Retaining agricultural nutrients in constructed wetlands—experiences under boreal conditions

Retaining agricultural nutrients in constructed wetlands*/

experiences under boreal conditions

Jari Koskiaho a,*, Petri Ekholm a, Mari Raty b, Juha Riihimaki a,Markku Puustinen a

a Finnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, Finlandb University of Helsinki, P.O. Box 27, FIN-00014 Helsinki, Finland

Received 26 May 2002; received in revised form 18 December 2002; accepted 30 January 2003

Abstract

Constructed wetlands are among the eutrophication abatement measures for which farmers may receive public

subsidies in Finland. To assess the performance of constructed wetlands under boreal conditions, we monitored the

retention of total suspended solids (TSS), total P (TP), dissolved reactive phosphorus (DRP), total N (TN), nitrate�/

nitrate-N (NOx -N) and ammonium-N (NH4-N) for 15�/26 months in three constructed wetlands located in southern

Finland. Annual retentions were �/5�/72% for TSS, �/6�/67% for TP, �/33�/33% for DRP, �/7�/40% for TN, �/8�/38%

for NOx -N and �/50�/57% for NH4-N. The constructed wetland with the longest water residence time (WRT) showed

the best performance, retaining annually about 25 kg of TP and 300 kg of TN per hectare. In contrast, the constructed

wetland with the shortest WRT functioned only occasionally and was a net source for DRP and NOx -N. In addition to

the WRT, high P sorption capacity of constructed wetland soil and high input concentrations appeared to promote

retention. Vegetation had a limited effect, input load being transported mostly outside the growing season. When

carefully designed and located, constructed wetlands may efficiently reduce nutrient loading from agriculture.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Constructed wetlands; Nutrients; Eutrophication; Agriculture; Finland

1. Introduction

Eutrophication caused by excessive nutrient

loading is a significant factor lowering the value

of surface waters for recreation, fisheries and

water supply. Currently, agriculture comprises

the principal nutrient source of lakes, rivers and

coastal waters in Finland (Valpasvuo-Jaatinen et

al., 1997). Most crop production areas*/as well as

water quality problems*/are found in the south-

ern parts of the country. In this region, the water

level of many lakes was, mainly in the 19th

century, lowered to obtain more land for agricul-

tural use (some lakes were drained completely).

For the same purpose, wetlands were drained

Abbreviations: CW, constructed wetland; DRP, dissolved

reactive phosphorus; EPC, equilibrium phosphorus

concentration; WRT, residence time of water.

* Corresponding author. Tel.: �/358-9-4030-0355; fax: �/

358-9-4030-0390.

E-mail address: [email protected] (J. Koskiaho).

Ecological Engineering 20 (2003) 89�/103

www.elsevier.com/locate/ecoleng

0925-8574/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0925-8574(03)00006-5

mailto:[email protected]

during the 1950s and 1960s. These land-usechanges may have partly exacerbated eutrophica-

tion; the decrease in water storage volume and

increased runoff velocities have probably reduced

the retention of nutrients cascading from fields to

finally enter the Baltic sea.

Several studies (e.g. Mitsch, 1992; Hunt et al.,

1999) show that wetlands retain sediment and

nutrients from agricultural runoff. The construc-tion, or re-establishment, of wetlands may thus

offer an option to ‘reverse’ the situation. Since

Finland’s accession to the EU in 1995, constructed

wetlands (CWs) have been among the eutrophica-

tion abatement measures for which farmers may

receive public subsidies in Finland (Valpasvuo-

Jaatinen et al., 1997). In the renewed agro-

environmental policy CWs are preferred to sedi-mentation ponds because*/in addition to water

quality amelioration*/they are considered to pro-

duce more ancillary benefits like aesthetic im-

provement of landscape and increased

biodiversity. There are a lot of encouraging

experiences on CWs in cold conditions (e.g.

Dellien and Wedding, 1997; Vymazal et al., 1998;

Uusi-Kamppa et al., 2000; Braskerud, 2001; Man-der and Jenssen, 2002). With this study, we aimed

at increasing the information in terms of the effect

of CW characteristics on their performance in the

treatment of agricultural runoff in boreal condi-

tions. Moreover, we wanted to give scientific

background for the design guidelines needed in

the implementation of the present agro-environ-

mental policy. With these objectives in mind, wemonitored the retention of total suspended solids

(TSS) and nutrients in three agricultural CWs

located in southern Finland.

2. Materials and methods

2.1. Climatic and hydrological background

In southern Finland, the mean temperature

range is from �/7 NC in January to 16 NC in

July. The long-term (1961�/1990) mean of annual

precipitation is 715 mm and the water equivalent

of snow in March 100 mm (Hyvarinen et al., 1995;

Hyvarinen, 1999). The growing season extends

from April/May to September/October, whereasmost of the agricultural nutrient load is trans-

ported at snowmelt in March/April (occasionally

in midwinter) and during autumn rains. The

weather in southern Finland during the study

years (1998�/2001) can be characterised as follows

(Hyvarinen, 1998�/2000). Summer 1998 was wet,

whereas summer 1999 was dry; winters 1998/1999

and 1999/2000 were both rich in snow but theformer was cold, followed by a warm spring, and

the latter was mild; April 2001 was warm and

rainy; all major autumn storms (in 1998, 1999 and

2001) occurred in October.

