encyclopedia of inland waters || urban aquatic ecosystems

Urban Aquatic EcosystemsJ L Meyer, University of Georgia, Athens, GA, USA

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Early urban centers were established near inlandaquatic ecosystems, which provided a reliable sourceof freshwater or served as important navigation routes.Hence, there is a long history of cities benefiting fromand impacting inland aquatic ecosystems (streams,rivers, ponds, lakes, and wetlands). Urban impactshave intensified as urban populations have expanded.The proportion of the human population living in

urban areas has increased considerably in the lastcentury. Only 10% of people were living in cities in1900; by 1950 the proportion had increased to29.8%; it was 47.2% in 2000; and projections for2010 are that 51.5% of people will live in cities. Mostof the future increase in urban populations will beoccurring in the developing world. For example,urban populations in Europe and North America areprojected to increase by only 10% from 1990 to2010, whereas urban populations in Africa andLatin America are projected to increase by 75%.There is no single definition of ‘urban.’ Three char-

acteristics are commonly used: population density(minimum 400–1000 persons per km2), populationsize (minimum 1000–5000), and occupation (a maxi-mum of 50–75% employed in agriculture). Urbansprawl is characteristic of urbanization in NorthAmerica, where the amount of land occupied byurban areas is increasing at a faster rate than is theurban population. This form of urbanization has sig-nificant consequences for aquatic ecosystems becauseof its associated extensive alteration of catchments.Several metrics have been used to quantify the extentof urbanization and relate that to its impacts onaquatic ecosystems (Table 1). The simplest is popula-tion density, with which all other metrics are corre-lated; but that metric does not capture the diversity ofdevelopment patterns and mechanisms of urbaninfluence, which range from the type of infrastructureto socioeconomic conditions.Cities differ greatly in geographic and climatic

setting, population and housing density, types ofindustry, modes of waste disposal, transportationand water infrastructure, and many other factorsthat influence aquatic ecosystems. Hence, the term‘urban aquatic ecosystem’ encompasses a diverserange of water bodies and impacts. This chapter pro-vides an overview of the diversity of impacts ofurbanization on inland, freshwater aquatic ecosys-tems. It begins with and focuses on urban rivers and

streams, then provides a shorter discussion of urbanlakes and ponds, and finally urban wetlands. Ground-water ecosystems are not discussed, although urbaniza-tion and resultant water withdrawal impact them; inparticular, saline intrusion is a result of over-extraction.Withdrawal of groundwater at rates greater than it isrecharged leads to declining water tables and drying upof springs and ponds. Urbanization also affects aquaticecosystems far from cities (e.g., via atmospheric trans-port of pollutants), but the focus here is on aquaticecosystems in cities as well as those in the urban fringe.The chapter ends with a summary of services providedto society by urban aquatic ecosystems, general conclu-sions, and gaps in our knowledge.

Rivers and Streams

Flowing waters in urban areas are impacted by multi-ple stressors resulting in a characteristic ‘urban streamsyndrome’ (Figure 1). Replacing vegetated cover withimpervious surfaces (e.g., roads, parking lots, roof-tops), altering network structure by burying streamsin culverts, encasing them in cement-lined straightchannels, and routing storm water through pipeschanges water movement across the landscape,which alters hydrologic regime, channel geomorphol-ogy, and temperature regime. More sediments, nutri-ents, pesticides, and contaminants are delivered tostreams. These alterations impact aquatic habitats,species assemblages, foodwebs, and ecosystem pro-cesses, resulting in a loss of the goods and servicesbenefiting humans that aquatic ecosystems provide.

Geomorphology

Urban development in the catchment impacts thephysical features of stream channels even when thechannel is not intentionally altered by activities suchas straightening and lining with concrete. Thesechanges occur in two stages: an initial increase insediment loss from the catchment and deposition inthe channel as roads and buildings are constructed;this is followed by declining sediment productionfrom the urbanized catchment, but enhanced runoffthat often results in channel enlargement. The initialsediment mobilization phase is associated with ratesof sediment production many times pre-developmentrates (Table 2), but sediment yields are lower whendevelopment is complete. For example, sediment

367

Increasedimperviousness

Humans

+

+

+

+

+

+

−

−

−−+

Vegetationloss

Flashyhydrology

Siltation Habitat

Primaryproduction

Contaminants,pesticides

Excessnutrients

Benthicorganicmatter

Altered andcontaminated

food webs;reduced

ecosystemservices

Figure 1 A simplified diagram of the urban stream syndrome showing the pathways of urbanization impacts; þ and – signs indicatethe direction of change. Human societies cause the changes indicated, and they are also impacted by the changes.

Table 1 Metrics of urbanization used to evaluate its impact on aquatic ecosystems, components included in each metric, and an

example of a study in which the metric was used

Metric Measures included Study

Population density Humans per km2 1

Percent urban land

use

% of catchment in urban land cover classes: high and low intensity urban, industrial, transportation 1

Percent impervious

cover

% of catchment covered by rooftops, roads, parking lots, and other impervious surfaces 2

Effective

imperviousness

% of catchment covered by impervious surfaces with a direct hydraulic connection to streams 3

Urban intensity index Infrastructure (road density, number of point source discharges, number of dams, number of Toxic

Release Inventory sites), land use (% urban and % forest + shrublands for entire basin and for

125 m buffer on each side of streams identified on 1:100 000 scale maps of the network), and

socioeconomic data (census counts for population, labor, income, and housing variables)

4

Common urban

intensity index

% basin in urban landuse, percent of basin in forested or shrubland, % of stream network buffer in

developed, % of stream network buffer in forest and shrub lands, and road density

5

1. Meyer JL et al. (2005) (see Further Reading).

2. Arnold CL and Gibbons CJ (1996) Impervious surface coverage: the emergence of a key environmental indicator. American Planners Association

Journal 62: 243–258.

