HYDROBIOLOGICAL BULLETIN 16(I): 21 - 33 (1982)

STUDIES ON DECOMPOSITION OF PHRAGMITES AUSTRALIS LEAVES UNDER LABORATORY CONDITIONS

ELLY P.H. BEST, M. ZIPPIN * AND J.H.A. DASSEN

(Limnological Institute, 'Vijverhof' Laboratory, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands)

INTRODUCTION

The littoral regions of small-size lakes may be very productive due to their occupation by macrophytes, and they may contribute therefore considerably in various ways to the entire lake metabolism. The macrophytes are only to a very limited extent grazed upon, and hence their share in the herbivorous diet is small (WESTLAKE, 1965). The energy fixed in these plants enters the animal food chain mainly through the decomposers and detritivores (e.g. PO LISI N I and BOYD, 1972; JOSSELYN and MATHI ESON, 1980; PIDGEON and CAIRNS, 1981).

Originally, most attention was paid to the primary production potential of littoral zones, in particular the emergent parts of it. However, more recently investigations were directed towards decomposition processes and their significance for the entire lake. Most studies centre on different aspects, where, on the one hand, the changes in the chemical composition of the macrophytes are described and on the other hand, only the effects on the ambient water are analyzed. The main topics are in this respect: oxygen consumption, loss of macro-nutrients, in either particulate or dissolved form, and the food source for detritivorer

In Lake Vechten, The Netherlands, the littoral region covered about one third of the total lake area. The largest, submerged part of this zone has already been studied for several years. Recently the role of the emergent macrophytes for the lake was investigated with respect to their production as well as their decomposition (BEST et al., 1982). In the present paper the changes in the chemical composition of Phragmites australis leaves during decomposition are studied. The experiments are performed under laboratory conditions. The effects of the decomposition processon the ambient water and its microorganisms are quantified and the importance of the sediment in relation to this is illustrated. A conservative estimate is made of the plant's contribution to the lake's DOM pools (C, N and P).

* Present address : State Foundation for Environmental Engineering, Rio de Janeiro, Brazil.

MATERIAL AND METHODS

Sample preparation and incubation. Last years' Phragmitesaustralis leaves were collected in March 1979 at the bottom of

Lake Vechten, freeze-dried, and clipped into small pieces of about 1-4 cm2; 3 g of the plant fragments were enclosed in litter bags (mesh size 1 mm) and placed in perspex vessels. The vessels (volume 890 ml) were filled with (a) water, (b) water and sediment, or (c) water, sediment and plant material (the last one in duplicate). The water and sediment were also collected at Lake Vechten in March 1979. The sediment was sampled from the upper 5 cm of the lake bottom in the littoral zone to ensure participation in the decomposition of the bacterial population which at that time is normally present in the water-sediment transition zone.

All vessels were incubated in growth chambers at a temperature of 15~ with a light intensity of 8.4 W m - 2 (Licor, UW quantum sensor 185 Li) and a photoperiod of 12L: 12D. The incubation conditions were chosen to simulate average summer conditions in the littoral zone of Lake Vechten, where the decomposition of the Phragmites leaves occurs normally.

21

Sampling procedure. After 65, 105 and 147 days of decomposition (DAVIS and VAN DER VALK, 1978) each

time vessels with the contents as mentioned above were sampled. Gas evolution in the vessels was recorded, and for gas analysis a sample was taken through a septum in the wall of the incubator, using a gas-tight syringe. Subsequently the lid was removed. When present the plant material was cautiously removed, and 100 ml medium was sampled (in duplicate) for oxygen determinations. The remaining medium was decanted without disturbance of the sediments. Part of the medium was filtered through 0.33 #m filters (Gelman, Acropore) and the filtrate was used for determination of dissolved nutrients (see water analyses). The particulate material on the filters was investigated with respect to photosynthetic pigments.

Analyses. a. Oxygen regime and gas evolution.

Oxygen was measured in the water by a modification of the Winkler method (GOLTERMAN, 1970). In the case of gas evolution, its composition was analyzed for 02, CO2, N 2 and CH4, as described by BESTetaI . (1977) using gas chromatography (Packard Becker, model 417). b. Bacteria and algae.