2.2. Site descriptions, water sampling and flow

monitoring

2.2.1. Hovi CW

The Hovi CW (60825?N, 24822?E, Table 1) was

constructed for demonstration and research pur-

poses. A geotechnical analysis, done prior to

construction, showed that the soil of the CW

area was heavy clay with a low water permeability

(3.0�/10�7�/1.5�/10�15 m s�1). The topsoil in

the catchment had a moderate concentration of

phosphorus (8.9 mg P l�1 soil) as determined withacid ammonium acetate extraction (Vuorinen and

Makitie, 1955). Since the CW area was comprised

of fieldland, the topsoil (0�/30 cm) was removed to

avoid P release. In addition to topsoil removal, a

deeper region was dug in the CW to facilitate

sedimentation and denitrification, and two spits of

land and an islet (Fig. 1a) were created from the

removed soil to introduce heterogeneity into thelandscape and to reduce flow velocity. The hydro-

graph of the Hovi catchment is characterised by

sudden peaks induced by snowmelt and heavy

rains, and prolonged dry periods in summer and

winter. The runoff volume 240 l s�1 km�2 (i.e.

2488 m3 per day inflow) represents a typical

situation during maximum spring flood at Hovi.

Under these conditions in the CW the maximumwater depth peaks to almost 2 m, and the nominal

water residence time (WRT), as calculated with

Eq. (1), is 39 h (Table 1):

WRT �24 �V

Q(1)

J. Koskiaho et al. / Ecological Engineering 20 (2003) 89�/10390

where V , water volume in the CW (m3), and Q ,

inflow (m3 per day).Construction left the CW area bare, and the

development of vegetation was, therefore, en-

hanced by planting and seeding. In the first 3

years, however, this development was minor

compared with the spontaneous propagation of

vegetation, mostly that of cattail. Table 2 showsTa

ble

1

Ch

ara

cter

isti

cso

fth

ew

etla

nd

s

Wet

lan

dY

ear

of

con

-

stru

ctio

n

Wa

terl

og

ged

are

aa

t

hig

hfl

ow

(ha

)

Catc

hm

ent

are

a(h

a)

Pro

po

rtio

no

fth

e

are

as

(%)

Fie

ldp

erce

nta

ge

of

the

catc

hm

ent

Mea

nsl

op

eo

fth

e

catc

hm

ent

(%)

Wet

lan

dv

olu

me

at

hig

hfl

ow

(m3)

No

min

al

WR

Ta

t

hig

hfl

ow

(h)

Ho

vi

19

98

0.6

01

25

.01

00

2.8

40

00

39

Ala

staro

19

96

0.4

89

00

.59

0B

/0.5

35

00

6

Fly

ttra

sk1

98

0s

(th

ep

re-

sen

tfo

rm)

60

20

00

3.0

35

No

da

taV

ary

ing

�/2

4

WR

Td

eno

tes

resi

den

ceti

me

of

wate

r.

Fig. 1. Schematic images of the Hovi (A), Alastaro (B) and

Flyttrask (C) wetlands in southern Finland.

J. Koskiaho et al. / Ecological Engineering 20 (2003) 89�/103 91

the most important species and the above ground

biomass measured on 17 Sepember 2001.

The performance of the Hovi CW was mon-

itored during April 1999�/May 2000 and separately

in April and October of 2001. The daily mean

flows entering and leaving the CW were derived

from the curves drawn by continuously recording

water level gauges at control weirs installed at the

inlet and outlet. During spring and autumn floods,

water samples were taken automatically from the

inflow and outflow at 8-h intervals and stored in a

refrigerator. At weekly visits, 2-5 days were chosen

for which daily composite samples for water

analysis were made by mixing three 8-h samples.

The choice was made according to the hydrograph

to obtain samples representing the increasing and

falling limbs of major runoff peaks. During winter,

freezing hazards forced us to desert the automatic

sampling and change over to manual sampling of

major flow events. In summer 1999, no samples

were taken as the CW got dried. The annual results

presented for the Hovi CW refer to the period

between 1 May 1999 and 30 April 2000. A total of

84 pairs of samples from the inlet and outlet were

analysed during the 15 months of observations.

2.2.2. Alastaro CW

The Alastaro CW (60856?N, 22856?E, Table 1)

was constructed by a single farmer with agro-

environmental subsidies. The P concentration of

the cultivated topsoil in the vicinity of the CW

ranged from 6.8 to 15 mg P l�1. Drainage waters

from an additional 13-ha area were also chan-

nelled to the CW. The CW consists of two parts:

an open-water basin is followed by a somewhat

smaller vegetated area (Fig. 1b). At a typical

annual maximum runoff (170 l s�1 km�2) inflow

is 13 200 m3 per day, and the maximum water

depth in the CW is slightly less than 1.5 m. Under

these conditions, nominal WRT in the CW is 6 h

(Table 1).

During the study period (29 May 1998�/29 May

2000) the vegetation in the second basin of the

Alastaro CW (see Table 2) was already relatively

well-developed; the moist areas had dense stands

of plants, but the drier areas had larger gaps in the

vegetation. A continuously recording water level

gauge and a control weir were present at Alastaro

only at the outlet. Water sampling was similar to

that in Hovi. In total, 126 pairs of samples were

analysed during the 24-month study period. The

Table 2

Dominant vegetation in the wetlands and estimated above ground biomass

Wetland Species Above ground biomass at autumn 2001 (g m�2 of

wetland)

Hovi Club-rush (Scirpus sylvaticus ) 44

Cattail (Typha latifolia )

Common waterplantain (Alisma plantago-aquatica )

Reed canary grass (Phalaris arundinacea ) Meadowsweet (Filipendula

ulmaria )

Yellow flag (Iris pseudacorus )

Compact rush (Juncus conglomeratus )

Alastaro Cattail (Typha latifolia ) 91

Thread rush (Juncus filiformis )

Soft-rush (J. effuses )

Meadow-foxtail (Alopecurus aequalis )

Common spikesage (Eleocharis palustris )

Beggartick (Bidens sp.)

