Journal of Hydrology (2006) 327, 226–234

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Field evidence for buoyancy-driven water flow ina Sphagnum dominated peat bog

E.B. Adema a,c,*, G.J. Baaijens a, J. van Belle c, C. Rappoldt b,A.P. Grootjans a, A.J.P. Smolders d

a Community and Conservation Ecology, Centre for Ecology and Evolutionary Studies, University of Groningen,P.O. Box 14, 9750 AA Haren, The Netherlandsb Alterra, P.O. Box 47, 6700 AA Wageningen, The Netherlandsc COPERNICUS Institute for Sustainable Development and Innovation, Environmental Sciences, Faculty of Geography,Utrecht University, Heidelberglaan 2, P.O. Box 80115, 3508 TC Utrecht, The Netherlandsd Aquatic and Environmental Biology, University of Nijmegen, The Netherlands

Received 28 April 2005; received in revised form 25 October 2005; accepted 10 November 2005

Summary Nocturnal buoyancy-driven water flow in bogs is proposed as a mechanism toreplenish the nutrient availability in the top of the acrotelm. In an earlier paper, we providedevidence for buoyancy-driven water flow on theoretical and experimental grounds. In thispaper, field evidence is given for the occurrence of nocturnal buoyancy cells. Measurementsin a small isolated bog in the Netherlands showed that temperature differences between dayand night and hydraulic conductivity resulted in a Rayleigh number that is sufficiently high toinduce buoyancy flow. Irregularities between diurnal heating and nocturnal cooling gave evi-dence that indeed convective heat exchange occurred. Moreover, we observed a temperaturerise at 5–10 cm depth around midnight, which would be expected on theoretical grounds ifbuoyancy-driven water flow occurs. Furthermore, the oxygen cycles at 5 cm depth showedan irregularity, which exactly matched the temperature irregularity. This also strongly suggeststhat buoyancy-driven water flow occurred. Nutrient samples taken from the same ombrotrophicbog showed a significant increase in the concentrations of NHþ4 , Ca

2+, Mg2+ and CO2 with depth,whereas K+ showed a significant decrease with depth indicating that buoyancy-driven waterflow may potentially redistribute these nutrients. The size of the buoyancy cells was assessedunder controlled conditions in a peat core taken from the bog. We observed two spots ofupward flow of warm water and two spots of downward flow of cold water. The cell sizes wereapproximately 25–100 cm2. We conclude that nocturnal buoyancy-driven water flow occurs inthe field were it may be an important mechanism for relocation of oxygen and nutrients.

�c 2005 Elsevier B.V. All rights reserved.

KEYWORDSBuoyancy-drivenwater flow;Hydraulic conductivity;Peat moss;Nutrients

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022-1694/$ - see front matter �c 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.jhydrol.2005.11.019

* Corresponding author. Tel.: +31 30 2534410; fax: +31 30 2532746.E-mail address: e.adema@geo.uu.nl (E.B. Adema).

Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog 227

Introduction

Sphagnum dominated bogs play an important role in the glo-bal carbon cycle. These peatlands may act as a net sink foratmospheric CO2 (Gorham, 1991; Clymo et al., 1998).Therefore, to retard global warming, conservation and res-toration of these peatlands have priority to optimise CO2 fix-ation. However, for successful conservation and restorationof Sphagnum dominated mires, knowledge about environ-mental constrains for Sphagnum growth is necessary.

Sphagnum plays an active role in creating its own environ-ment (Kilham, 1982). Awell-developed Sphagnum bog createsnutrient-poor acid conditions and thereby gains competitiveadvantage over other plant species (Malmer et al., 1994).The ability of Sphagnum species to deal with very low nutrientavailability is often attributed to their pronounced capacity toexchange hydrogen ions formineral cations (Daniels and Eddy,1985). This gives the plant the opportunity to grow in an envi-ronment with very low nutrient concentrations. Anotherimportant feature to survive in nutrient poor habitats is recy-cling of nutrients (Van Breemen, 1995).

