TECHNICAL REPORTS

1318

Th e eff ect of ice cover on sediment resuspension and internal total P (Tot-P) loading was studied in the northern temperate Kirkkojärvi basin in Finland. Th e gross sedimentation and resuspension rates were estimated with sediment traps during ice-cover and ice-free periods. After ice break, the average gross sedimentation rate increased from 1.4 to 30.0 g dw m−2 d−1. Resuspension calculations showed clearly higher values after ice break as well. Under ice cover, resuspension ranged from 50 to 78% of the gross sedimentation while during the ice-free period it constituted from 87 to 97% of the gross sedimentation. Consequently, the average resuspension rate increased from 1.0 g dw m−2 d−1 under ice-cover to 27.0 g dw m−2 d−1 after thaw, indicating the strong eff ect of ice cover on sediment resuspension. To estimate the potential eff ect of climate change on internal P loading caused by resuspension we compared the Tot-P loading calculations between the present climate and the climate with doubled atmospheric CO

2 concentration relative to the present day values (ice cover

reduced from current 165 to 105 d). Th e annual load increased from 7.4 to 9.4 g m−2. In conclusion, the annual internal Tot-P loading caused by resuspension will increase by 28% in the Kirkkojärvi basin if the 2 × CO

2 climate scenario comes true.

The Contribution of Ice Cover to Sediment Resuspension in a Shallow Temperate Lake:

Possible Eff ects of Climate Change on Internal Nutrient Loading

Juha P. Niemistö* and Jukka Horppila University of Helsinki

The rate of sediment resuspension in shallow lakes is mostly

regulated by wind-driven waves and water currents (Bloesch,

1982; Evans, 1994). In shallow areas, such water motion can

result in continuous resuspension, thus being the major factor

contributing to the total settling fl ux (Rosa, 1985; Evans, 1994;

Weyhenmeyer, 1996; Horppila and Nurminen, 2001; Niemistö

et al., 2005). Under ice cover, the wind stress is absent and thus

the amount of annually resuspended sediment strongly depends

on the duration of ice cover. Th e contribution of ice cover to

sediment resuspension has been studied rarely, although it has

been predicted that climate change may have strong eff ects on ice-

cover formation. According to diff erent predictions, the formation

of ice cover will be delayed, ice-cover period will be considerable

shorter, and during winter there will be ice-free periods (Kuusisto,

1989; Huttula et al., 1992; Fang et al., 1998).

A widely used model is the 2 × CO2 scenario in which atmo-

spheric CO2 values double relative to present day values. If this

scenario comes true, it will aff ect the freezing and thaw dates and

thus the duration of ice cover of lakes in temperate areas (Kuusis-

to, 1989; Fang et al., 1998). Fang and Stefan (1998) predicted that

the duration of ice cover will shorten about 45 d in the northern

contiguous USA in the 2 × CO2 climate. Similarly, climatological

predictions made in Finland indicate that the ice-cover period will

be from 40 to 60 d shorter (Kuusisto, 1989). Th e eff ects of climate

change on wind velocities are diffi cult to predict, but it is assumed

that with increasing temperatures the probability of storms, which

are particularly important resuspension-causing events (Bengtsson

et al., 1990), increases (Melillo et al., 1990).

Sediment resuspension is often the main factor contributing

to redistribution of sediment and gross sedimentation of shallow

lakes (Hilton et al., 1986; Evans, 1994), and thus can be consid-

ered one of the governing processes that regulate P fl uxes within a

lake. Numerous studies have shown that resuspension is an impor-

tant factor contributing to the Tot-P and soluble reactive P (SRP)

concentrations of lake water (de Groot, 1981; Kristensen et al.,

1992; Søndergaard et al., 1992; Koski-Vähälä and Hartikainen,

2001). However, studies on the eff ects of shortened ice-cover du-

ration on internal P loading via eff ects on sediment resuspension

are scarce. Consequently, investigating the eff ect of ice cover on

sediment resuspension is of great importance, especially in areas

Abbreviations: SS, concentration of suspended solids; Tot-P, total phosphorus; PSS,

phosphorus bound to suspended solids; SRP, soluble reactive phosphorus.

Department of Biological and Environmental Sciences/Limnology, P.O. Box 65

(Viikinkaari 1), FIN 00014, Univ. of Helsinki, Finland.

