Temporal export of nitrogen from a constructed wetland: influence of hydrology and senescing submerged plants

Ecological Engineering 23 (2004) 233–249

Temporal export of nitrogen from a constructed wetland:influence of hydrology and senescing submerged plants

Ann-Karin Thorena,∗, Catherine Legranda, Karin S. Tonderskib

a Department of Biology and Environmental Science, University of Kalmar, Barlastgatan 11, SE 392 31 Kalmar, Swedenb IFM—Biology, Linkoping University, Link¨oping, Sweden

Received 19 February 2004; received in revised form 27 August 2004; accepted 8 September 2004

Abstract

Nitrogen export was measured during monthly monitoring in an 18-ha constructed wetland in southeast Sweden 1998–2001.To investigate the influence of increased water flow on wetland nitrogen export, we performed intensive sampling (eight samplesper day) of total-N, urea-N, NH4+-N, NO3

−-N in January–March 2001. Investigations of wetland plant distribution 1997–2001and submerged plant biomass were combined with investigations of plant nitrogen content 1998–2001, to test the hypothesis thatnitrogen export was linked to wetland plant decomposition. Nitrogen was exported from the wetland (3.3 kg N ha−1 d−1), duringthe end of the intensive sampling period (19 February–12 March), coinciding with increased water flow (from 0.2 to 0.6 m3 s−1).Plant cover expanded rapidly and nitrogen assimilated in the submerged plant community was estimated to be 39 kg ha−1 in July2001. After senescence in April the following year, plant biomass was reduced by 75%. We concluded that a significant part(40%) of this wetland nitrogen export might be associated with the release of organic and inorganic nitrogen from the senesceds e may bel©

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ubmerged plant community. Moreover, we hypothesize that nitrogen removal in wetlands in cold temperate climatimited by temporal nitrogen export induced by high water flow velocity when wetland plants are decomposing.

2004 Elsevier B.V. All rights reserved.

eywords:Nitrogen export; Constructed wetland; Hydrology; Submerged plants; Senescence; Decomposition; Nitrogen release

. Introduction

The potential of wetlands to reduce nitrogen (N)n sewage water and in water from diffuse pollu-ion sources has been demonstrated (Moshiri, 1993;

∗ Corresponding author. Tel.: +46 480 447322;ax: +46 480 447305.E-mail address:[email protected] (A.-K. Thoren).

Fleisher et al., 1994; Huang et al., 2000; Reilly et2000; Braskerud, 2002; Mitsch et al., 2002; Paludaal., 2002; Koskiaho et al., 2003) and much advancment has been made in design methodology oveyears (e.g.Kadlec and Knight, 1996; Kadlec et a2000). However, temporal and spatial variations in wland nitrogen removal capacity still restrict our abito achieve predictable and sustained levels of wquality improvement. Water flow (m3 s−1) or hydraulic

925-8574/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.ecoleng.2004.09.007

234 A.-K. Thoren et al. / Ecological Engineering 23 (2004) 233–249

loading (m d−1) is identified as one critical factor fornitrogen removal efficiency. Flood events induce tem-poral export of N that significantly influences the an-nual N-removal in wetlands (Raisin et al., 1997; Spielesand Mitsch, 2000). Further, there may be wetlands withnet resuspension rather than net removal of nitrogen onan annual basis, due to short residence time (Arheimerand Wittgren, 2002; Koskiaho et al., 2003).

A second factor of importance for overall wetlandnitrogen removal is plant cover. However, the quanti-tative influence of emergent and submerged plants fornitrogen removal efficiency in wetlands is not clear, ascontradictory results are often presented in the litera-ture. High rice productivity explained the major partof wetland nitrogen removal (Liu et al., 2000), whileother authors conclude that plant assimilation of nitro-gen had no significant influence on nitrogen removal(Saunders and Kalff, 2001). Accumulated plant mate-rial was a possible factor explaining decreasing wet-land nitrogen removal in seven Norwegian wetlands asthe wetlands were ageing (Braskerud, 2002). Harvest-ing of emergent plants and limitations of plant cover-age improved wetland nitrogen removal (Sartoris et al.,2000; Thullen et al., 2002) and harvesting of submergedplants reduced internal loading of N and P (Asaeda etal., 2000). In contrast, other authors found that harvestof wetland vegetation has no significant impact on ni-trogen removal (Tcobanoglous, 1987; Wetzel, 2001).

In the present study, we wanted to investigate ifand why a constructed wetland, receiving water froma rilye ortr sa tivef dingt thea mett on-i rt( eret ters oval.H portm ase.T ift gen.S s ac ater

flow, and finally, we wanted to test the hypothesis thatthe wetland nitrogen export was influenced by nitrogenfrom decaying plants. We investigated this by perform-ing more frequent water sampling during the winterseason and by determining changes in submergedplant distribution, biomass, and plant N-content.