3. Walsh CJ et al. (2005) (see Further Reading).

4. McMahon G and Cuffney TF (2000) Quantifying urban intensity in drainage basins for assessing stream ecological conditions. Journal of the American

Water Resources Association 36: 1247–1261.

5. Tate CM, Cuffney TF, Giddings EM et al. (2005) Use of an urban intensity index to assess urban effects on streams in three contrasting environmental

settings. American Fisheries Society Symposium 47: 291–315.

368 Applied Aspects of Inland Aquatic Ecosystems _ Urban Aquatic Ecosystems

production from developing basins in Maryland aver-aged 16 times pre-development rates but only 1.7 timespre-development rates in basins where urbanizationwas complete. The source of sediment differs in thetwo phases: hillslope erosion is the largest sedimentsource in the initial aggradation phase, whereas chan-nel and bank erosion is the largest sediment source inthe subsequent erosional phase.Altered delivery of water and sediments from the

catchment results in changes in channel form. Datacollected around the world indicate larger (i.e., widerand often also deeper) channels in urbanizing rivers,

although this generalization has many exceptions.Urban streams also have reduced sinuosity. Whereflow has increased and sediment supply has not, bedcoarsening is observed; but where accelerated erosionoccurs during construction, stream beds are chokedwith silt and sand. As data are collected from urbanstreams in different hydroclimatic settings, regionaldifferences in these trends are becoming apparent.For example, reduction in channel capacity becauseof decreased depth has been observed in humid tropi-cal streams in African and Asian cities; in contrast,British rivers tend to become narrower and deeper

Applied Aspects of Inland Aquatic Ecosystems _ Urban Aquatic Ecosystems 369

with urbanization; and stream channel response tourbanization in arid environments is highly variablespatially with channel widening at some points andincision elsewhere. Differences in channel slope anderodibility of geologic materials as well as bridgeand road construction lead to spatial variation indegree of incision.Urbanization results not only in changes in channel

form but also changes in the structure of the entireriver network. Headwater channels are filled or buriedand encased in pipes, leading to a reduction in drain-age density (length of stream/area of catchment). Forexample, drainage density of natural stream channelswas reduced by 58% in an urbanMaryland catchmentand by 33% in an urban Georgia catchment. Whenroads and storm drains are included in the rivernetwork, drainage density increases by 50–>800%.Hence, small streams, which slow the downstreammovement of water, sediments and nutrients, arereplaced by an enhanced network of pipes, which aredesigned to rapidly transport water downstream. Thischange has profound consequences for the hydrology,chemistry and biology of the river network.

Hydrology

When natural vegetation is replaced by impervioussurfaces, the movement of water through the land-scape is altered (Table 3). Increasing imperviouscover results in decreased infiltration and a greaterproportion of precipitation leaving as runoff. Not

Table 2 Rates of sediment production in urbanizing landscapes

Location (% of catchment disturbed) Catchment area (km2)

Maryland, USA (100%) 0.0065


New Jersey, USA (100%) 0.075

Papeete, Tahiti (100%) 0.85Colorado, USA (53%) 3.7


New South Wales, Australia (23%) 83.8Maryland, USA (17%) 0.24

Virginia, USA (7%) 24.6

Data are from studies reviewed by Chin (2006). Increase over background is c

Table 3 Changes in the water budget (fate of precipitation) with incr

and Meyer (2001))

Impervious cover (%) Evapotranspiration (%) Shallow

<10 40 25

10–20 38 21

35–50 35 20

75–100 30 10

only is the total amount of runoff increased, but itspattern is also altered. Urban streams are character-ized as having flashier flows, i.e., floods are morefrequent and flows reach peak discharge more rapidly(Figure 2). Peak discharges are also higher in urbanstreams; e.g., discharge during a flood likely to occurevery two years in an urbanWashington stream is equalto discharge during a flood likely to occur only everyten years in a forested stream. A recent analysis ofhydrologic regime in catchments with >15% urbanland cover in the southeastern and northwestern UnitedStates found increased peak flows, decreased minimumflows, and increased flow variability. Urban peak flowswere 3–4 times those in agricultural regions, andannual flood peaks based on daily average dischargeswere magnified 22–84% in urbanized catchments.

With the decreased infiltration characteristic of ele-vated imperviousness (Table 3), one might expectlower baseflow; but this is not consistently observedbecause of additional inputs from septic systems,lawn and garden watering, and wastewater treatmentplant effluents. Wastewater can constitute a largefraction of urban stream discharge; e.g., effluent is69% of annual discharge and 100% of dischargeduring low flow conditions in the Platte River belowDenver, Colorado. Effluent-dominated streams arecommon in cities around the world.

Loss of riparian vegetation, runoff from heatedimpervious surfaces, direct discharge of heated efflu-ent from power-generating plants, and the urban‘heat island’ effect contribute to warmer streams in

Sediment yield (t km�2 year�1) Increase over background

54 056 300�30 889 140�1194 47�7300 120�2913 30�236 4�3829 120�9267 30�

12 549 3�

alculated from pre-development reaches upstream or nearby.

easing impervious cover in urban catchments (modified from Paul

infiltration (%) Deep infiltrations (%) Runoff (%)

25 10

21 20

15 30

5 55

0

5

10

15

20

25S

trea

m d

isch

arge

(L/

s)

0Days

8642

Figure 2 Typical hydrograph after a one-day storm in an urban

stream (dashed line) and a forested stream (solid line). Discharge

rises and falls faster in the urban stream and reaches a highermaximum.


cities. For example, urban streams have warmer sum-mer temperatures (5–8 �C), cooler winter temperatures(1.5–3 �C), and greater diel change in temperature.These differences are particularly apparent duringsummer storms, when temperature pulses can be10–15 �C warmer than forested streams.

