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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    Ecosysteemeffecten van lokaal veranderde slibgehaltes in de westelijke Waddenzee
    Brinkman, A.G. - \ 2015
    Den Helder : IMARES (Rapport / IMARES C157/15) - 121
    algen - schaaldieren - troebelheid - modellen - silt - aquatische ecosystemen - waddenzee - algae - shellfish - turbidity - models - silt - aquatic ecosystems - wadden sea
    Door middel van modelberekeningen is onderzocht hoe gevoelig primaire (algen) en secundaire (schelpdier‐)productie in de westelijke Waddenzee is voor een veranderde troebelheid. De studie is onderdeel van een gezamenlijke studie van Deltares‐IMARES naar antropogene invloeden op de primaire en secundaire productie. In de voorliggende studie worden ten behoeve van die analyse veel grotere troebelheidsveranderingen verondersteld dan welke in de praktijk kunnen voorkomen.
    Ems-Dollard primary production research Concise summary
    Brinkman, A.G. ; Jacobs, P. ; Jak, R.G. ; Riegman, R. - \ 2015
    IJmuiden : IMARES Wageningen UR (Report / IMARES Wageningen UR C163/14) - 78
    kaderrichtlijn water - waterbeleid - waterkwaliteit - modder - troebelheid - ecologie - eems - eems-dollard - water framework directive - water policy - water quality - mud - turbidity - ecology - river ems - eems-dollard
    The Water Framework Directive (WFD) requires EU member states to achieve good ecological and chemical status of all designated water bodies (rivers, lakes, transitional and coastal waters) by 2015. Therefore Rijkswaterstaat Waterdienst has initiated the project ‘Research mud dynamics Ems Estuary’ . The aim of this project, carried out by Deltares and IMARES, is to (1) improve our knowledge on the mud dynamics in the Ems Estuary, (2) to identify the reasons for the increase in turbidity and (3) to quantify measures to improve the ecological status of the estuary.
    Ems-Dollard primary production research: Full data report
    Brinkman, A.G. ; Riegman, R. ; Jacobs, P. ; Kuhn, S. ; Meijboom, A. - \ 2015
    IJmuiden : IMARES Wageningen UR (Report / IMARES Wageningen UR C160/14) - 297
    kaderrichtlijn water - richtlijnen (directives) - waterbeleid - waterkwaliteit - modder - troebelheid - ecologie - eems - eems-dollard - water framework directive - directives - water policy - water quality - mud - turbidity - ecology - river ems - eems-dollard
    The Water Framework Directive (WFD) requires EU member states to achieve good ecological and chemical status of all designated water bodies (rivers, lakes, transitional and coastal waters) by 2015. Therefore Rijkswaterstaat Waterdienst has initiated the project ‘Research mud dynamics Ems Estuary’ . The aim of this project, carried out by Deltares and IMARES, is to (1) improve our knowledge on the mud dynamics in the Ems Estuary, (2) to identify the reasons for the increase in turbidity and (3) to quantify measures to improve the ecological status of the estuary.
    Rwzi-effluent gebruiken voor vismigratie
    Foekema, E.M. ; Winter, H.V. ; Kleissen, F. ; Slijkerman, D.M.E. - \ 2012
    H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 45 (2012)1. - ISSN 0166-8439 - p. 34 - 36.
    waterzuivering - zuiveringsinstallaties - lozing - waterverontreiniging - troebelheid - vissen - palingen - vismigratie - water treatment - purification plants - disposal - water pollution - turbidity - fishes - eels - fish migration
    Zeewaarts migrerende schieralen laten een gedragsverandering zien wanneer zij geconfronteerd worden met een lozingspluim van rwzi-effluent. Ruim de helft van de vissen verandert van zwemrichting en tracht langs de rand van de pluim te passeren. Het risico dat een permanente migratiebarrière ontstaat is bij de meeste rwzi's niet groot door het dynamische karakter van de lozingspluim. Bij rwzi's die lozen op wateren met een beperkt afvoerdebiet, kunnen wel migratiebarrières ontstaan, maar met relatief eenvoudige aanpassingen kan de kans daarop worden verkleind. De onderzoeksresultaten geven aan dat rwzi-effluentpluimen potentie hebben om te worden ingezet voor de geleiding van stroomafwaarts migrerende vissen naar een veilige route langs gevaarlijke situaties zoals koelwaterinlaten, gemalen en waterkrachtcentrales.
