Staff Publications

Staff Publications

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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.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    The role of environmental shocks in shaping prosocial behavior
    Duchoslav, Jan - \ 2017
    Wageningen University. Promotor(en): E.H. Bulte, co-promotor(en): F. Cecchi. - Wageningen : Wageningen University - ISBN 9789463431477 - 190
    environment - behaviour - economic development - social behaviour - stress conditions - environmental temperature - physical properties - social environment - milieu - gedrag - economische ontwikkeling - sociaal gedrag - stress omstandigheden - omgevingstemperatuur - fysische eigenschappen - sociaal milieu

    All economic activity requires some degree of cooperation, and the process of economic development involves many social dilemmas. It is therefore crucial to understand how the preferences which guide our behavior vis-à-vis these situations are shaped. The ability and willingness to work for the benefit of the group rather than just one's own has evolved over many generations, and is – to some extent – innate to any healthy human being. At the same time, individual prosocial preferences are – also to a certain extent – endogenous to the physical and social environment within which we operate. This thesis identifies several ways in which environmental changes affect intrinsic prosocial preferences, and outlines a possible direction for fixing any such negative effects.

    In Chapter 1, I introduce the topic of prosocial preferences. I briefly describe how prosociality has been viewed over the course of scientific history, and summarize the current state of knowledge about the formation of social preferences. I further outline how extrinsic incentives can influence prosocial behavior without affecting the preferences which underpin it. Finally, the chapter contains an overview of the methodologies used throughout this thesis.

    In Chapter 2, I focus on an early formative factor of prosocial preferences—their fetal origins. I study how temperature shocks faced by pregnant women affect their children's later-life prosocial preferences. I find that exposure to higher than usual ambient temperatures during gestation reduces a child's probability of contribution to the public good, with the negative effect lasting into adulthood.

    Chapter 3 continues in the same vein as Chapter 2, looking at the fetal origins of prosocial preferences. In this chapter, I investigate how prenatal stress induced by random violence affects the preferences for cooperation among children born during an armed conflict. To do so, I exploit variations in the ratio of the lengths of the index and ring fingers—a marker of in utero hormone exposure negatively associated with high maternal distress during early fetal development. I show that prenatal stress reduces the probability that children contribute to the public good.

    In Chapter 4, I move away from the physical aspects of human environment, focusing instead on the social ones. I study the effects of a sudden introduction of a formal institution on individual cooperative behavior within informal arrangements. In particular, I look at how an NGO intervention which helped create a mutual health insurance affected cooperative behavior in a public goods game. I find that the introduction of formal insurance reduces contributions to the public good. This reduction in cooperation levels is, however, not due to the adopters of the formal insurance who may now have less need for informal reciprocal networks, and who therefore (partially) withdraw from them. It is instead the non-adopters who become less cooperative towards the adopters.

    To outline a possible direction for remedying the negative environmental effects on prosocial behavior described in the previous three chapters, I illustrate one of the ways in which prosocial behavior can be incentivized with a relatively simple and easily implementable policy. In Chapter 5, I evaluate the impact of introducing performance-based financial incentives on staff effort and, consequently, on allocative efficiency and output in healthcare provision. I show that in the case under investigation, financial incentives conditioned on output and worth roughly 5% of total expenditures increased staff effort to the extent that output rose by over 25%, without any detectable drop in the quality of the provided services. This not only shows the potential of incentive-compatible financing to improve the performance of underfunded healthcare systems in developing countries, but also that extrinsic motivation can be used to foster behavior which benefits the society rather than just the individual.

    Finally, I combine the main findings from the core chapters of the thesis in Chapter 6. I discuss their policy implications, and point out the some of the outstanding questions, outlining the directions for future research.

