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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Frozen desert alive : the role of sea ice for pelagic macrofauna and its predators : implications for the Antarctic pack-ice food we
Florentino De Souza Silva, A.P. - \ 2009
University of Groningen. Promotor(en): W.J. Wolff, co-promotor(en): Jan Andries van Franeker. - [S.l. : S.n. - ISBN 9789064643392 - 240
arctische gebieden - antarctica - ijs - kril - voedselwebben - mariene ecologie - macrofauna - arctic regions - ice - krill - food webs - marine ecology
Effects of an increasing filter feeder stock on larval abundance in the Oosterschelde estuary (SW Netherlands)
Troost, K. ; Gelderman, E.A.C. ; Kamermans, P. ; Smaal, A.C. ; Wolff, W. - \ 2009
Journal of Sea Research 61 (2009)3. - ISSN 1385-1101 - p. 153 - 164.
suspension-feeding bivalves - mussel mytilus-edulis - pelagic food-web - cerastoderma-edule - crassostrea-virginica - vertical-distribution - boundary-layer - ingestion - oyster - mechanisms
Predation by adult bivalves on bivalve larvae has been suggested to reduce larval abundance in areas with high bivalve filter-feeder biomass. Although the occurrence of larviphagy is well-studied in the laboratory, its effects in the field have scarcely been studied. We studied larviphagy at different spatial scales in the Oosterschelde estuary. On the scale of individuals, we confirmed that larviphagy occurs in Crassostrea gigas and Mytilus edulis in the Oosterschelde estuary, by examining stomach contents of adult bivalves. On a local scale, we studied effects of larviphagy by a Pacific oyster (C. gigas) bed on presence of larvae in the overlying water column by sampling larvae with fixed plankton nets. Abundance of blue mussel (M. edulis) larvae was significantly reduced by the oyster. Abundance of C. gigas larvae did not seem to be reduced by the oyster bed, but spawning by the adult oysters during the sampling period may have affected the results. On estuary-scale, the effect of larviphagy on larval abundance of C. gigas and M. edulis was studied using existing monitoring data over 6 years for M. edulis and 13 years for C. gigas. Numbers of M. edulis larvae showed no significant trend over the 6 years studied. Abundance of C. gigas larvae declined with an increasing filter feeder stock (that was mainly caused by an increase in C. gigas stock). This decline may be due to direct effects of larviphagy or indirect effects such as lowered food levels, and was not compensated by an increased larval production. All results combined, complemented with a theoretical estimate of the effect of larviphagy on estuary-scale, strongly suggest that larviphagy is major source of mortality for bivalve larvae in the Oosterschelde estuary.
Feeding current characteristics of three morphologically different bivalve suspension feeders, Crassostrea gigas Mytilus edulis and Cerastoderma edule , in relation to food competition
Troost, K. ; Stamhuis, E.J. ; Duren, L.A. ; Wolff, W. - \ 2009
Marine Biology 156 (2009)3. - ISSN 0025-3162 - p. 355 - 372.
particle image velocimetry - boundary-layer - filtration-rates - blue mussel - clearance rate - algal concentration - current speed - body size - flow - phytoplankton
Introduced Pacific oysters (Crassostrea gigas) have shown rapid expansion in the Oosterschelde estuary, while stocks of native bivalves declined slightly or remained stable. This indicates that they might have an advantage over native bivalve filter feeders. Hence, at the scale of individual bivalves, we studied whether this advantage occurs in optimizing food intake over native bivalves. We investigated feeding current characteristics, in which potential differences may ultimately lead to a differential food intake. We compared feeding currents of the invasive epibenthic non-siphonate Pacific oyster to those of two native bivalve suspension feeders: the epibenthic siphonate blue mussel Mytilus edulis and the endobenthic siphonate common cockle Cerastoderma edule. Inhalant flow fields were studied empirically using digital particle image velocimetry and particle tracking velocimetry. Exhalant jet speeds were modelled for a range of exhalant-aperture cross-sectional areas as determined in the laboratory and a range of filtration rates derived from literature. Significant differences were found in inhalant and exhalant current velocities and properties of the inhalant flow field (acceleration and distance of influence). At comparable body weight, inhalant current velocities were lower in C. gigas than in the other species. Modelled exhalant jets were higher in C. gigas, but oriented horizontally instead of vertically as in the other species. Despite these significant differences and apparent morphological differences between the three species, absolute differences in feeding current characteristics were small and are not expected to lead to significant differences in feeding efficiency.
Larviphagy in native bivalves and an introduced oyster
Troost, K. ; Kamermans, P. ; Wolff, W. - \ 2008
Journal of Sea Research 60 (2008)3. - ISSN 1385-1101 - p. 157 - 163.
suspension-feeding bivalves - mytilus-edulis-l - marine invertebrate larvae - crassostrea-gigas - oxygen-consumption - cerastoderma-edule - swimming behavior - veliger larvae - pacific oyster - mussel larvae
