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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Variatie in biodiversiteit in sloten binnen een polder
Heukelum, A. van; Peeters, E.T.H.M. - \ 2012
H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 45 (2012)24. - ISSN 0166-8439 - p. 45 - 47.
aquatische ecologie - biologisch waterbeheer - polders - sloten - biodiversiteit - drainagesystemen - aquatic ecology - biological water management - ditches - biodiversity - drainage systems
Met meer dan 300.000 kilometer aan poldersloten en weteringen vormen de Nederlandse polders een uitgebreid potentieel leefgebied voor onder andere water- en oeverplanten, vissen, amfibieën en insecten. Toch ziet men poldersloten nog vaak slechts als uniforme transportwegen van eutroof water en kreeg de natuurfunctie binnen polders pas recentelijk meer aandacht. In het kader van een afstudeeropdracht van Wageningen Universiteit is onderzoek verricht naar de ruimtelijke verspreiding van de biodiversiteit in één polder. Verschillen in slootdimensies, landgebruik en onderhoud en beheer hebben invloed op het watermilieu en daarmee op de waterfauna. Dit maakt dat een polderdrainagesysteem een hogere ecologische diversiteit heeft dan wellicht gedacht wordt. Juist de kleine poldersloten tonen een hoge ecologische potentie, mits het waterbeheer daarop weet te anticiperen. Dit artikel laat zien hoe en waar ecologische zones binnen de polder ontstaan die verschillen in rijkdom en variabiliteit.
Zuiverend riet in sloten : veelbelovende aanpak voor betere waterkwaliteit
Buck, A.J. de; Bolt, F.J.E. van der; Wijk, T. van - \ 2010
biologisch waterbeheer - phragmites - oppervlaktewaterkwaliteit - stikstof - fosfor - helofytenfilters - landschapsbeheer - biological water management - surface water quality - nitrogen - phosphorus - artificial wetlands - landscape management
Poster met onderzoeksinformatie. In 2009 is onderzoek opgestart naar het gebruik van riet in sloten als helofytenfilter om stikstof en fosfor weg te vangen uit het oppervlaktewater.
Biofilter haalt gewasbeschermingsmiddelen uit water
Werd, H.A.E. de - \ 2010
filters - biologische behandeling - gewasbescherming - biologisch waterbeheer - waterzuivering - biological treatment - plant protection - biological water management - water treatment
Mogelijkheden voor het gebruik van biofilters om gewasbeschermingsmiddelen uit het water te halen.
DEB model for cockles (Cerastoderma edule) in the Oosterschelde
Wijsman, J.W.M. ; Brummelhuis, E.B.M. ; Smaal, A.C. - \ 2009
Yerseke : IMARES (Report / Wageningen IMARES C048/09) - 44
schaaldieren - populatie-ecologie - economische hulpbronnen - biologische productie - klimaatverandering - biologisch waterbeheer - shellfish - population ecology - economic resources - biological production - climatic change - biological water management
The natural and cultured shellfish populations in the Oosterschelde are of considerable biological and economical interest. The hydromorphology of the Oosterschelde is continuously changing and adapting in response to natural anthropological and climatological changes. In view of these changes and of possible future developments in the region, questions arise with regard to the impact on the system’s carrying capacity for shellfish populations. Dynamic shellfish models can help to answer these questions. In this study, a Dynamic Energy Budget (DEB) model for cockles (Cerastoderma edule) in the Oosterschelde estuary is presented.
Land, Water and Ecosytems Management in the Khrisna River Basin : Report of the inception visit and stakeholder Report of the inception visit and stakeholder
Brandenburg, W.A. ; Gevers, G.J.M. - \ 2007
Wageningen : Plant Research International / Wageningen International - 33
watervoorraden - beschikbaar bodemwater - beschikbare watercapaciteit - waterbeheer - india - biologisch waterbeheer - vollegrondsgroenten - water resources - available water - available water capacity - water management - biological water management - field vegetables
India, as the world’s major irrigating country uses the largest share of its water resources for irrigation purposes. The demand for water from both the agricultural and the non_agricultural sectors is growing rapidly, causing an increased pressure on available water resources. In many areas overexploitation of the resources and environmental degradation are already experienced. The national and state governments of India, therefore, face huge challenges in water management India, as the world’s major irrigating country uses the largest share of its water resources for irrigation purposes
Regelgeving effluent en restmateriaal uit biobedden
Werd, H.A.E. de - \ 2007
Wageningen : Praktijkonderzoek Plant & Omgeving B.V., Bloembollen, Boomkwekerij & Fruit (PPO / Rapport ) - 8
waterbeheer - afvalwater - wetgeving - biologisch waterbeheer - Nederland - water management - waste water - legislation - biological water management - Netherlands
Deze notitie verkent de mogelijkheden omtrent de vraag: Hoe kan effluent en organisch restmateriaal uit biobedden en hiervan afgeleide systemen binnen de bestaande wet- en regelgeving verwijderd worden? Binnen het onderzoek naar emissiereductie van gewasbeschermingsmiddelen en biociden wordt onderzocht of biologische zuiveringsystemen zoals biobedden of hiervan afgeleide systemen effectief en praktisch toepasbaar zijn in Nederland. Voor het bepalen van de praktische toepasbaarheid van biobedden en afgeleide systemen, is het nodig te weten hoe effluent en restmateriaal uit een biologische zuiveringsinstallatie verwijderd kan worden. En aan welke normen het effluent moet voldoen om het niet tegen hoge kosten af te hoeven voeren.
Herstelmaatregelen in ondiepe meren: zijn de verbeteringen blijvend?
Nes, E.H. van; Lammens, E. ; Roijackers, R.M.M. ; Veeningen, R. - \ 2007
H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 40 (2007)1. - ISSN 0166-8439 - p. 29 - 32.
