Hydrobiologia 506–509: 135–145, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Role of sediment and internal loading of phosphorus in shallow lakes
Martin Søndergaard, Jens Peder Jensen & Erik Jeppesen
National Environmental Research Institute, Department of Freshwater Ecology, Vejlsøvej 25, P.O. Box 314, DK8600 Silkeborg, DenmarkE-mail: ms@dmu.dk
Key words: biomanipulation, iron, recovery, redox, release mechanisms, retention

Abstract
The sediment plays an important role in the overall nutrient dynamics of shallow lakes. In lakes where the externalloading has been reduced, internal phosphorus loading may prevent improvements in lake water quality. At highinternal loading, particularly summer concentrations rise, and phosphorus retention can be negative during most of
the summer. Internal P loading originates from a pool accumulated in the sediment at high external loading, andsignificant amounts of phosphorus in lake sediments may be bound to redox-sensitive iron compounds or fixed in
more or less labile organic forms. These forms are potentially mobile and may eventually be released to the lakewater. Many factors are involved in the release of phosphorus. Particularly the redox sensitive mobilization from theanoxic zone a few millimetres or centimetres below the sediment surface and microbial processes are considered
important, but the phosphorus release mechanisms are to a certain extent lake specific. The importance of internal
phosphorus loading is highly influenced by the biological structure in the pelagic, and lakes shifting from a turbid toa clearwater state as a result of, for example, biomanipulation may have improved retention considerably. However,internal loading may increase again if the turbid state returns. The recovery period following a phosphorus loading
reduction depends on the loading history and the accumulation of phosphorus in the sediment, but in some lakes
a negative phosphorus retention continues for decades. Phosphorus can be released from sediment depths as low
as 20 cm. The internal loading can be reduced significantly by various restoration methods, such as removal ofphosphorus-rich surface layers or by the addition of iron or alum to increase the sediment’s sorption capacity.

Introduction
Phosphorus availability is regarded as the most important factor for determining the water quality oflakes. Numerous studies have shown that high loadingof phosphorus leads to high phytoplankton biomass,
turbid water and often undesired biological changes.
The latter includes loss of biodiversity, disappearance of submerged macrophytes, fish stock changes,
and decreasing top-down control by zooplankton on
phytoplankton.
In order to reverse the eutrophication of lakes,
much effort has been made to reduce the external loading of phosphorus. Some lakes respond rapidly to such
reductions (Sas, 1989), but a delay in lake recovery isoften seen (Marsden, 1989; Jeppesen et al., 1991; vander Molen & Boers, 1994), one of the reasons beingthat phosphorus accumulated in the sediment during

the period of high loading needs time to equilibratewith the new loading level. Phosphorus release fromthe sediment into the lake water may be so intenseand persistent that it prevents any improvement of water quality for a considerable period after the loading
reduction (Granéli, 1999; Scharf, 1999).
Compared to deep lakes where a redox dependent accumulation of phosphorus occurs in the anoxic
hypolimnion during stratification, shallow lakes areusually well mixed and oxidized throughout the water column. Nonetheless, the sediment of shallow
lakes has often been demonstrated to release phosphorus to oxic lake water (Lee et al., 1977; Boström
et al., 1982; Jensen & Andersen, 1992), suggesting that other factors than redox conditions at the
sediment–water interface are involved. The importance of sediment–water interactions in shallow lakes
is furthermore enhanced by the high sediment sur-136
face:water column ratio, which means that the potential influence on lake water concentrations is stronger
than in deeper lakes. The direct contact with the photic
zone throughout the year and the regular mixing regime guarantee stable and near optimum conditions
for primary production (Nixdorf & Deneke, 1995).
Often the phosphorus pool in the sediment is morethan 100 times higher than the pool present in the lakewater, and lake water concentrations therefore depend
highly on the sediment–water interactions.
In this paper we give a short review of the phosphorus retention in lake sediments and the mechanisms and factors suggested to be vital to phosphorus
release from lake sediments. Our aim is to show thatthe sediment of shallow lakes is the domicile of numerous highly dynamic processes that may have very
substantial effects on the total phosphorus budget andlake water quality.

