Aquatic Botany, 12 (1982) 1--12 1 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
D E C O M P O S I T I O N O F POTAMOGETON CRISPUS L.: T H E E F F E C T S O F
D R Y I N G O N T H E P A T T E R N O F M A S S A N D N U T R I E N T L O S S
K.H. ROGERS* and C.M. BREEN
Pongolo River Research Group, Department of Botany, University of Natal, Pieter- maritzburg (Republic of South Africa)
(Accepted for publication 1 June 1981)
ABSTRACT
Rogers, K.H. and Breen, C.M., 1982. Decomposition of Potamogeton crispus L. : the ef- fects of drying on the pattern of mass and nutrient loss. Aquat. Bot., 12: 1--12.
A study was made of the pattern of decomposition ofPotamogeton crispus L., con- talned in gauze bags which were incubated in the field. Plant material was collected from the field when the population began its annual decline. Either by placing the material directly into bags or by air-drying first, it was possible to compare the patterns of mass and nutr ient los/following natural senescence, with that following unnatural death.
Natural senescence and decay was characterized by a linear and rapid loss of mass (4% day -1 ) while dried plants showed a rapid initial loss (18% in 2 h) followed by a slower loss (2% day- l ) . However, the overall rate of mass loss from dried plants was slower than that of senescent plants with 35 and 10% of the original mass remaining after 24 days, respec- tively:
There was little change in the nutrient status of the senescent material during the ex- periments. However, marked changes in the P and K concentrations (attributed to leaching) and Ca and Mg concentrations (attributed to adsorption) occurred in the dried material within 2 h. Losses of nutrient stocks were also markedly altered by drying. These rapid changes in nutr ient status of the dried material appeared to discourage colonization by detritivorous snails and reduce microbial catabolism of the plant tissues. The slower rate of loss from the bags would thus be attributed to the different pathways of energy and nutrient transfer which became operative when plant material was dried.
Different mathematical functions were required to define the two decomposition rates and it was clear that interference with the events of senescence by drying of the normally submerged plants could impinge on the predictive potential of such models.
It is concluded that in aquatic, particularly submerged macrophyte communities, the processes of detritus production and utilization cannot be considered in isolation from the preceding process of senescence.
INTRODUCTION
T h e e l u c i d a t i o n o f t h e i m p o r t a n c e o f t h e l i t t o r a l , a n d i ts m a c r o p h y t e v e g e t a t i o n , as a s o u r c e o f d e t r i t u s fo r l ake m e t a b o l i s m (Wetze l a n d H o u g h , 1 9 7 3 ; r e v i e w e d Wetze l , 1 9 7 5 ) has s t i m u l a t e d d e t a i l e d i n v e s t i g a t i o n o f d e c o m -
*Present address: Department of Botany and Microbiology, University of the Witwatersrand, Johannesburg, Republic of South Africa.
position processes. Present understanding is based largely on the study of the disappearance of particulate matter and nutrients from dead plant materi- al either contained in mesh bags and incubated in the field (Boyd, 1970; Wohler et al., 1975; Howard-Williams and Junk, 1976; Godshalk and Wetzel, 1978b; DaneU and SjSberg, 1979; Howard-Williams and Davies, 1979) or in- cubated in the laboratory, in vitro (Jewell, 1971; Wohler et al., 1975; God- shalk and Wetzel, 1978a, b, c; Carpenter and Adams, 1979).
The plant material, actively growing or senescent, has usually been killed by drying (Wohler et al., 1975, Howard-Williams and Junk, 1976; Howard- Williams and Davies, 1979) or lyophilization (Godshalk and Wetzel, 1978b; Carpenter and Adams, 1979) to prevent its growth during experiments and also to permit storage until experiments could be carried out. While this may represent the natural situation for many emergent macrophytes, most submerged plant production senesces and dies in the aqueous environment.
Natural senescence is an orderly and programmed sequence of events which leads to the death of a plant or plant part (Woolhouse, 1967). It in- cludes events such as the gradual loss of cell membrane integrity and the re- sultant leaching of dissolved substances (Eilam, 1965) and could therefore be an important aspect of decomposition (Gallagher, 1978).
In this study the natural pattern of senescence and decay of an annual population of the submerged macrophyte Potamogeton crispus L. was com- pared with that obtained when plants were artificially killed by drying. The study formed part of an autecological study of P. crispus in the shallow lakes of the subtropical Pongolo River floodplain in South Africa (27 ° 30'S/32 ° 15' E). The floodplain is characterized by summer (November--February) floods which recharge the small lakes formed behind the levees (Breen et al., 1978). This study was conducted at Tete, a 100 ha, 1.5 m deep lake in which dense growths of P. crispus occur in winter and spring (Rogers and Breen, 1980).