Copyright © 2007 by the American Society of Agronomy, Crop Science

Society of America, and Soil Science Society of America. All rights

reserved. No part of this periodical may be reproduced or transmitted

in any form or by any means, electronic or mechanical, including pho-

tocopying, recording, or any information storage and retrieval system,

without permission in writing from the publisher.

Published in J. Environ. Qual. 36:1318–1323 (2007).

doi:10.2134/jeq2006.0487

Received 8 Nov. 2006.

*Corresponding author (juha.niemisto@helsinki.fi ).

© ASA, CSSA, SSSA

677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS: SURFACE WATER QUALITY

Published online July 17, 2007

Niemistö & Horppila: Contribution of Ice Cover to Sediment Resuspension 1319

where the annual duration of ice cover in the present climate is

considerable and several shallow lakes exist.

Th is study aims to show the eff ect of ice break on sediment

resuspension and its possible consequences on internal Tot-P

loading. We hypothesize that if climate change proceeds as

predicted, the shortening of ice-cover duration will increase

the amount of annually resuspended sediment and thus the

internal Tot-P loading.

Material and Methods

Study Area and DesignTh e study was performed from 8 Feb. to 25 May 2005 in

the Kirkkojärvi basin of Lake Hiidenvesi in south-western

Finland (60°24′ N, 24°18′ E) (Fig. 1). In the shallow and

eutrophic Kirkkojärvi basin (area 1.6 km2, mean depth 1.1 m,

max depth 3.5 m, average open-water season Tot-P concentra-

tion 80–100 μg L−1) strong phytoplankton blooms are com-

mon during summer (Tallberg and Horppila, 2005). Th e bot-

tom sediment is quite inorganic (organic content ca. 10–20%

in the topmost 1 cm) being mainly clay.

Sampling was conducted at four diff erent sampling stations.

Water depths at Stations 1 to 4 were 1.5, 2.0, 3.5, and 1.0 m,

respectively (Fig. 1). Gross sedimentation and sediment resuspen-

sion at each station were estimated with cylindrical sediment traps

(plastic tube, diameter = 5.4 cm, aspect ratio = 6:1, a buoyant

collar keeping the trap in the upright position). Traps were located

0.5 m above the lake bottom. Th e same type of sediment traps

have been used before at the same basin (Horppila and Nurminen,

2001, 2003, 2005) and were used again to get comparable results.

Th e aspect ratio of the traps we used was adequate to avoid sedi-

ment resuspension from the trap (Bloesch and Burns, 1980), and

cylindrical traps have the best avoidance of undertrapping and

overtrapping (Hargrave and Burns, 1979). Samples from three

replicate traps at each station were collected during two ice-covered

periods with 28 d exposure (8 February–8 March and 8 March–5

April) and during two ice-free periods with 14 d exposure (27

April–11 May and 11–25 May) intervals. Th e ice break-up oc-

curred during 19 to 22 April. Trap exposure period of 14 d has

been used in earlier studies in the Kirkkojärvi basin during summer

(Niemistö et al., 2005; Tallberg and Horppila, 2005). During the

ice-covered dates the trap exposure period had to be extended to 28

d so that enough material was trapped. Mineralization of organic

matter in such a long interval, however, was considered minimal as

it has been shown to be very slow at low water temperatures (<4°C)

(Hargrave, 1969; Granéli, 1978).

Th e whole content of each trap was collected in the plastic

jar at the fi eld. Samples settled 24 h and most of the super-

natant liquid was siphoned off before taking subsamples

(two out of each jar). Dry weight of trap material and surface

sediment was measured after drying samples at 105°C and

the organic fraction by ignition at 550°C. Surface sediment

samples (the topmost 0- to 1-cm layer) were collected at each

trap location with a Limnos sediment sampler.

Seston (T) samples for concentration of suspended solids

(SS) were collected with a Limnos tube sampler (height 1 m,

volume 7.5 L) at the beginning and end of each trap exposure

period. Th ree replicate water samples from each station were

fi ltered through Whatman GF/C fi lter (pore size 1.2 μm), dried

at 105°C, and measured for dry weight. Th e organic fraction of

suspended solids was determined by ignition at 550°C.

