Dynamics and ecological significance of daily internal load of phosphorus in shallow Lake Balaton, Hungary

Dynamics and ecological significance of daily internalload of phosphorus in shallow Lake Balaton, Hungary

VERA ISTVANOVICS,* ANDRAS OSZTOICS † AND MARK HONTI*

*Water Resources Management Group of the Hungarian Academy of Sciences, Department of Sanitary and Environmental

Engineering, Budapest University of Technology and Economics, Budapest, Hungary†Department of Civil and Environmental Engineering, Budapest University of Technology and Economy, Budapest, Hungary

SUMMARY

1. As supported by field data, turbidity recorded by light scattering sensors could reliably

be converted into concentration of suspended particulate matter (SPM) and coefficient of

vertical light attenuation (Kd) in Lake Balaton.

2. Autocorrelation analysis revealed that proper determination of SPM concentration and

Kd required daily sampling. To approximate daily rate of resuspension, 15 min or more

frequent measurements were needed. Thus, routine monitoring provides very little insight

into environmental variability of shallow lakes as habitats for phytoplankton.

3. The internal P load was estimated from daily rate of resuspension and P desorption

capacity of sediments. The latter was assumed to be proportionate to the potentially mobile

inorganic P content of SPM. A comparison with net primary production and nutrient

status of phytoplankton showed that the proposed method of estimating time series of

internal P load captured seasonal trends.

4. The daily rate of resuspension was high whereas that of internal P load was low in Lake

Balaton relative to other shallow lakes. The latter reflects favourable behaviour of the

calcite-rich sediments. As a consequence, carrying capacity of Basin 1 of Lake Balaton was

P-determined.

5. The timing of external and internal loads was radically different. While the former

showed mostly seasonal changes, large pulses characterised the latter. As a consequence,

internal load may supply more P to phytoplankton growth during the critical summer

months than external load. However, the relative importance of these sources may show

substantial interannual variability.

6. Large resuspension events often followed each other during periods of 10–15 days.

It has been shown that disturbances in this frequency range are of key importance in

maintaining the diversity of phytoplankton. We propose that resuspension can be

perceived not only as a disturbance factor but also as a factor that periodically relaxes

nutrient stress. The former feature may dominate the instantaneous effect, whereas the

latter may determine the persistent effect of resuspension on succession of phytoplankton.

Keywords: light attenuation, nutrient status of algae, P desorption, pulses in P load, resuspension,turbidity

Introduction

In most shallow lakes, internal P load is the principal

component of P supply for phytoplankton growth

during summer. When the carrying capacity of a lake

is P-determined (Reynolds, 1992), internal load is a

key factor in regulating year-to-year variability of

Correspondence: Vera Istvanovics, Department of Sanitary and

Environmental Engineering, Budapest University of Technology

and Economics, H-1111 Budapest, Muegyetem rkp. 3., Hungary.

E-mail: [email protected]

Freshwater Biology (2004) 49, 232–252

232 � 2004 Blackwell Publishing Ltd

algal biomass (van der Molen & Boers, 1994; Istva-

novics & Somlyody, 2001). At the same time, internal

P load has been recognised to be a major cause of the

resistance of shallow lakes to recover from eutrophi-

cation (Sas, 1989). Thus, internal P load is of prime

importance from the viewpoint of both phytoplankton

ecology and eutrophication management. An order-

of-magnitude or a time-integrated (annual, seasonal,

monthly) estimate of net internal load normally

suffices for managerial purposes and the majority of

studies on internal P load satisfy this need (Lijklema

et al., 1986; Sas, 1989; Kozerski & Kleeberg, 1998;

Søndergaard, Jensen & Jeppesen, 1999; Istvanovics &

Somlyody, 2001). Evaluation of the role of internal P

load in algal dynamics, however, requires temporal

resolution that matches specific growth rates of

phytoplankton. Although numerous direct and indir-

ect approaches have been developed for estimating

internal P load in shallow lakes, none of these

methods allow the appropriate time scale over

prolonged periods of time to be achieved.

As Bostrom, Jansson & Forsberg (1982) have

pointed out, internal P load is a function of various

mobilisation and transport processes. The relative

importance of different P mobilisation mechanisms

differs among lakes in accordance with the geochem-

ical composition of their sediments (Bostrom, 1984).

Wind induced sediment resuspension is the most

important transport process in wind-exposed shallow

lakes (Lijklema et al., 1986; Søndergaard, Kristensen

& Jeppesen, 1992; Evans, 1994; Ogilvie & Mitchell,

1998). Mobilisation and transport of sedimentary P

can be characterised by different temporal scales.

While mobilisation results in relatively smooth

changes over several days to a few months, sediment

resuspension may fluctuate widely over hours.

Considering fast kinetics of P desorption from

suspended sediments with initial rate constants of

<1 min)1 (Williams, Syers & Harris, 1970; Li et al.,

1972; Kuo & Lotse, 1974), the internal load via P

desorption must closely follow the dynamics of

sediment resuspension. Consequently, if the aim is

to relate algal growth and/or phytoplankton succes-

sion to P supply originating mostly from P de-

sorption, conventional studies on P mobilisation and

P release from sediments are less relevant than

studies on the dynamics of sediment resuspension.

Resuspension has manifold and contrasting influ-

ences on algal dynamics. Reduced light availability

has been shown to retard algal growth (Somlyody &

Koncsos, 1991). Simultaneously, however, transport

of meroplanktonic/benthic species may temporarily

increase the biomass of phytoplankton (Padisak,

G.-Toth & Rajczy, 1988, 1990). Besides instantaneous

effects, increased nutrient availability upon resuspen-

sion may stimulate algal growth (Rijkeboer, De Bles

& Gons, 1991; Ogilvie & Mitchell, 1998). In a number

of lakes, large resuspension events are followed by a

measurable change in the concentration of soluble

reactive P (SRP) (Søndergaard et al., 1992; Hamilton

& Mitchell, 1997; Ogilvie & Mitchell, 1998). Depend-

ing on the equilibrium concentration between sedi-

ment and water, this change may either be negative

or positive. In highly calcareous Lake Balaton,

resuspension fails to detectably increase typically

low concentrations (<5 mg P m)3) of SRP. The P

uptake experiments, however, have suggested that P

deficient phytoplankton in the lake could not main-

tain net P uptake for longer than a few tens of

minutes in the absence of a desorptive P flux from

resuspended sediments (Istvanovics & Herodek,

1995).

In the present study, we examined sediment

resuspension in Lake Balaton by high frequency

recording of turbidity during two vegetation periods.

Using additional information about phosphorus

desorption capacity of the sediments, we estimated

the internal P load and related internal P supply to the

nutrient status of phytoplankton.

