Search

The authors present an ecophysiological model of the bivalve Cerastoderma edule that simulates individual growth and reproduction under ambient conditions in temperature and food availability in the Oosterschelde estuary, SW Netherlands. The model contains feedback loops in the uptake and metabolism of food and in the partitioning of carbon to the internal state variables: somatic tissue, storage, organic shell matrix and gametes. The model was calibrated for 24 parameters, based on random distributions of parameter values. This procedure includes an estimate of confidence intervals of the output variables. The simulated growth of shell length and animal wet and dry weight reflected the observed values of growth in the field for the period 1993–1997. The model is a tool for the integration of ecophysiological knowledge of this species and also for carrying-capacity studies of shellfish culture and for environmental management of populations in estuarine and coastal areasWe present an ecophysiological model of the bivalve Cerastoderma edule that Simulates individual growth and reproduction under ambient conditions in temperature and food availability in the Oosterschelde estuary, SW Netherlands. The model contains feedback loops in the uptake and metabolism of food and in the partitioning of carbon to the internal state variables: somatic tissue, storage, organic shell matrix and gametes. The model was calibrated for 24 parameters, based on random distributions of parameter values. This procedure includes an estimate of confidence intervals of the output variables. The simulated growth of shell length and animal wet and dry weight reflected the observed values of growth in the field for the period 1993-1997, The model is a tool for the integration of ecophysiological knowledge of this species and also for carrying-capacity studies of shellfish Culture and for environmental management of populations in estuarine and coastal areas. (c) 2005 Elsevier B.V. All rights reserved.

UItleg over een nieuw ecologisch beoordelingssysteem voor brakke binnenwateren, aansluitend bij de al bestaande STOWA-beoordelingssystemen voor andere watertypen. Op grond van het ecologisch functioneren en een geformuleerd ecologisch streefbeeld worden beïnvloedingsfactoren, karakteristieken en maatstaven onderscheiden, waarmee voor verschillende hoofdtypen en subtypen brakke binnenwateren een ecologisch profiel kan worden opgesteld. Zowel het zoutgehalte als de dynamiek blijken belangrijke differentiërende factoren tussen brakwatersystemen; morfologie en voedselrijkdom spelen een ondergeschikte rol

Uitleg over de totstandkoming van de door het College voor de Toelating van Bestrijdingsmiddelen (CTB) gehanteerde normen voor het bepalen van de risico's van bestrijdingsmiddelen voor het aquatisch ecosysteem, en hoe deze toelatingsnormen zich verhouden tot de in het waterkwaliteitsbeleid gehanteerde maximaal toelaatbare risiconiveaus (MTR's). Beide normen worden vergeleken met de waargenomen effecten van stoffen (insecticiden, herbiciden, fungiciden) in veldexperimenten. Daaruit blijkt dat de CTB-normen voldoende bescherming bieden voor waterorganismen

Weer een nieuw meetsysteem voor Nederlandse natuur. Om de natuurwaarde van oppervlaktewateren te meten wordt een natuurwijzer, opgebouwd uit positieve en negatieve biologische indicatoren, voorgesteld. De natuurwaarde wordt afgemeten aan een referentiebeeld, d.w.z. een stabiele, ecologisch optimale situatie zoals o.a. nog in Poolse wateren is te vinden

De graadmeter natuurwaarde geeft inzicht in de toename of afname van de totale soortendiversiteit. De graadmeter is afgestemd op deelgebieden die bestaan uit een combinatie van fysisch-geografische regio's, met daarbinnen natuurtypen. Per type wordt bekeken, in welke mate (indicator) soorten nog voorkomen. In dit WOT Natuur & Milieu project is gekeken op welke wijze de graadmeter voor aquatische natuur verbeterd kan worden

De graadmeter natuurwaarde geeft inzicht in de toename of afname van de totale soortendiversiteit. De graadmeter is afgestemd op deelgebieden die bestaan uit een combinatie van fysisch-geografische regio's, met daarbinnen natuurtypen. Per type wordt bekeken, in welke mate (indicator) soorten nog voorkomen. In dit WOT Natuur & Milieu project is gekeken op welke wijze de graadmeter voor aquatische natuur verbeterd kan worden

The first objective of the present study is to analyze the biodiversity of the aquatic ditch vegetation in relation to various envirotunental factors. A second objective is to test whether macrophyte biodiversity is highest at an intermediate level of ditch cleaning frequency

Onderhavige studie is een eerste praktische uitwerking voor vijf regionale watersystemen van de zogenaamde natuurwaarde-index. De natuurwaarde is in deze index gedefinieerd als een functie van de natuurkwaliteit en het areaal van een (water)systeem. Voor de natuurkwaliteit kunnen verschillende variabelen gebruikt worden, bijvoorbeeld de vegetatie (zoals in de voorliggende studie). De huidige natuurwaarde wordt bepaald als percentage van een referentiewaarde (uit het verleden). In deze studie wordt daarbij voor de huidige situatie gesproken van de ‘resterende natuurwaarde’, omdat in alle besproken regionale watersystemen sprake is van achteruitgang ten opzichte van de referentiewaarde. Het voorliggende onderzoek is eveneens bedoeld als een soort pilot-uitwerking van de Europese Kaderrichtlijn Water, welke per oppervlaktewatersysteem vraagt om indeling in vijf kwaliteitsklassen, waarbij vegetatie één van de kwaliteitsvariabelen vormt. De doelstelling van voorliggende studie is: Op basis van vegetatie voor vijf regionale watersystemen de resterende natuurwaarde in de huidige situatie bepalen ten opzichte van de referentiewaarde De vijf watersystemen waar het om gaat, zijn: * Vennen in hoog Nederland; * Beken in hoog Nederland; * Heuvellandbeken; * Plassen in het laagveengebied; en * Sloten in net laagveengebied