Th e gross sedimentation rate (S) was determined on the

basis of entrapped material. Sediment resuspension (R) was

calculated according to Gasith (1975):

TR

TS

ff

ffSR

−−

=

where

S = gross sedimentation (g dw m−2 d−1)

fS = organic fraction of S

fR = organic fraction of the surface sediment

fT = organic fraction of seston T collected from the

water column

Th ree replicate water samples for concentration of Tot-P and

SRP were collected at the same time with SS sampling (a Limnos

tube sampler). Total P and SRP were determined using the meth-

ods by Koroleff (1979) and Murphy and Riley (1962) (Lachat

autoanalyzer, QuickChem Series 8000, molybdenum blue–ascor-

bic method). Before analyzing SRP, samples were fi ltered through

membrane fi lters (Schleicher & Schuell ME 25, mixed cellulose

ester, pore size 0.45 μm). To determine the amount of P bound

to SS (PSS), Tot-P was also determined after fi ltering the water

samples through Whatman GF/C fi lter. Th e Tot-P concentration

of the surface sediment at each trap location was determined using

the methods by Koroleff (1979) after wet combustion with sulfuric

acid and hydrogen peroxide. Th e concentration of dissolved oxy-

gen at each of the stations was measured with a YSI-6600 sonde.

Th e external loading of suspended solids and Tot-P from the

infl owing river Vihtijoki was estimated from the data received

from Water and Environment of Western South Finland (Ranta

and Jokinen, 2006). Th e monthly river load was calculated

using discharge measurements (taken once every day) and sus-

pended solids concentrations (monthly water samples).

Fig. 1. Map of the study area. Water depths at Stations 1 to 4 were 1.5, 2.0, 3.5, and 1.0 m, respectively. Dash line ellipses show the locations of sediment traps of the study of Horppila and Nurminen (2005).

1320 Journal of Environmental Quality • Volume 36 • September-October 2007

Phosphorus Loading CalculationsTh e internal Tot-P loadings caused by resuspension, in both

the present and the predicted 2 × CO2 climate, were calculated

using the average resuspension rate of this study for the ice-cover

season and the average resuspension rates of the study of Horp-

pila and Nurminen (2005) for the ice-free season (trap locations

in Fig. 1). Th e eff ect of macrophyte-covered areas (39% of the

whole lake area; Horppila and Nurminen, 2005) on resuspension

during the ice-free season was taken into account. At the begin-

ning and end of the open-water season, a 30-d macrophyte-free

period was assumed due to normal macrophyte succession in

the Kirkkojärvi basin (L. Nurminen, personal communication,

2005) and thus diff erent average resuspension rates were used

for the growing season and for the period when no macrophytes

existed. In the present study no traps were located on macro-

phyte-covered areas. Th e loading in the present climate was calcu-

lated assuming 165 d ice-cover duration, and the loading in the

predicted climate was calculated assuming 60 d shorter ice cover

duration (Kuusisto, 1989) and thus 60 d longer growing season.

To estimate the annual internal Tot-P loading, the amount of an-

nually resuspended sediment was multiplied by the P concentra-

tion of the surface sediment (Horppila and Nurminen, 2005).

Th e equation for P loading calculation was:

(I × IR + N × N

R + M × M

R) × P

where

I = duration of ice cover (present climate 165 d, 2 × CO2

climate 105 d)

IR = average resuspension rate for ice-cover season

N = duration of macrophyte-free period during ice-free

season (60 d)

NR = average resuspension rate for ice-free season

without macrophytes

M = duration of the growing season (present climate 135

d, 2 × CO2 climate 195 d)

MR = average resuspension rate for the growing season

of macrophytes

P = the average P concentration of the surface sediment

Statistical AnalysesTh e concentration of SS, Tot-P, PSS, SRP, the rate of gross

sedimentation, and the rate of resuspension between ice-cover

and ice-free periods were all tested using analysis of variance

(SAS Institute, 1990). In all analyses, the presence or absence

of ice cover was considered as a treatment so that all values of

every variable analyzed were pooled to ice-cover or ice-free ob-

servations. Th e data collected on 5 April for SS, Tot-P, PSS, and

SRP were excluded from statistical analyses, because (apart from

SRP) the values diff ered markedly from the values of all the

other dates due to brief interval of high spring water discharge.

