PRIMARY RESEARCH PAPER

Biological availability and humic properties of dissolvedorganic carbon in Lake Balaton (Hungary)

Noemi Toth Æ Lajos Voros Æ Andrea Mozes ÆKatalin V.-Balogh

Received: 14 November 2006 / Revised: 29 March 2007 / Accepted: 11 May 2007 / Published online: 21 July 2007� Springer Science+Business Media B.V. 2007

Abstract The biological availability of dissolved

organic carbon (DOC) was experimentally stud-

ied in water samples collected at the mouth of the

River Zala and in the western and eastern basin

of Lake Balaton (Hungary) in four seasons. The

water samples were filter-sterilized and inocu-

lated with the in situ bacterial population. The

concentration of fulvic and humic acids were

analyzed at the beginning of the experiment. The

bacterioplankton biomass and DOC concentra-

tion were measured on day 0 and 28. The

decrease in the DOC concentration and the ratio

of the bacterial C/initial DOC concentration

showed the microbial utilization of DOC. No

seasonal changes in DOC availability were found

at any of the sampling stations. The DOC

bioavailability was higher in the river than in

the lake water. A strong positive correlation was

found between the bioavailability and the humic

properties of DOC. The observed changes in the

organic matter composition of Lake Balaton

support the view that much of the not readily

utilizable ‘refractory’ DOM resides in the non-

humic pool in standing waters.

Keywords Bacterioplankton � Bacterial C �Biodegradable dissolved organic carbon

(BDOC) � Humic substances (HS)

Introduction

Dissolved organic matter (DOM) is the largest

reservoir of organic matter in most aquatic eco-

systems and the primary substrate fueling bacte-

rioplankton activity (Pomeroy, 1974; Azam et al.,

1983; Wetzel, 1992). There is a continuous supply

of DOM to freshwaters, both from terrestrial input

(allochthonous material) as well as from primary

production (autochthonous material). Bacteria

utilize DOM partly for their energy requirements

and partly for the synthesis of new cell material.

The availability of DOM to heterotrophic bacteria

likely depends on its biochemical composition and

molecular size, inorganic nutrient concentrations

and other environmental factors, such as temper-

ature (Amon & Benner, 1996a).

Allochthonous DOC is to a large extent

composed of coloured humic and fulvic acids

and has therefore been held to be largely

Handling editor: J. Cole

N. Toth � L. Voros � K. V.-Balogh (&)Balaton Limnological Research Institute of theHungarian Academy of Sciences, P.O. Box 35, 8237Tihany, Hungarye-mail: vbalogh@tres.blki.hu

A. MozesFaculty of Science, PhD School, Experimental PlantBiology, Eotvos Lorand University, Budapest,Hungary

123

Hydrobiologia (2007) 592:281–290

DOI 10.1007/s10750-007-0768-5

recalcitrant, whereas autochthonous DOC from

phytoplankton and macrophytes is less coloured

and believed to be more labile (Wetzel, 1983).

Previously, relatively labile autochthonous mate-

rial was thought to be the dominant source of

substrates for bacterial growth (Cole et al., 1982).

More recently, it has become clear that allochth-

onous (or humic) organic matter can also be a

major source of energy and carbon for bacterial

growth in many freshwaters (Tranvik, 1988; Mo-

ran & Hodson, 1990). It has also been suggested

that humic substances are more important com-

ponents of the biodegradable DOM pool than

previously thought (Amon & Benner, 1996b;

Volk et al., 1997; Jansson et al., 2000; Kritzberg

et al., 2006).

In most natural waters a major portion of DOC

is dominated by dissolved humic substances (HS),

which may reach up to 80% of DOC (Thurman,

1985). Aquatic HS are polar, coloured organic

acids derived from soil, terrestrial and aquatic

plants (Schulten, 1995).