Flyttrask Common reed (Phragmites australis ) 1330

Water loosestrife (Lysimachia thyrsiflora )

Purple loosestrife (Lythrum salicaria )

Milk parsley (Peucedanum palustre )

Water sedge (Carex aquatica )

Common scullcap (Scutellaria galerigulata )


period at Alastaro between 1 June 1998 and 31May 1999 is referred as the first study year, and

that between 1 June 1999 and 31 May 2000 as the

second study year. During winter, freezing of the

equipment not only forced us to resort to manual

sampling but also to intermit the continuous water

level recording. Although there was mostly little

runoff during winter, some peaks during occa-

sional warmer periods were missed.

2.2.3. Flyttrask CW

Flyttrask (60808?N, 24805?E, Table 1) is a big,

seminatural wetland. As indicated by its Swedish

name (‘trask’ means ‘lake’), the area was a lake

until the 1910s when it was drained. The present

from of the CW was established when it was the

ditched to avoid flooding of the nearby fields

during high-flow periods, and dammed to main-tain an adequate water level during intermediate-

and low-flow periods so as to preserve a waterfowl

habitat. Five ditches lead to the CW from the

catchment as presented in Fig. 1c. The subcatch-

ments of the inlet ditches 1, 2, 3, 4 and 5 cover 140,

710, 200, 180 and 750 ha, respectively. The

proportions of arable land in the subcatchments

were similar and that arable areas were relativelyflat, whereas forested areas had large differences in

land elevations. In addition to arable and forested

land, the two largest subcatchments included small

bogs. In the CW, the ditches form 5�/8-m-wide and

0.9�/1.3-m-deep canals that gradually unite into

one large outlet (Fig. 1c) equipped with a control

weir and a graduated scale of water level. Water

flows through the CW predominantly in thesecanals, but during floods the water covers large

areas of the CW, consequently increasing its water

volume. Thus, the WRT first decreases and then

increases with flow, ranging from B/12 h at

intermediate runoff (15�/25 l s�1 km�2) to �/24

h at typical annual maximum runoff (90 l s�1

km�2).

The vegetation in Flyttrask (Table 2) wasdominated by a large reed area at the middle of

the CW. Even if the open-water areas of the Hovi

and Alastaro CWs were excluded, they had less

biomass per unit area than the aged Flyttrask CW.

Manual water sampling with simultaneous

water-level observations was performed during

the 26-month study period (23 April 1998�/24May 2000) once per month to four times per

week, depending on the flow. In total, 85 pairs of

samples were analysed. The mean input concen-

tration was estimated from a composite sample

formed by mixing water from the five inflowing

ditches in proportion to the sizes of their sub-

catchments. In Flyttrask, the period between 1

May 1998 and 30 April 1999 is referred to as thefirst study year and that between 1 May 1999 and

30 April 2000 as the second study year. Although

the large catchment and lower proportion of fields

dampens flow variations in Flyttrask and thus

partly justifies the use of manual sampling and

non-continuous water-level monitoring, the data

collected from Flyttrask are less accurate than

those from Alastaro and Hovi.

2.2.4. Vegetation analyses

In all CWs, emergent macrophyte vegetation

was sampled for biomass estimation in September2001 from four 0.5�/0.5-m2 situated on shallow,

vegetated parts of the CWs. The samples were

taken to a laboratory, dried in an oven at 60 8C for

7 days, and weighed. The predominant species

(Table 2) were determined earlier for each CW.

2.3. Water and soil analyses

Water samples were analysed for TSS, total P

(TP), dissolved reactive phosphorus (DRP), total

N (TN), nitrate�/nitrate-N (NOx -N), ammonium-

N (NH4-N) and pH. TSS were determined grav-imetrically according to the European standard

EN872 (Finnish Standards Association SFS,

1996), except for the filters used. In all filtrations

made for this study, Nuclepore polycarbonate

membranes with 0.4-mm pore size were used.

DRP was analysed from filtered samples using

the molybdate blue method (Murphy and Riley,

1962). In TP determination, the sample wasdigested with K2S2O8 before analysis with ammo-

nium molybdate. Particulate P (PP) was obtained

as TP�/DRP and the P content of TSS as PP/TSS.

NH4-N was analysed colourimetrically with hypo-

chlorite and phenol and NOx -N was analysed by

reduction of NO3� (Hg�/Cd or Cu�/Cd) followed


by NO2� determination. TN was analysed as NO3

�

after digestion of the sample with K2S2O8.

Before the topsoil removal in Hovi, soil samples

for more detailed P analysis were taken from 0 to

30 and 30�/60 cm soil layers from five sites in the

CW area. Each sample was divided into two

aliquots, one air-dried, homogenised and sieved,

and the other only sieved and stored moist at 5 8C.

The inorganic P content of the Hovi soil sampleswas analysed using Chang and Jackson (1957)

procedure according to Hartikainen (1979). The

concentrations of amorphous oxides of Al and Fe

were determined according to Niskanen (1989).

Equilibrium phosphorus concentration (EPC) was

obtained by suspending dry and moist soil in a set

of P standard solutions (0�/3 mg P l�1; solution to

soil ratio 50:1) and determining the point at whichno net desorption or adsorption occurred from a

Freundlich isotherm as modified by Fitter and

Sutton (1975). Above the EPC adsorption of P

should prevail, whereas the opposite applies when

P concentration is below the EPC. Finally, water-

extractable P was obtained from the suspensions

with no added P.

2.4. Calculation of material fluxes

Daily fluxes (kg per day) of nutrients and TSS to

and from the CWs were calculated by multiplying

the corresponding daily observed or estimated

concentration by the mean flow (derived from

flow-water level curves) for the same day. Con-

centrations for days between the sampling occa-

sions were estimated with linear interpolation. Thesame method was used to estimate the missing

flow values in Flyttrask. Daily retentions were

calculated by subtracting daily output from the

daily input. Retentions for longer periods*/exam-

ined here as percentage of the input and as

kilograms retained per hectare of CW area*/

were calculated by summing up the daily reten-

tions. Seasonal retentions were calculated for 3-month-periods as follows: spring from March to

May, summer from June to August, autumn from

September to November, and winter from Decem-

ber to February.