Chemistry

Urban streams receive a wide variety of chemical com-pounds from wastewater treatment plant (WWTP)effluents, industrial discharges, storm sewers, and sep-tic systems, as well as runoff from lawns, gardens,roads, and parking lots. As a consequence, concentra-tions of both inorganic and organic compounds areusually elevated in urban streams, although the type ofchemical pollution varies greatly depending on thenature of human activity in the catchment. Althoughtreatment technologies in WWTP have improved, sys-tems still fail, permitted discharges are exceeded, andmany cities still have combined sewer and stormwaterpipes so that combined sewer overflows are commonduring rainstorms. Rivers are the most frequent recipi-ent of effluents from WWTPs; e.g., of 248 urbanWWTPs in the United States, 84% discharge into riv-ers. Non-point sources (e.g., runoff from lawns, roads)are also ubiquitous in urban settings.High concentrations of phosphorus, nitrogen, and

other ions are commonly observed in urban streams.Elevated phosphorus concentrations are observedbelow WWTPs as well as from fertilizers applied tolawns and gardens. Erosion of soils during construc-tion can carry a considerable load of sediment-boundphosphorus to streams. High concentrations of nitrate

and ammonium can extend far downstream of urbancenters. Concentrations of other ions such as calcium,sodium, potassium, and chloride, are also commonlyelevated in urban streams. For example, runoff fromroad de-icing in the northeastern U.S. has resulted inchloride concentrations in streams that are elevatedthroughout the winter, reaching peak concentrationsequivalent to 25% sea water. Even during the summer,chloride concentrations remain at levels 100 timesthose observed in forested watersheds. Elevated elec-trical conductivity in urban streams is not unique tonorthern cities, but is a generally useful indicator ofurban influence.

Metals such as zinc, copper, lead, chromium, cad-mium, and nickel, frequently occur at higher concentra-tions in urban than in less disturbed streams. Althoughindustrial discharges contribute to these high concen-trations, non-point sources such asbrake linings (nickel,chromium, lead and copper) and tires (zinc, lead, chro-mium, copper and nickel) are a greater source. Metalconcentrations are generally higher in sediments than inthe overlying water, particularly fine-grained sedimentswith high organic matter content.

Pesticides such as insecticides, herbicides, and fun-gicides have a high detection frequency in urbanstreams. Pesticide concentrations in urban streamsediments in the United States and in France fre-quently exceed those observed in agricultural areas.Pesticides are used on lawns, gardens, and golfcourses as well as in homes and industrial or com-mercial buildings. Urban pesticide use accounts for athird of total use in the United States.

Other organic compounds such as polychlorinatedbiphenyls (PCBs), polycyclic aromatic hydrocarbons(PAHs), and petroleum-derived hydrocarbons arealso found in urban streams. PAHs are largely fromorganic solvents used in industry and delivered tostreams via industrial discharges. In contrast, hydro-carbons from automobiles and trucks enter streamsvia runoff from impervious surfaces. The amount ofhydrocarbons delivered by rivers to the ocean can beconsiderable; e.g., 48 500 l of oil enters NarragansettBay via rivers each year.

Pharmaceuticals and compounds from personal-care products (e.g., shampoo, deodorants) are alsocommonly detected in urban streams. Antibiotics,caffeine, chemotherapeutic drugs, analgesics, narcot-ics, psychotherapeutic drugs, and contraceptiveshave been detected, although their impact on aqua-tic biota and ecosystems is only beginning to beexplored. In laboratory experiments where test ani-mals (e.g., fathead minnows, stoneflies) are exposedto water from urban streams, increased mortalityrate and altered reproductive characteristics havebeen observed.


Biology

The organisms in urban streams are impacted by thealterations in geomorphology, hydrology and chemis-try described above. Elevated concentrations of coli-form bacteria are often recorded, particularly in wetweather and where storm and sanitary sewers arecombined. Antibiotic-resistant bacteria have alsobeen observed. Iron bacteria are a common featureof urban streams where anoxic, iron-rich groundwater reaches the surface.Naturally vegetated riparian zones are less com-

mon in urban environments. Their elimination alterslight and temperature regimes, bank stability, sedi-ment and nutrient delivery, inputs of leaf litter, andhabitat availability for plants and animals character-istic of the streamside environment. Even when aforested riparian zone is present, it is often so narrowthat it is less effective and vulnerable to extremeevents (e.g., wind storms) and bypassed by storm-water pipes so that little removal of sediments, nutri-ents or contaminants occurs.Increased light and nutrients in some urban settings

can result in elevated algal biomass in urban streams;however, algal biomass may also be lower in urbanstreams because of the presence of metals and herbi-cides as well as unstable substrates, variable flowregimes, and high turbidity resulting from excess finesediments. Pollution-tolerant algal taxa are moreabundant in urban streams.The invertebrate fauna of urban streams is charac-

terized by decreased abundance (and often absence)of sensitive taxa (e.g., Ephemeroptera, Plecoptera,and Trichoptera) and increased abundance of toleranttaxa (chironomids and oligochaetes). Taxa richnessdeclines with increasing urbanization (Table 4). Lossof habitat (e.g., excess sedimentation), dischargeextremes, elevated water temperature, low oxygen,toxic chemicals, poor food quality, and presence ofnon-native competitors and predators are some of themany factors responsible for the observed declines.Indices of biotic integrity based on benthic inverte-brates reveal threshold effects on invertebrate assem-blages when impervious surface cover is 5–18% ofthe catchment, although linear declines rather than athreshold are also observed (Table 4). Effectiveimperviousness (the area of impervious surfaceswith direct hydraulic connection to streams) is a bet-ter predictor of urban impact on invertebrates thantotal impervious cover. Recognition of the importanceof effective imperviousness has significant implicationsfor management of urban streams. It suggests that theeffects of impervious surfaces on aquatic ecosystemscan be reduced by disconnecting impervious surfacesfrom streams through the installation of features such

as rain gardens and infiltration basins rather thanstormwater pipes.