Bacteria were counted by means of epifluorescence microscopy, according to Z I MM E R MAN N (1977). At the beginning of the experiment an estimate was made of all bacteria inoculated in the vessels. In order to count the bacteria in the sediment, the freshly collected sediment was rinsed thoroughly with lake water, in the same ratio of sediment to water as used in the experiment. Sediment and water were stirred together on a series of filters with sequential pore sizes of 1, 0.085, 0.044 and 0.015 mm. The resulting suspension was handled like the other water samples. During the decomposition period only the bacteria in the water were counted.

Attention was paid to the composition of the photosynthetic organisms in the water, the algae as well as the bacteria. Therefore the 0.33 #m filters, and their adhering particulate fraction were dissolved in 100 % methanol and extracted for 20 minutes at 60~ The methanolic extracts were centrifuged and the absorption spectra (330 -780 nm) of the supernatants were recorded with a spectrophotometer (Pye Unicam, SP 1800 UV) in a 1 cm celt. The pigments specific to the different algal and bacterial groups were identified, according to HALLEG RAEFF (1976), using paper chromatography (modified according to STEKETEE, 1979). These paper chromatograms were only made when enough particulate material was present. c. Plant material.

The plant material was freeze-dried. It was ground into a fine powder with liquid nitrogen in a IKA mill (Janke and Kunkel). In the dry matter the ash (450~ and the C, N and P concentrations were determined. C and N were analyzed using a HCN analyzer (Perkin Elmer model 240) and P was determined according to a modified molybdate method (GOLTERMAN, 1970). d. Chemical analyses of the water.

In the water phase were determined: total organic carbon, total nitrogen and, after filtration over 0.33/Jm filters, dissolved organic carbon, dissolved nitrogen in the forms of nitrite, nitrate and ammonia, and orthophosphate. Total organic carbon was determined as chemical oxygen demand (GOLTERMAN, 1970). For the determination of total nitrogen the organic components of the water sample were destructed with strong acid (H2SO 4 with selenyloxyde as catalyst, (SCHEINER etal . , 1978) and all ammonia was measured with the indolophenol method (VERDOUW etal. , 1978). Nitrite-N was determined according to FREIER (1964), and likewise, nitrate-N after reduction to nitrite-N with the Cd/Cu method (WOOD et al., 1967). Dissolved organic carbon was measured using a DC-54 Ultra Low Level TOC-analyzer (Dohrmann). In this way the total and purgeable carbon were determined. Orthophosphate was measured colorimetrically with the molybdate antimonium method (GOLTERMAN, 1970). e. Sediment.

No analyses were carried out on the sediment.

R ESU LTS

22

Oxygen regime and gas evolution. The oxygen concentrations in the vessels changed considerably during the incubation period

(Table I). The experimental conditions proved favourable for algal growth. This was reflected in the increase in oxygen concentration in the vessels which were only filled with lake water, despite the use of a much lower light intensity than found outdoors. In the presence of sediment the consumption of oxygen, especially by bacteria, initially prevailed the oxygen production by the algae, but later on oxygen production increased strongly. This tendency was even more distinct in the presence of decomposing plant litter where the leaking of nutrients stimulated bacterial activity greatly, resulting in temporary oxygen depletion.

After 65 days of incubation substantial gas formation occurred in the vessels with water, sediment and plant material. Here, the gas composition, i.e. about 50 % methane, indicated the presence of a large number of methanogenic bacteria. A strong H2S smell and black color of the surface layer of the sediment proved the presence of sulfur bacteria. This indicated the establishment of reducing conditions soon after the aerobic beginning of the experiment. In this vessel a gradual conversion of methane to carbon dioxide occurred with the increase in oxygen concentration during the whole incubation period, i .e. 147 days. After 65 days of incubation only traces of methane were detected in all vessels including the one filled with decomposing plant material.

1.5-

1.O-

0 . 5 -

0

Fig. 1.

I 350 450 550 650 750

WAVELENGTH (nm)

LIGHT ABSORPTION

(relative)

I 8 5 0

Light absorption spectrum of lake water prior to incubation. A volume of 30 I was centrifuged at low speed 115,000 RPM) in a continuous rotor; the resulting suspension was concentrated by a second centrifugation and the pellet was extracted with methanol as described under Methods (according to C.L.M. Steenbergen, unpubl.).