MATERIALS AND METHODS
Potamogeton crispus produces vegetative propagules (turions) when the plants mature (Rogers and Breen, 1980). Intensive grazing of turions by waterfowl results in most of the standing crop being broken from the stolon or uprooted before the plants die (Rogers, 1979). The use of detached plants in decomposition studies would therefore represent the natural situation.
Material was collected by dragging an anchor behind the boat just above the sediment surface. The sample thus included both undamaged and duck- damaged plants of various ages. To ensure a study of the entire process of senescence, death and decay, only apparently healthy plants were selected, while those already showing evidence of senescence were discarded. Sedi- ment was removed by gentle washing in lake water and all invertebrates, roots and reproductive structures were removed. The excess water was then removed by spinning the material in a commercial spin-dryer (Rogers and Breen, 1980). Subsamples (100 g, about 10 g dry mass) were rapidly weighed
and transferred to plastic-coated fibre-glass gauze decomposition bags (400 X 250 ram, 2.25 mm 2 mesh size) standing in lake water to prevent drying out. Additional subsamples were air-dried before being placed in the decomposi- tion bags. Further subsamples, of varying mass, were oven dried at 60 ° C. These data were used to determine the fresh mass/dry mass ratio so that all fresh mass estimates could be converted to an oven dry mass basis.
The bags were transferred to four stations on the lake, situated 30 m apart within the area colonized by P. crispus. They were attached to a marker by a cord long enough to permit them to sink to the bottom. Bags containing plants which were not dried tended to float for 1--2 days before sinking to the sediment, as do naturally uprooted plants.
One bag of 'fresh' and one of dried material was collected from each station at approximately 5<lay intervals. The contents of the bags were rins- ed rapidly in distilled water and the snails and other invertebrates which had entered the bags during incubation were removed and counted. All ma- terial was dried at 60°C and weighed. The four samples for each treatment were then bulked, ground and analyzed by emission spectrometry (Philips 1410 semi-automatic X-R spectrometer) for phosphorus, potassium, calcium and magnesium. Total nitrogen was determined as ammonia with a Beckman ion-specific electrode (Meldal-Johnsen, 1975) following Kjeldahl digestion (Paech and Tracey, 1955).
Sequential estimates of standing crop (oven dry weight m -2) were used to follow the growth pattern of the P. crispus community during 1976 and 1977. Samples were taken with an electro-mechanical sampler as described in detail in Rogers and Breen (1980).
RESULTS
1976 Experiments
These experiments were designed to establish a routine for collection of material that was about to enter the senescent phase, so that young actively growing plants were not used in the decomposition studies.
P. crispus is a winter producing annual with maximum standing crop oc- curring in early spring (Fig. 1), after which the population declines as a result of senescence and decay.
Material placed, without prior drying, into decomposition bags on 21 August (Fig. 1, el) lost mass rapidly (83%) during the first 17 days before increasing significantly (P = 0.05, Fig. 2a). Inspection showed that some of the shoots in the bags had started to grow and it was evident that not all of the material placed in the bags had been entering the senescent phase. How- ever, when apparently healthy material (i.e. plants already senescing were discarded) was collected 30 days after maximum standing crop (Fig. 1, e2) and the experiment repeated, there was a rapid loss of mass which continued at a constant rate of ca. 4% day -1, until only 9% remained after 27 days (Fig.
4
~ 6 0 ~3 ~ e 1
U 4 0
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/ e 3
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1976 1977
Fig. 1. The growth cycle of the P. crispus community in a subtropical lake as determined by sequential estimates of standing crop. Arrows el, e 2 and e 3 indicate the times at which de- composition experiments were initiated.
ioo
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~6o
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(a) ( b )
~ L S D
6 10 15 20 25 30 0 5 10 15 20 25 D a y s Days
Fig. 2. Loss of dry weight during decomposition of P. crispus shoots expressed as percent remaining in decomposition bags with time. (a) Experiment started when the community was at maximum standing crop. (b) Experiment started 30 days after maximum standing crop. (LSD, least significant difference between the means; P = 0.05.)