In particular, Sphagnum growth may be stimulated byrecycling of nitrogen and phosphorus, which are consideredlimiting plant growth (Malmer and Nihlgard, 1980; Aertset al., 1992; Malmer, 1993). However, also oxygen andCO2 transport is of importance. Oxygen transported to thebottom of the acrotelm (defined by Ingram, 1978) may in-crease decay rates, thereby increasing nutrient release.CO2 formed by this decay may stimulate peat growth iftransported upward (Smolders et al., 2001). Moreover, in-creased oxidation of the acrotelm may lower CH4 emissionrates from the peat, an important greenhouse gas. Sundhet al. (1995) and Frenzel and Karofeld (2000) found thatCH4 emission from a raised bog could decrease due to oxida-tion of CH4 to CO2.

Transport can take place inside the plants (internaltransport) or outside the plants (external transport). Earlierresearch revealed that some internal nutrient transporttakes place, probably through submicroscopic perforationsin the end walls of stem parenchyma cells (Rydin and Clymo,1989). However, Sphagnum lacks the structure for notableinternal transport. Hence, external relocation is probablyof greater importance. Capillary water flow is an importantexternal transport mechanism in Sphagnum dominated eco-systems, firstly for water supply (Clymo and Hayward,1982), however, also solutes are transported with thewater. Brehm (1971) mentioned relocation trough capillarywater flow for K+, Mg2+ and Ca2+. However, Malmer (1993)considered upward transport of nitrogen and phosphorustrough capillary transport insignificant due to the low con-centrations of these nutrients in the pore water. Rappoldtet al. (2003) provided evidence for another external trans-port mechanism in a water-saturated peat moss layer. Theyshowed that buoyancy-driven water flow could occur if thetemperature difference between day and night is suffi-ciently large. They studied this phenomenon in a peat mosslayer by means of a mathematical model, numerical simula-tion and laboratory measurements.

During the night the bog surface cools, leading to a situ-ation with relatively dense cold water on top of warmerwater. If the temperature drop is sufficiently large, the cold

water sinks and the warm water wells up. This type of flowis called buoyancy-driven and it implies convective trans-port of heat and solutes carried with the water. Buoyancy-driven water flow often occurs as ‘‘cells’’ consisting ofadjacent regions with upward and downward flow leadingto mixing of the water and dissolved nutrients in the acrot-elm. Nocturnal buoyancy cells may redistribute nutrientsthat were released from Sphagnum litter at the lower partsof the acrotelm, replenishing the nutrient availability in thetop of the acrotelm where Sphagnum is growing. Especiallyscarce nutrients such as nitrogen and phosphorus that wereleached from Sphagnum litter (Verhoeven et al., 1990) canstimulate the Sphagnum growth.

In this paper, we focus on evidence for the occurrence ofnocturnal buoyancy cells in the field and try to assess itsimportance for external nutrient transport.

Materials and methods

Site description

Field measurements were carried out in the ‘Reeenven’, asmall bog (1.5 ha) in the north of the Netherlands(52�50 0N, 6�27 0E). The underground consisted of a boulderclay layer formed in the penultimate glacial period, the Saa-lian glaciations. This layer impedes ground water exchangewith deeper soil layers. On top of this boulder clay anapproximately 1 m thick sand layer has been deposited inthe last glacial, the Weichsel. The ‘Reeenven’ is situatedin a depression in the sand layer that is filled with Sphagnumpeat. Large parts of the vegetation are dominated by Sphag-num species. In the lower parts Sphagnum cuspidatum Ehrh.ex Hoffm. occurs with some Utricularia minor L. Sphagnummagellanicum Brid. and Sphagnum palustre L. dominate thehigher parts. Erica tetralix L. and Calluna vulgaris (L.) HuIlare accompanying vascular plants. Sphagnum fallax(Klinggr.) Klinggr. dominates the intermediate zone in asso-ciation with Rhynchospora alba (L.) Vahl, Oxycoccus palus-tris Pers. and Drosera rotundifolia L.

