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  • 1
    Electronic Resource
    Electronic Resource
    A multivariate analysis of phytoplankton and food web changes in a shallow biomanipulated lake (1996)
    Oxford BSL : Blackwell Science Ltd
    Freshwater biology 36 (1996), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. Phytoplankton dynamics, food chain changes and resilience in Lake Zwemlust, a shallow lake in The Netherlands, are described for the period 1986–94.2. After biomanipulation in 1987, the lake moved through two alternative states, while the external nutrient loadings were maintained. A clear-water phase, mostly dominated by macrophytes, persisted from 1987 to 1991, and a rather turbid state, dominated by algae, occurred in the summers of 1992–94, after several consecutive and sustained perturbations affecting different parts of the food web in the lake. These two periods were characterized by different community structures.3. The phytoplankton assemblage gradually changed in a pattern that reverted in later years towards that of the pre-biomanipulation stage, although the same species composition was not regained. This agrees with some mathematical models. During the clear-water phase, nutrient shortage, light climate and zooplankton feeding selected in favour of small, high surface : volume ratio and rapidly reproducing algae. However, in mid-summer of 1992–94, nutrient availability and cladoceran grazing on edible algae favoured cyanophytes.4. Nutrients were transferred to higher trophic levels or lost from the system at relatively high rates when the lake was in a piscivore–macrophyte-dominated state, while they tended to accumulate in the algae in a planktivore-dominated chain without macrophytes. The role of weed beds was central for nutrient competition (mostly nitrogen) with algae, as well as a refuge and a base for alternative food sources to grazers. Weed beds seemed to have a strong effect in increasing connectedness, resilience and stability of the lake community.5. The complete return of Zwemlust to a turbid state dominated by phytoplankton seems to have depended upon turnover of the limiting nutrient, which was retarded by macrophytes and stimulated by planktivorous fish and waterfowl.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2427.1996.d01-511.x
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of fish and nutrient additions on food-web stability in a charophyte-dominated lake (2004)
    Oxford, UK : Blackwell Science Ltd
    Freshwater biology 49 (2004), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. The response of major food-web constituents to combinations of nutrient addition and zooplanktivorous fish abundance was tested during two subsequent years in the shallow charophyte-dominated lake Naardermeer in the Netherlands, using in situ enclosures.2. Treatment effects differed sharply between study years. In 1998, when the summer temperature was low (17–21 °C), high algal biomass only developed at high nutrient levels in the presence of fish, but with no major effect on Chara biomass. In 1999, when the summer temperature was relatively high (20–25 °C), algal blooms occurred at high nutrient levels regardless of fish abundance, and were associated with a drastic decline in Chara biomass.3. Differences between years in temperature and initial zooplankton composition and biomass were likely to contribute to the varying relative importance of top-down and bottom-up effects in these enclosure experiments.4. The results suggest that when nutrient loads are increased towards levels where the macrophyte-dominated state is being destabilised, a ‘switch’ is more likely to occur in warm summers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2427.2004.01301.x
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  • 3
    Electronic Resource
    Electronic Resource
    Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment (2004)
    Oxford, UK : Blackwell Science Ltd
    Freshwater biology 49 (2004), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe.2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year-to-year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers.3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites.4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L−1) when grazer biomass was high (〉80–90 μg dry mass L−1) or accounted for 〉30% of the grazer community.5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass.6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2427.2004.01300.x
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  • 4
    Electronic Resource
    Electronic Resource
    Effects of artificial ultraviolet-B radiation on experimental aquatic microcosms (1999)
    Oxford, UK : Blackwell Science Ltd
