Reinforcing Recovery

To reinforce recovery, numerous physicochemical and biological restoration methods have been developed. Here we present the most frequently applied methods and an overview of some of the syntheses and notable case studies on each method (Table 1). Informative books or overview papers on the response of lakes to nutrient loading reduction and restoration are listed in 'Further reading' section.

Physico-Chemical Methods

Various physicochemical methods have been used to reduce internal P loading. These include sediment removal in shallow lakes and chemical treatment of the sediment with alum, calcium, or iron salts in both deep and shallow lakes. In deep lakes, injections with oxygen or nitrate to the bottom layer or continuous destabilization of the thermocline by effective circulation of the water column have been employed. Also withdrawal of hypolimnion water or flushing has been used in several case studies.

Lake ordered by increasing tw

Low/natural input

Increased loading

Reduced loading

Increased loading

Reduced loading

Total phosphorus (mg P g-1 DW)

Figure 1 Ratio of annual mean total phosphorus (TP) concentration measured and predicted in the surface water of lakes at maximum nutrient loading and 5, 10, 15, and 20 years after loading reduction (a); hydraulic retention time (tw) and mean depth (Zmean) of the lakes (b); and slope (mean ± SE) of linear regressions (forces through the origin) of observed versus predicted annual mean TP concentration in different years following loading reduction in shallow (c) and deep lakes (d). The slopes for 5 and 10 years in shallow

Total phosphorus (mg P g-1 DW)

Figure 1 Ratio of annual mean total phosphorus (TP) concentration measured and predicted in the surface water of lakes at maximum nutrient loading and 5, 10, 15, and 20 years after loading reduction (a); hydraulic retention time (tw) and mean depth (Zmean) of the lakes (b); and slope (mean ± SE) of linear regressions (forces through the origin) of observed versus predicted annual mean TP concentration in different years following loading reduction in shallow (c) and deep lakes (d). The slopes for 5 and 10 years in shallow

Sediment removal by dredging Dredging is an efficient, but a relatively costly technique to reduce an internal loading problem when the right equipment is used (cutterhead suction dredging is often efficient: Figure 2). It may also serve to deepen a lake that is gradually filling in. Due to the potential content of toxic substances in the sediment a key problem of dredging is how to dispose of the sediment. Other concerns are to obtain sufficient storage capacity during dredging, disturbance of wildlife during the process and release of toxic substances. Also, redistribution of sediment during the dredging period may take several years depending on lake size. Typically, the upper 20-60 cm is removed, sometimes modified so that more sediment is removed from TP 'hot spot' areas.

Sediment removal may also be used following an entire or partial water-level drawdown. Removal of sediment has in many cases reduced the internal loading immediately and substantially - a pioneering and notable example is that of Lake Trummen in Sweden. However, long-term success has frequently been hindered by a continuously too high external loading. Sediment removal is probably the most reliable method to reduce internal loading, as it permanently removes the source of nutrient loading. Other methods designed to eliminate internal loading have often proved less stable in the long term. Dredging is most useful in shallow lakes and reservoirs, in the latter preferably combined with water level drawdown. However disposal of dredged sediment may be a problem at places as the concentration of various toxic substances may exceed critical levels.

Hypolimnetic withdrawal and flushing Hypolim-netic withdrawal aims at removing nutrient-enriched hypolimnion water to maximize nutrient concentrations exported in the lakes outlet. Coincidentally, the retention time in the hypolimnion is shortened, reducing the risk of oxygen depletion. Typically, a pipe is installed in the deepest part of the lake with an outlet downstream below the lake level enabling the pipe to act as a siphon. Destratification and lake level reduction should be avoided by using a low outflow rate from the pipe. The method has generally generated good results. In most cases, hypolimnion TP decreases and so does the depth of anoxia layer, leading to reduced internal loading. After some years also the epilimnion TP will typically decline, the effect being stronger the longer the pipe has been in action. The drawback of the method is potential pollution of downstream systems (high loading of TP, ammonia, low oxygen concentration and occasionally an obnoxious smell of H2S). The most notable case is Lake Kortowo, Poland, where hypolimnion withdrawal has been conducted since 1956 (it was the first example) and is still ongoing due to still too high external loading. The method is particularly easy to apply in reservoirs where withdrawal can be established at the dam, but is also useful in summer stratified lakes. An alternative method is flushing of lake water with water, low in nutrient or rich in binding substances like iron or calcium, which are added at the time when the lake water concentration is high. Examples are Green Lake, Seattle, USA and Lake Veluwe, The Netherlands.