    In vivo 1H NMR methods to study dynamics of chloroplast water and thylakoid membrane lipids in leaves and in photosynthetic microorganisms
    Pagadala, Shanthi - \ 2017
    Wageningen University. Promotor(en): H. van Amerongen, co-promotor(en): H. van As. - Wageningen : Wageningen University - ISBN 9789463431569 - 130
    cell membranes - membranes - chloroplasts - thylakoids - photosynthesis - in vivo experimentation - stress conditions - stress - proteins - lipids - mobility - dynamics - celmembranen - membranen - chloroplasten - thylakoïden - fotosynthese - in vivo experimenten - stress omstandigheden - stress - eiwitten - lipiden - mobiliteit - dynamica

    Dynamics of thylakoid membranes and mobility of pigment-protein complexes therein are essential for survival of photosynthetic organisms under changing environmental conditions. The published approaches to probe mobility of the thylakoid membrane lipids and protein complexes are either dependent on the use of external labels or are used only for in vitro studies. Here, we present non-invasive 1H NMR methods (DOSY and DRCOSY) to study dynamics of water in chloroplasts, lipids in oil bodies and in thylakoid membranes and pigment-protein complexes under complete in vivo conditions in leaf disks of F. benjamina and A. platanoides and in suspensions of the green alga Chlamydomonas reinhardtii and blue-green alga Synechocystissp.PCC 6803.

    In leaf disks of Ficus benjamina and Acer platanoides, water in chloroplasts could be clearly discriminated from other pools. Both water in chloroplasts, and water in vacuoles of palisade and spongy cells showed resonances in the high field part of the spectra (with respect to pure water), in contrast to what has been reported in literature. Subepidermal cells (present only in F. benjamina but not in A. platanoides) may act as a water storage, buffer pool during drought. This pool prevented the fast loss of water from the chloroplasts. Nutrient stress and excess salt stress resulted in accumulated lipid bodies and in striking differences in the dynamics and spectra/composition of the different components. T2 values of the different components are compared with those observed in suspensions of Synechocystissp.PCC 6803. The differences in membrane composition (ratio of the different membrane lipids) were clearly observed in the DANS of the oil bodies and the (thylakoid) membranes, but the diffusion coefficients were quite comparable. Also the DANS of the component that is assigned to the pigment-protein complexes are quite different, reflecting the differed composition. The diffusion coefficients of this component in isolated spinach thylakoids and in C. reinhardtii are very comparable, but about a factor of 10 lower with respect to that of Synechocystis at short diffusion times. The dynamics of these complexes in these systems are thus quite different.

    Endogene stress : stressoren (deel 3)
    Leenstra, S.H. ; Scheerboom, J. - \ 2014
    Aquacultuur 29 (2014)4. - ISSN 1382-2764 - p. 22 - 23.
    aquacultuur - vissen - stressreactie - stress omstandigheden - aquaculture - fishes - stress response - stress conditions
    Een vishouder dient te weten dat een vis gedurende het leven ontwikkelingen doormaakt, die stress voor het dier betekenen. Hierbij worden - ter afsluiting van vorige bijdragen over stressoren (zie Aquacultuur 2014, nr. 2 en 3) - voorbeelden van endogene stress beschreven.
    QTL-based physiological modelling of leaf photosynthesis and crop productivity of rice (Oryza sativa L.) under well-watered and drought environments
    Gu, J. - \ 2013
    Wageningen University. Promotor(en): Paul Struik; H. Wang, co-promotor(en): Xinyou Yin; Tjeerd-Jan Stomph. - S.L. : s.n. - ISBN 9789461735300 - 181
    oryza sativa - fotosynthese van het kroondak - gewasproductie - stress omstandigheden - loci voor kwantitatief kenmerk - genetische merkers - plantenveredeling - simulatiemodellen - oryza sativa - canopy photosynthesis - crop production - stress conditions - quantitative trait loci - genetic markers - plant breeding - simulation models

    Key words: Drought, ecophysiological crop modelling, GECROS, genotype, G×E interaction, modelling, Oryza sativa L., photosynthesis, quantitative trait locus, rice.

    Improving grain yield of rice (Oryza sativa L.) crop for both favourable and stressful environments is the main breeding objective to ensure food security. The objective of this study was to amalgamate crop modelling and genetic analysis, to create knowledge and insight useful in view of this breeding objective.