Field measurements

Along two transects in the mire, electric conductivity (EC25)and temperature were measured for every 10 m width. Thetwo parameters were collected at 20 cm depth intervals byusing a 2 m long probe (Van Wirdum, 1984). One transectwas laid out in the north–south direction of the mire, thesecond transect was directed east–west. In the lower part,where Sphagnum cuspidatum dominates, in situ measure-ments of oxygen concentration, redox potential, and tem-perature were conducted continuously at three depths; 5,10, and 20 cm in the period of 21–24 September 1999.The first two parameters were measured using stainlesssteel needle electrodes (Van Gemerden et al., 1989), thetemperature was measured with thermistors. Data were col-lected using a Campbell scientific CR10X multi-channeldatalogger. The datalogger measured every 2.5 s and storedthe mean value of these measurements each minute. In Sep-tember 1999, soil–water samples were taken at the samelocation where the in situ measurements were conducted.The water samples were collected at three depths; 10, 20

Figure 1 Schematic representation of the Sphagnum core.We measured the hydraulic conductivity of the total columnand the rest (total-layer) of the column. The hydraulicconductivity of the layer below the cut line was calculated byformula (1.2).

228 E.B. Adema et al.

and 50 cm using ceramic cups to investigate the potentialfor nutrient transport by buoyancy-driven water flow. With-in 48 h after sampling, the water samples were analysed ona scalar analytical auto-analyser tor NHþ4 , NO�3 , Na+, K+,Ca2+, Mg2+, Cl�, pH and EC25. During the sampling day thesamples were stored in 50-ml airtight PVC bottles at 4 �C.Samples tor analyses of CO2 and HCO�3 were stored in Eppen-dorf cups at 4 �C prior to analyses. The analyses were car-ried out using an infrared carbon analyser (model 0525HR, Oceanography International).

Vertical saturated hydraulic conductivity

An intact peat monolith was collected in a 25 cm long PVCtube with an internal diameter of 10.8 cm, from the mea-suring site after the field measurements were finished.The core was sealed by two PVC caps and stored at 4 �C untilanalysis. Vertical hydraulic conductivity was measured usinga modified constant head method for different layers in theSphagnum monolith.

The core was placed on a grid in the middle of a largercontainer, filled with water, to enable a free flow of watertrough the peat column. The water level in the containerwas established a few centimetres above the surface levelof the Sphagnum core. After installation of the core a con-stant water flow was established by using a variable speedpump (Cole Parmer EW-07620-00). The flow rate was mea-sured with a flow meter (100–1500 ml min�1 Cole Parmer32458-32) and adjusted until measurable differences inhydraulic head occurred between the water level abovethe peat core and in the container. The hydraulic conductiv-ity was calculated by:

k ¼ QL

DhAð1:1Þ

where k is the hydraulic conductivity (m s�1), Q is flow rate(m3 s�1), L is length of the peat column (m), Dh is hydraulichead (m) and A is surface area of the peat column (m2).

The measurement was repeated twice with slightly in-creased flow rates resulting in three replicated measuresfor each length.

This procedure was repeated for different lengths of theSphagnum monolith by removing parts of the monolith atthe bottom.

The hydraulic conductivity for each separate layer wascalculated by (Kutılek and Nielsen, 1994; Fig. 1):

kðlayerÞ ¼LðlayerÞ

LðtotalÞkðtotalÞ

� LðrestÞkðrestÞ

ð1:2Þ

We calculated the Rayleigh number for each layer accordingto Rappoldt et al. (2003) by using the calculated hydraulicconductivity and a temperature difference of 8 �C betweenday and night. All other parameter values for calculating theRayleigh number were identical to those in Rappoldt et al.(2003).

Laboratory measurements

A cylindrical peat core (Ø30 cm, h 55 cm), with an approxi-mately 13 cm thick acrotelm, collected from the ‘Reeen-ven’ was placed in a climate room. In the process of

collecting, the spongy top layer inevitably got disturbed,but the peat moss had recovered after two weeks in a cli-mate chamber. Radial heat transport was prevented by10 cm of insulating foam around the cylinder. The climateroom was programmed to a 12-h light 12-h dark cycle witha temperature of 22 �C during the light period and 10 �C atthe dark period. A temperature profile was measured with8 copper–constantan–copper thermocouples and a therm-istor for reference. The thermocouples were placed at 1,6, 14, 25, 39, 56, 76, and 99 mm depth. The reference weldsand the thermistor were placed at 150 mm depth. The dataof the thermistor were collected using a Campbell ScientificCR10X multi-channel datalogger. The datalogger measuredevery second and stored the mean value of these measure-ments each minute. The data of the thermocouples werecollected using a Campbell scientific CR21 multi-channeldatalogger with the same frequency as the thermistor.