According to our earlier results DOC is the

dominant fraction (82–96%) in the total organic

carbon pool of Lake Balaton; the HS made up

75% of DOC at the mouth of the River Zala and

at least 50% in the eastern part of the lake

(V.-Balogh et al., 2003). The aim for this study

was to determine the spatial and temporal

changes of the concentration of the biologically

available dissolved organic carbon (BDOC) in

Lake Balaton and in its major inflow, the River

Zala. The connection between BDOC and the

humic properties of DOC was also considered.

Methods

Study site

Lake Balaton has an elongated shape (length

78 km, average width 7.6 km, surface area

596 km2, average depths 3.2 m). Most of its

tributaries are in the western part of the lake,

the main one being the River Zala (with more

than 50% of the water supply). The River Zala

flows through two reservoirs, one of which, the

lower Kis-Balaton reservoir is located in a former

peat–marsh area. Large amounts of humic

substances (HS) are exported from this area into

the western basin of the lake (V.-Balogh & Voros,

2001). The water mass moves slowly eastward and

leaves the lake through the Sio Canal. The

average water residence time is 4.71 years, in the

lake as a whole, but it is only 0.28 year in the

western basin. The western basin is the most

productive part of the lake, where the maximum

chlorophyll a concentration was 61 lg l–1 in 2003,

while it was only 13 lg l–1 in the less productive

eastern basin (Mozes et al., 2006).

Water samples were collected from three

sampling stations: the mouth of the River Zala

(46�42¢19.3¢¢ N, 17�15¢52.3¢¢ E), the western (Kes-

zthely) basin of Lake Balaton (46�44¢05.8¢¢ N,

17�16¢32.0¢¢ E) and the eastern (Siofok) basin of

Lake Balaton (in Hungary) (46�55¢19.0¢¢ N,

17�55¢53.6¢¢ E) in four seasons: in the spring

(May 13), in the summer (July 21), in the autumn

(October 14) of 2003 and in the winter (January

27) of 2004. The whole period was characterized

by severe drought.

Biological availability experiment

The water samples were taken with a 3 m long

sampling tube providing a vertical sample from

the whole water column. The samples were

transported in the dark in a coolbox to the

laboratory, which took 2 h, and the experiments

were started immediately. The DOC availability

experiment was carried out according to Servais

et al. (1989) and Waiser & Robarts (2000). After

prescreening through a 50 lm plankton mesh to

remove large zooplankters, the water was filtered

through a combusted GF-5 glass fibre filter (nom-

inal pore-size 0.45 lm). Then the water was filter

sterilized through a 0.2 lm Nuclepore polycar-

bonate filter (previously rinsed with copious

amounts of distilled water). 160 ml of inoculated

(ratio of 1:9) water was placed into autoclaved

500 ml Erlenmeyer flasks. The bacterial inoculum

consisted of screened water that had been passed

through a combusted GF/C filter (nominal pore-

size 1.2 lm). Two experimental series with three

parallels were used. Half of the flasks received

sterile additions of 7.1 · 10–5 mol l–1 nitrogen (as

NaNO3) and 3.2 · 10–6 mol l–1 phosphorus (as

KH2PO4) at 3-d intervals to prevent inorganic

282 Hydrobiologia (2007) 592:281–290

123

nutrient limitation. The flasks were well shaken

after inorganic nutrient additions. All flasks were

incubated at 24�C in the dark for 4 weeks and

continuously shaken.

Organic carbon analysis

The humic substances (HS) were isolated by low

pressure chromatographical XAD-method at the

beginning of the experiment (Standard Methods,

1995). Amberlite XAD-7 resin and Pharmacia C

type low pressure chromatography column (10 ml

bed volume) were used. The humic acids were

separated by precipitation with concentrated HCl

(pH of l.0). After precipitating for 24 h, the

samples were centrifuged. The substances in the

solution were fulvic acids and the precipitate

consisted of humic acids.