Retention and phosphorus in the sediment
Retention of phosphorus
During steady state conditions a certain amount of thephosphorus entering a lake is retained in the sediment(Fig. 1). The retention percentage depends on the hydraulic retention time, as demonstrated through different simple, empirically established models of the Vollenweider type: Plake =Pin /(1+tw0.5 ), relating in-lakephosphorus (Plake ) to inlet concentrations (Pin ) andhydraulic residence time (tw) (Vollenweider, 1976;
OECD, 1982). These models cannot, however, adequately describe the transient phase after reduced
loading when the system is not in equilibrium andstrongly influenced by the internal loading of phosphorus. The net retention of phosphorus is the difference between two processes with large opposite directed flux rates: (i) the downward flux caused mainly
by sedimentation of particles continuously entering
the lake or produced in the water column (algae, detritus etc.) and (ii) the upwards flux or gross releaseof phosphorus driven by the decomposition of organic
matter and the phosphorus gradients and transportmechanisms established in the sediment. Phosphorus
sedimentation from the lake water can be enhanced
in productive lakes via co-precipitation with calcium
carbonate (House et al., 1986; Driscoll et al., 1993;
Golterman, 1995; Hartley et al., 1997).
The importance of internal loading of phosphorus
during lake recovery is demonstrated by the findingthat phosphorus concentrations often increase during
summer in shallow eutrophic lakes (Sas, 1989; Phillips et al., 1994; Welch & Cooke, 1995; Ekholm etal., 1997). In most cases this increase can only be
the result of increased sediment loading, implying thatsummer phosphorus concentrations are largely controlled by internal processes (Jeppesen et al., 1997;
Ramm & Scheps, 1997; Kozerski & Kleeberg, 1998).
The most pronounced impact is often found in themost eutrophic lakes in which summer concentrations
typically exceed winter concentrations by 200–300%
from June until October (Søndergaard et al., 1999).
Mass balance calculations have shown that phosphorus retention exhibits a seasonal pattern mimicking
the seasonal variation in lake water phosphorus. In a
study of 16 Danish lakes, Søndergaard et al. (1999)
showed that the retention was positive during winterirrespective of the eutrophication level, while it was
negative during part of the summer. Even lakes witha phosphorus concentration below 0.1 mg P l−1 had
a 2-month negative retention in mid-summer, but the
duration of negative retention increased to 5 months
in lakes with a mean summer phosphorus concentration above 0.2 mg P l−1 . In the most eutrophic
lakes, a strongly negative retention occurred in May,suggesting that the onset of the increasing biologicalactivity in spring triggered the release of some ofthe phosphorus retained during winter. In early summer retention was found to be less negative owing
to the occurrence of a clearwater phase following
late-spring development of a high zooplankton biomass and its grazing on phytoplankton (Sommer etal., 1986; Luecke et al., 1990; Jeppesen et al., 1997;
Blindow et al., 2000).

Forms of phosphorus in the sediment
When entering the sediment, phosphorus becomes apart of the numerous chemically and biologically mediated processes and is ultimately either permanently
deposited in the sediment or released by various mechanisms and returned in dissolved form to the watercolumn via the interstitial water. It should be emphazised, however, that lake sediments can be very different and highly variable regarding chemical composition. Parameters such as dry weight, organic content,
and content of iron, aluminum, manganese, calcium,
clay and other elements with the capacity to bind andrelease phosphorus may all influence sediment–waterinteractions (Søndergaard et al., 1996).

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Figure 1. Schematic presentation of phosphorus pathways when entering a lake and some of the most important phosphorus compounds foundin the sediment (from Søndergaard et al., 2001). Adsorbed phosphorus is indicated by ads.