UItleg over een nieuw ecologisch beoordelingssysteem voor brakke binnenwateren, aansluitend bij de al bestaande STOWA-beoordelingssystemen voor andere watertypen. Op grond van het ecologisch functioneren en een geformuleerd ecologisch streefbeeld worden beïnvloedingsfactoren, karakteristieken en maatstaven onderscheiden, waarmee voor verschillende hoofdtypen en subtypen brakke binnenwateren een ecologisch profiel kan worden opgesteld. Zowel het zoutgehalte als de dynamiek blijken belangrijke differentiërende factoren tussen brakwatersystemen; morfologie en voedselrijkdom spelen een ondergeschikte rol

UItleg over een nieuw ecologisch beoordelingssysteem voor brakke binnenwateren, aansluitend bij de al bestaande STOWA-beoordelingssystemen voor andere watertypen. Op grond van het ecologisch functioneren en een geformuleerd ecologisch streefbeeld worden beïnvloedingsfactoren, karakteristieken en maatstaven onderscheiden, waarmee voor verschillende hoofdtypen en subtypen brakke binnenwateren een ecologisch profiel kan worden opgesteld. Zowel het zoutgehalte als de dynamiek blijken belangrijke differentiërende factoren tussen brakwatersystemen; morfologie en voedselrijkdom spelen een ondergeschikte rol

Onderhavige studie is een eerste praktische uitwerking voor vijf regionale watersystemen van de zogenaamde natuurwaarde-index. De natuurwaarde is in deze index gedefinieerd als een functie van de natuurkwaliteit en het areaal van een (water)systeem. Voor de natuurkwaliteit kunnen verschillende variabelen gebruikt worden, bijvoorbeeld de vegetatie (zoals in de voorliggende studie). De huidige natuurwaarde wordt bepaald als percentage van een referentiewaarde (uit het verleden). In deze studie wordt daarbij voor de huidige situatie gesproken van de ‘resterende natuurwaarde’, omdat in alle besproken regionale watersystemen sprake is van achteruitgang ten opzichte van de referentiewaarde. Het voorliggende onderzoek is eveneens bedoeld als een soort pilot-uitwerking van de Europese Kaderrichtlijn Water, welke per oppervlaktewatersysteem vraagt om indeling in vijf kwaliteitsklassen, waarbij vegetatie één van de kwaliteitsvariabelen vormt. De doelstelling van voorliggende studie is: Op basis van vegetatie voor vijf regionale watersystemen de resterende natuurwaarde in de huidige situatie bepalen ten opzichte van de referentiewaarde De vijf watersystemen waar het om gaat, zijn: * Vennen in hoog Nederland; * Beken in hoog Nederland; * Heuvellandbeken; * Plassen in het laagveengebied; en * Sloten in net laagveengebied

Uitleg over de totstandkoming van de door het College voor de Toelating van Bestrijdingsmiddelen (CTB) gehanteerde normen voor het bepalen van de risico's van bestrijdingsmiddelen voor het aquatisch ecosysteem, en hoe deze toelatingsnormen zich verhouden tot de in het waterkwaliteitsbeleid gehanteerde maximaal toelaatbare risiconiveaus (MTR's). Beide normen worden vergeleken met de waargenomen effecten van stoffen (insecticiden, herbiciden, fungiciden) in veldexperimenten. Daaruit blijkt dat de CTB-normen voldoende bescherming bieden voor waterorganismen

Het project Ecologische streefbeelden voor stromende wateren Veluwe & Vallei heeft ten doel om toetsbare streefbeelden te ontwikkelen voor de stromende wateren in het beheersgebied van de waterschappen Veluwe en Vallei & Eem. Het is het vervolg van het project Maatweb stromende wateren Veluwe & Vallei. In het maatwebproject zijn de verschillende macrofaunagemeenschappen van de stromende wateren in het beheersgebied beschreven. Voor de in totaal dertien onderscheiden gemeenschapstypen uit dit rapport zijn negen streefbeelden ontwikkeld. Deze bestaan uit een beschrijving van de gewenste abiotische condities in combinatie met biotische indicatorsoorten van macrofauna en macrofyten. Daarnaast zijn de te verwachten vissen, amfibieën en vogels bij de streefbeelden vermeld. Selectie van de macrofauna heeft plaatsgevonden met behulp van literatuuronderzoek. Tevens zijn, door middel van statistische analyse, ecologische preferenties van macrofaunataxa voor een aantal voor het streefbeeld relevante milieuvariabelen bepaald.