In Hovi, inflow exceeded outflow during flood

periods in 1999 and 2000. Since evaporation at

these relatively cold seasons cannot account forthe discrepancy, some bypass or*/more likely*/

seepage of water through the Hovi CW had

occurred. Therefore, to avoid overestimation of

retentions in the Hovi CW, part of the outflow

data was rejected and the retentions for April

1999, September�/October 1999 and February�/

April 2000 were calculated by using inflow for

both input and output load. In Alastaro andFlyttrask inflow was assumed to equal outflow.

The probability of seepage in these two CWs was

lower than in Hovi due to their shorter WRTs (see

Kovacic et al., 2000).

2.5. Statistical significance of the retentions

Due to the of presence of skewness and auto-correlation in our datasets, nonparametric sign

test was used. The tests were performed for the

observed daily pairs of inputs and outputs for

cases when runoff exceeded the mean runoff in

each CW (low-runoff periods had negligible con-

tribution to the material fluxes).

3. Results

3.1. Total suspended solids

The input concentrations of TSS (Table 3) were

substantially higher in Hovi than in Alastaro or in

Flyttrask, indicating high soil erosion in the clayey

and relatively inclined catchment of Hovi. During

May 1999�/April 2000, the Hovi CW retained24 300 kg ha�1 TSS, i.e. 68% of the input (Table

4). Although TSS were captured throughout the

period, the bulk of their annual input, as well as

mass retention, occurred during the snowmelt

period in April 2000. Mass retention was also

substantial in the preceding April (1999), but lower

in the following (2001). In October 2001, a slightly

negative TSS retention was found.During the first study year (1998/1999), the

Alastaro CW retained 20 600 kg TSS ha�1 (41%

of the input), virtually all in April 1999. However,

during spring 2000, the TSS output exceeded their

input (Table 5). Since the same also applied in the

preceding winter and in both autumns, the mean


retention during the second study year (1999/2000)

was negative (�/1800 kg ha�1).

At Flyttrask, most of the retention occurred*/

as in the other CWs*/during the snowmelt,

particularly in April 1999 (Table 6). During the

first and second study years, 16 and 8% of the TSS

input were retained by the Flyttrask CW, respec-

tively.

Both in Alastaro and Flyttrask, where the study

period covered 2 years, the performance was better

in the first year, when a relatively higher propor-

tion of runoff occurred in summer and a lower

proportion in winter, than in the second year (see

Tables 5 and 6). According to the sign test, TSS

retention was significant only in the Hovi CW

(Table 7).

3.2. Phosphorus

Most of the P entering the CWs was bound to

eroded soil particles, the mean proportions of

DRP in TP concentration being 8, 19 and 14% in

Hovi, Alastaro and Flyttrask, respectively. The

input concentration of P (Table 3) as well as its

annual retention was highest in the Hovi CW (24kg ha�1 per year; 62%, Table 4). In all the CWs,

DRP was retained at a lower mean rate than TP.

In Hovi, the retention percentage of DRP ex-

Table 3

Arithmetic means and standard deviations of the input concentrations of the wetlands between 1 May 1999 and 30 April 2000

Wetland TSS (mg l�1) TP DRP (mg l�1) TN NOx -N NH4-N

Mean S.D. Mean S.D. Mean S.D. Mean S.D. Mean S.D. Mean S.D.

Hovi 530 970 510 730 38 28 9800 9100 7900 8900 105 99

Alastaro 56 54 122 104 26 30 8400 6700 6800 5800 44 112

Flyttrask 29 30 67 47 9 7 3100 2900 2400 2700 90 53

Table 4

Seasonal and annual (1 May 1999�/30 April 2000) retentions in the Hovi wetland

Period Retention (kg ha�1) Runoff mean/max (l s�1 km�2)

TSS TP DRP TN NOx -N NH4-N

Spring 1999a 8200 (44%) 17 (60%) 0.79 (38%) 107 (50%) 11 (13%) 0.06 (4%) 35.6/255

Summer 1999b �/ �/ �/ �/ �/ �/ �/

Autumn 1999c 760 (76%) 1.1 (72%) 0.19 (85%) 100 (51%) 91 (51%) 0.18 (39%) 4.6/95.7

Winter 1999/2000 650 (19%) 1.2 (20%) 0.66 (33) 86 (24) 90 (27) �/0.60 (�/107%) 20.5/242

Spring 2000d 22 900 (73%) 22 (70%) 0.32 (15%) 92 (44%) 42 (32%) 1.1 (45%) 27.1/185

Annual 24 300 (68%) 24 (62%) 1.2 (27%) 280 (36%) 220 (35%) 0.7 (20%) 10.8/255

April 2001 4100 (36%) 7.8 (53%) 0.66 (47%) 63 (48%) 39 (46%) 1.3 (21%) 50.5/170

October 2001 �/111 (�/5%) 0.52 (17%) 0.34 (64%) 4.4 (9%) 5.8 (17%) 1.1 (68%) 21.8/130

Figures in parentheses denote the percentages of retention from the input loading. The means and maximums of daily mean runoff (l

s�1 km�2) for each period are presented in the last column. The annual retentions correspond to the retentions of the second study

year of the Alastaro and Flyttrask wetlands.a 1 April�/31 May.b No flow.c 1 October�/30 November.d 1 March�/30 April.


ceeded that of TP in summer, autumn and winter.