Methods

Sampling site

Lake Balaton is a large (596 km2), shallow

(zmean ¼ 3.2 m), elongated lake that can be divided

into four basins (Fig. 1). The largest tributary, the Zala

River (mean flow at the mouth section is 7.8 m)3 s)1)

enters the smallest (38 km2) and shallowest

(zmean ¼ 2.3 m) Basin 1. The only outflow connects

Basin 4 with the Danube. Basin 1 has become

hypertrophic during the 1970s (Herodek, 1986) and

recovered surprisingly quickly after a series of man-

agement measures taken from the mid-1980s (Istva-

novics & Somlyody, 2001).

We built a permanent sampling station 100 m

offshore at the outskirts of the town of Keszthely

(Fig. 1). The water depth was about 1.5 m in early

Dynamics of internal P load 233

� 2004 Blackwell Publishing Ltd, Freshwater Biology, 49, 232–252

spring and decreased to about 1.2 m during the

summer in both 2001 and 2002. The station is situated

in the area where fine-grained sediments with the

highest concentration of P and cyanobacterial spores

tend to accumulate (Mate, 1987; Gorzo, 1991). Occa-

sional comparisons indicated that the chemical

composition of water (chlorophyll, suspended solids,

pH, forms of P, vertical light attenuation), as well as

species composition of phytoplankton (J. Padisak,

personal communication), were similar at our near-

shore station and at various open water locations of

Basin 1.

Turbidity and meteorological records

In mid-July 2001, three light scattering sensors

(Wetlabs, Philomath, OR, U.S.A.) were installed at

the sampling station. Two of the sensors were fixed

0.7 m above the sediment. A third, less sensitive

sensor was inserted at 0.2 m above the sediment

surface. The average of six turbidity measurements

was recorded each hour until mid-October. In 2002,

turbidity was followed between the end of March and

October. Measurements were interrupted from time

to time by electrical problems caused by other

Fig. 1 Lake Balaton and its catchment area. The sampling station is shown in the insertion.

234 V. Istvanovics et al.


instruments operated at the station. Until mid-April,

sensors were installed at about 0.1, 0.7 and 1 m above

the sediment. Thereafter five sensors were applied at

about 0.05, 0.1, 0.4, 0.7 and 0.9 m above the sediment.

The least sensitive sensor was always the one closest

to the sediment surface. Turbidity was recorded every

15 min. The sensors were cleaned with ethanol every

3–4 days, a procedure that prevented biofilm forma-

tion on the measuring surface. When the measuring

window of a sensor was found to be overgrown, data

from between two cleanings were excluded.

The maximum output signal of each sensor was

5 V. This corresponded to a maximum turbidity of

150–170 NTU in the case of the sensitive sensors and

about 500 NTU in the case of the low sensitivity

sensor. Sensors were calibrated against the concen-

tration of suspended particulate matter (SPM) and the

coefficient of downward vertical light attenuation

(Kd, m)1) in the laboratory (see below).

Turbidity data were collected using a Keithley

(USA) multiplexer programmed by TestPoint (Capital

Equipment Corporation, Billerica, MA, U.S.A.). From

September 2002, the system could also be checked and

controlled remotely.

In order to calibrate light scattering sensors, 20

sediment cores were taken in a 100 m2 area around

the sampling station. The upper 1 cm layer of the

cores was pooled into a single sample. During the first

calibration trial, small amounts of known weight of

sediment were added incrementally to 5.3 L of What-

man GF/F filtered lake water in a large, black

container. The suspension was continuously stirred

with a magnetic stirrer. The water content of the

original sediment was known, and the concentration

of SPM calculated in the suspension. Light scattering

was measured after each addition at a low surface

light intensity. Photosynthetically active radiation

(PAR, lmol quanta m)2 s)1) was detected with a 4punderwater quantum sensor (Li-Cor, Lincoln, NE,

U.S.A.) at two fixed depths. During the latter meas-

urement, incident surface radiation was 180 lmol -

quanta m)2 s)1. During the second calibration trial,

the slowly sedimenting fraction of the sediment was

collected by suspending about 100 mL of the bulk

sediment in 500 mL Whatman GF/F filtered lake

water in a measuring cylinder. Water was siphoned

off carefully from above the settled material after

10 min. The sedimented material was resuspended

again in filtered water and this procedure was

repeated until an insignificant amount of sediment

remained in suspension. To concentrate slowly sedi-

menting fraction (vsed < 12–14 m day)1), water with

the resuspended matter from all successive resuspen-

sions was centrifuged at 1500 g for 20 min. Calibra-

tion was performed as during the first trial, except

that aliquots of the concentrated fine sediment with

known content of dry matter were pipetted into the

measuring vessel.

Continuous turbidity records were converted into

SPM concentration using the regression obtained

during the second calibration trial. The SPM time

series was smoothed by calculating time weighted 7

point moving averages. Weights were taken from the

autocorrelation function of SPM (see Results), and the

SPM content of the water column per unit surface area

was computed. Because of the lack of appropriate

information, we neglected changes in water depth.

Resuspension was estimated as an increase in the SPM

content of the water column between two subsequent

records. Daily rates of resuspension (Rd, g m)2 day)1)

were summed from the individual resuspension

events. The P desorption from resuspended sediments

was calculated using the assumptions that (i) rate of P

desorption from the resuspended sediments was a

linear function of the sum of the NH4Cl-RP and NaOH-

RP fractions of SPM and (ii) time-averaged concentra-

tion of these fractions corresponded to a desorption of

5 lg P g)1 dry mass (cf. Lijklema et al., 1986). The

fractional composition of P in SPM was determined

according to Hieltjes & Lijklema (1980; see below).

A small meteorological station was set up at the

sampling station by the HWI Elektronika Company,

Hungary. Global radiation, wind speed, wind direc-

tion and water temperature at three to five depths

were measured every 10 s. Mean values were saved at

15-min intervals on a data logger.

Manual sampling programme

Manual sampling started in mid-March and contin-

ued until mid-October.

Photosynthetically active radiation was measured

weekly with a 4p underwater quantum sensor in the

air and at 10 cm depth intervals in the water between

07:00 and 08:00 hours. Simultaneously, water samples

were taken from the middle of the water column with

a Meyer flask. Processing of samples started immedi-

ately after transportation to the shoreline laboratory of



the West Transdanubian Water Authority (less than

half an hour). Unless indicated otherwise, Whatman

GF/F glass fibre filters (Whatman, Maidstone, U.K.)

were used for filtration.

The concentration of SPM was determined gravimet-

rically after filtering an appropriate volume of water

(200–800 mL) onto dried, preweighed filters (105 �C,

24 h). Organic carbon content of SPM was approxima-

ted by measuring total C content after fuming the filters

with concentrated HCl for 10 min. The C concentration

was determined with a CHN autoanalyser.