Bacteria and algae. The light absorption spectra of the extracts made of the 0.33 #m particulate fractions

confirmed on the whole the gas analyses. Originally, a population consisting of green algae and diatoms was present in the lake water which was used for incubation (Fig. 1). The green algae predominated. During the incubation period the diatoms increased relatively as was indicated by

23

IT ABSORpTIO

tO

0 5

0 '

t,O

0.S

0 0 4

0

wJ m Jmen

350 450 s s o e,,~,o ~so B ~

WAVELENGZ H , nm l

LIGHT A B~.ORPT ~0~

1 5 -

v 0 -

0 5 -

0

O.S-

0

O~ l -

| Z,%% ~

w ~ E LE~'IGTH L . ~

L incubation content oxygen time vessel (rag 1-1) (days)

0 water 10.23 4- 65 10.75 +_

106 39.74 4- 147 19.73 •

0 water ~- 10.23 4- sediment

65 9.19 4- 106 9.91 4- 147 15.62 4-

0.22 0.71 1.88 0.18

0.13

0.02

0.18

0 water+ 10.23 4- 0.12 sediment

65 4- pl 0.60 4- 0.18 106 0 147 2.88 + ' 0.16

relative gas composition N2:CH4:CO2:O 2

photosynthetic pigments A, Pb, U

(+) (+) (+) (+)

(+)

(+ +)PB (+) (+) (+ +)

(+)

(+) PB ( + + ) U (+ +)PB (+) (+ +)PB (+)

bacteria (numberxlO--lO I--1)

w : w +sed

A 85.6 : 0 : 1 4 . 4 A

80.5:0.010:0.8:18.6 A 76.3:0.004:0.6:23.1 A

A

85.6:0 : 0 : 1 4 . 4 A 87.4:0.070:1.7:11.1 A 76.8:0.002:1.0:22.1 A

A

45.0:50.7 : 4 . 3 : 0 (H2S(+)) A 73.4:0.200:5.9:20.5 72.4:0.020:9.3:18.1

8.41 + 12.25 5.56 •

31.60 _

8.41 4- 23.37 t- 81.51

0.97 • 1.92 4-

0.21

0 1.17

W :

w + secl

0.21 8.44

0.50 1.92

8.41 4- 0.21 22.72 4- 7.50 32.43 14.97 + 4.06 18.24 4- 10.81

Table I. Oxygen concentration, gas composition and development of photosynthetic organisms and bacteria in the water of vessels, filled with lake water only, lake water and sediment, or lake water, sediment and plant material. Average values and S.E. A : algae; Pb : photosynthetic bacteria; U : unidentified photosynthetic bacteria; w : water; sed : sediment; pl : plant material.

an increase in diadinoxanthin and fucoxanthin compared to the pigments specific to green algae, vi~ chlorophyll a and b, lutein, neoxanthin and ~-carotene (see Figs. 3, 4 and Table I I). The total quantity of algae increased greatly, but also death of algae occurred, illustrated by a relative drop in chlorophyll a and b compared to the substantial rise in phaeophytin.

In the presence of sediment a small bloom of photosynthetic bacteria occurred during the first 65 days of incubation (Figs. 2, 3, 4, and Table II). However, these organisms disappeared afterwards.

The decomposing plant material stimulated greatly the development of a diverse population of photosynthetic organisms. During the first 65 days of incubation the bacterial Chromatium sl0p. predominated, followed by Chlorobium spp. in the period up to 106 days. Afterwards, both bacterial groups decreased strongly and were replaced by green algae and diatoms (Fig. 2 and Table II).

24

LIGHT k~ORPTlOI

(~V~ve) iS- rater* ~Im~l*

w l ~ r ~ E . r ~ t

w i l e r

~ 1 ~ r I W~LENGTH 1~1

Fig. 2. Light absorption spectra of the particulate fractions of 200 ml water sampled from vessels filled with lake water, lake water and sediment, or lake water, sediment and plant material. A:65, B: 106; C: 147 days of incubation.

pigment

chlorobactene bact. carotene deriv. bact. carotene bact. chlorophyll a phaeophytin

chtorophyll deriv.

chlorophyll a bact. chlorophyll c neoxanthin chlorophyll c diedinoxanthin fucoxanthin tuteln carotene chlorophyll b ketocarotenoid unidentified

absorption maximum (nm)