meren - eutrofiëring - baggeren - chlorofyl - fosfaten - nitraten - evaluatie - onderzoek - herstel - vissen - nederland - denemarken - biologisch waterbeheer - maatregelen - eutrophication - dredging - chlorophyll - phosphates - nitrates - evaluation - research - rehabilitation - fishes - netherlands - denmark - lakes - biological water management - measures
In deze studie evalueren we gegevens van 28 Nederlandse en 50 Deense meren waar effectgericht maatregelen zijn toegepast, waaronder voornamelijk actief biologisch beheer door afvissen van brasem en blankvoorn. Daarnaast zijn in sommige meren ook andere maatregelen toegepast, zoals baggeren en chemische fosfaatfixatie in het sediment. In meer dan de helft van de gevallen was actief biologisch beheer succesvol op de korte termijn. In sommige gevallen nam niet alleen het chlorofyl-, maar ook de fosfor- en stikstofgehaltes sterk af. Op de lange termijn was de heldere toestand in de meeste gevallen echter niet stabiel
Dispersie : herstelde petgaten en de rol van dispersie
Didderen, K. - \ 2007
Wageningen : Alterra (Alterra-rapport 1564) - 67
aquatische gemeenschappen - moerassen - plassen - herstel - dispersie - invasies - ecologie - vissen - waterkwaliteit - macrofauna - noordwest-overijssel - biologisch waterbeheer - aquatic communities - marshes - ponds - rehabilitation - dispersion - invasions - ecology - fishes - water quality - biological water management
Als herstelmaatregelen in oppervlaktewateren succesvol worden uitgevoerd, blijkt vaak dat de gewenste soorten niet of slechts gedeeltelijk terugkeren, ondanks gunstige abiotische omstandigheden. De oorzaak ligt waarschijnlijk in de mogelijkheden van de soorten, om de afstand af te leggen tussen het herstelgebied en dichtstbijzijnde populatie. In dit onderzoek is specifiek nagegaan hoe het dispersievermogen is van indicatorsoorten in petgaten
Onderzoek ten behoeve van het herstel en beheer van Nederlandse laagveenwateren; eindrapportage 2003-2006
Lamers, L. ; Geurts, J. ; Bontes, B. ; Sarneel, J. ; Pijnappel, H. ; Boonstra, H. ; Schouwenaars, J.M. ; Klinge, M. ; Verhoeven, J. ; Ibelings, B. ; Donk, E. van; Verberk, W. ; Kuijper, B. ; Esselink, H. ; Roelofs, J. - \ 2006
Ede : LNV-DK (Rapport DK nr. 2007/057-O) - 286
veenmoerassen - laagveengebieden - ecosystemen - ecologisch herstel - biodiversiteit - hydrologie - waterkwaliteit - waterbeheer - plassen - nederland - aquatische ecosystemen - biologisch waterbeheer - natura 2000 - bogs - fens - ecosystems - ecological restoration - biodiversity - hydrology - water quality - water management - ponds - netherlands - aquatic ecosystems - biological water management
Dit rapport presenteert de resultaten en conclusies van het onderzoek aan laagveenwateren binnen het kader van het Overlevingsplan Bos en Natuur in de eerste fase (obn, 2003-2006). In Hoofdstuk 3 wordt een overzicht gegeven van de onderzoekslocaties. Vervolgens worden in Hoofdstuk 4 de belangrijkste bevindingen van het correlatieve onderzoek naar de samenhang tusen biodiversiteit en milieukwaliteit gepresenteerd, met in Hoofdstuk 5 de rol van hydrologie in het laagveenlandschap. In de daaropvolgende hoofdstukken 6 tot en met 10 staan de onderzoeksvragen, methoden, resultaten en conclusies van de verschillende deelonderzoeken weergegeven met betrekking tot visstandsbeheer (Hoofdstuk 6), water- en veenkwaliteit (Hoofdstuk 7), verlanding en veenvorming (Hoofdstuk 8), voedselwebrelaties (Hoofdstuk 9) en fauna (Hoofdstuk 10). Ten slotte worden in Hoofdstuk 11 de belangrijkste conclusies van het onderzoek in de eerste fase samengebracht en bediscussieerd, in relatie tot de directe betekenis voor het laagveenbeheer. Dit zal uitgewerkt worden aan de hand van de nieuw gegenereerde kennis en bestaande literatuur over de betreffende milieuproblemen (‘ver’-thema’s), en de voor- en nadelen van beschikbare OBN-maatregelen. Als afsluiting wordt aan de hand van de resultaten in fase 1 aangegeven welke onderzoeksvragen geprioriteerd zijn voor de tweede fase
De successen van de ecologen in het waterbeheer
Wal, B. van der - \ 2004
In: Ecologisch Waterbeheer in de toekomst met een blik uit het verleden. - Wageningen : WUR - p. 9 - 19.
waterbeheer - ecologie - waterverontreiniging - geschiedenis - nederland - biologisch waterbeheer - aquatische ecosystemen - water management - ecology - water pollution - history - netherlands - biological water management - aquatic ecosystems
Enkele milieuhygiënische en biologische aspecten van het waterbeheer
Nieuwe uitdagingen vragen nieuwe concepten
Gardeniers, J.J.P. ; Peeters, E.T.H.M. ; Franken, R.J.M. - \ 2004
In: Ecologisch Waterbeheer in de toekomst met een blik uit het verleden. - Wageningen : WUR - p. 35 - 56.
waterbeheer - waterkwaliteit - ecologie - biologische technieken - biologisch waterbeheer - aquatische ecosystemen - water management - water quality - ecology - biological techniques - biological water management - aquatic ecosystems
Een historisch overzicht van de ontwikkeling van het vakgebied van de aquatische ecologen en uitdagingen voor de toekomst
Biomanipulation in the Netherlands : 15 years of experience
Meijer, M.L. - \ 2000
Agricultural University. Promotor(en): Marten Scheffer. - S.l. : S.n. - ISBN 9789058082268 - 208
eutrofiëring - hydrobiologie - waterbeheer - nederland - biologisch waterbeheer - aquatische ecosystemen - eutrophication - hydrobiology - water management - netherlands - biological water management - aquatic ecosystems
<p>Up to the mid 1950's Dutch lakes were characterised by clear water and luxurious macrophyte growth. In the 1960's eutrophication changed this situation and the lakes suffered from cyanobacterial blooms, turbid water and poor submerged vegetation. Restoration programmes were aimed at reducing the rates of the external phosphorus loading. Due to these measures, the nutrient- and chlorophyll-a concentrations decreased. The transparency of the water, however, increased only slightly. In the past one decade it has been recognised that eutrophication had given the turbid state various stabilising mechanisms through which it is very resistant to restoration measures centred on nutrient reduction.</p><p>A drastic reduction of the fish stock, known as biomanipulation, was suggested as an alternative approach. This thesis examines and evaluates the potential of biomanipulation as an additional restoration method and tries to understand and explain the mechanisms involved.</p><p><em>Alternative stable states (Chapter 2)</em></p><p>The turbidity of lakes is generally considered to be a smooth function of their nutrient status. However, models and observations in lakes suggest that over a range of nutrient concentrations, shallow lakes can have two alternative equilibria: a clear state dominated by aquatic vegetation, and a turbid state characterised by high algal biomass. This bi-stability has important implications for the possibilities of restoring eutrophied lakes. The turbid state is stabilised by planktivorous fish consuming large zooplankton, the grazer of algae and by benthivorous fish resuspending the bottom sediment in search for food. Nutrient reduction alone may have only a little impact on water clarity, but an ecosystem disturbance like fish stock reduction may contribute to factors to bring the lake back to a clear state. Once the clear water state has led to colonisation of the lakes with macrophytes, the macrophytes will stabilise the clear water state by various mechanisms. They may increase sedimentation and reduce resuspension, they may provide refuges for zooplankton against fish predation, in addition the macrophytes will compete with algae for nutrients, especially nitrogen and some macrophyte species may excrete allelopathic substances, that may inhibit algal growth.</p><p><em>The first Biomanipulation experiments (Chapter 3 and 4)</em></p><p>Biomanipulation in the Netherlands started with an experiment in small ponds (Chapter 3). Ten ponds of 0,1 ha were divided into two compartments; one half of each pond was stocked with 0+ cyprinids, the other served as a reference. This replicated, controlled experiment demonstrated significant differences in transparency, algae and zooplankton between compartments with fish and without fish. Despite the repeated addition of nutrients in the ponds no eutrophic conditions could be realised, probably due to increased sedimentation in the relatively small mesocosms. Although the aim of the experiment was to demonstrate the impact of planktivorous 0+ cyprinids on zooplankton and algae, the 0+ carp appeared to resuspend the bottom sediment as well, as the turbidity could not be explained by algal biomass only.