2b). Since no g rowth o f p lants was ev ident in the bags all mater ia l was con- s idered to have been a b o u t t o senesce when the e x p e r i m e n t was ini t ia ted. Thus it a p p e a r e d t h a t the p a t t e r n o f mass loss o f P. crispus dur ing na tu ra l senescence and decay could be fo l lowed when mate r ia l was co l lec ted ca. 30 days a f t e r m a x i m u m s tanding c r o p had been a t ta ined .
1977 Experiments
Material was co l lec ted a t the same stage o f the annua l cycle (Fig. 1, e3) fo r a c o m p a r i s o n o f the p a t t e r n o f na tura l senescence and decay wi th t h a t ob- t a ined w h e n plants were killed by drying. T o faci l i ta te discussion the t e r m s ' s enescen t ' and ' d r i ed ' p lan ts are used to descr ibe the mate r ia l used in the t w o sets o f l i t te r bags.
Senescen t p lan ts did n o t show a signif icant r e d u c t i o n in mass dur ing the first 24 h (Fig. 3) b u t t he rea f t e r loss p r o c e e d e d a t a c o n s t a n t ra te , ca. 4% d a y -1 o f the original mass , and a f t e r 24 days on ly 10% o f the original mass
~oo- - -
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20 "t
0 I ~%" 5 I() I'5 2'0 2'5 Days
Fig . 3. Loss o f d r y w e i g h t o f senescen t P. cri~pus s h o o t s ( d o t t e d l i n e ) a n d d r i e d s h o o t s (solid l ine) expressed as percent remaining in litter bags with time. (Vertical bars = 1 s t anda rd deviation from t he m e a n . )
remained. The decay rate was thus the same as that measured in 1976 (Fig. 2b).
Dried material lost 18% of its mass within 2 h (Fig. 3). No significant loss occurred during the next 22 h, after which it proceeded at a constant rate, 2% day -1 of the original mass. This was slower than the loss from senescent material and, after 24 days, a significantly greater proport ion of the original mass (35%) remained in the bags containing dried material.
Nutrient concentrat ion in senescent material remaining in the bags showed little change during decomposi t ion, although phosphorus and magnesium de- creased slightly (Fig. 4a). All nutrients were, therefore, lost from the bags at~ s imi le rates. Significant reduction in nutrient stock could not be detected after 24 h, but loss proceeded at a constant rate of ca. 4% day -1 during the following 24 days (Fig. 4b).
Nitrogen concentration in the dried material remained fairly constant during decomposi t ion and was thus similar to that in senescent material (Fig. 4a). The concentrations of other nutrients, however, changed markedly during the first 2 h and were therefore consistently different from those in senescent material (Fig. 4a). Phosphorus and potassium concentrations de- creased during the first 2 h whilst those of calcium and magnesium each in- creased by almost two-thirds. During~the period 1--24 days the concentra- tions of phosphorus and magnesium declined slightly with calcium and potassi- um remaining fairly constant.
The rapid loss of mass during the first 2 h, associated with differential rates of nutrient loss, resulted in marked differences between the nutrient stocks re- maining in decomposing senescent and dried material. The stocks of nitrogen, phosphorus and potassium in dried plant material decreased by 7, 54 and 95%, respectively, within 2 h (Fig. 4b). Thereafter, nitrogen and phosphorus stocks were reduced at constant rates (3.7 and 3.3% day -1, respectively) whilst no further losses in potassium were recorded. Reduct ion of nitrogen stocks be- tween 2 h and 24 days were more rapid in the case of senescent than dried material, so that after 24 days dried material contained more nitrogen than did senescent material. In contrast, after the initial rapid loss of phosphorus, the rate of loss was slower than that from senescent material and at the end of the experiment the amounts remaining were similar.
Fig. 4. Nutrient dynamics of senescent (dotted line) and dried (solid line) P. crispus plants in decomposition bags. (a) Concentration (%) of each nutrient in the plant material remaining in the bags at each sampling interval. (b) Percent of the original stock of each nutrient re- maining at each sampling interval.
TABLE I
Mean numbers and mass of snails (Bulinus natalensis Kiist.) found in litter bags containing (1) senescent plants and (2) plants which were dried before re-immersion in the lake. (LSD, least significant difference between the means.)
The stocks of calcium and magnesium in dried material increased markedly during the first 24 h before declining at almost constant rates of 3.3% day -1 of the original mass (Fig. 4b). The stocks of calcium and magnesium were consistently higher in dried than in senescent material.