However, during the snowmelt*/when the bulk of

the DRP was transported*/its retention was

markedly lower, which resulted in an annual

retention of only 27% (1.2 kg ha�1, Table 4).

The Alastaro CW was a source rather than a

sink for DRP. Negative monthly retentions were

observed throughout the study period except for

summer 1998, yielding a �/22% retention for the

entire study period (Table 5). In Flyttrask, 10% of

the incoming DRP (0.15 kg ha�1 per year) was, on

average, retained. Retention expressed as percen-

tage of the input load tended to increase with input

concentration at Flyttrask (r2�/0.41, P B/0.001

when calculated from monthly values). According

to the sign test, significant P retention occurred in

Hovi but not in the other CWs (Table 7). The

DRP release in Alastaro was also confirmed by the

test.

In Hovi, the mean P content of TSS in the

inflow was about the same as in the outflow (1.2

and 1.1 mg kg�1, respectively). Since small

particles tend to be enriched with P, this observa-

tion suggests that particles were not selectively

retained in terms of their size. The same also

appeared to hold for the other CWs, although the

P content of TSS was higher in Flyttrask (in: 1.8

mg kg�1, out: 1.9 mg kg�1) and in Alastaro (in:

2.1 mg kg�1, out: 2.3 mg kg�1) than in Hovi.

In the Hovi CW, removal of topsoil exposed asoil layer extremely low in desorbable P; the water-

extractable P in moist soil samples was negligible

(0.1�/0.2 mg kg�1) compared with the total con-

tent of inorganic P (560�/770 mg kg�1). The low

desorbability can be attributed to the low amount

of adsorbed P in relation to sorption components.

The reserves of ‘Al�/P’ were 9�/31 mg kg�1 and

those of ‘Fe�/P’ 55�/200 mg kg�1, whereas theconcentrations of amorphous oxides of Al were

2870�/8290 mg kg�1 and of Fe 4960�/9550 mg

kg�1. The EPC value of both air-dried and moist

soil was 3 mg l�1 (two of five sites were analysed

for EPC). On the other hand, the topsoil removed

was richer in P, ‘Al�/P’ being 29�/95 mg kg�1,

‘Fe�/P’ 125�/317 mg kg�1 and water-extractable P

0.3�/2.8 mg kg�1. The EPC values of the topsoilwere 8�/112 mg l�1.

3.3. Nitrogen

In all the areas, N was mostly in dissolved form:NOx -N constituted 77, 82 and 71% of the input

concentrations of TN in Hovi, Alastaro, and

Flyttrask, respectively (Table 3). The highest input

concentrations of TN and NOx -N were observed

in Hovi. The proportions of NH4-N in TN were

negligible, except for Flyttrask (3%).

Table 5

Seasonal and annual retention in the Alastaro wetland between June 1998 and May 2000



Summer 1998 800 (20%) 2.9 (22%) 0.48 (18%) 80 (9%) 43 (6%) �/0.75 (�/62%) 6.6/54.7

Autumn 1998 �/447 (�/6%) �/4.3 (�/20%) �/4.8 (�/124%) �/107 (�/8%) �/113 (�/9%) �/1.61 (�/53%) 7.4/49.9

Winter 1998/1999a 418 (64%) 0.5 (59%) �/0.03 (�/44%) �/2 (�/17%) �/3 (�/30%) 0.02 (8%) 0.2/0.9

Spring 1999 19 850 (53%) 22.2 (28%) �/3.9 (�/21%) 40 (4%) 15 (2%) �/3.38 (�/12%) 20.6/155

Summer 1999b

Autumn 1999c �/350 (�/9%) �/0.9 (�/8%) �/0.40 (�/13%) �/440 (�/23%) �/284 (�/17%) 0.34 (67%) 7.3/50.6

Winter 1999/2000a �/248 (�/8%) �/1.0 (�/11%) �/0.09 (�/5%) 5 (0.5%) �/102 (�/13%) 0.18 (25%) 3.7/36.8

Spring 2000 �/1214 (�/4%) �/2.9 (�/5%) �/0.61 (�/5%) �/78 (�/6%) �/60 (�/6%) �/8.7 (�/55%) 16.3/143

First study year 20 600 (41%) 21 (19%) �/8.3 (�/33%) 11 (0%) �/58 (�/2%) �/5.7 (�/17%) 8.8/155

Second study year �/1 800 (�/5%) �/4.8 (�/6%) �/1.1 (�/6%) �/520 (�/12%) �/450 (�/14%) �/8.5 (�/50%) 6.9/143

Figures in parentheses denote retention percentages from the input loading of the corresponding period. The means and maximums

of daily mean runoff (l s�1 km�2) for each period are presented in the last column.a Data based on manual sampling with simultaneous water height observations.b No flow.c 12 October�/30 November.


All CWs had less effect on TN than on TSS and

TP. In the Hovi CW, 36% (280 kg ha�1 per year)

of the annual TN load was retained (Table 4).

Retention occurred in all seasons, being highest in

April 1999, October 1999 and April 2000. The

same applied to NOx-N, although its retention in

spring was weaker than that of TN. In winter

1999�/2000 and in October 2001 dissolved inor-

ganic N (NOx -N�/NH4-N) retention exceeded TN

retention, suggesting an outflow of organic N.On average, the Alastaro CW had no effect on

TN during the first study year, whereas a negative

retention was observed during the second (Table

5). More TN left the CW than entered it (Table 5),

especially in autumns of 1999 and 2000 and in

spring 2000. NOx -N showed behaviour similar tothat of TN with a slightly higher percentage of

negative retention.