The response of stream fish assemblages to urbani-zation is similar to that described for other taxa: lossof sensitive and native species and increased abun-dance of tolerant, generalist species, including morenon-native species introduced either by accident orfor sport fishing. In some cases species richness declineswith increasing urbanization, but if urbanizationresults in invasion of native or exotic generalists,richness may increase (Table 4). The numbers of indi-viduals with parasites and lesions often increases withurbanization. These changes are in response to thesame kinds of factors causing invertebrate declines asidentified in the previous paragraph. Fish-basedindices of biotic integrity decline as impervious coverincreases, particularly when impervious cover exceeds�10% or when effective impervious cover is above athreshold of 8–12% (Table 4). Urban rivers offer fish-ing opportunities, although many carry advisorieswith recommendations for very limited consumptionof fish that are caught because of contamination withcompounds such as mercury and PCBs.

Declines in other vertebrate taxa have also beenobserved in urban streams and riparian zones: amphib-ians, birds, small mammals andmarsupials (e.g., platy-pus). Several factors can lead to amphibian declines inurban streams; these include toxic chemicals, alteredconductivity and pH, excess siltation, loss of terrestrialhabitat, elevatedmortality at road crossings, and intro-duction of competitors and predators. For example,declines in amphibian richness in southern Californiastreamswere related to invasion of an exotic crayfish instreams where flows were perennial because of urbandevelopment; amphibians could persist in the inter-mittent streams characteristic of this climate, but theexotic crayfish could not.

Ecosystem Processes

Changes in hydrology, chemistry and biology result inaltered ecosystem processes in urban streams,although functional changes have been studied lessthan structural changes. Rates of removal of nutrientsfrom stream water are lower in urban streams as aresult of reduced storage of benthic organic matter orlower algal biomass. Accelerated rates of leaf break-down have been observed in urban streams as a con-sequence of enhanced erosive capacity rather thanbiological decomposition. Trends in primary produc-tivity and system respiration with urbanization havenot been consistent, probably because rates of meta-bolism reflect a response to several factors (e.g., nutri-ent and organic matter supply, light availability,

Table 4 Examples of the responses of stream invertebrate (I) and fish (F) assemblages to urbanization

Location of metropolitanarea studied

Nature of response Source

Maryland, USA I: #diversity with " ISC (1 to 17%) 1

F: #diversity at ISC >12–15%; absent at ISC >30–50%

Virginia, USA I: # diversity with " ISC (15–25%) 2Washington, USA I: IBI # with " ISC (1 to 6%); no # if riparian intact 3

California, USA I: # EPT richness and % abundance in EPT taxa with " % urban land cover 4

California, USA I: all invertebrate metrics lower in concrete-lined streams than in natural or

channelized streams

5

Utah, USA I: # richness metrics and " tolerant taxa with " UII 6

Victoria, Australia I: # richness metrics with "effective ISC 7

Minas Gerais, Brazil I: depauperate fauna below urban untreated sewage; 8

F: fewer native and more exotic species below cityNew York, USA I: # biotic indices with "% urban land cover; 9

F: no significant change in indices with % urban land cover but # abundance

with "road density

Maryland, USA I: metrics go from good to poor at 15% ISC 10F: #diversity when ISC >10–12%

Massachusetts, USA I: # richness metrics and " tolerant taxa with " UII 6, 11

F: # species richness and fluvial specialists with " UIIAlabama, USA I: # richness metrics and " tolerant taxa with " UII 6, 11

F: # species richness and endemic species richness with " UII

N. Carolina, USA F: # IBI with " % urban land cover 12

Georgia, USA F: # species richness and " relative abundance of centrarchids with " % urbanland cover

13

Wisconsin, USA F: threshold at 8–12% effective ISC; # richness and IBI above threshold 14

Illinois and Wisconsin, USA F: low IBI when urban land cover >25% 15

Georgia, USA F: #IBI and " fin lesions with " % urban land cover 16Ontario, Canada F: #IBI at ISC >10%; less impact if riparian intact 17

New York, USA F: egg and larval density#to 10% urban land use; absent above that 18

ISC: impervious surface cover; effective ISC as defined in Table 1; IBI: Index of Biotic Integrity; EPT: Ephemeroptera, Plectoptera, Trichoptera; UII: urban

intensity index as defined in Table 1; ": increase; #: decrease.Sources

1. Klein (1979) in Paul and Meyer (2001) (see Further Reading).

2. Jones and Clark (1987) in Paul and Meyer (2001) (see Further Reading).

3. Horner RR, Booth DB, Azous A, et al. (1997) in Paul and Meyer (2001) (see Further Reading).

4. Carter JL and Fend SV (2005) Setting limits: The development and use of factor-ceiling distributions for an urban assessment using macroinvertebrates.

pp. 179–192. In: Brown LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

5. Burton CA, Brown LR, and Belitz K (2005) Assessing water source and channel type as factors affecting benthic macroinvertebrate and periphyton

assemblages in the highly urbanized Santa Ana River Basin, California. pp. 239–262. In: Brown LR, Gray RH, Hughes RM, et al. (ed.) (see Further Reading).