Aluminum, iron, and calcium treatment This method aims at supplying new sorption sites for phosphate onto the surface sediment. Phosphate adsorbs readily to calcite (CaCO3) and hydroxides of oxidized iron (Fe3+) and aluminum (Al3+). Phosphate precipitation with calcite has been used in hardwater bodies. An example is Frisken lake, British Columbia, Canada. However, the method appears unpredictable because pH often drops to below 7.5 in the sediment at which level calcite is dissolved. Phosphate adsorption onto Fe3+ and Al3+ is widely used to precipitate P in waste water treatment plants. Here, a molar (e.g., Al:P) precipitation ratio of 1:1 can be obtained due to high concentrations of phosphate when the ions are added, but in lakes the metal ions will normally form hydroxides with a lesser binding capacity for lakes and after 5 years in deep lakes were significantly different from the slopes for years 0 and 15 and 10 years in deep lakes (A). From Jeppesen etal., 2005 (full reference in Further Reading). Conceptual scheme describing the changes in retention and internal loading of phosphorus in a shallow lake with changing external loading. Different thickness of arrows indicates different loading and release rates (B). Sediment profiles of total phosphorus (TPsed) in Lake Sobygaard in 1985 (square), 1991 (triangle), and 1998 (star) (C). The sediment phosphorus profile of Lake Sobygaard changed markedly during 13 years. In the upper 25-30 cm of the sediment, TPsed has decreased at all depths. During the first 6 years, phosphorus was primarily released from the very high concentrations found at 15-20 cm depth, but during the latter 7 years TPsed has decreased at all depths. At most depths down to 25-30 cm, TPsed has been reduced by 3-4 mg P g-1 DW. Calculations based on comparisons of the 1985 and 1998 profiles show that a total of 57 g P m-2 has been released from the upper 20 cm sediment. In the same period, mass balance measurements show a total release of approximately 40 g P m-2. At the present release rate this means that another 15-20 years will pass before the lake will be in equilibrium, implying that the transient phase after reduced external loading in total will last for more than 30 years. Sediment cores were sampled from a central location and sectioned into 2-cm slices. Sediment from three different cores was pooled into one sample before analysis. Sediment profiles were adjusted to the 1985 level using a sedimentation rate of 0.6 cm years-1. From Sondergaard M, Jensen JP, and Jeppesen E (1999 ) Internal phosphorus loading in shallow Danish lakes. Hydrobiologia 408/409, 145-152.

Table 1 Some key references to papers on the different restoration methods discussed in the paper

Methodology Reference

Physical methods

Water level drawdown Beard TD (1973) Owenwinter drawdown. Impact on the aquatic vegetation in Murphy Flowage,

Wisconsin. Technical Bulletin No. 61. Wisconsin Department of Natural Resources, Madison.

Ploskey GR, Aggus LR, and Nestler JM (1984) Effects of water levels and hydrology on fisheries in hydropower storage, hydropower mainstream and flood control reservoirs. Technical Report E-84-8, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS., NTIS No. AD A146 239.

Randtke SJ, de Noyelles F, Young DP, Heck PE, and Tedlock RR (1985) A critical assessment of the influence of management practices on water quality, water treatment and sport fishing in multi-purpose reservoirs in Kansas. Kansas Water Resources Research Institute, Lawrence.

Hypolimnion withdrawal or Dunalska JA, Wisniewski G, and Mientki C (2007) Assessment of multi-year (1956-2003) hypolimnetic flushing withdrawal from Lake Kortowskie, Poland. Lake and Reservoir Management 23, 377-387.

Hosper H (1985) Restoration of Lake Veluwe, The Netherlands by reduction of phosphorus loading and flushing. Water Science and Technology, 17, 757-768

Lathrop RC, Astfalk TJ, Panuska JC, and Marshall DW (2004) Restoring Devil's Lake form the bottom up. Wisconsin Natural Resources 28, 4-9.

McDonald RH, Lawrence GA, and Murphy TP (2004) Operation and evaluation of hypolimnetic withdrawal in a shallow eutrophic lake. Lake and Reservoir Management 20, 39-53.

