Summary We conducted a paired large‐scale predation experiment over 32 months in two streams being seasonally shaded by deciduous riparian trees, using the benthivorous fish species gudgeon (Gobio gobio) and stone loach (Barbatula barbatula) as top predators. The biomass of benthic grazers and periphyton in the presence/absence of fish was measured and the periphyton production was compared with the consumption rates using a model‐based approach. A three‐level trophic cascade from benthivorous fish via benthic grazers to periphyton was evident from the field experiment. Integrated over the whole study period, fish reduced the biomass of benthic grazers and indirectly increased the periphyton biomass. Scenario analyses, using a simple dynamic model, indicated top‐down control of periphyton to be strongest during autumn, when periphyton growth was light‐limited, and weaker in the spring, when periphyton growth was not light‐limited. The seasonal light supply variation was caused by shading due to deciduous riparian trees during the vegetation period. This asymmetry in temporal processes weakened the top‐down control in a natural benthic community. Even though grazer biomass is naturally reduced in summer, due to the emergence of the most abundant species (mayflies), a grazer biomasses high enough to reduce the spring periphyton peak could not be sustained by the low summer periphyton growth. We suppose that the temporal decoupling of grazer biomass from periphyton biomass might be caused by the very short generation time of the primary producers (days) compared with the long generation time of the primary consumers (mostly 1 year).

Flexibility is an important adaptive attribute of the feeding periodicity of grazing mayfly larvae because most natural environments offer a wide variation in local predation risk in terms of space, time or predator species. In this study any changes of diel feeding periodicity and consumption rates of Baetis rhodani (Ephemeroptera) were analysed in response to different densities of benthivorous fish (Barbatula barbatula, Gobio gobio) by quantifying gut fullness using the fluorescence of algal pigments. Laboratory experiments with the grazer species B. rhodani were conducted by using different concentrations of chemical fish cues. In order to assess the transferability of the results to a larger scale, the experimental results were compared with field observations in two second order streams using different densities of freely foraging benthivorous fish. During the presence of chemical fish cues in the laboratory experiments the feeding periodicity of the B. rhodani larvae were mostly diurnal while in the absence of fish chemicals nocturnal feeding was observed. The same patterns could be detected in the field during the experiments with the different fish densities. These findings indicate that the larvae were able to assess variations in the predation risk and to alter their feeding habits by making flexible behavioural adjustments. The results from the laboratory experiments further suggested that the behavioural response is controlled by fish density. Behavioural changes were observed for medium and high concentrations of the fish cues but not for a very low concentration. In the field however, the mere presence of fish seemed to be sufficient to induce the observed behavioural shifts. Although the presence of benthivorous fish seemed to cause a lower consumption rate of the B. rhodani larvae in the field, such a reduction could not be found in the laboratory experiments. A conclusion from this study is that the identification of behavioural modifications is an essential component needed for a better understanding of complex trophic interactions in benthic communities. Accurate evaluation and detailed observation of direct and indirect effects cannot be made without the consideration of such behavioural mechanisms.

Summary1. In an extensive field study, we tested the hypothesis that warming during a sensitive period alters trophic interactions in pelagic food webs of dimictic lakes: Matching of predation on the key herbivore Daphnia by vertebrate (Perca fluviatilis) and invertebrate (Leptodora kindtii) predators would destabilise Daphnia populations during summer. To predict food‐web‐mediated effects of climate warming, we relied on phase‐specific warming trends (during winter, early thermal stratification, summer stagnation) of an 11‐year (1999–2009) period to study instantaneous or time‐delayed ecological responses.2. Warming during the period of early thermal stratification (here corresponding to May) rather than during winter or summer was found to generate complex, time‐delayed cascading effects on Daphnia via timing and rates of predation. In contrast to our hypothesis, warming exceeding a critical mean temperature (14 °C) during May increased the stability of the Daphnia population during summer.3. The termination of predation on Daphnia by older perch and L. kindtii was advanced even stronger (14 days per degree Celsius warming during May) than the start of predation. Warming during May thus shortened the period of synchronised predation on Daphnia by older perch, young‐of‐the‐year perch and L. kindtii (match index) by up to 4 weeks, thereby decreasing total rates of predation on Daphnia during July by up to 40%.4. The timing of the top‐down control of Leptodora dynamics by perch proved to be a key process for population stability of Daphnia during summer compared to the less important direct cascading effect of fish on Daphnia and temperature effects on Leptodora growth and consumption or on Daphnia reproduction.5. We conclude that predicting the consequences of climate warming for plankton dynamics requires a mechanistic understanding of lake‐specific temperature‐driven changes in trophic cascades, especially when an invertebrate predator is dynamically coupled both to the key herbivore and to their predators.