During one nocturnal period a rectangular temperaturegrid with a mesh of 1 by 1 cm was measured twice at39 mm depth. The first grid was measured 3 h after the lightwas turned off and the temperature had changed. A row of12 copper–constantan–copper thermocouples and meltingice as a reference were used to measure the grid. The dis-tance between the thermocouples was 2 cm. This row ofthermocouples will be further referred to as the y-axis.On the x-axis 20 measurements were carried out with aninterval of 1 cm. Then the row of thermocouples was moved1 cm in the y-direction and we measured the 20 steps backin the x-direction. In this way, the temperature was mea-sured in a cm-grid of 19 by 23 cm. Two hours later a secondmeasurement was conducted in a 45� rotated grid, in orderto study the stability of the buoyancy cells (Fig. 2).

Data analysis

Data of in situ oxygen measurements were fitted to themodel x ¼ �x þ a sin ð2pðtþ bÞÞ, in which x = the tested

Figure 2 Schematic drawing of the 19 by 23 cm rectangulartemperature grid measurement with 1 · 1 cm grid cells. Firstly,12 rows with 1 cm intervals were measured in the direction ofthe x-axis). Therefore, the row of thermocouples, with aninterval of 2 cm, (1) was moved in direction A along the x-axis(black lines). Then the row of thermocouples was moved 1 cmalong the y-axis (B) before the measuring was continued in theC direction along the x-axis (gray lines). A second measurementwas conducted in a 45� angle in order to study the stability ofthe buoyancy cells.

Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog 229

variable; t = time in days, and �x, a, and b are scalablefactors to test whether a periodical cycle of 24 h waspresent. The significance of the regression was tested byanalyses of variance after the data passed the test for anormal distribution (P > 0.05; one-sample Kolmogorov–Smirnov test; SPSS for Windows version 11 by SPSS Inc.)and a test for homogeneity of variance (P > 0.05; Levenetest for homogeneity of variance; SPSS for Windows ver-sion 11 by SPSS Inc.). The model fitting and analyses ofvariance on the regression was conducted with SigmaPlot8.0 by SPSS Inc. Data of ion concentrations, CO2, EC25

and pH were analysed for differences in depth by analysesof variance after the data passed the tests for a normaldistribution(P > 0.05; one-sample Kolmogorov–Smirnovtest) and for homogeneity of variance(P > 0.05; Levenetest for homogeneity of variance). This data analyseswas conducted with SPSS for Windows version 11 by SPSSInc.

Figure 3 Electric conductivity and temperature

Results

Field measurement

TransectsThe electric conductivity (EC25) along transects was low,indicating oligotrophic water (Fig. 3a). Only at the borderof the bog the values exceeded 125 lS cm�1. Towards thecentre of the bog the EC25 decreased gradually. The temper-ature measurements showed a similar pattern as the EC25measurements (Fig. 3b). The lowest temperatures werefound at the border of the bog whilst the central part ofthe bog showed a kernel of relatively warm water.

In situ measurementsThe continuous oxygen measurements showed a decline inoxygen with depth. The average oxygen concentrations overthe measuring period are, respectively, 145, 66 and5 lmol l�1 for 5, 10 and 20 cm depth (Fig. 4). Although alldepths showed variation in time, only the 5 cm depthshowed a significant 24-h cycle, in which the oxygen con-centration increased during day time and decreased duringthe night (P < 0.0001).

The temperature measurements also showed a daily cy-cle at all three depths with amplitude decreasing with depth(Fig. 5). Moreover, the temperature profile reversed duringthe night, which induced an unstable water column withdenser cold water over lighter warm water. Noteworthy isthe hump shaped temperature rise around midnight at adepth of 5 cm, and to a lesser extent at 10 cm in the twonocturnal cooling periods. This irregularity, though less pro-nounced, also occurred in the 5 cm deep oxygen cycle,which was measured at short distance from the temperatureprobes. Combining the two 5 cm deep cycles of temperatureand oxygen shows that the temperature rise and oxygenconcentration change occurred at the same time (Fig. 6).