Suspended particulate matter was concentrated for

sequential extraction of P from 800 to 2000 mL of lake

water by centrifugation at 1500 g for 20 min. Loss of

SPM (usually <5%) was determined gravimetrically

by filtering the supernatant onto preweighed filters.

Visual inspection showed that the loss could primar-

ily be attributed to poorly sedimenting cyanobacteria.

Fractional distribution of P was determined in

duplicates of the concentrated SPM sample according

to the sequential extraction scheme of Hieltjes &

Lijklema (1980). Total P content of the NaOH extract

was measured according to the method of Menzel &

Corwin (1965). When calculating the P concentration

of various fractions (lg P g)1 dry mass), we corrected

for the loss of SPM during centrifugation. Fractional

composition of P was determined in triplicates from

the uppermost 1 cm sediment layer collected monthly

in the same manner as described above. In 2002, the

fine-grained, slowly sedimenting fraction of this layer

was also extracted sequentially.

The concentration of chlorophyll a was measured

spectrophotometrically after extracting the filters in

90% cold acetone in the dark and correcting for

phaeopigments by acidification (Lorenzen, 1967).

Forms of P were measured in triplicate. Concentration

of SRP was determined according to Murphy & Riley

(1962) using 10 cm vials. Total P and total dissolved P

were analysed according to Menzel & Corwin (1965).

Surplus P (SP) content of the phytoplankton was

estimated by filtering 10–30 mL of water onto pre-

washed cellulose acetate membrane filters of 0.2 lm

pore diameter (Whatman) and extracting the filters in

boiling distilled water for 1 h (Fitzgerald & Nelsson,

1966). Concentrations of ammonium, nitrite and

nitrate were measured according to standard methods

(FBA, 1978). Total N and total dissolved N were

determined in triplicate by second order derivative

spectroscopy after oxidation to nitrate with alkaline

persulfate solution (Crumpton, Isenhart & Mitchell,

1992). Concentrations of total inorganic carbon, Ca2+

and Mg2+ were obtained by titration.

Light dependence of steady-state photosynthesis (PI

curve) was determined biweekly using the 14C method

of Lewis & Smith (1983). Two MBq of NaH14CO3

(Amersham, Little Chalfont, U.K.) were added to

300 mL of lake water at a PAR of about 0.2 lmol

quanta m)2 s)1. The sample was distributed in 5 mL

portions into 43 original glass scintillation vials

(Wheaton, Millville, NJ, U.S.A.), and these were

incubated for 30 min at photon flux density between

0.6 and 700 lmol quanta m)2 s)1 in a photosynthetron

that was thermostated at the ambient water tempera-

ture (±1.5 �C). Five additional background samples

were prepared by immediately poisoning 5 mL aliqu-

ots with 0.1 mL of buffered formalin. Photosynthesis

was stopped by poisoning the samples. Unused 14C

was removed by vigorously shaking the samples for

2 h with 0.25 mL of 6 NN HCl. Following neutralisation,

10 mL of Optiphase Highsafe scintillator (Wallac,

Boston, MA, U.S.A.) was added. Five total count

samples were prepared by adding 0.1 mL of labelled

water to a mixture of 5 mL unlabelled lake water and

10 mL scintillator that contained 0.2 mL of phenetil-

amin. Radioactivity was detected in a RackBetaII

liquid scintillation counter for 10 min. Efficiency of

counting was determined by the channel ratio method.

Parameters of the PI curves were obtained by non-

linear fitting of the Jassby & Platt (1976) model to the

data:

PBðIÞ ¼ PBmax � tanh aB � I=PB

max

� �� RB

where PB and PBmax [lg C (lg chl a))1 h)1] are the

actual and the maximum rates of biomass-specific

photosynthesis, respectively; I (lmol quanta m)2 s)1)

is the photon flux density; aB [lg C (lg chl a))1 h)1

(lmol quanta m)2 s)1))1] is the slope of the initial

linear portion of the PI curve and RB [lg C (lg chl

a))1 h)1] is the biomass specific rate of dark respir-

ation assumed to be independent of light conditions.

In order to estimate area-specific net primary

production, turbidity was converted into Kd based

on the regression obtained during the second calibra-

tion trial. Incident PAR was calculated as 47% of

global radiation with the assumption that 1 mol

photon in the visible range was equivalent to 218 kJ

(Reynolds, 1997). From the ratio of PAR measured in



the air and immediately below the water surface, we

assumed a mean albedo of 25% that could be due to

wave action. Photon flux density was calculated every

15 min at depth intervals of 10 cm. PBmax and concen-

tration of chlorophyll a were changed half way

between two measurements, whereas aB was estima-

ted from the regression obtained between this value

and Kd. Respiration was assumed to equal 2% of

biomass specific maximal photosynthesis (cf. Falkow-

ski & Raven, 1997; Reynolds, 1997).

Daily external load data from the Zala River were

obtained from the West Transdanubian Water

Authority. This component represents 90–95% of

total nutrient load to Basin 1 of Lake Balaton (Istva-

novics, Somlyody & Clement, 2002).

Results

Sediment resuspension and internal P load

The relationship between concentration of SPM and

turbidity was linear during the first calibration trial

(Fig. 2a). The regression line, however, was unreal-

istically steep [1.86 g m)3 (NTU))1] and resulted in a

significant overestimation of the SPM concentration

measured weekly during our manual samplings. We

suspected that the discrepancy could be caused by a

large difference in grain size distribution of the bulk

sediment used for calibrating the sensors and that of

the naturally resuspended particles. Therefore we

separated the fine-grained fraction of the uppermost

sediment layer during the second trial. This trial

resulted in a nearly 1 : 1 relationship between

turbidity and SPM concentration (Fig. 2a). A com-

parison with field data indicated that conversion

based on this second regression line was satisfactory

(Fig. 2a).

Temperature profiles (not shown) indicated that the

water column was homogeneously mixed. The verti-

cal distribution of SPM was also nearly uniform in the

upper 1 m of the water (Fig. 3a). Elevated concentra-

tions of SPM were recorded at 0.2 m above the

sediment surface or closer. This difference tended to

increase with increasing SPM concentration (Fig. 3b).

The concentration of SPM was higher in the near-

bottom water than in the upper water column by a

mean factor of 1.9 ± 1.1 with the exception of a 2-week

period starting in late September 2002 when unusu-

ally calm weather resulted in an up to 20-fold lower

SPM concentration in the water column relative to the

near-bottom layer (Fig. 3b).

