Clearing Lakes
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ClearingLake s anecosystem approach tothe restoration and managementof shallow lakes in theNetherlands CENTRALE LANDBO UW CATALO GU S 0000 0751 3092 Promotor: dr. L. Lijklema hoogleraar in het Waterkwaliteitsbeheer ^„^Ol.ûa-SH ClearingLake s an ecosystem approach tothe restoration and managementof shallow lakes in theNetherlands HarryHosper Proefschrift terverkrijgin g van degraa d van doctor op gezagva n derecto r magnificus van deLandbouwuniversiteit Wageningen, dr. C.M. Karssen, in het openbaar te verdedigen op 13 mei 1997 desnamiddag s tevie r uur in deAula . Ministry ofTransport , PublicWork s andWate r Management Institute for Inland Water Management andWast eWate r Treatment (RIZA) P.O. Box 17820 0A A Lelystad,th e Netherlands. ISBN 90-5485-682-3 Cover design: Harry Hosper 8cCarlo s Seguel BIBLIOTHEEK LANDBOUWUNIVERSITEIT WAGENINGEN ffiJot^o^ ÎASH Stellingen 1. Hoeweldiscussie sron d het Nederlandse mestbeleid wellichtander sdoe n geloven,i s de strijd tegen de 'groene soep' ind e Nederlandse meren en plassen nog niet verloren. 2. Bloeiva n draadvormigecyanobacterië n (zoals Oscillatoria agardhii) en overmaat aan planktivoree n benthivore vis,zij n degrootst e blokkadesvoo r ecologisch herstelva n meren en plassen, nadat deextern efosfaatbelastin g eenmaal isgereduceerd . 3. Hetster k uitdunnen van devisstan d inee n eenmalige wintervisserij, isvoo rkleinere , afgesloten plassen een effectieve en efficiënte maatregel voor bestrijding van algen- bloei,zelf s bijrelatie f hoge fosfaatgehalten. 4. Ingrot e meren- en plassengebieden kunnen reguliere wintervisserijen, gerichto p het verlagen van destan d aan planktivoree n benthivore vis,ee n bijdrage leveren aan de bestrijding van algenbloei. 5. pe Ecologische Hoofdstructuur (EHS) verdient tenminste dezelfde publiekee n politieke aandacht alsander e grote infrastructurele projecten, zoalsd e Betuwelijn, de Hoge Snelheidslijn (HSL)e nee n tweede Schiphol. 6. Deleu s 'Lelystad,ee n dijk vanee n stad'zo u sterk aan geloofwaardigheid winnen,e n waarschijnlijk zelfs overbodigzij n geweest, alsdez e stad aan dedij k was gebouwd. 7. Inee n 'open plan proces' maakt 'Jutlandisering' een goede kans hett e winnen van 'Nederland-Distributielan d' . 8. Voorkennismanagemen t geldtda the tfeitelijk e leiderschapza lligge n bijdiegen edi ed e relaties in een netwerk van mensen op de meest aantrekkelijke wijze weet te voeden. JosephW.M .Kessels ,He tCorporat eCurriculum ,Inaugurel eRed e2 3februar i 1996,Rijk sUniversitei tLeiden . 9. Voord eontwikkelin gva n de Europesegedacht e isee n 'eurotaai' belangrijker dan een 'euromunt'. 10.Symptoombestrijdin g wordtaantrekkelijker , alsd e oorzaken alzij n weggenomen. Stellingenbi jhe tproefschrif t 'Clearinglakes ,a necosyste mapproac ht oth erestoratio nan dmanagemen t ofshallo w lakesi nth eNetherlands' . HarryHosper ,Wageningen , 13me i 1997 Contents Abstract 6 1. General introduction 7 2. Shallowlake si n the Netherlands: searching for lake restoration objectives 15 3. Multi-lake studies: external nutrient loading and lakerespons e 25 4. Whole-lake studyo fVeluwemeer : lake flushing for control of Oscillatoriabloom s and internal phosphorus loading 47 5. Biomanipulation in shallowlakes : concepts,cas estudie s and perspectives 73 6. Whole-lake study of Wolderwijd: biomanipulation for promoting the clearwate r state 95 7. Guidinglak erestoratio n and management 117 References 135 Summary 153 Samenvatting 159 Met dank aan 165 Over deauteu r 167 Abstract Eutrophication of lakes isa major problem for water management in the Netherlands. Lake restoration has been focused on the control of external P loading from point sources. However, this approach did not result in the water quality desired. The algae-dominated turbid water state may be extremely stable, and then additional measures are necessary to remove certain 'blockages', such as: the persistent bloom of Oscillatoria algae, the bloom- mediated P release from the sediments, and the abundance of fish, preventing zooplankton and submerged macrophytes from developing. This thesisaddresses : (1) the lake conditions prior todisturbance , (2)th erelationship sbetwee nnutrien t loadingan dlak eresponse , (3) the control of P release from the sediments and Oscillatoriabloom s bylak e flushing and (4) the perspectives offis h stockmanagemen t for the restoration of shallowlakes . .. we have presented a version of eutrophication and of ecosystembehavior devoid of all assumptionsbut one, that phosphorus is important. Well, whatis the answer? Theanswer is that wetoo mustplugour ears andsail past theSirens. We must learn tojuggle all of the balls, ifnot atonce, thena few ata time... Joseph Shapiro (1979) Chapter1 . General introduction Identification of