    Effect of calcium chelators on physical changes in casein micelles in concentrated micellar casein solutions
    Kort, E.J.P. de; Minor, M. ; Snoeren, T.H.M. ; Hooijdonk, A.C.M. van; Linden, E. van der - \ 2011
    International Dairy Journal 21 (2011)12. - ISSN 0958-6946 - p. 907 - 913.
    skim-milk - physicochemical characterization - phosphates - viscosity - equilibria - turbidity - gelation - ions - precipitation - dissociation
    The effect of calcium chelators on physical changes of casein micelles in concentrated micellar casein solutions was investigated by measuring calcium-ion activity, viscosity and turbidity, and performing ultracentrifugation. The highest viscosities were measured on addition of sodium hexametaphosphate (SHMP), because it cross-linked the caseins. For the weak calcium chelator disodium uridine monophosphate (Na2UMP), physical changes in the solutions were negligible. Disodium hydrogen phosphate (Na2HPO4), trisodium citrate (TSC), and sodium phytate (SP) caused similar increases in viscosity, but had different effects on turbidity. The increase in viscosity was attributed to swelling of the casein micelles (i.e., increased voluminosity) at decreasing calcium-ion activity. The major decrease in turbidity was due to dissociation of the casein micelles. The extent of micellar dissociation was dependent on the type and concentration of calcium chelator. It seems that the micelles were dissociated in the order of SHMP = SP > TSC > Na2HPO4 > Na2UMP
    Estimating Actual Evapotranspiration from Satellite and Meteorological Data in Central Bolivia
    Seiler, C. ; Moene, A.F. - \ 2011
    Earth Interactions 15 (2011)12. - ISSN 1087-3562 - p. 1 - 24.
    formulation - evaporation - radiation - turbidity - model
    Spatial estimates of actual evapotranspiration are useful for calculating the water balance of river basins, quantifying hydrological services provided by ecosystems, and assessing the hydrological impacts of land-use practices. To provide this information, the authors estimate actual evapotranspiration in central Bolivia with a remote sensing algorithm [Surface Energy Balance Algorithms for Land (SEBAL)]. SEBAL was adapted for the effects of topography (particularly for elevation, slope, and aspect) and atmospheric properties on incoming solar radiation. Instantaneous fluxes are converted to daily and annual fluxes using reference evapotranspiration. The required input data consist of meteorological data and satellite data. Results show more evapotranspiration for humid regions and less evapotranspiration for dry regions and deforested land. Actual evapotranspiration estimates are compared with annual precipitation measurements from 27 meteorological observations. In case annual actual evapotranspiration is estimated correctly, it must be lower than the precipitation measurements. This is the case for 23 stations. The remaining four stations are all located at higher altitudes (>2700 m). Unfortunately, no actual evapotranspiration measurements are available for additional validation purposes.
    Effects of submerged vegetation on water clarity across climates
    Kosten, S. ; Lacerot, G. ; Jeppesen, E. ; Motta Marques, D.M.L. ; Nes, E.H. van; Mazzeo, N. ; Scheffer, M. - \ 2009
    Ecosystems 12 (2009)7. - ISSN 1432-9840 - p. 1117 - 1129.
