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Wetlands in a Dry Land:
Understanding for Management
W.D. Williams (Editor)
Environment Australia
Land and Water Resources
Research and Development
Corporation
Biodiversity Group
Land & Water
Resources
Research&
Development
Corporation
~
The effects of drying and reflooding on
nutrient release from wetland sediments
Arthur McComb and Song Qiu
Environmental Science,
Murdoch University,
Murdoch WA 6150
WATER REGIME
This paper discusses the implications of drying and
flooding effects on nutrient release from sediments
in wetlands. It discusses the role of sediment
properties, phosphorus and nitrogen transformations, carbon cycling, drawdown and macrophyte
control, and modelling. Finally, comments on
management issues are made. In summary:
Drying and refilling cause substantial changes in
water quality. The effect depends on 1. sediment
properties (sediment composition; nutrient and
organic content); 2. type of drawdown (gravity or
evaporative); 3. severity of drying (proportion of
drying area, rate of drawdown, degree of
dewatering, temperature and time of sediment
drying and weathering); 4. conditions of refilling
(origin of water, degree of sediment disturbance).
Sediment properties are the key to physical,
chemical and biological changes during drying
and reflooding, as they affect nutrient transformations and exchange between sediment and water.
Changes may include particle aggregation and
sediment consolidation, phosphorus adsorption
and desorption; organic degradation, nitrogen
transformation, and stimulation of microbial
Phosphorus
transformations
in
processes.
sediments during drawdown depend on many
factors. There is a higher probability of stimulating
phosphorus release than of reducing phosphorus
concentration during the reflooding of dried
sediments, particularly in organic-rich wetlands.
Stimulation of phosphorus release and algal
production is most likely to result from an evaporative drawdown event. Concentrations of
inorganic nitrogen (ammonia and nitrate) often
increase during or after drawdown, although the
148
WATER REGIME
effect may be short-term. Such release has the
potential to stimulate N-Jimited algal blooms.
Drawdown has been used effectively in controlling
macrophytes. The levels of sediment dewatering,
freezing or hot climate conditions generally determine the levels of macrophyte reduction.
However, control of macrophytes is often species
specific. The elimination of macrophytes may
increase the probability of algal blooms.
This paper is concerned with the implications of
drying and reflooding for nutrient cycling. For our
purposes, a 'water regime' is considered as a
complete cycle in wetland water-level. Changes in
nutrient availability are considered to occur in
two-phases: the first is drawdown, exposure of
surface sediments and dewatering; the second is
reflooding of the exposed, dried sediments and
their return to the earlier, waterlogged state.
Nutrient .behaviour during drawdown and
reflooding has been only sparsely reported, and
little is known about the underlying causes of the
changes. One indirect source of information is
the relatively better-documented nutrient
processes in rice soils, which suggests that drying
and rewetting increases nutrient availability.
Hundreds of years ago Japanese farmers dried
soils under the sun to increase soil fertility and
improve rice production (De Datta, 1981). The
concentration of water-soluble phosphorus in
submerged rice soils was often appreciably
increased compared with dried soils (Ponnamperuma, 1972). Nitrogen transformations were also
altered in reflooded rice soils, with an accumulation of ammonia and other forms of inorganic
nitrogen (De Datta, 1981).
Further information comes from soil experiments, in which increased solubility or extractability
of various nutrients, including P, Ca, Mg, K, Fe and
Mn, have often been reported after soil samples
have been air-dried and rewetted (Bartlett and
James, 1980; De Datta, 1981; Payne and Rechcigl,
1989). Drying soils in the laboratory has been
reported to stimulate phosphorus and nitrogen
cycling, and microbial activity (Birch, 1958;
Soulides and Allison, 1961; Marumoto et al., 1982).
The effects in wetlands of drying and
reflooding depend on many factors. In some
shallow wetlands, drawdown and drying of
sediment decreased phosphorus concentration at
refilling, but in other cases it increased phosphorus
concentrations (Nichols, 1975; Cooke, 1980;
Jacoby et al., 1982; Siver et al, 1986; De Groot and
Wijck, 1993).
It is difficult to draw general conclusions about
effects of water regime on nutrient availability. Such
difficulties may lie in the validity of making comparisons between different studies because of uncontrolled or unspecified pre- and post-drawdown
conditions (such as temperature and pH) and to the
complexity of the biotic activities involved in the
processes. In many cases, sediment composition
may be a dominant factor determining differences
in responses to drying and reflooding.