The total DOC concentration (water was

filtered through 0.45 combusted GF-5 glass fibre

filter) and the concentration of different DOC

fractions, fulvic acids (FA) and non humic

substances (NHS) were measured using an Ele-

mentar High TOC analyser. The DOC concen-

tration of humic acids was calculated:

DOC – (DOCNHS + DOCFA). The total DOC

concentration was also measured at the end of

the 4 week experiment. Acid-washed glassware

was used for organic carbon analyses.

The water colour (Pt unit) (Cuthbert & del

Giorgio, 1992) was also determined at the begin-

ning of the experiment with a Shimadzu UV 160A

spectrophotometer at wavelength 440 nm (Kirk,

1994; Cuthbert & del Giorgio, 1992).

Bacterioplankton measurement

The water samples for the enumeration of bacte-

rioplankton were fixed immediately by adding

glutaraldehyde to a final concentration of 1%(v/

v). Two milliliters of the subsamples were stained

with acridin orange and filtered onto a 0.2 lm

pore size Millipore black polycarbonate filter and

were examined by epifluorescence microscopy

(Hobbie et al., 1977). Bacterial cell volumes were

estimated based on length and width measure-

ments and geometric formulas. For the calcula-

tion of the bacterial carbon production the

biomass (V = total cell volume) was converted

to carbon with a conversion factor of

2.2 · 10–13 g C lm–3 (Bratbak, 1985).

Availability measurement

The biologically available dissolved organic car-

bon (BDOC) concentration was determined in

each of the experiments by measuring the

decrease in the DOC concentration due to the

carbon oxidization caused by bacteria (Servais

et al., 1989; Waiser & Robarts, 2000). Another

method of biological availability was the estima-

tion based on the produced bacterial carbon (BC)

per the initial DOC concentration (Leff & Meyer,

1991).

Chlorophyll a and nutrient concentrations

The chlorophyll a concentration was determined

spectrophotometrically after hot methanol extrac-

tion (Wetzel & Likens, 1991). The PO4-P, NO3-N

and NH4-N values were provided by the Central

Transdanubian Environmental and Water

Authority and the West Transdanubian Environ-

mental and Water Authority.

Statistical method

Bacterial growth and DOC bioavailability data

were compared with ANOVA (OriginPro 7.5

software) to examine the effects of the treatment

and of spatial and seasonal differences. Differ-

ences were considered significant when P < 0.05.

Results

Humic properties of the waters

The total dissolved organic carbon (DOC) con-

centration (Fig. 1) varied between 7.7–

16.5 mg l–1. The DOC concentration of humic

substances ranged from 3.5 to 10.8 mg l–1, which

amounted to 45–66% of the total DOC. The

contribution of the different fractions of humic

substances to the total DOC was the following:

fulvic acids (FA) 3.0–8.7 mg l–1 DOC (31–57%)

and humic acids (HA) 0.02–3.1 mg l–1 DOC (0.2–

19%). The higher DOC values were found at the

Hydrobiologia (2007) 592:281–290 283

123

mouth of the River Zala. The DOC concentra-

tions were similar at the two basins of Lake

Balaton.

The water colour value – mostly due to the

chromophoric dissolved organic matter (CDOM)

– ranged between 71 and 95 mg Pt l–1 at the

mouth of the River Zala (Table 1). Compared to

the river, the water was even lighter in the lake.

The colour values ranged 12–19 mg Pt l–1 in the

western basin and <5–10 mg Pt l–1 in the eastern

one (Table 1).

DOC availability

The concentration of biologically available dis-

solved organic carbon (BDOC), that is the

decrease in DOC concentration, was between

1.43–1.86 mg l–1 at the mouth of the River Zala

(Table 2). The contribution of the BDOC to the

total DOC concentration varied between 9.3 and

14.6%. There were no significant differences

found in the BDOC values either between

seasons or between treatments in the experiments

carried out with the water of River Zala.