Introduced Pacific oysters Crassostrea gigas have expanded rapidly in the Dutch Oosterschelde estuary, while stocks of native bivalves declined slightly. As a consequence, total filtration pressure increased significantly, which may affect the mortality of bivalve larvae. Better escape abilities in Pacific oyster larvae might be a contributing factor to their rapid geographic expansion. To study whether C. gigas larvae are filtered less than larvae of native bivalves, we investigated filtration and ingestion of the larvae of the native Mytilus edulis and introduced C. gigas by the adults of C. gigas and M. edulis as well as the native Cerastoderma edule. We measured filtration rates of C. gigas and M. edulis larvae by the adult bivalves (C. gigas, M. edulis and C. edule), and compared these to filtration rates of algae. Additionally, we studied the fate of filtered larvae. All three adult species filtered both C. gigas and M. edulis larvae. M. edulis larvae were filtered by all three bivalve species with the same filtration rates as algae, whereas filtration rates of C. gigas larvae were roughly 50% lower than filtration rates of algae. This suggests that C. gigas larvae can somehow reduce their filtration risk, whereas larvae of M. edulis cannot. The majority of filtered C. gigas and M. edulis larvae were ingested.
Can bivalve veligers escape feeding currents of adult bivalves?
Troost, K. ; Veldhuizen, R. ; Stamhuis, E.J. ; Wolff, W. - \ 2008
Journal of Experimental Marine Biology and Ecology 358 (2008)2. - ISSN 0022-0981 - p. 185 - 196.
marine invertebrate larvae - particle image velocimetry - mytilus-edulis-l - swimming behavior - cerastoderma-edule - copepod nauplii - mussel larvae - responses - ingestion - predation
While the stock of introduced Pacific oysters (Crassostrea gigas) increased in the Oosterschelde estuary (SW Netherlands), so did the filtration pressure of all bivalve species together. In the same period, stocks of native bivalves declined slightly. The expansion of Pacific oysters in Dutch estuaries might be partially due to better abilities of their larvae to avoid or escape filtration, compared to larvae of native bivalves. In this context, escape and swimming abilities of Pacific oyster larvae and the larvae of the native blue mussel (Mytilus edulis) were compared. Swimming behaviour of C. gigas larvae and larvae of M. edulis was recorded in still water and in a suction current mimicking a bivalve feeding current, in a horizontal and in a vertical plane. Larval swimming behaviour in a suction flow field was reconstructed by subtracting local water movement vectors from the total movement of larvae, yielding movement paths due to larval swimming alone. Swimming speeds and the rate of displacement in vertical direction of C. gigas and M. edulis larvae were related to larval shell length, and to the pitch of up- or downward swimming. Larvae of both species did not show escape reactions in a suction flow field. With increasing shell length, larval swimming speeds of both species increased significantly. Swimming speeds of C. gigas larvae were significantly higher than swimming speeds of M. edulis larvae, resulting in a faster vertical displacement. The ability to migrate to more favourable water layers faster may offer C. gigas an advantage over native bivalves with slower swimming larvae.
Scratching the Surface
Franeker, J.A. van; Florentino De Souza Silva, A.P. ; Fijn, R.C. ; Wolff, W. - \ 2007
ijs - flora - vegetatie - fauna - bodemfauna - ice - vegetation - soil fauna
Beknopt overzicht van een onderzoek naar het leven in het water onder het ijs. Er is door middel van een speciale methode onderzoek gedaan naar de flora en fauna onder het ijs
AAgro-Stacks ammoniakdepositiemodel voor veehouderijen: Modelbeschrijving, achtergronden en vergelijking
Hofschreuder, P. ; Wolff, P. de; Erbrink, H.J. ; Ogink, N. - \ 2007
Wageningen : Animal Sciences Group - 93 p.
Time allocation of Orius sauteri in attacking Thrips palmi on an eggplant leaf
Yano, E. ; Jiang, N. ; Hemerik, L. ; Mochizuki, M. ; Mitsunaga, T. ; Shimoda, T. - \ 2005
Entomologia Experimentalis et Applicata 117 (2005)3. - ISSN 0013-8703 - p. 177 - 184.
wolff heteroptera anthocoridae - parasitoid encarsia-formosa - karny thysanoptera - poppius heteroptera - taiwan - japan - temperature - predation - behavior
Orius sauteri (Poppius) (Heteroptera: Anthocoridae) is a polyphagous predator used as a biological control agent of palm thrips, Thrips palmi (Karny) (Thysanoptera: Thripidae). We studied O. sauteri's searching efficiency, time allocation on a leaf, leaving tendency, and attacking of prey. Approximately 78% of the encountered prey was eaten. Searching for prey was concentrated for 86% of the time on the lower leaf side, where palm thrips are usually found. Patch residence times on empty leaves were different from those on leaves with T. palmi larvae. Walking activity was not affected by the thrips density, and walking took place during 64% of the total search time. The leaving tendency of O. sauteri was affected by the time from patch entry and the presence or absence of palm thrips, but not by the thrips density. If prey were present, the leaving tendency decreased (multiplication factor 0.327), resulting in longer giving-up times than when no prey was present. The fact that the leaving tendency increases when patch exploitation lasts longer suggests that not much time is wasted on patches where encounters with prey are scarce
Digital 3D reconstruction of two parahissian accessory bundles in a case of Wolff-Parkinson White syndrome
Steffen, C. ; Schaepman, M.E. ; Hardmeier, T. ; Schweitzer, W. - \ 2004
International Journal of Legal Medicine 118 (2004)2. - ISSN 0937-9827 - p. 101 - 105.
cardiac conduction system - sudden-infant-death - catheter ablation - pathways - children - localization - diagnosis - age