    meren - waterkwaliteit - waterplanten - troebelheid - vegetatie - zuid-amerika - lakes - water quality - aquatic plants - turbidity - vegetation - south america - shallow lake restoration - plant biomass - aquatic macrophytes - planktivorous fish - temperate lakes - eutrophic lake - florida lakes - zooplankton - phytoplankton - biomanipulation
    A positive feedback between submerged vegetation and water clarity forms the backbone of the alternative state theory in shallow lakes. The water clearing effect of aquatic vegetation may be caused by different physical, chemical, and biological mechanisms and has been studied mainly in temperate lakes. Recent work suggests differences in biotic interactions between (sub)tropical and cooler lakes might result in a less pronounced clearing effect in the (sub)tropics. To assess whether the effect of submerged vegetation changes with climate, we sampled 83 lakes over a gradient ranging from the tundra to the tropics in South America. Judged from a comparison of water clarity inside and outside vegetation beds, the vegetation appeared to have a similar positive effect on the water clarity across all climatic regions studied. However, the local clearing effect of vegetation decreased steeply with the contribution of humic substances to the underwater light attenuation. Looking at turbidity on a whole-lake scale, results were more difficult to interpret. Although lakes with abundant vegetation (>30%) were generally clear, sparsely vegetated lakes differed widely in clarity. Overall, the effect of vegetation on water clarity in our lakes appears to be smaller than that found in various Northern hemisphere studies. This might be explained by differences in fish communities and their relation to vegetation. For instance, unlike in Northern hemisphere studies, we find no clear relation between vegetation coverage and fish abundance or their diet preference. High densities of omnivorous fish and coinciding low grazing pressures on phytoplankton in the (sub)tropics may, furthermore, weaken the effect of vegetation on water clarity.
    Climate-related differences in the dominance of submerged macrophytes in shallow lakes
    Kosten, S. ; Kamarainen, A. ; Jeppesen, E. ; Nes, E.H. van; Peeters, E.T.H.M. ; Lacerot, G. ; Scheffer, M. - \ 2009
    Global Change Biology 15 (2009)10. - ISSN 1354-1013 - p. 2503 - 2517.
    meren - waterkwaliteit - klimaat - stikstof - troebelheid - voedingsstoffen - diepte - waterplanten - zuid-amerika - europa - noord-amerika - fosfor - temperatuur - fytoplankton - vegetatie - lakes - water quality - climate - nitrogen - turbidity - nutrients - depth - aquatic plants - south america - europe - north america - phosphorus - temperature - phytoplankton - vegetation - phosphorus concentrations - aquatic macrophytes - eutrophic lakes - fish - restoration - communities - nutrient - growth
    It has been suggested that shallow lakes in warm climates have a higher probability of being turbid, rather than macrophyte dominated, compared with lakes in cooler climates, but little field evidence exists to evaluate this hypothesis. We analyzed data from 782 lake years in different climate zones in North America, South America, and Europe. We tested if systematic differences exist in the relationship between the abundance of submerged macrophytes and environmental factors such as lake depth and nutrient levels. In the pooled dataset the proportion of lakes with substantial submerged macrophyte coverage (> 30% of the lake area) decreased in a sigmoidal way with increasing total phosphorus (TP) concentration, falling most steeply between 0.05 and 0.2 mg L-1. Substantial submerged macrophyte coverage was also rare in lakes with total nitrogen (TN) concentrations above 1–2 mg L-1, except for lakes with very low TP concentrations where macrophytes remain abundant until higher TN concentrations. The deviance reduction of logistic regression models predicting macrophyte coverage from nutrients and water depth was generally low, and notably lowest in tropical and subtropical regions (Brazil, Uruguay, and Florida), suggesting that macrophyte coverage was strongly influenced by other factors. The maximum TP concentration allowing substantial submerged macrophyte coverage was clearly higher in cold regions with more frost days. This is in agreement with other studies which found a large influence of ice cover duration on shallow lakes' ecology through partial fish kills that may improve light conditions for submerged macrophytes by cascading effects on periphyton and phytoplankton. Our findings suggest that, in regions where climatic warming is projected to lead to fewer frost days, macrophyte cover will decrease unless the nutrient levels are lowered
    Calcium-binding capacity of organic and inorganic ortho- and polyphosphates
    Kort, E.J.P. de; Minor, M. ; Snoeren, T.H.M. ; Hooijdonk, A.C.M. van; Linden, E. van der - \ 2009
    Dairy Science and Technology 89 (2009). - ISSN 1958-5586 - p. 283 - 299.
