, species interactions, and sublethal study (question 2). Urbanization effects often are responses to urbanization is required (question 3). reduced or confounded when urbanization occurs on land previously used for agriculture (Fitzpatrick et al. Stream functional responses 2004, Van Sickle et al. 2004, Heatherly et al. 2007). In their synthesis of research in 9 metropolitan areas Stream ecosystem functional responses to urbani- across the US, Brown et al. (2009) found no detectable zation are less studied than are structural responses, responses of water quality (N and herbicides), algae, but this imbalance is beginning to shift. One frequent and fishes to urbanization of previously agricultural finding is that leaf breakdown rates are higher in lands. Presumably, this lack of response was observed some urban streams than in nonurban streams, but because sensitive members of these groups already putative mechanisms differ. Paul et al. (2006) attrib- had been eliminated by stressors associated with uted higher breakdown rates to physical abrasion, a agricultural activities. result supported by the findings of Swan et al. (2008). Responses vary by assemblage group, and macro- However, Imberger et al. (2008) demonstrated that invertebrates often show the highest sensitivity to microbial activity, not physical abrasion, was the urbanization (Brown et al. 2009, Walters et al. 2009). primary driver. In streams with significant contami- However, this pattern might be a result of the higher nation by Zn, Cu, and other metals, leaf decomposi- diversity of macroinvertebrates than of fishes, and our tion might be reduced as a result of reduced greater knowledge of invertebrate taxa than of algal abundances of shredders (Duarte et al. 2008, Roussel taxa. Some studies have demonstrated responses at et al. 2008). Thus, leaf breakdown rates can show a very low levels of urbanization, such as 4% total unimodal response to increasing urbanization in urban area (fish; Miltner et al. 2004), 4.4% TI which rates increase with higher nutrients and (invertebrates; Ourso and Frenzel 2003), and even microbial activity up to some threshold level of urban 1% EI (diatoms; Walsh et al. 2005a). The spatial impact and then decrease in response to high toxicant arrangement of impervious cover and urban devel- concentrations at high levels of urban impact (Chad- opment in a watershed might be a significant wick et al. 2006). However, Imberger et al. (2008) determinant of the magnitude of stream ecosystem found increased leaf breakdown despite reduced response (King et al. 2005, Moore and Palmer 2005). shredder abundance, perhaps in part because urban Given the range of response patterns and thresholds riparian zones in some locations are dominated by 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1085

TABLE 2. Twenty-six key research questions in urban stream ecology. Questions are organized by the order referenced in the text, not by importance.

Question number Question 1 What is the relationship between urbanization and the structure and function of microbial communities and their associated services? 2 How will climate change affect structural and functional responses to urbanization? 3 What behavioral and other sublethal life history changes occur to species under urbanization, and how do these affect species interactions? 4 How do transport, retention, removal and transformation of nutrients and C vary among urban streams and compare to less disturbed streams? 5 To what extent do urban-induced changes in lower trophic levels affect upper trophic levels? 6 How do primary and secondary productivity vary among urban streams and compare to less disturbed streams? 7 How do hydrologic budgets and stream hydrologic regimes vary based on density of urbanization, stormwater mitigations, application of low-impact development principles, riparian and forest retention policies, age of infrastructure, climate, and soil conditions? 8 What is the contribution of aging infrastructure (e.g., leaking sewer and water lines) to stream flow, nutrient levels, and toxin levels? 9 What are channel geomorphic responses at different stages of urbanization, are the responses predictable, and do urban streams eventually reach a new stable state? 10 What are the characteristics of structure and function within piped and concrete-lined streams, especially with regard to biogeochemical processing? 11 How do piped and concrete-lined streams affect ecosystem structure and function in downstream reaches? 12 To what degree has elevated temperature contributed to biotic assemblage shifts in urban streams across various regions? 13 Does elevated temperature in urban streams contribute to increases in metabolic rates? 14 Which toxicants have significant effects on urban stream ecosystem structure and function and at what concentrations? 15 Do urban toxicants alter the community or functional composition of microbial communities, thereby affecting critical ecosystem functions? 16 Does alteration of terrestrial inputs have significant effects on urban stream ecosystems, and under what circumstances? 17 How does urbanization affect movement of aquatic organisms and populations both within and beyond urban areas? 18 How can we best identify clusters of covarying stressors (e.g., by causal pathway analysis, management approach, etc.) to minimize the effort required for evaluating individual stressors? How do clusters of covarying stressors vary based on region, type of development, and stage of development? 19 What are the interactions and synergies among multiple stressors and multiple responses? 20 Can retrofitted, dispersed stormwater treatment measures in existing urban areas mimic some of the important ecological and hydrological processes previously performed by headwater streams that are now buried? 21 What are urban stream management strategies that provide both stream ecosystem improvements and other societal benefits? 22 Which management actions are likely to achieve improved ecological condition under different levels of impervious cover and different current stream conditions? 23 What are cost-effective restoration strategies for different stream conditions and different stressors, and when and how should they be applied? 24 What are appropriate indicators of ecosystem structure and function to use as management targets under different circumstances? 25 How do structure and function in urban streams combine to produce ecosystem goods and services, and how do those services map to those desired by the public and decision makers? 26 How can we improve communications between scientists, managers, planners, engineers and stakeholders?

exotic species with more labile leaves than native capacity of these streams for attenuating increased N species. We have seen no published studies of woody loads might become diminished or saturated com- debris breakdown rates in urban streams. pared with biotic capacity in streams draining natural N processing in urban streams is an area of active landscapes (e.g., Martı´ et al. 2004, Grimm et al. 2005, research, and some patterns are emerging (reviewed Gibson and Meyer 2007, Klocker et al. 2009). In some by Bernhardt et al. 2008). Urban streams tend to cases, the capacity of streams to assimilate N decreases transport elevated N loads (Groffman et al. 2004, Lewis at even low levels of suburban development (Kaushal and Grimm 2007, Kaushal et al. 2008b), and the biotic et al. 2006). Denitrification might be substantial and 1086 S. J. WENGER ET AL. [Volume 28 spatially variable in many urban streams (e.g., Baker et growth, but algae were regularly removed by fre- al. 2001, Groffman et al. 2005), and denitrification rates quent flow events generated by as little as 1.3 cm of 2 increase with increasing NO3 concentrations, albeit rainfall (Murdock et al. 2004). High algal biomass in not enough to keep pace; i.e., efficiency declines with urban streams has been variously explained by increasing concentrations (Mulholland et al. 2008). increased nutrients (Catford et al. 2007) and increased Where channel incision lowers floodplain water tables, light (e.g., Roy et al. 2005a). Conversely, high levels of denitrification in the riparian zone might be reduced toxicants are associated with reduced algal biomass because of a lack of saturated, anoxic riparian soils or (Hill et al. 1997), so algal biomass and primary decreased hydrologic connectivity between the stream production might follow the same unimodal relation- channel and hyporheic zone (Groffman et al. 2003, ship with urbanization as leaf breakdown rates. Many Kaushal et al. 2008a). Conversely, nitrification might be questions about the effects of urbanization on stream elevated in urban streams, particularly those receiving metabolism remain unanswered (e.g., question 6). wastewater effluent (Merseberger et al. 2005), unless dissolved O is severely depressed. Elucidating the 2 Urban Stream Stressors relative importance of different environmental vari- ables influencing N retention and removal in urban The stressors reviewed in this section are loosely streams is still a central area of research (question 4). organized to proceed from physical to chemical and Moreover, an emerging picture is that unless all species biological in nature. This organization reflects the of N are tracked, what appear to be reductions or losses concept that changes to the physical environment are might simply be conversion to other forms. often fundamental and affect many other stressors, Studies of stream ecosystem respiration and me- but does not mean that physical stressors are always tabolism have not yielded a clear trend with urban- the most important drivers of changes to biota and ization (Meyer et al. 2005a, Iwata et al. 2007, von function. Schiller et al. 2008). However, streams receiving untreated or partially treated wastewater discharg- Hydrologic alteration es—a category that includes many urban streams in the developing world, and even in Europe and North Impervious surfaces increase surface runoff, which America—have high respiration rates because of alters stream hydrology by increasing the frequency inputs of high-quality C and nutrients (Ometto et al. and magnitude of high-flow events (e.g., Leopold 2004, Izagirre et al. 2008). Mechanisms affecting 1968, Schueler 1994, Booth and Jackson 1997). The metabolism are not always straightforward. For result is often a profound change to the disturbance example, bed sediment instability, which can result regime of urban streams. The degree to which the from increased stormwater runoff in urban areas, disturbance regime changes depends heavily on the reduces both primary production and community nature of stormwater management systems (Booth respiration (Uehlinger et al. 2002). Metabolism is and Jackson 1997, Walsh et al. 2005a), including linked to light, temperature, nutrient availability, whether stormwater is collected and routed directly organic matter, and channel dynamics, so responses to streams, is routed into sewer lines in a combined vary among streams depending on how urbanization wastewater system, is temporarily detained in ponds has affected each of those factors. or other structures, or is infiltrated onsite. Base flows Some urban streams have higher algal biomass sometimes are reduced with urbanization because of relative to less disturbed streams (e.g., Taylor et al. lack of groundwater recharge, although lawn water- 2004, Catford et al. 2007), and might have higher ing, septic effluent, point-source discharges, leaking primary production. In some cases, nutrient and C water and sewer lines, and deforestation associated concentrations in urban streams stimulate sufficient with urbanization might augment base flows (Lerner periphyton growth to alter fundamentally the struc- 2002, Konrad and Booth 2005, Roy et al. 2009). In other ture of the benthic habitat (Murdock et al. 2004). These regions, flows can be reduced locally by groundwater changes to primary productivity might have cascad- and surface-water withdrawals (Konrad and Booth ing effects on higher trophic levels (question 5). 