An autocorrelation analysis was performed on the

SPM time series obtained in 2002. The autocorrelation

function indicated that a recording frequency of

60 min was suitable to properly determine concentra-

tion of SPM (Fig. 4a). This was the recording fre-

quency adopted in 2001. Derivative of the SPM

concentration was noisy due to both stochasticity of

resuspension/sedimentation and small measuring

errors in turbidity. Therefore we smoothed the time

Fig. 2 (a) Calibration of light scattering sensors (turbidity)

against concentration of suspended particulate matter (SPM)

and (b) coefficient of vertical light attenuation (Kd) [regressions

are as follows: Trial 1: (SPM) ¼ 1.86NTU-5.61, r2 ¼ 1.00, n ¼ 244

and Kd ¼ 0.091NTU-0.785, r2 ¼ 1.00 n ¼ 39 for NTU £ 100. Trial

2: (SPM) ¼ 1.14NTU-6.20, r2 ¼ 1.00, n ¼ 44 and Kd ¼0.085NTU-0.078, r2 ¼ 1.00 n ¼ 44. Field data: (SPM) ¼1.12NTU-1.42, r2 ¼ 0.81, n ¼ 93 and Kd ¼ 0.078NTU + 0.612,

r2 ¼ 0.89 n ¼ 92].



series of SPM concentration by calculating time-

weighted 7 point moving averages.

To test the influence of sampling frequency on Rd, we

also derived time-weighted 5 point moving averages of

SPM concentration from the original data set by

assuming a decreasing frequency of recording up to

75 min. Estimated Rd decreased exponentially with the

decreasing frequency (Fig. 4b). A 60-min frequency of

recording resulted in a 46% lower value than 15-min

recording. Extrapolation of the regression between

sampling frequency and Rd indicated that a 1-min

recording could result in a 16% higher value than a

frequency of 15 min. Consequently, the uncertainty

because of unaccounted short-term and seasonal chan-

ges in water depth may be more substantial than the

error caused by ‘low’ frequency of recording in 2002.

In 2002, daily resuspension averaged 64 g m)2

(Table 1). The maximum exceeded the mean Rd by a

Fig. 3 (a) Concentrations of suspended particulate matter (SPM) at various depths above the sediment surface and (b) the ratio of

SPM in the near-bottom layer (SPMB) and in the upper water column (SPMW).



factor of 6. In 2001, resuspension averaged

31 g m)2 day)1. Statistical analysis suggested that

the difference in frequency of recording turbidity (cf.

Fig. 4b) was responsible for the lower Rd in 2001

compared with 2002. We present internal load data

that have been corrected for this underestimation of

resuspension during 2001.

To account for changes in P desorption capacity of

resuspended sediments, we followed the variability in

fractional composition of P in SPM. Time-averaged

data indicated that resuspension of the fine-grained

sediment fraction selectively transported particles into

the water that were richer in potentially mobile

inorganic P (NH4Cl-RP and NaOH-RP) than the

uppermost 1 cm layer of the sediments (Fig. 5a). The

SPM was further enriched in mobile inorganic P over

the fine-grained fraction. This suggested that mobili-

sation of P took place after resuspension. NaOH-

extractable organic P (NaOH-nRP) content of SPM

exceeded that of the sediment by a mean factor of 3.

The mobile inorganic P content of SPM increased

significantly during the summer blooms of phyto-

plankton (Fig. 5b). No such seasonal variability could

be observed in the sediments (not shown). This was

another indication of P mobilisation following resus-

pension. As organic P (NaOH-nRP) is not directly

available for assimilation by algae, we assumed that

the P desorption capacity of the resuspended

sediment was proportional to the pool of potentially

mobile inorganic P in SPM.

The mean concentration of potentially mobile inor-

ganic P was 283 lg P g)1 dry mass. We assumed a

mean P desorption capacity of 5 lg P g)1 that repre-

sented 1.8% of the mobile P pool. The minimum P

desorption was 2 lg P g)1, whereas the maximum

reached 11 lg P g)1.

The internal P load averaged 0.28 ± 0.31 mg P

m)2 day)1 in 2001 and 0.39 ± 0.43 mg P m)2 day)1

in 2002 (Table 1). Considering the whole period of

study, the mean external load of SRP slightly excee-

ded the internal load in 2002. In 2001, the internal P

load reached only half of the external load of SRP

(Table 1).

The timing of external and internal loads differed

significantly (Fig. 6). While the former showed relatively

Fig. 4 (a) Autocorrelation function of concentration of suspen-

ded particulate matter and (b) the influence of frequency of

recording on the daily rate of resuspension (Rd) (in b R15d is the

rate estimated from 15-min recording and Rfid is that at

decreasing frequencies. The regression line is y ¼ 1.18 · 10)0.33x,

r2 ¼ 0.99).

Table 1 Mean rate of resuspension (Rd) and P load (both values were corrected for the low frequency of recording in 2001)

Period

Rd

(g m)2 day)1)

P load (mg P m)2 day)1)

External

Total P Soluble reactive P Internal

17 July–31 August 2001 59 ± 46 0.79 ± 0.50 0.63 ± 0.41 0.24 ± 0.21

1 September–9 October 2001 65 ± 84 0.44 ± 0.13 0.36 ± 0.12 0.32 ± 0.40

17 July–9 October 2001 62 ± 66 0.63 ± 0.41 0.51 ± 0.34 0.28 ± 0.31

27 March–30 June 2002 57 ± 67 1.06 ± 0.60 0.70 ± 0.48 0.41 ± 0.43

1 July–31 August 2002 73 ± 57 0.40 ± 0.10 0.29 ± 0.08 0.44 ± 0.51

1 September–21 October 2002 61 ± 58 0.29 ± 0.11 0.15 ± 0.05 0.29 ± 0.31

27 March–21 October 2002 64 ± 61 0.68 ± 0.55 0.45 ± 0.41 0.39 ± 0.43



smooth seasonal changes, sharp spikes at irregular

intervals were characteristic of the latter (Fig. 6). As a

consequence of the contrasting temporal distributions,

the internal P load supplied as much or more P to algae

as external load during the critical summer months and

thereafter during 2002 (Table 1, Fig. 6a).

Our internal load estimates were nearly continuous

from 15 May to 21 October 2002. Twelve days were

excluded because 15-min turbidity recording was

interrupted by technical problems. During this

5-month period, seven pulses of desorptive internal

P load could be identified that exceeded mean load by

more than a factor of 2 (Fig. 6b). In addition to this, a

large pulse of internal P load could be estimated from

the sudden increase in the concentration of SRP

during early August that was not related to resus-

pension (see later). Of these eight pulses, six were

followed by a net increase in algal biomass within a

few days. The mean external load of SRP was equal to

that of internal P load during the selected time period,

but only two small peaks were observed in June

(Fig. 6b). As to the contrasting influence of resuspen-

sion on algal growth, the second bloom of phyto-

plankton was particularly interesting. The large peaks

Fig. 5 (a) Time-averaged fractional distribution of P according to the method of Hieltjes & Lijklema (1980) and (b) variation of the

potentially mobile inorganic P content (NH4 Cl-RP and NaOH-RP) of suspended particulate matter.



in desorptive internal P load were associated with an

exceptionally big resuspension event (Rd ¼ 150 to

230 g dry mass m)2 day)1) during late August that

increased the coefficient of vertical light attenuation

up to 15 m)1. In spite of the extremely poor light

availability, improved P supply resulted within a few

days in a persistent increase in algal biomass (Fig. 6b).

