INTRODUCTIONDuring the past 50 years, numerous lake restorationmethods have been developed and tested all over the world(Born 1979; Cook et al. 1993; Phillips et al. 1999). The pur-pose has most frequently been to combat eutrophication,which has led to a high abundance of phytoplankton, turbidwater and an overall deterioration of lake water quality andbiological diversity.

Lakes that do not respond to a reduction in external nutrient loading, even when nutrient loading has beenreduced to a level so low that an improvement in water quality should be observable, have become special subjectsof study. The resilience to change may be chemicallyinduced through the release of phosphorus from a pool

accumulated in the sediment during the period of high load-ing (Marsden 1989; Phillips et al. 1994; Søndergaard et al.,in press, 1999). It may also be biologically induced and aresult of a biological structure established during the periodof high nutrient loading; typically, a fish community domin-ated by zooplanktivorous and benthivorous species or thedisappearance of submerged macrophytes, which are bothfactors that favour the turbid state (Benndorf 1990; Jeppesenet al. 1990; Lauridsen et al. 1993). The potential of using lakerestoration to establish clear-water conditions has recentlybeen encouraged by the theory of alternative stable statesin shallow lakes. The theory, which suggests that a lake mayalternate between a turbid and a clear-water state within agiven nutrient level (Scheffer et al. 1993; Scheffer &Jeppesen 1998; Bachmann et al. 1999), has given inspirationto and induced several management-orientated lake restora-tion projects with the purpose of shifting the lake from theturbid to the clear-water state.

Lakes & Reservoirs: Research and Management 2000 5: 151–159

Lake restoration in DenmarkMartin Søndergaard,* Erik Jeppesen, Jens Peder Jensen and Torben Lauridsen

National Environmental Research Institute, Department of Lake and Estuarine Ecology, Vejlsøvej 25, DK-8600 Silkeborg,Denmark

Abstract

Lake restoration in Denmark has involved the use of several different restoration techniques, all aiming to improve lake waterquality and establishing clear-water conditions. The most frequently used method, now used in more than 20 lakes, is thereduction of zooplanktivorous and benthivorous fish (especially roach (Rutilus rutilus) and bream (Abramis brama)) with theobjective of improving the growth conditions for piscivores, large-sized zooplankton species, benthic algae and submergedmacrophytes. Piscivore stocking (mainly Esox lucius (pike)), aiming especially at reducing the abundance of young-of-the-year fish, has been used in more than 10 lakes and frequently as a supplement to fish removal. Hypolimnetic oxidation, withoxygen and nitrate, has been undertaken in a few stratified lakes and sediment dredging, with the purpose of diminishingthe internal phosphorus loading, has been experimented with in one large, shallow lake. Submerged macrophyte implantationhas been conducted in some of the biomanipulated lakes to increase macrophyte abundance and distribution. Overall, theresults from lake restoration projects, in the mainly shallow Danish lakes, show that external nutrient loading must be reducedto a level below 0.05–0.1 mg P L–1 under equilibrium conditions to gain permanent effects on lake water quality. By using fishremoval, at least 80% of the fish stock should be removed over a period of not more than 1–2 years to obtain a substantialeffect on lower trophic levels and to avoid regrowth of the remaining fish stock. Stocking of piscivores requires high densities(>0.1 individuals m–2) if an impact on the plankton level is to be obtained and stocking should be repeated yearly until a stable clear-water state is reached. The experiments with hypolimnetic oxygenation and sediment dredging confirm that internal phosphorus loading can be reduced. Experience from macrophyte implantation experiments indicates that protectionagainst grazing by herbivorous waterfowl may be useful in the early phase of recolonization.

Key wordsbiomanipulation, hypolimnetic oxidation, lake restosation, macrophyte implantation, sediment removal.

*Corresponding author. Email: ms@dmu.dk

Accepted for publication 15 February 2000.

In Denmark, different types of lake restoration projectshave been undertaken and in the present study, we give asurvey of the methods applied and the results obtained sofar. We also attempt to draw some general conclusions basedon Danish experiments, including recommendations on theconditions that need to be fulfilled prior to and during anongoing restoration project. As most restoration projectshave been undertaken during the past 5–10 years, long-termeffects and stability have not yet been fully elucidated.Furthermore, the Danish experiences are primarily basedon shallow lakes.