What needs to be determined is the dryinginduced changes in nutrient availability pre- and
post drawdown, taking into account not only
physical and chemical properties, but also in
biological components. What can be observed
directly from the field (such as changes in P
concentrations pre- and post-drawdown) are the
net result of complicated, dynamic processes
which may be controlled by different factors at
different stages. Thus, an observed effect may not
be caused directly by drawdown, but by differences in environmental conditions, which may
either be a consequence of the water regime or to
changes such as weather conditions (temperature,
rainfall or wind). Even if a change were shown to
be caused by drawdown, this may not be a direct
impact, but result from drying-induced changes in
sediment properties, biological activity, or even
secondary pH changes resulting from changes in
bioactivity. Without control of conditions under
which various drying-related changes may take
place, it is impossible to reach reliable conclusions.
For instance, algal blooms may occur after
drawdown and refilling, which have been attributed to increased light penetration following
macrophyte reduction (Cook, 1980), but increased
nutrient release may be partly responsible.
NATIONAL WETLANDS RESEARCH AND DEVELOPMENT PROGRAM
WATER REGIME
What might be the cause of an increase in
nutrients after reflooding? Two major processes
appear to be directly involved. The first is physicochemical, dominated by phosphorus sorption. It is
the traditional concept that phosphorus concentration in a sediment-water system is determined
essentially by interactions between phosphorus
and Fe, AI hydroxides (Mortimer, 1941; Williams
et al., 1971; Fox, 1993; Walbridge and Struthers,
1993). These interactions can be strongly affected
by several key factors, including redox, pH and
temperature.
The second process mediating nutrient concentrations is essentially biological, and includes
uptake and release of phosphorus by biological
material, mineralisation of organic matter and
generation of phosphate by microorganisms; this
may create anoxic conditions at the sediment-water
interface, under which phosphorus release can be
greatly accelerated. This process has been extensively reviewed by Bostrom et al. (1988) and
Gachter and Meyer (1993). It may be anticipated
that these two processes exert major influences on
the drying-induced changes in nutrient concentrations, especially in phosphorus concentration.
One approach to understanding effects on
nutrient behaviour is to make detailed observations as wetlands progress through a cycle of
natural drying and rewetting. An advantage of
such studies is that they embrace all features of the
natural environment as the seasons progress, but
the disadvantage is that they do not allow the
contributions of different factors to nutrient
relations to be disentangled. At best, such investigations allow hypotheses to be erected as to the
factors likely to be important in controlling
sediment behaviour. Further insight requires a
more experimental approach in which enclosures
are set up, either in the field or laboratory, in
which the water column is held under controlled
conditions, and those conditions manipulated, or
at least measured, to test hypotheses. Sometimes
large field enclosures are employed, such as domes
which encompass some of the statistical variation
of the sediment surface, but the simplest approach
is to use columns collected from the field which
have an intact sediment core overlain by an intact
149
water column. Such cores can then be studied
under conditions of wet storage or drying. The
reliability of such techniques to disentangle
nutrient behaviour is very much dependent on
how well the factors that affect nutrient behaviour
are controlled. Minimising the potential interference involved in the systems is one of the major
objectives for this type of study.
Sediment compaction
Physical, chemical and biological changes occur
at higher rates during drawdown and sediment
drying of sediments. These include sediment
compaction, sediment particle aggregation,
phosphorus and nitrogen transformations, and
mineralisation of organic matter. All these are
closely related to the properties of the original
sediment. Fox et al. (1977) reported that peat-type
lake sediments lost almost no water upon drying
(7% in 180 days) but organic-rich sediment
consolidated 40-50% during the same period of
drying. Plotkin (1979) observed a similar consolidation of highly organic sediment. In some cases,
the compaction and fine particle aggregation
caused by drying benthic sediments is not
reversed after refilling (Fox et al., 1977; Plotkin,
1979; Twinch, 1987). Turbidity may be lowered
by drying and reflooding, and this has been
attributed to the aggregation of small particles
and the formation of a crust on the sediment
surface (Fox et al., 1977).
Particle aggregation in dried sediments as been
reported to reduce silt content by 28% and clay
content by 71% (Twinch, 1987). Sand content
increased by 29% in comparison with wet
sediments. In general, dried sediments had less
buffering capacity for external phosphate loading,
and phosphorus release to overlying water
increased.