</p><p>Lake Bleiswijkse Zoom was biomanipulated by dividing the small lake into two parts, and removing more than 75% of the original fish biomass from one part (Chapter 4). In the treated part the removal of bottom-stirring activity by benthivorous fish and the increase in zooplankton grazing causing low chlorophyll-a concentrations led to an increase of Secchi depth transparency from 20 cm to the bottom of the lake. Within two months the Characeae became abundant. The two parts differed significantly in inorganic suspended solids, algal biomass, transparency and total nutrient concentrations. Compared with the control compartment the number of fish species in the treated part increased.</p><p><em>Impact of benthivorous fish on turbidity (Chapter 5)</em></p><p>In two biomanipulated small lakes (Bleiswijkse Zoom and Noorddiep) the increase in transparency was not only caused by a decrease in algal biomass but also by a decrease in sediment resuspension by benthivorous fish. The biomass of benthivorous fish and the concentration of inorganic suspended solids were positively related. A model was used to calculate the impact of resuspended inorganic suspended solids on the turbidity. By combining these relations a direct effect of benthivorous fish on the Secchi depth was calculated. In addition, it is argued that the algal biomass was also indirectly influenced by removal of benthivorous fish, as the benthivorous fish prevented macrophytes from settling, whereas macrophytes were essential to keep the water clear.</p><p><em>Comparison of results with a Danish lake (Chapter 6)</em></p><p>In Chapter 6 the five year results of three biomanipulated Dutch lakes (Lake Zwemlust, Noorddiep and Bleiswijkse Zoom) are compared with one Danish biomanipulated lake (lake Vaeng). The fish stock reduction led in general to a low fish stock, low chlorophyll-a, increase in Secchi disk transparency depth and a high abundance of macrophytes. Large Daphnia became first abundant, but their density soon decreased due to food-limitation and predation by fish. The total nitrogen concentrations decreased due to N-uptake by macrophytes and enhanced denitrification. In lake Bleiswijkse Zoom the water transparency decreased and the clear water state was unstable. The fish stock increased and the production of young fish in summer was high. Clear water occurred only in spring . Large Daphnia decreased markedly in summer and the macrophytes dissappeared. Except in Bleiswijkse Zoom the water remained clear in all lakes during the first five years. In summer of the sixth year transparency decreased in the hypertrophic lake Zwemlust. Also in the less eutrophic lake Vaeng, a short-term turbid state (6 weeks) occurred in summer 1992 after a sudden collapse of the macrophytes. Deterioration of the water quality seems to start in summer and is apparently related to a collapse in macrophytes. At a low planktivorous fish stock the duration of the turbid state is shorter than in presence of a high planktivorous fish biomass which became apparent comparing Lake Vaeng with lake Zwemlust.</p><p><em>Development of fish communities after biomanipulation (Chapter 7)</em></p><p>The development of fish communities after biomanipulation has been studied in detail in three small lakes. In lake Zwemlust, after the biomanipulation, the introduced pike could not control the explosive growth of the introduced rudd. In lake Noorddiep and Bleiswijkse Zoom the fish population became more diverse. Bream and carp became less dominant and were partly replaced by roach and perch. The main predator pike-perch was strongly reduced and replaced by pike and perch. The share of piscivorous fish increased at all sites. The recruitment of young-of-the-year fish was similar or even higher in the clear overgrown areas than in the turbid water before the measures, but the recruitement of young -of-the-year to older species differed between the species. Predation by pike and perch could not control the young-of-the-year cyprinids, but their predation may have contributed to the shift form bream to roach, because of selective predation on bream in the open water, while roach was hiding in the vegetation. The macrophytes provided new refugia and feeding conditions that favour roach and perch, but offer relatively poor survival conditions for bream and carp.</p><p><em>Biomanipulation on a large scale (Chapter 8)</em></p><p>The drastic fish removal in Lake Wolderwijd (2670 ha) showed that also in a very large lake biomanipulation cause water clarity to increase. In this large lake in total 425 tons of fish were removed, i.e. ca. 75% of the original fish stock. In addition 625.000 specimen of 0+ pike were added to the lake. The success of biomanipulation in the lake was transient. In spring 1991 the transparency of the water increased as a result of grazing by <em>Daphnia galeata</em> . The clear-water phase lasted for only six weeks. Macrophytes did not respond as expected and most of the introduced young pike died. However, from 1991 to 1993 the submerged vegetation gradually changed. Chara began to spread over the lake (28 ha in 1991 to 438 ha in 1993). The water over the Chara meadows was clear, probably as a result of increased net sedimentation in these areas, due to reduced resuspension.</p><p><em>Evaluation of Biomanipulation Projects (Chapter 9)</em></p><p>Evaluation of eighteen biomanipulation projects showed that biomanipulation can be a very effective method to increase the transparency of the water in lakes. In almost half of all projects a return to the clear water state was obtained and in only ten per cent of the projects the biomanipulation failed to cause an increase in the Secchi depth. In all other cases, the water transparency increased significantly. The increase in Secchi depth was significantly stronger than the general trend observed in Dutch lakes where no measures have been taken. The improvement in Secchi depth and chlorophyll-a was also more marked in lakes where only the phosphorus load had been reduced.</p><p>The critical factor for obtaining clear water was the extent of the fish reduction in winter. Significant effects were observed only after &gt; 75% fish reduction. Success seems to require substantial fish reduction. In such strongly biomanipulated lakes, wind resuspension of the sediment did not influence water clarity. No conclusions can be drawn with respect to the possible negative impact of cyanobacteria or <em>Neomysis</em> on grazing by <em>Daphnia</em> and consequently on water clarity. In all lakes, with an additional phosphorus loading reduction, the fish stock had been reduced less drastically. In these lakes, the effect on transparency was less pronounced than in the lakes with &gt; 75% fish removal.</p><p>Daphnia grazing seems responsible for spring clear water in all but one clear lakes. The factors that determine the clear water in summer are less obvious and more diverse, but a high macrophyte cover seems to play an important role in all the lakes.</p><p>The decrease of the Secchi depth over the years in almost all manipulated eutrophic lakes supports the idea that the clear, vegetation dominated state is not stable at high nutrient conditions. However, the return time to the turbid water state is long ( &gt; 8 years). As we have not applied biomanipulation in mesotrophic lakes, we could not investigate if under those conditions the clear water state would be stable on the long term. For the stability of the clear water state it would be good to have a large piscivorous population. Under the present condition in The Netherlands, with "constant" waterlevels, scarce emergent vegetation and relatively high nutrient concentrations, no dense population of piscivores are likely to develop.</p><p><em>Development of ideas (Chapter 10)</em></p><p>How our ideas have evolved on biomanipulation on biomanipulation in the past fifteen years is subject of the last chapter. Biomanipulation seems a more effective tool than it was thought when it was started, especially for eutrophic lakes. Expected negative aspects like increase in inedible algae have hardly been found. Also the ideas on the working of biomanipulation have evolved. In accordance with earlier ideas, the increase in water clarity mostly occurs in spring and <em>Daphnia</em> grazing seems crucial for this spring clear-water phase. Reduced resuspension after removal of benthivores has been shown to be important in three out of the eighteen lake studies. However, in most lakes the fish removal caused a reduction in nutrient concentrations, which may have been caused by a reduction in the benthivores or by an increase of bottomalgae.