METHODSDanish lakes

The mean water depth is lower than 1.6 m in half of theDanish lakes whereas it is higher than 5 m in only 10%(Fig. 1). Most lakes are nutrient-rich because of the highsewage input from urban and cultivated areas. Even thoughgreat efforts have been made to reduce nutrient loadingduring the past two decades, not the least from sewage plants(Jeppesen et al. 1999), most lakes are still eutrophic as 50%of them have a summer average of total phosphorus above0.15 mg P L–1 (Fig. 1). The average summer Secchi depth islower than 0.85 m in half of the lakes and only 13% of thelakes have a summer Secchi depth above 2 m.

Compared with unaffected lakes, the high nutrient supplyhas led to various changes in biological structure (Jeppesenet al. 1999, 2000). The fish community has changed from having a high abundance of predatory fish (generally perch (Perca fluviatilis) and pike (Esox lucius)) to almostcomplete dominance by zooplanktivorous species. In particular, roach (Rutilus rutilus) and bream (Abramisbrama) have become dominant and generally constituteapproximately 80% or more of the fish stock biomass in nutrient-rich lakes. The zooplankton is dominated by smallcladocerans (Bosmina etc.) and cyclopoid copepods, whilethe number of large-sized cladocerans and more efficientphytoplankton grazers (especially Daphnia) has declined. Asthe phytoplankton biomass increases concurrently withincreasing nutrient levels, the zooplankton : phytoplanktonratio biomass decreases and the zooplankton is no longercapable of controlling the abundance of phytoplankton.Finally, the enhanced turbidity leads to a significant declinein the abundance or even complete disappearance of submerged macrophytes.

Sampling and analysisSampling procedures were generally conducted accordingto the guidelines of the Nation-wide Monitoring Programme(Kronvang et al. 1993); that is, lake water was usually sam-pled twice a month in summer (1 May to 1 October) and

monthly during winter. The programme included samplingof water chemistry (nutrients, chlorophyll a, turbidity),phytoplankton (species, biomass) and zooplankton (species,biomass). The composition and relative abundance of fishwere investigated by using standardized fishing methodsand multiple mesh-sized (6.25–75 mm) gill nets. Submergedmacrophyte abundance was determined in August whenbiomass was at a maximum. A description of the samplingprogramme can be found in Jeppesen et al. (2000).

Restoration measures: Aims and methodsSix different types of restoration measures have beeninitiated in Danish lakes (Table 1). In all cases, the mainobjective has been to increase water transparency, either by

152 M. Søndergaard et al.

Fig. 1. Frequency distribution (%) of the (a) mean depth of Danish

lakes (n 5 500), (b) total phosphorus concentration (n 5 200) and

(c) Secchi depth (n 5 180).

limiting the internal loading of phosphorus from the lakesediment or by stimulating the grazing food chain byincreasing the grazing pressure from zooplankton on phyto-plankton.

Biomanipulation via fish stock manipulation has beenundertaken in more than 20 Danish lakes and is thus themost frequently applied method. The manipulations haveinvolved either the removal of zooplanktivorous fish (mainlyroach and bream) or stocking of predatory fish (generallypike or, less frequently, perch), or both. The purpose of fishmanipulation was to reduce the fish predation pressure on zooplankton and thus increase the growth potential oflarge-sized zooplankton and thereby its ability to limit the

abundance of phytoplankton and to promote increased abun-dance of predatory fish (Jeppesen et al. 1990; Søndergaardet al. 1990). Selective removal of zooplanktivorous fish hasusually been carried out by using pound nets or trawling,but fish traps, gill nets and electro-fishing have also beenused. The extent of fish removal in the different lakes has varied significantly, from 10 to 80 g m–2 and between 5 and 80% of the estimated total fish biomass. The stockingof predatory fish, the main purpose of which was to affect the recruitment and survival of YOY (young-of-the-year) planktivorous fish, has involved stocking of pike fingerlings in spring along the littoral zone (Berg et al.1997; Søndergaard et al. 1997). Also, the stocking of pike

Lake Restoration in Denmark 153

Table 1. Restoration measures used in Danish lakes > 5 3 104 m2 during the past 15 years

Restoration No. restoration Lake size, mean depth

measures projects and phosphorus concentration Main objectives of the restoration

Fish removal 20–30 10–850 3 104 m2 To reduce the number of zooplanktivorous and

1.1–4.3 m benthivorous fish in order to improve the conditions

0.08–0.70 mg P L–1 for large-sized zooplankton and piscivores.