Three-dimensional reconstruction of digitized histological serial sections of the cardiac conduction system yielded two accessory pathways in a case of a 24-day-old male infant who died after a short period of illness with ECG findings of Wolff-Parkinson-White syndrome. In infants, the differential diagnosis of possible accessory pathways connecting the AV conduction system, atrial or ventricular septum includes dispersed conduction system tissue without connecting features. This is why three-dimensional reconstruction is necessary in order to refute or establish connectivity of cell groups as found in histological serial slice images.
Do two closely related petrel species have a different breeding strategy in Antarctica?
Creuwels, J.C.S. ; Franeker, J.A. van - \ 2003
In: Antarctic biology in a global context. / Huiskes, A.H.L., Gieskes, W.W.C., Rozema, J., Schorno, R.M.L., van der Vies, S.M., Wolff, W.J., Leiden : Backhuys - p. 144 - 147.
fauna - broedgedrag - ecologie - ornithologie - stormvogel - zuidpoolgebied - Antarctica
Responses of seabirds, in particular prions (Pachyptila sp.), to small-scale processes in the Antarctic Polar Front
Franeker, J.A. van; Brink, N.W. van den; Bathmann, U.V. ; Pollard, R.T. ; Baar, H.J.W. de; Wolff, W.J. - \ 2002
Deep-Sea Research. Part II, tropical studies in oceanography (2002). - ISSN 0967-0645
fauna - ecologie - fauna - ornithologie - zeevogels - zuidpoolgebied - Antarctica
Small-scale distribution patterns of seabirds in the Antarctic Polar Front (APF) were investigated in relation to other biological, physical, and chemical features during the ANT-XIII/2 research cruise of R.V. Polarstern from December 1995 to January 1996. The APF is characterized by steep gradients in sea-surface temperature and salinity. Within the APF, gradient zones were closely associated with elevated levels of primary production, chlorophyll-a (chl-a) concentrations, and zooplankton densities.Even broad-billed prions (Pachyptila vittata group), which dominated the seabird community by 83% in carbon requirements, showed small-scale distributional patterns that were positively related to primary production, chl-a, and total zooplankton densities. The findings demonstrate a close, direct link between fine-scale physical processes in the APF and biological activity through several food web levels up to that of zooplankton-eating seabirds. Broad-billed prions appeared to forage on very small copepods (Oithona spp.) in close association with the front. Fish- and squid-eating predators showed poor correlations with small-scale spatial structures of the APF. However, in a wider band around the APF, most top predators did occur in elevated densitiesshowing gradual spatio-temporal diffusion of the impact of the APF on higher trophic levels.
Species richness and distribution of benthic tidal flat fauna of the Banc d'Arguin, Mauritania
Wijnsma, G. ; Wolff, W.J. ; Meijboom, A. ; Duiven, P. ; Vlas, J. de - \ 1999
Oceanologica acta : European journal of oceanology 22 (1999)2. - ISSN 0399-1784 - p. 233 - 243.
The Banc d'Arguin in Mauritania, West Africa, is an area of tidal flats and shallow inshore waters bordering the sandy desert of the Sahara. The project Banc d'Arguin 1985-1986 investigated predominantly benthic biomass and production, the same data allow investigation of the species diversity and the distribution of the tidal flat fauna of the Banc d'Arguin. We sieved and analysed 410 core samples (diameter 10 cm) from 82 locations with 0.6 mm mesh size. A total of 111 taxa were identified. Polychaeta were most diverse with 42 species belonging to 20 families. Other large species groups were Gastropoda (20 species, 14 families), Amphipoda (14 taxa, 8 families) and Bivalvia (15 species, 10 families). Densities ranged from 152 to 5635 ind. m-2, with an average of 1404 ind. m-2. Six taxa accounted for more than half of the total density. H' values were generally low with values ranging from 0 to 2.78 nats. Low evenness values indicated a high degree of dominance. The total density was independent of any sediment characteristic, probably because of the large range of species. Densities of dominant species did show a correlation with sediment size. A fraction of the individuals could have been lost with our use of a 0.6 mm sieve. Many species occurredonly at a few stations. An explanation for this could be that stations are dominated by different species, while the diversity within a station is relatively low. The occurrence of opportunistic species supports this explanation. The Banc d'Arguin is a reversed estuary, and the stress caused by high temperatures and salinity could result in opportunistic assemblages. The (theoretical) high turnover rate of these opportunistic assemblages could be the key to the importance of the Banc d'Arguin for wintering shorebirds. VA:IBN
Zingiber G.R. Boehmer
Wolff, X.Y. ; Astuti, I.P. ; Brink, M. - \ 1999
In: Plant Resources of South-East Asia 13 : Spices / de Guzman, C.C., Siemonsma, J.S., Leiden : Backhuys Publishers - p. 233 - 238.
Trachyspermum roxburghianum (DC.) H. Wolff
Siemonsma, J.S. ; Jansen, P.C.M. - \ 1999
In: Plant Resources of South-East Asia 13 : Spices / de Guzman, C.C., Siemonsma, J.S., Leiden : Backhuys Publishers - p. 223 - 225.
The smell of water : grazer-induced colony formation in Scenedesmus
Lürling, M. - \ 1999
Agricultural University. Promotor(en): W.J. Wolff; E. van Donk. - S.l. : s.n. - ISBN 9789058080462 - 270
scenedesmus - daphnia - aquatische gemeenschappen - waterorganismen - fytoplankton - schadelijke waterplanten - spreiding - ecologie - begrazing - onderzoek - scenedesmus - daphnia - aquatic communities - aquatic organisms - phytoplankton - aquatic weeds - spread - ecology - grazing - research - cum laude