</p><p>According to early views, colonisation of macrophytes seems crucial for maintaining the clear water in summer. In lakes without macrophyte growth, the water frequently became turbid again in summer, if predation of 0+ fish on <em>Daphnia</em> was high. At least a coverage of 50-&gt;70% of the lake with macrophytes is required to keep the water clear in the whole lake. At lower coverage only locally clear water above the macrophytes may be achieved. Macrophytes induce many changes in the ecosystem, which help stabilising the clear water state. The expected increase of piscivore control of 0+ fish has not been found in Dutch lakes. Rather, the macrophytes seem to stimulate the production of 0+ fish in eutrophic lakes. Macrophytes can provide refuge for zooplankton, but this effect may be small if high densities of 0+ fish or Neomysis are present within the vegetation. In large, shallow lakes reduced resuspension and increased sedimentation between the macrophytes may significantly contribute to the water clarity. Furthermore, nitrogen limitation of the algal growth has proven to be important in some lakes, but in others a strong reduction of the chlorophyll-a nutrient ratio's suggest that other factors than nutrient limitation are responsable for the low algal biomasses in presence of plants. Those can be related to zooplankton grazing, allelopathic effects or increase of filtering zebramussels.An increase in benthic algae may play a role in this too, but there is not yet much knowledge on this aspect.</p><p><em>Implication for the water quality manager</em></p><p>This thesis shows that biomanipulation can be a very effective restoration method provided the fish biomass reduction is substantial. Indeed, biomanipulation seems to be more effective in increasing water transparency than reducing the phosphorus load. Biomanipulation may even lead to a substantial reduction of the nutrient concentrations in the water column. Provided that a substantial fish reduction is carried out, the system may shift to a clear water state even at high nutrient concentrations.</p><p>The costs of biomanipulation are low compared to the costs of phosphorus reduction measures (Boers et al, 1997). Although this study shows that in eutrophic lakes no long-term success can be obtained, the long return time to the turbid state implies that even in highly eutrophic lakes, where the clear state is not stable, biomanipulation may be an cost-effective management strategy, as in some lakes it may suffice to reduce the fish stock drastically once every five year.</p><p>However, biomanipulation may not work in all conditions. In open systems with a a lot of boats, it may be difficult to remove a substantial part of the fish stock. In those lakes repeated fish reduction may also have effect (as shown in Finland and Sweden), but in The Netherlands there is not yet much experience with this method. Furthermore in lakes with a very high density of inedible cyanobacteria, and a high resuspension of sediment the potential of the method is still to be tested.</p><p>With future applications attention should be paid to removal of more than 75% of the fish stock, removal of small fish in adjacent ditches, removal or disturbance of spawning fish to reduce the production of 0+ fish and to reservation of money for additional fisheries. Furthermore the potential for macrophyte growth should be investigated beforehand, and if necessary macrophyte stimulating measures should be taken, like reducing the water depth or addition of oospores or seeds of plants.</p><p>The challenge for the future is to create a shift to the clear water state by means of biomanipulation in more open systems, in lakes with a high density of inedible cyanobacteria or floating layers of cyanobacteria and in lakes with a high resuspension of loose sediment.</p>
Biomanipulation in shallow lakes in The Netherlands: an evaluation of 18 case studies
Meijer, M.L. ; Boois, I. de; Scheffer, M. ; Portielje, R. ; Hosper, H. - \ 1999
Hydrobiologia 408-409 (1999)0. - ISSN 0018-8158 - p. 13 - 30.
meren - evaluatie - nederland - biologisch waterbeheer - lakes - evaluation - netherlands - biological water management
Eighteen shallow lakes in The Netherlands were subjected to biomanipulation, i.e. drastic reduction of the fish stock, for the purpose of lake restoration. The morphology and the nutrient level of the lakes differed, as did the measures applied. In some lakes biomanipulation was accompanied by reduction of the phosphorus loading. In all but two lakes, the Secchi disk transparency increased after the fish removal. Eight lakes (no phosphorus loading reduction, except for one lake) showed a strong and quick response to the measures: the bottom of the lake became visible (`lake bottom view'') and there was a massive development of submerged macrophytes. In eight other lakes the water transparency increased, but lake bottom view was not obtained. In the biomanipulated lakes the decrease in total phosphorus and chlorophyll aand the increase in Secchi disk transparency were significantly stronger than the general trend occurring in Dutch lakes where no measures had been taken. The improvement in the Secchi depth and chlorophyll awas also stronger than in lakes where only the phosphorus load was reduced. The critical factor for obtaining clear water was the extent of the fish reduction in winter. Significant effects were observed only after >75% fish reduction. Success seems to require substantial fish manipulation. In such strongly biomanipulated lakes, wind resuspension of the sediment never prevented the water from becoming clear. No conclusion can be drawn with respect to the possible negative impact of cyanobacteria or Neomysison grazing by Daphniaand consequently on water clarity. In all lakes where there had been a high density of cyanobacteria or years with a high density of Neomysisother factors such as insufficient fishery may explain why lake bottom view was not obtained. In all lakes with additional phosphorus loading reduction the fish stock has been reduced less drastically (15–60%). In these lakes the effects on transparency were less pronounced than in the lakes with > 75% fish removal. Daphniagrazing seems responsible for spring clearing in all clear lakes but one. In three lakes the reduction of benthivorous fish also increased the transparency. The factors that determine water clarity in summer are less obvious. In most clear lakes a low algal biomass coincided with a macrophyte coverage of more than 25% of the lake surface area. However, it was not clear what mechanism caused the low algal biomass in summer, although inorganic nitrogen concentrations were regularly found to be very low. Daphniagrazing in open water seemed to be of little importance for suppressing the algal biomass in summer. Although in most lakes the total phosphorus concentration decreased after the biomanipulation, the dissolved phosphorus concentration remained too high to cause phosphorus limitation of the algal growth. In four out of six clear lakes for which there are long-term data the transparency decreased again after 4 years. In one lake with lower nutrient levels the Secchi disk transparency increased over the years. However, the number of lakes with low nutrient levels is too small for conclusions to be drawn regarding the impact of nutrient levels on the stability of the clear water state.
Clearing lakes : an ecosystem approach to the restoration and management of shallow lakes in the Netherlands
Hosper, H. - \ 1997
Agricultural University. Promotor(en): L. Lijklema. - S.l. : Hosper - ISBN 9789054856825 - 168
waterkwaliteit - waterbeheer - hydrologie - limnologie - meren - plassen - nederland - water - reservoirs - biologisch waterbeheer - water quality - water management - hydrology - limnology - lakes - ponds - netherlands - biological water management