Water analysesNutrient samples taken from the same ombrotrophic bogshowed a significant increase in concentrations of NHþ4 ,Ca2+, Mg2+ and CO2 with depth, whereas K+ showed a signif-icant decrease with depth. Na+, Cl�, and NO�3 did not show asignificant trend with depth (Table 1). The EC25 increasedslightly with depth, but the EC25 did not exceed 55 lS cm�1.The pH was between 4.3 and 4.5 and did not show any sig-nificant trend.

measurements in the South–North transect.

Figure 4 Daily oxygen cycles at three depths. Oxygensaturation decreased with depth. Only the oxygen saturationat 5 cm depth showed a significant daily cycle in oxygen.

Figure 5 Temperature cycles at three depths over more than2 days. The amplitude of the differences decreased with depthresulting in an inverted temperature profile at night in whichcold water lay on top of warm water. Due to differences indensity this may lead to instability, which may result inbuoyancy cells. Note the small temperature increase just aftermidnight at 5 cm depth.

Figure 6 Comparison of temperature cycle with oxygencycle. The temperature increase during the night that startedat 1:30 h agreed with an increase in oxygen at the same time(arrows). Both were indications that convective water flow isinitiated at that time. The temperature rise is probably due tothe water flow of warm water from deeper layers. Thereplenishment of oxygen at 5 cm may also be attributed tothe mixing of oxygen rich surface water with water from deeperlayers, thus increasing the oxygen saturation at deeper layers.

Table 1 Water analyses of the pore water; daily averagewith standard error, ion concentrations are given in lmol l�1

and EC25 is given in lS cm�1, n = 8

10 cm Depth 20 cm Depth 50 cm Depth

x SE x SE x SE

CationsCa2+ 11.5 1.6 15.9 1.4 20.4 1.8Mg2+ 19.1 2.9 20.1 1.0 25.6 1.0K+ 32.9 4.2 11.5 0.8 10.4 1.2Na+ 222 6 259 3 252 12NHþ4 52.8 1.1 32.7 0.4 113.6 0.7

AnionsNO�3 2.48 0.23 1.89 0.32 1.82 0.04HCO�3 2.84 0.62 3.26 0.26 8.43 0.78Cl� 527 48 512 32 617 32

Dissolved gasCO2 148 8 280 16 439 27

EC25 49.6 1.3 45.5 1.3 54.9 1.1pH 4.52 0.07 4.34 0.04 4.49 0.04

230 E.B. Adema et al.

Vertical saturated hydraulic conductivity

The vertical hydraulic conductivity of the Sphagnum mono-lith is was high in the upper layer (0.25 m s�1) and de-creased with depth. Calculations of the Rayleigh numbers(Rappoldt et al., 2003), as shown in Table 2, indicated thatonly in the upper layer (0–8 cm) the hydraulic conductivityis sufficient to support buoyancy-driven water flow. In the8–12 cm layer, the hydraulic conductivity will rapidly de-crease to values that do not support buoyancy-driven waterflow. Below this second layer the k values are so low that wemay not expect buoyancy-driven water flow at all.

Laboratory measurements

The temperatures measured at 8 different depths showed asteady daily cycle over the measuring period of 3 days. Theaverage 24-h temperature decreased with depth (Fig. 7).

Grid measurementsThe temperature grid measurements in the cylindrical peatcore showed two warm and two cold spots (Fig. 8). In thewarm spots, the temperature was up to 1.5 �C higher thanthe average temperature of the grid. In the cold spots,the temperature was up to 1 �C lower compared to the

Table 2 Density and saturated hydraulic conductivity of different layers from the Sphagnum monolith

Layer (cm) Density (kg m�3) Hydraulic conductivity(m s�1)

Rayleigh number

Sphagnum Other plants Total Average SE Average SE

0–8 9.35 0.31 9.66 0.25 0.09 167 588–12 17.87 0.06 17.93 0.0063 0.0022 4.2 1.412–16 14.31 3.20 17.51 0.0077 0.0002 5.1 5.116–19 29.83 8.22 38.05 0.0104 0.0003 6.9 6.9

The density of the layer is divided in a fraction of Sphagnum and a fraction of other plant species, consisting manly of roots from vascularplants. The saturated hydraulic conductivity was calculated from three replicate measurements at different flow rates. The Raleighnumber was calculated according to Rappoldt et al. (2003) with a DT of 8 �C.