    Photosynthesis is fundamental to biomass production, but the process is very sensitive to abiotic stresses, including drought. Upland rice cv. Haogelao, lowland rice cv. Shennong265, and 94 of their introgression lines (ILs) were studied under drought and well-watered conditions to analyse the genetics of leaf photosynthesis. After correcting for microclimate fluctuations, significant genetic variation was found in this population, and 1-3 quantitative trait loci (QTLs) were detected per photosynthesis-related trait. A major QTL was mapped near marker RM410 on Chromosome 9 and was consistent for phenotyping at flowering and grain filling, under drought and well-watered conditions, and across field and greenhouse experiments. These results suggest that photosynthesis at different phenological stages and under different environmental conditions is, at least to some extent, influenced by the same genetic factors.

    To understand the physiological regulation of genetic variation and resulting QTLs for photosynthesis detected in the first study, 13 ILs were carefully selected as representatives of the population, based on the QTLs for leaf photosynthesis. These 13 ILs were studied under moderate drought and well-watered conditions in the experiment where combined gas exchange and chlorophyll fluorescence data were collected to assess CO2 and light response curves. Using these curves, seven parameters of a photosynthesis model were estimated to dissect photosynthesis into stomatal conductance (gs), mesophyll conductance (gm), electron transport capacity (Jmax), and Rubisco carboxylation capacity (Vcmax). Genetic variation in light saturated photosynthesis and the major QTL of photosynthesis on Chromosome 9 were mainly associated with variation in gs and gm. Furthermore, relationships between these parameters and leaf nitrogen or dry matter per unit area were shown valid for variation across genotypes and across water treatments. In view of these results and literature reports, it was argued that variation in photosynthesis due to environmental conditions and to genetic variation shares common physiological mechanisms.

    QTL analyses were further extended to other physiological parameters of rice. Molecular marker-based estimates of these traits from estimated additive allele effects were used as input tothe mechanistic crop model GECROS. This marker/QTL-based modelling approach showed the ability of predicting genetic variation of crop performance within ILs for a diverse set of field conditions. This approach also showed the potential of extrapolating to a large population of recombinant inbred lines from the same parents. Most importantly, this model approach may improve the efficiency of marker-assisted selection, as it provides a tool to rank the relative importance of the identified markers in determining final yield under specific environmental conditions.

    To examine the extent to which natural genetic variation in photosynthesis can contribute to increasing biomass production and yield of rice, the GECROS crop model was used again to analyse the impact of genetic variation in photosynthesis on crop biomass production. It was shown that in contrast to other studies a genetic variation in photosynthesis of 25% can be scaled up equally to crop level, resulting in an increase in biomass of 22-29% across different locations and years. The difference with earlier studies seems related to the fact that variation in both Rubisco-limited and electron transport-limited photosynthesis were observed in our IL population.

    This thesis has contributed to closing the gap between genotype and phenotype by integrating crop physiology and genetics through an innovative QTL/marker-based modelling approach. This approach can contribute to making the use of genomics much more efficient in practical plant breeding.