Resuspension stimulated the growth of N2 fixing

cyanobacteria (V. Istvanovics, A. Osztoics & M. Honti,

unpublished data).

Internal P load related to net primary production

and nutrient status of algae

As supported by field data, turbidity could satisfacto-

rily be converted to Kd using the regression obtained

Fig. 6 (a) Internal P load estimated from desorption and external P load of soluble reactive P. (b) Internal and external P loads

over a background level of 0.7 mg P m)2 day)1 (this background corresponded to the mean P load within the examined period times 2.

The arrow indicates the conservatively estimated rate of redox-related internal P load).



during our calibration trials in the laboratory (Fig. 2b).

Unlike the relationship between turbidity and SPM,

the relationship between turbidity and Kd was not

sensitive to grain size distribution. A systematic error

of unknown origin, however, emerged during our

first calibration trial over NTU values of about 100.

The vertical light attenuation coefficient measured

manually or estimated from turbidity averaged about

4 m)1 in both 2001 and 2002. Maxima during large

storms exceeded 20 m)1 (not shown). Characteristic

seasonal changes were observed in the mean under-

water photon flux density experienced by the phyto-

plankton in the fully mixed water column (Fig. 7b).

Before early July and after late September light

availability was typically much higher than during

the summer months.

Fig. 7 (a) Seasonal variation in aB and mean vertical light attenuation coefficient during a 3-day period before measuring photo-

synthesis (Kd). (b) Net primary production and mean photosynthetically active radiation (PAR) in the water column (weighted 5 point

moving average with weights taken from the autocorrelation function is presented for the latter).



Maximum rate of biomass specific photosynthesis

varied between 4 and 9 mg C (mg chl a))1 h)1 with

peak values during the summer (not shown). aB

ranged from 0.03 to 0.13 lg C (lg chl))1 h)1 (lmol -

quanta m)2 s)1))1. Changes in aB were best correlated

with the mean of vertical light attenuation coefficients

during a 3-day period preceding the measurement of

photosynthesis (Fig. 7a; r2 ¼ 0.59, n ¼ 19). Net pri-

mary production was estimated as 103 and 91 g C m)2

in the period 17 July to 9 October in 2001 and 2002,

respectively. During the period 15 May and 21 October

2002, net primary production reached 174 g C m)2.

We recalculated the concentration of chlorophyll a

from net primary production under the assumptions

that (i) the C : chl a ratio was 50 and (ii) 80% of net

production was used to compensate for various

losses. In spite of the manifold insufficiencies of

these assumptions as well as those behind estima-

ting net production, the simple calculation reason-

ably reflected seasonal trends in measured

concentration of chlorophyll a (Fig. 8a). A major

discrepancy was found during the July bloom in

2002 that might be caused by neglecting photoinhi-

bition.

Fig. 8 (a) Recalculation of concentration of chlorophyll a from net primary production (for details, see text). (b) The ratio of net

primary production and the sum of external and desorptive internal P loads (weighted 5 point moving average with weights taken

from the autocorrelation function is presented).



The C : P ratios were derived from net primary

production and P load, the latter being the sum of

desorptive internal loads of P and external loads of

SRP (Fig. 8b). Before the exponential growth phase of

the summer phytoplankton, relatively low C : P ratios

prevailed (Fig. 8b). A substantial increase in the C : P

ratios was indicative of an increasing P deficiency that

eventually led to the collapse of the blooms in both

2001 and July 2002. The bloom in late August 2002

seemed to be less P deficient than the previous ones.

Transient decreases in the C : P ratios corresponded

to pulses of internal P supply (Figs 5b and 8). A

sudden cooling by 6 �C in 3 days (not shown) could

be the prime reason for the collapse of the August

bloom.

Relatively high external P loads during the

spring of 2001 manifested in concentrations of SRP

that exceeded the average concentration (about

3 mg P m)3) by a factor of 3 (Fig. 9). A sharp increase

in SRP was also observed in early August 2002.

This latter peak was neither related to external P load

nor to enhanced P desorption following resuspen-

sion, but coincided with the absolute maximum of

mobile inorganic P content of SPM (cf. Fig. 5b). The

loosely adsorbed NH4Cl-RP fraction was primarily

responsible for the maximum. Apart from these

periods, concentrations of SRP below 5 mg P m)3

suggested the possibility of P deficiency (cf. Reynolds,

1992).

The ratio of dissolved inorganic N (DIN) to SRP

fluctuated widely between 7 and 300 by weight,

primarily because of sharp fluctuations of DIN (not

shown). Ratios approaching the Redfield ratio of 7

and probably indicative of N deficiency in Lake

Balaton were characteristic in May and early June

2001. The DIN to SRP ratios averaged about 23 during

the exponential growth phase of each summer bloom.

This again suggested that P might be the nutrient that

most often limited growth of phytoplankton during

the summer.

To test which of the particulate nutrient ratios could

be best used to assess nutrient status of algae in

shallow Lake Balaton, we performed regression ana-

lyses between concentrations of various particulate

nutrients and that of either SPM or chlorophyll. We

included into the analysis results of weekly measure-

ments during 2000 that were carried out at our

sampling station according to the same protocol as

in 2001 and 2002. Particulate P was strongly related to

SPM (Table 2) and so C : P and N : P ratios were

rejected as poor indicators. Concentration of particu-

late N covaried with the biomass of algae and it was

relatively insensitive to sediment resuspension.

Although the concentration of SP was weakly and

non-linearly related to chlorophyll, SP was the par-

ticulate nutrient that was the least influenced by the

concentration of SPM (Table 2). Taken overall,

chlorophyll specific SP content of algae (SPB) and

Fig. 9 Comparison of the external and internal P loads with concentration of soluble reactive P (SRP) (SRP below 5 mg P m)3 is

potentially indicative of P deficiency, Reynolds, 1992).



particulate N to SP ratios seemed to be the most

reliable indicators of nutrient status.

As with dissolved nutrients, high values of SPB

and low PN : SP ratios suggested that P supply was

sufficient during the spring (Fig. 10). An abrupt

switch from N to P deficiency occurred in mid-July

2001 and early June 2002, coinciding with the drop in

external P load. In contrast to the C : P ratios (Fig. 8),

Fig. 10 (a) Comparison of the external and internal P loads with biomass specific surplus P content of algae (SPB), and (b) with the

ratio of particulate N and surplus P (PN : SP) [we consider that SPB around 0.5 lg P (lg chl a))1 and PN : SP around 30 lg N (lg P))1

are potentially indicative of P deficiency].