<p>In the 1950 <em>s</em> and 1960 <em>s,</em> most shallow lakes in the Netherlands shifted from macrophyte-dominated clear water lakes, towards algae-dominated turbid water lakes. Eutrophication, i.e. increased nutrient loading, is the main cause of the deterioration of the lake ecosystems. Other perturbations, such as the loss of lake-marginal wetlands (nutrient filters, habitat for pike, Esox lucius) and chemical pollution toxic to zooplankton, will have reinforced the effects of nutrient enrichment. The lake restoration strategy has been concentrated on the reduction of the external phosphorus (P) loading. However, so far this approach did not result in the water quality (in terms of transparancy, phytoplankton species, fish stock etc.) desired. A more comprehensive approach to lake ecosystem functioning may provide additional tools for lake restoration. Algal blooms in lakes will develop when the algal production is high and the algal losses are low. Production is controlled by the supply of nutrients and light. Consumption of algae by zooplankton is a major loss process. In this thesis attention is focused on both sides of the algal balance: (1) the control of the external and internal (from the sediments) P loading, and (2) biomanipulation, the manipulation of fish communities aiming at increased consumption of algae. Reduction of the planktivore fish stock may enhance the zooplankton and thus the grazing on algae. Bream <em>(Abramis brama)</em> and roach ( <em>Rutilus rutilus</em> ) are among the major fish species in many turbid Dutch lakes. Large bream feeds on zooplankton, as well as benthic organisms. Reduction of the benthivore fish stock results in reduced sediment resuspension and P release and less disturbance of rooted macrophytes.<p><strong>Development of the lake restoration strategy (chapter 1)</strong><br/>The strategy for lake restoration gradually evolved from solely P stripping from sewage, towards a more comprehensive and ecosystem-based approach. In the Netherlands, P removal at sewage treatment plants is common practice and polyphosphates in household detergents have been replaced by less harmful compounds. TP levels in surface waters (including the Rhine river) are going down, however, in lakes algal blooms persist. Presently, additional measures directed at the reduction of non- point sources, treatment or removal of P-rich sediments and fish stock management come into focus. Other negative influences such as the ongoing loss of lake-marginal wetlands and (locally) the chemical pollution toxic to zooplankton, should be reversed as well.<p><strong>Shallow lakes in the Netherlands: searching for lake restoration objectives (chapter 2)</strong><br/>Shallow lakes which are not, or only slightly, influenced by man, may provide clues to define key variables and processes expressing ecological sustainability. Historical studies may therefore be useful for finding specific objectives for lake restoration. One natural peat lake (Naardermeer) and two lake areas resulting from peat mining (Reeuwijk lakes and Oude Venen) were selected for such a reference study. It is well-known that many lakes in the Netherlands showed major changes in water transparency in the 1950 <em>s</em> and 1960 <em>s.</em> The information collected refers mainly to the period 1930-1950. At that time, all three lakes were dear with abundant submerged vegetation. Naardermeer, with its extremely low nutrient loading, showed the strongest indications for a stable dear water state (a high diversity of Chara species). Nutrient loading of the other two lakes was probably higher. It is speculated that the intensive commercial fishing in the Reeuwijk lakes (by 30-50 families), and the excellent pike habitat in the Oude Venen (due to natural water level fluctuations), played a significant role in maintaining the dear water state in these two lakes. <strong></strong><p><strong>Multi-take studies: external nutrient loading and lake response (chapter 3)`</strong><br/>Multi-lake studies proved to be helpful in developing management criteria for eutrophication control. In 1976-1977, the first national eutrophication survey of the shallow eutrophic lakes of the Netherlands was conducted, including ca. 65 lakes, of which 14 lakes with reliable water- and nutrient budgets. More up-to-date relationships for take management were<br/>derived by including the results of the eutrophication surveys for 1983-1985 and 1980-1988. According to the steady-state loading-response models, the in-lake TP concentration shows a proportional or 'somewhat' less than proportional response to a decrease in TP loading. Secchi depth-chlorophyll relationships and upper limits for chlorophyll in relation 4 to TP, TN and underwater light climate could be established. Lakes with an excess of TP V611 show a 'threshold response', after P loading reduction: algal biomass only goes down if the TP concentration approaches the limitation line, CHL = -24 + 1.04 TP (CHL and TP in mg m <sup>-3</SUP>). <em>Oscillatoria</em> -dominated lakes may produce more algal biomass per unit P (CHL = 1 .54 TP).<br/>Objectives for Secchi depth were derived, (1) for triggering a collapse of <em>Oscillatoria</em> blooms, and (2) for creating proper light conditions for the restoration of submerged vegetation. However, due to the highly variable non-algal turbidity, only upper limits can be given for the chlorophyll and TP levels necessary. <strong></strong><p><strong>Whole-lake study of Veluwemeer: lake flushing for control of Oscillatoria blooms and internal phosphorus loading (chapter 4)</strong><br/>Lake Veluwemeer (3,356 ha, mean depth 1.25 m) has suffered from a year-round <em>Oscillatoria</em> bloom from 1971 onwards. Early in 1979, the P loading of the lake was reduced from 2.7 to 1.5 g P m <sup>-2</SUP>y <sup>-1</SUP>. Monthly TP budgets, however, showed a substantial net P release from the sediments during summer and therefore the prospects for recovery were poor. Summer peaks in TP coincided-with extremely high pH (pH 9-10) and it was hypothesized that the <em>Oscillatoria</em> bloom supported a self-perpetuating process of algal activity, high pH, P release and even more algal activity. Winter flushing with water that was low in TP and high in Ca <sup>2+</SUP>and HC0 <sub>3-</sub> could interrupt this vicious cycle. The effects were spectacular. After the first winter flushing in 1979-80, summer pH dropped by one whole unit, TP and chlorophyll more than halved and summer algal growth has been P-limited ever since. However, Secchi depth only increased from 0.20 to 0.30 m. This disappointing transparency could be explained by reduced chlorophyll content per unit algal biovolume. <em>Oscillatoria</em> , which prefers dim light conditions, lost its dominant position after the cold winter of 1985. After the species shift, summer transparency was still limited to 0.40-0.50 m. Model calculations showed a decrease of summer internal P loading from 1.0-8.4 before to 0.0-1.7 mg m <sup>-3</SUP>d <sup>-1</SUP><br/>after the measures. Low summer pH values could be explained by the enhanced CaC0 <sub>3</sub> precipitation during the spring. Apparently, the low pH resulted in reduced P release from the sediments and P-limited algal growth during summer. The reduced algal biomass and consequently lower sediment oxygen demand, and maybe also increased N0 <sub>3</sub><sup>-</SUP>levels during early spring, will have reinforced the better binding of P to the sediments. Manipulation of the carbonate system proved to be an effective tool in controlling internal P loading. During winter, conservative behavior (net growth = 0) of the <em>Oscillatoria</em> population may be assumed. Therefore, <em>Oscillatoria</em> blooms can be effectively (>95% <em>)</em> removed from wellmixed lakes by flushing in November-February, with three times the lake volume.<p><strong>Biomanipulation in shallow lakes: concepts, case studies and perspectives (chapter 5)</strong><br/>High fish stocks in algae-dominated lakes tend to impose a homeostasis on the lake ecosystem, which then resists the recovery of the lake. In today's turbid lakes, the common phenomenon of the 'spring clear water phase', induced by zooplankton grazers, fails to appear. Large numbers of planktivorous bream and roach, throughout the year, as well as high densities of 'inedible' filamentous cyanobacteria, prevent the peaking in population of the efficiently grazing large-bodied <em>Daphnia.