The BDOC concentration was between 0.41

and 0.82 mg l–1 in the water of the western basin

(Table 3). The contribution of BDOC to the total

DOC concentration was between 4.2 and 8.7%.

No significant differences were found in the

values of the BDOC concentration between the

treatments – inorganic nutrient addition and

original water – in the experiments carried out

with the water of the western basin.

The BDOC concentration ranged between

0.09 and 0.7 mg l–1 in the water of the eastern

basin (Table 4). These extreme BDOC values

corresponded to the total DOC concentration of

1.2 and 7.9%, respectively. The BDOC values

were significantly (P < 0.05) higher in the sum-

mer than in the spring in the experiments with

the inorganic nutrient treatment. The BDOC

concentration values did not show any significant

seasonal differences otherwise in the experi-

ments carried out with the water of the eastern

basin.

As regards bacterial biomass expressed as

bacterial carbon production, no significant differ-

ences were found between the original (no N + P)

and the inorganic nutrient treated (N + P) exper-

iments either in the case of the water of the River

Zala (Table 2) or of the water of western basin of

the lake (Table 3). However, the increase in

biomass was significantly higher in the inorganic

nutrient treated (N + P) experimental variants

than in the original (no N + P) water from the

eastern basin in spring (P < 0.05) and in autumn

(P < 0.05) (Table 4). The average values in the

differences in the increase of the bacterioplank-

ton biomass showed an increasing tendency

(Fig. 2) as opposed to the decreasing ambient

Table 1 Water colour concentrations at the beginning of the biological availability experiments

Colour (mg Pt l–1)

Spring Summer Autumn Winter

Mouth of the River Zala 81.2 ± 2.5 95.4 ± 1.7 87.9 ± 2.1 71.1 ± 1.2Western basin of Lake Balaton 13.2 ± 0.7 11.5 ± 0.5 12.4 ± 0.4 19.1 ± 0.8Eastern basin of Lake Balaton <5 9.0 ± 0.2 5.0 ± 0.2 9.9 ± 0.3

(Mean of triplicates and SD)

02468

1012141618

pS

ring

Su

em

mr

Au

utn

m

iW

tnre

pS

rin g

Su

em

mr

Au

utn

m

iW

tnre

pS

ring

Su

em

mr

Au

utn

m

iW

t nre

Mouth of the River Zala Western basin Eastern basin

( C

OD

mg

l 1-)

HA FA NHS

Fig. 1 Spatial and seasonal changes of the concentrationof dissolved organic carbon (DOC) fractions (NHS-nonhumic substances, FA-fulvic acids, HA-humic acids)at the beginning of the biological availability experimentscarried out with water samples from the mouth of theRiver Zala and two basins of Lake Balaton

284 Hydrobiologia (2007) 592:281–290

123

phosphorus (PO4-P) and nitrogen (NH4-N) con-

centrations (Fig. 3).

The organic carbon availability, expressed as

bacterial carbon produced (BC) per initial ambi-

ent DOC concentration varied between 1.3 and

5.2 at the mouth of the River Zala (Table 2). The

ratios of BC/DOC ranged from 0.8 to 10.9 in the

water of western basin (Table 3) and 0.11–11.6 in

the water of eastern basin (Table 4). The BC/

DOC ratios were significantly higher in the inor-

ganic nutrient treated (N + P) variants than in the

experiments carried out with the original water

Table 2 Summary of results from 4 week long dissolved organic carbon availability experiments at the mouth of the RiverZala

Season Treatment Decrease inDOC (mg l–1)

AvailableDOC (%)

Increase inbiomass (lg C l–1)

Increase in BC/initialambient DOC (lg mg–1)

Spring No N + P 1.5 ± 0.4 11.2 ± 2.6 42 ± 19.8 3.0 ± 1.4N + P 1.6 ± 0.5 11.2 ± 3.8 27.9 ± 1.1 2.0 ± 0.1