2. Material and methods

2.1. Site description

The constructed pond–marsh–pond system KalmarDamme is located on the South East coast of Swe-den in the Tornebybacken drainage basin 2 km fromthe stream outlet to the Baltic Sea. The drainage basin(48 km2) is mainly agricultural land with a few smallerurban areas and a regional airport, Kalmar Airport.Urea (H2N–CO–NH2) is spread as pellets to keepairport runways free from ice. During rainfall andsnowmelt, urea infiltrates gravel ditches along the run-ways. From the ditches, drainage pipes collect andtransport the water to Tornebybacken stream. In natu-ral soil and waters, urea is rapidly transformed to NH4

+

and HCO3− in the presence of microbes with the en-

zyme urease (Mobley and Hausinger, 1989; Swensenand Singh, 1998). After approximately 2 km of streamtransport, at a wooden dam, the water is lifted (0.8 m)into the constructed wetland, by three pumps. A pres-sure gauge at the streambed regulates the pumps andk et-l openc het ndt tud-i

.5 md tie reed( es-t 8 haa wet-l 3.9a(

, G,H . In

gricultural areas and airport run-off, was temporaxporting nitrogen. During winter season, the airpun-off is rich in urea, H2N–CO–NH2, which is used ade-icing agent on airport runways. Design objec

or the wetland was to reduce nitrogen correspono 50% of the annual amount of urea-N used atirport. This design criterion of 50% reduction was

he first year, 1998, according to results from the mtoring program for the wetland run by Kalmar Airpomonthly sampling). In the following years, there wendencies of wetland nitrogen export during wineason severely affecting annual nitrogen remowever, it was not clear whether this nitrogen exight be a measurement artifact or actually the che aim of this study was, at first, to investigate

he wetland acted as a temporal source of nitroecondly, we wanted to investigate if there waorrelation between wetland nitrogen export and w

eeps a minimum flow in the stream. Both at the wand inlet and outlet, the water passes through anhannel with a thin-plate weir (Thomson weir). Transport of de-icing urea from Kalmar Airport aransformation of urea in the wetland has been sed inThoren et al. (2003).

The wetland consists of several ponds that are 1eep (A–E, G, H, J, K and M, inFig. 1), and one tha

s 0.5 m deep (F). Area marked I and L (Fig. 1) is cov-red with dense (>30 years) stands of commonPhragmites australis). Average water depth here isimated to 0.1 m. The complete wetland area is 1nd the theoretical water residence times for the

and (including 0.1 m water level in reed stands) arend 1.7 days during medium (0.20 m3 s−1) and high0.60 m3 s−1) water flow, respectively.

Submerged plants were introduced to pond B, and I, just after the wetland was constructed

A.-K. Thoren et al. / Ecological Engineering 23 (2004) 233–249 235

Fig. 1. The 18-ha constructed wetland Kalmar Damme in southeast Sweden. Ponds A–E, G, H, J, K, M are 1.5 m deep and pond F is 0.5 m deep.In ponds A–H and pond J submersed vegetation was widely distributed. Area marked I and L was dense elder stands of common reed.

late September 1996, fresh plants of the Canadian(common) water weed (Elodea canadensis) werethrown into each pond (approximately 20 kg perpond). Simultaneously, approximately 20 kg of thenative submerged plant spiked (Eurasian) water milfoil(Myriophyllum spicatum) was planted into pond H.

2.2. Sampling

The Swedish Civil Aviation Administration oper-ated a monitoring program for the wetland during1998–2001. Once a month, in the open channels at thewetland inlet and outlet, respectively, 300 ml of wa-ter were sampled every hour for 24 h using automaticsamplers (ISCO 6700C). From this water, 1000 mlwas collected in acid–washed vials and sent for nitro-gen analysis. During the high frequency sampling inJanuary–March 2001, additional automatic samplers,containing a rack of 12 bottles, were collecting 50 mlwater (eight samples per day) in separate bottles, ex-cept during severe frost events. Water samples werecollected every 36 h in acid washed vials and frozenprior to analysis within one hour after emptying thesamplers. Eight water samples per day were analyzed.

2.3. Analyses

Results from analysis of total-N, NH4+-N andNO3

−/NO2−-N in water samples from monthly sam-

pling were obtained from the Swedish Civil AviationA cial

laboratory according to SS-EN ISO 11 905-1 (total-N), SS-EN ISO 117 32 (NH4+-N) (Swedish StandardsInstitute, 1998and 2000).

NO3−-N was analyzed spectrophotometrically with

Flow Injection Analysis after reduction to NO2−-N ina cadmium column (Tecator, 1992). NO2

−-N was an-alyzed according toTecator (1992).

Water samples from high frequency sampling wereanalyzed at the Department of Biology and Environ-mental Science. Total-N was analyzed as NO2

−-Nafter oxidation with potassiumperoxodisulphateaccording to SS-EN ISO 11 905-1 (Swedish StandardsInstitute, 1998). Urea was analyzed by the diacetyl-monoxime method followed by spectrometric readingwith a detection limit of 0.05-mg l−1 urea-N (Priceand Harrison, 1988). Ammonium was analyzed bythe salicylate-hypochlorite method, followed by spec-trometric reading (Bower and Holm-Hansen, 1980).Detection limit was 0.07 mg l−1 (5�M) ammonium-N.Nitrate/nitrite was analyzed on an ion chromatograph,Dionex DX-120 with a detection limit 0.5 mg N l−1

(concentrations were never below 2 mg l−1 NO3−-N).

Quality assurance standards were included in theanalyses.

2.4. Hydrology

Stream water flow data at the Tornebybacken mouthwere obtained from the Swedish Meteorological andHydrological Institute (SMHI) and were modeled witht l
dministration and were analyzed by a commer he PULS-model (Carlsson et al., 1987). The mode


was driven by precipitation and temperature data, ob-tained from a site at Kalmar airport. The pumps liftingwater from the stream into the wetland were calibratedregularly and pump operation time was used to vali-date the modeled PULS data. Inflowing water amountsaccording to PULS data were 5% less compared withflows calculated from pump operation data during thesampling period 1 February–12 March.