    hexametaphosphate oxalate citrate - metal-ion interactions - casein micelles - uridine 5'-monophosphate - complexing agents - phosphate - milk - nucleotides - stability - turbidity
    The aim of this research was to determine the calcium-binding capacity of inorganic and organic ortho- and polyphosphates. This calcium-binding capacity can be used to influence the stability of, for example, casein micelles in dairy systems. Four phosphates were selected: disodium uridine monophosphate (Na2UMP, organic orthophosphate), disodium hydrogen phosphate (Na2HPO4, inorganic orthophosphate), sodium phytate (SP, organic polyphosphate), and sodium hexametaphosphate (SHMP, inorganic polyphosphate). Concentrations of up to 100 mmolL-1 phosphate were added to a 50 mmolL-1 CaCl2 solution. The samples were prepared at pH 8.0 and were analyzed before and after sterilization for calcium-ion activity, conductivity, pH, sediment, and turbidity. Both SHMP and SP are strong chelators, as calcium ions bind to these phosphates in the ratio of 3:1 and 6:1, respectively. Calcium ions also strongly bind to Na2HPO4, but in a ratio of 3:2 with insoluble Ca3(PO4)2 complexes as result. The equilibrium position of Na2UMP is not strong towards the chelated complex, and significant levels of free calcium and free phosphate can exist. An equilibrium constant of Lmol-1 was determined for calcium uridine monophosphate (CaUMP) complexes. Both calculation of the equilibrium constant and analysis on the CaUMP precipitate confirmed a reactivity of 1:1 between calcium and Na2UMP. The CaUMP complexes are well soluble at ambient temperature, and insoluble complexes appear after sterilization, because the solubility of CaUMP decreases during heating. Finally, we concluded that the structure of phosphate molecules determines their calcium-binding capacity rather than organic or inorganic origin of phosphates
    Video imaging in turbid water
    Dalfsen, J.A. van - \ 2008
    Den Helder : IMARES (IMARES report C024/08) - 11
    remote sensing - technieken - video-opnames - waterbodems - habitats - nederlandse wateren - troebelheid - remote sensing - techniques - video recordings - water bottoms - habitats - dutch waters - turbidity
    In order to describe seabed habitats and to identify and quantify spatial patterns information is needed at different scales. Whereas ground-truthing techniques are used to gain point information, remote sensing techniques are used to obtain information on a larger scale. To link seabed samples taken with grabs and cores to a wider patterns remote sensing techniques as side scan sonar, multibeam and video imaging are often used. For the latter, however, the Dutch waters are often too turbid to obtain good images. In order to improve information obtained by photography and video a “water lens” system was developed and tested.
    Comparison of MODIS and Landsat TM5 images for mapping tempo-spatial dynamics of Secchi disk depths in Poyang Lake national nature reserve, China
    Wu, G. ; Leeuw, J. de; Skidmore, A.K. ; Prins, H.H.T. ; Liu, Y. - \ 2008
    International Journal of Remote Sensing 29 (2008)8. - ISSN 0143-1161 - p. 2183 - 2198.
    total suspended matter - water-quality - thematic mapper - multispectral data - satellite imagery - coastal waters - chlorophyll-a - turbidity - clarity - calibration
    Landsat has successfully been applied to map Secchi disk depth of inland water bodies. Operational use for monitoring a dynamic variable like Secchi disk depth is however limited by the 16-day overpass cycle of the Landsat system and cloud cover. Low spatial resolution Moderate Resolution Imaging Spectroradiometer (MODIS) image captured twice a day could potentially overcome these problems. However, its potential for mapping Secchi disk depth of inland water bodies has so far rarely been explored. This study compared two image sources, MODIS and Landsat Thematic Mapper (TM), for mapping the tempo-spatial dynamics of Secchi disk depth in Poyang Lake National Nature Reserve, China. Secchi disk depths recorded at weekly intervals from April to October in 2004 and 2005 were related to 5 Landsat TM and 22 MODIS images respectively. Two multiple regression models including the blue and red bands of Landsat TM and MODIS respectively explained 83% and 88% of the variance of the natural logarithm of Secchi disk depth. The standard errors of the predictions were 0.20 and 0.37 m for Landsat TM and MODIS-based models. A high correlation (r = 0.94) between the predicted Secchi disk depth derived from the two models was observed. A discussion of advantages and disadvantages of both sensors leads to the conclusion that MODIS offers the possibility to monitor water transparency more regularly and cheaply in relatively big and frequently cloud covered lakes as is with Poyang Lake.