Summer No N + P 1.7 ± 0.4 10.5 ± 2.7 34.0 ± 20.0 2.1 ± 1.21N + P 1.6 ± 0.4 9.5 ± 2.2 25.0 ± 1.1 1.5 ± 0.1

Autumn No N + P 1.4 ± 0.4 9.3 ± 2.4 33.8 ± 9.9 2.2 ± 0.7N + P 1.5 ± 0.5 9.5 ± 3.2 19.2 ± 3.3 1.3 ± 0.2

Winter No N + P 1.5 ± 0.1 11.6 ± 0.5 55.7 ± 8.2 4.4 ± 0.6N + P 1.9 ± 0.3 14.6 ± 2.2 66.1 ± 3.3 5.2 ± 0.3

(Mean of triplicates and SD)

Table 3 Summary of results from 4 week long dissolved organic carbon availability experiments at the western basin ofLake Balaton

Season Treatment Decrease inDOC (mg l–1)

AvailableDOC (%)

Increase inbiomass (lg C l–1)

Increase in BC/initialambient DOC (lg mg–1)

Spring No N + P 0.5 ± 0.1 6.4 ± 1.2 27.9 ± 17.8 3.6 ± 2.3N + P 0.7 ± 0.3 9.4 ± 3.9 43.1 ± 16.5 5.5 ± 2.1

Summer No N + P 0.6 ± 0.3 6.1 ± 3.5 27.8 ± 32.3 3.0 ± 3.5N + P 0.7 ± 0.1 7.1 ± 0.9 7.3 ± 4.5 0.8 ± 0.5

Autumn No N + P 0.8 ± 0.3 8.7 ± 2.8 22.2 ± 10.7 2.4 ± 1.2N + P 0.6 ± 0.1 6.8 ± 0.7 15.3 ± 3.7 1.6 ± 0.4

Winter No N + P 0.4 ± 0.3 4.2 ± 2.9 88.4 ± 11.7 9.1 ± 1.2N + P 0.4 ± 0.1 4.3 ± 1.2 105.4 ± 11.0 10.9 ± 1.1

(Mean of triplicates and SD)

Table 4 Summary of results from 4 week long dissolved organic carbon availability experiments at the eastern basin ofLake Balaton

Season Treatment Decrease inDOC (mg l–1)

AvailableDOC (%)

Increase inbiomass (lg C l–1)

Increase in BC/initialambient DOC (lg mg–1)

Spring No N + P 0.1 ± 0.1 1.2 ± 0.1.3 62.0 ± 16.6 8.1 ± 2.2*N + P 0.3 ± 0.2* 3.2 ± 1.4 85.7 ± 5.3 11.1 ± 0.7*

Summer No N + P 0.5 ± 0.2 5.9 ± 2.2 1.0 ± 0.8 0.1 ± 0.1N + P 0.7 ± 0.2* 7.9 ± 2.4 17.6 ± 12.3 2.0 ± 1.4

Autumn No N + P 0.3 ± 0.2 3.5 ± 2.6 30.2 ± 25.0 3.5 ± 2.9*N + P 0.3 ± 0.2 4.0 ± 1.9 82.9 ± 22.4 9.7 ± 2.6*

Winter No N + P 0.4 ± 0.2 4.3 ± 2.2 71.7 ± 7.6 8.5 ± 0.9N + P 0.3 ± 0.0 3.6 ± 0.3 97.4 ± 57.3 11.6 ± 6.8

(Mean of triplicates and SD; * P < 0.05)

Hydrobiologia (2007) 592:281–290 285

123

(no N + P) in the spring (P < 0.05) and in the

autumn (P < 0.05) in the water of eastern basin.