6. Cuffney TF, Zappia H, Giddings EM, et al. (2005) Effects of urbanization on benthic macroinvertebrate assemblages in contrasting environmental settings:

Boston, Massachusetts; Birmingham, Alabama; and Salt Lake City, Utah. pp. 361–408. In: Brown LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

7. Walsh CJ (2004) Protection of in-stream biota from urban impacts: Minimize catchment imperviousness or improve drainage design? Marine and

Freshwater Research 55: 317–326.

8. Pompeu PS, Alves CBM, and Callisto M (2005) The effects of urbanization on biodiversity and water quality in the Rio das Velhas Basin, Brazil. pp.

11–22. In: Brown LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

9. Limburg KE, Stainbrook KM, Erickson JD et al. (2005) Urbanization consequences: Case studies in the Hudson River watershed. pp. 23–38. In: Brown

LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

10. Schueler TR and Galli J (1992) in Paul and Meyer (2001) (see Further Reading).

11. Meador MR, Coles JF, and Zappia H (2005) Fish assemblage responses to urban intensity gradient in contrasting metropolitan areas: Birmingham,

Alabama and Boston, Massachusetts. pp. 409–423. In: Brown LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

12. Kennen JG, Chang M, and Tracy BH (2005) Effects of landscape change on fish assemblage structure in a rapidly growing metropolitan area in North

Carolina, USA. pp. 39–52. In: Brown LR, Gray RH, Hughes RM, et al. (ed.) (see Further Reading).

13. Walters DM, Freeman MC, Leigh DS et al. (2005) Urbanization effects on fishes and habitat quality in a southern Piedmont river basin. pp. 69–86. In:

Brown LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

14. Wang L, Lyons J, Kanehl P, et al. (2001) Impacts of urbanization on stream habitat and fish across multiple spatial scales. Environmental Management

28: 255–266.

15. Fitzpatrick FA, Diebel MW, Harris MA et al. (2005) Effects of urbanization on the geomorphology, habitat, hydrology and fish Index of Biotic Integrity of

streams in the Chicago area, Illinois and Wisconsin. pp. 87–116. In: Brown LR, Gray RH, Hughes RM et al. (ed.) (see Further Reading).

16. Helms BS, Feminella JW, and Pan S (2005) Detection of biotic responses to urbanization using fish assemblages from small streams of western

Georgia, USA. Urban Ecosystems 8: 39–57.

17. Steedman RJ (1988) in Paul and Meyer (2001) (see Further Reading).

18. Limburg KE and Schmidt RE (1990) in Paul and Meyer (2001) (see Further Reading).



substrate instability, pesticides, turbidity); all ofthese factors generally increase with urbanization,but some stimulate whereas others decrease prim-ary productivity and ecosystem respiration. Theresponse of invertebrate secondary production tourbanization is unknown because it has not beenmeasured along a gradient of urbanization.

Lakes and Ponds

Many of the impacts of urbanization just describedfor rivers and streams are also observed in urban lakesand ponds such as altered temperature regimes, ele-vated concentrations of nutrients and contaminants,reduced total species richness, and a greater propor-tion of exotic species. Water withdrawals from urbanlakes or their tributary streams can significantlyreduce lake levels; e.g., lake levels of Lake Chapalain Guadalajara Mexico are currently 7 m below thelevel of the 1930s, exposing extensive mudflats.Urban lakes and ponds receive of inputs of nutrientsand contaminants from both atmosphere and catch-ment. Alteration of urban catchments and the physi-cal, chemical and biological characteristics of urbanstreams described above result in enhanced deliveryof sediments, nutrients, metals, and organic contami-nants from streams and stormwater conduits, as wellas direct runoff from impervious surfaces. Air pollu-tion further adds to the contaminant load of urbanlakes and ponds.One of the earliest incidences of urban impacts on

aquatic ecosystems is discernible in the sediments ofan Italian lake, Lago di Monterossi. Construction ofthe Via Cassia (a Roman highway) about 2000 yearsago resulted in elevated rates of sedimentation, andhigher sediment nutrient content. This is an exampleof cultural eutrophication, commonly resulting fromanthropogenic inputs of nutrients. The excess nitro-gen (N) and phosphorus (P) usually comes fromagricultural or urban sources with higher N:P ratiosthan in reference settings. Symptoms of culturaleutrophication include increased algal biomass andproductivity, a shift from algal species that are pal-atable to herbivorous zooplankton to inedible cya-nobacteria, and an increased incidence of fish kills.Lake Washington in Seattle, Washington, USA is aclassic example of this phenomenon, where increasedalgal blooms and decreased Secchi disc depths wereassociated with inputs of P from municipal sewage.When these inputs were diverted, algal productivitydecreased and Secchi disc depths increased. Simplydiverting inputs is not always effective, as wasobserved in Lake Trummen, Sweden. Because of accu-mulated P in lake sediments, sediment skimming and

elimination of carp (they disturb sediments, therebyreleasing P) was also necessary before improvementswere observed.