Het project Ecologische streefbeelden voor stromende wateren Veluwe & Vallei heeft ten doel om toetsbare streefbeelden te ontwikkelen voor de stromende wateren in het beheersgebied van de waterschappen Veluwe en Vallei & Eem. Het is het vervolg van het project Maatweb stromende wateren Veluwe & Vallei. In het maatwebproject zijn de verschillende macrofaunagemeenschappen van de stromende wateren in het beheersgebied beschreven. Voor de in totaal dertien onderscheiden gemeenschapstypen uit dit rapport zijn negen streefbeelden ontwikkeld. Deze bestaan uit een beschrijving van de gewenste abiotische condities in combinatie met biotische indicatorsoorten van macrofauna en macrofyten. Daarnaast zijn de te verwachten vissen, amfibieën en vogels bij de streefbeelden vermeld. Selectie van de macrofauna heeft plaatsgevonden met behulp van literatuuronderzoek. Tevens zijn, door middel van statistische analyse, ecologische preferenties van macrofaunataxa voor een aantal voor het streefbeeld relevante milieuvariabelen bepaald.

Uitleg over de totstandkoming van de door het College voor de Toelating van Bestrijdingsmiddelen (CTB) gehanteerde normen voor het bepalen van de risico's van bestrijdingsmiddelen voor het aquatisch ecosysteem, en hoe deze toelatingsnormen zich verhouden tot de in het waterkwaliteitsbeleid gehanteerde maximaal toelaatbare risiconiveaus (MTR's). Beide normen worden vergeleken met de waargenomen effecten van stoffen (insecticiden, herbiciden, fungiciden) in veldexperimenten. Daaruit blijkt dat de CTB-normen voldoende bescherming bieden voor waterorganismen

Uit de bijdragen in dit themanummer blijkt duidelijk dat poelenaanleg inmiddels in grote delen van Nederland ingeburgerd is als middel om op kleinschalige wijze relatief snel natuurwinst te bereiken. Dit kan zowel binnen de bestaande natuurgebieden als daarbuiten, waarmee poelenaanleg een goed middel is om de verweving tussen natuur- en cultuurlandschap te versterken. De vraag dringt zich op of poelenprojecten aan de verwachtingen voldoen. Is het graven van standaard veedrinkpoelen de juiste weg of moeten er wellicht meer nieuwe inzichten ingepast worden?

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.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.Alternative stable states (Chapter 2)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.The first Biomanipulation experiments (Chapter 3 and 4)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.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.Impact of benthivorous fish on turbidity (Chapter 5)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.Comparison of results with a Danish lake (Chapter 6)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.Development of fish communities after biomanipulation (Chapter 7)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.Biomanipulation on a large scale (Chapter 8)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 Daphnia galeata . 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.Evaluation of Biomanipulation Projects (Chapter 9)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.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 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 Neomysis on grazing by Daphnia 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 > 75% fish removal.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.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 ( > 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.Development of ideas (Chapter 10)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 Daphnia 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.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 Daphnia was high. At least a coverage of 50->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.Implication for the water quality managerThis 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.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.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.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.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.

Uit de bijdragen in dit themanummer blijkt duidelijk dat poelenaanleg inmiddels in grote delen van Nederland ingeburgerd is als middel om op kleinschalige wijze relatief snel natuurwinst te bereiken. Dit kan zowel binnen de bestaande natuurgebieden als daarbuiten, waarmee poelenaanleg een goed middel is om de verweving tussen natuur- en cultuurlandschap te versterken. De vraag dringt zich op of poelenprojecten aan de verwachtingen voldoen. Is het graven van standaard veedrinkpoelen de juiste weg of moeten er wellicht meer nieuwe inzichten ingepast worden?

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.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.Alternative stable states (Chapter 2)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.The first Biomanipulation experiments (Chapter 3 and 4)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.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.Impact of benthivorous fish on turbidity (Chapter 5)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.Comparison of results with a Danish lake (Chapter 6)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.Development of fish communities after biomanipulation (Chapter 7)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.Biomanipulation on a large scale (Chapter 8)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 Daphnia galeata . 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.Evaluation of Biomanipulation Projects (Chapter 9)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.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 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 Neomysis on grazing by Daphnia 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 > 75% fish removal.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.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 ( > 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.Development of ideas (Chapter 10)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 Daphnia 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.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 Daphnia was high. At least a coverage of 50->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.Implication for the water quality managerThis 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.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.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.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.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.

Uit de bijdragen in dit themanummer blijkt duidelijk dat poelenaanleg inmiddels in grote delen van Nederland ingeburgerd is als middel om op kleinschalige wijze relatief snel natuurwinst te bereiken. Dit kan zowel binnen de bestaande natuurgebieden als daarbuiten, waarmee poelenaanleg een goed middel is om de verweving tussen natuur- en cultuurlandschap te versterken. De vraag dringt zich op of poelenprojecten aan de verwachtingen voldoen. Is het graven van standaard veedrinkpoelen de juiste weg of moeten er wellicht meer nieuwe inzichten ingepast worden?

Weer een nieuw meetsysteem voor Nederlandse natuur. Om de natuurwaarde van oppervlaktewateren te meten wordt een natuurwijzer, opgebouwd uit positieve en negatieve biologische indicatoren, voorgesteld. De natuurwaarde wordt afgemeten aan een referentiebeeld, d.w.z. een stabiele, ecologisch optimale situatie zoals o.a. nog in Poolse wateren is te vinden

Weer een nieuw meetsysteem voor Nederlandse natuur. Om de natuurwaarde van oppervlaktewateren te meten wordt een natuurwijzer, opgebouwd uit positieve en negatieve biologische indicatoren, voorgesteld. De natuurwaarde wordt afgemeten aan een referentiebeeld, d.w.z. een stabiele, ecologisch optimale situatie zoals o.a. nog in Poolse wateren is te vinden