Nurnberg GK (1987) Hypolimnetic withdrawal as lake restoration technique. Journal of Environmental Engineering 113, 1006-1016.

Destratification Fast AW (1973) Effects of artificial destratification on primary production and zoobenthos of El

Capitan Reservoir, California. Water Resources Research 9, 607-623.

Gachter R (1987) Lake restoration. Why oxygenation and artificial mixing cannot substitute for a decrease in the external phosphorus loading. Schweizerische Zeitung fur Hydrologie 49, 170-185.

Pastorak RA, Lorenzen MW, and Ginn TC (1982) Environmental aspects of artificial aeration and oxygenation of reservoirs: a review of theory, techniques and experiences. Technical Report No. E-82-3. U.S. Army Corps of Engineers, Vicksburg, MS.

Sediment removal Bjork S (1974) European Lake Rehabilitation Activities. Institute of Limnology Report. Sweden:

University of Lund.

Hinsman WJ and Skelly TM (1987) Clean lakes Program Phase 1 Diagnostic/Feasibility Study for the Lake Springfield Restoration Plan. Springfield City Water, Light and Power, Springfield, IL.

Fast AW, Moss B and Wetzel RG (1973) Effects of artificial aeration on the chemistry and algae of two

Michigan lakes. Water Resources Research 9, 624-647. Gachter Rand Wehrli B (1998) Ten years of artificial mixing and oxygenation: No effect on the internal

P loading of two eutrophic lakes. Environmental Science and Technology 32, 3659-3665. McQueen DJ and Lean DRS (1984) Aeration of anoxic hypolimnetic water: effect on nitrogen and phosphorus concentrations. Verhandlungen der Internationale Vereinigung der Limnologie 22, 268.

Pastorak RA (1981) Prey vulnerability and size selection by Chaoborus larvae. Ecology 62, 1311-1324. Noon TA (1986). Water quality in Long Lake, Minnesota, following Riplox sediment treatment. Lake and Reservoir Management 2, 131. Ripl W (1976) Biochemical oxidation of polluted lake sediment with nitrate - A new restoration method. Ambio 5, 132-135. Ripl W (1986) Internal phosphorus recycling mechanisms in shallow lakes. Lake and Reservoir Management 2, 138.

Sondergaard M, Jeppesen E, and Jensen JP (2000) Hypolimnetic nitrate treatment to reduce internal phosphorus loading in a stratified lake. Lake and Reservoir Management 16, 195-204.

Chemical methods

Iron Boers PJ, Vand der Does J, Quaak M, and Van der Vlugt J (1994) Phosphorus fixation with iron(III)

chloride: A new method to combat internal phosphorus loading in shallow lakes? Archiv fur Hydrobiologie 129, 339-351.

Hayes CR, Clark RG, Stent RF, and Redshaw CJ (1984) The control of algae by chemical treatment in a eutrophic water supply reservoir. Journal of the Institute of Water and Engineering Science 38, 149-162.

Young SN, Clough WT, Thomas AJ, and Siddall R (1988) Changes in plant community at Foxcote Reservoir following use of ferric sulphate to control nutrient levels. Journal of the Institute of Water and Engineering Science 2, 5-12.

Oxygenation Air/oxygen

Nitrate

Continued

Table 1 Continued

Methodology Reference

Aluminum Barko JW, James WF, Taylor WD, and McFarland DG (199G) Effects of alum treatment on phosphorus and phytoplankton dynamics in Eau Galle Reservoir: A synopsis. Lake and Reservoir Management 4, 63-72.

Rydin E, Huser B, and Welch EB (2GGG) Amount of phosphorus inactivated by alum treatments in Washington lakes. Limnology & Oceanography 1, 226-23G.

Jernelov A (Ed.) (1971) Phosphate reduction in lakes by precipitation with aluminum sulphate. In: 5th International Water Pollution Research Conference, San Francisco, CA, 26 July-1 May 197G. New York: Pergamon Press.

Reitzel K, Hansen J, Andersen F0, Hansen KS, and Jensen HS (2GG5) Lake restoration by dosing aluminum relative to mobile phosphorus in the sediment. Environmental Science and Technology 39, 4134-414G.