Regulating mobility by actively entering the drift under imminent predation risk is an avoidance strategy employed by aquatic macroinvertebrate species that is widely accepted within the scientific community. This response was most evident with respect to diurnal predators that feed in the water column, such as many salmonids. We investigated the role of the nocturnal benthivorous gudgeon [Gobio gobio (L.)] on the drift activity of two macroinvertebrate species known to display this behaviour: Baetis rhodani (PICTET) and Gammarus pulex (L.). Laboratory drift experiments using gudgeon kairomones were conducted with the results determining significant altered activity in the presence of gudgeon kairomones for both macroinvertebrate species. B. rhodani showed reduced drift activity in the kairomone treatment compared to the kairomone-free control, with a distinct nocturnal pattern being observed for both. G. pulex shifted from a similar day/night movement pattern to a nocturnal movement pattern with decreased activity during the day. Reduce activity during the day, whilst maintaining normal activity at night would not reduce the probability of encountering a nocturnal predator under natural conditions and therefore appears to not be a meaningful anti-predator response. To assess the relevance of these findings under natural conditions, we compared the experimental results with drift measurements from field observations. These show a significant reduction in drift activity for G. pulex and slight tendencies for reduced night-time drift for B. rhodani, under seasonal variations. We conclude that the behaviour in response to the physical contact or the hydrodynamic stimuli of nocturnal predators is the most likely explanation for the differences between the results from our laboratory experiment and the field observation. We further discuss that the observed migration patterns might have different species specific consequences for density stabilisation on a population level.

Dammed drinking water reservoirs with their catchment areas and the downstream rivers are dynamic systems that change permanently under the influence of many factors. Their multifunctional use for drinking water supply, flood control, energy production, nature conservation and recreation as well as ecological constraints for the rivers downstream requires an integrative management considering and balancing between different requirements. Thus, an optimal reservoir management has to take into account scenarios of external influences and must be based on predictions of prospective raw water qualities. Furthermore, the impacts of short- and long-term changes of the raw water quality on drinking water treatment have to be considered. The problem is very complex and cannot be solved intuitively but requires the application of hydrological, ecological and process models. This approach was followed in the work presented here, as a tool to predict and evaluate the impacts of different reservoir management strategies in an integrative way is currently not available. The developed decision support procedure (DSP) allows for the estimation of the effects of different hydrological and water quantity management scenarios on raw water quality, water processing costs and ecology in the downstream river. Extreme hydrological events or changing boundary conditions (e. g. climate change) are taken into account.

To predict the coherence in local responses to large-scale climatic forcing among aquatic systems, we developed a generalized approach to compare long-term data of dimictic water bodies based on phenomenologically defined hydrographic events. These climate-sensitive phases (inverse stratification, spring overturn, early thermal stratification, summer stagnation) were classified in a dual code (cold/warm) based on threshold temperatures. Accounting for a latitudinal gradient in seasonal timing of phases derived from gradients in cumulative irradiation (2.2 days per degree latitude), we found a high spatial and temporal coherence in warm-cold patterns for six lakes (84 %) and the Baltic Sea (78 %), even when using the same thresholds for all sites. Similarity to CW-codes for the North Sea still was up to 72 %. The approach allows prediction of phase-specific warming trends and resulting instantaneous or time-delayed ecological responses. Exemplarily, we show that warming during early thermal stratification controls food-web-mediated effects on key species during summer.

P>1. We conducted an experimental study of predation by benthivorous fish on a natural community of stream invertebrates using a reach-scale approach. Over a 2-year period (experimental phase), the benthic invertebrate community of a stretch containing two species of benthivorous fish was compared with a fishless stretch. Thereafter, all fish were removed and benthic community structure was analysed again to account for natural differences between the two stretches (reference phase).2. Benthivorous fish at the moderate densities investigated did not affect total benthic biomass or density, but did alter species composition. In addition, the fish effect differed between pool and riffle habitats, with larger effects in the pools indicating a habitat-specific predation effect. In the reference phase, when all fish were removed from the stream, the difference between the two stretches was reduced.3. The benthivorous fish reduced the densities of four taxa (Pisidium sp., Dugesia gonocephala, Gammarus pulex, Limoniidae), representing 29% of total biomass. It is possible that density reductions of other species were masked by prey migration despite the relatively large spatial scale. Indeed, higher drift activity in the upstream fishless stretch could have increased the density of Baetis rhodani in the fish stretch, as indicated by the results of a drift model.4. Our results provide insights into stream food web ecology because fish predation showed effects even in a natural system where habitat complexity was high, environmental factors were highly variable and many predator and prey species interacted and because benthivorous fish were the focus, whereas the majority of previous predation experiments in streams have used drift-feeding trout.