Finally, in the Flyttrask CW, the TN load was

decreased by 8% during the study period (47 kg

ha�1 per year, Table 6). Seasonally, the highest

amount was retained in spring 1999, whereas the

highest proportion of input loading was captured

in summer 1999. Compared with TN, NOx-N wasretained at similar rates. As much as 55% of the

incoming NH4-N was removed during the study

period. In general, N retention was higher in the

first than in the second study year in Alastaro and

Flyttrask. According to the sign test, both TN and

NOx -N retentions in Hovi and Flyttrask (and

NH4-N in the latter) were significant, but the

negative TN retention found in Alastaro was not(Table 7).

4. Discussion

4.1. Particulate matter

Generally, the Hovi CW retained a high portion

of the input particulate matter regardless of season

or runoff volume. The poor TSS retention in

October 2001 was probably related to the extra-

ordinary green algae (Cladophora sp.) occurrence

in the preceding summer; a thick algal mat covered

Table 6

Seasonal and annual retention in the Flyttrask wetland between May 1998 and May 2000



Spring 1998a 157 (27%) �/0.03 (�/4%) �/0.02 (�/32%) 2.6 (6%) 4.7 (15%) 0.2 (26%) 12.9/32.5

Summer 1998 178 (20%) 0.22 (11%) 0.01 (4%) 20 (23%) 17 (27%) 1.0 (54%) 8.2/36.5

Autumn 1998 �/261 (�/9%) 0.26 (5%) 0.12 (19%) 9.9 (6%) 12 (11%) 1.9 (55%) 15.7/57.5

Winter 1998/1999 29 (9%) 0.11 (15%) 0.02 (11%) �/0.3 (�/1%) �/0.1 (0%) 0.9 (37%) 6.1/7.0

Spring 1999 1047 (41%) 1.41 (27%) 0.18 (17%) 26 (13%) 16 (11%) 6.6 (63%) 24.9/95.2

Summer 1999 37 (38%) 0.19 (43%) 0.06 (68%) 2.5 (38%) 0.8 (35%) 0.3 (56%) 1.9/4.0

Autumn 1999 84 (29%) 0.18 (31%) 0.01 (19%) 15 (27%) 13 (28%) 0.5 (37%) 4.4/10.8

Winter 1999/2000 58 (2%) 0.41 (7%) �/0.12 (�/23%) 4.8 (1%) 8.3 (2%) 4.0 (59%) 30.3/79.9

Spring 2000 217 (18%) 0.51 (25%) 0.04 (21%) 4.7 (5%) 6.1 (9%) 0.9 (40%) 11.4/37.2

First study year 1100 (16%) 1.9 (15%) 0.30 (15%) 57 (11%) 48 (14%) 10 (57%) 14.2/95.2

Second study year 410 (8%) 1.3 (14%) �/0.00 (�/0%) 27 (5%) 28 (6%) 5.8 (53%) 12.1/79.9

Figures in parentheses denote retention percentages from the input loading of the corresponding period. The means and maximums

of daily mean runoff (l s�1 km�2) for each period are presented in the last column.a 23 April�/31 May.

Table 7

Statistical significance of the retentions in the Hovi, Alastaro

and Flyttrask wetlands according to the sign test

Retention of Hovi Alastaro Flyttrask

TSS * ns ns

TP *** ns ns

DRP *** ***a ns

TN *** nsa *

NOx -N * *a ***

NH4-N ns nsa ***

Following symbols for the significance levels were used: ***,

P B/0.001, **, P B/0.01 and *, P B/0.05.a Negative retention.ns signifies not significant


almost the entire CW since mid-July. The decom-

posing algae may have increased the particulate

organic matter (and TSS) in the outflow in

October. This assumption is supported by lower

TN than the coexistent inorganic N retention,

which suggests a net outflow of organic N.

Flyttrask and particularly Alastaro showed

marked temporal variation in TSS retention, and

sometimes more TSS left these CWs than entered

them. Such incidents were probably attributed to

the resuspension of deposited matter induced by

excessive flow velocity. The Alastaro CW was

simply too small to adequately decelerate flow,

and the high potential of the large Flyttrask CW

was partly wasted due to canalisation. Canalisa-

tion also explains the lower retention of TSS per

CW area in Flyttrask than in Alastaro: the

retention achieved in Flyttrask was divided by a

huge area, which*/for most of the time*/was not

even in contact with inflowing water.The apparently high TSS retention in the

Alastaro CW in spring 1999 may be an over-

estimation due to dilution of the inflowing water

with melting snow in the CW, which was covered

with ice and a thick layer of snow after a cold

winter. By contrast, the following winter was

milder, and the amount of ice and snow at the

beginning of spring flooding*/and hence the

dilution effect*/consequently lower. Moreover,

the bottom of the CW was probably more

susceptible to resuspension, which may explain

the negative TSS retention.

Differences in seasonal performance, which

might be attributed to the annual development of

vegetation, were observed in Alastaro and Fly-

trask. For example, in June and August 1998

(during the growing season), TSS were decreased

in Alastaro at the rising limb of the hydrograph,

but at or after the peaks, TSS concentrations in the

outflow equalled those in the inflow (Fig. 2a and

b). On the other hand, in October 1998 (after the

growing season), TSS retention was negative at the

very beginning of flooding, suggesting resuspen-

sion of earlier deposited matter (Fig. 2c). At this

time, the decaying vegetation was not able to

protect the bottom, nor markedly lower the flow

velocity. As also found in Flyttrask in 1998, TSS at

equal flow volumes were retained in summer butexported in autumn.

Vegetation has been considered important for

proper functioning of CWs (Brix, 1997). For

example, macrophytes have been found to reduce

flow velocity near the wetland bottom and increase

the sedimentation of suspended clay particles

(Petticrew and Kalff, 1992). However, under

conditions occurring in Finland vegetation mayplay a role for the most part in summer, when

agricultural runoff is typically low.