To improve water clarity, enhance growth of

submerged macrophytes, benthic algae and the

abundance of benthic invertebrates.

Piscivore stocking 10–20 10–850 3 104 m2 To reduce the number of zooplanktivorous and

1.2–3.5 m benthivorous fish in order to improve

0.08–0.27 mg P L–1 the potentials of large-sized zooplankton

To improve water clarity, enhance growth of

submerged macrophytes, benthic algae and the

abundance of benthic invertebrates.

Macrophyte implantation 5 13–150 3 104 m2 To increase the dispersal potential and abundance of

0.8–2.6 m submerged macrophytes in order to stabilize the

0.1–0.5 mg P L–1 clear-water stage.

To enhance the day-time refuge for large-bodied

zooplankton.

Sediment dredging 1 150 3 104 m2 To reduce the internal loading of phosphorus by

0.8 m removing phosphorus-rich sediment.

0.9 mg P L–1

Hypolimnetic aeration 2 8–340 3 104 m2 To reduce the internal loading of phosphorus by

5.0–13.1 m improving the redox conditions in the hypolimnion

0.1–0.5 mg 3 104 P L–1 and surface sediment.

Hypolimnetic nitrate 1 10 3 104 m2 To reduce the internal loading of phosphorus by

addition 2.4 m improving the redox conditions in the hypolimnion

0.5 mg P L–1 and surface sediment.

has varied greatly from 0.005 to 0.36 individuals m–2

year–1.Macrophyte implantation has been used in five lakes with

the purpose of increasing the abundance and distributionpotential of submerged macrophytes. Despite the shallow-ness, the natural stock of macrophytes has often disappearedbecause of the high nutrient loading and turbid water(Jeppesen et al. 1999). Re-establishment after improvedtransparency may be slow, possibly because of a low or complete lack of seed banks and lack of nearby locationsfrom where plants may spread. Also, waterfowl grazing may be responsible for slow re-establisment (Søndergaardet al. 1998). Based on the assumption that plants are capable of long-term and long-range spreading, implantationhas generally been used as a supplement to other restora-tion methods and has so far often been limited to a small part of the total lake area. The most comprehensiveexperiment was made in Lake Engelsholm, where 900 m2

of macrophytes were established inside enclosures. Nativeand eutrophication-tolerant species, such as Potamogetonpectinatus and Potamogeton crispus, were usually used.

Large-scale sediment removal has only been undertakenin shallow Lake Brabrand, near the city of Aarhus (lake area150 3 104 m2, average depth 0.8 m). The objective was toreduce the sediment release of phosphorus by removing theupper nutrient-rich sediment layer because mass balancemeasurements showed that lake internal loading was high(Jørgensen 1998). Simultaneously, the sediment removal wasaimed at preventing the filling in of this recreationally impor-tant lake, that has an annual sediment increase of about 1 cm. Prior to, or concurrently with, sediment removal, phosphorus removal was introduced at all major sewageplants in the lake catchment. Sediment was removed byusing a dredge (Mudcat®, Ellicot Machine Corp), which waspulled in tracks over the parts of the lake from which sediment was to be removed. Thereafter, the nutrient-richsediment was pumped to depositing basins near the lake. In total, approximately 500 000 m3 mud was removed over a7-year period.