    Impact of human activities on water level and clarity and underwater light climate of Vallisneria spiralis L. in Poyan Lake, China
    Wu, G. - \ 2008
    Wageningen University. Promotor(en): Andrew Skidmore; Herbert Prins, co-promotor(en): J. de Leeuw. - [S.l.] : S.n. - ISBN 9789085049074 - 119
    meren - china - water - oppervlaktewater - waterkwaliteit - watervogels - waterplanten - lichtsterkte - vallisneria spiralis - troebelheid - voedselplanten - groei - remote sensing - oppervlaktewaterkwaliteit - waterstand - aquatische ecosystemen - menselijke invloed - lakes - china - water - surface water - water quality - waterfowl - aquatic plants - light intensity - vallisneria spiralis - turbidity - food plants - growth - remote sensing - surface water quality - water level - aquatic ecosystems - human impact
    Almost 95% of the world population of Siberian crane (Grus leucogeranus) winter in Poyang Lake, China. Here they forage on the tubers of the submerged aquatic macrophyte Vallisneria spiralis L. The growth and production of V. spiralis are regulated by the local hydrology, which might also be influenced by the changed hydrology of the Yangtze River induced by engineering projects. This thesis investigated the impact of dredging activities in the northern Poyang Lake and the Three Gorges Dam in the Yangtze River on the water turbidity and water level of Poyang Lake, analyzed how these hydrological changes influenced the underwater light climate of V. spiralis, and discussed their potential impacts on the growth and productivity of V. spiralis and further on the food habitat of Siberian crane in Poyang Lake. The principal results obtained can be summarized as follows:
    (1) Moderate resolution imaging spectroradiometer (MODIS) and Landsat thematic mapper (TM) images were compared for mapping the tempo-spatial dynamics of water clarity. Two multiple regression models including the blue and red bands of Landsat TM and MODIS respectively explained 83% and 88% of the variation of the natural logarithm of Secchi disk depth. On the basis of the comparison of water clarity predicted from Landsat TM and MODIS data as well as the advantages and contras of both sensors, it is concluded that MODIS offers the possibility to monitor the dynamics of water clarity more regularly and cheaply in relatively big and frequently cloud covered lakes like Poyang Lake.
    (2) The performance of the various Landsat TM bands for detecting dredging ships was explored in the turbid water of the northern Poyang Lake. The result revealed that the TM bands 1 to 4 had limitation especially in turbid water, while bands 5 and 7 better discriminated vessels from surrounding waters. Therefore, it is recommended using the mid-infrared bands of Landsat TM for operational ship monitoring in turbid water.
    (3) The possibility to strengthen inference of dredging impact while simultaneously monitoring vessels and water turbidity with remote sensing techniques was investigated in the northern Poyang Lake. Time-series MODIS images revealed a significant increase in water turbidity from 2001 onwards, while Landsat TM image analysis indicated a simultaneous increase in the number of vessels. Regression analysis further showed a highly significant positive relation (R2 = 0.92) between water turbidity and vessel number. On the basis of discussion of ship-related resuspension, final destinations of vessels and coincidence of vessel clusters with irregular dented patterns and turbid plume development, it is confirmed that dredging caused the increase in water turbidity. Therefore, it is concluded that simultaneously monitoring water turbidity and vessels enhanced the strength of evidence in remotely sensed dredging impact assessments.