An ecological study of wetlands was undertaken in northern Lake Victoria (East Africa) between 1993 and 1996 with a major aim of characterising shallow vegetation-dominated interface habitats, and evaluating their importance for fish, in particular, for the stocked and socio-economically important Oreochromis niloticus LINNÉ (the Nile tilapia). From field and laboratory experiments, five major habitat types could be defined by the type of the dominant emergent macrophyte at the shore from the more than 40 identified plant species along a 110 km shoreline. These were: Cyperus papyrus L. (papyrus), Phragmites mauritianus Kunth (reeds), Typha domingensis Pers. (bulrush), Vossia cuspidata (Roxb.) (hippo grass), and the alien floating Eichhornia crassipes (Martius) Solms-Laubach (water hyacinth).From digital data, considerable long term changes in the shoreline wetland landscape of the lake were discerned and appeared to be primarily associated with increasing human activity (e.g., agriculture, biomass harvests) which had resulted into a 5 % reduction of wetland cover. Inspite of the absence of a well developed euhydrophyte community (e.g., Potamogeton and Ceratophyllum ), and increasing infestations with E. crassipes mats, the width of the littoral zone was established by secchi transparency as being about 50 - 70 m away from the shallow (less than 1 m deep) vegetation fringe sloping to between 2 and 4 m in depth at its outer fringe. Hydrological influences associated with seasonal changes (the alternation of rainy with dry periods) explained most of the observed variation in abiotic (e.g., Si, tot.-P, soluble reactive-P, NO 3 -N, pH, temperature) and biotic (phytoplankton, macrofauna, fish) factors, but there was also significant (p < 0.05) variation due to vegetation, distance from the shore out towards open water and interaction effects between these factors.At least 30 species of fish were identified from the shallower (2.5 m) vegetated habitats in contrast to 10 species from the deeper (4 - 8 m) open water habitats. There were other significant (p < 0.05) spatial and temporal differences in habitat use by fish. Species diversity was dominated by haplochromine species but three stocked species (the Nile perch, Lates niloticus L., O. niloticus and Tilapia zilli ) contributed at least 90% of the estimated numerical and biomass densities of which, the Nile tilapia was the most important component making up 45 - 65 % of the biomass of all fish. Season was a major factor in size-related abundance patterns but generally, most of the Nile tilapia biomass was associated with Phragmites-Vossia-Typha -dominated habitats which were also important for small (

An ecological study of wetlands was undertaken in northern Lake Victoria (East Africa) between 1993 and 1996 with a major aim of characterising shallow vegetation-dominated interface habitats, and evaluating their importance for fish, in particular, for the stocked and socio-economically important Oreochromis niloticus LINNÉ (the Nile tilapia). From field and laboratory experiments, five major habitat types could be defined by the type of the dominant emergent macrophyte at the shore from the more than 40 identified plant species along a 110 km shoreline. These were: Cyperus papyrus L. (papyrus), Phragmites mauritianus Kunth (reeds), Typha domingensis Pers. (bulrush), Vossia cuspidata (Roxb.) (hippo grass), and the alien floating Eichhornia crassipes (Martius) Solms-Laubach (water hyacinth).From digital data, considerable long term changes in the shoreline wetland landscape of the lake were discerned and appeared to be primarily associated with increasing human activity (e.g., agriculture, biomass harvests) which had resulted into a 5 % reduction of wetland cover. Inspite of the absence of a well developed euhydrophyte community (e.g., Potamogeton and Ceratophyllum ), and increasing infestations with E. crassipes mats, the width of the littoral zone was established by secchi transparency as being about 50 - 70 m away from the shallow (less than 1 m deep) vegetation fringe sloping to between 2 and 4 m in depth at its outer fringe. Hydrological influences associated with seasonal changes (the alternation of rainy with dry periods) explained most of the observed variation in abiotic (e.g., Si, tot.-P, soluble reactive-P, NO 3 -N, pH, temperature) and biotic (phytoplankton, macrofauna, fish) factors, but there was also significant (p < 0.05) variation due to vegetation, distance from the shore out towards open water and interaction effects between these factors.At least 30 species of fish were identified from the shallower (2.5 m) vegetated habitats in contrast to 10 species from the deeper (4 - 8 m) open water habitats. There were other significant (p < 0.05) spatial and temporal differences in habitat use by fish. Species diversity was dominated by haplochromine species but three stocked species (the Nile perch, Lates niloticus L., O. niloticus and Tilapia zilli ) contributed at least 90% of the estimated numerical and biomass densities of which, the Nile tilapia was the most important component making up 45 - 65 % of the biomass of all fish. Season was a major factor in size-related abundance patterns but generally, most of the Nile tilapia biomass was associated with Phragmites-Vossia-Typha -dominated habitats which were also important for small (

Afscheid Lijklema

In het water- en natuurbeleid worden verschillende methoden gehanteerd voor het stellen van ecosysteemdoelen in grote wateren. Verschillen in uitgangspunten leiden tot problemen in de beleidsafstemming en ook in een eenduidige toetsing van de beleidsresultaten. Alle grote wateren vallen in de Ecologische Hoofdstructuur, zodat het integraal waterbeheer voor de taak gesteld is om de nationale en internationale biodiversiteit veilig te stellen. Een aantal acties is geformuleerd om tot een betere afstemming tussen het integrale waterbeleid en het sectorale natuurbeleid te komen.