The limnological significance of sediment
porosity is based on the exchange flux between
pore fluids and the overlying water, and is relevant
to drawdown and sediment drying in at least two
ways: dewatering during drawdown, and rewetting
after refilling. During sediment compaction, the
WETLANDS IN A DRY LAND: UNDERSTANDING FOR MANAGEMENT
.....
150
WATER REGIME
pore fluid might be transported out of the
sediment (a phenomenon often observed during
storage of sediment samples where a layer of water
comes to overlay the sediment). Mud-type
sediments or sediment containing a high content
of organic matter may have a relatively greater
capacity for compaction than sands and coarsetextured sediments.
During evaporative drawdown, pore fluid may
first be transported across the sediment surface,
and then evaporate to the atmosphere. The more
evaporable components of pore fluid, such as CH4 ,
H2S and NH3, may be partly transferred into the
atmosphere if evaporation proceeds for sufficiently
long. The remaining, concentrated components
would be subjected to physical, chemical and
biological reactions under rigorous conditions,
including exposure to oxygen, high temperature,
solar radiation and other extreme meteorological
conditions. Clearly some changes will be accelerated under these conditions.
Properties of Fe and AI oxides
Both colloidal and non-colloidal sediment
fractions are significant in phosphorus sorption.
For example, amorphous and poorly crystalline
ferric hydroxide, which has a high binding
capacity for phosphorus, can occur as a surface
coating on sediment particles (Ceding and Turner,
1982; Pertersen et al., 1993); these fine-grained
components are important in controlling
phosphorus exchange between sediment and
water. Studies of phosphorus sorption in soils and
sediments have often established correlation
between phosphorus sorption properties and the
extractability of Fe and Al (Ryan et al., 1985;
Richardson, 1985; Redshaw et al., 1990). Ryan et
al. (1985) found 'that phosphorus sorption in
20 calcareous soils was significantly correlated
with oxalate extractable Fe. Similar relations were
found in freshwater wetlands, in which the
phosphorus sorption was correlated with both
oxalate extracted Fe and Al, of which oxalate-Al
was better correlated with phosphorus adsorption
(Richardson, 1985). Comparison of extractability
between moist and dried soils revealed that drying
can change the concentrations of various chemically extractable elements, such as Fe, Al, P, Ca
and Mn (Haynes and Swift, 1985; Payne and
Rechcigl, 1989).
As drying and rewetting can markedly change
the properties of phosphorus binding materials,
drying-induced changes in phosphorus sorption
can be considered as a secondary process, which
is mediated by primary changes in the binding
materials, It is more likely that several primary
changes (e.g. Fe, Al, and microbial activity), and
several concurrent secondary change (e.g. concentrations of P, N and other nutrients) are involved
during drying and reflooding. The questions are:
1. what major primary or secondary processes
may mediate nutrient concentrations during
reflooding? and 2. how, and by how much, can
these processes affect phosphorus exchange
between sediment and water?
Two approaches were used by Qj.u (1995) to
examine phosphorus related processes during
drying and rewetting. Firstly, sediment solutions,
either 'wet' or air-dried, were examined for the
changes in solution properties, as some of these
were expected to provide information relevant to
changing phosphorus sorption on drying.
Secondly, oxalate and CDB (citrate-dithionitebicarbonate) extractions were applied to 'wet' and
air-dried sediments, to examine changes in iron
and aluminium extractability after drying. The
ratio between oxalate and CD B extractability
(Feo/Fed) was used to assess "crystallisation" of iron
oxides, which is related to surface morphology and
particle stability, and to phosphorus sorption by
the iron oxides.
Air-drying significantly altered the pH,
turbidity, colour and soluble Fe(III) in rewetted
sediment solutions. It was concluded that air-drying
increased oxalate- or CDB- extractable Fe of the
sediments; there was also an increase in crystallisation during drying. Thus an effect on phosphorus
sorption can occur in response to the changes in Fe
crystallinity. On the other hand, the amount of both
oxalate- and CDR-extractable AI was unchanged
after air-drying. Oxidation-related effects during
sediment exposure and drying are therefore
considered the main cause for such changes.