the problem Lake restoration is one of the major issues in water management in the Netherlands. Hundreds of millions of guilders are being spent yearly for nutrient control and additional lakerestoratio n measures (Anonymous, 1995).I nth efirs t halfo fthi scentury ,th emajorit y of shallow lakes was clear and the lake bottom was covered with vegetation. Now, most lakes lookmurk y andgree nbecaus eo fa nexcessiv ealga lgrowt han d theresuspensio n of sediments by fish and wind. Wildlife and recreational values have been severely negatively affected. Submerged plants have disappeared and the fish stock is characterized by large numbers of onlya fe wspecie so fpre yfish ,lik ebrea m and roach,an d relativelylo wnumber so f predatory fish,suc ha spike ,pike-perc h andperch .Eutrophication ,i.e .increase d nutrient loading,i sth e main cause for the deterioration of the lake ecosystems. Other perturbations, such as chemical pollution toxic to zooplankton, and the loss of lake-marginal wetlands, will have reinforced theeffect s ofnutrien t enrichment.Th elak erestoratio n strategyi n the Netherlands has been focused on reduction of the external phosphorus loading. However, so far the control of external loading has not resulted in the water quality desired (Van Liere &Gulati , 1992;D eDecker e etal, 1996). Theturbi d lakeecosyste m tendst ob eresistan tt orecover yan d solelyreducin go fexterna l nutrient loading seemed tob einsufficien t for attaining clearwate r conditions. A comprehensive approach to lake ecosystem functioning may provide additional tools for lake restoration. General concepts and scopeo f this thesis The development of the lake restoration strategy started with the 'Vollenweider approach', which aimed at criteria for external nutrient loading from the analysis of large numbers of different lakes (multi-lake studies) (Vollenweider, 1968). It then became clear that lake sediments, where most of the phosphorus (P) had accumulated, could act as a source of P, once the external loadingha d been reduced (Golterman, 1977).So ,th ereductio n in external loading may be counteracted by (enhanced) internal loading, giving a marginal response in water quality. Additionally, ecologists argued that the biotic structure of the turbid water ecosystem (e.g. large numbers of zooplankton-eating and benthos-eating fish and no submerged macrophytes) also contributes to the resistance of the lake to recovery (Shapiro, 1980;Moss , 1983). Shapiro etal.(1975 )ha d alreadyemphasize d thenecessit yo ftreatin glake s asecosystems ,rathe r than 'containers ofalga ean d phosphorus'. For understanding the algal dynamics and the mechanisms causing resistance in lakerecovery , an ecosystem approach is needed. Fig. 1.1 shows a simple model ofphytoplankto n biomass, as a result of production and loss processes. Production by photosynthesis is controlled by external and internal nutrient loading and the availability of sunlight. Loss processes are consumption by filter- Chapter 1 feeders (such as zooplankton and mussels), mortality (due to factors such as limiting resourceso r parasites),an d sinkingo falga et oth e sediments. Inflow and outflow are omitted in this model. nutrients Fig. 1.1Algal biomass isthe net result of production and lossprocesses. The algaear e part of the lakeecosyste m and the feedback from this ecosystem to the algaei s lackingi n the above model.Alga eaffec t the physical (e.g.wate r turbidity),chemica l (e.g.p H and oxygen levels) and biological lake conditions (e.g. food web structure, abundance of submerged macrophytes) and this effect is dependant on both the species composition and biomasso fth ealgae .Th echange dlak econdition s may,i n turn,promot e thealgae ,leadin g to a self-perpetuating process ofincreasin g algalbloom s (Fig. 1.2). © lake algae ecosystem Fig. 1.2 Interaction ofalgae and thephysical, chemicaland biologicalconditions of the lake ecosystem. Finally, light availability, which depends on lake depth (or mixing depth for stratified lakes) and non-algal turbidity, determines the upper limit of algal biomass. Understanding these feedback mechanisms iso f primary importance for restoration and management of shallow lakes,a sca n beillustrate d byth e following examples: General introduction Physical conditions Algae make the water turbid. Beyond a certain threshold of lake turbidity, populations of submerged macrophytes in shallow lakes collapse, and with that the 'clearing effects' of the submerged vegetation. Providing refuge to grazing zooplankton, competition for nutrients between macrophytes and algae,an d protecting the sediments against wind- or fish-induced resuspension, areamon g themajo r mechanismsb ywhic hvegetatio n makesth ewate r clearer. Chemical conditions Algal production leads to a high sediment oxygen