Afscheid Lijklema

Afscheid Lijklema

Afscheid WJ Wolff, IBN

In het water- en natuurbeleid worden verschillende methoden gehanteerd voor het stellen van ecosysteemdoelen in grote wateren. Verschillen in uitgangspunten leiden tot problemen in de beleidsafstemming en ook in een eenduidige toetsing van de beleidsresultaten. Alle grote wateren vallen in de Ecologische Hoofdstructuur, zodat het integraal waterbeheer voor de taak gesteld is om de nationale en internationale biodiversiteit veilig te stellen. Een aantal acties is geformuleerd om tot een betere afstemming tussen het integrale waterbeleid en het sectorale natuurbeleid te komen.

Sri Lanka has more than 12.000 artificial reservoirs and no natural lakes as is true for many other areas in SE Asia. These reservoirs cover a total surface area of around 175.000 ha. They are shallow with maximum depths often less than 5 m and exhibit considerable fluctuations in water level. In general the fish production, estimated from commercial catches, is high in these reservoirs, but lower than would be expected on the basis of the high primary production. This situation changed drastically when around 1952 the exotic tilapias O.mossambicus and O.niloticus were introduced. From that moment onward the fish yield increased and the proportion of inland fisheries to total commercial fisheries rose from almost 0% up to the current 20%. This freshwater fish yield is dominated by the introduced tilapias. The cause of this successful introduction of the tilapias has long been debated. Two different hypotheses exist. Firstly, it has been suggested that the relatively low fish production (based on the yield) in the reservoirs before the introduction of the tilapias was probably due to the origin of the fish, mainly indigenous riverine carps, which may not be optimally adapted to their new conditions. According to this hypothesis the introduced tilapias were competitively dominant and were able to capture niches formerly occupied by the indigenous fish species. Secondly, the alternative hypothesis assumes that the tilapias are able to occupy an empty niche when introduced in the reservoirs and consequently did not compete with the indigenous fish species. Determination of the correct theory is important, both from a theoretical and applied (fisheries management) point of view. This is attempted by studying, for the first time, the effects of the exotic lacustrine tilapias on the indigenous riverine fish species, within the framework of a tropical fish community.The present study was conducted at Tissawewa reservoir. Tissawewa is a typical shallow lowland irrigation reservoir (average depth = 1.2 m) with a surface area of about 200 ha in the dry SE corner of Sri Lanka. In this study the total biological fish production was estimated at 2430 kg/ha/yr of which about 20% consisted of the exotic tilapias. In contrast, about 80% of the commercial catches of 242 kg/ha/yr consisted of these species. This strongly suggests that the importance of tilapias in terms of biological fish production was markedly overestimated not only in Sri Lanka but most probably in all of SE Asia because all estimates of fish production were based on commercial catches. The gross primary production was estimated at about 13.000 kg C/ha/yr of which 1.9% is transferred into fish. This efficiency is high compared to efficiencies between 0.2% and 1.6% reported for other lakes and reservoirs. This high transfer efficiency of fish production can be explained by the short food chain. Most of the fish biomass (ca 45%) consists of a small indigenous cyprinid, A.melettinus, which feeds predominantly on phytoplankton and detritus. The tilapias (which make up ca. 9% of the fish biomass) are also herbivorous. These primary consumers represent about 64% of the biological fish production in the reservoir. The secondary consumers represent 36% of the biologic production while only 0.3% of the fish production is made up by the third trophic level, that of the tertiary consumers.The structure and fuinctioning of the fish community can be regulated according to several mechanisms such as a deterministic versus a stochastic regulation or, when deterministically regulated, by bottom-up (regulated by primary producers) versus top-down (regulated by predators) control. A deterministically regulated community is generally at an equilibrium, with population levels at carrying capacity determined by resource limitations and coexisting species avoiding competitive exclusion through biotic interactions such as resource partitioning. In contrast, a community is stochastically regulated when the environment is not stable enough to allow an equilibrium to persist. The abundances of species in such a community are determined largely through differential responses to unpredictable environmental changes, rather than through biotic interactions. Based on the disturbances, caused by water level fluctuations, acting on the fish community and community characteristics such as resistance and resilience, the Tissawewa fish community is considered to be deterministically regulated. However, extreme low water levels or the drying up of the reservoir which occur occasionally can cause a relatively brief time of stochastic regulation. Both these phenomena could be studied because during the sampling period, due to a long period of extreme drought, the water level decreased until the reservoir was completely dry. After filling up a different ecosystem had evolved. Before the drought (Period 1) Tissawewa had vegetation only in the shallow, littoral zone and a high turbidity due to resuspension of the thick layer of detritus on the bottom. After the drought (Period 2) vegetation was found all over the reservoir, covering the entire water column, and the water was significantly clearer due to a lower concentration of suspended detritus and phytoplankton. During Period 1 the average fish biomass was about 1700 kg/ha, while during Period 2 this was only 770 kg/ha. The community was at equilibrium at the beginning of the sampling period but as the water level decreased below 1.5 in maximum depth, this equilibrium was disturbed. About one year after the reservoir refilled, a new equilibrium different from the former equilibrium was established. The species mainly affected by these environmental changes was A.melettinus. Its share in the total fish biomass decreased from on average 45% during Period I to 9% during Period 2. This was mainly caused by a decrease of its main food source, that of suspended detritus, from respectively 7.3 g C/m 3to 2.1 g C/m 3. This also illustrates the importance of bottom-up regulation for the structure of the Tissawewa fish community.The fact that the structure of the Tissawewa fish community is deterministically regulated together with the importance of bottom-up regulation implies that competition and resource partitioning are important mechanisms determining the abundance of the different populations. Therefore the niche occupation of the ten most important fish species was