Smeltzer E, Kirn RA, and Fiske S (1999) Long-term water quality and biological effects of alum treatment of Lake Morey, Vermont. Lake and Reservoir Management 15, 173-1S4.

Welch EB and Cooke GD (1999) Effectiveness and longevity of phosphorus inactivation with alum. Lake and Reservoir Management 15, 5-27.

Biological methods

Fish removal/stocking Benndorf J (1995) Possibilities and limits for controlling eutrophication by biomanipulation.

Internationale Revue der gesamten Hydrobiologie SG, 519-534.

Drenner RW and Hambright KD (1999) Review: Biomanipulation of fish assemblages as a lake restoration technique. Archiv fur Hydrobiologie 146, 129-165.

Gulati RD, Lammens EHHR, Meijer M-L, and van Donk E (199G) Biomanipulation, tool for water management. Hydrobiologia 2GG/2G1, 1-62S.

Hansson L-A, Annadotter H, Bergman E, Hamrin SF, Jeppesen E, Kairesalo T, Luokkanen E, Nilsson P-A, S0ndergaard M and Strand J (199S) Biomanipulation asan application of food-chain theory: constraints, synthesis, and recommendations for temperate lakes. Ecosystems 1, 55S-574.

Lazzaro, X. (1997). Do the trophic cascade hypothesis and classical biomanipulation approaches apply to tropical lakes and reservoirs? Verhandlungen Internationale Vereinigung der Limnologie 26, 719-73G.

McQueen DJ (199S) Freshwater food web biomanipulation: A powerful tool for water quality improvement, but maintenance is required. Lake and Reservoir Management 3, S3-94.

Mehner T, Benndorf J, Kasprzak P, and Koschel R (2GG2) Biomanipulation of lake ecosystems: Successful applications and expanding complexity in the underlying science. Freshwater Biology 47, 2453-2465.

Meijer ML, De Boois I, Scheffer M, Portielje R, and Hosper H (1999) Biomanipulation in Shallow Lakes in the Netherlands: An Evaluation of 18 Case Studies. Hydrobiologia 4G9, 13-3G.

Perrow MR, Meijer M-L, Dawidowicz P, and Coops H (1997) Biomanipulation in shallow lakes: State of the art. Hydrobiologia 342/343, 355-365.

Shapiro J (1979) The need for more biology in lake restoration. In: U.S. Environmental Protection Agency National Conference on Lake Restoration. USEPA44G/5-79-GG1. pp. 161-167.

Macrophyte protection/ Cooke GD, Welch EB, Peterson SA and Nichols SA (2GG5) Restoration and management of lakes and transplantation Reservoirs, 3rd edn. Boca Raton, FL: Taylor & Francis.

Lauridsen TL, Sandsten H, and M0ller PH (2GG3) The restoration of a shallow lake by introducing Potamogeton spp. The impact of waterfowl grazing. Lakes & Reservoirs: Research and Management S, 177-187.

Smart RM, Dick GO, and Doyle RD (1998) Techniques for establishing native aquatic plants. Journal of Aquatic Plant Management 36, 44-49.

S0ndergaard M, Bruun L, Lauridsen TL, Jeppesen E, and Vindbœk Madsen T (1996) The impact of grazing waterfowl on submerged macrophytes. In situ experiments in a shallow eutrophic lake. Aquatic Botany 53, 73-84.

Macrophyte removal Engel S (199G) Ecological impacts of harvesting macrophytes in Halverson Lake, Wisconsin. Journal of Aquatic Plant Management 28, 41-45.

Pieterse A and Murphy K (Eds.) (199G) Aquatic Weeds. The Ecology and Management of Nuisance Aquatic Vegetation. Oxford, UK: Oxford University Press.

phosphate. The formation of tri-valent hydroxides from reaction with water results in a marked drop in lake water pH, depending on the alkalinity level. Indeed, lake water alkalinity limits the amount of acid salts that can be added since pH has to be in the range of 5.5-9 to obtain hydroxide floc formation. Thus, in most cases of aluminum treatment of lakes the dose has been determined by the lake water alkalinity to avoid a pH drop to below 6 (for safety reasons).