Figure 7 Differences between 24-h temperatures means atthe different depths compared to means at 1 mm depth during2 days. Markers represent averages of three replicated mea-surements with standard errors. The temperature decreasewith depth implies that nocturnal heat loss exceeded diurnalheating. In a constant environment, regarding diurnal temper-ature regime, such as the climate room, this would only bepossible if not only conductance but also another heat transportmechanism would be active during the night, in this caseconvective water flow.

Figure 8 Temperature grid measured at 39 mm depth in apeat core in a climate room after 3 h into the cold period. Thecore shows two warm and two cold spots, which possiblycontribute to upward and downward water flow, respectively.

Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog 231

average. The second measurement conducted at an angle of45� showed the same pattern as the first measurement (datanot shown). The two cold and the two warm spots occurredat the same positions as in the first measurements althoughthe average temperature was 2� lower due to 2 additionalhours of nocturnal cooling. The cells showed some variabil-ity in shape and size. The horizontal cell sizes were between25 and 100 cm2.

Discussion

The measurements of the electrical conductivity indicatedthat the ‘Reeenven’ is an almost isolated water body withonly little influence of incoming shallow groundwater fromthe edges of the bog. The temperature measurements sup-ported this observation (Fig. 3b). These measurementsshowed that an isolated warm water body was still presentin the middle of the bog as a result of high summer temper-

atures. If the bog were not isolated, water movement wouldhave resulted in a more homogeneous temperature distribu-tion in the transects. These data, therefore, indicated thatthere was hardly any exchange of water with the surround-ing groundwater and that the ‘Reeenven’ can be consideredan isolated ombrotrophic bog.

The in situ temperature measurements conducted in the‘Reeenven’ showed that the differences between day andnight temperatures are large enough to induce buoyancy-driven water flow. Under the observed temperature condi-tions and with an 8 cm thick layer in which hydraulic con-ductivity supports water movement, Rayleigh numberslarger than 44 indicate that the water column will becomeunstable and buoyancy-driven flow will occur (Pieters,2004). With, in our case, a temperature difference betweenday and night exceeding 8 �C at the peat surface, an esti-mate of the Rayleigh number will be 167 for the 0–8 cmlayer and will decrease to 12 in the 8–12 cm layer basedon the measured hydraulic conductivity. These numberssuggest that buoyancy-driven water flow will occur to a cer-tain depth in the 8–12 cm layer. Hence, the hump in thetemperature measurements at 5–10 cm depth could becaused by buoyancy-driven upward flow of warm water. Atthe same time the oxygen content at the 5 cm depth rises,

232 E.B. Adema et al.

conspicuously (Fig. 6). This can also be related to buoyancy-driven water flow if the oxygen electrode is positioned in a‘‘cold cell’’, since in such a cell cold oxygen-rich water willflow downwards. The distortion of the temperature distribu-tion with depth during the cooling process was also observedunder experimental conditions in a climate room and is con-sistent with numerical model simulations (Rappoldt et al.,2003). If only conductive heat transport would take place,both processes, cooling and heating, would occur at thesame rate, and the mean daily temperature would be thesame at al depths. However, if nocturnal buoyancy-drivenwater flow occurs, the nocturnal cooling is more effectiveresulting in a decreasing mean daily temperature withdepth, as we have found. More evidence for nocturnal buoy-ancy-driven water flow is provided by the grid measure-ments showing two warm and two cold spots. The warmspots may represent places where an upward flow of warmwater occurs, whilst in the cold spots dense surface wateris sinking to deeper layers.

Nocturnal water flow by buoyancy cells will not only mixthe water but also the solutes in it. Since transport by diffu-sion is a slow process (the diffusion coefficient for oxygen is1.96 · 10�5 cm2 s�1 at 20 �C and ammonium 1.95 ·10�5 cm2 s�1 at 20 �C; Boudreau, 1997) convective transportcan play an important role in oxygen and nutrient replenish-ment. Since the concentration of two important nutrients,NHþ4 and CO2 increase with depth, net upward transportby buoyancy-driven water flow of these nutrients is possi-ble. This may have major consequences for the internalnutrient cycle in the bog ecosystem. Moreover, oxygentransport to the lower parts of the acrotelm will acceleratethe decomposition rate of the senescent Sphagnum plantshere, thus increasing nutrient releases. If transported,these nutrients may become available to the growing capit-ulum of Sphagnum.