Five days earlier, no peak in water discharge was yet observed.

Resuspension values as a percentage of total fl ux were arcsin

transformed before testing the diff erences between ice-cover and

ice-free trap exposure periods. Th e relationship between SS and

Tot-P was studied by linear regression (5 April data excluded).

Results

Gross Sedimentation and ResuspensionUnder ice cover, the gross sedimentation rate ranged from 0.3

to 1.3 g dw m−2 d−1 during the fi rst sampling period (8 February–8

March) and from 1.3 to 3.8 g dw m−2 d−1 during the second one

(8 March–5 April) (Fig. 2). Sampling Station 1 showed the highest

gross sedimentation values during both exposure periods under ice.

Fig. 2. Gross sedimentation and resuspension rates during ice-covered periods (a) 8 February to 8 March, (b) 8 March to 5 April, and (c) ice-free periods 27 April to 11 May and (d) 11 to 25 May (+95% confi dence limits) (note diff erent scales in ice-free and ice-cover fi gures).

Niemistö & Horppila: Contribution of Ice Cover to Sediment Resuspension 1321

Th e further the sampling station was located from the inlet of the

river Vihtijoki, the lower was the gross sedimentation rate (Fig. 2).

Th e resuspension rate was highest at Station 1 and followed the

same pattern as the gross sedimentation under ice cover. From 8

February to 8 March the resuspension rate ranged between 0.2 and

1.0 g dw m−2 d−1 (51–78% of the gross sedimentation rate) and

from 8 March to 5 April between 0.2 and 4.0 g dw m−2 d−1 (13–

108% of the gross sedimentation rate) (Fig. 2). Under ice cover,

the contribution of external loading of suspended solids from the

river Vihtijoki to gross sedimentation was from 70 to 100%.

After ice break, the highest gross sedimentation and resuspen-

sion rates were associated with depth rather than distance from

the river inlet. From 27 April to 11 May, the gross sedimentation

ranged between 9.1 and 20.0 g dw m−2 d−1. Th e resuspension rate

was between 8.2 and 19.0 g dw m−2 d−1, constituting 87 to 95%

of the total fl ux. During the second ice-free period, from 11 May

to 25 May, the gross sedimentation showed the highest values of

the study, ranging from 27.9 to 74.5 g dw m−2 d−1. Th e resuspen-

sion rate was from 27.0 to 68.0 g dw m−2 d−1, constituting 92 to

97% of the gross sedimentation (Fig. 2). External SS loading of

the infl owing river constituted 20% of the gross sedimentation

during the fi rst and 4% during the second ice-free period.

Th e average gross sedimentation rate during ice-free period

was more than 20 times higher and the average resuspension

rate was 27 times higher compared with the average rates under

ice cover. Statistically, gross sedimentation rate (p < 0.0001), re-

suspension rate (p < 0.0001), and resuspension as percentage of

the total fl ux (p < 0.0001) increased signifi cantly after ice break.

Concentration of Suspended Solids, Total Phosphorus,

Phosphorus Bound to Suspended Solids, and Soluble

Reactive PhosphorusAt each station the concentration of SS was higher on ice-

free sampling dates, 10.9 to 19.6 mg L−1, than on ice-cover

dates, 2.9 to 10.9 mg L−1 (p < 0.0001), with a peak value of

56.4 mg L−1 measured on 5 April, excluded from the statistical

analysis (Fig. 3). At most of the stations Tot-P (p < 0.0001) as

well as PSS (p < 0.0001) showed higher values after ice break.

Total P values ranged more widely, from 38 to 98 μg L−1 under

ice (up to 168 on 5 April) and from 68 to 90 μg L−1 during

ice-free exposure periods. Th e concentration of PSS varied from

4 to 17 μg L−1 before (up to 103 on 5 April) and from 27 to

54 μg L−1 after ice break (Fig. 3). In contrast to Tot-P values,

the SRP values were signifi cantly lower during ice-free periods

(p < 0.0001), ranging from 17 to 35 μg L−1 under ice (down

to 15 on 5 April) and from 5 to 17 μg L−1 after ice break (Fig.