Hypolimnetic oxidation has been undertaken in twoDanish lakes. The most comprehensive restoration so farwas made in deep, stratified, Lake Hald in central Jutland(lake area 340 3 104 m2, average depth 13 m, maximum depth31 m). For 12 years, pure oxygen was pumped into thehypolimnion in summer when the lake was stratified(Rasmussen 1998). Oxygen was dispersed in the hypo-limnion via eight diffusers (each having approximately50 000 holes at 1 mm in diameter) placed at four differentlocations on the lake bottom. The purpose was to increasethe sediment’s phosphorus-binding capacity via oxidation ofiron and to enhance survival of animals (particularly the

endangered chironomid larvae, Chironomus anthracinus)living in the profundal zone. On average, 210 3 103 kg O2

were added yearly, corresponding to 140 mg m–3 day–1 duringthe period of stratification.

Hypolimnetic addition of calcium nitrate has been usedin one lake only. In the 10 3 104 m2 large and summer-stratified Lake Lyng, situated near the town of Silkeborg(mean depth 2.4 m and maximum depth 7.6 m), calciumnitrate (Ca(NO3)2) was added to the hypolimnion during two summer periods (Søndergaard et al. unpubl. data, 2000).The dosages were 8–10 g N m–2 and the nitrate was added either in a dissolved or granulated form at 5-m depthsin areas greater than 5 m. Dosing was undertaken approxi-mately once a week from late June until late August. Thepurpose was to increase the capability of the sediment to retain phosphorus under the anaerobic conditions that developed shortly after the onset of stratification via the oxidation effects of nitrate, on iron in particular (Ripl1978).

RESULTS AND DISCUSSIONIn spite of a significant variation in lake morphometry, nutri-ent loading, intensity and scale of intervention (Table 1),some general patterns seem to emerge from Danish restor-ation projects, especially regarding fish manipulation onwhich comprehensive sets of data exist. The data obtainedprimarily cover a brief post-restoration period and do notallow an adequate validation of long-term effects.

The impact of fish manipulation on both the fish com-munity and the remaining trophic levels depends highly onthe scope of the manipulation (Tables 2,3). No effects or veryfew were observed, while large-scale and intensive manipu-lation often had marked effects on several biological andchemical variables used as indicators of improved waterquality (Table 2). The results suggest that at least 80% of thezooplanktivorous fish stock should be removed, if an impacton trophic levels other than fish is to be obtained. This observation supports the results obtained from several otherinternational experiments (Perrow et al. 1997; Hansson et al.1998; Meijer et al., in press, 1999). If the effect is to cascadeto lower trophic levels, the critical fish biomass seems to beapproximately 10 g m–2 in eutrophic lakes, a level alsorecorded elsewhere by Seda and Kubecka (1997). However,the removal of large amounts of fish does not necessarilymean that this manipulation has effectively improved thewater quality. The time scale is an important factor. In thecase of long-term, but less intensive, interventions, theremaining fish will largely compensate for the removal viaincreased growth and reproduction. Thus, during therestoration of a 270 3 104 m2 large lake conducted over a5-year period, twice as many fish were removed as estimated

154 M. Søndergaard et al.

prior to the intervention, without it having any apparenteffects on the fish stock biomass (Mæhl 1998). Therefore,fish removal should preferably not last much longer than 1–2 years (Hansson et al. 1998). In northern temperate lakes,it is particularly important to reduce the abundance ofbream, as bream, as well as reducing the abundance of zooplankton, also markedly reduces the number of benthicinvertebrates (Andersson et al. 1978; Brönmark et al. 1997).The loss of benthic invertebrates may have a serious impacton perch, whose growth largely depends on and is positivelycorrelated with the abundance of macroinvertebrates(Persson 1983; Diehl 1993). Thus, at high bream abundance,a competitive bottleneck at the macroinvertebrate feedingstage occurs (Persson & Greenberg 1990), preventingperch from reaching the predatory stage. By removingbream, the growth of perch increases. This has been illus-trated by Danish experiments where perch, in only 2 years,reached the size usually obtained over a 5-year period in atypical Danish lake (Müller & Jensen unpubl. obs., 1998).

Finally, when searching for food in the sediment, roach andbream may also have a direct, negative influence on sus-pended matter and lake turbidity because of the resuspen-sion of sediment (Breukelaar et al. 1994; Tátrai et al. 1997)or enhanced nutrient release (Brabrand et al. 1990; Havens1991).