350, 470 350, 480, 500, 670 360, 480, 510, 750 360, 580, 776 412, 668

426, 66

430, 665 44O, 665, 78O 442, 470 445 448, 475 449 452, 480 454 454, 650 460 420, 675

Table II.

relative composition (% of pigments in particulate fraction of the water phase)

water

t = 6 6 t : 1 4 7

3.6 24.8 4.9

10.2 (+ 6.0%

46.9 1.2

1.8 2.5 ( 6.0 +)

7.6 14.1 4.5

5.8 28.0 5.0 10.0

12.3 4.0

6.8

water + seal.

t = 6 5 t=147

7.6 7.1 9.0

12.8 (+ 6.0 +)

38.1 36.6

2.8 1.7 ( 6 . 0 + )

10.0 5.9 9.4

7.0 9.0 6.0 7.6 9.0 6.2

1.5 6.7

water + sediment -I- plant material

t = 6 5 t = 1 0 6 t=147

5.7 2.8 13.0 14.4 3.1 35.9 5.6

1.1 3.1 42.5

8.5 (+ 6.2 +) 3.6 17,0

15.1 44.2 2.6 2.9

( 6.2 +) 0.9 5.7 6.3

8.6 1.5 3.8 8.0

5.6 11.3

9.9 4.8 B = 5.5

Pigment diversity, occurring in vessels filled with lake water, lake water and sediment, or lake water, sediment and plant material, incubated for different periods under standard conditions. 4- : no separation of chlorophyll derivatives and chlorophyll c. 5 : unidentified bacterial pigment; t in days.

tJ me (days)

0 65

106 147

ashfree ash C N P C: N C: P {g dr.wt.) (% dr.wt.) (% ash-free {% ash-free (% ash-free (xl03)

dr.wt.) dr,w+.) dr.wt.) /

2.628 12.40 47,96 1.76 0.082 1 30.77 1.54 2.379 6.47 47_40 1.42 0.043 1 39.50 2.82 2.129 12.27 49.56 1.64 0.059 34.42 2.17 1.688 7.02 49.95 1.49 0.049 37.82 2.60

Table III. Changes in organic weight, nutrients and the C:N and P:N atomic ratio of Phragmites leaves during decomposition under standard experimental conditions= Average values of two samples.

25

Fig. 3.

~ 5 7

5

1 ~ CHLOROPHYLL DERIVATIVES

2 rJTr~ BACTERIAL CHLOROPHYLL C

3 ~ BACTERIAL CHLOROPHYLL A

4 I-'-1 CHLOROPHYLL A

5 F==I DERIVATIVES BACTERIAL CAROTENE

6 ~ BACTERIAL CAROTENE

7 ~ CHLOROBACTENE

Diversity of pigments in a population of different algal species, developed in vessels with lake water or lake water and sediment. Incubation occurred under standard experimental conditions for 147 days. Species-specific pigments are: 2 + 3 for Prasinophyceae, Chlorophyceae, Euglenophyta; 2 for Xanthophyceae, Chloromonadophyceae, 4 -I- 6 for Chlorophyta, Euglenophyta; 5 for Diatomeae, Chrysophyceae, Pyrrophyta; 8 for all algae. Chlorophyll derivatives are naturally derived degradation products of chlorophyll, probably coming into existence in the process of ageing. Phaeophytin is chlorophyll without the Mg2+-ion.

Bacterial counts at the beginning of the experiment showed that the upper layer of the sediment contained considerably more bacteria than the overlying water, i.e. 22x1010 and 8x1010 cells 1-1, resp. (Table I). In the vessels filled with water the bacteria only increased in number during the initial period of incubation, After 106 days there seemed to be a strong decrease in their number, which was probably an artifact. By that time the bacteria were mainly attached to the algae which were growing against the walls of the vessels. Although the bacteria-algae complexes were scraped from the walls, it proved impossible to make a homogeneous suspension for preparation of microscopic slides of the bacteria.

26

In the presence of sediment, the bacteria in the water increased dramatically in number. Probably mainly aerobic strains were involved since the oxygen concentration was still high. Subsequently, their number in the.water decreased. This was partly due to colonization of the sediment by anaerobic strains which were not counted.

When decoml~osing plant litter was present, the bacterial number remained initially fairly constant and decreased slightly afterwards. In this case the sediment as well as the plant material in the litterbag was colonized by anaerobic bacteria. Since many bacteria fell out of the plant material during the sampling they were only partly counted in the water.