Chemical sequential extractions have been widelyused in order to describe the many different forms inwhich phosphorus occurs in the sediment (Williams
et al., 1971; Hieltjes & Lijklema, 1980; Psenner etal., 1988; Golterman & Booman, 1988). The aim isoften to give a more precise description of the potentials for phosphorus release from the sediment andto predict its future influence on lake water concentrations (Lijklema, 1993; Seo, 1999). By fractionation phosphorus is characterized as being bound tothe variety of inorganic sediment components as described above (Stumm & Leckie, 1971; Boström etal., 1982) or in organic phosphorus compounds. Organically bound phosphorus occurs in more or lesslabile forms or in a refractory form that is not released
during mineralization and which constitutes a fraction permanently buried in the sediment. Fractionation
schemes usually yield operationally defined fractions,but it may be debated which type of sediment phosphorus the different fractionations actually measure(Pettersson et al., 1988; Jáugeui & Sánches, 1993).
Often, loosely sorbed organic and inorganic fractions
as well as the iron-bound and redox-sensitive sorption of phosphorus are considered potentially mobile(Boström et al., 1982; Søndergaard, 1989; Søndergaard et al., 1993; Rydin, 2000). Petticrew et al.
(2001) found a close connection between total phosphorus release rates and the iron-bound phosphorus
components in the sediment, and during lake recovery,
this fraction can constitute a majority of the phosphorus released (Fig. 2). While fractionation schemes
may provide relevant information on the overall andlong-term conditions for phosphorus sorption expected to prevail in the sediment, it has been difficult so
far to establish general relationships between phosphorus forms and the intensity and duration of internalloading. Knowledge coupling the mechanisms behindinternal loading with sediment characteristics seems
inadequate (Phillips et al., 1994; Welch & Cooke,1995).
Importance of biological structure
The biological structure of a lake can significantly
influence its phosphorus concentrations and retention(Beklioglu et al., 1999). For example, clearwaterconditions resulting from increased top-down control
on phytoplankton often ensure considerably lower inlake nutrient concentrations (Søndergaard et al., 1990;
Benndorf & Miersch, 1991; Nicholls et al., 1996).
A positive relationship between clearwater conditions
and increased phosphorus retention was documented
by the changes recorded in Danish Lake Engelsholm
after biomanipulation involving a 66% removal of
the fish stock (Søndergaard et al., 2002a). Here, decreased turbidity led to significantly lower phosphorus
concentrations and higher phosphorus retention. Furthermore, the period with negative retention during
summer was reduced from 6 to 4 months. The annual
net retention changed from −2.5 to +3.3 g phosphorus
m−2 y−1 . These observations indicate that if a lake
returns to a turbid state after being clear for someyears, it has the risk of suffering from a high internalloading again. It also suggests that when biomanipulation is considered to improve lake water quality after aloading reduction it might be advantageous to wait forsome years until the influence of internal phosphorus
loading is reduced.
Several mechanisms are probably involved in theincreased phosphorus retention when the clearwater
state is achieved. These include the reduced sedimentation of organic matter, which again reduces oxygen

138
consumption and prevents low redox conditions. In addition, the improved light conditions enhance benthic
primary production and with it the phosphorus uptake and oxidation of the sediment surface (van Luijnet al., 1995; Woodruff et al., 1999). If submerged
macrophytes become abundant they assimilate phosphorus, but may also effect the retention in other ways
(see below). Benthivorous fish as for example bream(Abramis brama) have a significant impact on the resuspension of sediment (Breukelaar et al., 1994) as
well as on the concentration of phosphorus and its
release from the sediment (Havens, 1991; Søndergaard et al., 1992). A reduction in their abundance
may be a factor of particular importance in biomanipulated lakes formerly dominated by bream or otherbenthivorous species (Persson et al., 1993; Hansson
et al., 1998). Fish-mediated phosphorus release fromthe sediment is sometimes believed to be stronger
and more important for lake water quality than that
achieved through reduced planktivory and top-down
control on phytoplankton (Havens, 1993; Horppila etal., 1998).
Duration of internal loading