The decomposing senescent and dried plant material both attracted snails (Bulinus natalensis Kiist.) with numbers and mass initially higher on the dried material. However, by the eighteenth day the senescent material support. ed more than twice the number and mass of snails (Table I).
DISCUSSION
The rate and pattern of mass loss from dried plant material was similar to that observed for a variety of aquatic plants (Boyd, 1970, 1971; Mason and Bryant, 1975; Howard-Williams and Junk, 1976; Godshalk and Wetzel, 1978b; Howard-Williams and Davies, 1979). The initial rapid loss of mass has been attributed mainly to the solubilization and leaching of minerals and or- ganic substances (Planter, 1970; Kaushik and Hynes, 1971; Godshalk and Wetzel, 1978a; Swift et al., 1979) and the later, slower loss of mass is at- tributed to the breakdown or catabolism of plant material by heterotrophic micro-organisms (Howard-Williams and Junk, 1976; Saunders, 1976; Godshalk and Wetzel, 1978c). However, when decomposition bags are used, mass loss during the latter phase is also related to the loss of particulate matter (com- minution, Swift et al., 1979) from the bags and the rate of catabolism of these particles is not recorded. Thus, as decomposition is the sum of leaching comminution and catabolism (Swift et al., 1979) the pattern of mass loss from decomposition bags in the field reflects the rates of production of both the particulate and dissolved organic detritus (Rich and Wetzel, 1978), and not the rate of complete decomposition.
Since dried P. crispus lost a greater proportion of its phosphorus and potassium than of its mass during the first 2 h, it is clear that these two nutri-
ents were rapidly leached. In contrast, the rate of loss of the nitrogen stock was the same as the rate of mass loss indicating that leaching was not impor- tant. This has been observed in other studies (Odum and de la Cruz, 1967; de la Cruz, 1975; Howard-Williams and Davies, 1979) and is attributed to the rapid binding of any soluble nitrogen that is released by either the microflora associated with the decomposing material, or as protein com- plexes on the dead material. As decomposition proceeds, these processes can also result in an increase in the concentration of nitrogen (Howard- Williams and Davies, 1979) but this was not evident in the present study, possibly because its duration was so short.
Howard-Williams and Junk (1976) observed a marked increase in the con- centration of calcium during the early stages of decomposition and attributed it to the adsorption of Ca 2÷ onto the surface of the plant material by cation exchange. The increase in stocks and concentration of both calcium and magnesium within 2 h during this study, suggests that dried P. crispus is also acting as a cation exchange surface.
Despite the close similarity between the patterns of nutrient and mass loss reported elsewhere and those shown here for dried P. crispus, they devi- ate markedly in most respects from those occurring in senescent material.
Thus senescent plant material did not act as a cation exchange surface and the loss of mass from senescent material did not show the initial rapid leaching. The latter might be expected since during natural ageing, leaching would be an extended process as individual cells senesce, die and lyse (God- shalk and Wetzel, 1978c), whereas when dried material is used, all cells are dead on re-immersion.
However, since leaching is a more rapid process than microbial breakdown of structural material, certain nutrients such as P and K, would be expected to show a decrease in concentration during decomposition (Swift et al., 1979). As this was not observed in the case of senescent material it is con- cluded that leaching was dependent on structural breakdown and thus nutrient loss by leaching was concurrent with loss of particulate matter by comminution. The presence of large numbers of snails in the bags contain- ing senescent material implicates them in the process. Indeed they have been shown to consume senescent plants (K.H. Rogers, 1978 unpublished data) and their involvement in the decomposition process will be reported in a later paper.
It is clear, however, that the mechanisms leading to a loss of mass and nutrients during these experiments were markedly altered by drying of the plant material. Harrison and Mann (1975) and Godshalk and Wetzel (1978a) have also observed that drying increased the rate of leaching, and the former authors commented that drying appeared to alter the organic matter in such a way that it was less rapidly attacked by microbes. The overall slower rate of mass loss from dried material in this study might also, therefore, be a re- sult of a change in the pattern of microbial breakdown.
Until recently, it was considered that colonization by microflora respon-
sible for decomposition began after death of the plant or plant part (Ol~h, 1972). Howard-Williams et al. (1978) and Robb et al. (1979) have, however, demonstrated a microfloral succession, dominated by bacteria, which begins on young plants and continues after death. Many of these bacteria are poten- tial decomposers as they produce extra-cellular cellulases (Robb et al., 1979) and there is evidence that they may be partly responsible for death of cells (Howard-Williams et al., 1978).