    Contrasting extremes in water-related stresses determine species survival
    Bartholomeus, R.P. ; Witte, J.P.M. ; Bodegom, P.M. van; Dam, J.C. van; Aerts, R. - \ 2012
    stress omstandigheden - droogte - zuurstoftekort - bodemwaterbalans - biodiversiteit - ecohydrologie - planten - bedreigde soorten - klimaatverandering - stress conditions - drought - oxygen deficiency - soil water balance - biodiversity - ecohydrology - plants - endangered species - climatic change
    Contribution to the EGU General Assembly 2012.
    Climate change hampers endangered species by stronger water-related stresses
    Bartholomeus, R. ; Witte, F. ; Bodegom, P. van; Dam, J.C. van; Aerts, R. - \ 2011
    Geophysical Research Abstracts 13 (2011). - ISSN 1029-7006
    klimaatverandering - bedreigde soorten - stress omstandigheden - droogte - zuurstoftekort - bodemwaterbalans - ecohydrologie - climatic change - endangered species - stress conditions - drought - oxygen deficiency - soil water balance - ecohydrology
    The international research community is beginning to realise that climate extremes may be more powerful drivers of vegetation change and species extinctions than slow-and-steady climatic changes, but the causal mechanisms of such changes are presently unknown. In order to quantify oxygen and drought stress with causal measures, we focused on interacting meteorological, soil physical, microbial, and plant physiological processes in the soil-plant-atmosphere system. Contribution to the EGU General Assembly 2011.
    Climate change threatens endangered plant species by stronger and interacting water-related stresses
    Bartholomeus, R.P. ; Witte, J.P.M. ; Bodegom, P.M. van; Dam, J.C. van; Aerts, R. - \ 2011
    Journal of Geophysical Research: Biogeosciences 116 (2011)G4. - ISSN 2169-8953
    klimaatverandering - bedreigde soorten - vegetatie - bodemwater - stress omstandigheden - droogte - ecohydrologie - climatic change - endangered species - vegetation - soil water - stress conditions - drought - ecohydrology - environmental variation - terrestrial ecosystems - hydraulic conductivity - regression quantiles - indicator values - soil-conditions - oxygen stress - root-growth - diversity
    Atmospheric CO2-concentration, temperature and rainfall variability are all expected to increase in the near future. The resulting increased dynamics of soil moisture contents, together with increased plant physiological demands for both oxygen and water, will lead to an increased occurrence of wet and dry extremes of plant stresses, i.e. of oxygen and drought stress, respectively, alone and in interaction. The use of indirect environmental variables of previous studies and their focus on one stress at a time has hampered understanding the causal impact of climate change on plant species composition through changes in abiotic site conditions. Here, we use process-based simulations of oxygen and drought stress and show that both stresses will increase (on average with ca. 20% at sites where both stresses occur) in a warmer and more variable future (2050) climate (applying a national downscaled version of IPCC scenarios). These stresses will increasingly coincide, i.e. both stresses will occur more often (but not at the same time) within the same vegetation plot. We further show that particularly this increased coincidence of water-related stresses will negatively affect the future occurrence of currently endangered plant species (a reduction of 16%), while such a decrease is not apparent for common species. Individual stresses did not affect the occurrence of endangered plant species. Consequently, the species that are already threatened under the current climate, will suffer most from climate change
    Uitdroging in keten grootste gevaar voor vaste planten
    Dalfsen, P. van; Dijkema, M.H.G.E. ; Gude, H. ; Miller, B. - \ 2007
    De Boomkwekerij 2007 (2007)1. - ISSN 0923-2443 - p. 12 - 13.
    stekken - kwekers - hergroei - gewaskwaliteit - dehydratie (fysiologisch) - export - stress omstandigheden - cuttings - growers - regrowth - crop quality - dehydration (physiological) - exports - stress conditions
    Na export hebben diverse soorten vaste planten problemen met de hergroei. Onderzoek werd gedaan door PPO naar de omstandigheden in de keten die tot deze problemen leiden, welke kunnen worden aangeduid als een algemene verslechtering van de plantkwaliteit. Uitdroging blijkt een belangrijke rol te spelen, een overzicht.
    Uitdroging in keten grootste gevaar voor vaste planten
    Dalfsen, P. van; Dijkema, M.H.G.E. ; Gude, H. ; Miller, B. - \ 2007
    BloembollenVisie 2007 (2007)107. - ISSN 1571-5558 - p. 20 - 21.
    stekken - kwekers - hergroei - gewaskwaliteit - dehydratie (fysiologisch) - export - stress omstandigheden - cuttings - growers - regrowth - crop quality - dehydration (physiological) - exports - stress conditions
    Diverse soorten vaste planten hebben na het exporteren problemen met de hergroei. PPO deed onderzoek naar omstandigheden in de keten, die een verslechtering van de kwaliteit van de planten kunnen geven. Uitdrogen blijkt daarin een belangrijke factor te zijn
    Monitoring drought affected crop yields based on ERS-scatterometer data : exploration of possibilities to integrate ERS-scatterometer derived soil moisture into the CGMS crop model for a Russian-Ukrainian study area
    Boogaard, H.L. ; Diepen, C.A. van; Savin, I. - \ 2000
    Wageningen : Alterra (Alterra-rapport 9) - 95
    gewasopbrengst - droogte - bodemwater - meteorologie - modellen - satellieten - monitoring - remote sensing - oogstfactoren - stress omstandigheden - agronomie - oekraïne - gerst - crop yield - drought - soil water - meteorology - models - satellites - monitoring - remote sensing - yield factors - stress conditions - agronomy - ukraine - barley
    In this study the possibilities of integrating ERS scatterometer-derived soil moisture into CGMS are explored. This remote sensed soil moisture is used to calculate drought stress in grains of barley for a Russian-Ukrainian study area. The results arecompared with drought stress based on the rainfall-driven water balance and with regional yields statistics of barley. The use of ERS scatterometer in CGMS seems promising, especially when additional input data such as sowing dates and crop parameters are improved and conversions of ERS scatterometer data into soil moisture are more specified for different soils and seasonal variations.
    Ecological and statistical evaluation of effects of pesticides in freshwater model ecosystems
    Brink, P.J. van den - \ 1999
    Agricultural University. Promotor(en): M. Scheffer; T.C.M. Brock; C.J.F. ter Braak. - S.l. : Van den Brink - ISBN 9789054859987 - 164
    ecosystemen - zoet water - verontreiniging - pesticiden - pesticidenresiduen - zoetwaterecologie - milieueffect - risicoschatting - habitats - modellen - herbiciden - fungiciden - insecticiden - waterinvertebraten - stress omstandigheden - aquatische ecosystemen - ecotoxicologie - ecosystems - fresh water - pollution - pesticides - pesticide residues - freshwater ecology - environmental impact - risk assessment - habitats - models - herbicides - fungicides - insecticides - aquatic invertebrates - stress conditions - aquatic ecosystems - ecotoxicology