~ 7

3

1 5

1 ~ CHLOROPHYLL DERIVATIVES

2 [TrT[TJ CHLOROPHYLL B

3 ~ CHLOROPHYLL A

4 ~ NEOXANTHIN

5 ~ DIADINOXANTH IN

6 ~ LUTEIN

7 ~ PHAEOPHYTIN

8 i 8 CAROTENE

Fig. 4. Diversity of pigments in a population consisting mainly of photosynthetic bacteria, developed in vessels with lake water, sediment and Phragmites leaves during an incubation period of 106 days under standard experimental conditions aerobic to anaerobic. Species- specific pigments are: 1 + 7 for Chlorobium; 3 4- 3 for Chromatium; 4 for algae.

27

. L 2, ,

z

I Z

~ Z

0 '~=

z

0 z

m

L

E "~ r , o

e~ ..~

u

r- o

0

+1

O O O

+I +1 §

O O O ~

-H§ ~-H

c~ c~ c~

O

44

O ~ O

O Q O O

-H-H -H +I !

O O ~

-H -H§

dc~o~

+~ ~E

m

O

-H

O ~ O O

-H -H-H 4-I

~

<

E

o .

. ' C I

L. oJ o ~ C v=

.E_ ~:

co r -

~ E

7:o

- ~ 0

~ E

o= >-

.~_ o

z~

28

Rant material. In current research on decomposition processes often degradation rates of plant litter are

expressed in terms of weight loss. Some data support a linear relationship between organic weight loss and time (MASON and BRYANT, 1975). Others, in which the initial rapid leaching period is included, fit better an exponential function with one or more rate constants (JEWE LL, 1971; HODKINSON, 1975; GODSHALK, 1977). According tO MINDERMAN (1968), however, only constants specific to the constituents of litter can be derived, for instance to cellulose, but not to the whole litter. However, the data on weight loss in the present study (Table II1") f i t ted more or less an exponential function with one rate constant, as given by HODKINSON (1975): W t = W o e - k t (in which W t = org.wt, at time t, in g; W o = initial org.wt., in g; t = time in days; k = rate constant); a k value of 0.003 g org.wt.day - 1 was found.

Substantial quantities of C, N and P leaked out of the decomposing plant material directly after immersion (Table III). This was reflected in the decrease in C, N and P concentration in the plant litter and the sudden changes in C: N and C: P ratio's. P was removed to a far greater extent than N, whereas C remained relatively stable in the plant material. After this initial leakage phase a stage was reached in which decomposition, i .e. release of nutrients, occurred more slowly. At 106 days of incubation a temporary increase in N and P had occurred possibly due to bacterial accumulation and excretion. The decrease of organic matter, on the contrary, proceeded more gradually.

Chemical analyses of the water. In all vessels a rise in the organic carbon concentration in the water occurred during the

incubation period (Table IV). Especially the particulate fraction increased, even though the carbon of the lake water at the beginning of the experiment consisted for about 80 % of DOC. The large particulate fraction indicated algal and bacterial growth whether or not either sediment or sediment and plant litter were present. No accumulation of DOC occurred in the vessels without plant material due to DOC utilization by bacteria under continually aerobic conditions. After 106 days of incubation the decrease in total organic carbon in the water is probably caused by attachment of bacteria-algae complexes to the walls of the vessels and, when present, by colonization of the sediment surface. Since the sediment contained a considerable number of bacteria, the carbon concentrations of the water in vessels filled with lake water and sediment were higher than those with water only.

The total nitrogen concentration of the water increased greatly during the incubation period (Table IV). This increase occurred mainly in the (particulate + organic) fraction due to algal and bacterial growth. As was mentioned already, the sediment provided a substantial number of bacteria and thus nitrogen, but the leachate of the decomposing plant litter also enhanced the nitrogen concentration in the overlying water. The dissolved inorganic fraction was always low compared with the total nitrogen concentration in the water, also when the oxygen concentration was still high. Immediately after incubation, the nitrate and the nitrite concentrations dropped, and remained subsequently fairly constant. Ammonia, on the other hand, increased substantially in the beginning and dropped suddenly after 147 dayr Apparently most of the dissolved (inorganic) nitrogen fraction was used by bacteria during the initial stages of decomposition.