Figure 2. Changes in sediment phosphorus profiles of hypertrophic
Lake Søbygaard, Denmark, during recovery (measured in 1985,
1991 and 1998). The external phosphorus loading to the lake was
reduced by 80–90% in 1982. Fractionation was conducted according
to Hieltjes & Lijklema (1980). Organic-P was calculated as Tot-P –(NH4 Cl-P+NaOH-P+HCl-P). After 20 years the annual retention ofphosphorus is still negative, and the total recovery period is estimated to last more than 30 years after the loading reduction. The upper
panel is from Søndergaard et al. (1999) and is reprinted with kindpermission from Kluwer Academic Publishers.

The duration and importance of internal loading relate
mainly to the flushing rate, loading history and chemical characteristics of the sediment (Marsden, 1989).
Some lakes respond rapidly to an external loading
reduction by an immediate or only shortly delayed
decline in lake concentrations following the changes
in loading (Edmonson & Lehman, 1981; Sas, 1989;
Welch & Cooke, 1995; Beklioglu et al., 1999). A
fast response may be ensured by a high flushing rate
provided that the period with high phosphorus loading
was relatively short. On the other hand, a long loading
history with a high loading rate is reflected in the sizeof the phosphorus pool accumulated in the sediment,
and, if large, a rapid flushing rate may not suffice toensure a fast return to low concentrations (Jeppesen etal., 1991).
The sediment depth interacting with the lake water is probably lake specific and highly dependent onlake morphology, sediment characteristics and windexposure. Most often, phosphorus in the upper approximately 10 cm is considered to take part in the wholelake metabolism (Boström et al., 1982), but mobilityof phosphorus from depths down to 20–25 cm has
been seen (Fig. 2, Søndergaard et al., 1999). The internal phosphorus loading may be very persistent andendure at least 10 years after an external loading re-

139
duction has been effected (Welch & Cooke, 1999). In
some lakes, phosphorus retention can remain negativeeven for 20 years or more after the nutrient loading
reduction (Søndergaard et al., 1999).

concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus
concentrations in the water, whereas in late summer
no effect was found.
Temperature