    Aquatic risk assessment of pesticides

    The first tier in the aquatic risk assessment procedure consists of a comparison between a Predicted Environmental Concentration (PEC) with a No Effect Concentration (NEC). A requirement for registration is that the PEC should not exceed the NEC. The NEC is calculated from the toxicity of the pesticide for defined standard test species (viz. algae Daphnia , fish) and an assessment factor, which accounts for potential differences between standard test species and indigenous species. The assessment factors used are 100 (to be multiplied with the acute EC50 of Daphnia and fish) or 10 (to be multiplied with the chronic NOEC of fish or EC50 of algae). Because this approach lacks ecological realism, the first aim of the present thesis was to validate the assessment factors used in the first tier by evaluating three chemicals with different modes of action (insecticide, herbicide, fungicide) as benchmark compounds.

    We compared the No Observed Effect Concentrations (NOECs), resulting from microcosm and mesocosm experiments using these compounds, with the NECs as used for the risk assessment procedure. Table 1 summarises the standards calculated from the first tier criteria set by the Uniform Principles (UP-standard), as well as the NOEC ecosystem for acute and chronic exposure regimes for the three substances. In addition, Table 1 lists the Dutch water quality standards. The assessment factors seem to protect the tested aquatic ecosystem against acute and chronic exposure to the insecticide chlorpyrifos and against chronic exposure to the herbicide linuron and the fungicide carbendazim (Table 1; chapters 2, 3 and 4). Dutch water quality standards for these three compounds were lower than the UP-standards and thus also seem to protect the aquatic ecosystems tested when exposed to individual compounds.

    A comparison between the UP-standards and the Lowest Observed Effect Concentration at the ecosystem level (LOEC ecosystem ) indicates that when the NEC is exceeded by a factor of 10, effects cannot be excluded in the case of chronic exposure. In the case of a single application of the insecticide chlorpyrifos, however, the assessment factor can be considered overprotective; an assessment factor of 10 instead of 100 would also seem to suffice. Two extensive literature reviews on the impact of insecticides and herbicides on aquatic microcosms and mesocosms also demonstrate that the first tier criteria of the Uniform Principles are generally adequate to protect different aquatic ecosystems from pesticide stress (Lahr et al., 1998; Van Wijngaarden et al., 1998). For compounds such as fungicides, however, hardly any information could be found in the open literature, so that validation of the assessment factors for these types of pesticide needs further attention.