There have been few studies of urban lakes along agradient of urbanization, but there are many studiesof individual lakes in urban settings. Urbanizationaffects not only lakes within city limits, but alsothose at the urban-rural fringe, where suburbs areexpanding. Although an ‘urban lake syndrome’ analo-gous to the ‘urban stream syndrome’ has not beenarticulated, its characteristics would include the symp-toms of cultural eutrophication described above com-bined with elevated concentrations of anthropogeniccontaminants (e.g., metals and hydrocarbons) and ahigher proportion of introduced species. In contrastto reference lakes, where most contaminants are fromatmospheric sources, increased inputs of nutrientsand contaminants from point (e.g., municipal andindustrial effluents) and non-point (e.g., septic sys-tems and stormwater) sources in the catchment alterurban lake chemistry. Concentrations of coliformbacteria can be high, and beach closures occur, espe-cially after storms that result in combined sewer over-flows. Many European and older North Americancities have conduits that carry both sewage and storm-water; intense rainstorms fill the pipes, overwhelmwastewater treatment plants, and dump untreatedwastes directly into receiving waters. In the develop-ing world, untreated municipal and industrial wastesare commonly discharged directly into aquatic eco-systems, resulting in highly degraded urban aquaticecosystems. The unique biodiversity of Lake Victoriain Africa is threatened by urban development aroundit because untreated wastes go directly into the lake.Hypoxic bottom waters are common in eutrophicurban lakes, with consequences for biogeochemicalcycles as well as benthic biota. The extent to whichthese symptoms are exhibited in an urban lakedepends not only upon urbanization intensity, butalso upon lake attributes that result from its geologi-cal and biological setting, such as area, volume,depth, water residence time, sediment characteristics,and species present.

Sediments from urban lakes provide a historicalrecord of contamination. Concentrations of metalsin sediments from urban lakes are considerablyhigher than in reference lakes. Stricter discharge lim-its were enacted in the United States in the 1970s,and concentrations of lead, cadmium, chromium andnickel in urban lake sediments generally declinedover the past three decades, whereas there has beenno consistent trend for copper and mercury, andincreases outnumber decreases for zinc. Concentra-tions of polycyclic aromatic hydrocarbons (PAHs)and chlordane in sediments from 38 urban and


reference lakes across the United States increased ascatchment urbanization increased. PAH concentra-tions increased over the past three decades, whereaschlordane increased in half the lakes and decreased inthe other half. Both compounds enter food webs andare the cause of many fish consumption advisories.Algal diversity is reduced, but biomass and produc-

tivity is usually high in urban lakes because of ele-vated rates of nutrient delivery and altered grazerassemblages. High algal biomass combined withaccelerated sediment delivery results in higher turbid-ity in urban lakes. Water temperature, depth of tur-bulent mixing, pH, and low N:P ratios in urban lakescan result in seasonal blooms of cyanobacteria thatare inedible to zooplankton, unsightly, often createtaste and odor problems, and can be toxic to humans,pets, and livestock. In 2007, crews skimmed morethan 6000 tons of cyanobacteria from Lake Taihu inChina in an attempt to keep them out of the drinkingwater of the city of Wuxi. Scientists have expressedconcern that such blooms will become even morecommon throughout the world’s urban areas becauseof global warming. Shallow urban lakes often sup-port dense stands of aquatic macrophytes, many ofwhich are invasive weeds (e.g., Hydrilla). Housingdevelopment along the shoreline can be extensivewith resulting loss of riparian forest and reducedinput of woody debris to the littoral zone. Thoselosses represent a loss of nearshore habitat for fishesand other biota. The absence of riparian and littoralvegetation in Wisconsin lakes in urban commercialsettings was identified as the factor resulting in fewerzooplankton taxa in those lakes than in lakes in urbanresidential or forested settings. Regionally commonzooplankton taxa were present, but rare taxa weremissing from the urban lakes. Artificial lights at nightin urban lakes and ponds can interfere with zooplank-ton migration patterns.Onondaga Lake near Syracuse, New York, pro-

vides a classic example of the impacts of urbanizationon a lake. Industrial discharges during the 19thand 20th centuries combined with increasing humanpopulation and inputs of sewage effluent resulted inelevated salinity, high concentrations of nutrients andorganic contaminants, toxic concentrations of freeammonia, severe oxygen depletion, frequent cyano-bacteria blooms, and the loss of native zooplanktonand fish species. As industrial andmunicipal dischargeshave been reduced and sediment cleanup programsbegun, lake water quality has improved, cyanobacteriablooms are less common, and native species ofDaphniahave returned. Fishing is allowed, but consumptionadvisories persist.In addition to naturally occurring lakes, artificially

created lakes and ponds are common in urban areas.

The European Union has classified 4% of its surfacewaters as artificial. Artificial lakes and ponds areoften fairly shallow and may be purely ornamental,serve as a municipal water source, store storm water,or enhance its infiltration. Reservoirs pooled behinddams provide water for generating electricity. Intro-duction of non-native plants, invertebrates, and fishesalter food webs and nutrient dynamics in these eco-systems. These introductions occur more frequentlyin ponds close to roads (a shorter distance to carry anaquarium before dumping). Introductions of bottom-feeding fishes (e.g., goldfish) may enhance culturaleutrophication by accelerating release of phosphorusfrom the sediments.

Wetlands

Urbanization has resulted in significant wetland lossthrough draining, dredging, and filling. Even if wet-lands are not completely eliminated, urban develop-ment fragments them with road crossings and impairswetland ecosystem function by altering hydrologicregime, increasing input of nutrients and toxins, andintroducing exotic species. Wetland species such asturtles and salamanders that spend part of their lifeon land and part in water are particularly vulnerableto urbanization. Not only does one life history stagehave to survive in an altered aquatic environment, butthe terrestrial stage has to survive in an often hostileterrestrial environment (e.g., migrating across roadsresults in high mortality rates). Studies have consis-tently found anuran abundance and species richnessto be negatively correlated with measures of urbani-zation: % urban land use, road density, % imper-viousness, and large inputs of stormwater. Similarfindings have been reported for wetland bird species.