2005). Some of the mechanisms by which urban- However, use of algal standing crops to infer primary induced hydrologic alteration affects stream ecosys- productivity is especially problematic in urban tems were reviewed by Konrad and Booth (2005). streams because frequent high flows can scour and Significant among these mechanisms are increased remove algal accumulations. In a Texas stream where scour of algal assemblages, rapid export of nutrients wastewater effluent comprised ,70% of baseflow, and organic matter (i.e., decreased retentiveness), and high nutrient and C levels contributed to rapid algal direct physical washout of fauna. Hydrologic alter- 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1087 ation influences channel geomorphology and water universal (Burcher and Benfield 2006). High levels of quality and indirectly affects biotic communities and suspended and bed sediment have multiple deleteri- ecosystem processes. Climate change might further ous effects on aquatic ecosystems (reviewed by exacerbate hydrologic alteration, especially in regions Newcombe and MacDonald 1991, Waters 1995, Wood where storms increase in frequency and severity. and Armitage 1997). Issues related to hydrologic budgets and hydrologic alteration are fundamental drivers of many of the Piping and filling channels changes shown in Fig. 1 (questions 7 and 8). Piping and filling of streams (i.e., stream burial) is an extreme form of modification that is common in Altered geomorphology highly urbanized areas, especially where stormwater As a result of hydrologic alteration and changes in is routed into combined sewer–stormwater overflow sediment supply, urban stream channels typically systems. The proportion of buried streams can be have increased bed and bank erosion that leads to quite high; in Baltimore City, O of all stream reaches increased widths and cross-sectional areas compared have been buried (Elmore and Kaushal 2008). The to nonurban streams unless artificially constrained degree of burial has not been assessed in most urban (Leopold 1968, Booth and Jackson 1997, Hession et al. areas, but in many regions, most buried reaches are 2003, Chin 2006). A long-standing paradigm is that ephemeral or intermittent headwater streams (Roy et urban stream channels first undergo a period of al. 2009), which are areas of high biological activity in sedimentation from construction, subsequently expe- undisturbed systems (Meyer and Wallace 2001). rience channel enlargement from increased storm Piping of headwaters causes downstream impacts flows, and eventually stabilize (Wolman 1967, Chin via increased flow velocities, altered C and nutrient 2006). Significant urban stream channel enlargement inputs, and amplified N transport; these effects can be has been documented, but numerous studies have exacerbated by increased climatic variability (Kaushal been unable to find a relationship between degree of et al. 2008b). In the US, interest in understanding how urbanization and indicators of enlargement (Doyle et headwater degradation affects downstream waters is al. 2000, Cianfrani et al. 2006, Yagow et al. 2008). particularly strong because of important ramifications Eventual stabilization of enlarged urban streams also for regulation under the US Clean Water Act has been described (Finkenbine et al. 2000, Henshaw (Leibowitz et al. 2008). Questions 10 and 11 relate to and Booth 2000), but stabilization might be rare, and stream burial. much work needs to be done to determine under what conditions a new stable form might occur (question 9). Increased temperature and light Channel enlargement (where it occurs) can affect Urban streams often have higher summer baseflow stream ecosystems by several mechanisms. Expand- water temperatures than do nonurban streams be- ing channels can be a major source of sediment cause of the urban heat island effect (increased air (Trimble 1997); unstable channels can provide poor temperatures in urban cores), release of stored water habitat for some organisms (Schiff and Benoit 2007); from shallow detention ponds, point discharges from and channel incision can lower the water table below wastewater treatment plants, and increased insolation microbially active soil zones (Groffman et al. 2003). from removal of riparian vegetation. In some climates, Urban stream geomorphology also can be affected urban streams also suffer pulses of high temperatures by direct channel modification and by large inputs of because runoff from heated impervious surfaces can sediment. Direct modification often takes the form of result in highly variable temperatures over short time placing a stream in a concrete-lined channel to scales (Van Buren et al. 2000, Nelson and Palmer prevent it from migrating or enlarging. This practice 2007). Increased insolation also can lead to increased causes extreme habitat simplification, and although it algal production (see Stream functional responses will stabilize the local reach, it will tend to exacerbate above). Elevated water temperatures can exceed hydrologic and geomorphic impacts downstream. tolerances of cold-water species or favor species Moreover, concrete channels separate the stream from metabolically adapted to higher temperatures, there- the floodplain and the hyporheic zone and eliminate by altering assemblage structure (Krause et al. 2004, important locations of microbial processing and other Nelson et al. 2009). Elevated water temperatures also biological activity. Many urban streams also have can affect whole-reach metabolism, especially respi- elevated suspended sediment levels compared to ration. However, controls on stream temperature are streams in undeveloped watersheds (e.g., Walters et numerous and complex (LeBlanc et al. 1997, Burk- al. 2003, Grimm et al. 2005), although this trait is not holder et al. 2008). Most research has focused on 1088 S. J. WENGER ET AL. [Volume 28 developing predictive models from small- or medi- questions about the importance of toxicants in urban um-scale data sets (e.g., Van Buren et al. 2000, Nelson streams remain unanswered (questions 14 and 15). and Palmer 2007), but research is needed to address questions related to the broad-scale patterns of Dissolved O2 temperature effects and whether elevated tempera- tures are contributing to higher metabolic rates In urban streams that receive insufficiently treated (questions 12 and 13). Answers to these questions wastewater, biological and chemical O2 demand can will contribute to our understanding of the interactive be elevated and can lead to O2 deficits (reviewed in Paul and Meyer 2001). O deficits also can occur in effects of climate change and urbanization on aquatic 2 urban streams with reduced baseflows or elevated communities and ecosystem functions. organic matter (e.g., Mallin et al. 2006, Pellerin et al. 2006), and in areas where increased sedimentation Increased toxicants leads to stagnant, anaerobic pools. Most lotic macro- We use toxicants broadly to mean chemical con- invertebrates and fishes are adapted to well-oxygen- taminants that cause lethal and sublethal effects on ated environments, so low dissolved O2 can reduce aquatic organisms. Our definition includes what are biotic integrity. However, anaerobic microbial pro- commonly called emerging contaminants (potentially cesses, such as denitrification, can be enhanced under harmful but traditionally unmonitored compounds) these conditions. and regulated trace metals and organic contaminants. Detrimental effects on ecosystem structure and Increased ionic concentrations function from toxicants has been well documented Stormwater runoff, deicing salt, point-source dis- (reviewed by Paul and Meyer 2001), but determining charges, leaking sewer lines, and improperly func- which toxicants are of most concern is usually tioning septic systems can increase concentrations of difficult. Toxicants common in urban streams include dissolved solutes and conductivity in urban streams. heavy metals (particularly Cd, Cr, Cu, Pb, and Zn), Conductivity values vary naturally among streams polycyclic aromatic hydrocarbons (PAHs), and a with different underlying geology, but conductivity range of pesticides (Bannerman et al. 1993, Beasley consistently increases along gradients of urbanization. and Kneale 2002, Gilliom et al. 2006). In US National Elevated salinity can be a stressor to freshwater Water Quality Assessment (NAWQA) surveys con- organisms, particularly mayflies (Kefford et al. 2003, ducted between 1992 and 2001, 83% of urban water Kaushal et al. 2005). In high-latitude locations where samples and 70% of urban bed sediment samples salt is used as a deicer, salinity and Cl2 concentrations exceeded aquatic life benchmarks for §1 pesticides reach toxic levels for many organisms and can have (Gilliom et al. 2006). Many other potential toxicants numerous secondary effects, including acidification can be present in urban streams, especially streams and mobilization of metals (Kaushal et al. 2005, Daley that receive point-source wastewater discharges (Kol- et al. 2009). Urban streams in lower latitudes also have pin et al. 2002). Interactions among compounds can elevated conductivities (e.g., Rose 2007), but salinities have synergistic or antagonistic effects that further are generally at least an order of magnitude lower complicate our ability to predict the consequences of than toxic levels. Nevertheless, conductivity is a these substances for stream structure and function, useful and inexpensive indicator of certain aspects although some of these issues are being addressed as of urban disturbance, especially wastewater inputs part of the NAWQA program (Belden et al. 2007). (Wang and Yin 1997). Toxicity tests of urban runoff on aquatic biota have shown mixed results, with some studies reporting Increased available nutrients high toxicity (e.g., Skinner et al. 1999) and others showing low biotic responses to relatively high According to US NAWQA data from 1992 to 2001, contaminant concentrations (Maltby et al. 1995). inorganic nutrient levels are higher in urban than in Sensitivity to toxins varies greatly among taxa forested streams and are similar between urban and 2 (reviewed by Beasley and Kneale 2002), so the choice agricultural streams, although NO3 and total N are of study organism in such tests is critical. In addition, highest in agricultural streams (Mueller and Spahr many contaminants are present at much