Experience regarding the stocking of pike fry is lesscomprehensive and in most cases only relatively low propor-tions have been stocked. It appears though, that if stockedin large numbers, cascading effects on lower trophic levelscan be achieved, primarily in the year of stocking.Experiments from Lake Lyng showed that pike abundancedid not depend on the number of pike stocked the previousyear (Berg et al. 1997; Søndergaard et al. 1997). The reasonis probably, as also shown in the Netherlands (Grimm &Backx 1990), that the size of the pike stock primarilydepends on the number of habitats, especially that of thelittoral and macrophyte-covered zones. Restoration by pikestocking should therefore primarily be considered in lakes

Lake Restoration in Denmark 155

Table 2. Effects on the fish stock after fish removal

Intensive fish removal over a short-term period Long-term but low intensity fish removal Modest fish removal

The growth rate of the remaining fish, Gradual reduction of the biomass Poor or no effect

especially perch, increases following proportion of bream. The biomass,

improved water transparency. however, remains high compared

with intensively fished lakes.

Perch often becomes the dominant

predatory fish, whereas the response of Occasionally increased percentages

pike remains unclear. of piscivores, especially caused

by increased biomass of perch.

The proportion of predatory fish Usually, however, the share of piscivores

increases significantly during the first remains below 20%.

1–2 years following fish removal because of

the increased abundance of perch.

Table 3. Effects on different trophic levels recorded after intensive fish removal

Parameter Effects

Zooplankton Increased abundance of large-sized species

Increased grazing pressure on phytoplankton

Phytoplankton Reduced abundance

Invertebrates Increased abundance

Increased food supply for perch

Submerged macrophytes Gradually higher distribution depending on i.a. depth conditions, waterfowl grazing and seed bank.

Waterfowl Increased abundance – including species feeding on submerged macrophytes (e.g. mute swan

(Cygnus olor) and coot (Fulica atra))

Nutrient content Declining concentrations caused by increased retention

Secchi depth Increased Secchi depth

where a few years of stocking and improved transparencycan lead to a shift in the biological structure towards onethat maintains a clear-water state (e.g. colonization of sub-merged macrophytes). Likewise, the results from Lake Lyngshowed that stocking densities must be much higher thanthe natural stock if any impact on other trophic levels is tobe achieved (Søndergaard et al. 1997). The results observedin Denmark and elsewhere (Meijer et al. 1995; Prejs et al.1997) indicate that stocking of at least 0.1 individualsm–2 year–1 is required, but more information is needed to optimize stocking strategies (Skov & Berg unpubl. data, 1999). Also, the potential of using piscivores seems highest if used in association with fish removal to impedemassive growth of YOY fish (Perrow et al. 1997; Hansson et al. 1998).

Lake water nutrient concentrations also seem susceptibleto fish manipulation. Often, both in-lake nitrogen and phos-phorus decrease markedly and retention increases if clear-water conditions are obtained (Søndergaard et al. 1990;Jeppesen et al. 1998), which has a positive effect on the waterquality of downstream lakes or fjords. The reasons for thecause in higher retention have not yet been identified, butvarious factors may be involved (Wright & Shapiro 1984;Jeppesen et al. 1998), including improved light conditionsand the fact that increased benthic primary productionexceedingly impedes phosphorus release from the sediment(Hansson 1992; Van Luijn et al. 1995).

When evaluating the effects of macrophyte implantation,it must be taken into consideration that the implantationexperiments usually only covered a very modest part of thetotal lake area. So far, no evident impact of implantation hasbeen found in the study of lakes, but the results of variousother Danish investigations suggest that plant-eating water-fowl may delay and/or impede macrophyte dispersal. In anumber of exclosure experiments, Lauridsen et al. (1993,1994) and Søndergaard et al. (1996, 1998) showed that thegrowth of unprotected plants was much lower than plantsprotected against waterfowl grazing. Lake Engelsholm,provided evidence that even when waterfowl densities are relatively low, macrophytes may be subjected to a considerable grazing pressure. There is thus ample reason to indicate that plant-eating birds may delay the re-establishment of submerged macrophytes. Furthermore,even when macrophytes are established, herbivory by birdsmay enhance the probability of a transition back to the phytoplankton-dominated stage by favouring inedible plantspecies with a shorter growing season and lower stabilizingeffects on the clear-water stage (Janse et al. 1998). Theimpact of macrophytes on a stabilization of the clear-waterstate seems considerable, especially when the plant volume infested with submerged macrophytes exceeds