Since the bulk of P was in particulate form, TP

retention followed the variation in the TSS reten-

tion. The highest annual TP retentions observed

here (21 kg ha�1 per year for Alastaro and 24 kg

ha�1 per year for Hovi) fall in the lower range of

those found for ponds and CWs in the Nordiccountries (20�/1160 kg ha�1 per year, Uusi-

Kamppa et al., 2000), yet are quite close to those

measured for constructed ponds by Dellien and

Wedding (1997) in southern Sweden (30 kg ha�1

per year) and by Haikio (1998) in central Finland

(37 kg ha�1 per year).

4.2. Dissolved nutrients

Despite its low proportion in TP, DRP is a

crucial P fraction, because it is directly available to

algae and thus contributes readily to eutrophica-

tion (Ekholm, 1998). However, DRP was substan-

tially retained only in Hovi. Two major removal

processes for DRP exist in CWs: it may be utilised

by biota or adsorbed by soil. Since vegetation in

the Hovi CW was sparse and most of the DRPentered outside the growing season, the incorpora-

tion of DRP in macrophytes appears unlikely. By

contrast, P adsorption was promoted by several

factors in Hovi. Removal of the topsoil from the

CW area exposed a soil layer extremely low in

secondary P on oxide surfaces and high in sorption

components. Accordingly, the EPC of the soil was

low, in fact invariably below the DRP in the inflow(Fig. 3). The topsoil was also removed in the

Alastaro CW. However, since the DRP concentra-

tion in the inflow*/as well as WRT*/was lower in

Alastaro (and in Flyttrask), the conditions for P

sorption were less favourable there than in Hovi.

In fact, release rather than retention of DRP


appeared to be the predominant process in Alas-

taro. Whereas the DRP retention in July 1998 (and

the corresponding summertime retention in Flyt-

trask) may have been due to biological uptake in

this CW with dense vegetation, the negative

retention in October 1998 may have resulted

from the release of P from the decaying biota.

Since rooted plants may take most of their P from

soil rather than water (Carignan and Kalff, 1980),

they may transfer soil P to water. In this respect,

the sparse vegetation in Hovi might even have

been beneficial. The substantial release of DRP in

Alastaro in April 1999 may have been attributed

to P liberated from thawing ice-broken plant cells

(Ulen, 1984). Alternative explanations include P

release from suspended or bottom sediments

induced by dilute snowmelt water (Rekolainen,

1989) and concomitant widening of the water�/soil

ratio (Yli-Halla et al., 1995), and decreased O2

concentration (e.g. Andersen, 1974), particularly

in the deeper first part of the CW. In addition, the

mean pH in the inflow was 7.0 (range 6.4�/8.9),

whereas optimal pH values for P adsorption are

below 6.5 (e.g. Lijklema, 1977; Verhoeven and

Meuleman, 1999). Finally, the P content of TSS in

the Alastaro inflow was almost twice that in Hovi.

With increasing P content, the P-binding ability of

soil particles tends to decrease.

In contrast to P, most of the inflowing N was in

dissolved form, mainly as NOx-N, in all the CWs.

Nitrogen retention was highest in Hovi. As

discussed above, significant biological uptake in

this CW appears unlikely, leaving denitrification

the principal N-removing process. In Hovi, WRT

was long and NOx-N concentrations high, which

should favour denitrification (Reddy and D’An-

gelo, 1997). On the other hand, denitrification is

also promoted by temperature (Xue et al., 1999),

which was low during floods, and by the supply of

organic C (Reddy et al., 1982; Schnabel et al.,

1996), also relatively low in this new CW. The

annual NOx-N loss in Hovi (about 250 kg ha�1)

corresponds to a daily value of 0.07 g m�2 per

day. If denitrification was the sole N retention

process, its rate in Hovi falls in the ranges reported

by Xue et al. (1999) and Fleischer et al. (1994):

0.05�/0.28 and 0.001�/0.48 g m�2 per day, respec-

tively. However, N was probably also removed by

sedimentation, as indicated by the higher TN than

dissolved inorganic N retention.

The richer, more mature vegetation in the

Alastaro versus Hovi CW should favour denitrifi-

cation by supplying more organic C. In addition,

the first basin of the Alastaro CW was deeper and

thus may have provided more sites with low O2

content. Furthermore, the concentration of NOx -

N in the inflow was at the same high level as in

Fig. 2. TSS concentration (mg l�1) in the inflow and outflow and runoff (l s�1 km�2) in the Alastaro wetland during three runoff

events in 1998.


Hovi. Nevertheless, N retention was poor in

Alastaro, probably due to the short WRT during

floods; the N released from the vegetation upon

senescence*/in addition to inflowing N*/has been

exported from the CW rather than denitrified.

Drainage pipes from an additional 13-ha area

were led into the Alastaro CW. These waters did

not contribute to the measured input concentra-

tions but did affect the output. Since drainage

waters tend to be richer in N than surface runoff

(Skaggs et al., 1994; Turtola and Paajanen, 1995),

the measured input TN and NOx-N*/and hence

their retentions*/may have been underestimated

in Alastaro. For TSS and P, probably no such bias

existed.

Due to the better performance during high-flow

periods, N retention was higher in Flyttrask than

in Alastaro, despite the low input NOx-N con-

centrations (often B/1000 mg l�1) in Flyttrask.

The higher N removal may be due to the longer

WRT. In addition, the bottom of this relatively old

CW with mature vegetation was probably rich in

easily degradable C for denitrifying bacteria. The

Alastaro CW, as well as the Hovi CW, were

recently excavated with their vegetation and or-

ganic sediment still at the build-up stage. The high

NH4-N reduction in Flyttrask not only suggests

the presence of biological uptake but also of

intensive nitrification.