    Table 1:Derived UP-standards, Dutch water quality standards and NOEC ecosystem observed in semi-field studies for the insecticide chlorpyrifos, the herbicide linuron and the fungicide carbendazim (all concentrations in µg/L). UP-standards were calculated from criteria set by the first tier of aquatic risk assessment. For references to toxicity values see Table 3 in chapter 1 of this thesis.
    UP-standardDutch water quality standardNOEC ecosystem / LOEC ecosystem
    Short-termLong-termAcute exposureChronic exposure
    Chlorpyrifos0.01 a0.01 c0.0030.1 / 0.9 (Chapter 2)0.01 d/ 0.1 e
    Linuron0.6 b*0.6 b*0.25- / -0.5 / 5 (Chapter 3)
    Carbendazim3.2 a1 c0.11- / -3.3 / 33 (Chapter 4)
    * Dutch standard would be 0.1 µg/L (0.1 x NOEC of the standard test algae; Anonymous, 1995); - No data available; a: 0.01 × LC50 Daphnia ; b: 0.1 × EC50 Algae; c: 0.1 × NOEC Daphnia; d: data from unpublished experiment, Van den Brink et al., in prep.; e: data from Van den Brink et al., 1995.

    Ecological effects and recovery

    One of the aims of the present thesis was to gain insight into long-term community responses and into the factors determining the recovery of affected populations after a single application of an insecticide in experimental ditches. As was expected from its mode of action, application of chlorpyrifos resulted in large adverse effects on arthropod taxa (chapter 2). Because this experiment was performed in relatively large, outdoor systems, the recovery of the affected populations could be investigated. The recovery of populations of individual species was highly dependent on their life-cycle characteristics, such as the number of generations per year, the presence of resistant life stages and the ability to migrate from one system to another. In chapter 2 this is illustrated by the responses of two mayflies, cladocerans and an amphipod. The mayflies Cloeon dipterum and Caenis horaria do not have life stages resistant to chlorpyrifos, but are able to migrate from one ditch to another. They are also almost equally susceptible to chlorpyrifos in the laboratory but showed a very different recovery pattern.

    The former species recovered within 12 weeks at the highest treatment level, whereas the latter species took 24 weeks to recover fully. This can be explained from the difference in the number of generations per year. C. dipterum has many generations per year and thus recolonises the ditch repeatedly, thus recovering as soon as the concentration of chlorpyrifos allows this. C. horaria , however, produces only one generation per year, so that recovery can only take place when the next generation recolonises the ditch. Unlike mayflies, Cladocerans are not able to migrate actively from one ditch to the other. They did, however, show a very fast recovery at the higher concentration (Chapter 2). This is possible because they have a short generation time and resistant life stages in the form of ephyppia. If a taxon is not able to recolonise an impacted system and does not have resistant life stages, the species can become extinct in isolated systems like the experimental ditches. This applies for the amphipod Gammarus pulex , which became extinct at the two highest concentrations and did not recover within the 55 week experimental period. No significant effects on the invertebrate community, with the exception of Gammarus, were found from week 24 after insecticide application onwards, suggesting recovery.

    As part of the third aim of the thesis, the long-term responses in ecosystem structure and functioning after chronic exposure to a herbicide and fungicide were studied in aquatic microcosms. The higher concentration of the photosynthesis-inhibiting herbicide linuron resulted in a decreased biomass of the macrophyte Elodea nuttallii and decreased abundance of most algal taxa (chapter 3). The dissolved oxygen and pH levels also decreased at lower pesticide concentrations as a consequence of inhibited photosynthesis. Although a decrease in the abundance of most algal taxa was observed after to the herbicide application, a net increase in chlorophyll-a was found for the phytoplankton, periphyton and neuston. This increase was completely caused by the green alga Chlamydomonas sp., which appeared to be relatively tolerant to linuron and also had the ability to develop a tolerance to relatively high concentrations within a week. As a result of this tolerance and the reduced competition for nutrients with macrophytes, the community in the microcosms shifted from macrophyte-dominated to an algae-dominated state, especially at the highest treatment level (150 µg/L). The Copepoda and Cladocera benefited from this increased food supply and showed elevated abundance values at the higher treatment levels. Some macrophyte-associated invertebrates decreased in abundance as a result of the decline of their habitat.