Release mechanisms
Numerous mechanisms have been proposed to be responsible for the release of phosphorus from lake
sediments. In the following we will give a short review of some of the most important bearing in mind,
however, that one should be careful when generalisingand that the phosphorus release can be governed byvery different mechanisms in different lakes.
Resuspension
In shallow lakes, wind-induced resuspension is amechanism that frequently causes increased concentrations of suspended solids in the lake water. Particulate bound forms of phosphorus settling to thebottom may be resuspended several times before permanent sedimentation (Ekholm et al., 1997). In very
shallow lakes, resuspension events increase, more orless continuously, the contact between sediment andwater (Kristensen et al., 1992; Hamilton & Mitchell,1997). An example is shown in Figure 3 from a shallow Danish lake in which suspended solids and total
phosphorus increased by a factor 5–10 within a few
days during two events of increasing wind. In someshallow lakes, year-to-year variation in internal phosphorus loading has been shown to be largely controlled
by wind mixing (Jones & Welch, 1990).
Resuspension increases turbidity, but does not necessarily lead to increased release of phosphorus. Thisis because the overall process depends on the actual
equilibrium conditions between sediment and waterand on the capability of phytoplankton to take up phosphorus (Søndergaard et al., 1992; Ekholm et al., 1997;
Hansen et al., 1997). In the example from Lake Vest
Stadil Fjord (Fig. 3), there was no or only a veryslight persistent increase in total phosphorus concentrations after the wind events. In other lakes it has
been shown that resuspension increases release rates(Fan et al., 2001), or at least during some parts of theseason may cause a release, while there may be noeffect later in the season due to changed concentrationsin the lake water (Søndergaard et al., 1992). Frommeasurements in a shallow Finnish lake, Horppila &Nurminen (2001) concluded that in early summer, theTemperature reflects many of the biologically mediated processes in the lake. The pronounced seasonality
in internal loading and retention capacity strongly
indicates that the release mechanisms are linked totemperature and biological activity (Jensen & Andersen, 1992; Boers et al., 1998; Søndergaard et al.,1999). These include stimulation of the mineralizationof organic matter, the release of inorganic phosphatewith increasing temperatures (Boström et al., 1982;
Jeppesen et al., 1997; Gomez et al., 1998), and increased sedimentation of organic material related tothe seasonal variation in phytoplankton productivity(Ryding, 1981; Istvánovics & Pettersson, 1998).
As organic loading increases during spring andmineralization processes are strengthened, the penetration depth of oxygen and nitrate into the sedimentdeclines (Tessenow, 1972; Jensen & Andersen, 1992).
Jensen and Andersen (1992) observed that the temperature effect on phosphorus release was strongest
in lakes with a large proportion of iron-bound phosphorus. They also noticed a decrease in the thicknessof the oxidised surface layer with increasing temperatures, suggesting a redox-sensitive release. The
thickness of the top oxic sediment can thereby influence the concentration of phosphorus in the wholewater body (Gonsiorczyk et al., 2001).
Redox
Redox conditions in the surface sediment are theclassical explanation of sediment water interactions.
Einsele (1936) and Mortimer (1941) very early described how the phosphorus release was determinedby redox-sensitive iron dynamics. In oxidised conditions, phosphorus is sorbed to iron (III) compounds,
while in anoxia iron (III) is reduced to iron (II) andsubsequently both iron and sorbed phosphate returned
into solution. In shallow lakes the whole water column
is usually oxic, which also establishes an oxic surface
layer of the sediment with a high capacity to bind
phosphorus. In agreement herewith, Penn et al. (2000)
suggested that an oxidized microlayer at the sediment–water interface partially inhibits sediment phosphorus
release under well-mixed conditions in spring and au-

140

Figure 3. Lake water changes in Lake Vest Stadil Fjord, Denmark, during 10 days of varying wind speed (from 0–2 to 5–7 to 2–3 m s−1 ). Lake
area is 450 ha and mean depth 0.8 m.

tumn. On the other hand, phosphorus trapped in theoxic microlayer can be freed when the microlayer ischemically reduced at the onset of anoxia. Then, highphosphorus release rates are observed. In this way, theoxidized microlayer may serve to regulate seasonality in rates of sediment phosphorus release, but does
not influence long-term sediment–water exchange. If
the oxic surface layer becomes saturated with phosphorus, phosphorus transported upwards from deeper
sediment layers may simply pass through the oxic
layers into the water column.
The presence of nitrate, which normally penetrates
deeper into the sediment than oxygen and, like oxygen, has the capability to keep iron in its oxidised
form, can also be important for the redox sensitive
sorption of phosphorus (McAuliffe et al., 1998; Duras& Hejzlar, 2001). For example Kozerski et al. (1999)
found that high summer phosphorus release rates wererelated to low nitrate input to Lake Müggelsee. In contrast, Jensen et al. (1992) showed that the presence of
nitrate during winter and early summer diminished therelease rates, whereas nitrate addition in late summer
enhanced the phosphorus release in the same lakes,
probably by stimulating the mineralization process.
pH
pH is particularly important in lake sediments where
the capacity to retain phosphorus depends on iron,because the phosphorus binding capacity of the oxy-