Figure 9 Scheme of the nutrient flow in the acrotelm of a peatdriven water flow. This will result in a net upward transport of nut

Ecological importance

The possible impact of nutrient redistribution through watertransport is maximal if external nutrient input is low. Innorth-western Europe, increased atmospheric nutrient depo-sition has probably overruled the subtle role of nutrient recy-cling in bog ecosystems. For example, the critical nitrogenload for the most sensitive ombrotrophic bogs is between 5and 10 kg nitrogen ha�1 yr�1, a value that is by far exceededin the Netherlands (Bobbink and Roelofs, 1995 and Lamerset al., 2000). However, most bogs are situated in areas withmuch lower atmospheric deposition rates. Recently McKen-zie (2005) showed that the thermal regime of peatlands insubarctic Eastern Canada is probably suitable to induce buoy-ancy-driven nutrient flow. They found that conduction wasthe main heat transport mechanism for deeper layers, upto 400 cm. Nevertheless, their data of air and peat surfacetemperature showed sufficient diurnal temperature differ-ences to support buoyancy-driven nutrient flow in the toplayer of the peat if the hydraulic conductivity of this layeris high enough. This conclusion is supported by older temper-ature data from Moore (1986) who measured for 2 years thetemperature at six peatland sites in Northern Quebec. Hefound summer surface temperatures of 15–20 �C in the peat.Hence, concerning the thermal expansion coefficient ofwater at these temperatures, buoyancy-driven nutrient flowmay occur with sufficient daily temperature differences.

In areas with low atmospheric deposition rates, nitrogenand/or phosphorus are often mentioned as limiting nutrientsin bogs (Roswall and Granhall, 1980; Aerts et al., 1992). Dam-man (1978, 1986) found that in unpolluted bogs the annualamount of nitrogen and potassium used for growth, exceededthe annual input, indicating the recycling of nutrients. Buoy-ancy-drivenwater flow inducing nutrient transport could be a

moss layer. The nutrient redistribution is caused by buoyancy-rients due to the efficient nutrient uptake by Sphagnum.

Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog 233

mechanism that plays an important role in nutrient recycling.This stresses the need for in situ measurements of nutrientconcentrations on a small scale without disturbing the noc-turnal buoyancy flow. Also oxygen and carbon dioxide trans-port have to be taken into account, the latter for its possiblepositive effect on Sphagnum growth. Carbon dioxide canstimulate Sphagnum growth under intermediate nutrient in-put levels (Smolders et al., 2001; Heijmans et al., 2001).

Much work has been done on the inter-specific competi-tion for nutrient between Sphagnum and vascular plants(Tomassen et al., 2004; Heijmans et al., 2002a,b; Limpenset al., 2003, 2004). Most of this work focuses on competitionbetween Sphagnum and vascular plants at high levels ofnutrient input. In this situation, Sphagnum is no longer ableto take up al the deposited nitrogen and a part of these nutri-ents becomes available for vascular plant growth (Lamerset al., 2000; Bragazza and Limpens, 2004). Under low atmo-spheric deposition almost all deposited nutrients are cap-tured by Sphagnum, whereas nutrients used by vascularplants originate mainly from the decay of organic matter(Aerts et al., 1992; Malmer et al., 1994). The root systemsof vascular plants are often situated at the bottom of theacrotelm were peat decomposition takes place. They are,therefore, the first to capture nutrients released by decom-position. However, if buoyancy-driven water flow occurs,nutrients released by decomposition may rapidly be translo-cated to the top of the acrotelm were the capitula of Sphag-num can intercept these nutrients efficiently due to theirhigh cation exchange capacity (Clymo, 1963; Fig. 9). Thismay be the reason why in lawns and hollows, where buoy-ancy-driven water flow takes place, the occurrence of vascu-lar plants is much lower compared to the higher parts of thebog, especially in initial stages of peat bog succession.

Acknowledgements

We thank Nelly Eck for measuring many of the water sam-ples and Jacob Hogendorf for his practical assistance duringthe experiment.

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