The highest annual TN retention in this study

(280 kg ha�1 in Hovi), as well as the annual TN

retention of 320 kg ha�1 reported by Haikio

(1998), is lower than the estimates of about 1000kg ha�1 reported by Dellien and Wedding (1997)

and attributable to the colder climate and thus less

favourable conditions for denitrification in Fin-

land.

4.3. Design considerations

The CW area in relation to its catchment

(effecting long WRT) appears to be the most

important single factor*/and a design para-

meter*/when high retentions are aimed for. High-

lighting this implication, our results were in

accordance with several other studies. For exam-

ple, in five Nordic CWs, all comprising (like the

Alastaro CW) less than 1% of their catchments,the annual TN retentions varied between 3 and

15% (Haikio, 1998; Braskerud, 2001). By contrast,

Whigham et al. (1999) and Kovacic et al. (2000)

reported annual TN retentions between 23 and

52% in a total of seven CWs, all covering (like the

Hovi CW) more than 3% of their catchments. In

Fig. 3. Input concentrations of dissolved reactive phosphorus (DRP) dots and equilibrium phosphorus concentration (EPC) of the soil

(horizontal reference lines) in the Hovi wetland.


the case of TP, the above studies showed similar*/

though not as consistent*/tendencies as TN.

If only nominal WRT (�/V/Q) is considered, a

deep pond would provide equal water volume with

smaller area (i.e. lower land acquisition costs) as a

shallow CW. However, in a deep pond the contact

area between the overflowing water and soil and

vegetation, and hence possibilities of N denitrifica-

tion and P adsorption, are lower. Moreover, there

are several other aspects that favour multiform

solutions*/i.e. CWs with both deep and shallow

parts, spits of land, islets etc.*/instead of uni-

formly deep sedimentation ponds. These include

(i) biodiversity: in addition to aquatic environ-

ment, multiform CWs feature more terrestrial and

littoral ecosystems, (ii) landscape: sedimentation

ponds are often crude, rectangular ‘widenings of

ditch’ whereas well-designed multiform CWs

smoothly assimilate into the surrounding land-

scape, and (iii) economy: deep ponds entail high

excavation and earth moving costs.

In addition to the long nominal WRT, the Hovi

CW also had a morphology with high hydraulic

efficiency, i.e. a form in which flow efficiently

utilised the CW area, and stagnated zones (with

little or no water movement) were small. Persson

et al. (1999) examined the hydraulic efficiency of

variously shaped wetland basins by a two-dimen-

sional hydraulic model. Of the 13 basins examined,

the basin with spits of land similar to those in the

Hovi CW, was ranked the second best. Moreover,

our own modelling exercises with basin morphol-

ogies of the Hovi and Alastaro CWs (Koskiaho,

2003) suggested higher hydraulic efficiency for the

former. Both our experiences and those of Persson

et al. (1999), however, suggest that an elongated,

rectangular shape is hydraulically not very ineffi-

cient, either. Thus, too small a size in relation to

the catchment rather than deficient shape was

responsible for the weak retention in the Alastaro

CW.

As previously discussed, our experiences

strongly speak for the removal of formerly culti-

vated topsoil from the site of a CW. This action

not only reduces the risk of P release but also*/in

a fortunate case*/creates long-lasting P-retaining

capacity for the CW.

5. Conclusions

Under cold, boreal conditions most agricultural

loading occurs during seasons when biological

activity is low. In such cases, CWs appear to

function primarily as sedimentation basins by

reducing flow velocity. Accordingly, all three

CWs retained nutrients more efficiently in parti-

culate than in dissolved forms. However, under

favourable conditions P adsorption and denitrifi-

cation also occur. By promoting all the retention

processes, WRT is a crucial factor in the perfor-

mance of CWs. To ensure adequately long actual

WRTs during floods, CWs should be large en-

ough, and designed so that water spreads evenly to

the entire CW area. If flow velocity increases

excessively, it not only prevents retention but

induces resuspension and export of the matter

formerly deposited. High input concentrations

may also promote retention. Therefore, CWs

should be located at the nutrient sources, i.e. in

the vicinity of fields, cowsheds, etc. However,

construction of wetland on a P-rich soil (cultivated

field, grassland, etc.) should be avoided, unless the

topsoil layer is removed.Since our study period was short and two of the

CWs were still young, the long-term performance

of CWs remains unknown. Moreover, there is an

inherent problem in assessing the effect of CWs.

During the transport from a field to a lake, for

example, nutrients are liable to several qualitative

and quantitative changes (e.g. Ekholm et al.,

2000). What fraction of the nutrients removed by

a CW would have been retained in any case during

transport? In other words, is the actual benefit of

CWs as high as observed in retention studies? For

example, coarse matter captured by a CW may

have settled to the bottom of ditches and streams

rather than entered larger bodies of water. On the

other hand, the significance of the DRP release

(negative retention) in CWs is also questionable,

unless it is caused by some process specific for

CWs, such as transfer of soil P to water by

macrophytes or by anaerobia. If the release occurs

from eroded soil particles, it may also occur*/

regardless of CWs*/during transport or in the

receiving body of water.


Acknowledgements

We are grateful to Professor Helina Hartikainen

for her advice in the soil P analysis, Dr Pertti

Seuna for his comments, Timo Nieminen for

processing the runoff data, and to the laboratory

staff of the Regional Environment Centres of

Uusimaa and Southwest Finland for the chemical

analyses. The authors also wish to thank the fieldpersonnel: Catrine Huhta, Bertil Lostedt, Britt

Lundqvist and Juhani Koskinen. This study was

partly funded by the Primrose-project (EVK1-

CT2000-00065) of the EU fifth framework pro-

gramme.

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Retaining agricultural nutrients in constructed wetlands—experiences under boreal conditions