In aquatic systems, the phytoplankton - zooplankton relation is of major importance because it is the first step in the pelagic food chain. It is well known that zooplankton feed with a highly variable success on phytoplankton, primarily owing to algal characteristics such as size, shape, cell wall texture, nutritional quality and toxicity. Algae are present in a broad variety of shapes and may express an enormous variability in their morphology, physiology and behavior depending on environmental variables. Because algae depend on solar energy they have to remain in the upper water layers as long as possible. Moreover, they have to compete with other algae for dissolved nutrients. This means that in an aquatic environment selection pressure exists for small organisms since these have the most efficient uptake of nutrients and light and lowest sinking losses. By contrast, mortality through grazing by an entire assemblage of protozoan and metazoan grazers will exert a strong selection for traits that reduce this mortality through grazing. An effective way to resist grazing is by a dramatic increase in size. However, this confronts the algae with conflicting allometries of selection pressures.

Since algae are small relative to their predatory enemies, they may not survive an encounter with a grazer. Therefore, it may be profitable to detect a grazer before they encounter each other in order to elicit a defensive strategy. In a predictable environment temperature and day length could be good predictor of danger. However, in aquatic systems, grazing fluctuates considerably on temporal and spatial scales and chemical cues may be used instead. All organisms exchange constantly chemicals with their environment and those chemicals that are essential in the biology of the grazer and are detectable by the algae may prove potential indicators of danger. They convey information and are referred to as infochemicals. This thesis focuses on the role of infochemicals in the interaction between algae and zooplankton, with emphasis on the Scenedesmus (algae) - Daphnia (waterflea) relation.