higher 2006). This pattern is less likely in many developing concentrations in sediments than in the water column, countries where fertilizer use is low and discharges of and sediment suspension during high flows can untreated wastewater in urban streams are high. + greatly increase toxicity (Christensen et al. 2006). NH4 concentrations tend to be highest in urban Despite an increasing volume of research, many streams (von Schiller et al. 2008), especially those 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1089 receiving inputs from wastewater treatment plants present, the functions of wood can be compromised in (Martı´ et al. 2004). In a recent review of N dynamics in urban streams (Larson et al. 2001). The degree to urban watersheds, Bernhardt et al. (2008) found that, which terrestrial inputs are altered and the extent to in many cities, the original source of most N is food, which this alteration drives observed ecosystem with significant additional contributions from fertil- changes are unclear (question 16). izer, drinking water, and atmospheric deposition, especially highly local deposition of combustion- Increased barriers to movement derived N produced by automobiles. In urban areas Streams in urban areas tend to have high densities of with effective wastewater treatment systems, most of instream obstructions that prevent movement of fish this N is exported to terrestrial systems as sludge, but and other aquatic organisms. Road crossings, particu- in areas with limited wastewater management much larly culverts with extreme slopes, velocities, pool sizes, N is discharged into streams and rivers (Bernhardt et or vertical drops, can prevent small organisms from al. 2008). Even where effective treatment systems passing upstream or downstream (Warren and Pardew exist, significant quantities of nutrients enter streams 1998, Schaefer et al. 2003). Culverts also can be barriers via leaking sewer and septic systems (Kaushal et al. to upstream dispersal of adult aquatic insects (Blakely 2006, Walsh and Kunapo 2009) and stormwater runoff et al. 2006), and absence of forested areas can prevent delivered to streams through stormwater drainage among-stream dispersal of adults (Smith et al. 2009). systems (Hatt et al. 2004, Bernhardt et al. 2008). The Even small channel modifications can interrupt move- actual contributions of various sources are often ment. For example, road crossings that divide discharge unclear (e.g., see question 10). Elevated nutrients into multiple sections can be unsurpassable barriers for and shifts in relative proportions of different nutrients 2 + neritid snails that rely on complex hydrologic cues for (e.g., N to P) or of forms of N (e.g., NO3 to NH4 ) can upstream migration (Blanco and Scatena 2006). Move- alter stream processes, including nutrient uptake, leaf ment barriers are of particular concern to migratory breakdown rates, and primary production. species, which include temperate anadromous species, such as salmon and eels, and a large proportion of Altered terrestrial inputs tropical fish, shrimp, and snail species (Ramı´rez et al. Reduced riparian cover and burying of streams in 2009). Thus, coastal cities that sit astride major rivers urban areas can reduce inputs of leaves, wood, and can affect aquatic communities far inland. However, the terrestrial invertebrates. However, leaves that fall overall importance of urban movement barriers has not onto roads in many urban areas ultimately wash into been well studied (question 17). streams via the storm drainage network. Thus, leaf inputs can be much higher in urban than in Evaluating multiple stressors undeveloped watersheds (Miller and Boulton 2005, Each of the stressors described above has been Carroll and Jackson 2009). Alteration of riparian observed in at least some urban streams, and each can vegetation in urban areas can significantly change significantly affect ecosystem structure and function the composition and timing of leaf inputs. For under certain circumstances. However, multiple stress- example, deciduous leaf inputs (and thus, N inputs) ors occur in most cases. This tendency for co-occurrenc- are higher in urban than in nonurban areas in the es interferes with our ability to connect observed northwestern US, and timing of inputs differs structural or functional changes to a single stressor, between urban and nonurban areas (Roberts and and herein lies the crux of the difficulty in understand- Bilby 2009). However, terrestrial inputs are more ing the urban stream syndrome. It might be that only a important in some systems than others. For example, few stressors (e.g., certain toxicants) are the proximal Amazonian stream ecosystems depend heavily on causes for most of ecosystem structural changes, but allochthonous inputs, and decreased riparian cover teasing these stressors apart from those that are and loss of terrestrial input are important factors in relatively unimportant will require an extensive series the decline of macroinvertebrates in urban streams in of reductionist studies. Therefore, we suggest that Brazil (Couceiro et al. 2007). Large wood inputs tend examining stressors that consistently covary also would to be lower in urban than in forested streams be a useful way to elucidate mechanisms (question 18). (Finkenbine et al. 2000, Elosegi and Johnson 2003), Furthermore, many stressors interact to produce non- and wood is sometimes removed to prevent flooding additive and synergistic effects, in which case manage- and damage to bridges. The lack of wood can affect ment for single stressors could cause unanticipated stream habitat and biological assemblages (reviewed effects. Thus, stressor interaction is an important area by Gurnell et al. 1995, Dı´ez et al. 2000). Even when for study (question 19). 1090 S. J. WENGER ET AL. [Volume 28

Urban Stream Management Overall, the importance of riparian degradation as an urban stressor is highly variable. In some regions, such Stressor sources and management tools as the Amazon, loss of riparian forests might be a major Many potential stressors are produced by a few key driver of stream ecosystem changes (Couceiro et al. sources, which in theory can be managed by a 2007), whereas in many more developed regions, loss of relatively small suite of tools. This principle is riparian forests is likely to be less important than illustrated in Table 3, which links the symptoms of stormwater runoff (e.g., Walsh et al. 2007). Riparian the urban stream syndrome with their specific causes degradation can be reduced by effective implementa- or sources and the prescriptions or management tion and enforcement of planning/zoning regulations practices used to address them. Here, we discuss the and riparian buffer ordinances. most widespread and significant stressor sources and Direct channel modification, resulting in buried or describe management tools used to control those concrete-lined streams, is a major stressor source in sources. many urbanizing areas. Direct regulation of stream Overall, the most significant stressor source in most burial is rare, but the practice sometimes is managed urban streams in developed countries is stormwater indirectly via riparian buffer ordinances and stream runoff (Walsh et al. 2005b). Researchers have long protection laws, such as the US Clean Water Act. recognized that increased stormwater runoff from Sometimes reach-scale restoration is used to return impervious cover is a key indicator of urban impact buried streams to the surface, but the restored stream on aquatic systems (e.g., Leopold 1968, Schueler 1994, might still be degraded (Purcell et al. 2002). Booth and Jackson 1997). Many of the stressors In most developed countries, point sources have described above are products of efficient delivery of been regulated for many decades, and management contaminant-laden stormwater runoff from impervious emphasis has shifted to nonpoint sources. Neverthe- surfaces to streams via the storm drainage systems less, in many urban areas, point sources remain (measured by EI; Brabec et al. 2002). Stormwater runoff significant sources of labile C and nutrients and a effects can be managed by planning/zoning regulations host of poorly studied toxicants that might be of that reduce EI, stormwater management ordinances that ecological importance, especially during baseflow require control or treatment of runoff with engineering conditions (Kolpin et al. 2002). In less developed or design solutions, and good housekeeping controls on countries, the level of point-source management the use and storage of toxicants, fertilizers, and other varies widely, and direct discharges of untreated contaminants. Stormwater controls might even be able waste is common in many cities (Bernhardt et al. to compensate for the loss of some of the ecosystem 2008). In such areas, point sources might be the most services provided by headwater streams (question 20). significant stressor sources. In some cases, control measures could have additional Of the remaining stressor sources, construction-site societal benefits. Stormwater harvesting provides a erosion is ubiquitous and one of the most obvious, and substantial low-energy water resource to cities (Fletcher for these reasons, it commonly is regulated to some et al. 2007), and biofiltration systems have the potential degree. Water withdrawals and impoundments are to cool urban microclimates that are affected by the very significant stressor sources in some regions. urban heat island effect (Endreny 2008). Identifying However, their effects and potential management often such win–win management strategies is another impor- transcend urban boundaries because withdrawals and tant area for future research (question 21). impoundments are not strictly urban phenomena and Riparian degradation is another source of multiple often are not managed at the scale of the city. Leaking stressors, or is a contributing factor that affects multiple septic systems, leaking sewer lines, and road crossings stressors. Intact, naturally vegetated riparian zones are stressor sources that are often of secondary have functions, such as trapping and processing importance but have potentially high local importance nutrients and toxicants, moderating temperatures, and (see Fig. 1, Table 3 for management strategies). Much contributing organic matter, that are important for research is needed to determine which management maintaining natural stream function and structure (for strategies are most effective for controlling stressors reviews see Wenger 1999, Broadmeadow and Nisbet under varying levels of impervious cover and differing 2004, Mayer et al. 2006). Urban streams commonly have biogeoclimatic conditions (question 22). reduced and altered riparian zones. Even intact urban riparian zones have limited opportunities to function Managing urban streams based on incomplete knowledge because much stormwater runoff is routed directly into streams via the stormwater conveyance network, which Answering the questions listed above will improve bypasses the riparian buffer (Roy et al. 2005b, 2006). management of urban streams by enabling us to 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1091

TABLE 3. Symptoms, causes, and mitigation for the urban stream syndrome.