When decomposition is studied using senescent material, the microflora would remain living and relatively undisturbed but when dried material is used it would, like the plant cells, have been killed by the drying process. Dried material would therefore have to be recolonized before decomposi- tion could proceed. Since nutrients and dissolved organic matter are leached rapidly during the first 2 h and they are known to influence the rate of bac- terial production (Fenchel and Harrison, 1976; Paerl, 1978) it is likely that recolonization would be nutrient-limited. Drying must, therefore, be expect- ed to alter both the type of micro-organisms colonizing the material and the rate of catabolism; hence the slower overall rate of mass loss from dried plants.
As snarls, including Bulinus, are known to feed on senescent plant material (Azevedo and Medeiros, 1955; Lammens and van der Velde, 1978) and are grazers of bacteria and other micro-organisms (Calow, 1974; Berrie, 1976), it must also, therefore, be expected that drying will influence snail numbers and the consequent pattern of comminution and detritus formation. The larger snail mass which was supported on the decomposing senescent ma- terial emphasizes the different pathways of energy and nutrient transfer which become operative if the material is dried prior to experimentation.
Decomposition has usually been modelled by an exponential function of the type y = ae-bk where y is the mass remaining after a time interval b, a is the initial mass and k is the rate constant (Jewell, 1971; Carpenter and Adams, 1979; Howard-Williams and Davies, 1979). Although an exponential model of this type fits the data for decomposition of dried P. crispus (coef- ficient of determination 0.99, Fig. 5a), inspection shows that in common
100
SO
'E 6o
~ 2o
(a)
lJO ll5 210 2'5 Days
1'o 1'5 2'o 2'. Days
Fig. 5. F u n c t i o n s of mass of p l a n t mate r ia l r ema in ing against t ime. Ac tua l da ta po in t s for 1977 e x p e r i m e n t s are shown. (a) Loss o f dr ied p l a n t mater ia l f r om d e c o m p o s i t i o n bags as descr ibed by a s imple e x p o n e n t i a l f u n c t i o n where y = a e - b k . (b) Loss of senescen t p lan t mater ia l f r om d e c o m p o s i t i o n bags as descr ibed by a l inear f u n c t i o n where y = a o - - a l x .
10
with data from other workers (Godshalk and Wetzel, 1978b; Howard- Williams and Davies, 1979), it does not fully account for the initial rapid loss by leaching. In a modification, Godshalk and Wetzel (1978b) noted that if k was considered to decrease exponentially (an exponential model of the type d W / d t = k W where k = ae -bt, W = percent of the initial mass remaining and t = days of decomposition) then no such discrepancy occurred. How- ever, k did not decrease exponentially in this study but rather changed very abruptly between 2 and 24 h.
The loss of mass from senescent material in both 1976 (not shown) and 1977 (Fig. 5b) was best described by a linear model of the type y = a o - a l x
(where y is the mass remaining after time x, a0 is the initial mass and al the rate constant) with coefficients of determination of 0.995 and 0.997, respectively. However, despite its usefulness in describing the pattern of mass loss from the decomposition bags, it is limited in that it does not account for the fate of fine particulate or the dissolved detritus. More detailed in- vestigations of the factors affecting production and utilization of this detri- tus are required before a biologically meaningful model can be produced. Nevertheless, the different functions required to describe the two sets of data suggest that drying of a submerged macrophyte such as P. c r i s p u s may bias a mathematical model of decomposition.
Since drying markedly affected the decomposition process, many previ- ous studies may have underestimated the rates of decomposition of sub- merged macrophytes and also the role of invertebrates in detritus formation and nutrient transfer within the littoral zone. Furthermore, in most plants production, senescence and decay follow sequentially in any organ and may occur simultaneously during the life span of a plant as older organs (particu- larly leaves) die and new ones are produced. Similarly, in a population, some plants will begin to senesce earlier than others. Consequently the rates of transfer of nutrients and organic matter within an ecosystem will change during the life-span of the population. To establish the importance of the various rates of decomposition to system metabolism, as suggested by God- shalk and Wetzel (1978c), will require an understanding of senescence within the population, and of the in situ utilization o f senescent and decaying ma- terial by higher trophic levels.
ACKNOWLEDGEMENTS
This work was supported by a grant from the Inland Waters Ecosystems Section of the National Programme for Environmental Sciences, Council for Scientific and Industrial Research. Dr. B.R. Davies and Professor D.F. Toerien made valuable comments on the manuscript.
11
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