In the presence of filtered medium from a Daphnia culture, the non-spiny Scenedesmus acutus formed numerous eight-celled colonies (coenobia) (Chapter 2). However, in control populations, i.e. in the absence of Daphnia -infochemical, S. acutus remained unicellular and formed only four-celled colonies when cultures reached stationary phase. The induced colony formation appeared reversible as eight-celled colonies gradually disappeared from the treated populations.

A prerequisite for further exploration of the phenomenon of Daphnia -induced colony formation is the development of a reliable biotest (Chapter 3). Inoculum algal density, carbon availability and filter-type are some of the factors that affected the Daphnia -induced colony formation. Analysis of filter extractables revealed that at least two detergents might cause S. acutus to shift rapidly from a completely unicellular population to one dominated with colonies. The production of the Daphnia -infochemical is related to the amount of food processed by the animals. Starved animals or animals fed with ingestible but non-digestible beads were ineffective in inducing colonies. Neither algal homogenates nor auxins and several organic carbon sources have colony inducing activity. The infochemical does seem to originate from the Daphnia -food interaction, or better from the grazer-algal food interaction as several herbivorous zooplankters were able to induce colonies in S. acutus , whereas carnivorous zooplankton and fish were ineffective (Chapter 3). Simple excretion products, such as ammonia and urea alone or in combination with organic carbon sources were ineffective as colony inducing agents (Chapters 3 & 4).

Scenedesmus plasticity has, however, not only been shaped by the activity of grazers, but also by other selective forces. Several factors are known that may influence the growth and morphological development in Scenedesmus and among them nutrient availability (Chapter 4) and temperature (Chapter 5) are important ones. In culture, with relatively high algal densities carbon limitation may occur. The availability of inorganic carbon appeared ineffective in inducing colonies, but had a clear effect on cell size. Neither N- nor P-limitation resulted in the formation of numerous, eight-celled coenobia. In general, under nutrient limitation cultures were dominated by unicells. However, despite the limitation, by adding Daphnia water, colonies still could be induced. One of the criticisms on use of artificial growth media is the excessive amount of nutrients in most of them. However, using media of various strengths showed no differences in morphological appearance of S. acutus , both in the absence and presence of Daphnia water.It appears that as long as cell division is not hampered grazer-induced colony formation may occur.

Temperature not only affected growth, but also the morphological development in S. acutus (Chapter 5). At low temperatures growth was reduced, but cell- and colony size increased. Under a broad range of temperatures from 9° to 29°C, the addition of Daphnia water significantly increased the proportion of eight-celled coenobia. The smaller size at higher temperature supports the hypothesis of a trade-off between sinking and size.