    Freshwater biology 42 (1999), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. The effects of prolonged ultraviolet-B (UVB) radiation on freshwater communities were studied in indoor microcosms (600 L) with artificial light sources, simulating a clear, shallow, mesotrophic aquatic ecosystem. A range of six intensities (in duplicate) of UVB radiation, ranging from 0 (control) to 9.56 kJ m−2 day−1 at the water surface, was applied for 8 weeks. The UVB radiation levels, attenuation, shading and scattering were comparable to those in Dutch shallow freshwater systems. Physical, chemical and biological variables were monitored weekly.2. The UVB treatment did not affect the abundance, species composition or biovolume of the phytoplankton or zooplankton communities, nor did it affect the periphyton or the macroinvertebrate community. A few species showed a significant response on some of the sampling dates, but there was no negative UVB effect at the community level. Overall, the ecosystems in the microcosms were not affected by the UVB treatment.3. In a bio-assay, a laboratory clone of Daphnia pulex, not subjected to UVB radiation, was fed with seston from the microcosms. Daphnia pulex feeding on seston from the control microcosms grew faster, had better survival and better reproduction than D. pulex feeding on seston from the UVB treated microcosms. The phytoplankton–zooplankton interaction may have been influenced by the UVB treatment.4. The dissolved oxygen content (DOC) concentrations in the microcosms were around 5 mg L−1. The DOC levels in Dutch systems rarely fall below 10 mg L−1. This might provide sufficient protection against the detrimental effects of increased UVB radiation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2427.1999.00478.x
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  • 5
    Electronic Resource
    Electronic Resource
    Effects of UVB-irradiated algae on life history traits of Daphnia pulex (1997)
    Oxford, UK : Blackwell Science Ltd
    Freshwater biology 38 (1997), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. The impact of ultraviolet-B (UVB)-irradiated phytoplankton on the life history parameters of Daphnia was studied. Three species of Chlorophyceae (Chlamydomonas reinhardtii, Scenedesmus acutus and S. subspicatus) and one species of Cryptophyceae (Cryptomonas pyrenoidifera) were cultured with and without exposure to UVB radiation (biologically effective dose of 3 kJ m–2 day–1 DNA weighted) and fed to Daphnia pulex.2. The daphnids feeding on UVB-irradiated algae tended to be smaller in size, to produce less and smaller offspring, and to have a reduced population growth rate.3. Fatty acid (FA) analysis of the phytoplankton showed a reduced total FA content under UVB radiation for all four species. Changes in the content of unsaturated fatty acids (UFA) were species specific.4. The results indicate that the energy transfer between the primary producers’ and grazers’ level might be negatively influenced by UVB radiation. Direct effects on food quality are modest but detectable, changes in quantity or species composition may be more important in the energy transfer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2427.1997.00221.x
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  • 6
    Electronic Resource
    Electronic Resource
    UV-induced changes in phytoplankton cells and its effects on grazers (1997)
    Oxford, UK : Blackwell Science Ltd
    Freshwater biology 38 (1997), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. This review addresses the effects of UV-radiation on the morphology and biochemistry of phytoplankton and the potential effects on grazers.2. UVA and UVB radiation inhibit the uptake of inorganic nutrients in phytoplankton. Reduced rates of ammonium and nitrate uptake in marine diatoms, and reduced uptake of phosphorus in freshwater flagellates are reported. The effects on cell stoichiometry are not settled.3. UVA and UVB radiation promote increased cell volumes owing to a decoupling between the photosynthetic processes and cell division. Loss or inactivation of flagellae and loss of motility are also reported for a number of phytoplankton species.4. UVA and UVB radiation may affect major biochemical constituents. Accumulation of intracellular, photosynthetic products (lipids or carbohydrates) is a common, although not unique, property of UV-stressed algae. Fatty acid (FA) profiles seem susceptible to UV radiation. A relative increase of short-chained, and a decrease in polyunsaturated FA (PUFA) are reported. The important membrane FA like eicosapentaenoic acid (EPA, 20 : 5ω3) and docosahexaenoic acid (DHA, 22 : 6ω3) seem particularly susceptible, owing to lipid peroxidation or reduced biosynthesis.5. UV-related responses are highly dependent on taxonomy, cell-cycle stage, nutrient-limitation and the UV : PAR (photosynthetic active radiation)-ratio.6. Nutrient deficiency, cell size, cell wall properties and FA can all have significant impacts on grazers. Thus the reported effects on cell morphology and biochemical constituents could have profound effects on grazers and energy transfer in aquatic foodwebs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2427.1997.00223.x
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  • 7
    Electronic Resource
    Electronic Resource
    Short-term and long-term effects of zooplanktivorous fish removal in a shallow lake: a synthesis of 15 years of data from Lake Zwemlust (2002)
    Oxford, UK : Blackwell Science Ltd
    Freshwater biology 47 (2002), S. 0 
    Details