</em> Submerged macrophytes play a key role in maintaining the clear water state throughout the summer. For the reestablishment of the submerged vegetation, it is necessary to restore the spring dear water phase. Biomanipulation, i.e. a substantial fish stock reduction, could trigger a shift from a stable turbid water state to an alternative stable clear water state. Nine case studies were evaluated for testing the applicability and perspectives of biomanipulation. It was concluded that a single substantial fish stock reduction (><em>75%)</em> during winter, offers good chances for achieving clear water.in the next spring. The filamentous cyanobacteria and the possible development of invertebrate predators <em>(Neomysis, Leptodora)</em> on <em>Daphnia</em> are uncertain factors for successful biomanipulation. Rapid colonization of submerged macrophytes, stabilizing the dear water state, has been demonstrated in small lakes (&lt; 30 ha). High nutrient levels, ultimately (in two cases, after five and seven years), lead to a shift back to the turbid water state. Top-down control by the predatory fish (pike and perch, <em>Percafluviatilis)</em> seems restricted to small lakes, with a high degree of 'patchiness' (patches of macrophytes and open water). An alternative stable clear water state may be expected in the TP range of 50-100 <em></em> mg m <sup>-3</SUP>(or higher TP levels for very small lakes). More and more lakes in the Netherlands approach these TP levels, so the chances for biomanipulation are improving. Fish control is more difficult in large lakes, and particularly in networks of interconnected lakes. Additionally, in large lake areas the reestablishment of vegetation takes more time. Winter fishing on a regular basis (rather than a single fish stock reduction) may then be promising, but case studies are needed for further evaluation.<p><strong>Whole-lake study of Wolderwijd: biomanipulation for promoting the clear water state (chapter 6)</strong><br/>Lake Wolderwijd (2,555 <em></em> ha, mean depth 1.60 m) has suffered from <em>Oscillatoria</em> blooms, turbid water and a poor submerged vegetation as a result of eutrophication since the early 1970s <em>.</em> From 1981-1984 <em></em> the lake was flushed (via Veluwemeer) during winter, with water low in TP and high in Ca <sup>2+</SUP>and HCO <sub>3</sub><sup>-</SUP>. TP and chlorophyll a in the lake more than halved, but Secchi depth in summer only increased from 0.20 to 0.30 m. In the hope of triggering a shift from the algae- dominated turbid water state to a macrophyte-dominated dear water state, the lake was biomanipulated during winter 1990-91. The fish stock, mainly bream and roach, was reduced from 205 to 45 kg ha <sup>-1</SUP>. In May 1991, 575,000 (217 ind ha <sup>-1</SUP>) pike fingerlings were introduced. From 1989 onwards, lake flushing was intensified in order to reduce TP and the Oscillatoria bloom. In spring 1991, the lake water cleared as a result of grazing by Daphnia galeata. The clear water phase lasted for only six weeks. Macrophytes did not respond as strongly as was expected on the basis of the results from the small-scale case studies (ch. 5). Most of the young pike died. However, from 1991 to 1993, the submerged vegetation has gradually changed. Characeae began to spread over the lake (from 28 ha in 1991 to 438 ha in 1993). The water over the Chara meadows was clear, probably as a result of increased net sedimentation within these areas and reduced mixing between (clear) water from vegetated areas and (turbid) water from non-vegetated areas. It is hypothesized that expansion of the Chara meadows might ultimately result in a shift of the whole lake to a long-lasting clear water state. In order to promote the Chara, the fish stock reductions, which aimed at a spring dear water phase, should be continued.<p><strong>Guiding lake restoration and management (chapter 7)</strong><br/>The relationships between nutrient loading and shallow lake response are complex and tend to be different for the process of eutrophication and the reverse process of oligotrophication. The bottom line is that both clear water lakes and turbid water lakes resist changes in nutrient loading, showing the phenomenon of hysteresis. In shallow lakes of moderate productivity, the submerged vegetation plays a key role in stabilizing the dear water state. In turbid water lakes, algal blooms and particularly <em>Oscillatoria</em> blooms (resistant to low P and low light, reduced edibility for <em>Daphnia</em> ), are self- reinforcing and therefore resistant to restoration efforts. Additionally, resuspension of sediments by wind and benthivorous fish may contribute to the stability of the turbid water state. Exceptional weather conditions or special actions ('switches') may trigger a shift from the one state into the other. The external nutrient loading should first of all be reduced in lake restoration. Furthermore, negative trends such as pollution with chemicals, toxic to <em>Daphnia</em> or macrophytes, should be reversed. The rehabilitation of lake-marginal wetlands will also contribute to a sustainable clear water state. However, as the turbid water state is extremely stable, additional measures may be necessary. Certain 'blockages' have to be removed to get the recovery process started:<p><em>(1) the Oscillatoria bloom</em><p>After external loading reduction, eliminating the <em>Oscillatoria</em> bloom should be the first priority in lake restoration. For many lakes it will be difficult to achieve TP &lt; 20-50 mg m <sup>-3</SUP>, which is needed for a collapse of the bloom. Therefore, additional switches are needed. Washout by flushing during winter is a promising tool (flushing with at least three times the lake volume, during November- February). <em></em><p><em>(2) the bloom-mediated P release from the sediments</em><p>Manipulation of the carbonate system through flushing with water low in TP, but rich in Ca <sup>2+</SUP>and HC0 <sub>3</sub><sup>-</SUP>, was successful in Veluwemeer. Sediment removal in the Dutch peat lake Geerplas failed to produce the desired results. Obviously, dredging is no panacea for solving problems of P release in shallow Dutch lakes. It seems wise to focus on external loading reduction and more appropriate in-lake measures.<p><em>(3) the abundance offish, preventing Daphnia and macrophytes from developing</em><p>After fish stock reduction (>75%) and clearing of the lake water, rapid colonization of submerged macrophytes has been demonstrated in small lakes (&lt; 30 ha). In large lakes and particularly in networks of interconnected lakes the fish stock is more difficult to control. Additionally, in large lake areas the reestablishment of submerged vegetation takes more time. Winter fishing on a regular basis, rather than a single fishing operation, may be promising then. However, case studies are needed for further evaluation of the efficacy of repeated fishing for promoting <em>Daphnia.</em> Prerequisites for successful biomanipulation are low TP levels (TP &lt; 100 mg m <sup>-3</SUP>, or higher TP levels for very small lakes) and low numbers of inedible algal species, such as <em>0scillatoria.</em>
Zwemlust: mechanismen van helder worden en van overgang naar de troebele toestand zes jaar na biomanipulatie.
Otte, A.J. ; Donk, E. van; Meulemans, J.T. - \ 1996
H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 20 (1996). - ISSN 0166-8439 - p. 607 - 612.
algen - biologisch waterbeheer - utrecht - algae - biological water management
Habitat evaluatie procedure: een bruikbaar instrument voor het (regionaal) waterbeheer? Verslag van een discussiemiddag.
Fellinger, M. ; Friedrich, J. ; Peeters, E.T.H.M. - \ 1996
Werkgroep Ecologisch Waterbeheer - 12
ecosystemen - kwaliteit - prestatieniveau - nederland - biologisch waterbeheer - ecosystems - quality - performance - netherlands - biological water management
Het resultaat van het project Watersysteemverkenning vormt de wetenschappelijke basis voor de Vierde Nota Waterhuishouding. Evenals de Derde Nota gebruikt het project Watersysteemverkenning een soortsgerichte benadering om de effecten op het ecosysteem te bepalen. Hiertoe wordt gebruik gemaakt van de zogenaamde Habitat Evaluatie Procedure (HEP) en de Habitat Geschiktheid Index (HGI). Het gebruik van de HEP en de HGI is afkomstig uit de Verenigde staten waar het al enige jaren wordt gebruikt, met name voor hogere organismen. Het is echter de vraag of en hoe bruikbaar de HEP en de HGI zijn voor het (regionaal) waterbeheer in Nederland. De subgroep Standaardisatie stelde deze vraag centraal in de discussiemiddag in het kader van de Algemene Leden Vergadering van de Werkgroep Ecotogisch Waterbeheer (WEW). Deze speciale uitgave van de nieuwsbrief van de WEW bevat een samenvatting van de verschillende presentaties en een versiag van de gevoerde discussies.