An analysis of potential costs associated with grazer-induced colony formation was initially directed on metabolic costs (Chapter 6). However, no reductions in growth and photosystem II efficiency were detected in induced colonies. Higher sinking losses of induced colonial Scenedesmus populations were measured. Hence, costs may be assigned to enhanced sinking out of the euphotic zone into darker and colder water layers, thereby significantly reducing growth rates. The strategy may, however, not be completely lethal, as Scenedesmus is known to be capable of surviving for prolonged periods on the sediments.

One of the prerequisites for interpreting the grazer-induced colony formation as an induced defense is that the response has to be effective in reducing mortality through grazing. In Chapter 7, the grazing success of several zooplankton species, such as the rotifers Keratella and Brachionus and the cladocerans Bosmina , Ceriodaphnia and Daphnia , was analyzed. Food intake was reduced in all smaller grazers, but not in the largest Daphnia species. Moreover, growth of the small Daphnia cucullata was reduced when offered colonial S. acutus . These reductions appeared not the result of an altered biochemical composition of induced colonies.

So far, the effect of grazing-associated infochemicals had only been examined for the non-spiny S. acutus . In Chapter 8, 23 different Scenedesmus strains, 9 different other chlorophytes, 2 diatom species and 5 strains of cyanobacteria were investigated. In 35% of the Scenedesmus a positive response to the addition of Daphnia water was observed. Most responding appeared the non-spiny strains, i.e. 64% in contrast to the 4% for spined Scenedesmus . Not only is the trait colony formation only one of the potential defensive traits, it also appeared that the biotest was only suited for examining non-spiny Scenedesmus .

The grazer-induced colony formation appeared not to be restricted to the genus Scenedesmus, since two Coelastrum strains were responsive too. Also in the diatom Synedra and the cyanobacterium Microcystis cell size was increased in the presence of Daphnia water. Moreover, the latter showed a tendency to higher toxin levels when cultured in the presence of medium from a Daphnia culture (Chapter 8).

Another phenomenon often observed in the presence of live Daphnia , is the aggregation of live cells onto fecal pellets (Chapter 8). These large aggregates will undoubtedly be inedible to grazers and may be an additional process affecting the energy flow from algae to their consumers.

In Chapter 9, experiments were performed to evaluate the ability of Daphnia to locate algae by means of chemical cues. No evidence for such a mechanism was detected. However, the animals did seem to avoid water with odors from congeners. Moreover, water from crowded Daphnia cultures had clear effects on growth and reproduction in two Daphnia species and may have an effect on the phytoplankton-grazer interaction.

Summarizing the various experiments described in this thesis, the phenomenon of grazer-induced colony formation in Scenedesmus can be interpreted as an inducible defense at the expense of higher sinking losses. The phenomenon is not restricted to Scenedesmus and because of the enormous plasticity in phytoplankton, numerous species may eventually turn out not only to respond to abiotic but to biotic agents as well.

Effects of ultraviolet-B radiation on phytoplankton - zooplankton interactions = [Effecten van ultraviolet-B straling op interacties tussen fytoplankton en zooplankton]
Lange, H.J. de - \ 1999
Agricultural University. Promotor(en): W.J. Wolff; E. van Donk. - Wageningen : De Lange - ISBN 9789058080158 - 187
zoölogie - aquatische gemeenschappen - fytoplankton - zoöplankton - ultraviolette straling - interacties - ozon - zoology - aquatic communities - phytoplankton - zooplankton - ultraviolet radiation - interactions - ozone

The decrease in stratospheric ozone concentration has received wide attention because the ozone layer protects the earth from harmful ultraviolet-B radiation (UVB, 280-320 nm). UVB radiation is harmful for organisms, and therefore scientific research into how UVB radiation affects organisms and ecosystems receives great interest. This thesis describes the effects of UVB radiation on interactions between phytoplankton (algae) and zooplankton (waterfleas) in freshwater ecosystems.

The underlying hypothesis in this thesis is that phytoplankton is directly affected by UVB radiation because it needs visible light (PAR, 400-700 nm) for photosynthesis, and is consequently also exposed to UVB. Zooplankton on the other hand is not dependent on light, and is able to move actively through the water. This capability of vertical migration, and the possible ability to detect UV radiation may allow zooplankters to regulate their exposure to UVB. Therefore, indirect effects on zooplankton through changes in phytoplankton (its food) may play a more important role than direct UVB effects.