NATIONAL WETLANDS RESEARCH AND DEVELOPMENT PROGRAM
WATER REGIME
Increased phosphorus release
In general, phosphorus availability in reflooded
soils is higher than that of nonflooded soils. This
increase has been attributed to the redox status of
soils; at low redox ferric phosphate was reduced to
ferrous phosphate (De Datta, 1981). With the
submergence of rice soils, crystallised iron
phosphate tends to change into colloidal iron
phosphate through solution-precipitation (Chang,
1976). It is believed that phosphate existing in a
more dispersed state, such as iron phosphate, is
more available to plants.
The view that drawdown and reflooding
increases phosphorus concentrations has been
supported by Schoenberg and Oliver (1988) and
Fabre (1988), who observed higher phosphate
concentrations in water overlying sediment which
had been air-dried. Phosphate concentration in
sediment suspensions was positively related to the
initial phosphorus concentration and pH of the
overlying water, and to the rate of refilling. Higher
agitation rates during refilling tended to increase
the concentration of soluble phosphate (Fabre,
1988).
Briggs et al. (1985) reported the effects of water
regime on nutrient and ionic concentrations of two
impermanent, freshwater wetlands in southwestern New South Wales. The wetlands were
allowed to dry for one to several months and then
reflooded. Total loads of iron, phosphate, nitrate
and sulfate increased as the wetlands reflooded.
Nitrate and phosphate concentrations also peaked
in their experimental tanks after reflooding.
Increased nutrient release from terrestrial soils
after reflooding has been reported in this area
(Charley, 1972).
In a study of changes in phosphorus availability
during a drying and reflooding cycle, ~u and
McComb (1994, 1995) dried intact sediment cores
from a small freshwater wetland for 40 days, and
then refilled the cores and incubated them under
controlled conditions. There was a five-fold increase
in soluble reactive phosphorus (SRP) after refllling
under aerated conditions. This effect was enhanced
by anaerobic conditions created by microbial
15:1.
activity. At concentrations of 0-2 mg P L-1, a range
encountered in most wetlands, phosphate sorption
was linearly related to phosphate concentration
in the water; in most organic-rich sediments
air drying and rewetting increased labile-P, a
fraction readily released to the water phase, and
equilibrium P concentrations. At higher concentrations (2-100 mg P L- 1), drying decreased the P
sorption maximum (Xm) and sorption constant (b)
of the sediment, and so increased the SRP concentrations in the water phase.
The forms of phosphorus in wetland sediments
are affected during drying (~u 1995). Chemical
fractionation indicated a general increase in
loosely-bound P (NH4Cl-P) and labile-P, measured
in the sorption experiments. There was also a
general increase in NaOH-P (bioavailable) in airdried sediments, while HCl soluble P (not bioavailable) decreased in most of the selected sediments
after drying. Thus, the probability of phosphorus
release appears to increase as a result of sediment
air-drying.
The impacts of drying on phosphate sorption
were investigated for Chaffey Dam, New South
Wales during 1995 (Darren Baldwin, pers. comm.).
Sediment samples were taken along a transect
covering the heavily dried, wet-littoral and
submerged sediments from both above and below
the oxycline. Dried sites showed the least adsorptive capacity for phosphate, and the sediments
beneath the oxycline the highest. These effects
were considered to result from oxidation of previously inundated sediment.
Bacteria are important contributors of P during
drying in two ways (~u and McComb, 1995):
1. when sufficient moisture and oxygen were available, bacteria rapidly removed soluble P from the
water and incorporated it into the particulate
phase, the amount of P taken up being positively
correlated with bacterial respiration; and 2. upon
drying, bacterially stored P was partly returned to
the water, the release increasing with increasing
bacterial uptake. The bacterial contribution of P
upon drying was investigated by sterilising airdried and wet sediments with a low dose of gamma
irradiation (10 kGy), to discriminate among P
contributed from the native (initial) microbial
WETLANDS IN A DRY LAND: UNDERSTANDING FOR MANAGEMENT
152
WATER REGIME
biomass (Pi) before drying, P released from the
increased (developed) microbial biomass (Pii)
during drying, and P stored in bacteria which had
survived air drying (P,). It was estimated that airdrying killed about 76% of the microbial biomass.
In general, Qju and McComb's results agree with
the findings of Sparling et al. (1985) on soil drying,
that "the whole P increase on air-drying was
derived from the killed microbial biomass".