    The fungicide carbendazim, which belongs to the bendimidazoles, is known to adversely affect microorganisms and worms. This property explains its effects on the "worm-like" taxa of the Turbellaria and Oligochaeta, but could not explain its effects on invertebrate groups like Amphipoda, Gastropoda and Cladocera (chapter 4). Unlike the direct effects of chlorpyrifos and linuron, therefore those of carbendazim on freshwater populations could not be completely deduced from the latter's taxonomic relation with the pest organisms, carbendazim it is supposed to control. The fungicide appeared to have the mode of action of a biocide rather than a chemical with a specific mode of action. Due to the decline of many invertebrates and the concomitant reduction in grazing pressure, the chlorophyll-a level and the abundance values of some phytoplankton taxa increased at the two highest concentrations (330 and 1000 µg/L).

    The "eutrophication-like" consequences of insecticide contamination have also often been reported and discussed in the literature (e.g. DeNoyelles et al., 1994, Cuppen et al., 1995). The increased abundance of algae due to a decrease in susceptible herbivores is a commonly reported consequence of insecticide contamination (Van Wijngaarden et al., 1998).

    In the present thesis, the occurrence of herbicides in the aquatic ecosystem is regarded as an undesirable side effect of its use on land. However, herbicides are also deliberately released into aquatic ecosystems for the control of nuisance aquatic vegetation (Pieterse and Murphy, 1990). Aquatic weeds are most commonly removed using compounds with a mode of action specific to macrophytes. Since algae are relatively tolerant to these chemicals (Lahr et al., 1998), they may increase their biomass due to reduced competition for nutrients (Kobriae and Whyte, 1996). Terrestrial weeds are, in the Netherlands, usually controlled by means of photosynthesis-inhibiting herbicides (NEFYTO, 1996). Although their mechanism is different, chapter 3 shows that prolonged exposure to the photosynthesis-inhibiting herbicide linuron may also result in a shift from macrophyte dominance to plankton dominance. The review published by Lahr et al. (1998) shows that this may be true for photosynthesis-inhibiting herbicides in general.

    The effects of fungicides are largely unstudied, but chapter 4 indicates that fungicide contamination can also cause elevated algal densities. This means that all three pesticides can contribute to "eutrophication-like" effects, though the mechanisms differ. The significance of realistic concentrations of pesticides in causing symptoms of eutrophication in surface waters, however, largely remains to be investigated.

    Tools to evaluate microcosm and mesocosm experiments

    Semi-field experiments are usually evaluated at the taxon level. Since many species normally have low abundance values and/or show high variability (Van Wijngaarden et al., 1996), this approach has the great disadvantage that only a limited number of species can be properly analysed. This means that a substantial part of the information gathered is not used for the evaluation. This thesis presents a new multivariate tool for the analysis of treatment effects at the community level. Multivariate techniques have already been used for a long time in ecology to analyse the relation between communities and their environment. The most commonly used ordination technique is correspondence analysis, which is based on the bell-shaped unimodal model. This model fits in with the theory of the rise and fall in a preference of a species along an environmental gradient, described by their optimum and tolerance.

    Chapter 7 indicates why clustering and ordination based on correspondence analysis are not suitable for the analysis of the ecotoxicological data sets presented in this thesis. It argues that species normally have no optimum along the environmental axis of a stressor such as pesticides. Their response is more accurately described by a linear method; expected direct effects will increase with the concentration. On the basis of laboratory tests, this relation between the endpoint and the concentration of stressor is assumed to be sigmoid, and it is argued that a linear response model is a good approximation of this.