Of the two hydroxides, iron has the higher affinity for phosphate, and P release from oxic sediment surfaces is often controlled by Fe3+ when present in a molar ratio higher than 8:1 relative to potentially mobile phosphate. Meanwhile Fe3+ is reduced to Fe2+ in oxygen-depleted sediments and, so far, all documented examples of iron addition have shown a time-limited effect. Also, Fe3+ is a high quality electron acceptor for bacterial respiration (as NO3 and O2) which may stimulate mineralization, releasing organic P that would otherwise be buried in the reducing sediment.

Contrary to Fe3+, Al3+ is stable independent of redox conditions. Only, the 'pH window' is narrower (6-9) due to formation of soluble hydroxides at lower or higher levels. Therefore, aluminum should normally not be applied to sediments that may be resus-pended in epilimnetic water where pH is high. A fresh Al(OH)3 floc strips lake water for phosphate while sinking to the bottom, but with ageing the floc loses some 75% of its binding capacity and affinity for phosphate due to crystal (gibbsite) formation. Still, it seems that permanent immobilization of P can be reached with a 10:1 ratio of Al and P. Al addition has been used for restoration in some 120 lakes in USA and Europe and the longevity of positive effects varies from a few to 10-15 years. Factors that may influence P adsorption onto Al(OH)3 are high concentrations of humic acids and silicate (e.g., in pore-water). Still, distinct layers of Al and aluminum-bound P can be found in the sediment decades after treatment and the longevity of improved conditions is, therefore, most likely limited by continuously high external P-loading, for example from diffuse run-off.

Of the three chemicals listed here, treatment with aluminum (Figure 3.) seems most promising and at the same time it provides a cost-efficient solution compared with sediment dredging. Toxic effects of the Al3+ ion in treated lakes have only been reported in cases where the application technique failed and pH dropped below 6 during the treatment. If chemical treatment is to be successful it generally requires that the main problem is a very limited wash out of excess P during internal loading events and that the overall annual external P-loading is low; otherwise, the new sorption sites added to the sediment will be rapidly saturated.

Hypolimnetic oxidation This method employs addition of oxidizers to the hypolimnion to improve the redox sensitive sorption of phosphate to iron in the sediment and thereby reduce internal P loading. Most often oxygen is used (an example in Figures 4 and 5), but alternatively electron acceptors such as nitrate can be applied. The presence of both oxygen and nitrate will maintain a redox potential above approximately 200 mV where iron is in its oxidized form (Fe3+). Both oxygen and nitrate can oxidize reduced iron species present in the sediment by microbial metabolism. Oxygenation may also serve to improve the living conditions for fish and invertebrates.

Oxidation has been conducted using different techniques: oxygen is added either as pure oxygen or as atmospheric air. Oxygen or atmospeheric air may be

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Figure 3 Examples of aluminium treatment. In Shadow Lake (WI, USA) aluminium (5.7 g Al m 3) was applied to hypollmnetlc water In 1978. The dose was calculated from lake water alkalinity and resembled approx. 11 g Al m-2. Concomitantly external P loading was reduced by 58%. TP declined instantly from 0.06 to 0.02 mg l-1 and phosphate from 0.02 to 0mg l-1. In 1988 concentrations had injected via a number of diffusers creating fine bubbles at the deepest part of the lake or hypolimnetic water may be pumped on land where it is oxidized before being returned to the deep waters. Also, an

'oxidation tower' that circulates hypolimnetic water while oxidizing it has been used in German lakes. When using nitrate, a liquid solution of nitrate is added by stirring it into the upper sediment layer or

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(a) 1950 1960 1970 1980 1990 2000 (b) 1950 1960 1970 1980 1990 2000

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Figure 4 Annual changes in oxygen concentrations close to the bottom of Lake Baldegg, Switzerland, before and after initiation of oxygenation in 1982 (upper). Below are the changes in P content at overturn (dots) and seasonal changes of hypolimnetic (z > 20 m) P content (lines) and annual P deposition rates derived from freeze core analyses. From Gachter R and Wehrli B (1998 ) Ten years of artificial mixing and oxygenation: No effect on the internal phosphorus loading of two eutrophic lakes. Environmental Science & Technology 32: 3659-3665.