Table 2 Results of regression analysis between concentration of various particulate nutrients and that of either SPM or chlorophyll a

Nutrient SPM (g m)3) Chlorophyll a (mg m)3)

P (mg P m)3) [PP] ¼ 0.82[SPM] + 24.9, r2 ¼ 0.82, n ¼ 100 [PP] ¼ 1.98[chl a] + 25.3, r2 ¼ 0.56, n ¼ 100

Organic C (g C m)3) [POC] ¼ 39.5[SPM] + 1.4, r2 ¼ 0.66, n ¼ 97 [POC] ¼ 98.3[chl a] + 1.4, r2 ¼ 0.48, n ¼ 97

Surplus P (mg P m)3) [SP] ¼ 0.07[SPM] + 11.6, r2 ¼ 0.22, n ¼ 94 [SP] ¼ 13.9log(chl a) - 1.2, r2 ¼ 0.38, n ¼ 94

N (mg N m)3) [PN] ¼ 3.7[SPM] + 157, r2 ¼ 0.40, n ¼ 97 [PN] ¼ 15.4[chl a] + 50.9, r2 ¼ 0.78, n ¼ 97

Chl a (mg m)3) [chl a] ¼ 0.20[SPM] + 9.2, r2 ¼ 0.36, n ¼ 102 –



neither SPB nor PN : SP ratios revealed difference in P

supply during the three summer blooms. Transient

relaxation of P deficiency due to pulses of internal P

load could not be observed in these indicators, with the

exception of the presumably redox-related enhance-

ment of internal P load during early August, 2002.

Discussion

Resuspension and internal P load

The light scattering sensors turned out to be surpris-

ingly reliable tools for estimating both the concentra-

tion of SPM and vertical light attenuation coefficient

in Lake Balaton. This was not a priori evident, as the

same approach has failed in other lakes. Thus, van

Duin (1992) reported that in polymictic Markemeer,

the Netherlands, turbidity sensors overestimated

concentration of SPM in the field by a factor of 5–50.

At the same time, Markensten & Pierson (2003)

reported that SPM concentrations could reliably be

approximated in shallow Lake Malaren, Sweden,

using transmissometer sensors. Composition and

grain size distribution of sediments are the likely

reason for such intersystem differences. In this

respect, the most influential features of Balaton

sediments are high magnesian calcite content (50–

65% of dry mass), very low organic content (<4%),

most particles are <100 lm and mean particle size is

10–40 lm (Mate, 1987).

Because we recorded turbidity at various depths

but at a single station, the influence of horizontal

transport on changes in SPM remains unknown.

Wind-induced sediment resuspension has been mod-

elled with success in Lake Balaton assuming local

equilibrium with the wind and negligible horizontal

transport (Luettich, Harleman & Somlyody, 1990).

Occasional samplings during variable wind condi-

tions indicated that SPM concentrations were not

significantly different between our permanent sam-

pling station and various locations in Basin 1. Thus,

horizontal transport was not likely to seriously distort

our estimates of resuspension.

The mean rate of resuspension was about

60 g m)2 day)1 in both years after correcting for the

50% underestimation of Rd in 2001 because of low

frequency (60 min) of recording. One must, however,

bear in mind that the water depth at our sampling

station (1.2 m) was only half of the mean depth of

Basin 1 of Lake Balaton (2.3 m). As in our station, a

significant increase in the SPM concentration was

restricted to a near-bottom layer of 0.2 m or so in

deeper areas of Lake Balaton (Somlyody, 1984). Thus,

basin-wide daily resuspension could be about twice as

high as that estimated for our shallow station.

Modelling sediment resuspension in Lake Balaton,

Luettich et al. (1990) found that a few millimetre thick

surface layer of the sediments behaved in a fully non-

cohesive manner. In agreement with this, the rate of

sediment resuspension was much higher in Lake

Balaton than in many other shallow lakes (Aalderink

et al., 1985; Evans, 1994; Kozerski & Kleeberg, 1998).

Luettich et al. (1990) speculated that bioturbation

and wind-induced agitation might be responsible for

the non-cohesivity. Extrapolation of our data (Fig. 3)

to a 2.3 m deep column showed that the mean SPM

content was 116 g dry mass m)2 with maxima

approaching 490 g m)2. On the assumption that water

content of the sediments subject to resuspension was

95%, stirring up of a 2.3 mm thick layer accounted for

the mean SPM content. The maximum corresponded

to 9.5 mm. Extrapolating again to the mean depth of

2.3 m, rate of daily resuspension would also corres-

pond to a 2.3 mm thick layer. Thus, the turnover of

the non-cohesive layer could be estimated as 1 day)1.

There are two obvious consequences of this rapid

turnover. First, it suggests that resuspension itself is a

crucial factor in maintaining non-cohesivity of surface

sediments. Secondly, replenishment of P desorbed

upon resuspension must be rapid provided that P

desorption capacity remains constant. Indeed,

Søndergaard et al. (1992) found that the rate of P

desorption was constant during repeated experimen-

tal resuspension of sediments and concluded that

replenishment of P was fast in Arresø, Denmark. No

comparable evidence is available from Lake Balaton.

However, intense microbial activity at the sediment/

water interface (Zlinszky, 1987) as well as bioturba-

tion and metabolic activity of macrozoobenthos sug-

gest that fast replenishment may also occur in our

lake.

The fine-grained sediment fraction was separated

from the bulk sediment on the basis of its slow

sedimentation. Fractional composition of P was

clearly indicative of selective resuspension of particles

that were enriched in potentially mobile inorganic P

and depleted in less mobile HCl-RP relative to the

bulk sediment (Fig. 5). The concentration of mobile



inorganic P further increased upon resuspension. This

can be explained by the finding that an increase in pH

was the most important factor of P mobilisation in

Lake Balaton (Pettersson & Bostrom, 1986; Istvanov-

ics, 1988).

The central idea of our study was that high

resolution internal load estimates can be derived from

the continuously recorded rate of sediment resuspen-

sion and nutrient desorption capacity of resuspended

sediments. It is likely that desorption of ammonium

occurs upon resuspension (Ogilvie & Mitchell, 1998).

However, as no information was available on ammo-

nium desorption in Lake Balaton, we have not paid

attention to the internal load of N.

We must admit that our assumptions of P

desorption of sediments were based on relatively

weak experimental evidence. These assumptions

were as follows: (i) P desorption was proportionate

to the potentially mobile inorganic P fraction of SPM

and (ii) mean capacity was 5 lg P g)1 dry mass.