    Chapters 2 and 3 use Redundancy Analysis (RDA) to elucidate the effects of pesticides at the community level. RDA is the constrained version of the well-known ordination technique Principal Component Analysis (PCA) and is based on a linear response model (Jongman et al., 1995). In chapters 2 and 3 the analysis is constrained to the variance explained by treatment, time and their interaction. It was concluded that RDA successfully summarised the effects of a pesticide on a community in a single diagram, and is very useful especially when combined with Monte Carlo permutation tests for the determination of the significance of treatment effects. Kersting and Van den Brink (1997), however, found that output from RDA can sometimes result in very cluttered diagrams.

    Chapter 5 presents a new method, termed the Principal Response Curves, which overcomes this problem. PRC is based on RDA and extracts the first principal component from the treatment variance, by excluding from the analysis the variance explained by time as well as differences between replicates. It results in an easy-to-read diagram, showing the deviations of all treatments from the control in time. In contrast to most other techniques, it also allows a quantitative interpretation down to the species level. Chapter 6 introduces the rank 2 model of PRC, this means that after the extraction of the first basic response pattern, a second pattern is extracted, which expresses the most important deviation from the first response present in the data set. The second pattern is of particular importance if no single dominant response pattern is present in a data set but several sub-dominant ones occur. In chapter 6 this is illustrated by an analysis of the invertebrate and phytoplankton data sets of a microcosm experiment with two stressors, the insecticide chlorpyrifos and nutrient additions. This example shows that PRC is also able to summarise several different response patterns in two diagrams.

    Microcosm and mesocosm experiments are often said to be of limited value due to ecological variability and noise. From the experiments and statistical tools as described in this thesis we can conclude that despite the noise clear response patterns are revealed, if experiments are properly designed and analysed. Chapters 2, 3 and 4 illustrate that, even with a limited number of replicates, an ecological threshold level (e.g. NOEC ecosystem ) and an effect-chain covering different trophic levels can be obtained.

    Suggestions for future research

    In normal agricultural practice, protection of crops from pest organisms is not achieved by the application of a single compound; usually, several different compounds with different target organisms are used. Some pesticides are also administered repeatedly. The effects of combinations of pesticides on freshwater ecosystems are, however, largely unstudied (Hartgers et al., 1998). Therefore, it is important to develop criteria for the ecological risk assessment of mixtures of compounds, using realistic pesticide treatment regimes for particular crops.

    The problem of combination toxicity becomes even more complex when other substances used in agricultural areas, such as fertilisers, are taken into account. The combined effects of eutrophication and contaminant stress are largely unknown. It can be expected, however, that the trophic status of an ecosystem will alter the effects of pesticides (Chapter 6; Kramer et al., 1997).

    The ecological effect chain resulting from the experiments with the herbicide linuron and fungicide carbendazim demonstrated that microcosm and mesocosm experiments with pesticides as stressors can be very useful tools to investigate trophic interactions in aquatic ecosystems. The results of these experiments are currently being used to build a food-web model (Traas et al., 1998). Such models are considered to hold great promise for an improved understanding of ecosystem functioning and may eventually provide the ability to predict effects of contaminants at ecosystem level (Health Council of the Netherlands, 1997). The greatest obstacles that have to be overcome are the lack of solid data on parameter values (data on for instance maximum growth rate) and the lack of validation. This means that the further development of food web models require not only laboratory research on parameters values but also semi-field research for the collection of validation data sets (Health Council of the Netherlands, 1997).

    The modeling of direct effects and recovery patterns at the population level can be of great use for an assessment of the risks and a ranking of the effects of pesticides. For the future, modeling treatment effects and recovery patterns may be of great value as a research tool but also as a predictive tool. Models have the advantage that they allow integration of ecological and ecotoxicological knowledge, something that was largely absent from ecotoxicology until a few years ago. Development of these models will allow to a better evaluation of microcosm and mesocosm experiments performed for scientific or registration purposes.

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