Reduced phosphorus after reflooding
In contrast to the emerging generalisation that
drying and reflooding increase nutrient concentrations, Cook and Power (1958) reported that
drainage reduced the nutrient content of marsh
soils, though the possibility remains that the
loosely attached, fine or colloidal fractions of the
surficial sediment were lost during draining. In a
saline tropical African lake characterised by high
pH, high conductivity, high CaC03 and exchangeable Na+, drawdown and mud exposure resulted
in a "loss" of organic matter, phosphorus and
exchangeable Na+ (McLachlan and McLachlan,
1969). Working on sediments from Lake Apopka,
Florida, Fox et al. (1977) found that drawdown,
drying and consolidation, followed by reflooding,
did not bring about a significant breakdown of
organic matter and nutrient release.
Another systematic evaluation was conducted
in a small 5.4 ha reservoir (Lake Laurel, Georgia)
by Barman and Baarda (1978). The reservoir was
drained and the sediment exposed, dried for six
months then refilled. Dissolved oxygen, turbidity,
sulfide, nitrogen and phosphorus were compared
for water quality pre- and post drawn. There were
significant falls in phosphorus concentration after
drawdown. Total phosphorus, total soluble
phosphorus and ortho-phosphate were reduced
from 90-100 ]lg P L- 1 predrawdown to less than
10 )lg L- 1 after drawdown. There were also significant reductions in phytoplankton biomass and
periphyton production after drawdown. In that
study, however, the confidence of the results seem
to be limited by uncontrolled environmental and
limnological conditions during the comparison.
Another study focussed on the shallow,
eutrophic Long Lake in Washington, USA, where
the water-level was reduced by almost 2m in four
months during the summer and fall of 1979. The
trophic status was expected to improve through
sediment consolidation and macrophyte reduction. Drawdown resulted in an 84% reduction of
macrophyte biomass in 1980, although there was
a minimal sediment consolidation (0.1 m). Before
drawdown, the summer phosphorus concentration
of the lake approached or exce.eded 100 )lg L-1, but
remained below 50 )lg L- 1 after drawdown (Jacoby
et al., 1982).
There is little detailed information about the
changes during drying and refilling. Although
reductions in phosphorus after water-level
drawdown have been reported, the effects did not
seem to be direct. The reduction in phosphorus
loading may be related to a decrease in water pH,
which might result from decreased macrophyte
photosynthesis. Decreased pH might further
reduce phosphorus release from sediments after
reflooding, through a mechanism which exchanges
phosphate on iron and aluminium hydroxides for
hydroxy ions (Lijklema, 1976; Fabre, 1988). For
this reason, improvements in lake trophic conditions through drawdown and drying can be short
term (Cooke, 1980; Stent, 1981).
Reddy and Patrick (1984) reviewed nitrogen transformation and loss in flooded soils and sediments,
and concluded that biological, chemical and
physical processes involved were: 1. mineralisation
of organic N; 2. nitrification of NH4-N; 3. NH3
volatilisation; 4. denitrification.
De Datta (1981) found that, in the absence of
oxygen, the inorganic nitrogen regime in flooded
soils is characterised by the accumulation of
ammonia. In a study of the effects of a drawdown
on a waterfowl impoundment in Michigan, USA,
Kadlec (1962) found a marked increase in
nitrogen; total nitrogen in sediment increased
from 2. 7 mg L- 1 before drainage to 15.9 mg L- 1
after drainage. In soil solution it increased from
0.05 mg L- 1 to 15.5 mg L- 1 after reflooding,
NATIONAL WETLANDS RESEARCH AND DEVELOPMENT PROGRAM
WATER REGIME
reflecting higher microbial activity in the
sediment. In the water column, although the
nitrate concentration was rather low, there was an
increase of nitrate from 0.05 to 0.16 mg N L- 1.
Breimer and Slangen (1981) reported a
substantial increase in nitrate in soil samples stored
at room temperature for two months, especially for
sandy soils with a relatively high organic content.
Gaudet and Muthuri (1981) studied the effect of
seasonal changes in water-level on the littoral zone
of the shallow Lake Naivasha, Kenya. The lake
edge was exposed during annual drawdown for
part of the year, and was subsequently inundated.
Nitrogen concentration was highest in the littoral
zone, and the distribution of nitrogen, phosphorus
and sulphur followed a decreasing gradient from
shore to open water. High consumption of nutrients occurred after the inundation of soil and plant
material which had been exposed by drawdown.