The relationship between humic properties,

phytoplankton and the biological availability

of DOC

Strong positive correlation was found between

the DOC and BDOC concentrations (P < 0.001)

(Fig. 4A), between the organic carbon concen-

tration of humic substances and the BDOC

concentration (P < 0.001) (Fig. 4B) as well as

between the colour and BDOC concentration

(P < 0.001) (Fig. 4C). As regards the humic

fractions, strong positive correlation was found

between the organic carbon concentration of

fulvic acids and BDOC (R2 = 0.857; P < 0.001),

but no significant relationship was found between

humic acids and BDOC (R2 = 0.292; P > 0.05). In

the case of the nonhumic DOC fraction, the

BDOC concentration increased with the increase

0

10

20

30

40

50

Mouth of theRiver Zala

Western basin Eastern basin

Ba

tcer

ial

ibm o

ass

(µC

gl1-

)

Treatment differences(N+P) - (No N+P)

Fig. 2 Mean (n = 12) differences in the bacterial biomassof the untreated (no N + P) and nutrient (N + P) enrichedexperimental variants at the end of the biological avail-ability experiments carried out with water samples fromthe mouth of the River Zala and two basins of LakeBalaton

0

0.5

1

1.5

2

2.5

3

3.5

Mouth of theRiver Zala

Western basin Eastern basin

Iron

inagc

tun ri

tnes

( L

µ gog

l 1-)

PO4-P NH4-N

Fig. 3 Spatial changes of average (n = 12; ± SD) PO4–Pand NH4–N concentration values at the beginning of thebiological availability experiments carried out with watersamples from the mouth of the River Zala and two basinsof Lake Balaton

y = 0.1691x - 0.9639

R2 = 0.82860.0

0.5

1.0

1.5

2.0

0 5 10 15 20DOC (mg l-1)

vA

aila

ble

( C

OD

mg

l 1-)

A

y = 0.2065x - 0.3972

R2 = 0.87260.0

0.5

1.0

1.5

2.0

0 5 10 15 20

HSC (mg l-1)v

Aai

labl

eC

OD

( m

gl

1-)

B

y = 0.0147x + 0.3313

R2 = 0.8886

0

1

1

2

2

0 20 40 60 80 100

Colour (mg Pt l-1)

vA

aila

ble

CO

D ( m

gl

1-)

C

Fig. 4 Relationship between the dissolved organic carbon(DOC) and biological available dissolved organic carbon(BDOC) (A); between the dissolved organic carbon ofhumic substances (HSC) and biological available dissolvedorganic carbon (BDOC) (B); the colour and biologicalavailable dissolved organic carbon (BDOC) (based on theresults of all experiments carried out with water from themouth of the River Zala, the western basin and the easternbasin of Lake Balaton)

286 Hydrobiologia (2007) 592:281–290

123

in the organic carbon of nonhumic substances, but

this relationship was not strong (R2 = 0.336).

However, the BDOC concentration did not cor-

relate (R2 = 0.0073; P > 0.05) with the phyto-

plankton biomass (chlorophyll a; Table 5).

Discussion

In order to understand the role of dissolved

organic matter (DOM) in aquatic environments,

it is important to consider the bioavailability of

DOC. In general, autochthonous DOC is more

rapidly and more completely utilized by bacteria

than allochthonous DOC (Boyer at al., 2006).

Jugnia et al. (2006) measured the spatial distri-

bution of bacterial abundance and production in a

recently flooded oligo-mesotrophic reservoir (the

Sep Reservoir, Puy-De-Dome, France) in relation

to concentrations of dissolved organic substances.

They concluded that DOM components originat-

ing from sources other than phytoplankton

primary production may be important in sustain-

ing bacterial growth on a short-time scale in a

recently flooded reservoir.

The River Zala, which is the main tributary of

Lake Balaton, flows through reservoirs flooded

13 years ago in a peat-marsh area, which still

produces high DOC concentrations (V.-Balogh &

Voros, 2001). The concentration of DOC is

usually one order of magnitude lower in alpine

lakes (Sommaruga, 2001), than in Lake Balaton

(7.8–9.7 mg l–1), but the observed values are quite

common in shallow lowland lakes. Similar DOC

concentrations (8.6–9.2 mg l–1) were found in

Northeastern Minnesota and in Florida (Brakke

et al., 1988).