    (4) Three extrapolation and interpolation methods were compared for predicting daily photosynthetically active radiation reaching the earth surface over the Poyang Lake national nature reserve. The result revealed that the triangulated irregular network (TIN) and inverse distance weighted (IDW)-based interpolation method produced more reliable result than the extrapolation methods.
    (5) The photosynthetically active radiation reaching the top of V. spiralis (PARlc) in Lake Dahuchi from 1998 to 2006 was modelled. The results revealed significant between year differences in PARtc, which were determined primarily by fluctuation in water level. Six years of Secchi disk depth records revealed seasonal switching of the lake from turbid at low water level in autumn, winter and spring to clear at high water level in summer. The highest PARtc occurred at intermediate water levels, which were reached when the Yangtze River forces Lake Dahuchi out of its turbid state in spring. The operation of the Three Gorges Dam, which will increase water level of Yangtze River from May to June may force Lake Dahuchi out of its turbid state earlier and increase the light intensity reaching the canopy and the production of V spiralis. It is concluded that the operation of the Three Gorges Dam might at the short term increase the light regime and productivity of V. spiralis and possibly affect the food habitat of Siberian crane in Poyang Lake.
    (6) A conceptual framework to integrate a simulation model of plant biomass for V. spiralis with remote sensing and a geographical information system was introduced. It is expected that the system, once implemented, could contribute to evaluate the impact of possible hydrological changes on the growth and productivity of V. spiralis, and assess whether tuber feeding bird populations are constrained by the regional availability of food resources in Lake Poyang.
    It is anticipated that these results provide a basis for the future research and necessary information for decision-making that need to be made by the responsible authorities to decide how to rationally manage this unique lake ecosystem.

    Cyclic state shifts in a restored shallow lake
    Rip, W. - \ 2007
    Wageningen University. Promotor(en): Marten Scheffer, co-promotor(en): Egbert van Nes; B. Beltman. - [S.l.] : S.n. - ISBN 9789085047063 - 216
    meren - waterplanten - herstel - ecosystemen - fosfaten - vegetatie - klimaat - nederland - troebelheid - aquatische ecosystemen - utrecht - lakes - aquatic plants - rehabilitation - ecosystems - phosphates - vegetation - climate - netherlands - turbidity - aquatic ecosystems - utrecht
    Rip onderzocht de oorzaak van de fluctuaties aan de hand van natuurreservaat Botshol ten zuidwesten van Abcoude. Ze vond er een relatie met de afwisseling van droge en natte winters. Vroeger waren de winters minder nat en ook niet zo vaak nat, waardoor de helderheid van de meren niet wisselde. De laatste dertig jaar valt er ’s winters echter steeds meer regen. Hierdoor stroomt er van het land meer fosfaatrijk water in de meren. Wanneer de concentratie en afstroming zo hoog zijn dat de fosfaatbelasting boven een drempel uitkomt, kan helder water ineens troebel worden. ‘Het moet vervolgens weer een aantal jaren achtereen droog zijn, wil het fosfaat uit het systeem gaan zodat het water terugkeert naar een heldere toestand’, zegt Rip
    Vertical distribution patterns of zooplanktivorous fish in a shallow, eutrophic lake, mediated by water transparency
    Mous, P.J. ; Densen, W.L.T. van; Machiels, M.A.M. - \ 2004
    Ecology of Freshwater Fish 13 (2004)1. - ISSN 0906-6691 - p. 61 - 69.
    juvenile sockeye-salmon - oncorhynchus-nerka - reactive distance - migration - netherlands - ijsselmeer - turbidity - growth - perch
    The vertical distribution pattern (VDP) of fish at shallow sites in eutrophic lake - Lake IJssel, the Netherlands - as affected by water transparency, was examined. The pattern was assessed by pair trawling at three depths and by hydroacoustics from June to August. Water transparency was estimated by light intensity and Secchi depth measurements. Hydroacoustics showed that fish were more dispersed at night than during the day. European smelt, (Osmerus eperlanus) and young-of-the-year Eurasian perch (Perca fluviatilis) comprised 97% of the fish in numbers. European smelt concentrated near the surface, at light intensities of >200 muW . cm(-2) when the water was turbid, and near the bottom when the water was clear (Secchi depth > 1.2 m). We concluded that low water transparency over-ruled the light sensitivity of European smelt, and caused European smelt to stay higher in the water column.