We assumed that wetland outflow was the sameas inflow during the sampling period February–March2001, since winter season evaporation was estimated to1 mm d−1 (1% and 3% of daily wetland inflow at aver-age flow and high flow, respectively) as the ponds wereexcavated in clay, allowing a minimum of infiltration togroundwater. Precipitation on the wetland surface was0.9 mm d−1 on average for the sampling period and atmost 6 mm d−1. Dilution due to precipitation was con-sequently 6% and 2% of wetland inflow at average flowand high flow, respectively.

2.5. Calculations of nitrogen and hydraulicloading

Loading of nitrogen (kg ha−1 d−1), water flow(m3 s−1), hydraulic load (m d−1), and removal rates(kg N ha−1 d−1) for the years 1998–2001 were calcu-lated from pump operation data and monthly total-Nconcentration. Nitrogen removal was calculated as thedifference between the loading at wetland inlet and out-l

ndM loadu ea-s ndd ula-t ,2

L

I N-std lad

2.6. Vegetation survey

Species distribution and density of emergent andsubmerged plant communities were surveyed dur-ing July–August in 1997, 1998, and 2001. Emer-gent species were investigated in seven fixed squares(2 m× 2 m) in pond A, E, F, G and I, and sub-merged plant communities were investigated in 14cubes (1 m× 1 m× 1 m) by boat in ponds A, C, E, F,G, I and L. The survey zones were chosen in 1997and revisited in the following years to follow plantgrowth and distribution. Plant density was categorizedas the percentage (0–5%, 25–0%, 50–5%, 75–00%)coverage of the survey areas, respectively. Plant den-sity of emergent communities was estimated (visualinspection) along anx- and y-axis (2 m× 2 m). Thesubmerged plants density was estimated, with the aidof a glass-bottomed view-box, alongx-, y- andz-axis(1 m× 1 m× 1 m), where thez-axis was 1 m or thepresent depth.

2.6.1. Plant biomassAt submerged plant biomass peak in the end of July

2001,E. canadensiswas the dominating submergedspecies. Based on results of the vegetation survey, 20areas ofE. canadensiswere chosen for biomass mea-surements. The density was estimated to be 75%–100%(10 areas) and 50%–5% (10 areas), respectively. Analuminium cylinder, 55 cm high and with a diameterof 30 cm, was gently forced down in the sediment. Allt .c andh sedf d at1 thep

rgedp re int erec weref

2n

i 98.I 2),s tem-b the

et (kg N ha−1 d−1).Wetland daily load of total-N during February a

arch 2001 was calculated as the cumulated dailysing the daily average of the N-concentrations mured (n= 8 per day) at the wetland inlet and outlet aaily average stream water flow (PULS data). Calc

ions were performed using the equation (House et al.001):

k(j) =k∑

i=1

�ti[ci(j)Qi + ci+1(j)Qi+1]

2

n thisLk(j) is the cumulated load of the respectivepecies (j), during the time periodk(e.g. February),�ti ,he sampling intervali (e.g. February 1–2),ci(j), theaily average concentration of the N-species (mg−1)ndQi , the daily average water flow (l s−1) from PULSata.

he plants appearing in the cylinder were removedE.anadensiswas loosely attached to the sedimentad a very low root biomass. The plants were rin

rom sediment, brought to the laboratory, and drie05◦C for 48 h. The biomass was determined aslant dry weight per m2 (g DW m−2).

Biomass determination of a senesced submelant community was made by the same procedu

he beginning of April 2002. For this, 10 areas whosen where the highest densities (75%–100%)ound in July 2001.

.6.2. N-content in E. canadensisSamples ofE. canadensisfor N-analysis were take

n pond A and J on nine occasions, the first in May 19n the following years (1999, 2000, 2001 and 200amples were taken twice a year, in May and Seper. On every occasion samples were taken from


same part of the pond. Plant samples were taken by anet, from 0.2 to 0.3 m below water surface at approx-imately 1 m depth. The plants were rinsed with pondwater and gently shaken from water and transportedto the laboratory in 3-l plastic bags, filled to 2/3 withplants. A minimum of 200 mg plant material needed foranalyses was dried in 60◦C for 24 h. N-content inE.canadensiswas analyzed using a CN analyzer (Auto-matic Elemental Analyzer CE Elantech NA 2500) andexpressed as milligram N per dry weight of plant.

2.7. Statistical calculations

We performed regression analyses of nitrogenspecies concentrations with time and water flow ve-locity as independent variables. When comparing inletand outlet concentrations we used the non-parametricWilcoxon signed rank test, since the concentrationswere not independent from one another and a pairedtest was not relevant due to varying time lag. We alsotested whether slopes of regression lines were sig-nificantly different by calculating the pooled resid-ual mean square as standard error for the samples,and then performing at-test using the standard errorand the regression coefficient (Zar, 1996). We consid-ered the tests robust since the sample size was large(d.f. < 170) and the probability to make a Type I er-ror (�-error, two-tailed) was very low (p< 0.0001).Results from the non-parametric Wilcoxon signedr res

n-t n

ponds. Regression analyses, non-parametric tests, andt-test were made using the statistical software packageSPSS 10.1.