Symptom Cause Mitigation Hydrologic alteration Reduced infiltration rates (through compaction, Disconnecting and minimizing Increased peak flow covering by impervious surfaces, and impervious surfaces magnitude piping of ephemeral channels) Infiltration practices Increased frequency of large Reduced evapotranspiration through loss of Bioretention practices to promote flows vegetation evapotranspiration Increased volume of storm Groundwater withdrawals Stormwater harvesting flows Imported water Forest cover Altered low flows (reduced or Infrastructure improvements to reduce increased) leakage Altered geomorphology Increased peak flows Riparian buffers Higher bed mobility Riparian disturbance Instream channel restoration Reduced bank stability Reduced channel roughness Stream engineering regulations Channel simplification Channelization Improved site design to minimize Increased fine sediment inputs Construction sediments stream disturbance Increased embeddedness Road runoff Zoning and planning Landscaping and ground disturbance Erosion and sediment control (see Channel incision and streambank retreat mitigation of hydrologic alteration) Direct channel manipulation Straightening channels Regulations preventing direct Lining with concrete and rip-rap manipulations Piping streams Change in perception of stream Filling streams appearance Restoration of floodplains to allow reconnection Bring piped streams to the surface Water chemistry changes Point-source discharges Regulations or other incentives to Increased nutrients Fertilization minimize fertilizer and pesticide use Increased pesticides Pesticide application Water quality treatment of wastewater Increased metals Contaminants from roads and stormwater (see mitigation of Increased petroleum byproducts Roofing materials hydrologic alteration) Elevated bacteria Pet waste Constructed wetlands Increased conductivity Leaky wastewater pipes Reduced sewer leakage Pharmaceuticals Septic tanks (many of the above transmitted Elimination of combined sewer overflows to streams via storm drains) Improved septic performance Increased water temperature and Riparian disturbance Riparian buffers (canopy cover) light Runoff from impervious surfaces Stormwater retention practices (see Discharges from ponds and treatment plants mitigation of hydrologic alteration) Urban heat island effect Altered terrestrial inputs Riparian disturbance Riparian buffers Reduced habitat complexity Wood removal Behavioral modification via homeowner High flows (flushing) education (see mitigation of hydrologic Road drainage alteration) Movement barriers Road culverts blocking passage Use of bridges rather than culverts Geomorphic alteration Better culvert design and placement Water quality changes Lost riparian habitat Riparian disturbance Riparian buffers (see mitigation of Exotic species hydrologic alteration and channel Channel incision modification)

target the causes of degradation with appropriate ment proceed with existing knowledge? We suggest 3 strategies and regulations. However, some of these simple steps (modified from Palmer et al. 2005): questions might never be answered fully from a 1) Identify the desired stream ecosystem state.—Poten- scientific perspective, and streams nevertheless must tial states of urban streams can be categorized into 3 be managed now. How might urban stream manage- groups: a) minimally altered streams in which near- 1092 S. J. WENGER ET AL. [Volume 28 natural ecosystem structure and function is preserved is possible is often difficult to answer, and additional or restored; b) moderately altered streams with signifi- research is needed to find practical, inexpensive, cantly modified ecosystem structure and function, but and effective stream management tools (questions 20– that still provide multiple ecosystem services (usually 22). the largest category of urban streams outside of dense 2) Identify major stressors or stressor sources and select urban cores); c) severely altered streams that have lost appropriate management actions.—The 2nd task is to all but the most tolerant species and provide few identify stressors that might prevent the stream from ecosystem services (often in concrete channels and being in the desired end state. In some cases, the managed purely for stormwater drainage and dilu- major stressors will be apparent, and in others, causal tion of contaminants). assessment tools, such as CADDIS, could be used to This categorization scheme is essentially a simpli- identify the stressors of concern. However, in many fied version of the Biological Condition Gradient cases, the available evidence will not indicate clearly (Davies and Jackson 2006) that is used by the US which stressors are critical. In such cases management EPA. Currently, most urban stream management can proceed on the basis of our understanding of proceeds without conscious declaration of goals, strong links between overarching stressor sources and and streams arrive at one of these states (usually ecosystem responses even when the intermediate moderately or severely altered) haphazardly. We mechanisms and specific stressors are poorly under- argue that explicit identification of the desired end stood. For example, using low-impact design tech- state of an urban stream is a necessary first step in its niques to infiltrate stormwater can manage stressors management to minimize misallocation of resources. ranging from hydrologic alteration to toxicants Each of these 3 states might be an appropriate choice, (Walsh et al. 2005a, Ladson et al. 2006, Murakami et depending on the situation. In a newly developing al. 2008). watershed where imperiled fish species are present, We wish to emphasize 2 basic principles for minimally altered would be the appropriate goal for selecting management approaches. First, a tool that most streams. In a heavily urbanized watershed with addresses stressors and their underlying sources is important downstream resources (say, a bay with a more likely to have long-term success than a tool that productive fishery), moderately altered might be treats symptoms. Second, preventative approaches reasonable, with a focus on minimized downstream that implement regulations in advance of develop- nutrient and contaminant transport rather than on ment are more cost-effective than retrofitting or maintaining local biotic integrity. In a heavily restoring developed watersheds. Therefore, we rec- urbanized city in a developing nation with few ommend that reach-scale stream restoration be used resources, the paramount objective could be to judiciously because it might not achieve desired goals provide improved drainage for public health pur- unless implemented as part of a broader management poses, and severely altered might be an acceptable strategy that controls the critical underlying stressors state. However, such a decision should not be made (Bernhardt and Palmer 2007). Stream restoration is a lightly. In many cases, developing cities might be multibillion dollar business, but most restoration able to avoid the mistakes of the developed world projects are not held to criteria for success (Palmer and implement new stormwater management strate- et al. 2005) or monitored (Bernhardt et al. 2005), gies that provide multiple environmental and social although notable exceptions exist (e.g., Kaushal et al. benefits, sometimes at lower financial cost than 2008a). In general, restoration of urban streams has conventional drainage practices (Bernhardt et al. relied far too heavily on structural approaches, such 2008, Roy et al. 2008). as channel reconfiguration or bank armoring, rather The choice of desired end state is one for society to than process-based approaches, such as restoration of make, perhaps through a public participation process natural flow and reestablishment of features, such as involving residents and stakeholders of each stream’s floodplain wetlands and infiltration areas, that protect catchment or region. Scientists can inform this infrastructure and promote desired ecosystem services decision by communicating the costs and benefits of (Kaushal et al. 2008a, Craig et al. 2008, Palmer 2009, setting different goals in terms of supplied ecosystem Klocker et al. 2009). In other words, restoration has services. The existing state of the stream is a key issue, focused on addressing reach-scale symptoms rather and scientists have the critical roles of assessing the than considering the underlying dynamics of the existing condition and the factors that led to it (step 2, watershed as a whole (Palmer 2009). More research is below) and advising decision makers on the kind of needed on how to design truly effective restoration improvement that is possible, given ecological, eco- strategies and how to integrate restoration of streams nomic, and political constraints. The question of what with upland stormwater management (question 23). 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1093