Bivalve grazing, nutrient cycling and phytoplankton dynamics in an estuarine ecosystem
Prins, T.C. - \ 1996
Agricultural University. Promotor(en): W.J. Wolff; P.H. Nienhuis. - S.l. : Prins - ISBN 9789054854975 - 151
Mytilidae - mossels - Bivalvia - dieren - voedingsgedrag - fytoplankton - vegetatie - groei - plankton - pelecypoda - oosterschelde - biologisch waterbeheer - mussels - animals - feeding behaviour - phytoplankton - vegetation - growth - eastern scheldt - biological water management
<br/>This thesis has considered the impact of the suspension feeding bivalve <em>Mytilus</em><em>edulis</em> on nutrient cycling and phytoplankton in an estuarine ecosystem. The research was started within the framework of an extensive research project with the objective to evaluate the changes in the Oosterschelde ecosystem as an effect of a coastal engineering project (Nienhuis & Smaal, 1994). The Oosterschelde estuary is a system where mussels are dominant consumers, which is at least partly due to the strong regulation of mussel biomass by fisheries. As a consequence, the Oosterschelde estuary is a typical example of an ecosystem where herbivores have a strong impact on the entire pelagic system.<p>The main emphasis in this study has been on the impact of the mussels on the exchange of material between the water column and the benthic system. This exchange has been studied with an <em>in situ</em> method designed to measure exchange of particulate and dissolved material between an undisturbed bivalve community and the water column, under natural conditions with respect to food supply, current speed, temperature etc. The method was evaluated in chapter 2. It was concluded that the method was suitable for <em>in situ</em> measurements of the fluxes of particulate and dissolved matter between the mussel bed and the water column.<p>Mussel beds filter considerable amounts of material. The amount of material filtered is determined by mussel biomass, mussel activity and the supply of particulate material to the mussel bed. The <em>in situ</em> experiments, carried out in the years 1987-1989, showed that the quantity and quality of the suspended particulate matter varied considerably at a short time scale. A tidal variation was observed, with the supply of relatively phytoplankton-rich water to the intertidal flats during flood tide. During ebb, phytoplankton concentrations in the water were generally reduced, probably as a consequence of depletion by suspension feeders. Superimposed on this tidal variation changes in seston quantity and quality at the time scale of hours were observed. This short-term variation was related to wind-induced resuspension of bottom material. The observed increases in SPM, POC and phaeophytin- <em>a</em> levels during these resuspension events indicated that resuspension of algal detrital material, probably biodeposits, occurred.<p>It was shown in Chapter 3 that under calm weather conditions a good correlation between the concentrations of SPM, POC and chlorophyll- <em>a</em> in the water column and the size of the respective fluxes to the mussel bed was observed. From a comparison of the composition of the fluxes with the composition of the seston it became clear that the fluxes contained a relatively high proportion of phytoplankton. After filtration, mussels are able to selectively ingest phytoplankton, while other particles are rejected through the pseudofaeces. As a result, the pseudofaeces have a reduced chlorophyll- <em>a</em> content (Kiørboe & Møhlenberg, 1981; Prins et al., 1991). Mussels eject the pseudofaeces into the water column. Pseudofaeces have a low settling velocity and are easily resuspended (Risk & Moffat, 1977; Nowell et al., 1981). This makes it probable that in the sequence of filtration, selection, ingestion and pseudofaeces formation, a fraction of the non-algal material that was filtered by the mussels and then rejected as pseudofaeces, was exported from the mussel bed. Consequently, the net flux of material to the mussel bed had a high proportion of chlorophyll- <em>a</em> , whereas part of the POC and of the particulate inorganic matter, filtered by the mussels, was exported immediately. Under rougher weather conditions sometimes a significant net export of SPM and POC from the mussel bed was observed as a result of wind-induced resuspension. The results indicated that only a part of the material that was filtered by the mussels, was stored in the mussel bed, due to immediate export of pseudofaeces by tidal currents or to more occasional wind-wave resuspension.<p>Previous estimates indicated that the mussel population in the Oosterschelde may filter the entire volume of the Oosterschelde in approximately 10 days (Smaal et al., 1986; Van Stralen & Dijkema, 1994). The generally low phytoplankton concentrations in the Oosterschelde have been attributed to the severe 'top-down' control exerted by the mussel, together with the other dominant bivalve suspension feeder <em>Cerastoderma edule</em> (Smaal et al., 1986; Herman & Scholten, 1990). The observed in situ filtration activity of the mussel beds, presented in chapter 3, agrees with the values used in earlier published estimates of mussel grazing pressure.<p>Recent ecophysiological studies have shown that bivalves respond to short-term changes in the quantity and quality of the seston by regulating ingestion rates. This regulation may be achieved by changes in clearance rates, or by rejection of variable amounts of filtered material as pseudofaeces in combination with pre-ingestive selection. The nature of this physiological response depends on both quality and quantity of the seston, and on acclimation (Bayne, 1993; Navarro & Iglesias, 1993). When exposed to diets with a low organic content, mussels mainly respond to these short-term changes by modifying the rate of pseudofaeces production (Bayne, 1993; Navarro & Iglesias, 1993). Our <em>in situ</em> experiments carried out in the Oosterschelde, showed no effect of the changes in seston concentration and composition during a tidal cycle on clearance rates (Chapter 3); this is consistent with the hypothesis that ingestion is regulated by pseudofaeces production when seston quality is low. The seasonal variation in clearance rates of a mussel bed was studied by monthly measurements on a semi- natural mussel bed (Chapter 6). Clearance rates showed a positive response to chlorophyll- <em>a</em> concentrations and were negatively affected by SPM concentrations. The relations observed in the latter experiment were probably the result of adaptation on a longer time scale (Bayne, 1993). In the mesocosm experiments presented in Chapter 7, mussel clearance rates were generally lower than rates observed in experiments with natural seawater, and showed considerable temporal variation. In the mesocosms the mussels were exposed to seston with a much higher proportion of algae than is commonly observed <em>in situ</em> . The reduced and variable clearance rates in the mesocosm experiment agree with the current idea that it is more optimal for bivalves to regulate ingestion by changes in clearance rates, when the animals are exposed to diets with a high organic content (Iglesias et al., 1992; Bayne, 1993; Navarro & Iglesias, 1993).<p>In addition to the effects of seston quantity and quality on the seasonal variation of clearance rates, a strong inhibitory effect of the presence of the alga <em>Phaeocystis sp</em> . on mussel clearance rates was observed. This inhibition of mussel clearance rates may lead to a breakdown of the top-down control of phytoplankton biomass and may increase the risk of massive algal blooms.<p>In general, a release by the mussel bed of ammonium, phosphate and silicate was observed during the <em>in situ</em> measurements. In June 1987 an experiment was carried A during a long period of high phytoplankton concentrations, resulting in a large flux of organic matter from the water column to the mussel bed. A high release of inorganic nutrients was observed during that experiment. In June 1988 chlorophyll- <em>a</em> concentrations in this part of the Oosterschelde estuary were low for a number of consecutive weeks. In that period a number of in situ measurements were carried out, all showing low uptake rates of chlorophyll- <em>a</em> , and a small release of inorganic nutrients. It was inferred from these results, that the difference in organic matter supply to the mussel bed was the cause for the difference in nutrient release between the observations from June 1987 and the results from June 1988. Moreover, it was shown that considerable day/night differences in fluxes occurred in June 1987. During daytime, much lower release of inorganic nutrients by the mussel bed was measured, which was attributed to immediate uptake of the nutrients by algae.<p>Budgets presented in Chapter 5, suggested that only a small proportion of the nutrient fluxes to the mussel bed were stored in mussel biomass. The remainder was stored in the sediment as biodeposits. Mineralization of the biodeposition and excretion by the mussels resulted in a release of inorganic nutrients. In the case of nitrogen, there was an approximate equilibrium between the net uptake of particulate N by the mussel bed and the release of dissolved inorganic N by the mussel bed, indicating that storage of N by the mussel bed was of minor importance. The contribution of direct excretion by the mussels to the DIN flux from the mussel bed was relatively small. Silicon fluxes showed some retention in spring, and predominantly a release in autumn. The uptake of particulate P by the mussel bed was somewhat higher than the release of phosphate. On average approximately 35% of the P uptake was retained by the mussel bed. In addition to storage in mussel biomass, which was estimated to be small in relation to the total flux of particulate P, sorption of phosphate on sediment particles might be responsible for some of the P retention.