Since in the case of phosphorus only ortho-P was measured in the water, not all fractions can be considered in this study. It was probably consumed mainly by the algae for their growth. Due to death of the algal population a large amount of P was released after 106 days of incubation which was used again for growth in the subsequent algal bloom (Tables I, II and II I). The same trend was also detected when sediment was present, but in this case the P released by algal death was associated to a greater extent with sediment and plant material. In the presence of plant litter mos, released P was initially bound by bacteria in the sediment, since it was not recovered in the water nor in the plant material. Afterwards, colonization by bacteria enhanced the P concentration in the plant material between 65 and 106 days of incubation.

29

DISCUSSION

During the early stages of decomposition significant quantities of oxygen were consumed by the bacteria which were originally present in the 5 cm surface layer of the lake bottom. Their metabolic activity as measured by this parameter was stimulated greatly in the presence of the plant litter, Initial oxygen uptake rates of 1.4/Jg 02 g-1 plant litter h -1 (at 15~ in low light intensity) were measured. Under the same experimental conditions 4.4/Jg 02 1-1 day -1 was produced in the water, whereas 18,4/Jg 02 1-1 day -1 was consumed by the sediment portion. The oxygen uptake rate of the plant litter was much lower than reported by ANDERSEN (1978) for Phragmites leaves at 10~ i.e. 200/Lg g-1 h-1; however, the experimental conditions were different.

Initially the bacteria belonged mainly to the methanogenic group. These were succeeded by purple bacteria, diatoms and green algae. Only a few fungal sporeswere present in the water. Concurrent with these changes in the microcosmic community an increase in oxygen concentration and a gradual conversion of methane to carbon dioxide occurred. This might be explained as follows. During aerobic metabolism oxygen serves as the ultimate hydrogen acceptor and is required for stoichiometric conversion of organic matter to carbon dioxide and water. Under anaerobic conditions, for instance in the sediments of a lake, oxidation occurs by utilization of other oxidants namely either inorganic or organic electron acceptors (R I CHARDS, 1965; STUMM and MORGAN, 1970; WHITFI ELD, 1971; RICH and WETZEL, 1978). Products of total decomposition are carbon dioxide, water and various mineral nutrients. In anaerobic decomposition not all organic matter is totally oxidized. Part of the products, volatile (methane and hydrogen) as well as various fatty acids, are formed in measurable quantities, and they are biologically or chemically oxidized if more energetically favourable electron acceptors, i.e. oxygen, become available completing the decomposition process.

The initial weight loss of the plant litter during the first month of decomposition (4.5 %) was much lower than the values reported in literature (MASON and BRYANT, 1975; ANDERSEN, 1978). This might be caused by the absence of macrofauna and wave action in the vessels. Another

factor that might have caused this relatively low weight loss rate is the tight packing of the plant litter, the low surface:volume ratio leaving ample space for bacterial colonization (PI DGEON and CAI RNS, 1977). During the initial decomposition stage P leaked most rapidly out of the plant material (98 % loss), followed by N (23 % loss). C was lost at about the same rate as the total organic matter (10 %). Apparently, the initial most substantial loss of soluble carbohydrates had already occurred in the lake (HUNTER, 1976). After an initial rise, the C: N and C: P ratio's remained fairly constant due to microbial immobilization and, in the case of N, comp!exing with tannic and other phenolic acids (KAUSHIK and HYNES, 1968; SUBERKROPP eta/., 1976). According to SAUNDERS (1976) the more rapid loss of N and P in later stages of decomposition indicates the dominant role played by microorganisms as opposed to that of chemical complexing since microbial activity decreases during later stages of the decomposition process. Microbial activity makes the material palatable for macroconsumers later on.

The decomposing plant litter affected the C, N and P concentrations in the water of the ' vessels considerably in that it enhanced mainly the DOM. Although the DOM fraction is only small compared to the total organic matter pool present in the water of the lake it includes substances such as organic acids and humic substances (also in particulate form) that play an important role in decomposition processes in sediments (DICKINSON and PUGH, 1974; KISTRITZ, 1978). Phenolic and lignin components of the plant material are probably the main precursors of humic substances (CHRISTMAN and OGLESBY, 1971). Humic substances are degraded by populations consisting of different bacterial strains, with species-specific enzymes (DAG LEY, 1975). Not readily degradable compounds must be converted first chemically, retarding the decomposition process (LADD and BUTLER, 1975). During decomposition also substances come into existence that slow down further degradation (COWLI NG and BROWN, 1969). Lignin is degraded only under aerobic conditions (SCHUBERT, 1965).