3) Identify appropriate monitoring indicators and well recognized (e.g., Pickett et al. 2008). However, manage adaptively.—As with any type of management discussions at the SUSE2 meeting revealed that activity, the degree to which goals are being met must scientists still have work to do to ensure we are be monitored, and a system is needed for adjusting contributing our share to effective communication. management approaches that are not delivering intended results. A discussion of these issues is Acknowledgements beyond the scope of our paper, but we note 2 key Funding for the SUSE2 meeting was provided by points. First, selection of indicators is not trivial the US EPA Office of Research and Development (question 24). Sometimes the choice of indicators is National Center for Environmental Assessment, the driven by government mandates, such as the total Baltimore Ecosystem Study Long-Term Ecological maximum daily loads of the US Clean Water Act, but Research (LTER) site, the Central Arizona–Phoenix mandated metrics are not necessarily useful surro- LTER, the US Geological Survey (USGS), and The gates for other ecosystem services of interest. The University of Georgia River Basin Center. We thank European Union Water Framework Directive (2000) Jerry McMahon of the USGS for assisting in meeting has made identification of indicators a high priority, planning. We thank 2 anonymous referees for helpful although to date, the emphasis has been more on comments on a draft of this manuscript. Last, we structural than functional indicators. Second, adaptive thank the many attendees of SUSE2 and the members management (Walters and Hilborn 1978) should be of the listserv who contributed to the development of considered from the outset. This management strate- these questions. gy is challenging in urban systems, where manage- ment decisions affect numerous stakeholders and Literature Cited might be controversial. Decision makers should be prepared from the beginning for the possibility that BAKER, L. W., D. HOPE,Y.XU,J.EDMONDS, AND L. LAUVER. management policies might have to be revisited. A 2001. Nitrogen balance for the central Arizona–Phoenix clear monitoring and assessment plan with triggers— (CAP) ecosystem. Ecosystems 4:582–602. BANNERMAN, R., D. OWENS,R.DODDS, AND N. HORNEWER. 1993. thresholds of monitoring results that require action— Sources of pollutants in Wisconsin stormwater. Water should be established early in the management Science and Technology 28:241–259. process and communicated to all involved parties. BEASLEY, G., AND P. KNEALE. 2002. Reviewing the impact of metals and PAHs on macroinvertebrates in urban The Need for Better Communication among watercourses. Progress in Physical Geography 26: Scientists, Managers, and Decision-Makers 236–270. BELDEN, J. B., R. J. GILLIOM,J.D.MARTIN, AND M. J. LYDY. 2007. Even complete scientific understanding will not How well can we predict the toxicity of pesticide benefit urban streams if the understanding is not mixtures to aquatic life? Integrated Environmental communicated effectively to planners, managers, and Assessment and Management 3:364–372. decision makers. One of the major themes of the BERNHARDT, E. S., L. E. BAND,C.J.WALSH, AND P. E. BERKE. SUSE2 meeting was the need to convey existing 2008. Understanding, managing, and minimizing urban scientific knowledge to on-the-ground practitioners. impacts on surface water nitrogen loading. Annals of A particular example is the need to describe the the New York Academy of Sciences 1134:61–96. BERNHARDT, E. S., AND M. A. PALMER. 2007. Restoring streams ecosystem services provided by urban streams, in an urbanizing world. Freshwater Biology 52:738–751. especially those of greatest societal value (question BERNHARDT, E. S., M. A. PALMER,J.D.ALLAN,G.ALEXANDER,K. 25). A more fundamental question is how to establish BARNAS,S.BROOKS,J.CARR,S.CLAYTON,C.DAHM,J. better multiway communications among all parties FOLLSTAD-SHAH,D.GALAT,S.GLOSS,P.GOODWIN,D.HART, with an interest in urban stream function and B. HASSETT,R.JENKINSON,S.KATZ,G.M.KONDOLF,P.S. management (question 26). Considerable time at the LAKE,R.LAVE,J.L.MEYER, T. K. O’DONNELL,L.PAGANO,B. SUSE2 meeting was devoted to seeking answers to POWELL, AND E. SUDDUTH. 2005. Synthesizing US river question 26. Ideas included fact sheets, workshops restoration efforts. Science 308:636–637. and training sessions, and cooperative programs to BLAKELY, T. J., J. S. HARDING,A.R.MCINTOSH, AND M. J. WINTERBOURN. 2006. Barriers to the recovery of aquatic test scientific questions with on-the-ground manage- insect communities in urban streams. Freshwater ment actions that include effective monitoring. Many Biology 51:1634–1645. existing government agencies, educational institu- BLANCO, J. F., AND F. N. SCATENA. 2006. The spatial tions, and nongovernmental organizations work arrangement of Neritina virginea (Gastropoda: Neritidae) actively in this arena, and the need for multidisci- during upstream migration in a split-channel reach. plinary, integrative approaches to urban ecology is River Research and Applications 22:1–11. 1094 S. J. WENGER ET AL. [Volume 28

BOOTH, D. B., AND C. R. JACKSON. 1997. Urbanization of CRAIG, L. S., M. A. PALMER,D.C.RICHARDSON,S.FILOSO,E.S. aquatic systems: degradation thresholds, stormwater BERNHARDT,B.P.BLEDSOE,M.W.DOYLE,P.M.GROFFMAN, detection, and the limits of mitigation. Journal of the B. A. HASSETT,S.S.KAUSHAL,P.M.MAYER,S.M.SMITH, American Water Resources Association 33:1077–1090. AND P. R. WILCOCK. 2008. Stream restoration strategies for BRABEC, E., S. SCHULTE, AND P. L. RICHARDS. 2002. Impervious reducing river nitrogen loads. Frontiers in Ecology and surfaces and water quality: a review of current literature the Environment 6:529–538. and its implications for watershed planning. Journal of DALEY, M. L., J. D. POTTER, AND W. H. MCDOWALL. 2009. Planning Literature 16:499–514. Salinization of suburbanizing New Hampshire streams BROADMEADOW, S., AND T. R. NISBET. 2004. The effect of and groundwater: spatial extent and impacts of climate riparian forest management on the freshwater environ- change. Journal of the North American Benthological ment: a literature review of best management practice. Society 28:929–940. Hydrology and Earth System Sciences 8:286–305. DAVIES, S. P., AND S. K. JACKSON. 2006. The biological BROWN, L. R., T. CUFFNEY,J.F.COLES,A.H.BELL,J.T.MAY,F. condition gradient: a descriptive model for interpreting FITZPATRICK,G.MCMAHON, AND J. STEUER. 2009. Urban change in aquatic ecosystems. Ecological Applications streams across the USA: lessons learned from studies in 16:1251–1266. nine metropolitan areas. Journal of the North American DI´EZ, J. R., S. LARRAN˜ AGA,A.ELOSEGI, AND J. POZO. 2000. Effect Benthological Society 28:1051–1069. of removal of woody debris on streambed stability and BURCHER, C. L., AND E. F. BENFIELD. 2006. Physical and retention of organic matter. Journal of the North biological responses of streams to suburbanization of American Benthological Society 19:621–632. historically agricultural watersheds. Journal of the DOYLE, M. W., J. M. HARBOR,C.F.RICH, AND A. SPACIE. 