    Lake age and water level affect the turbidity of floodplain lakes along the lower Rhine
    Roozen, F.C.J.M. ; Geest, G.J. van; Ibelings, B.W. ; Roijackers, R.M.M. ; Scheffer, M. ; Buijse, A.D. - \ 2003
    Freshwater Biology 48 (2003). - ISSN 0046-5070 - p. 519 - 531.
    geologische sedimentatie - waterkwaliteit - troebelheid - chlorofyl - vegetatie - oppervlaktewater - uiterwaarden - waterstand - rijn - geological sedimentation - water quality - turbidity - chlorophyll - vegetation - surface water - river forelands - water level - river rhine - wind-induced resuspension - sediment resuspension - benthivorous fish - shallow lakes - hydrological connectivity - phytoplankton biomass - community structure - chlorophyll-a - plankton
    1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity. 2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS). 3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton. 4. Water clarity was positively related to lake depth and the presence of vegetation. 5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important. 6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency. 7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes. 8. In conclusion, in-lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine.
    Rheology and structure of ovalbumin gels at low pH and low ionic strength
    Weijers, M. ; Sagis, L.M.C. ; Veerman, C. ; Sperber, B.L.H.M. ; Linden, E. van der - \ 2002
    Food Hydrocolloids 16 (2002). - ISSN 0268-005X - p. 269 - 276.
    hen ovalbumin - turbidity
    The objective of this study was to relate the rheological behavior of ovalbumin gels at low pH and low ionic strength to their mesoscopic structure, using rheological measurements and Transmission Electron Microscopy (TEM). Varying pH, ionic strength and protein concentration, we obtained transparent solutions, transparent gels, opaque gels, or turbid gels, upon heating and subsequent cooling of the ovalbumin solutions. At equal pH and increasing salt concentration we found an increase in turbidity, suggesting that the structure of the aggregates changed from linear or branched to more clustered aggregates. The gel strength increased with increasing salt concentration. A similar trend was observed at equal ionic strength and increasing pH. TEM micrographs of ovalbumin aggregates at pH 2 and 15 mM NaCl show that the ovalbumin aggregates are linear. Increasing the pH from 2 to 3.5 results in structures that are still linear, but with a higher degree of clustering. No random aggregates were observed. TEM micrographs of the gel phase at pH 3.5 and 30 mM NaCl (turbid) show that the structure consists of linear aggregates organized in large clusters of approximately 230–350 nm. At pH 2 and 30 mM NaCl (transparent) the gel consists of single strands with a diameter of about 3.3–3.9 nm, i.e. one or at most two monomers thick.
    Sturing van de deeltjesverwijdering bij de fysisch-chemische voorzuivering van afvalwater
    Mels, A.R. ; Nieuwenhuijzen, A.F. van - \ 2002
    Afvalwaterwetenschap 1 (2002)2. - ISSN 1568-3788 - p. 33 - 42.
    afvalwaterbehandeling - waterzuivering - filtratie - polymeren - voorbehandeling - geactiveerd slib - afvalwater - stedelijke gebieden - verwijdering - troebelheid - stikstof - chemisch zuurstofverbruik - waste water treatment - water treatment - filtration - polymers - pretreatment - activated sludge - waste water - urban areas - removal - turbidity - nitrogen - chemical oxygen demand
    Dit artikel beschrijft experimenten met een voorzuiveringssysteem op laboratoriumschaal bestaande uit een polymeerdoseer- en mengsysteem en een bezinker. In het systeem werd afvalwater uit het dorp Bennekom (voor)behandeld
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