genated sediment layer decreases with increasing
pH as hydroxyl ions compete with phosphorus ions
(Lijklema, 1976). The impact of pH on releasehas been illustrated by Koski-Vahala and Hartikainen(2001), who demonstrated that high pH, which is common in eutrophic lakes during summer, may markedly
increase the internal phosphorus loading risk whenlinked with intensive resuspension. In the sediment ofeutrophic lakes, photosynthetically elevated pH canestablish more phosphorus, which is loosely sorbed
to iron, and thus increase release rates (Lijklema,
1976; Søndergaard, 1988; Welch & Cooke, 1995;
Istvánovics & Pettersson, 1998).
Iron:phosphorus ratio
The combined ferric oxides and hydroxides available
in the sediment may bind phosphate very effectively.
The involvement of iron in the dynamic equilibrium
between the sediment and water has led to the suggestion that an iron dependent threshold exists forthe sediment’s ability to bind P. Jensen et al. (1992)
showed that the retention capacity was high as longas the Fe:P ratio exceeds 15 (by weight), and when
above this ratio internal phosphorus loading may be
prevented by keeping the surface sediment oxidised.
Caraco et al. (1993) suggested that the Fe:P ratio
should exceed 10 if it was to regulate phosphorus release. The presence of a threshold is supported by
the strong positive relationship between the concentra-

141
tions of phosphorus and iron in the surface sediment ofshallow lakes (Søndergaard et al., 2001). In hardwaterlakes, iron may be less important for the phosphorus
release compared to the solubilisation of apatite due
to decreased pH during mineralization (Golterman,
2001). Yet, it has been suggested that even in calcareous systems, iron and aluminium, when present
in high concentrations, are involved in regulating thephosphorus cycling (Olila & Reddy, 1997).
Chemical diffusion and bioturbation
The interstitial water of the sediment, which normallycontains less than 1% of the sediment’s total phosphorus pool, is important for the phosphorus transport
between sediment and water as interstitial phosphate
constitutes the direct link to the water phase above andthe solid–liquid phase boundary between water andsediment (Boström et al., 1982; Löfgren & Ryding,1985). An upward transport of phosphorus is created via a diffusion-mediated concentration gradient,normally appearing just below the sediment surface.
Bioturbation from benthic invertebrates or throughgas bubbles produced in deeper sediment layers during the microbial decomposition of organic matter
may significantly enhance the process (Ohle, 1958,
1978; Fukuhara & Sakamoto, 1987). There is someevidence that bioturbation from benthic chironomids
can enhance phosphorus release rates, particularly in
sediments low in total iron (Phillips et al., 1994).
Benthic invertebrates can also inhibit phosphorus release by supplying oxic water into the sediment andincreasing the oxidised surface layer of the sediment(Boström et al., 1982). Similarly, low phosphorus flux
rates can be recorded despite a steep interstitial water gradient, provided that the top centimetres of thesediment either have a high phosphorus sorption capacity (Moore et al., 1998), or its chemical processes
are controlled by a photosynthetically active biofilm(Woodruff et al., 1999).
Mineralization and microbial processes
In shallow and eutrophic lakes, the sediment continuously receives high amounts of freshly produced
organic material that is not decomposed before reaching the sediment. Thus, sediment bacteria may havea significant role in the uptake, storage and releaseof phosphorus (Pettersson, 1998). High organic input creates the potential for a high mineralization rate,
provided that the supply of oxiders such as oxygen or