The biomass of bacteria increased both in the

river and in the lake water in the course of the

DOC availability experiments. The N and P

enrichment did not influence the growth of

bacteria in the river water, where the inorganic

phosphorus and nitrogen concentrations were the

highest, while in the eastern basin of the lake the

bacterioplankton was limited by inorganic nutri-

ents. This lake area is the least productive part of

the lake, showing a mesotrophic character (Mozes

et al., 2006), and the phytoplankton growth is

limited by inorganic nutrients (Herodek, 1986;

Istvanovics & Herodek, 1995). Our results suggest

that the planktonic bacteria are also limited by

inorganic nutrients and compete with algae for

these resources.

Measurements of bacterioplankton production

in humic lakes resulted in very clear positive

correlation with epilimnetic DOC concentrations

(Jonsson et al., 2001), and a large pool of

dissolved humic substances, typical for many

boreal lakes, could be of great importance as a

bacterial substrate, even utilized with low effi-

ciency (Tranvik & Hofle (1987). The bacterial

growth efficiency (BGE: BDOC/biomass in-

crease) varied between 2 and 25%, which means

that most of the utilised DOC was respired. The

observed BGE values fell in the range of values

(0.15–40%), which are commonly reported for

aquatic ecosystems (Anderson & Turley, 2003;

Eiler et al., 2003; Biddanda et al., 2001). The

inorganic nutrient (N + P) enrichment did not

cause a significant increase of BGE in the

different water bodies.

The biological DOC availability, as quantified

by the yield of bacterial C (lg) per milligram of

DOC present, was 1.3–5.2 for the River Zala

and 0.11–11.6 for Lake Balaton. Leff & Meyer

(1991) measured the changes in the biological

availability of DOC for the native bacterial

assemblages (bacterial C/initial ambient DOC)

along the Ogeechee River in Georgia. The

BDOC values (0.1–1.0) found by Leff & Meyer

(1991) were in the lower values of our range for

River Zala.

Table 5 Chlorophyll aconcentrations at thebeginning of thebiological availabilityexperiments

Chlorophyll a (lg l–1)

Spring Summer Autumn Winter

Mouth of the River Zala 4.5 22.5 1.4 2.7Western basin of Lake Balaton 8.3 13.5 8.5 13.0Eastern basin of Lake Balaton 3.3 2.9 3.9 1.1

Hydrobiologia (2007) 592:281–290 287

123

The biologically available dissolved organic

carbon (BDOC) concentration, as quantified by

the decrease of DOC, was the highest (1.5–

1.9 mg l–1) in the river water, and it became

significantly lower (0.3–0.8 mg l) in the lake.

Waiser & Robarts (2000) have found a similar

BDOC interval, 1.2 mg l–1–1.9 mg l–1, in the

freshwater creeks of the Redberry Lake.

Generally, only a small fraction of the total

DOM is readily available for bacterial uptake,

which, in average is 14% of the carbon in lakes

(Søndergaard & Middelboe, 1995). Our results for

the biologically available carbon in Lake Balaton

were lower; it varied from 4 to 8% for the western

basin and from 1(3%) to 6%(8%) (values in

parentheses represent N + P treatments) for the

eastern basin. The BDOC% ranged from 9.3 to

14.6% for the River Zala. Similar BDOC values

(9 and 14%) were found by Waiser & Robarts

(2000) for creeks in non inorganic nutrient exper-

iments, but the BDOC values were greater in the

case of N + P treatments for all inflows. The

BDOC concentration values did not show any

significant seasonal differences in our water

bodies, which could be explained by the relatively

constant DOC and HS concentrations in the river

and lake water, the drought and the long water

residence time in the lake. Contrary to this Boyer

et al. (2006) found lower BDOC values in the dry

season than in the wet season (5.56% vs. 16.86%)

in shallow oligotrophic waters of Florida Bay. This

may be explained by the distinct chemical char-

acteristics of the DOM produced at different

times of year (Maie at al., 2006).