increased to around 50% of the pre-treatment values, likely reflecting the reduced external loading (a). Reproduced from Cooke et a/. (2005) (full reference in Further Reading). In Lake Sonderby (Denmark), the aluminium dose was calculated from potentially mobile P (P in lake water and loosely adsorbed P, iron-bound P, and NaOH-extractable organic P in the surface 10 cm of the sediment) by assuming that a 4:1 Al:P ratio would immobilize the pool. The dose was 31 g Al m~2. TP declined from 1.60 to 0.08 mg r1 and phosphate from 1.50 mg r1 to 0 in the year after treatment. TP and seasonal internal loading increased slowly during the following 5yearsand reached a TP value of 0.3 mg r1 in summer 2006. Lake Sonderby had 15 times higher lake water P concentrations than Shadow Lake and was probably under-dosed since the ratio of Al:P should have been 10:1. Shadow Lake probably received a higher dose than needed to immobilize the P pool in water and sediment, but burial of the reactive aluminium may limit the effectiveness for trapping new P to a few years. Slowly, a new surface sediment with a reactive P pool built up in both lakes as new P is supplied from external sources and (at least in Lake Sonderby) from deeper sediment layers (b). Sequential chemical extraction is used to quantify P pools and reactive Al and Fe in sediments. Depth distributions of iron-bound P (PBD), aluminium-bound P (PNaOH), total sediment P (TP), and eactive aluminium (AlNaOH) in Lake Sonderby demonstrate accumulation of aluminium-bound P in the surface 4 cm following the aluminium treatment in 2001. No change in iron-bound P has been observed (c). Reproduced from Reitzel et a/. (2005 , full reference in Table 1).

by injecting it into the water just above the sediment. The advantage of using nitrate instead of oxygen is that much higher oxidation equivalents can be added because being a salt nitrate is more soluble than oxygen (a gas). Thus, nitrate will penetrate the sediment to a greater depth than oxygen. In contrast to oxidation of bottom water by oxygen, the objective of using nitrate is not only to oxidize the surface sediment, but also to increase the turnover of organic matter via denitrification and, with it, rapidly reduce the sediment oxygen consumption and increase its P binding potential.

The results obtained for oxidation are variable. While some oxidation experiments have clearly led to lower accumulation of P and reduced elements (e.g., ammonium) in the hypolimnion, others showed neither reduced release of P from the sediment nor its higher retention due to increased hypolimnetic dissolved oxygen concentrations. The explanation is that oxygenation enhances P retention only if the sulphide production is lowered and more ferrous phosphate (e.g., vivianite) and less FeS are deposited in the anoxic sediment. Also, oxygenation needs to be conducted for many years. Basically, hypolimnetic oxygenation treats the symptoms rather than the underlying and fundamental problems, namely the sediment's large consumption of oxygen and the release of

P from a mobile pool. Only when the redox conditions are permanently improved or the P pool is buried deep down the sediment, can the sediment oxygen consumption be expected to decline, leading to increasing and more permanent P retention. The method is potentially applicable to eutrophic deep and stratified lakes.

Water level alterations Water level management has been used extensively as a tool to improve the habitats for waterfowl and promote game fishing and water quality. The ultimate regulation is a complete draw-down, which has been used to control nuisance plant growth. It may also facilitate a shift to clear-water conditions in nutrient-rich turbid lakes, at least in the short term, as drying out may consolidate the sediment. Moreover, fish kills mediated by the drawdown enhance zooplankton grazing on phytoplank-ton, which in turn improves water clarity and thus growth conditions for submerged macrophytes and food resources for waterfowl. Changes in the water level may also influence lakes indirectly by affecting fish recruitment. Lack of flooding of marginal meadows in spring has been suggested as an important factor for poor recruitment of pike (Esox lucius) in regulated lakes. Short-term partial draw-down has been used to improve game fishing, which enhances the biomass and size of predatory fish at the expense of planktivorous and benthivorous fish. This may be because the lower water table augments the predation risk or that it dries the fertilized eggs in their early ontogeny. Water level regulation as a restoration tool may be particularly useful in shallow lakes and in reservoirs.

Biological Methods

In recent years a number of biological methods (often termed biomanipulation in the literature) have been developed of which especially fish manipulation has been widely used.