2000. North American Benthological Society 25:356–369. Examining the effects of urbanization on streams using BURKHOLDER, B. K., G. E. GRANT,R.HAGGERTY,T.KHANGAON- indicators of geomorphic stability. Physical Geography KAR, AND P. J. WAMPLER. 2008. Influence of hyporheic 21:155–181. flow and geomorphology on temperature of a large, DUARTE, S., C. PASCOAL,A.ALVES,A.CORREIA, AND F. CASSIO. gravel-bed river, Clackamas River, Oregon, USA. 2008. Copper and zinc mixtures induce shifts in Hydrological Processes 22:941–953. microbial communities and reduce leaf litter decompo- CARROLL, G. D., AND C. R. JACKSON. 2009. Observed sition in streams. Freshwater Biology 53:91–101. relationships between urbanization and riparian cover, ELMORE, A. J., AND S. S. KAUSHAL. 2008. Disappearing shredder abundance, and stream leaf litter standing headwaters: patterns of stream burial due to urbaniza- stocks. Fundamental and Applied Limnology (in press). tion. Frontiers in Ecology and the Environment 6: CATFORD, J. A., C. J. WALSH, AND J. BEARDALL. 2007. Catchment 308–312. urbanization increases benthic microalgal biomass in ELOSEGI, A., AND L. B. JOHNSON. 2003. Wood in streams and streams under controlled light conditions. Aquatic rivers in developed landscapes. American Fisheries Sciences 69:511–522. Society Symposium 37:337–353. CENTER FOR WATERSHED PROTECTION. 2003. Impacts of imper- vious cover on aquatic ecosystems. Watershed Protec- ENDRENY, T. 2008. Naturalizing urban watershed hydrology tion Research Monograph No. 1. Center for Watershed to mitigate urban heat-island effects. Hydrological Protection, Ellicott City, Maryland. (Available from: Processes 22:461–463. http://www.cwp.org/Store/guidance.htm) EUROPEAN UNION WATER FRAMEWORK DIRECTIVE. 2000. Directive 2000/60/EC of the European Parliament and the CHADWICK, M. A., D. R. DOBBERFUHL,A.C.BENKE,A.D. HURYN,K.SUBERKROPP, AND J. E. THIELE. 2006. Urbaniza- Council of 23 October 2000 establishing a framework tion affects stream ecosystem function by altering for community action in the field of water policy. 22/ hydrology, chemistry, and biotic richness. Ecological 12/2000. Official Journal of the European Communities Applications 16:1796–1807. L 327:1–73. CHIN, A. 2006. Urban transformation of river landscapes in a FINKENBINE, J. K., J. W. ATWATER, AND D. S. MAVINIC. 2000. global context. Geomorphology 79:460–487. Stream health after urbanization. Journal of the Amer- CHRISTENSEN, A. M., F. NAKAJIMA, AND A. BAUN. 2006. Toxicity ican Water Resources Association 36:1149–1160. of water and sediment in a small urban river (Store FITZPATRICK, F. A., M. A. HARRIS,T.L.ARNOLD, AND K. D. Vejlea, Denmark). Environmental Pollution 144:621– RICHARDS. 2004. Urbanization influences on aquatic 625. communities in northeastern Illinois streams. Journal CIANFRANI, C. M., W. C. HESSION, AND D. M. RIZZO. 2006. of the American Water Resources Association 40: Watershed imperviousness impacts on stream channel 461–475. condition in southeastern Pennsylvania. Journal of the FLETCHER, T. D., G. MITCHELL,A.DELETIC,A.LADSON, AND A. American Water Resources Association 42:941–956. SE´ VEN. 2007. Is stormwater harvesting beneficial to COUCEIRO, S. R. M., N. HAMADA,S.L.B.LUZ,B.R.FORSBERT, urban waterway environmental flows? Water Science AND T. P. PIMENTEL. 2007. Deforestation and sewage and Technology 55:265–272. effects on aquatic macroinvertebrates in urban streams GIBSON, C. A., AND J. L. MEYER. 2007. Nutrient uptake in a in Manaus, Amazonas, Brazil. Hydrobiologia 575: large urban river. Journal of the American Water 271–284. Resources Association 43:576–587. 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1095

GILLIOM, R. J., J. E. BARBASH,C.G.CRAWFORD,P.A.HAMILTON, streams draining urban and agricultural watersheds in J. D. MARTIN,N.NAKAGAKI,L.H.NOWELL,J.C.SCOTT,P. central Japan. Limnology 8:243–250. E. STACKELBERG,G.P.THELIN, AND D. M. WOLOCK. 2006. IZAGIRRE, O., U. AGIRRE,M.BERMEJO,J.POZO, AND A. ELOSEGI. The quality of our nations’s waters–pesticides in the 2008. Environmental controls of whole-stream metab- nation’s streams and ground water, 1992–2001. US olism identified from continuous monitoring of Geological Survey Circular 1291. US Geological Survey, Basque streams. Journal of the North American Reston, Virginia. Benthological Society 27:252–268. GRIMM, N. B., R. W. SHEIBLEY,C.L.CRENSHAW,C.N.DAHM,W. KAUSHAL, S. S., P. M. GROFFMAN,L.E.BAND,C.A.SHIELDS,R. J. ROACH, AND L. H. ZEGLIN. 2005. N retention and P. MORGAN,M.A.PALMER,K.T.BELT,C.M.SWAN,S.E.G. transformation in urban streams. Journal of the North FINDLAY, AND G. T. FISHER. 2008a. Interaction between American Benthological Society 24:626–642. urbanization and climate variability amplifies water- GROFFMAN, P. M., D. J. BAIN,L.E.BAND,K.T.BELT,G.S. shed nitrate export in Maryland. Environmental Science BRUSH,J.M.GROVE,R.V.POUYAT,I.C.YESILONIS, AND W. and Technology 42:5872–5878. C. ZIPPERER. 2003. Down by the riverside: urban riparian KAUSHAL, S. S., P. M. GROFFMAN,G.E.LIKENS,K.T.BELT,W.P. ecology. Frontiers in Ecology and the Environment 1: STACK,V.R.KELLY,L.E.BAND, AND G. T. FISHER. 2005. 315–321. Increased salinization of fresh water in the northeastern GROFFMAN, P. M., A. M. DORSEY, AND P. M. MAYER. 2005. United States. Proceedings of the National Academy of Nitrogen processing within geomorphic structures in Sciences of the United States of America 102: urban streams. Journal of the North American Bentho- 13517–13520. logical Society 24:613–625. KAUSHAL, S. S., P. M. GROFFMAN,P.M.MAYER,E.STRIZ, AND A. GROFFMAN, P. M., N. L. LAW,K.T.BELT,L.E.BAND, AND G. T. J. GOLD. 2008b. Effects of stream restoration on denitri- FISHER. 2004. Nitrogen fluxes and retention in urban fication in an urbanizing watershed. Ecological Appli- watershed ecosystems. Ecosystems 7:393–403. cations 18:789–804. GURNELL, A. M., K. J. GREGORY, AND G. E. PETTS. 1995. The role KAUSHAL, S. S., W. M. LEWIS,JR, AND J. H. MCCUTCHAN. 2006. of coarse woody debris in forest aquatic habitats: Land use change and nitrogen enrichment of a Rocky implications for management. Aquatic Conservation: Mountain watershed. Ecological Applications 16: Marine and Freshwater Ecosystems 5:143–166. 299–312. HATT, B. E., T. D. FLETCHER,C.J.WALSH, AND S. L. TAYLOR. KEFFORD, B. J., P. J. PAPAS, AND D. NUGEGODA. 2003. Relative 2004. The influence of urban density and drainage salinity tolerance of macroinvertebrates from the Bar- infrastructure on the concentrations and loads of won River, Victoria, Australia. Marine and Freshwater pollutants in small streams. Environmental Manage- Research 54:755–765. ment 34:112–124. KING, R. S., M. E. BAKER,D.F.WHIGHAM,D.E.WELLER,T.E. HEATHERLY, T., M. R. WHILES,T.V.ROYER, AND M. B. DAVID. JORDAN,P.F.KAZYAK, AND M. K. HURD. 2005. Spatial 2007. Relationships between water quality, habitat considerations for linking watershed land cover to quality, and macroinvertebrate assemblages in Illinois streams. Journal of Environmental Quality 36: ecological indicators in streams. Ecological Applications 1653–1660. 15:137–153. KLOCKER, C. A., S. S. KAUSHAL,P.M.GROFFMAN,P.M.MAYER, HENSHAW, P. C., AND D. B. BOOTH. 2000. Natural restabiliza- tion of stream channels in urban watersheds. Journal of AND R. P. MORGAN. 2009. Nitrogen uptake and denitri- the American Water Resources Association 36: fication in restored and unrestored streams in urban 1219–1236. Maryland, USA. Aquatic Sciences (in press). OLPIN URLONG EYER HURMAN HESSION, W. C., J. E. PIZZUTO,T.E.JOHNSON, AND R. J. HORWITZ. K , D. W., E. T. F ,M.T.M ,E.M.T ,S. 2003. Influence of bank vegetation on channel morphol- D. ZAUGG,L.B.BARBER, AND H. T. BUXTON. 2002. ogy in rural and urban watersheds. Geology 31:147–150. Pharmaceuticals, hormones, and other organic waste- HILL, B. H., J. M. LAZORCHAK,F.H.MCCORMICK, AND W. T. water contaminants in US streams, 1999–2000: a national WILLINGHAM. 1997. The effects of elevated metals on reconnaissance. Environmental Science and Technology benthic community metabolism in a Rocky Mountain 36:1202–1211. stream. Environmental Pollution 95:183–190. KONRAD, C. P., AND D. B. BOOTH. 2005. Hydrologic changes in IMBERGER, S. J., C. J. WALSH, AND M. R. GRACE. 2008. More urban streams and their ecological significance. Amer- microbial activity, not abrasive flow or shredder ican Fisheries Society Symposium 47:157–177. abundance, accelerates breakdown of labile leaf litter KRAUSE, C. W., B. LOCKARD,T.J.NEWCOMB,D.KIBLER,V. in urban streams. Journal of the North American LOHANI, AND D. J. ORTH. 2004. Predicting influences of Benthological Society 27:549–561. urban development on thermal habitat in a warm water IWATA, T., T. TAKAHASHI,F.KAZAMA,Y.HIRAGA,N.FUKUDA,M. stream. Journal of the American Water Resources HONDA,Y.KIMURA,K.KOTA,D.KUBOTA,S.NAKAGAWA,T. Association 40:1645–1658. NAKAMURA,M.SHIMURA,S.YANAGIDA,L.XEU,E. LADSON, A. R., C. J. WALSH, AND T. D. FLETCHER. 2006. FUKASAWA,Y.HIRATSUKA,T.IKEBE,N.IKENO,A.KOHNO, Improving stream health in urban areas by reducing K. KUBOTA,K.KUWATA,T.MISONOU,Y.OSADA,Y.SATO,R. runoff frequency from impervious surfaces. Australian SHIMIZU, AND K. SHINDO. 2007. Metabolic balance of Journal of Water Resources 10:23–33. 1096 S. J. WENGER ET AL. [Volume 28