determined along three dimensions: the trophic, the spatial and the temporal dimension. The position along the trophic dimension depends on the diet of the species, the position along the spatial dimension on the distribution over the various habitats in the reservoir and the position along the temporal dimension depends on the time of day the species were actively foraging. Because the body size of fish can increase several orders of magnitude during ontogeny which, in turn, can have considerable consequences for the niche occupation of an individual, each species was subdivided into size-classes for which niche occupation was determined separately. Not only the size-specific niche occupation along each dimension was determined in this study, but also the interaction between the different dimensions. In order to comprehend the impact of these interactions two types of competition should be distinguished: exploitative competition and interference competition. The former describes how species affect each other through the exploitation of the same food sources, the latter deals with a more direct interference through the capturing of space.Exploitative competition mainly depends on the interaction between the trophic and spatial dimension. In other words: what food is taken from where while the time at which this happens is unimportant. Interference competition depends on the interaction between the spatial and temporal dimension: where and when is a certain species active. With regard to the time of day a species is actively foraging, seven diurnal species were observed, two nocturnal piscivores while one species was mainly active during dusk and dawn. When actively foraging the distribution of a fish is determined by a trade-off between foraging profitability and risk of predation. For those species or size-classes subject to predation, avoidance of predation is the main factor determining their distribution even if this results in suboptimal foraging. The juveniles of all species avoid predation by using the cover provided by the vegetation. The main adult prey, A.melettinus, H. gaimardi and R.daniconius avoid predation by evading the bottom layer of the reservoir where the piscivores reside. For those individuals which are not foraging their distribution is aimed at avoiding interference competition and predation. This is realised by all fish through a migration upward along the vertical plane. Environmental changes affecting the risk of predation and foraging rate are reflected in the distribution patterns of the fish species involved, confirming the ability of species to facultatively respond to these factors. Because fish density and consequently interference competition was lower after the drought, the upward migration also decreased.For the Tissawewa fish community a correlation was observed between the morphology of a fish and its diet. This diet was apparently determined by only a few morphological characters. The gape width constrained the maximum size of the food particles, the gut length was related to the potential of utilising vegetable matter and the orientation of the mouth and the presence of barbels were correlated with the position of the food along the vertical gradient. Assuming functional relationships, the potential niches of the species were established, and within each species the main ontogenetic differences could be explained. The realised trophic niches of each population both before and after the drought coincided with what could be expected from the relative positions of the potential niches of these populations along the trophic resource dimension assuming niche segregation.It was observed that size-specific measures of calculating niche breadths and niche overlaps, in which ontogenetic changes can be incorporated, present considerable advantages to the conventional measures when studying intra- and interspecific competition. For example, niche breadth which can be considered an indicator of the flexibility of a species to adjust to changes in food availability is markedly affected by ontogenetic changes; species with large ontogenetic changes would be considered more flexible then they actually are. Species of which the juvenile stages appeared to compete for resources would not be considered potential competitors based on the conventional measures. Also, possible competition for food was observed between the zooplanktivorous juvenile stages of several species with non- zooplanktivorous large adults, and the adult zooplanktivores: the juvenile competitive bottleneck. The abundance of a species is determined by the combination of intra- and interspecific competition. The calculated niche overlap measures were compared: 1) within one species between ontogenetic stages; 2) between species; 3) for the entire fish community between the periods before and after the drought. This strongly suggested that: 1) intra-specific competition is lower for species with large ontogenetic changes; 2) inter-specific competition is lower for specialists; and 3) competition for resources exists. The latter was concluded because during Period 1 when fish biomass was highest, niche segregation was highest and consequently average niche overlap was lowest. For the Tissawewa fish community segregation along the trophic dimension was more important than along the spatial or temporal dimension.Based on niche overlap along three resource dimensions, the main potential indigenous competitor of the exotic introduced tilapias is the small pelagic cyprinid, A . melettinus. However, a more detailed analysis of the foraging behaviour of these potential competitors shows that A.melettinus mainly feeds on the fine particulate detritus and phytoplankton, suspended in the water column, while the tilapias graze the deferral aggregate from the bottom. This implies that A . melettinus is not a true competitor of the tilapias and might even benefit from the tilapias breaking down the flocculant material from the bottom into smaller, better suspendable, particles. Therefore, the successful introduction of the lacustrine tilapias can be explained from the ability of these species to occupy a previously unfilled niche, and as a consequence the indigenous fish community was most probably not harmed by this introduction.The acquired knowledge of the Tissawewa ecosystem pertaining to the effects of, among others, competition, predation and water level fluctuations on the fish community is used to discuss the consequences of various management measures on the fish production and fish yield. It was concluded that an additional exploitation of van the small pelagic fish species, either directly, or through introduction and exploitation of a large (controllable) piscivore, will most probably enhance the existing fishery on tilapias. Based on the Tissawewa case study it is concluded that at least for Tissawewa, but most probably for all similar tropical waterbodies a markedly higher utilisation efficiency (yield/ production) can be achieved than the 10% that is currently assumed.