Fish manipulation Various methods have been used to overcome biological resistance. The typical measure is removal of plankti-benthivorous fish. This method has been extensively used in north temperate lakes in Europe, such as The Netherlands and Denmark, during the past 20 years. Removal of approx. 75% of the planktivorous and benthivorous fish stock during a 1-2 year period has been recommended to avoid regrowth and to stimulate the growth of potentially piscivorous fish (Figure 6). A simple, feasible strategy of fish removal is to catch passive fish with active gear and active fish with passive gear using information on the seasonal behavior of fish, such as spawning or foraging migration and shoaling of the target species. An alternative or supplementary method to fish removal is stocking of piscivores. The basic tools are stocking with nursery or pond-raised fingerlings, catch regulations and habitat management. Stocking with pelagic species (zander, Stizostedion lucioperca, walleye, Sander vitreum) should be accompanied by littoral species (pike, largemouth bass, Micropterus salmoides). Otherwise, the nursery areas of the target species are not affected. Stocking of zander or walleye should be accompanied by catch and mesh size limits for fishing.

Dramatic, cascading short-term effects are generally achieved in eutrophic lakes and reservoirs with efficient fish reduction in the form of reduced phytoplankton biomass, dominance by large sized zooplankton and improved transparency (Figures 6 and 7). The effects of fish manipulation may cascade to the nutrient level as well. From 30% to 50% reduction in lake concentrations of TP has been recorded in the relatively more successful fish manipulation experiments in shallow and stratified eutrophic lakes. So far, the long-term perspectives are less promising. A gradual return to the turbid state and higher abundance of zooplanktivorous fish after 5-10 years have been reported in many case-studies; however, also long term or permanently lower abundance of benthivores, such as bream (Abramis brama), has occurred, resulting in improved water clarity in the long term. It is therefore recommended to repeat the fish removal at intervals to maintain the clear-water state, expectedly involving a gradual diminishing of efforts. A drawback of the method is that the lakes retain more P in the clear-water state after manipulation and therefore have more P available for internal loading if the system returns to the turbid state than if fish manipulation had not been conducted. Stocking of piscivorous fish has often been less successful than fish removal.

Fish manipulation is potentially useful for all lakes and is probably particularly effective in the temperate zone when TP has reached concentrations below 0.05 mg P in shallow lakes and below, say, 0.02 mg P in deeper lakes and reservoirs. However, the exact values may vary along a gradient in climate and may also depend on the external N loading.

Protection of submerged macrophytes and transplantation Construction of exclosures to protect macro-phytes against waterfowl grazing has been employed as an alternative or supplementary restoration tool to fish manipulation. The exclosures enable the macrophytes to grow in a grazer-free environment from where they may spread seeds, turions or plant fragments augmenting colonization. Moreover, the exclosures also serve as a daytime refuge from fish predation for the zooplankton. The usefulness of plant refuges as a restoration tool is probably particularly high in small lakes although 11 ha enclosures have been established in the large Lake Wuli in China. Transplantation of plants or seeds is an alternative method. The methods described are particularly useful in shallow lakes.

Combating nuisance plant growth Although reestablishment of submerged macrophytes is the goal of many a lake restoration project, dense plant beds appearing in nutrient-enriched lakes may occasionally be considered a nuisance since they impede navigation and reduce the recreational value for anglers. Moreover, excessive growth of invading species, like the Eurasian milfoil, Myriophyllum spicatum, Pistia stratiotes, or Eichhornia crassipes in many lakes in the US, South America, and Africa, or the North American Elodea canadensis in Europe, may substantially alter lake ecosystems and constitute a serious threat to the native flora and fauna. Methods to combat such nuisance plant growth are manual harvesting, introduction of specialist phytophagous insects such as weevils or herbivorous grass carps (Ctenopharyngon idella), water level draw-down, coverage of the sediment with sheets, or chemical treatment with herbicides. Often, harvesting and water level draw-down have only a transitory effect because of fast regrowth of the plant community and

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0 20 40 60 80 100 Chlorophyll a (ig l-1)

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1 2 3 Transparency (m)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Chlorophyll a/total P

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Before

Before

Figure 6 Transparency, chlorophyll a, TP and chlorophyll a/TP ratio in lakes before and a few years after an effective fish removal (upper). Below the amount of fish removed during biomanipulation of eutrophic European lakes dominated by planktivorous and benthivorous fish. White circles denote the annual catch in cases with effective fish removal leading to an improvement in water quality (increased transparency and decline in biomass of cyanobacteria) at least in the short term or an increase in piscivorous perch. Black dots indicate lakes in which fish removal was too limited to have an effect on water quality or fish density. Data compiled by Jeppesen and Sammalkorpi (2002 ). In: Perrow MR and Davy AJ Handbook of Ecological Restoration, Vol. 2: Restoration in practice, pp. 297-324.