Assumption (i) was likely to cause less uncertainty, as

variability in Rd highly exceeded that in mobile

inorganic P content of SPM as indicated by the

coefficients of variation (0.97 and 0.36, respectively).

As a consequence, temporal changes in desorptive P

load were more strongly influenced by the dynamics

of resuspension than by slower, mostly seasonal

changes in mobile inorganic P content of SPM. The

experimental basis for assumption (ii) was far from

strong because of the difficulty in detecting changes

accurately in low SRP concentration during P desorp-

tion experiments that are carried out within the

realistic range of SPM concentrations (Lijklema et al.,

1986). Lijklema et al. (1986) concluded that the P

desorption capacity of sediments varied between

5 and 10 lg P g)1 in Lake Balaton. Our assumptions

resulted in a reasonably similar range (2 and

11 lg P g)1). Nevertheless, the choice of a different

mean value of P desorption capacity would induce a

proportionate change in the absolute value of internal

P load. These uncertainties led us to testing our

estimate against net primary production and nutrient

status of phytoplankton.

Net primary production was a rough approxima-

tion, as several features of dynamic photosynthetic

response of phytoplankton were neglected (cf. Fal-

kowski & Raven, 1997). We only accounted for

photoacclimation by changing the value of aB accord-

ing to the relationship between measured aB and

mean Kd during a 3-day period preceding the meas-

urement of photosynthesis (cf. Kiefer & Mitchell,

1983). In spite of these oversimplifications, the

estimate agreed well with other estimates of primary

production in Basin 1 of Lake Balaton (L. Voros,

personal communication). Moreover, the seasonal

trend of measured concentrations of chlorophyll a

could reasonably be recalculated from our estimate.

Thus, seasonal changes in C : P ratios derived from

net primary production and from the sum of external

and internal P loads are likely to reflect real changes in

P availability. Absolute values of C : P ratios were not

conclusive because of the substantial recycling of P

within the water column (Herodek, 1986; Voros,

V-Balogh & Herodek, 1996).

The comparison of internal P load with indicators

of nutrient status obtained from chemical measure-

ments was no less problematic than the above

approach. In general, dissolved nutrients and their

ratios are inferior indicators of nutrient status of

phytoplankton relative to ratios of particulate nu-

trients. In Lake Balaton, however, the interference of

resuspension cannot be neglected. Regression analy-

sis revealed that only particulate N and SP were

more strongly related to changes in algal bio-

mass than to changes in inorganic SPM (Table 2).

Pettersson (1980) suggested from his measurements

in stratified Lake Erken, Sweden, that SPB values

around 0.2 lg P (lg chl a))1 were indicative of P

deficiency. We suggest that this threshold was about

0.5 lg P (lg chl a))1 in shallow Lake Balaton. Simi-

larly, the PN : SP ratio varied between 10 and

20 lg N (lg P))1 in P sufficient cultures of Cylindro-

spermopsis raciborskii. Increasing P deficiency induced

a tenfold increase in this ratio (V. Istvanovics,

A. Osztoics & M. Honti, unpublished data). This N2

fixing cyanobacterium was dominant during the

bloom in 2001 and co-dominant in 2002 (J. Padisak,

personal communication). In the field, PN : SP ratio

ranged from 10 to 50 lg N (lg P))1. Values over

30 lg N (lg P))1 might be indicative of P deficiency.

As a consequence of all of the above-mentioned

uncertainties, seasonal trends rather than absolute

values of internal P load could be tested against

independent data sets.

Various indicators (Figs 8–10) resulted in a similar

conclusion about seasonal changes in the nutrient

status of phytoplankton. The external load during the

spring was high enough to sustain P sufficiency of the



low algal biomass present, and even to increase the

SRP concentration in 2001. Internal P load together

with the diminished external load during the summer

was too low to maintain P sufficiency. Thus, summer

blooms were P deficient, and the yield was P-deter-

mined sensu Reynolds (1992). The agreement between

C : P ratios and nutrient ratios suggested that our

method of estimating internal P load captured the

seasonal trends correctly.

We must note that within the frame of the present

study, diurnal changes in photosynthetic activity of

various algal groups were followed using a novel

technique, the on-line delayed fluorescence excita-

tion spectroscopy (Gerhardt & Bodemer, 2000). We

plan to test our internal load estimates against this

data set that has the same temporal resolution as

our turbidity time series. However, the results

obtained by DF spectroscopy first require a critical

evaluation.

The internal P load averaged 0.39 mg P m)2 day)1

in 2002 and 0.28 mg P m)2 day)1 in 2001. The ampli-

tude of larger pulses varied between 1 and

2.5 mg P m)2 day)1. Release rates of P reached the

amplitude of these pulses from both batch systems

(Pettersson & Bostrom, 1986) and intact sediment

cores (Istvanovics, 1988). As discussed previously,

basin-wide Rd and in turn, internal P load were about

twice as high as in our shallow station. Nevertheless,

the difference between the rate of desorptive P load

and release rates measured in laboratory remained

surprisingly small. For example, Søndergaard et al.

(1992) estimated an internal P load of 60–

70 mg P m)2 day)1 in Arresø, Denmark from resus-

pension experiments in laboratory. This load was

20–30 times greater than P release from undisturbed

sediment cores. Of resuspended P, 12% was

desorbed. Desorption experiments with sediments of

Lake Balaton showed an order of magnitude lower P

desorption capacity (Lijklema et al., 1986) than sedi-

ments of Arresø. Accordingly, the rate of internal P

load was also very much lower (Table 1, Fig. 6). This

is another indication of the favourable behaviour of

highly calcareous sediments of Lake Balaton that has

been inferred from the evaluation of long-term data

by Istvanovics & Somlyody (2001).

Unlike our estimates of internal P load, area-specific

external P load is independent of water depth.

Consequently, basin-wide mean internal P load might

be similar to the external load of total P during the

study period in 2002, and exceeded the latter by a

factor of 2 during summer (Table 1). Previous experi-

mental and modelling studies (Pettersson & Bostrom,

1986; Somlyody, 1986; Istvanovics, 1988) also conclu-

ded that on an annual basis external and internal

loads were similar. The comparison suggests that

the magnitude of our internal P load estimate was

realistic.

The proportion of external to internal P load may

show considerable interannual fluctuations. Our

study period was too short to assess long-term

variability in internal P load. The external load in

2001 reached only 42% of the mean load during the

period 1993–2002. In 2002, the respective value was

30%. The reason was the extreme deficit in precipi-

tation during the last 3 years, and closure of the weir

at the mouth of the Zala River in the summer. This

intervention resulted in the sudden drop of external P

load in mid-July 2001 and early June 2002 (Figs 6, 9

and 10). Internal load together with the diminished

external load failed to ease the increasing P deficiency

of summer blooms (Figs 9 and 10). Thus, closure of

the weir efficiently reduced carrying capacity of Basin

1. In years of mean or high flow, the relative

importance of internal P load is expected to be lower.