Recognition of the impacts of urban stormwaterrunoff on aquatic ecosystems has resulted in regula-tions requiring the construction of stormwater reten-tion, detention, or infiltration ponds and wetlands inthe United States and the European Union. Wetlandshave also been constructed to treat sewage and storm-water. These artificial wetlands can be effective innutrient removal, but provide habitat that is lessdesirable than naturally occurring wetlands. Artificialwetlands are characterized by elevated concentra-tions of contaminants and a high proportion of exoticflora and fauna.

Ecosystem Services

Urban aquatic ecosystems provide a wide range ofecosystem services, which are the goods and services

Table 5 Examples of ecosystem services provided by intact urban aquatic ecosystems. Services are organized according to the

framework used in the Millennium Ecosystem Assessment (2005)

Type of ecosystem service Services provided by aquatic ecosystems

Provisioning Produce food (e.g., fisheries)

Fresh water for human uses

Regulating Natural hazard regulation (e.g., flood protection)Water purification (e.g., retention of sediments; retention and transformation of nutrients,

contaminants and organic matter)

Cultural Inspiration and aesthetic values; spiritual renewal and a sense of placeEducational opportunities (e.g., interesting habitats and biota)

Recreational opportunities (e.g., boating, fishing, swimming, wildlife viewing)


produced by ecosystems that are beneficial to humans(Table 5). The impacts of urbanization describedin the previous sections have reduced the capacityof aquatic ecosystems to provide these services. Forexample, increasing impervious cover reduces ground-water recharge, storage of floodwaters and sediments,capacity for nutrient and contaminant removal, allof which impactwater quality and aquatic biodiversity.As city dwellers recognize the value of ecosystem ser-vices, there is growing interest in preservation (e.g.,greenways) and rehabilitation of urban aquatic eco-systems (e.g., riparian planting, daylighting streamspreviously encased in culverts). Ecologically sensitivedevelopment of urban waterfronts and trails alongwaterways can provide both economic and ecologicalbenefits to city dwellers. As the proportion of thehuman population living in urban areas continuesto increase, aquatic ecosystems in the city offer placesfor spiritual renewal as well as valuable opportuni-ties to enjoy and learn about the natural world.

Conclusions

An ever-increasing proportion of the growing humanpopulation lives in cities and both impacts anddepends upon the ecosystem services provided bystreams, rivers, lakes, ponds, and wetlands. Urbaniza-tion impacts the physical, chemical, and biologicalcharacteristics of these ecosystems. Alterations includeincreased frequency and magnitude of floods; greaterrange in water temperature; increased sedimentation;altered structure of stream channels and river net-works; increased concentration of ions (salinization),nutrients (cultural eutrophication), and contaminants(metals, pesticides, hydrocarbons, pharmaceuticals);reduced capacity for nutrient removal; increased algalbiomass with more frequent nuisance algal blooms; agreater proportion of tolerant species of algae, inver-tebrates, and fishes; fewer amphibian and wetlandbird species; and increased prevalence of non-native

species. As a consequence of these changes, the abilityof urban aquatic ecosystems to provide servicesbenefiting humans has been degraded.

Knowledge Gaps

Scientific understanding of urban aquatic ecosys-tems has advanced considerably in the past decade,but the complexity of interactions between humaninfrastructure, institutions, and aquatic ecosystemshas only begun to be explored. Effective manage-ment and rehabilitation of urban aquatic ecosystemsrequires improved scientific understanding in thefollowing areas:

. Measures of aquatic ecosystem processes in a di-verse array of cities. Rates and patterns of nutrientcycling, ecosystem metabolism, and secondary pro-ductivity along gradients of urbanization in differ-ent geographical and cultural settings are largelyunknown; yet these are the processes providingvalued ecosystem services. Generalizations abouturban impacts are primarily derived from Temper-ate Zone cities in the developed world, whereasmost of the growth in urban populations is occur-ring in tropical cities in the developing world.

. Influence of type and pattern of development.Alternative building designs and development pat-terns (e.g., clustered housing) are being proposed toreduce urban impacts on aquatic ecosystems; theseshould be viewed as catchment experiments to ex-plore their impact on physical, chemical, and bioticcharacteristics of urban waters.

. Effectiveness of rehabilitation practices. Manycities have invested heavily in projects to improveconditions in aquatic ecosystems; yet there hasbeen relatively little evaluation of the effectivenessof different practices. For example, given the find-ings on importance of effective imperviousness,will reducing the hydraulic connectivity betweenimpervious surfaces and streams result in improvedecological conditions?


. Link ecological, engineering, and socio-economicanalyses. Urban aquatic ecosystems are impactedby human actions and institutions. Better under-standing and management of these ecosystemsrequires collaborative interdisciplinary studies andmodels. For example, how can economic values beassigned to the ecosystem services provided byurban aquatic ecosystems? Urban systems includeprocesses and pathways that are not found in un-managed systems and that are influenced by factorsnot traditionally considered by ecologists such aseconomic conditions and human decisions on lawnand garden design. Studies of urban aquatic eco-systems are part of the broader discipline of urbanecology, which recognizes that collaboration out-side the natural sciences is essential to advanceunderstanding of urban systems.

Glossary

Catchment – Area of the land that is drained by astream network or land area from which waterflows into a lake.

Cyanobacteria – Photosynthetic bacteria (formerlycalled blue-green algae) that can form denseblooms; some produce toxins and some are able tofix atmospheric nitrogen.

Eutrophic – Very productive.

Hypoxic – Low concentration of dissolved oxygen.

Imperviousness – The extent to which a catchment iscovered by surfaces (e.g., roofs, paved roads, andparking lots) that do not allow water to penetrateinto the ground.

Infiltration – Gradual movement of water into soil.