<p>The relatively low retention of nutrients by the mussel bed may seem surprising, and contradictory to the observed growth of mussels and accumulation of biodeposits. However, the observed mussel production at the mussel lots in the Oosterschelde is close to or lower than the biomass seeded at the lots on an annual basis (Van Stralen & Dijkema, 1994), as the growth of individual mussels is balanced by mortality. This means that net storage of nutrients in mussel biomass is of minor importance. The accumulation of nutrients in biodeposits was more or less balanced by mineralization and resuspension. It should be realized, however, that as a consequence of the large fluxes of material towards and from the mussel bed it was not possible to detect small differences between uptake and release. Retention by the mussel bed of a relatively small fraction of the filtered material could still result in the accumulation of a considerable amount of organic matter in the sediment of the mussel bed.<p>The net result of uptake and release processes is a rapid cycling of organic matter, with a conversion of particulate organic matter into dissolved inorganic nutrients. It was estimated in Chapter 4 that the amount of nitrogen, regenerated by the mussel beds in the central part of the Oosterschelde, was of the same order of magnitude as regeneration due to mineralization in all other sediments, that cover a much larger area. A comparison of the rates of nitrogen regeneration by mussel beds to estimates of total mineralization (benthic + pelagic) in the central part of the Oosterschelde was presented in Chapter 5, and indicated that the mussels contributed significantly to the nitrogen mineralization. The rapid recycling of nutrients by the mussels may stimulate phytoplankton growth rates in summer, when primary production is limited by low levels of N and Si (Wetsteyn & Kromkamp, 1994).<p>A mesocosm experiment was carried out to explore the relations between mussel grazing, nutrient cycling and phytoplankton development under more controlled conditions. Four mesocosms were used with different densities of mussels to establish a gradient in grazing pressure. The development of phytoplankton biomass was inversely related to mussel biomass, showing the strong effect of mussel grazing on phytoplankton standing stock. In all mesocosms phytoplankton growth was P-limited.<p>In spite of a significant contribution to P-regeneration by the bivalves in the mesocosms with high mussel density, estimates indicated that nutrient regeneration by the mussels was not the most important source of regenerated P. A tentative P balance suggested that external loading and pelagic mineralization contributed significantly to the regeneration of nutrients, and overall nutrient regeneration was higher in the low mussel biomass mesocosms. Still, the availability of phosphate was highest in the mesocosms with the highest mussel density. The experiment demonstrated the major impact that grazing may have on the various nutrient pools. In the mesocosms with low mussel density a major fraction of P was stored in phytoplankton biomass. In the mesocosms; with high mussel density, grazing resulted in a reduction of phytoplankton biomass, a consequently lower storage of P in phytoplankton biomass and an increase of the dissolved inorganic nutrient pool. As a consequence of the increased availability of phosphate in the latter mesocosms the phytoplankton community showed increased growth rates. As was shown by Sterner (1989), even without regeneration of nutrients by the grazers, grazing will increase nutrient availability, simply by preventing the monopolization of this resource by the algal community. The increased phytoplankton growth rates in the high mussel biomass mesocosms coincided with a shift in the composition of the phytoplankton community towards a dominance of diatoms. The results showed that grazing resulted in a transfer of nutrients from the phytoplankton pool to the pools of dissolved inorganic nutrients and grazer biomass, a change in phytoplankton composition, and a change in phytoplankton growth rates.<p>Many temperate coastal ecosystems have large populations of bivalve suspension feeders. Densities of bivalve suspension feeders, typical for bivalve dominated systems, are in the range of 2-8 g ADW m <sup><font size="-2">-3</font></SUP>. This includes systems like San Francisco Bay, Bay of Marennes-Oléron, Western Wadden Sea and Oosterschelde estuary (Smaal & Prins, 1993). The initial mussel biomass in the mesocosm experiment ranged from 0.5 to 3.8 g ADW m <sup><font size="-2">-3</font></SUP>, and the treatments with the highest mussel biomass were comparable to the above-mentioned systems with respect to bivalve density.<p>Results presented in this thesis demonstrated that bivalves affect the pelagic system in various ways. Our mesocosm experiment showed that grazing may induce shifts in phytoplankton species composition towards faster growing species. Moreover, grazing has an effect on the relation between nutrient supply and phytoplankton production. Under nutrient- limiting conditions, bivalve grazing has been shown to have a positive effect on phytoplankton growth rates. This is caused by an increase in nutrient availability. As our observations in the Oosterschelde showed, the mussel population in that estuary filters particulate nutrients, and recycles dissolved inorganic nutrients. The contribution of nitrogen mineralization on mussel beds to total mineralization may be significant, even on the scale of an estuary like the Oosterschelde. The mesocosm experiment demonstrated that regeneration of nutrients by the grazers is not the only factor leading to an increase of the dissolved inorganic nutrient pool. Cropping of the algal community by grazing reduces the accumulation of nutrients in the phytoplankton, and this alone may be sufficient to enlarge the pool of dissolved nutrients (cf. Sterner, 1989).<p>Our <em>in situ</em> observations of grazing rates of the mussels confirmed the hypothesis on top- down control of phytoplankton by bivalve grazing in the Oosterschelde (e.g. Smaal et al., 1986; Herman & Scholten, 1990). Our mesocosm experiment showed a strong regulation of phytoplankton biomass by bivalve grazing. From this it can be inferred that in bivalve dominated systems phytoplankton biomass is determined by grazing, even when nutrients are not limiting. This leads to the conclusion that the response of phytoplankton to changes in external nutrient load will be limited. Similar phenomena have been observed in freshwater systems with dominating large herbivores (e.g. Mazumder, 1994; Mazumder & Lean, 1994). As was argued by Herman & Scholten (1990), top-down control of phytoplankton biomass by bivalve grazing makes a system more resilient to increases in the external nutrient loading, and in this sense the bivalve population acts as a eutrophication control. Eutrophication control by using bivalve suspension feeders has been suggested as a means to combat algal blooms, both in marine and freshwater systems (Takeda & Kurihara, 1994; Ogilvie & Mitchell, 1995). However, it should be realized that grazing will also enlarge the pool of inorganic nutrients. As was pointed out by Herman & Scholten (1990), this large pool of unused nutrients may be profitable to any primary producer that is less susceptible to grazing by the bivalve, for example macro-algae like <em>Ulva sp</em> . or the colony-forming <em>Phaeocystis sp..</em> Eutrophication control by bivalves involves the risk of a sudden shift in an ecosystem towards another, equally undesirable state, and should therefore be accompanied by nutrient input reduction.
Coprecipitation of phosphate with calcium carbonate in Lake Veluwe.
Danen-Louwerse, H.J. ; Lijklema, L. ; Coenraats, M. - \ 1995
Water Research 29 (1995). - ISSN 0043-1354 - p. 1781 - 1785.
meren - reservoirs - plassen - water - waterverontreiniging - waterkwaliteit - eutrofiëring - fosfaten - fosforpentoxide - derivaten - nederland - biologisch waterbeheer - lakes - ponds - water pollution - water quality - eutrophication - phosphates - phosphorus pentoxide - derivatives - netherlands - biological water management
Diatomeeengezelschappen in Overijssel
Maasdam, R. ; Roijackers, R.M.M. ; Cate, J.H. van - \ 1993
Wageningen etc. : LU [etc.]
bacillariophyta - biocenose - biologische technieken - kanalen - rivierwater - rivieren - waterlopen - water - waterkwaliteit - nederland - aquatische ecosystemen - biologische eigenschappen - biologisch waterbeheer - biologische monitoring - overijssel - biocoenosis - biological techniques - canals - river water - rivers - streams - water quality - netherlands - aquatic ecosystems - biological properties - biological water management - biomonitoring
Naar een ecologische indeling van sloten, weteringen en "genormaliseerde" laaglandbeken in Gelderland
Verdonschot, P.F.M. ; Wetering, B. van de - \ 1993
Wageningen : IBN-DLO (IBN - rapport 030) - 119
biologische technieken - waterkwaliteit - hydrobiologie - biocenose - rivieren - waterlopen - waterwegen - rivierregulering - stroomnormalisatie - nederland - biologische monitoring - aquatische ecosystemen - biologisch waterbeheer - gelderland - biological techniques - water quality - hydrobiology - biocoenosis - rivers - streams - waterways - river regulation - stream training - netherlands - biomonitoring - aquatic ecosystems - biological water management
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