UVB, UVA (320-400 nm) and PAR radiation were measured in 19 aquatic systems in the Netherlands. In most systems the penetration of UVB radiation was limited to the upper decimetres. High phytoplankton biomass or high concentrations of humic acids caused the limited light penetration. Lake Maarsseveen was the clearest system in this study with a vertical attenuation coefficient (K d ) for UVB of 9.1 (m -1). This corresponded to a 1% UVB penetration depth of 51 cm. The effect of UVB radiation on a system will be a combination of penetration depth, mixing processes, and depth of the system.

There is a number of UV-mediated qualitative changes in phytoplankton that may affect herbivorous zooplankton. Changes in phytoplankton species composition, increase in cell size and increase in cell wall thickness will negatively affect ingestion and digestion by zooplankton. Phytoplankton cell biochemistry is also affected by UVB radiation, namely a decrease in lipids and proteins, and an increase in carbohydrates. This will strongly influence the cells nutrient quality. The potential negative effect of UV on fatty acids (FA) in phytoplankton call for special attention. FA play a major role in nutrition for most animals, and as such UV-mediated lipid peroxidation or reduced biosynthesis of essential FA could be a major determinant of food quality for aquatic herbivores.

Experiments with zooplankton grazing on UVB-stressed phytoplankton showed varying effects on grazing rates. Grazing rates of the large waterflea Daphnia were not affected by UVB-stressed phytoplankton. The small waterflea Bosmina and small rotifer Brachionus had both increased and decreased grazing rates on UVB-stressed phytoplankton. These experiments suggest that possible effects of enhanced UVB radiation on phytoplankton-zooplankton interactions are not straightforward predictable from grazing experiments. Moreover, extrapolation of data to predict ecosystem response to UVB stress seems unjustified.

Life history experiments with Daphnia feeding on several species of UVB-stressed phytoplankton showed that life history traits were negatively affected. Effects of UVB-stressed phytoplankton on the population growth rate of Daphnia were not significant. Effects on clutch sizes and quality of offspring were significant. In general, a smaller number of juveniles of poorer quality was produced in the UVB treatments. This may have implications for the food web functioning. The UVB effect was dependent on the phytoplankton species. The UVB effects may be caused by a change in food quality combined with possible reduced digestibility of UVB stressed phytoplankton.

No UVB effects were found in the experiments with indoor model ecosystems. UVB radiation had no significant effect on the phytoplankton, zooplankton, periphyton or macro-invertebrate communities in these model ecosystems. A bio-assay with Daphnia feeding on phytoplankton from the model ecosystems showed that phytoplankton from the UVB treatments had a negative effect on Daphnia growth and survival, and to a lesser extent on fecundity. These results indicate that the transfer of energy from phytoplankton to zooplankton can be negatively influenced by UVB radiation. Overall, the model ecosystems were not affected by the UVB stress. From these results it can be concluded that a natural ecosystem with similar penetration of UVB can be resistant to UVB radiation presently occurring at temperate latitude.

Field experiments at 3 different latitudes (in the Netherlands, Norway and Spitsbergen) showed that present levels of UVB radiation may negatively affect phytoplankton and zooplankton. The different phytoplankton species showed different responses in growth rate to UVB, UVA and PAR radiation. This implies that increased levels of UVB radiation could lead to shifts in phytoplankton community structure. Chlamydomonas (a green algae with flagella) responded to UVB radiation with a loss of flagella, whereas the growth rate was not affected.

Daphnia in Lake Zwemlust (the Netherlands) responded to UVB with a decreased grazing rate and smaller body size, whereas the survival was not affected. Daphnia grazing rates were affected in all three locations, though variance was large and differences marginal. This shows that solar UVB radiation in potential can reduce the transfer of energy from phytoplankton to zooplankton. UVB effects were comparable between the three locations. The magnitude of the UVB effects was quite different, due to very different weather conditions.