Figure 6 Transparency, chlorophyll a, TP and chlorophyll a/TP ratio in lakes before and a few years after an effective fish removal (upper). Below the amount of fish removed during biomanipulation of eutrophic European lakes dominated by planktivorous and benthivorous fish. White circles denote the annual catch in cases with effective fish removal leading to an improvement in water quality (increased transparency and decline in biomass of cyanobacteria) at least in the short term or an increase in piscivorous perch. Black dots indicate lakes in which fish removal was too limited to have an effect on water quality or fish density. Data compiled by Jeppesen and Sammalkorpi (2002 ). In: Perrow MR and Davy AJ Handbook of Ecological Restoration, Vol. 2: Restoration in practice, pp. 297-324.

high external loading. Grass carp may have a strong effect on plant growth and is currently used in many parts of the world to reduce macrophyte abundance, but a shift to a turbid state is a typical side effect (Figure 8). The method should therefore be used with caution. Moreover, before planning plant removal one has to bear in mind that these plants generally have a positive effect on the water clarity and biodiversity of the lakes.

Mussels and lake restoration Mussels are efficient filter feeders in lakes. Large unioids like Anodonta, Unio and Hyridella are sometimes abundant in well-mixed macrophyte-dominated lakes and can filter the entire water volume in a few days. They often disappear, however, in turbid lakes probably due to predation by fish larvae. Re-introduction of these species may therefore be a useful tool but it has so far received little attention. Also, the zebra mussel,

Figure 7 Mean (± 1 SD, n = 10) above-ground biomass (<1.7 m = black bars, >1.7 m = white bars) and cover (black squares) of the total submerged vegetation in areas above 3 m water depth at 10 sites in Lake Finjasjoen (12 km2 surface area), Sweden during the period 1992-97. Fish removal (393 kg ha-1) was conducted during 1992- 1994 (a). Bars or squares that share a common letter are not significantly different. Mean (± 1 SD, n = 10) outer depth of the submerged vegetation at 10 sites during the period 1992-97 (b). Squares that share a common letter are not significantly different. Relation between two submerged macrophyte variables (above-ground biomass and cover), and Secchi depth, chlorophyll a, TP and PO4-P for the years 1992-97. TP, Po4-P, Secchi depth and chlorophyll a are summer means (c). From Strand JA and Weisner SEB (2001) Dynamics of submerged macrophyte populations in response to biomanipulation. Freshwater Biology 46: 1397- 1408.

Figure 7 Mean (± 1 SD, n = 10) above-ground biomass (<1.7 m = black bars, >1.7 m = white bars) and cover (black squares) of the total submerged vegetation in areas above 3 m water depth at 10 sites in Lake Finjasjoen (12 km2 surface area), Sweden during the period 1992-97. Fish removal (393 kg ha-1) was conducted during 1992- 1994 (a). Bars or squares that share a common letter are not significantly different. Mean (± 1 SD, n = 10) outer depth of the submerged vegetation at 10 sites during the period 1992-97 (b). Squares that share a common letter are not significantly different. Relation between two submerged macrophyte variables (above-ground biomass and cover), and Secchi depth, chlorophyll a, TP and PO4-P for the years 1992-97. TP, Po4-P, Secchi depth and chlorophyll a are summer means (c). From Strand JA and Weisner SEB (2001) Dynamics of submerged macrophyte populations in response to biomanipulation. Freshwater Biology 46: 1397- 1408.

Dreissena polymorpha, which colonized Europe in the 19th century and more recently also in Great lakes in the United States, may have a major impact on water clarity when abundant. Significant effects on water clarity have also been shown in enclosure experiments. However, spreading Dreissena to other water systems is problematic as demonstrated in North America where lack of natural enemies in the rapid colonization phase allows Dreissena to reach enormous densities. This has resulted in significantly reduced chlorophyll of the Great lakes, but also in fouling of water intakes in reservoirs and uncontrolled impacts on the entire lake ecosystem. The method is potentially most useful in shallow lakes, but the long term perspective is not clear.

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