A survey is given of the work and life of Cornelis den Hartog up to the date in 1996 on which he retired from his position as a professor at the University of Nijmegen. Cornelis (Kees) den Hartog made important contributions to aquatic ecology in the widest sense, e.G. On brackish water typology, meiofauna (microturbellaria), macroinvertebrates, littoral algae, sea-grasses and aquatic macrophytes. He favoured the ecosystem approach in aquatic ecology by studying structure and functioning in an integrated way. He led 31 students to their doctor's degrees (PhD).

Sri Lanka has more than 12.000 artificial reservoirs and no natural lakes as is true for many other areas in SE Asia. These reservoirs cover a total surface area of around 175.000 ha. They are shallow with maximum depths often less than 5 m and exhibit considerable fluctuations in water level. In general the fish production, estimated from commercial catches, is high in these reservoirs, but lower than would be expected on the basis of the high primary production. This situation changed drastically when around 1952 the exotic tilapias O.mossambicus and O.niloticus were introduced. From that moment onward the fish yield increased and the proportion of inland fisheries to total commercial fisheries rose from almost 0% up to the current 20%. This freshwater fish yield is dominated by the introduced tilapias. The cause of this successful introduction of the tilapias has long been debated. Two different hypotheses exist. Firstly, it has been suggested that the relatively low fish production (based on the yield) in the reservoirs before the introduction of the tilapias was probably due to the origin of the fish, mainly indigenous riverine carps, which may not be optimally adapted to their new conditions. According to this hypothesis the introduced tilapias were competitively dominant and were able to capture niches formerly occupied by the indigenous fish species. Secondly, the alternative hypothesis assumes that the tilapias are able to occupy an empty niche when introduced in the reservoirs and consequently did not compete with the indigenous fish species. Determination of the correct theory is important, both from a theoretical and applied (fisheries management) point of view. This is attempted by studying, for the first time, the effects of the exotic lacustrine tilapias on the indigenous riverine fish species, within the framework of a tropical fish community.The present study was conducted at Tissawewa reservoir. Tissawewa is a typical shallow lowland irrigation reservoir (average depth = 1.2 m) with a surface area of about 200 ha in the dry SE corner of Sri Lanka. In this study the total biological fish production was estimated at 2430 kg/ha/yr of which about 20% consisted of the exotic tilapias. In contrast, about 80% of the commercial catches of 242 kg/ha/yr consisted of these species. This strongly suggests that the importance of tilapias in terms of biological fish production was markedly overestimated not only in Sri Lanka but most probably in all of SE Asia because all estimates of fish production were based on commercial catches. The gross primary production was estimated at about 13.000 kg C/ha/yr of which 1.9% is transferred into fish. This efficiency is high compared to efficiencies between 0.2% and 1.6% reported for other lakes and reservoirs. This high transfer efficiency of fish production can be explained by the short food chain. Most of the fish biomass (ca 45%) consists of a small indigenous cyprinid, A.melettinus, which feeds predominantly on phytoplankton and detritus. The tilapias (which make up ca. 9% of the fish biomass) are also herbivorous. These primary consumers represent about 64% of the biological fish production in the reservoir. The secondary consumers represent 36% of the biologic production while only 0.3% of the fish production is made up by the third trophic level, that of the tertiary consumers.The structure and fuinctioning of the fish community can be regulated according to several mechanisms such as a deterministic versus a stochastic regulation or, when deterministically regulated, by bottom-up (regulated by primary producers) versus top-down (regulated by predators) control. A deterministically regulated community is generally at an equilibrium, with population levels at carrying capacity determined by resource limitations and coexisting species avoiding competitive exclusion through biotic interactions such as resource partitioning. In contrast, a community is stochastically regulated when the environment is not stable enough to allow an equilibrium to persist. The abundances of species in such a community are determined largely through differential responses to unpredictable environmental changes, rather than through biotic interactions. Based on the disturbances, caused by water level fluctuations, acting on the fish community and community characteristics such as resistance and resilience, the Tissawewa fish community is considered to be deterministically regulated. However, extreme low water levels or the drying up of the reservoir which occur occasionally can cause a relatively brief time of stochastic regulation. Both these phenomena could be studied because during the sampling period, due to a long period of extreme drought, the water level decreased until the reservoir was completely dry. After filling up a different ecosystem had evolved. Before the drought (Period 1) Tissawewa had vegetation only in the shallow, littoral zone and a high turbidity due to resuspension of the thick layer of detritus on the bottom. After the drought (Period 2) vegetation was found all over the reservoir, covering the entire water column, and the water was significantly clearer due to a lower concentration of suspended detritus and phytoplankton. During Period 1 the average fish biomass was about 1700 kg/ha, while during Period 2 this was only 770 kg/ha. The community was at equilibrium at the beginning of the sampling period but as the water level decreased below 1.5 in maximum depth, this equilibrium was disturbed. About one year after the reservoir refilled, a new equilibrium different from the former equilibrium was established. The species mainly affected by these environmental changes was A.melettinus. Its share in the total fish biomass decreased from on average 45% during Period I to 9% during Period 2. This was mainly caused by a decrease of its main food source, that of suspended detritus, from respectively 7.3 g C/m 3to 2.1 g C/m 3. This also illustrates the importance of bottom-up regulation for the structure of the Tissawewa fish community.The fact that the structure of the Tissawewa fish community is deterministically regulated together with the importance of bottom-up regulation implies that competition and resource partitioning are important mechanisms determining the abundance of the different populations. Therefore the niche occupation of the ten most important fish species was determined along three dimensions: the trophic, the spatial and the temporal dimension. The position along the trophic dimension depends on the diet of the species, the position along the spatial dimension on the distribution over the various habitats in the reservoir