The available DOC (as quantified by the

decrease of DOC) showed a significant positive

correlation with the concentration of humic sub-

stances (HS) and the colour of the water, but the

non humic substances (NHS) did not show

(R2 = 0.249) a significant correlation with BDOC.

These results suggest that the coloured humic

substances are the major source of biologically

available carbon for bacteria in these waters. The

experiments of Tranvik & Hofle (1987) also

showed that the amount of BDOC was two times

higher in humin rich water than in clear water. In

addition, several studies have suggested that

humic substances are more important compo-

nents of the biodegradable DOM pool than

previously thought (Volk et al., 1997; Jansson

et al., 2000; Kritzberg et al., 2006).

The planktonic algae, which are the main

primary producers in Lake Balaton, showed

significant spatial and temporal differences, but

in our experiment the BDOC seemed to be

independent from the phytoplankton biomass.

This result does not mean, however, that the role

of phytoplankton is negligible concerning organic

carbon production. The lack of an apparent

relationship between phytoplankton biomass

and BDOC can be explained by the very rapid,

immediate uptake of the autochthonous DOM by

bacteria, which is not detectable by the kind of

experiments we have performed in this study.

There was a rapid decrease in the DOC

concentration of the inflowing river water in the

western basin of Lake Balaton. The observed

decrease is primarily attributable to the signifi-

cant loss of humic substances caused by bacteri-

ological and photochemical processes, as it has

been demonstrated by Moran et al. (2000). Wa-

iser & Robarts (2000) showed that lake DOM is

lower in aromaticity, percentage of chromophoric

moieties, fluorescence and molecular weight than

its creek counterpart. The concentration of the

DOC exhibited only minor alterations in the lake

during the long (4 year) residence time, while the

optical properties, composition and reactivity

showed a marked spatial tendency, as it has

previously been demonstrated (V.-Balogh et al.,

2003). The contribution of humic carbon to the

total DOC pool decreased and the colour of the

water disappeared. The inverse relationship

between water colour and retention time has also

been documented for lakes in the Swedish forest

region (Meili, 1992).

The observed stable and relatively high DOC

concentration in the lake is most probably the

result of the photobleaching of the coloured

DOC. There was the water clear in the eastern

basin of Lake Balaton, while only a minor portion

(1–6%) of the DOC was available for the aquatic

bacteria. The intensity of the photochemical

processes decreased due to the low absorption

of the solar radiation as also indicated by the

decrease in the H2O2 production (V.-Balogh

et al., 2006). Experiments with surface-water

DOM in oceans provide evidence that sunlight-

288 Hydrobiologia (2007) 592:281–290

123

induced photo-transformations of natural seawa-

ter DOM can produce DOM that is resistant to

microbial degradation (Benner & Biddanda,

1998). Amon and Benner (1996b) reported that

across a spectrum of environments with DOM of

varying sources and composition, the reactive

pool of high-molecular-weight (HMW) DOC is

typically larger than the reactive pool of low-

molecular-weight (LMW) DOC. During decom-

position, organic matter continuously becomes

less bioreactive and smaller in size (Amon and

Benner, 1996b). The observed changes in the

organic matter composition of Lake Balaton

support the view that much of the not readily

utilizable ‘refractory’ DOM resides in the non-

humic pool in standing waters.

Acknowledgements This work has been financiallysupported by the Balaton Project grant of the Office ofthe Prime Minister (MeH). We wish to thank the CentralTransdanubian Environmental and Water Authority andthe West Transdanubian Environmental and WaterAuthority for the PO4-P, NO3-N and NH4-N values andto Kati Voros for correcting the English text.

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