3. Results

3.1. Variation in wetland nitrogen removal

Nitrogen removal in Kalmar Damme wetland washighest during the first year of monitoring. Annual ni-trogen removal in 1998 was 440 kg N ha−1 year−1 andfor the other years (1999–2001) 140, 240 and 180 kgN ha−1 year−1, respectively (Table 1). During the 4-year period, the highest urea amounts were applied atthe airport during the winter 1998–1999 (Table 1). Mostof this urea-N was applied in December 1998 and Jan-uary 1999 (Fig. 2a).

From an annual average of 2.4 kg N ha−1 d−1 dur-ing 1998, nitrogen removal decreased to 0.7, 1.2 and0.9 kg N ha−1 d−1 in 1999, 2000 and 2001, respectively(Fig. 2a). This noticeable decrease in nitrogen removalwas not explained by the changes in the magnitudeof oscillations in total-N load and water flow, sincethose were similar for the 4 years 1998–2001, withhigh total-N load in October–April and low load inMay–September (Fig. 2b).

Four-year average nitrogen removal was most effi-cient during autumn October–December, expressed inspecific removal and in relation to total-N load at wet-l gJD re-m av-

TT land to atK

Y Rem(%)

1 312 12 1

A 1S

W ort app

ank test and t-test of regression slopes weimilar.

A two-samplet-test was used to test if nitrogen coent inElodea canadensisvaried significantly betwee

able 1otal-N load at wetland inlet and outlet, respectively (kg), wetalmar Airport, during 1998–2001

ear Wetland loadtotal-N (kg)

Removal total-N (kgN ha−1 year−1)

Inlet Outlet

998 22800 14700 440999 39400 37000 140000 30900 26700 240001 30400 27100 180

verage 30800 26400 250.D. 6000 8000 133

etland total-N removal is as well expressed in relation to airp

and inlet (3.6 kg N ha−1 d−1 and 54%) and least durinanuary–April (0.2 kg N ha−1 d−1 and 3%) (Fig. 2a).uring summer months May–September, nitrogenoval was high in relative removal (54%, 4–year

tal-N removal (kg N ha−1 year−1, %) and application of urea-N (kg)

oval total-N Applied at airporturea-N (kg)

Tot-N removal inpercentage ofapplied urea-N (%)

6 16300 506 13800 174 9900 421 7600 43

7 11900 383400

lied urea-N (%).


Fig. 2. (a) Variation in urea-N application at Kalmar Airport standardized on wetland area per day (kg ha−1 d−1) and total-N removal rate(kg ha−1 d−1) and (b) inlet water flow (m3 10−3 d−1) and inlet total-N load (kg ha−1 d−1) during monthly sampling 1998, 1999, 2000, and 2001.

erage), but relatively low in specific removal (0.8 kgN ha−1 d−1) most likely due to low nitrogen loading.

3.2. Variation in N-species concentration

Nitrate was the dominating form of nitrogen(70%) in wetland influent water during the years

1998–2001 (Table 2). Even the effluent water wasnitrate-dominated (70%) as an annual average, indi-cating that urea transformation and nitrification wasnot limiting the nitrogen retention in the wetland dur-ing these years. Seasonally, there were differencesin N-species composition between winter and sum-mer. Occasionally during winter, associated with air-


Fig. 3. (a) Variation in nitrogen species concentration (total-N, urea-N, NO3−-N, and NH4

+-N, mg l−1) at wetland inlet, airport urea application(date) and (b) variation in precipitation (mm d−1) and temperature (◦C) during daily sampling in January–March 2001.

port urea application, the wetland influent water con-tained proportionally less nitrate and more organic-Nand ammonium-N. During May–September dissolvedorganic nitrogen dominated wetland effluent water(Thoren et al., unpublished data).

During sampling 13 February–13 March, in 2001,there was a significant increase in nitrate concentrationswhen the water passed through the wetland system(compared slopes, d.f. = 190,170,p< 0.001) (Fig. 4).

On the contrary, concentrations of urea-N and NH4+-N

were significantly reduced during the passage throughthe wetland (compared slopes, d.f. = 253,p< 0.01 andd.f. = 254,p< 0.001, respectively, data not shown).

Nitrogen in wetland influent and effluent was dom-inated by nitrate (78%–81%) during the sampling pe-riod in January–March 2001 (Table 2). Urea-N andammonium-N concentrations during January–March2001 were low at the wetland inlet. An exception was


Table 2Annual (1998–2001) and daily (1 February–13 March) average N-species concentration, total-P, chemical oxygen demand (CODCr)(mg l−1), and the pH range in the influent and effluent wetland water

Parameter Annual 1998–2001 February–March 2001

Inlet Outlet Inlet Outlet

Total-N 6.4 4.3 7.5 8.0NO3

−-N 3.9 3.2 6.0 6.5NH4

+-N 0.8 0.4 0.5 0.3Urea-N no data no data 0.5 0.2Total-P 0.06 0.06 no data no dataCODCr 53.4 52.2 no data no datapH (range) 6.1–7.8 6.3–8.0 no data no data

one distinct peak in urea-N concentration in response tode-icing at airport runways 21 and 23 January (Fig. 3a).The response in urea-N concentrations at wetland inleton urea application at the airport was direct since pre-cipitation fell (8 mm d−1) on 24 January and tempera-ture was well above 0◦C and no ground frost (Fig. 3b).