LARSON, M. G., D. B. BOOTH, AND S. A. MORLEY. 2001. MEYER, J. L., M. J. PAUL, AND W. K. TAULBEE. 2005a. Stream Effectiveness of large woody debris in stream rehabil- ecosystem function in urbanizing landscapes. Journal of itation projects in urban basins. Ecological Engineering the North American Benthological Society 24:602–612. 18:211–226. MILLER, J. E., G. R. HESS, AND C. E. MOORMAN. 2007. Southern LEBLANC, R. T., R. D. BROWN, AND J. E. FITZGIBBON. 1997. two-lined salamanders in urbanizing watersheds. Ur- Modeling the effects of land use change on the water ban Ecosystems 10:73–85. temperature in unregulated urban streams. Journal of MILLER, W., AND A. J. BOULTON. 2005. Managing and Environmental Management 49:445–469. rehabilitating ecosystem processes in regional urban LEIBOWITZ, S. G., P. J. WIGINGTON,M.C.RAINS, AND D. M. streams in Australia. Hydrobiologia 552:121–133. DOWNING. 2008. Non-navigable streams and adjacent MILTNER, R. J., D. WHITE, AND C. YODER. 2004. The biotic wetlands: addressing science needs following the integrity of streams in urban and suburbanizing Supreme Court’s Rapanos decision. Frontiers in Ecology landscapes. Landscape and Urban Planning 69:87–100. and the Environment 6:364–371. MOORE, A. A., AND M. A. PALMER. 2005. Invertebrate LEOPOLD, L. B. 1968. Hydrology for urban planning. US biodiversity in agricultural and urban headwater Geological Survey, Washington, DC. (Available from: streams: implications for conservation and manage- http://eps.berkeley.edu/people/lunaleopold/) ment. Ecological Applications 15:1169–1177. ERNER L , D. N. 2002. Identifying and quantifying urban MUELLER, D. K., AND N. E. SPAHR. 2006. Nutrients in streams recharge: a review. Hydrogeology Journal 10:143–152. and rivers across the nation: 1992–2001. US Geological LEWIS, D. B., AND N. B. GRIMM. 2007. Hierarchical regulation Survey Scientific Investigations Report 2006-5107. US of nitrogen export from urban catchments: interactions Geological Survey, Reston, Virginia. of storms and landscapes. Ecological Applications 17: MULHOLLAND, P. J., A. M. HELTON,G.C.POOLE,R.O.HALL,S. 2347–2364. K. HAMILTON,B.J.PETERSON,J.L.TANK,L.R.ASHKENAS,L. LUSSIER, S. M., R. W. ENSER,S.N.DASILVA, AND M. W. COOPER,C.N.DAHM,W.K.DODDS,S.E.G.FINDLAY,S. CHARPENTIER. 2006. Effects of habitat disturbance from V. GREGORY,N.B.GRIMM,S.L.JOHNSON,W.H. residential development on breeding bird communities MCDOWELL,J.L.MEYER,H.M.VALETT,J.R.WEBSTER,C. in riparian corridors. Environmental Management 38: P. ARANGO,J.J.BEAULIEU,M.J.BERNOT,A.J.BURGIN,C.L. 504–521. CRENSHAW,L.T.JOHNSON,B.R.NIEDERLEHNER,J.M. MALLIN, M. A., V. L. JOHNSON,S.H.ENSIGN, AND T. A. O’BRIEN,J.D.POTTER,R.W.SHEIBLEY,D.J.SOBOTA, AND S. MACPHERSON. 2006. Factors contributing to hypoxia in M. THOMAS. 2008. Stream denitrification across biomes rivers, lakes, and streams. Limnology and Oceanogra- and its response to anthropogenic nitrate loading. phy 51:690–701. Nature 452:202–205. MALTBY, L., A. B. A. BOXALL,D.M.FORROW,P.CALOW, AND C. MURAKAMI, M., N. SATO,A.ANEGAWA,N.NAKADA,A.HARADA, I. BETTON. 1995. The effects of motorway runoff on fresh- T. KOMATSU,H.TAKADA,H.TANAKA,Y.ONO, AND H. water ecosystems 2. Identifying major toxicants. Envi- FURUMAI. 2008. Multiple evaluations of the removal of ronmental Toxicology and Chemistry 14:1093–1101. pollutants in road runoff by soil infiltration. Water MARTI´, E., J. AUMATELL,L.GODE´ ,M.POCHE, AND F. SABATER. Research 42:2745–2755. 2004. Nutrient retention efficiency in streams receiving inputs from wastewater treatment plants. Journal of MURDOCK, J., D. ROELKE, AND F. GELWICK. 2004. Interactions Environmental Quality 33:285–293. between flow, periphyton, and nutrients in a heavily impacted urban stream: implications for stream resto- MATTSSON, B. J., AND R. J. COOPER. 2006. Louisiana water- thrushes (Seiurus motacilla) and habitat assessment as ration effectiveness. Ecological Engineering 22:197–207. cost-effective indicators of instream biotic integrity. NELSON, K. C., AND M. A. PALMER. 2007. Stream temperature Freshwater Biology 51:1941–1958. surges under urbanization and climate change: data, MAYER, P. M., S. K. REYNOLDS,M.D.MCCUTCHEN, AND T. J. models, and responses. Journal of the American Water CANFIELD. 2006. Riparian buffer width, vegetative cover Resources Association 43:440–452. and nitrogen removal effectiveness: a review of current NELSON, K. C., M. A. PALMER,J.E.PIZZUTO,G.E.MOGLEN,P.L. science and regulations. EPA/600/R-05/118. National ANGERMEIER,R.H.HILDERBRAND,M.DETTINGER, AND K. Risk Management Laboratory, Office of Research and HAYHOE. 2009. Forecasting the combined effects of Development, US Environmental Protection Agency, urbanization and climate change on stream ecosystems: Cincinnati, Ohio. from impacts to management options. Journal of MERSEBERGER,G.C.,E.MARTI´, AND F. SABATER. 2005. Net changes Applied Ecology 46:154–163. in nutrient concentrations below a point source input in NEWCOMBE, C. P., AND D. D. MACDONALD. 1991. Effects of two streams draining catchments with contrasting land suspended sediments on aquatic ecosystems. Canadian uses. Science of the Total Environment 347:217–229. Journal of Fisheries Management 11:72–82. MEYER, J. L., AND J. B. WALLACE. 2001. Lost linkages and lotic OMETTO, J. P., L. A. MARTINELLI,A.GESSNER,P.B.CAMARGO,M. ecology: rediscovering small streams. Pages 295–317 in C. BERNARDES, AND A. W. KRUSCHE. 2004. Macroinverte- M. C. Press, N. J. Huntly, and S. Levin (editors). brate community distribution in two sub-tropical Ecology: achievement and challenge. Blackwell Science, watersheds, southern Brazil. Ecohydrology and Hydro- Oxford, UK. biology 4:35–47. 2009] TWENTY-SIX URBAN STREAM RESEARCH QUESTIONS 1097