approximately 20% (Schriver et al. 1995; Meijer et al. in press,1999). However, in a study on diurnal horizontal migrationof zooplankton in a 21 3 104 m2 shallow lake, Lauridsen et al. (1996) estimated that the establishment of a 3% coverage with 2 m diameter patches of dense Potamogetonpectinatus would be sufficient to double the density ofCeriodaphnia spp. and Bosmina longirostris in open water atnight, which would subsequently have a significant impacton zooplankton grazing on phytoplankton.

Danish experience with physicochemical methods islimited. The sediment removal in Lake Brabrand led to anincrease in water depth and a reduction of phosphorusrelease from the lake bottom. Although the lake still suffersfrom internal loading, both the duration and extent of theinternal phosphorus loading seemed to decline significantlyafter the intervention, compared with other lakes where theexternal loading has been reduced (Jørgensen 1998). Thelake remains, however, in the turbid state because the exter-nal nutrient loading has not been reduced sufficiently.Oxidation of the hypolimnion in Lake Hald has had a markedeffect on the oxidation level and internal phosphorusloading and together with a reduction of external loadingoccurring simultaneously with the oxidation, this has led to higher transparency. Recent results, however, indicate that further oxidation beyond the now finished 12-yearperiod is necessary to avoid increased internal loading (K. Rasmussen, pers. comm., 1999). Hypolimnetic nitrateaddition in Lake Lyng showed that it is possible to limit theinternal release and accumulation of phosphorus in thehypolimnion, even when using relatively low doses of nitrate(Søndergaard et al. unpubl. data, 2000), but if permanenteffects are to be obtained, the treatment should probably be continued.

CONCLUSIONSAn important prerequisite for a successful and stablerestoration intervention in northern-temperate shallowlakes seems to be that lake nutrient loading should bebrought to a level of 0.05–0.1 mg P L–1 under equilibriumconditions, as previously concluded (Jeppesen et al. 1990,1999). The probability of a successful intervention isexpected to increase with declining nutrient levels. Clear-water conditions may be obtained even at high nutrientconcentrations, but the risk of a return to the turbid state ishigh if the intervention is not continued.

By using biomanipulation in northern-temperate lakes,approximately 80% of the prey fish should be removed overa 1–2-year period, if increased growth of the remaining fish stock is prevented and if significant effects are to be obtained. The stock of zooplanktivorous fish needs to bereduced below approximately 10 g m–2. Stocking of pike fry

156 M. Søndergaard et al.

to combat the YOY of planktivorous fish must be extensive(0.1 m–2) if effects cascading to lower trophic levels are tobe obtained. Also, pike fry stocking is only effective the yearin which it is made and must therefore be repeated until stabilization is achieved. Generally, the long-term stabilityof biomanipulated restoration is still very poorly elucidated,locally and internationally. One of the future challengeswithin this field is to determine the stability of the clear-waterstate, taking into account the often significant interannualvariations in fish recruitment and growth of submergedmacrophytes mediated by, for instance, variations in climate.

Experience with implantation of submerged macrophytesindicates that protection against waterfowl grazing in the early phase of implantation can be useful. Danishexperience with large-scale sediment removal andhypolimnetic oxygenation is limited. The results seem toconfirm other findings showing that it is possible to reduce both the duration and size of internal nutrientloading, but that hypolimnetic oxygenation needs to beconducted for many years in order to gain permanent effects.

ACKNOWLEDGEMENTSThe assistance of the technical staff of the NationalEnvironmental Research Institute, Silkeborg, Denmark, isgratefully acknowledged. The authors also wish to thankfield and laboratory assistance provided by L. Hansen, J.Stougaard-Pedersen, B. Lausten, J. Glargaard. K. Jensen, L. Nørgaard, K. Thomsen and S. B. Nielsen from theNational Environmental Research Institute, Denmark.Manuscript and linguistic assistance was provided by A. M. Poulsen. The authors also wish to thank the Danish Counties for access to some of the data used in theanalyses.

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