3.3. Wetland nitrogen release

Data from the monthly sampling program indi-cated periods with nitrogen release from the wetland inJanuary–April 1999, 2000, and 2001 (Fig. 2a). In 1999,

this coincided with large amounts of urea applied at theairport, but this was less obvious for the years 2000and 2001. Daily sampling in February–March 2001confirmed the wetland N-release event indicated in themonthly sampling in March 2001 (Fig. 2a andFig. 5a).In the middle of the sampling period (19 February),the wetland started to release total-N continuously, ex-pressed as the difference between cumulated daily inletand outlet load (Fig. 5a). The total-N-release from thewetland started 19 February, increased distinctly on the28 February, and then the cumulated outlet load of total-N exceeded the cumulated inlet load of total-N duringin total 22 days. According to the residual betweencumulated daily influent total-N and cumulated dailyeffluent total-N, the wetland exported 1300 kg nitrogen(3.3 kg N ha−1 d−1) during the period 19 February–13March (Fig. 5a).

The discrepancy between total-N load and nitrate-Nload seen inFig. 5a was most likely due to dilutedorganic nitrogen, since concentrations of urea-N andNH4

+-N were consistently too low to explain the500 kg discrepancy (Fig. 3 and Fig. 6). Most of thenitrogen released was nitrate (800 kg), according to thedifference between cumulated daily inlet and outletload (Fig. 5a). Since concentrations of urea-N andNH4

+-N were low at the inlet of the wetland duringthis period, load of urea-N and NH4+-N was low

F nd wet ch.N er (Wilc

ig. 4. Variation in nitrate-N concentrations at wetland inlet (�) aitrate concentrations at wetland outlet were significantly high

land outlet (�) (mg l−1) during the period 13 February–13 Maroxon signed ranks test,N= 190,170,p< 0.001).


Fig. 5. (a) Difference between cumulated daily wetland inlet and outlet load of total-N (�) and the difference between cumulated daily inlet andoutlet load of NO3

−-N (©) and (b) variation in water flow (m3 s−1) 1 February–13 March 2001.

(28 and 16 kg, respectively) (Fig. 3a), these amountsof urea-N transformed and NH4+-N nitrified cannotexplain the increase of 800 kg nitrate-N in outlet loadduring 1–13 March (Fig. 5a).

3.4. Variation in water flow

Variation in water flow significantly affected wet-land nitrogen removal. Nitrogen release was coincidingwith a rapid increase in water flow. When water flowincreased above 0.2 m3 s−1 the N-release started the 19

February when the difference between cumulated inletand outlet load of total-N became negative. A distinctpeak in nitrogen release is visible during 5 days (28February–5 March) when there was a flood event in-creasing water flow from 0.3 to >0.6 m3 s−1 (Fig. 5aand b). During this period, water residence time in thewetland decreased from 4 days to <2 days and the hy-draulic load increased from 0.2 to 0.5 m d−1.

Nitrate concentrations in wetland influent and ef-fluent were decreasing from February to March, dueto completed airport de-icing (last application 13


February) (Fig. 3a and Fig. 4). Furthermore, inletnitrate-N concentration was significantly decreasingwith increasing water flow most likely due to dilution(r2 = 0.49, d.f. = 22,p< 0.001;Fig. 6). In contrast, dilu-tion or high water flow did not have a significant effecton outlet nitrate concentrations according to the regres-sion model (r2 = 0.11, d.f. = 22,p= 0.055). Moreover,water flow had no effect on ammonium-N concentra-tions, either at wetland inlet or wetland outlet (Fig. 6).Variations in nitrate-N concentrations were not signif-icantly correlated to air temperature (data not shown).

3.5. Plant cover and biomass

Initially in July 1997, the first vegetation seasonafter planting, patches ofE. canadensiscovered lessthan 1% of the wetland area, and 56% of the wet-land (10 ha) were bare sediment (Figs. 7 and 8a).Phragmites australiscovered 43% of the wetland area(8 ha) and theMyriophyllum spicatumplanted simul-taneously withE. canadensis,covered less than 1%of the wetland area (Figs. 7 and 8a). The following

year 1998,E. canadensispatches, with a density of75%–100%, covered 20% of the wetland area and theCharaceaeNitellasp. had spontaneously colonized 5%of the wetland area (0.5 ha) in pond E (Fig. 1 andFig. 8b). Coverage of theM. spicatumand commonreed was in 1998 unchanged compared to 1997. Threeyears later, 2001, theE. canadensisplant coverage hadexpanded to 46% of the wetland area while the distri-bution ofM. spicatumhad decreased to <0.5% and theCharaceae (Nitella sp.) had completely disappeared.On the other hand, the broad-leaved pondweed (Pota-mogeton natans) had spontaneously colonized pondA-I (Fig. 1 and Fig. 8b). The distribution ofP. aus-tralis had barely changed. The cover of the submergedplantE. canadensisexpanded rapidly in the wetland(Table 3). When the increase inE. canadensiscover-age (m2) was plotted over the years 1997, 1998, and2001, the estimated expansion (growth rate) followedan exponential curve (y=−4050x2 + 2e7x− 2e10) andvalues for 1999 and 2000 could be interpolated(Table 3). Between the first years (1997–1998), theexpansion was 32,400 m2 year−1 and between the last

F asing wn ly depe mc nd inle

ig. 6. Nitrate-N concentrations at the wetland inlet was decreitrate concentrations at the wetland outlet were not significantoncentration was not dependent on water flow, either at wetla

ith increasing water flow water flow (r2 = 0.49, d.f. = 22,p< 0.001) whilendent on water flow (r2 = 0.11, d.f. = 22,p= 0.055, see text). Ammoniut or at the wetland outlet.