Secchi depth – A measure of turbidity in water; thevertical distance that a Secchi disk (black and whitedisc about the size of a small dinner plate) can belowered into the water before it disappears from anobserver’s view; clear water has a large Secchi depthwhereas it is small in turbid water.

Urban sprawl – Low-density development on theedges of urban areas usually characterized bysingle-family homes whose residents are dependenton personal automobiles for transportation.

See also: Aquatic Ecosystem Services; Bioassessment of

Aquatic Ecosystems; Biogeochemistry of Trace Metals

and Mettaloids; Chloride; Cyanobacteria; Eutrophication;Eutrophication of Lakes and Reservoirs; Harmful Algal

Blooms; Mercury Pollution in Remote Freshwaters;Nitrogen; Phosphorus; Restoration Ecology of Rivers.

Further Reading

Azous AL and Horner RR (2001) Wetlands and Urbanization.Boca Raton: Lewis Publishers.

Booth DB and Jackson CR (1997) Urbanization of aquatic systems:

degradation thresholds, stormwater detection, and the limits of

mitigation. Journal of the American Water Resources Associa-tion 33: 1077–1090.

Brown LR, Gray RH, Hughes RM, et al. (eds.) (2005) AmericanFisheries Society Symposium 47: Effects of Urbanization on StreamEcosystems. Bethesda, Maryland: American Fisheries Society.

Chin A (2006) Urban transformation of river landscapes in a global

context. Geomorphology 79: 460–487.

Dodds WK (2002) Freshwater Ecology: Concepts and Environ-mental Applications. Academic Press.

Dodson SI, Lillie RA, andWill-Wolf S (2005) Land use, water chem-

istry, aquatic vegetation, and zooplankton community structure of

shallow lakes. Ecological Applications 15: 1191–1198.Effler SW (1996) Limnological and Engineering Analysis of a

Polluted Urban Lake: Prelude to Environmental Managementof Onondaga Lake. New York: Springer.

Grimm NB, Faeth SH, Golubeiwski NE, et al. (2008) Globalchange and the ecology of cities. Science 319: 756–758.

Kaushal SS, Groffman PM, Likens GE, et al. (2006) Increased

salinization of fresh water in the northeastern United States.

Proceedings of the National Academy of Sciences 102:13517–13520.

Knutson MG, Sauer JR, Olsen DA, et al. (1999) Effects of land-

scape composition and wetland fragmentation on frog and toadabundance and species richness in Iowa and Wisconsin, U.S.A.

Conservation Biology 13: 1437–1446.

Mahler BJ, VanMetre PC, and Callender E (2006) Trends in metals

in urban and reference lake sediments across the United States,1970 to 2001. Environmental Toxicology and Chemistry 25:

1698–1709.

Meyer JL, Paul MJ, and Taulbee WK (2005) Stream ecosystem

function in urbanizing landscapes. Journal of the NorthAmerican Benthological Society 24: 602–612.

Meyer JL and Wallace JB (2001) Lost linkages in lotic ecology:

rediscovering small streams. In: Press M, Huntly N, and Levin S(eds.) Ecology: Achievement and Challenge, pp. 295–317.

Oxford, UK: Blackwell.

Millennium Ecosystem Assessment (2005) Ecosystems and HumanWell-Being. Current State and Trends, Vol. 1. Washington D.C.:Island Press.

Paul MJ and Meyer JL (2001) Streams in the urban landscape.

Annual Review of Ecology and Systematics 32: 333–365.Poff NL, Bledsoe BP, and Cuhaciyan CO (2006) Hydrologic varia-

tion with land use across the contiguous United States: Geomor-

phic and ecological consequences for stream ecosystems.

Geomorphology 79: 264–285.Riley SPD, Busteed GT, Kats LB, et al. (2005) Effects of urbaniza-

tion on the distribution and abundance of amphibians and inva-

sive species in southern California streams. ConservationBiology 19: 1894–1907.

Scott MC and Helfman GS (2001) Native invasions, homogeni-

zation, and the mismeasure of integrity of fish assemblages.

Fisheries 26: 6–15.VanMetre PC andMahler BJ (2005) Trends in hydrophobic organic

contaminants in urban and reference lake sediments across the

United States, 1970–2001. Environmental Science and Technol-ogy 39: 5567–5574.

Walsh CJ, Roy AH, Feminella JW, et al. (2005) The urban streamsyndrome: current knowledge and the search for a cure. Journalof the North American Benthological Society 24: 708–723.


Wang L, Lyons J, Kanehl P, et al. (2000) Watershed urbanization

and changes in fish communities in southeastern Wisconsinstreams. Journal of the American Water Resources Association36: 1173–1189.

Relevant Websites

http://beslter.org/ – Baltimore Ecosystem Study Long-term Ecologi-

cal Research site.

http://caplter.asu.edu/CentralArizona – Phoenix Long-term Eco-

logical Research site.

http://ec.europa.eu/environment/water/water-framework/ – Water

Framework Directive for the European Union, which describesthe water information system for Europe.

http://water.usgs.gov/nawqa/ – National Water Quality Assessment

Program, with projects that include the impact of urbanization

on aquatic ecosystems.http://www.maweb.org/ – Millennium Ecosystem Assessment with

information on global trends in urbanization.

http://www.unhabitat.org/ – Provides information on the World

Urban Forum and on aquatic ecosystems in the developingworld.

http://beslter.org/

http://caplter.asu.edu/CentralArizona

http://ec.europa.eu/environment/water/water-framework/

http://water.usgs.gov/nawqa/

http://www.maweb.org/

http://www.unhabitat.org/

encyclopedia of inland waters || urban aquatic ecosystems

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