30

Carbon, nitrogen and phosphorus have several functions in lake metabolism. Depending on pH and redox conditions they can be present in inorganic or organic forms. The ratio between both forms is regulated by microfloral action, bacteria as well as fungi. Low availability in the water might limit productivity.

A preliminary estimate was made of the contribution to the DOM pools of Lake Vechten by decomposition of Phragmites leaves. By personal observation it was known that all shoot material ends up in the lake. Without taking root biomass into consideration, this would mean a compartment representing, respectively, 298 kg C, 6 kg N and 1.4 kg P year -1 or 6.34g C m -2 , 0.13g N m - 2 and 0.03 g P m - 2 year -1. All these nutrients become available with different rates~ The potential input by decay of plant material must be seen in comparison to the lakes' whole dissolved organic matter pools. The average DOM values of Lake Vechten amounted to 29.9 g DOC m -2, 1.9 g DON m - 2 and 0.11 g PO4-P m -2, respectively, (cf. BEST etal., 1968; VERDOUW and DEKKERS, 1982; WlSSELO, 1980). This comparison illustrates the small contribution to the lakes' DOM pools by decomposition of Phragmites litter.

ACKNOWLEDGEMENTS

Mrs. M.Zippin was supported by a post-graduate grant from the Ministry of Education and Science in The Netherlands. Mrs. G.Wiegers carried out the pigment analyses, and gave technical assistance. Mr. E.M.J.Dekkers performed the C and N determinations.

SUMMARY

The decomposition of Phragmites leaves was studied under experimental conditions in vessels during 147 days. This process was compared in vessels filled with (a) lake water, (b) lake water and sediment, and (c) lake water, sediment and plant material.

In the course of time the ash-free dry weight and the N and C concentration in the plant material decreased gradually to, respectively, 64, 54 and 66 % of the initial values. The P concentration fluctuated due to accumulation of bacteria and their excretion products. Almost all C and N which had disappeared from the plant material during the first 100 days of incubation was recovered in the water. Subsequently, these nutrients accumulated in the sediment. Only 10 % of the C and N in the water was soluble ( < 0.33 #m). Ortho-P increased substantially from 60 to 100 days of incubation in the vessels (b) and (c), possibly given off by the sediment or originating from decaying algae and bacteria. Only a minor part of the plantoP was recovered as ortho-P in the water.

The effect of the decomposing plant material on the diversity within the microscopical primary producers was studied using paper chromatography of the pigments. The changes in bacterial numbers were followed by epifluorescence microscopy.

REFERENCES

ANDERSEN, F.~)., 1978. Effects of nutrient level on the decomposition of Phragrnites communis Trin. Arch.Hydrobiol., 84:42-54.

BEST, E.P.H., A.H.PIETERSE, R.SOEKARJO.and L.DE LANGE, 1977. A preliminary study of the internal gas composition of Lemna gibba L. Acta bot.Neerl., 26:109 -113.

BEST, E.P.H., M.C.I.BLAAUBOER, Th.CAPPENBERG et al., 1978. Towards an integrated study of the ecosystem of Lake Vechten. Hydrobiol. Bull., 12:107-118.

BEST, E.P.H., M.ZIPPIN and J.H.A.DASSEN, 1982. Growth and production of Phragmitesaustralis in Lake Vechten (The Netherlands). HydrobioI.Bull., 15:165 - 174.

31

CHRISTMAN, R.F. and R.T.OGLESBY, 1971. Microbiological degradation and the formation of humus. In: K.V.Sarkanen and C.H.Ludwig, Eds., Lignins. Occurrence, formation, structure and reactions. Wiley-lnterscience, N.Y.p.769-796.

COWLING, E.B. and W.BROWN. 1969. Structural features of cellulosic material in relation to enzymatic hydrolysis. In: G.J.Hajny and E.T.Reese, Eds., Cellulases and their applications Adv.Chem.Ser.Publ. 95, Am.Chem.Soc., Washington. p.152-187.

DAGLEY, S., 1975. Microbial degradation of organic compounds in the biosphere. Am.Scient., 63:681-689.

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