Fig. 7. Variation in vegetation cover (%) of submerged species in relation to bare sediment, in the part of the wetland (10 ha) not covered byPhragmites australis,over the years 1997, 1998, and 2001.

Table 3Distribution of the submerged plantElodea canadensisin the wetland (m2), the yearly expansion (m2 year−1), the amount of nitrogen (kg)temporarily stored in plants in relation toE. canadensis, contribution to internal load of N (kg) from plant decomposition for the years 1997–2001

Year Elodea canadensiscover (m2)

Elodea canadensisex-pansion (m2 year−1)

Temporary N-storage inE. Canadensis(kg)

Contribution to internalload of N (kg)

1997 1800 1800 15 111998 34200 32400 300 2251999 60000a 25800 450 3372000 75000a 15000 580 4352001 82800 7800 700 525

Cover area was measured in July and plant N-content in May and September every year.a Interpolated values from an exponential curve fitted to values from 1997, 1998 and 2001 (y=−4050x2 + 2e7x− 2e10).

years (2000–2001) the expansion had decreased to7800 m2 year−1 (Table 3).

In July 2001, the biomass of denseE. canadensisbeds was on average 232 g DW m−2 (n= 10, std = 40.7)and the less dense mixed population (E. canadensisandP.natans) beds had a biomass of 136 g DW m−2 (n= 10,std = 7). According to these results the less dense mixedpopulation community, which was estimated to have abiomass of 50%–75%, was 60% of the biomass in thedense single speciesE. canadensiscommunity, indi-cating that the method of determining density duringvegetation survey was adequate in our study.

There was no significant difference in the aver-age N-content of plants between the inlet pond A andthe pond J further downstream in the system, indi-

cating low spatial variation (t-test, d.f. = 17,p< 0.001,Table 4). The within–year and between–year varia-tion in plant N-content was also low (Table 4). TheN-content inE. canadensisfrom Kalmar Damme wet-

Table 4Nitrogen content inElodea canadensis(mg g−1 DW) in KalmarDamme sampled May and September 1998–2001 (this study) andin a Danish study for comparison

Location N-content inElodeaCanadensis(mg g−1 DW)

Kalmar Damme, pond A 35.0± 3.5 (n= 9)Kalmar Damme, pond J 32.9± 1.8 (n= 9)Nutrient rich stream (Denmark),

Madsen and Cedergreen (2002)47.6± 2.1 (n= 5)


Fig. 8. Distribution and expansion of submerged species andPhragmites australisin the wetland for the years (a) 1997; (b) 1998, and (c) 2001.


land was lower than the N-content ofE. canadensisfrom Danish streams.

Annual plant N-uptake was estimated by combin-ing annual cover ofE. canadensiswith biomass datafrom 2001 and mean nitrogen content inE. canaden-sis from 1998 to 2001. In 1997, 15 kg N was as-similated and stored inE. canadensisin the system(Table 3). In 2001 plant assimilated N-amount hadincreased to 700 kg N distributed over 82 800 m2

(8 g N m−2). Wetland N-removal was highest in 1998(440 kg N ha−1 year−1 or 2.4 g N m−2 year−1) whenthe expansion ofE. canadensiswas the highest (≈32000 m2 year−1) (Tables 1 and 3).

After senescence and before spring growth in April2002, the biomass ofE. canadensiswas reduced to58 g DW m−2 (n= 10, std = 19.2). This was equivalentto only about 25% of the summer biomass and plant as-similated N-amount was probably reduced correspond-ingly.

4. Discussion

4.1. Nitrogen release

Our results showed that Kalmar Damme wetlandwas occasionally acting as a source of nitrogen.The temporal nitrogen export documented during 22days February–March 2001 corresponded to 40%(72.2 kg N ha−1) of the annual wetland N-removal of1 tedt landd en-t val(

rre-l portso ofh hy-d ds.I r inO eo xi-mw rtoI s int uch

higher hydraulic loads of 1.2 and 5.3 (Wedding, 2000),and at loads of 0.9–2.7 m d−1 (Braskerud, 2002). Inaddition, those episodic N-exports were of minorimportance for the annual N-removal in the latterwetlands.

Annual average N-removal in Kalmar Damme of180–440 kg N ha−1 year−1 was lower than reported N-removal from wetlands receiving non-point run-off.Braskerud (2002)presented data from five wetlandsremoving 510–2800 kg N ha−1 year−1 and Wedding(2000) reported N-removal from three wetlands re-moving 360–3200 kg N ha−1 year−1. Annual averagenitrogen removal more similar to Kalmar Damme (thisstudy) was reported from wetlands receiving agricul-tural run-off removing 220 kg N ha−1 year−1 in Aus-tralia (Raisin et al., 1997) and 300 kg N ha−1 year−1

in Finland (Koskiaho et al., 2003) and two Amer-ican wetlands treating river water, removing 700 kgN ha−1 year−1 (Spieles and Mitsch, 2000).