3). Total P and SS collected from the water column during the

study correlated positively (R2 = 0.38, p = 0.004) (Fig. 4).

Phosphorus Loading Caused by ResuspensionTh e average resuspension rate for the ice-cover season was

1.0 g dw m−2 d−1. For the ice-free season the average resuspen-

sion rate was 32.0 g dw m−2 d−1 when no macrophytes existed

and 28.9 g dw m−2 d−1 during the growing season. Th e average

P concentration of surface sediment samples of all the sampling

stations, 1.23 g kg−1, was used in internal Tot-P loading calcula-

tions because the values measured during ice-cover and ice-free

periods showed no signifi cant diff erence (p = 0.509). In the pres-

ent climate the amount of annually resuspended sediment in the

Fig. 3. Concentration of (a) suspended solids (SS), (b) total phosphorus (Tot-P), (c) P bound to suspended solids (PSS), and (d) soluble reactive phosphorus (SRP) in the water samples at the beginning and end of trap exposure periods at Stations 1 to 4 (+95% confi dence limits).

1322 Journal of Environmental Quality • Volume 36 • September-October 2007

Kirkkojärvi basin was 6.0 kg dw m−2 yr−1, whereas in the 2 × CO2

climate (60 d shorter ice-cover duration assumed) the amount

would be 7.7 kg dw m−2 yr−1. Correspondingly, the annual in-

ternal Tot-P loading caused by resuspension would thus increase

from 7.4 to 9.4 g m−2 yr−1 (28% higher loading of Tot-P).

Discussion

Eff ect of Resuspension on Suspended Solids, Total

Phosphorus, and Soluble Reactive Phosphorus ConcentrationsWind-driven waves and currents are considered the main fac-

tors causing sediment resuspension in shallow lakes (Hilton et al.,

1986; Evans, 1994). Th is phenomenon has also been observed in

the Kirkkojärvi basin and resuspension has been demonstrated to

be the main factor contributing to high gross sedimentation values

(Horppila and Nurminen, 2001; Niemistö et al., 2005). Conse-

quently, as expected, the gross sedimentation and resuspension

rates were markedly higher after ice break than under ice cover.

During winter, ice cover prevented the infl uence of winds on sedi-

ment resuspension. Although the external SS loading from River

Vihtijoki constituted a large portion of the gross sedimentation

under ice cover, quantitatively the loading was rather low. More-

over, because the external SS loading constituted only a small pro-

portion of the gross sedimentation during the ice-free periods, we

concluded that the occurrence of ice cover was of great importance

to the rate of sediment resuspension, which in turn played a major

role in water quality and gross sedimentation rates after ice break.

Th e resuspension rate in shallow lakes is highly dependent on

the occurring winds and water currents (Evans, 1994) and because

of that two very diff erent exposure periods can follow as was ob-

served during the present study. However, the average resuspension

rate of the two ice-free periods that we measured corresponded

well to seasonal averages observed before at the Kirkkojärvi basin

(Horppila and Nurminen, 2005; Tallberg and Horppila, 2005).

Kristensen et al. (1992) pointed out a clear connection between

high resuspension rates and high SS concentrations of shallow,

eutrophic Lake Arresø in Denmark. Th e same study also showed

a connection between high resuspension rate and high Tot-P con-

centrations. Results similar to these were recorded in the Kirkko-

järvi basin during the ice-free periods. After ice break, when resus-

pension increased 27-fold, SS and Tot-P concentrations increased

markedly. Because SS loading from River Vihtijoki was low at the

time, sediment resuspension caused by wind-induced waves and

currents was considered to be the main factor contributing to high

SS values. Furthermore, a large proportion of P was bound to SS,

and external P loading from the river constituted only 7 to 16% of

the Tot-P values, so it could be concluded that sediment resuspen-

sion dominated P input into the water column during the ice-free

period. Th e contribution of bioturbation, caused by benthivorous

fi sh (Breukelaar et al., 1994), to resuspension and P loading was

assumed to be only of minor importance since the dominant cypri-

nid fi sh inhabited the littoral spawning areas during spring.