Gouleau et al. (1993) studied the effect of
drying and rewetting on distribution of nitrogen in
the sediments of a small pond and found that, at
the end of a drying sequence, the concentration of
dissolved ammonium in the 'interstitial water
interface' was 45 times more than that at the beginning of drying. The authors concluded that after
drying and rewetting the ammonium fluxes intensified across the sediment-water interface.
High concentrations of phosphorus and
nitrogen were reported in Washington Park Lake,
New York, after drawdown and subsequent
dredging (Hardt, 1989). Lake depth increased,
transparency improved, and weed growth was
eliminated. Another massive drawdown was
reported by Stent (1981) in Grafham Water Reservoir, Cambridge, England. Detailed monthly
records of pH, P04 and N03 over a five year
period indicated an unprecedented peak of P04
and N03 after the 1976 drawdown, but this was
attributed to high nutrient loadings by river inflow
during refilling.
Qj.u and McComb (1996) reported a series of
changes in nitrogen-related activities in intact cores
after reflooding. They suggest that drying
enhanced mineralisation of organic matter and
hence accumulated a source of ammonium N.
Upon reinundation, this pool of ammonium N was
153
rapidly released into the overlying water and
stimulated nitrification when there was sufficient
dissolved oxygen. Two implications may be
drawn. Firstly, drawdown of water-level and
drying of organic lake sediment may temporarily
increase concentrations of ammonium and nitrate
in the overlying water during refilling, so
increasing the probability of N limited algal
blooms. However, the subsequent stimulation of
nitrification, followed by nitrate penetration into
anaerobic sediments may increase denitrification
in the sediments and promote nitrogen loss. Thus
drying and reflooding of organic-rich sediments
offers potential for nitrogen removal from
eutrophic wetlands.
Detrital organic matter from aquatic plants,
degraded to various extents, is relatively finegrained or colloidal. This fraction may be important during a drying and rewetting cycle for two
reasons. First, the organic substances, whether as
coatings or infillings between sediment particles,
will support benthic microbial activity by
supplying respirable organic carbon, and release
phosphorus and nitrogen due to their decomposition. It also creates anaerobic condition, which, as
noted above, favour phosphorus release. Secondly,
coatings of these organic materials may change the
surface charges of the sediment mineral particles
and thus probably 'block' phosphorus sorption
(Holtan et al., 1988; Gall, 1990).
Birch and Friend (1956) studied the effect of
drying and rewetting on humus decomposition in
East Mrican soil. When soil was air-dried and then
rewetted there was a marked increase in respiration; it was established that drying at high temperature had a similar but enhanced effect on humus
degradation. Repeated drying followed by rewetting produced marked increases in decomposition
of soil organic matter. Birch (1958) and Soulides
and Allison (1961) reported similar results in
humus decomposition and nitrogen availability
after soil drying. A fresh flush of decomposition of
organic matter was observed after each successive
drying and wetting cycle. Repeated air drying had
WETLANDS IN A DRY LAND: UNDERSTANDING FOR MANAGEMENT
154
11
I
WATER
REGIME
a cumulative effect in the decomposition of
organic matter, while prolonged drying increased
the rate of decomposition. Drying increased nitrification and the release of ammonia, but was more
destructive to bacteria than freezing. Swift et al.
(1987) suggested that several species of bacteria
were capable of using humic acid as the sole
source of carbon and nitrogen. Bacterial numbers
increased for two and six weeks during the incubation, and chromatography elution patterns showed
a shift in molecular weight distribution towards a
low molecular weight after biodegradation.
The decomposition of organic matter in the
sediment, however, is generally a rather slow
process, especially for organic matter with high
molecular weight. The above experiments
indicated only relatively higher rates with easily
degradable organic matter, including carbohydrates, amino acids and short chain hydrocarbons.
In order to judge the validity in reduction of lake
internal nutrient loading, it is necessary to take into
consideration the vertical distribution of chemicals
and biological materials in sediment. During field
drying, deeper layers of sediments may remain
anaerobic, and degradation is much slower and
less complete than in the upper layers. If there is
significant organic enrichment in the deeper part
of the sediment, it is probably necessary to
increase exposure of deeper layers to accelerate its
biodegradation. This is, however, difficult to
achieve by drawdown alone, and it thus may
become a major limitation to oxidation of organic
materials in natural systems.