The fact that Kalmar Damme wetland temporar-ily receives nitrogen in the form of urea makes wa-ter residence time in the wetland especially crucialsince urea transformation to ammonium and nitrifi-cation of ammonium must proceed before denitrifica-tion can take place. This is most obvious in the resultsfrom winter season 1998–1999, when weather condi-tions made it necessary to use large amounts of ureaat the airport. The total-N wetland inlet load was nothigher than for other years, but the proportion of urea-N and ammonium-N was larger (Fig. 2a and unpub-l arch1 itha inD withh ncet ndt liedu trateb ,2

lit-e omw hend nearr in as we-d dK u-

83.3 kg N ha−1. Consequently, this severely affeche annual wetland nitrogen removal and the wetid not meet design expectations and environm

al legislative demands on 50% nitrogen remoTable 1).

Furthermore, the nitrogen export event was coated to increased water flow. There are several ref wetland nitrogen export coinciding with periodsigh water flow or high hydraulic load although theraulic load was very different between the wetlan

n two constructed wetlands, treating river watehio, USA,Spieles and Mitsch (2000)reported nitratutflow that exceeded inflow with 400% at a maum hydraulic load of 0.1–0.2 m d−1. This is in lineith our study (Kalmar Damme) where nitrogen expoccurred at a maximum hydraulic load of 0.3 m d−1.

n contrast, in Scandinavian constructed wetlandhe agricultural landscape, N-export occurred at m

ished data). The N-release visible in January–M999 was likely influenced by high load of N wlarge proportion of urea-N from airport de-icingecember 1998 and January 1999, coincidingigh water flow and short wetland water reside

ime (Fig. 2a). Weather conditions (precipitation aemperature) regulated the transport of airport apprea and the transformation to ammonium and niefore the water reached the wetland (Thoren et al.003).

Clearly, based on the results from this study andrature, the ability of wetlands to remove nitrogen frater is dependent on hydrological dynamics. Wata was standardized for water flow, there was a lielation between N-concentration and N-removal,tudy of 5 rivers, 23 lakes, and 23 wetlands in Sen, Denmark, Switzerland and Canada (Saunders analff, 2001). However, there are more factors infl


encing the variation in N-removal efficiency. In tem-perate regions, e.g. Scandinavia, low N run-off and Nload occur during the warmer period when assimila-tion in biota is high and wetland N transformation ismost active. High N load generally occurs during thecolder periods when the microbial activity is lower dueto lower temperatures. In addition, this is the periodwhen senescence of macrophytes and mineralizationof plant litter takes place, which may add to the total-Nload of a wetland.

4.2. Plants as temporal N-storage

The submerged plant cover ofElodea canadensisin Kalmar Damme increased with a factor 19 between1997 and 1998, the first years of plant growth, andwith a factor 2.4 between 1998 and 2001 (Table 3).Rapid expansion ofE. canadensishas previouslybeen described in a shallow lake in southern Sweden(Strand and Weisner, 2001) and in constructed pondsin Finland (Kankaala et al., 2000). When emergentmacrophyte growth was doubling, high N-removal wasdocumented in a Californian wetland (Sartoris et al.,2000). A linear relationship between biomass produc-tivity (g DW m−2) and N and P uptake as percentage ofwetland nutrient removal was observed in field studiesin China. Very high rice productivity, 1800–2000 gDW m−2 explained the major part (70%) of the mea-sured N and P removal (Liu et al., 2000). Thus, duringperiods of rapid plant growth and expansion, largea y bea is al ae,f d isc np r fort dd

imitw Theq or ag iron-mb sedi im-i ntlya nu-t

4.3. Plant decomposition and N-release

When macrophytes die, they accumulate in the sed-iment at the bottom. In the succeeding decompositionprocess, organic and inorganic nutrients are released.

In Kalmar Damme wetland, the submerged commu-nity ofE. canadensiswas reduced from 232 g DW m−2

in July to 58 g DW m−2 in April the following year.Similarly,Sand-Jensen et al. (1989)found that the sum-mer biomass of submerged macrophytes (E. canaden-sisandCallitriche cohocarpa) varied between 175 and225 g DW m−2, while the winter season biomass var-ied between 5 and 50 g DW m−2 in the Danish riverSusa over 3 years. In addition to the decomposition ofthe plant community itself, there is also decompositionof particles captured by the plant communities duringsummer.

In our study there was a temporal wetland releaseof 1300 kg total-N (3.3 kg N ha−1 d−1). Approximately800 kg was explained by an increase in nitrate-N con-centration, but there was still a discrepancy of 500 kgN not accounted for when wetland inlet and outletloads were compared (Fig. 5). Release and export ofdissolved organic nitrogen (DON) might explain thisdiscrepancy.

Plant–stored nitrogen was calculated to 700 kgN (39 kg N ha−1 of total wetland area) in summer2001, in Kalmar Damme wetland (Table 3). In April2002, 75% of theE. canadensissummer biomasswas reduced. Assuming a linear relationship betweenb .2 kgN ings com-p andl wasn rgedp as inf

-t leiw ys,a nts(t wasl tm wast wayf rm

mounts of nitrogen are assimilated and this man important removal pathway. In addition, there

arge initial uptake of N by microbiota (bacteria, algungi, protists) when a newly constructed wetlanolonized (Wetzel, 2000). Rapid assimilation of N ilants and microbiota may be one explaining facto

he higher N-removal in the Kalmar Damme wetlanuring 1998 (Table 1andFig. 2a).