Resuspension can be the main factor contributing to P fl ux

between water and sediment, and thus it plays a very important

role in the internal P loading of shallow lakes (Søndergaard et al.,

1992). In the Kirkkojärvi basin, resuspension was clearly the domi-

nant factor increasing the concentration of particulate Tot-P in the

water column after ice break. However, several factors aff ect the

mobility of resuspended P. Diff erent studies have reported positive

(Rippey, 1977; Søndergaard et al., 1992), negative (Lennox, 1984),

or negligible (Peters and Cattaneo, 1984) eff ects of resuspension

on P release. Th e release of P from the suspended particles depends

strongly on the pH of water (Andersen, 1975). In the water col-

umn the concentration of solids is markedly lower compared with

sediment and thus the buff ering capacity of pH is weaker (Drake

and Heaney, 1987). Increase in pH, due to algal blooms, and

decrease in soluble P, due to assimilation, enhances the release of P

from resuspended particles. Consequently, resuspension and algal

assimilation together can serve as a mechanism that pumps out P

from the sediment (Søndergaard et al., 1992). When and in which

conditions the liberation of P from the suspended particles occurs

in the Kirkkojävi basin is still a subject to be studied.

Th e O2 depletion in eutrophic waters due to long-term ice

cover may be an important factor for P release. However, O2

depletion has not been a problem in the Kirkkojärvi basin due

to short theoretical retention time, approximately 10 d. In the

present study no O2 depletion or high SRP concentrations were

observed in the end of the ice-cover season. Close to the lake

bottom, the concentration of O2 was >4 mg L−1 at the deepest

station on 5 April. However, if O2 depletion occurred during

winter, the shortening of ice-cover duration would decrease the

anoxic P release due to better mixing and reaeration (Huttula

et al., 1992). Th e decrease in SRP concentration after ice break

was attributable to uptake by the increasing diatom biomass

observed every spring in the Kirkkojärvi basin (Tallberg and

Horppila, 2005), and probably partly due to adsorption of P to

suspended particles (Koski-Vähälä and Hartikainen, 2000).

The Development of the Internal Phosphorus LoadingAlthough the liberation of P from resuspended particles

depends on many factors, during the open-water season resus-

pension continuously mixes P-rich sediment particles into the

water column (Koski-Vähälä and Hartikainen, 2000), creating

the potential for P release in the Kirkkojärvi basin. If climate

change proceeds as predicted (Kuusisto, 1989), the shorter

ice-cover duration will result in a higher amount of annually

resuspended sediment and particulate P.

According to simulations of Kallio et al. (1996) the mean an-

Fig. 4. The relationship between suspended solids (SS) and total phosphorus (Tot-P), R2 = 0.38, p = 0.004 (data of ice-covered and ice-free dates pooled, 5 April data excluded).

Niemistö & Horppila: Contribution of Ice Cover to Sediment Resuspension 1323

nual runoff in Finland will increase only slightly due to climate

change. However, the simulations also showed that high spring

runoff shifts to occur in winter. Th e annual pattern of nutrient

losses from the watershed followed that closely (Kallio et al.,

1996). It was also discussed that the change in the seasonal pat-

tern of runoff could result in higher P concentration in surface

runoff due to shorter periods of frozen ground and snow cover

during winter (Kallio et al., 1996). Consequently, if external P

loading from watersheds increases in the future, there will be

even more P in the sediments of shallow lakes to resuspend.

Th e prediction model applied in the present study (Kuusisto,

1989) contains an assumption that no change in wind conditions

will occur in the 2 × CO2 climate. Th erefore, in our calculations,

resuspension and Tot-P loading caused by it increased linearly with

respect to shortening of ice-cover duration. However, there is a pos-

sibility of stronger winds and more frequent storms in the future

(Melillo et al., 1990). Meijer et al. (1990) showed strong evidence

that in shallow lakes, high resuspension rates resulted in high water

turbidity, restricting macrophyte colonisation. Moreover, a previous

study at the Kirkkojärvi basin indicated that macrophyte stands

considerably prevented sediment resuspension (Horppila and Nur-

minen, 2005). Th erefore, we conclude that if the Kirkkojärvi basin

becomes more turbid in the future, the annual internal Tot-P load-

ing at the basin will be even higher than we have estimated.

AcknowledgmentTh is study was fi nancially supported by the Academy of

Finland (project no. 205365). Jouko Sarén helped with the

fi eld work and Raija Mastonen with the laboratory work.

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