At the same time, evaluation of long-term data

suggested that in years of extreme blooms of C.

raciborskii, internal load exceeded external one during

summers (Istvanovics & Somlyody, 2001; Istvanovics

et al., 2002).

Although P desorption following sediment resus-

pension is the key process of internal P load in

wind-exposed shallow lakes (Lijklema et al., 1986;

Søndergaard et al., 1992), it is not the only component.

During the present study, we also followed nutrient

gradients in the interstitial water by the dialysis

technique. Cylindrical rod samplers separated into

nine chambers of 3 cm length were inserted into the

sediment at monthly intervals at our station. The

upward diffusive flux of P peaked at

0.2 mg P m)2 day)1 in the upper 10 cm sediment

layer during the summer. The simultaneous flux of

iron, however, was high enough to retain P flux from

deeper sediments provided that Fe(III) could precipi-

tate in the oxidised surface layer (cf. Lofgren, 1987;

Lofgren & Bostrom, 1989). The SRP concentration at

1.5 cm below the sediment surface exceeded that at

1.5 cm above the surface by a factor of 2–3. Thus,

neither advection of pore water nor diffusion of P



could, in general, significantly contribute to internal P

load. The first week of August 2002 was an exception.

In that week, SRP concentration suddenly increased in

the water by 8 mg P m)3. Such a phenomenon has

never been reported during summer in Lake Balaton.

We immediately checked and found that SRP showed

a similar peak at various locations of Basin 1. The

most likely explanation was that extreme weather

conditions led to the reduction of sediment surface.

This event was an obviously important source of

internal P load that temporarily eased the P deficiency

of phytoplankton. We certainly underestimated the

rate of internal load during this period because of

both P assimilation by algae and the possibility that

enhanced turbulent diffusion was restricted to a few

days instead of a week.

Implications for phytoplankton ecology

Autocorrelation analysis of SPM time series revealed

that an appropriate characterisation of SPM concen-

tration required daily sampling. The same holds true

for another derivative of turbidity, Kd. Considering

that (i) short-term variability in underwater light

field induces a series of photoacclimation reactions

(Falkowski & Raven, 1997), (ii) variability over

1–2 weeks repeatedly disrupts phytoplankton succes-

sion (Padisak, G.-Toth & Rajczy, 1988; Padisak et al.,

1990) and (iii) seasonal changes represent a major

driving force for succession (Reynolds, 1997), one

must conclude that routine monitoring programmes

yield very little insight into shallow lakes as habitats

for phytoplankton. Moreover, sufficiently precise

estimates of sediment resuspension and the associ-

ated desorptive flux of P require an even higher

frequency of sampling (15 min or higher in Lake

Balaton). This is because the higher the frequency of

recording, the closer we approximate gross resuspen-

sion. Automatic recording of turbidity or other

parameters convertible to SPM and Kd are the only

means that will provide sufficient information about

growth conditions of algae in wind-exposed shallow

lakes.

Apart from hypertrophic lakes where self-shading

of phytoplankton significantly contributes to vertical

light attenuation, maximal light availability is expec-

ted to occur during the summer. In this respect, the

only difference between a stratified deep lake and a

polymictic shallow lake is that frequent resuspension

induces sharp oscillations in light in the latter. In

his lake typology, Reynolds (1997) referred to Lake

Balaton as one of the illustrative examples of

polymictic lakes. However, our data do not support

this expectation. Mean photon flux density in the

water column was lowest, albeit widely fluctuating,

between early July and late September in both 2001

and 2002. This increase in vertical light attenuation

might have two causes. First, calcite precipitation is

very intense in this highly calcareous lake. Biogen-

ically induced carbonate precipitation is proportion-

ate to photosynthesis that peaks in summer.

Concentration of Ca was depleted from about 60 to

30 g m)3 during the summer of both years. Sedi-

mentation of freshly precipitated, small calcite crys-

tals is presumably extremely slow (2 m day)1 or less,

cf. Somlyody & Koncsos, 1991). Secondly, mean

sediment resuspension was somewhat higher during

the summer than during the cold seasons at least

in 2002. Diminished light availability during the

summer must be an important factor in selection

for shade tolerant cyanobacteria in Lake Balaton

(Padisak & Reynolds, 1998), as well as in other

calcareous lakes.

Strong resuspension events induce well defined

restructuring of phytoplankton assemblages in Lake

Balaton (Padisak et al., 1988, 1990; Padisak, 1993): the

immediate effect includes an increase in the biomass

of meroplanktonic species and, above an unspecified

threshold effect of wind, a substantial decrease in the

biomass of filamentous cyanobacteria. Resuspension

is followed by an outburst of small, shade tolerant,

r-selected species, including picoalgae. After a few

days, these are gradually replaced by larger, slowly

growing K-selected species. These patterns can best be

identified when large resuspension events follow each

other in 10–15 days. Our data showed that the

frequency of large resuspension events often matched

this criterion. We also observed that out of eight

pulses of internal P load between May and October

2002, six were followed by a net biomass increase

within a few days. Either the pulses were too low, or

the sampling frequency was too long, the enhanced P

supply failed to induce detectable changes in nutrient

status of phytoplankton. Seasonal changes in C : P

ratios (Fig. 8b), however, suggested that exponential

development of summer blooms was only possible at

the expense of previously stored P. Our findings

together with those of Padisak (1993) and Padisak



et al. (1988, 1990) indicate that pulses of P were large

enough to select for K species with a high P storage

capacity. Thus, sediment resuspension can be per-

ceived not only as a disturbance factor but also as a

factor that periodically relaxes nutrient stress. The

former feature may dominate the instantaneous effect,

whereas the latter may determine the persistent effect

of resuspension on succession of phytoplankton.

Acknowledgments

This study was financially supported by the grant

EVK1-CT-1999-00037 ‘Phytoplankton-om-line’ and by

the Balaton Project of the Office of the Prime Minister.

We are indebted to Dr Laszlo Somlyody, Dr Kurt

Pettersson and two anonymous referees for criti-

cally reading this manuscript. We thank the West-

Transdanubian Water Authority and the Balaton

Limnological Research Institute of the Hungarian

Academy of Sciences for their permission to use their

laboratory facilities. We are indebted to Dr Volkmar

Gerhardt for installing the light scattering sensors at

our station. Mr Norbert Turay and Mr Gabor Poor

provided excellent technical assistance.

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(Manuscript accepted 15 December 2003)



Dynamics and ecological significance of daily internal load of phosphorus in shallow Lake Balaton, Hungary

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