Eventually, live plant storage pools approach a lhere little further annual net uptake is possible.uantity of nutrients able to be assimilated is finite fiven plant species, nutrient regime, and set of envental conditions (Howard-Williams, 1981). This maye one explaining factor to why N-removal decrea

n the wetland from 1999. In addition, nutrients asslated during plant growth are not stored permanend as the plants grow old they contribute more to

rient release during decomposition.

iomass and N-content–as much as 525 kg N (29ha−1 total wetland area) had an origin from decay

ubmerged plants. This N-release due to plant deosition is likely underestimated since N-storage

eakage from decomposition of emergent speciesot included in the calculations, since the submelants dominated the system and consequently w

ocus at the planning stage of the studies.Nitrogen release from submerged plants (Potamoge

on pectinatus)in the shallow eutrophic Lake Swartvas 40% of the N-content during the first 30 dand 70% after 90 days during litter bag experimeHoward-Williams and Davies, 1979). It was foundhat although tissue concentration of N and Pow, the large biomass (300 mg DW m−2) of the plan

eant that next to the sediments the macrophyteshe largest reservoir of nutrients. The major pathrom the plants was back to the water in soluble fo


(Howard-Williams and Allanson, 1981). Modelling thegrowth and decomposition of denseP. pectinatusbedsbased on data from the lake Swartvlei studies showedthat N-release from the sediment would increase with275% during 10 years when the plant material was ac-cumulated in the sediment, whereas N-release wouldincrease with 10% during 10 years, ifP.pectinatuswereharvested yearly at peak growth (Asaeda et al., 2000).

Leaching of dissolved organic nutrients from plantand algal tissue has been documented in freshwater(Mann and Wetzel, 1996; Wetzel, 2000; Stepanauskaset al., 2000) and increased brackish water sedimentfluxes of DOC, DON, and NH4+ were observed im-mediately after addition ofZostera marinaleaves(Pedersen et al., 1999). Microbial activity and plantlitter C:N ratios are regulating mineralisation and N-fluxes from brackish water sediment (Lomstein et al.,1998). When plant litter was high in C:N ratio, mostN was incorporated in bacterial and algal cells and lit-tle N was released from sediment. This implies thatN-fluxes might be higher during mineralization of sub-merged plants with a low C:N ratio likeE. canadensisin the Kalmar Damme wetland.

Decomposition of macrophytes and the release ofnutrients from macrophytes to the water column in awetland or shallow water system were closely linkedto hydrological parameters (Atkinson and Cairns,2001). Exchange of organic and inorganic nitrogen be-tween macrophytes and surrounding water is controlledby diffusive boundary layer and water flow velocity( fsat asedf tesa ro-ba ta thewa on-t thes andr

ch2 s lowa ithi ht

be released by physicochemical mobilization of partic-ulate detritus into solution during wetland litter, soil,and sediment freezing (Edwards et al., 1986) and fromwetland microorganisms damaged by frost (Kingsburyand Moore, 1989). While plant coverage, plant decom-position, and freeze-thawing of cells are important inproducing soluble organic nitrogen, hydrology is likelyto control nitrogen export on ecosystem and landscapescale (Sand-Jensen et al., 1989; Wetzel, 2001; Quallsand Richardson, 2003).

5. Conclusions

In this study, we found a temporal wetland export ofnitrate and diluted organic nitrogen induced by a snowmelt flood rapidly raising water flow velocity and wet-land water levels. We argue that a significant proportionof this nitrate and dissolved organic nitrogen originatedfrom the release of organic and inorganic nitrogen dur-ing decomposition and freeze-thawing of plant materialand microorganisms. The temporal nitrogen exportcorresponded to 40% of the wetland annual removal of183 kg N ha−1 during 2001. Thus, we hypothesize thatshallow water systems dominated by submerged plantsin temperate and cold temperate climate, might tempo-rally act as sources of nitrogen. Our results indicate thatN-removal efficiency in constructed wetlands mightbe improved by regulating water flow oscillations.F dingt tedw t ofp

A

A dM h-n ity)f oanS re-v an-c dgeF ona

Stevens and Hurd, 1997). Within natural stands oubmerged plants, water could be stagnant (Madsennd Warnacke, 1983; Loose and Wetzel, 1993) and in

he wetland soils and sediments, inorganic N relerom the organic pool via ammonification, accumulas NH4

+-N rather than NO3−-N, because of the anaeic status of the flooded soil or sediment system (Reddynd Patrick, 1984) and diffusion limitations (Reddy el., 1980). When water flow velocity increases andater levels are raised, stored NH4

+ will be mobilizednd can be nitrified. High water flow may also c

ribute to oxygenization of the water, sediment, andenesced plant community, promoting nitrificationeducing denitrification (Tiedje, 1988).

In Kalmar Damme, during February and Mar001, there were several days with temperatures as−10 to−15◦C and the wetland was covered w

ce and snow (Fig. 3b). Diluted organic nitrogen mig

urthermore, our results support studies concluhat nitrogen export from macrophyte dominaetlands might be prevented by regular harveslants.

cknowledgements

We thankAsa Blixte (Vatten-och SamhallsteknikB), Claes-Goran Kungberg (Kalmar Airport) anarlene Norrby (Kalmar University) for valuable tecical support, and Patrick Dinnetz (Kalmar Univers

or advice with statistical analysis. We also thank JThullen (U.S. Geological Survey, Denver) and

iewers for valuable comments on manuscript. Finial support was obtained from the Swedish Knowleoundation, the Swedish Civil Aviation Administratind the Faculty Board of Kalmar University.


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Documents

Temporal export of nitrogen from a constructed wetland: influence of hydrology and senescing submerged plants