OURSO, R. T., AND S. A. FRENZEL. 2003. Identification of linear ROY, A. H., M. C. FREEMAN,B.J.FREEMAN,S.J.WENGER,W.E. and threshold responses in streams along a gradient of ENSIGN, AND J. L. MEYER. 2005b. Investigating hydrologic urbanization in Anchorage, Alaska. Hydrobiologia 501: alteration as a mechanism of fish assemblage shifts in 117–131. urbanizing streams. Journal of the North American PALMER, M. A. 2009. Reforming coastal watershed restora- Benthological Society 24:656–678. tion. Estuaries and Coasts 32:1–17. ROY, A. H., M. C. FREEMAN,B.J.FREEMAN,S.J.WENGER,J.L. PALMER, M. A., E. S. BERNHARDT,J.D.ALLAN,P.S.LAKE,G. MEYER, AND W. E. ENSIGN. 2006. Importance of riparian ALEXANDER,S.BROOKS,J.CARR,S.CLAYTON,C.N.DAHM,J. forests in urban catchments contingent on sediment and F. SHAH,D.L.GALAT,S.G.LOSS,P.GOODWIN,D.D.HART, hydrologic regimes. Environmental Management 37: B. HASSETT,R.JENKINSON,G.M.KONDOLF,R.LAVE,J.L. 523–539. MEYER, T. K. O’DONNELL,L.PAGANO, AND E. SUDDUTH. ROY, A. H., S. J. WENGER,T.D.FLETCHER,C.J.WALSH,A.R. 2005. Standards for ecologically successful river resto- LADSON,W.D.SHUSTER,H.W.THURSTON, AND R. R. ration. Journal of Applied Ecology 42:208–217. BROWN. 2008. Impediments and solutions to sustainable, PAUL, M. J., AND J. L. MEYER. 2001. Streams in the urban watershed-scale urban stormwater management: les- landscape. Annual Review of Ecology, Evolution and sons from Australia and the United States. Environ- Systematics 32:333–365. mental Management 42:344–359. PAUL, M. J., J. L. MEYER, AND C. A. COUCH. 2006. Leaf SCHAEFER, J. F., E. MARSH-MATTHEWS,D.E.SPOONER,K.B. breakdown in streams differing in catchment land use. GIDO, AND W. J. MATTHEWS. 2003. Effects of barriers and Freshwater Biology 51:1684–1695. thermal refugia on local movement of the threatened PELLERIN, B., S. S. KAUSHAL, AND W. H. MCDOWELL. 2006. Does leopard darter, Percina pantherina. Environmental Biol- anthropogenic nitrogen enrichment increase organic ogy of Fishes 66:391–400. nitrogen concentrations in runoff from forested and SCHIFF, R., AND G. BENOIT. 2007. Effects of impervious cover at human-dominated watersheds? Ecosystems 9:852–864. multiple spatial scales on coastal watershed streams. PICKETT, S. T. A., M. L. CADENASSO,J.M.GROVE,P.M. Journal of the American Water Resources Association GROFFMAN,L.E.BAND,C.G.BOONE,W.R.BURCH,C.S.B. 43:712–730. GRIMMOND,J.HOM,J.C.JENKINS,N.L.LAW,C.H.NILON, SCHUELER, T. R. 1994. The importance of imperviousness. R. V. POUYAT,K.SZLAVECZ,P.S.WARREN, AND M. A. Watershed Protection Techniques 1:100–111. WILSON. 2008. Beyond urban legends: an emerging SKINNER, L., A. DE PEYSTER, AND K. SCHIFF. 1999. Developmen- framework of urban ecology, as illustrated by the tal effects of urban storm water in medaka (Oryzias Baltimore Ecosystem Study. BioScience 58:139–150. latipes) and inland silverside (Menidia beryllina). Ar- PURCELL, A. H., C. FRIEDRICH, AND V. H. RESH. 2002. An chives of Environmental Contamination and Toxicology assessment of a small urban stream restoration project 37:227–235. in northern California. Restoration Ecology 10:685–694. SMITH, R. F., L. C. ALEXANDER, AND W. O. LAMP. 2009. RAMI´REZ, A., R. DE JESU´ S-CRESPO,D.MARTINO´ -CARDONA,N. Watershed urbanization and stream-inhabiting insects: MARTI´NEZ-RIVERA, AND S. BURGOS-CARABALLO. 2009. Urban role of the terrestrial environment. Journal of the North streams in Puerto Rico: what can we learn from the American Benthological Society 28:1022–1037. tropics? Journal of the North American Benthological SWAN, C. M., B. HEALEY, AND D. C. RICHARDSON. 2008. The role Society 28:1070–1079. of native riparian tree species in decomposition of ROBERTS, M. L., AND R. E. BILBY. 2009. Urbanization alters invasive tree of heaven (Ailanthus altissima) leaf litter in litterfall rates and nutrient inputs to small Puget an urban stream. Ecoscience 15:27–35. Lowland streams. Journal of the North American TAYLOR, S. L., S. C. ROBERTS,C.J.WALSH, AND B. E. HATT. 2004. Benthological Society 28:941–954. Catchment urbanisation and increased benthic algal ROSE, S. 2007. The effects of urbanization on the hydro- biomass in streams: linking mechanisms to manage- chemistry of base flow within the Chattahoochee River ment. Freshwater Biology 49:835–851. Basin (Georgia, USA). Journal of Hydrology 341:42–54. TRIMBLE, S. W. 1997. Contribution of stream channel erosion ROUSSEL, H., E. CHAUVET, AND J. M. BONZOM. 2008. Alteration to sediment yield from an urbanizing watershed. of leaf decomposition in copper-contaminated freshwa- Science 278:1442–1444. ter mesocosms. Environmental Toxicology and Chem- UEHLINGER, U., M. NAEGELI, AND S. G. FISHER. 2002. A istry 27:637–644. heterotrophic desert stream? The role of sediment ROY, A. H., A. L. DYBAS,K.M.FRITZ, AND H. R. LUBBERS. 2009. instability. Western North American Naturalist 62: Urbanization impacts the extent and hydrologic perma- 466–473. nence of headwater streams in a Midwestern US UTZ, R. M., R. H. HILDERBRAND, AND D. M. BOWARD. 2009. metropolitan area. Journal of the North American Identifying regional differences in threshold responses Benthological Society 28:911–928. of aquatic invertebrates to land cover gradients. ROY, A. H., C. L. FAUST,M.C.FREEMAN, AND J. L. MEYER. Ecological Indicators 9:556–567. 2005a. Reach-scale effects of riparian forest cover on VAN BUREN, M. A., W. E. WATT,J.MARSALEK, AND B. C. urban stream ecosystems. Canadian Journal of Fisheries ANDERSON. 2000. Thermal enhancement of stormwater and Aquatic Sciences 62:2312–2329. runoff by paved surfaces. Water Research 34:1359–1371. 1098 S. J. WENGER ET AL. [Volume 28

VAN SICKLE, J., J. BAKER,A.HERLIHY,P.BAYLEY,S.GREGORY,P. across multiple spatial scales. Environmental Manage- HAGGERTY,L.ASHKENAS, AND J. LI. 2004. Projecting the ment 28:255–266. biological condition of streams under alternative sce- WANG, X. H., AND Z. Y. YIN. 1997. Using GIS to assess the narios of human land use. Ecological Applications 14: relationship between land use and water quality at a 368–380. watershed level. Environment International 23:103–114. vON SCHILLER, D., E. MARTI´,J.L.RIERA,M.RIBOT,J.C.MARKS, WARREN, M. L., AND M. G. PARDEW. 1998. Road crossings as AND F. SABATER. 2008. Influence of land use on stream barriers to small-stream fish movement. Transactions of ecosystem function in a Mediterranean catchment. the American Fisheries Society 127:637–644. Freshwater Biology 53:2600–2621. WATERS, T. F. 1995. Sediment in streams: sources, biological WALSH, C. J., T. D. FLETCHER, AND A. R. LADSON. 2005a. Stream effects and control. American Fisheries Society Mono- restoration in urban catchments through redesigning graph 7. American Fisheries Society, Bethesda, Mary- stormwater systems: looking to the catchment to save land. the stream. Journal of the North American Benthological WEBB, J. A., AND E. L. KING. 2009. A Bayesian hierarchical Society 24:690–705. trend analysis finds strong evidence for large-scale WALSH, C. J., AND J. KUNAPO. 2009. The importance of upland temporal declines in stream ecological condition around flow paths in determining urban impacts on stream Melbourne, Australia. Ecography 32:215–225. ecosystems. Journal of the North American Benthologi- WENGER, S. J. 1999. A review of the literature on riparian cal Society 28:977–990. buffer width, extent and vegetation. Office of Public WALSH, C. J., A. H. ROY,J.W.FEMINELLA,P.D.COTTINGHAM, Service and Outreach, Institute of Ecology, University of AND P. M. GROFFMAN. 2005b. The urban stream syn- Georgia Institute of Ecology, Athens, Georgia. (Avail- drome: current knowledge and the search for a cure. able from: Institute of Ecology, University of Georgia, Journal of the North American Benthological Society 24: Athens, Georgia 30602-2202 USA.) 706–723. WOLMAN, M. G. 1967. A cycle of sedimentation and erosion WALSH, C. J., K. A. WALLER,J.GEHLING, AND R. MAC NALLY. in urban river channels. Geografiska Annaler 49A: 2007. Riverine invertebrate assemblages are degraded 385–395. more by catchment urbanization than by riparian WOOD, P. J., AND P. D. ARMITAGE. 1997. Biological effects of deforestation. Freshwater Biology 52:574–587. fine sediment in the lotic environment. Environmental WALTERS, C. J., AND R. HILBORN. 1978. Ecological optimization Management 21:203–217. and adaptive management. Annual Review of Ecology and Evolution 9:157–188. YAGOW, G., W. C. HESSION,T.E.WYNN, AND B. BEZAK. 2008. Developing strategies for urban channel erosion quan- WALTERS, D. M., D. S. LEIGH, AND A. B. BEARDEN. 2003. Urbanization, sedimentation, and the homogenization tification in upland coastal zone streams: final report. of fish assemblages in the Etowah River Basin, USA. VT-BSE Document No. 2008-0005. Virginia Coastal Hydrobiologia 494:5–10. Zone Management Program, Richmond, Virginia. (Available from: http://www.deq.state.va.us/coastal/ WALTERS, D. M., A. H. ROY, AND D. S. LEIGH. 2009. Environmental indicators of macroinvertebrate and fish documents/task83-05.pdf) assemblage integrity in urbanizing watersheds. Ecolog- ical Indicators 9:1222–1233. WANG, L., J. LYONS,P.KANEHL, AND R. BANNERMAN. 2001. Received: 16 December 2008 Impacts of urbanization on stream habitat and fish Accepted: 19 June 2009