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: SUMMARY 1. Removal of zooplanktivorous fish (mainly bream) in 1987 from a shallow eutrophic lake in the Netherlands, Lake Zwemlust, resulted in a quick switch from a turbid state with cyanobacteria blooms to a clear state dominated by macrophytes.2. The clear state was not stable in the long term, however, because of high nutrient loadings.3. In 1999, another removal of zooplanktivorous fish (mainly rudd) had similar effects as in 1987, although macrophytes returned more slowly.4. In the years directly following both interventions there was a ‘transition period’ of very clear water with high densities of zooplanktonic grazers in the absence of macrophytes; low oxygen concentrations indicate that during those years primary production was low relative to heterotrophic activity.5. The transition period appears to provide the light climate necessary for the return of macrophytes.6. Reduction of nutrient loading is necessary to improve water quality in Lake Zwemlust in the long term. In the short term, repeated fish stock reduction is a reasonable management strategy to keep Lake Zwemlust clear.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2427.2002.01006.x
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  • 8
    Electronic Resource
    Electronic Resource
    Defenses in phytoplankton against grazing induced by nutrient limitation, UV-B stress and infochemicals (1997)
    Springer
    Aquatic ecology 31 (1997), S. 53-58 
    Details
    ISSN: 1573-5125
    Keywords: algal cell wall ; colony formation ; Daphnia ; digestibility ; grazing resistance ; induced defence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract It is becoming increasingly evident that the efficiency of zooplankton grazing on algae is not only a matter of quantity of the grazer relative to its food. Planktonic primary producers are not defenseless food-particles that are easily harvested by the consumers. Several algal species are able to adjust their phenotype (colony formation, spines, size) in such a way that it results in a reduced grazing pressure. It was recently demonstrated that morphological changes in the cell wall of green algae, induced by nutrient limitation and UV-B stress, may reduce their digestibility. A high fraction of induced cells pass intact and viable through the gut of the zooplankters, such that the grazing impact on the population is strongly reduced. It was also found that the presence of exudates (infochemicals) released by daphnids may change the morphology of algae. Unicellular green algae of the genus Scenedesmus were induced to form eight-cell coenobial types, heavily armed with spines, within three to five days after adding filtered water from an algal culture with Daphnia present. Both defence mechanisms may play an important role in zooplankton production and competition, and may serve as an example of highly efficient strategies to resist heavy grazing pressure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1009951622185
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  • 9
    Electronic Resource
    Electronic Resource
    Biogeography ofFreshwater Algae, J. Kristiansen (1997)
    Springer
    Aquatic ecology 31 (1997), S. 342-342 
    Details
    ISSN: 1573-5125
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1009956010333
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  • 10
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    Evaluating early-warning indicators of critical transitions in natural aquatic ecosystems (2016)
    National Academy of Sciences
    Details
    Publication Date: 2016-11-22
    Description: Ecosystems can show sudden and persistent changes in state despite only incremental changes in drivers. Such critical transitions are difficult to predict, because the state of the system often shows little change before the transition. Early-warning indicators (EWIs) are hypothesized to signal the loss of system resilience and have been shown to precede critical transitions in theoretical models, paleo-climate time series, and in laboratory as well as whole lake experiments. The generalizability of EWIs for detecting critical transitions in empirical time series of natural aquatic ecosystems remains largely untested, however. Here we assessed four commonly used EWIs on long-term datasets of five freshwater ecosystems that have experienced sudden, persistent transitions and for which the relevant ecological mechanisms and drivers are well understood. These case studies were categorized by three mechanisms that can generate critical transitions between alternative states: competition, trophic cascade, and intraguild predation. Although EWIs could be detected in most of the case studies, agreement among the four indicators was low. In some cases, EWIs were detected considerably ahead of the transition. Nonetheless, our results show that at present, EWIs do not provide reliable and consistent signals of impending critical transitions despite using some of the best routinely monitored freshwater ecosystems. Our analysis strongly suggests that a priori knowledge of the underlying mechanisms driving ecosystem transitions is necessary to identify relevant state variables for successfully monitoring EWIs.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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