It might be useful to point out that drawdown has
long been used for controlling undesirable aquatic
plants (Beard, 1973; Nichols, 1975; Jacoby et al.,
1982). Cooke (1980) concluded that the technique
retarded macrophyte growth by destroying seeds
and vegetative reproductive structures through
exposure of these structures to drying or freezing
(in winter), and altering the substrate through
dewatering
and
sediment
consolidation.
Dewatering of sediments, exposure of the thallus
and reproductive structure to hot or freezing
conditions are essential for elimination of the
rooted plants.
Two drawbacks that interfere with macrophyte
management are the selection of desiccation-resistant macrophytes, and invasion by terrestrial
plants. In addition, the effectiveness of macrophyte
control appears to be species-specific and environment-dependent. In order to achieve a well
planned and satisfactory result, it is necessary to
1. assess the drawdown conditions, e.g. drawdown
level, weather condition during drawdown, degree
of dewatering, period and area of exposure, degree
and time of refilling, and 2. identify the species
composition of the targeted macrophyte populations before drawdown is applied.
When a dynamic balance has been established
in an ecosystem, it is always difficult to reverse by
a single-step management effort, since each
process or sub-process of the balance has its own
buffering capacity. It has been suggested that
consecutive drawdown events, sufficiently low to
expose weed-beds, would be more effective than a
single drawdown treatment (Richardson, 1975). A
number of lake restoration programs in the USA
have recommended the combination of drawdown
with other restoration methods to improve the
results of lake restoration (Bateman and Laing,
1977; New Jersey Department of Environmental
Protection, 1983; Florida State Department of
Environmental Regulation, 1983; Arkansas Game
and Fish Commission, 1983).
A critical step in understanding the nutrient
relations during sediment drying and reflooding is
the development of conceptual models to encapsulate the interrelations among the various pools
and processes in the systems of interest. Fig. 1 is of
particular relevance as a conceptual model which
indicates points impacted by wetting and drying.
The question arises as to whether attempts
should be made to turn such conceptual models
into numerical models. We support the view that
this should be done, as it forces one to sort out the
magnitude of the various pools and processes, and
allows sensitivity analyses to determine which
NATIONAL WETLANDS RESEARCH AND DEVELOPMENT PROGRAM
WATER REGIME
155
Figure 1. A conceptual model for the impacts of sediment drying and reflooding on P availability.
Drawdown and sediment drying
_____________P_~~~c~:_c_h_ern~~~I_P!~c:e_s~!l~ ______________________________ ~i~~o~!c_a~ .P!~c_e~~e_e_ ______________ _
o,;d,tioo
1
Fe (II)
.,._
Initial stage of drying
(sufficient moisture in sediment)
Fe (Ill)
~
erobic to aerobic
Destroy
anaerobic bacteria
Stimulate aerobic bacteria
•
•
Increase loosely bound P and NaOH-P
Accumulate P in bacterial biomass
~------------ -f~~t-11~~ ~;y~~~ ~~~ ~~~~~~~t~~------------ -·----------------------
---------------------,---------------------------,
Feo/Fed
Destroy plankton
•
Decrease P adsorption
t
Destroy bacteria
Refloodingjreinundation
Release loosely bound P
and Fe-bound P
t
Release cell-bound P
t
Increase P in water
parameters affect model behaviour most critically.
Model validation would be needed, using field and
laboratory measurements.
Nutrient availability can be strongly affected by a
drying and reflooding, and this has considerable
management potential. Although this is widely
recognised, realising the potential is not straightforward. Various schools of thought have developed. For example, some managers may hold the
view that, as water is the most characteristic feature
of wetlands, every effort should be made to
maintain water-levels, if necessary diverting scarce
water resources from other uses. Another reason
for maintaining water-levels might be to avoid a
high concentration of dissolved substances during
drying, and consequent increases in algal blooms
and avian botulism. Maintaining the levels may in.
some cases be aesthetic, such as preserving a
wetland landscape in a metropolitan area, and
avoiding 'unsightly' bare sediment, even though
this may have been a perfectly normal occurrence
under pristine conditions.
At the opposite end of the spectrum is the view
that wetlands which have been subjected to drying
may have better water quality, in terms of water
clarity and the absence of severe blue-green
· blooms, as compared with wetlands in the same
area which did not to dry out. Both ends of the
spectrum of opinions might be held for wetlands
in the same region, for example on the Swan
Coastal Plain. Clearly it is not yet possible to make
generalisations for all wetlands, even in the one
area.