and the position along the temporal dimension depends on the time of day the species were actively foraging. Because the body size of fish can increase several orders of magnitude during ontogeny which, in turn, can have considerable consequences for the niche occupation of an individual, each species was subdivided into size-classes for which niche occupation was determined separately. Not only the size-specific niche occupation along each dimension was determined in this study, but also the interaction between the different dimensions. In order to comprehend the impact of these interactions two types of competition should be distinguished: exploitative competition and interference competition. The former describes how species affect each other through the exploitation of the same food sources, the latter deals with a more direct interference through the capturing of space.Exploitative competition mainly depends on the interaction between the trophic and spatial dimension. In other words: what food is taken from where while the time at which this happens is unimportant. Interference competition depends on the interaction between the spatial and temporal dimension: where and when is a certain species active. With regard to the time of day a species is actively foraging, seven diurnal species were observed, two nocturnal piscivores while one species was mainly active during dusk and dawn. When actively foraging the distribution of a fish is determined by a trade-off between foraging profitability and risk of predation. For those species or size-classes subject to predation, avoidance of predation is the main factor determining their distribution even if this results in suboptimal foraging. The juveniles of all species avoid predation by using the cover provided by the vegetation. The main adult prey, A.melettinus, H. gaimardi and R.daniconius avoid predation by evading the bottom layer of the reservoir where the piscivores reside. For those individuals which are not foraging their distribution is aimed at avoiding interference competition and predation. This is realised by all fish through a migration upward along the vertical plane. Environmental changes affecting the risk of predation and foraging rate are reflected in the distribution patterns of the fish species involved, confirming the ability of species to facultatively respond to these factors. Because fish density and consequently interference competition was lower after the drought, the upward migration also decreased.For the Tissawewa fish community a correlation was observed between the morphology of a fish and its diet. This diet was apparently determined by only a few morphological characters. The gape width constrained the maximum size of the food particles, the gut length was related to the potential of utilising vegetable matter and the orientation of the mouth and the presence of barbels were correlated with the position of the food along the vertical gradient. Assuming functional relationships, the potential niches of the species were established, and within each species the main ontogenetic differences could be explained. The realised trophic niches of each population both before and after the drought coincided with what could be expected from the relative positions of the potential niches of these populations along the trophic resource dimension assuming niche segregation.It was observed that size-specific measures of calculating niche breadths and niche overlaps, in which ontogenetic changes can be incorporated, present considerable advantages to the conventional measures when studying intra- and interspecific competition. For example, niche breadth which can be considered an indicator of the flexibility of a species to adjust to changes in food availability is markedly affected by ontogenetic changes; species with large ontogenetic changes would be considered more flexible then they actually are. Species of which the juvenile stages appeared to compete for resources would not be considered potential competitors based on the conventional measures. Also, possible competition for food was observed between the zooplanktivorous juvenile stages of several species with non- zooplanktivorous large adults, and the adult zooplanktivores: the juvenile competitive bottleneck. The abundance of a species is determined by the combination of intra- and interspecific competition. The calculated niche overlap measures were compared: 1) within one species between ontogenetic stages; 2) between species; 3) for the entire fish community between the periods before and after the drought. This strongly suggested that: 1) intra-specific competition is lower for species with large ontogenetic changes; 2) inter-specific competition is lower for specialists; and 3) competition for resources exists. The latter was concluded because during Period 1 when fish biomass was highest, niche segregation was highest and consequently average niche overlap was lowest. For the Tissawewa fish community segregation along the trophic dimension was more important than along the spatial or temporal dimension.Based on niche overlap along three resource dimensions, the main potential indigenous competitor of the exotic introduced tilapias is the small pelagic cyprinid, A . melettinus. However, a more detailed analysis of the foraging behaviour of these potential competitors shows that A.melettinus mainly feeds on the fine particulate detritus and phytoplankton, suspended in the water column, while the tilapias graze the deferral aggregate from the bottom. This implies that A . melettinus is not a true competitor of the tilapias and might even benefit from the tilapias breaking down the flocculant material from the bottom into smaller, better suspendable, particles. Therefore, the successful introduction of the lacustrine tilapias can be explained from the ability of these species to occupy a previously unfilled niche, and as a consequence the indigenous fish community was most probably not harmed by this introduction.The acquired knowledge of the Tissawewa ecosystem pertaining to the effects of, among others, competition, predation and water level fluctuations on the fish community is used to discuss the consequences of various management measures on the fish production and fish yield. It was concluded that an additional exploitation of van the small pelagic fish species, either directly, or through introduction and exploitation of a large (controllable) piscivore, will most probably enhance the existing fishery on tilapias. Based on the Tissawewa case study it is concluded that at least for Tissawewa, but most probably for all similar tropical waterbodies a markedly higher utilisation efficiency (yield/ production) can be achieved than the 10% that is currently assumed.

Afscheid WJ Wolff, IBN

Een groot deel van de natuurgebieden in Nederland is sterk in kwaliteit achteruitgegaan en genivelleerd als gevolg van atmosferische depositie. Het duurzaam voortbestaan en herstel van de genivelleerde en bedreigde ecosystemen en soorten is op lange termijn slechts mogelijk indien maatregelen worden genomen die de luchtverontreiniging doen afnemen. ln dit kader vormen de Oisterwijkse Vennen een belangrijk onderzoekgebied.