A field study was done in the province of Saskatchewan, Canada, comparing seston (suspended live and dead particles) quantity and quality of ponds and lakes with different light penetration properties. Multivariate analysis suggested that the phytoplankton species composition was influenced by the light climate of the studied system. A standardized laboratory experiment with Daphnia showed that the phospholipid content of the seston was the best predictor of Daphnia growth, because of the high nutritional value of phospholipids. It was hypothesized that in the studied systems, light climate and especially the penetration of ultraviolet radiation was important in determining the phospholipid content of the seston. The proposed relationships between light climate, phospholipid content of the seston, and Daphnia growth need experimental confirmation.

Summarizing the different experiments described in this thesis, the effects of UVB on the phytoplankton-zooplankton interactions were present, detectable, and mostly negative. The magnitude of the UVB effects was not large. It was not possible to make generalizations because of the species-specific reactions to UVB. The experiments were all short-term, and it may be erroneous to make long-term predictions based on these results. However, the subtle differences found in this study may be important in determining ecosystem functioning. UVB radiation interacts with other environmental variables (such as temperature, nutrient concentrations, and vertical mixing), and is already playing a role in the functioning of an ecosystem.

See also:project descriptionandc.v. of the author.
Ecology of wintering terns
Brenninkmeijer, A. ; Klaassen, M. ; Stienen, E.W.M. - \ 1998
In: Waders in Guinea-Bissau, October 1992 - May 1993. The end of the East-Atlantic Flyway / Wolff, W.J.,
Spatial variation and seasonality in growth and reproduction of Enhalus acoroides (L.f.) royle populations in the coastal waters off Cape Bolinao, NW Philippines
Rollón, R.N. - \ 1998
Agricultural University. Promotor(en): W. van Vierssen; W.J. Wolff, co-promotor(en): E.D. de Ruyter van Steveninck. - Rotterdam etc. : Balkema - ISBN 9789054104124 - 135
hydrocharitaceae - kustgebieden - marien milieu - mariene ecologie - plantengeografie - milieufactoren - ruimtelijke variatie - seizoengebondenheid - groei - voortplanting - mariene biologie - populatiebiologie - hydrocharitaceae - coastal areas - marine environment - marine ecology - phytogeography - environmental factors - spatial variation - seasonality - growth - reproduction - marine biology - population biology
Lake Victoria wetlands and the ecology of the Nile tilapia, Oreochromis niloticus Linne
Balirwa, J.S. - \ 1998
Agricultural University. Promotor(en): W.J. Wolff; P. Denny; R.M.M. Roijackers. - Rotterdam : Balkema - ISBN 9789054104117 - 247
hydrologie - limnologie - meren - plassen - hydrobiologie - biocenose - wetlands - polders - Percidae - rivierbaars - Cichlidae - Tilapia - Lutjanidae - ecologie - diergedrag - gewoonten - Centraal-Afrika - water - reservoirs - aquatische ecosystemen - hydrology - limnology - lakes - ponds - hydrobiology - biocoenosis - wetlands - polders - Percidae - perch - Cichlidae - Tilapia - Lutjanidae - ecology - animal behaviour - habits - Central Africa - water - reservoirs - aquatic ecosystems

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 (<15.0 cm) and juvenile fish.

The shallow vegetated habitats were found to be ecologically important for the Nile tilapia for sheltering and feeding, and, it was also found that Lake Victoria Nile tilapia were significantly more fecund (with 3723 ± 147 eggs.female -1) and had a higher condition index than populations of the species in Lake Kyoga (also stocked) and Lake Albert (a native habitat). It was inferred that these differences could be a result of a better nutritional base in Lake Victoria where the species was shown to be omnivorous (with detrital and animal foods as major dietary items) contrary to the previously believed herbivorous (phytoplankton) habits. Successional patterns associated with water hyacinth and the strong hydrological influences on shallow vegetated habitats imply that basin disturbances could therefore be a major threat to water quality and the fisheries.

Changes in biodiversity of the Wadden Sea and adjacent habitats: The Netherlands, Germany, and Denmark
Wolff, W.J. - \ 1996
In: Biodiversity in managed landscapes. Theory and practice / Szaro, R.C., Johnston, D.W., Oxford : Oxford University Press - p. 531 - 547.
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