This review suggests that the effects of drying
and reflooding are directly or indirectly related to
sediment properties (sediment composition;
nutrient and organic content) and the rigour of
drying and reflooding. Sediment properties may
well determine the nutrient concentration in the
reflooded phase.
WETLANDS IN A DRY LAND: UNDERSTANDING FOR MANAGEMENT
:156
WATER REGIME
In different management contexts, the same
impact may be deemed desirable or undesirable.
For example, increased nitrate concentration after
reflooding may be undesirable if it stimulates Nlimited algal blooms, but is desirable if denitrification and N loss is a priority management target.
Account should therefore be taken of the possibility of utilising drying to achieve particular
management objectives, such as promoting denitrification, and using nitrate to buffer effects of
oxygen depletion on phosphorus release. A
· simple, unified management strategy suitable for
all types of wetlands appears unlikely. Sound
management requires clear management targets,
and identification of differences between the
individual wetlands in a number of key factors.
Key factors
In general, the key factors to be specified for
management purposes are wetland/ sediment
types, and the severity of changes in water regime;
each of these could consist of several elements.
Although the roles of inorganic components such
as Fe and AI, Ca, and Mn are important in determining the impacts, severe impacts from an altered
water regime on nutrient availability are more
likely to result from a combination of abiotic and
biotic effects.
Because microbial processes are so important,
they will in many cases override sorption-related
processes in controlling or modifying nutrient
availability. There may well be higher vulnerability
to altered water regime in wetlands with higher
level of organic-enrichment and primary productivity, and in wetlands which are heavily used as
waterfowl habitat. For this reason, it would be
sensible to focus on a suite of selected, vulnerable
wetlands rather than seek generalisations for all.
long-term changes
As pointed out earlier, we have taken a 'water
regime' to be one complete seasonal cycle of
drying and rewetting, and most data presented in
this review are relatively short-term. However, one
can envisage the pattern of drying and reflooding
being more or less repeated periodically with a
long-term mean behaviour. The question then
arises, how might one predict the consequences of
changing this long-term mean? Such a change
might be brought about by extended lowering of
the watertable, flooding through increased
drainage, and changes in rainfall and river flow.
The effects on nutrient availability will depend not
only on the rigour of drying and reflooding, but
also on the frequency and extent with which the
cycles occur in the long-term. To improve our
understanding, one might compare the trophic
status and nutrient properties of two groups of
wetlands in the same geographical region, which
have a long recorded history, one group frequently
subjected to drying and reflooding, and the other
group permanently inundated. The accumulation
of sediment nutrients and organic matter might be
the focus of such a comparison.
This is a challenging area where it would be
advantageous to have numerical models for
summarising and predicting the relationships
between drying and reflooding on nutrient behaviour. This is needed for the long-term management
of Australian wetlands.
1. How can wetlands be categorised according to
2.
3.
4.
5.
their vulnerability (e.g. using their sediment
properties) to altered water regime, to predict
impacts of management?
Do catchment soils respond in the same way
as sediments to drying, contributing in consequence increased soluble nutrients loads to
wetlands through rainfall and surface runoff?
What is the implication of the time of
increased nutrients in wetlands in different
geographic areas? For instance, an ammonium
or nitrate pulse after reflooding may last a few
days to a few weeks in the field; does this pose
risks of triggering algal blooms, especially of
nuisance species?
What are the effects of drying and reflooding
on other nutrients such as silicon?
How can drying and reflooding be managed to
remove organic matter and nitrogen from
wetlands?
NATIONAL WETLANDS RESEARCH AND DEVELOPMENT PROGRAM
W AT E R R E G I M E
6. What processes in a wetland may dissipate or
deflect the effects? Can we manage the drying
to control nutrient enrichment during reinundation?
7. How do the impacts of drying on nutrient
availability affect ecological processes in
wetlands? What is the role of an altered water
regime, for example, increased periods of
drying or reflooding, on the long-term sustainability of wetland ecology?
8. How might numerical models be used to
predict effects of drying and reflooding on
nutrient release?
9. An holistic, overall assessment of the drying
effect is needed to assist decision and policymaking processes for the management of
Australian wetlands. This should incorporate
current knowledge of the effect on nutrient
availability and ecological responses such as
biodiversity, and long-term impacts on ecological sustainability.
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