nitrate is sufficient. Subsequently, the typical sedimentprofile will have oxygen penetrating a few millimetresinto the sediment, followed by nitrate which can befound several centimetres into the sediment dependingon the decomposition rate and the nitrate input.
If nitrate concentrations are low, but sulphatelevels and the supply of biodegradable organic matter high, desulphurication and sulphur cycling may
become important parts of the sediment processes(Holmer & Storkholm, 2001). Hydrogen sulphide
formed from sulphate reduction induces the formationof iron sulphide and decreases the potential of phosphorus sorption and thereby the potential phosphorus
release from the sediment (Ripl, 1986; Phillips et al.,
1994; Kleeberg & Schubert, 2000; Perkins & Underwood, 2001). The internal, dynamic P-release fromlake sediments may thereby be determined by the ligand exchange of phosphate against sulphide with iron.
During winter, low sedimentation rates and sufficient
supply of oxygen or nitrate to the sediments establish ahigh redox potential, maintaining the sedimentary ironin its oxidized form.
Submerged macrophytes
In shallow lakes, submerged macrophytes have the potential of being very abundant with a high plant-filled
volume. Macrophytes may, however, influence thephosphorus cycle both negatively and positively. Decreased release is seen when oxygen released from theroots increases the redox-sensitive phosphorus sorption to iron-compounds (Andersen & Olsen, 1994;
Christensen et al., 1997), and when high abundanceof macrophytes diminishes the resuspension rate andreduces the phosphorus release from the sediment(Granéli & Solander, 1988; Van den Berg et al.,1997). Increased phosphorus release may be recorded
in dense macrophyte beds and beneath macrophyte
canopies due to low oxygen concentrations (Frodge
et al., 1991; Stephen et al., 1997), or due to increased pH (James et al., 1996). From experiments
and measurements in the Broads in U.K., Stephen
et al. (1997) concluded that if rooted macrophytes
have a significant effect on phosphorus release they
increase it. Barko & James (1997) have given a comprehensive review of the effects of submerged aquatic
macrophytes on nutrient dynamics, sedimentation andresuspension.

142
Concluding remarks
Because of their strong impact on lake water concentrations, it is clear that knowledge of sediment–waterinteractions and the processes behind retention and release of phosphorus is fundamental for understanding
the function of shallow lakes. Many different mechanisms may be involved in the sediment release, but
two types are often of particular importance: (i) redoxdependent release of phosphorus bound to iron and(ii) microbial processes. The redox-dependent releasemechanism is also relevant in well-mixed eutrophic
lakes with an organic-rich sediment. Here, the oxic
surface layer is often too thin to prevent a releasefrom deeper parts of the sediment. Microbial processes
being fuelled by degradable matter accumulated in
the sediment or sediment settling from the lake water, together with the supply of oxidizers (oxygen,nitrate and sulphate), are important for the cycling ofphosphorus.
Presently, we do not have sufficient knowledge todevelop general models for the release mechanisms ofshallow lakes, descriptions that could be used as a predictive tool in lake management following a nutrient
loading reduction. More clear relationships between
easily measurable sediment characteristics and net release rates of phosphorus have to be established. It
should also be noted that phosphorus release mechanisms to some extent are lake specific: resuspension
being important particularly in very shallow and windexposed lakes, redox-sensitive release in iron-richsystems, etc.
In order to combat internal phosphorus loading andaccelerate lake recovery after decreased external loading, numerous lake restoration techniques have beendeveloped and tested (Dunst et al., 1974; Born, 1979;
Cooke et al., 1993; Phillips et al., 1999; Welch &Cooke, 1999; Søndergaard et al., 2000; Perkins & Underwood, 2001). They comprise both physical measures, such as sediment dredging by which nutrient richsediment is removed, as well as chemical methods.
The chemical methods aim to influence the redoxdependent phosphorus fixation by either improving thesorption capacity of the elements already present in
the lake/sediment or by adding new sorption capacity,
as for example iron, alum or calcium (Søndergaard etal., 2002b). For all types of restoration measures, an
important prerequisite for obtaining success and longterm effects is elimination of the underlying reasonsfor the impoverished water quality, i.e. a sufficient reduction of the external phosphorus loading (Benndorf,

1990; Jeppesen et al., 1990; Hansson et al., 1998;
Søndergaard et al., 2000).

Acknowledgements
This work was partly financed by the EU-project BUFFER (EVK1-CT-1999-00019). The technical staff at
the National Environmental Research Institute, Silkeborg, are gratefully acknowledged for their assistance.
Field and laboratory assistance was provided by J.
Stougaard-Pedersen, B. Laustsen, L. Hansen, L. Nørgaard, K. Jensen and L. Sortkjær. Layout and manuscript assistance was provided by A. M. Poulsen and
T. Christensen. Data were partly collected and madeavailable by local county authorities.

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