Although acidification in freshwaters reduces the richness of aquatic species in general, dragonflies are less affected. However, detailed knowledge regarding the effects of very acidic (pH < 4.0) and highly conductive (>700 mu S cm(-1)) water on dragonfly species richness and composition is still scarce. To assess this, 19 anthropogenically influenced waters with a wide range of pH (2.64-6.81) and conductivity (113-2620 mu S cm-(1)) were investigated in the Muskau Arch area (western Poland, eastern Germany). Of the 41 dragonfly species found, 31 were autochthonous. Both total (S-t) and autochthonous (S-a) species richness correlated positively with pH and negatively with conductivity. However, the correlations for autochthonous species were strongly influenced by the samples from the extremely acidic (pH 2.64-2.86) and most ion-rich (conductivity > 1200 mu S cm(-1)) waters, where no species developed. The S-a values from acidic waters with slightly higher pH values (between 3.0 and 4.0) did not differ significantly from those found in neutral waters. Nevertheless, species preferring acidic or neutral conditions, respectively, were clearly separated, showing a direct or indirect effect of pH on the community structure in the area. We thus conclude that only pH values below 3.0 and conductivity above 1200 mu S cm(-1) have a detrimental effect on dragonflies. Other acidic waters are suitable habitats for specialists, which do not develop in neutral waters. Thus, moderate acidification enhances the dragonfly species richness of a region like the Muskau Arch area.

Many predation experiments in streams are carried out in enclosures. Hence, the relevance of their results to predict population dynamics is often unclear due to the relatively small spatial and temporal scale of the experiments. To enhance the transferability of experimental results on the ecosystem scale the impact of fish predators on a prey population was observed in a reach scale approach over 2 years in a natural stream. A 400-m reach inhabited by the small benthivorous fishes gudgeon (Gobio gobio) and stone loach (Barbatula barbatula) was compared with a fishless reference reach.

In many lakes and reservoirs, problems caused by off-flavours are known to be particularly associated with the occurrence of planktonic and benthic cyanobacteria. Frequently observed objectionable taste and odorous products of cyanobacteria are geosmin and 2-methylisoborneol.Investigations focused on the littoral zone of Wahnbach Reservoir (Germany) revealed that benthic cyanobacteria were present in this oligotrophic drinking water reservoir. Benthic cyanobacteria were found in the depth horizon between 1.75 m and 11 m, particularly on south-exposed slopes. This spatial distribution indicates a possible key role of the underwater light climate. Moreover, cell-bound and dissolved geosmin were detected in corresponding littoral samples. Both fractions were subjected to spatial and primarily temporal variations with maximum concentrations at the end of summer. However, a substantial lowering of the water level caused a diminution of cyanobacterial growth. Due to the drawdown of the water level concentrations of cell-bound geosmin and pigments (as a proxy of cyanobacterial biomass) were remarkably reduced, and dissolved geosmin was never detected during this phase. Except for the influence of water level fluctuation no other abiotic variables had a significant influence on pigment and geosmin concentrations. From geosmin concentrations detected in the littoral zone, the probability of serious episodes of odour events in the raw water of the Wahnbach Reservoir was estimated. Hence, the probability that the raw water was affected by geosmin was minor, which was supported by routine flavour profiles. Nevertheless, the study shows that odorous episodes caused by benthic cyanobacteria are likely to develop even in an oligotrophic lake or reservoir when these cyanobacteria, and consequently odorous production, proliferate. In principle, such a proliferation cannot be excluded as nutrients are available from the sediment pore water, and underwater light at the sediment surface in the sublittoral is sufficiently high due to very low phytoplankton-induced turbidity under oligotrophic conditions. Thus, management-induced fluctuations of the water level seem to be the main control variable to generate light conditions at the sediment surface fluctuating in a given depth horizon faster than cyanobacteria can develop there. (C) 2011 Elsevier Ltd. All rights reserved.

Changes of the benthic Microcystis community during the time of reinvasion and its influence on the planktonic inoculum were investigated in a laboratory experiment and in a shallow lake (Quitzdorf Reservoir, Germany). Reinvasion was quantified by the diminution of benthic Microcystis abundance, and moreover, by the increase of Microcystis in the water column. Laboratory investigations revealed that the migration of Microcystis from the sediments was elevated by the increase of water temperature from 9 to 15 degrees C. The extent of reinvasion was more pronounced in light than in dark treatments. Moreover, the reinvasion was accompanied by losses of Microcystis which disappeared from the sediments. In the laboratory experiment it was demonstrated that Microcystis got lost especially under dark conditions. This was supported by the lower photochemical vitality of dark-exposed Microcystis compared to the light-exposed ones.As a consequence of the recruitment from sediments to the water column the morphospecies composition of the Microcystis community changed. Furthermore, the mcyB genotypes of the Microcystis morphospecies were determined. From this morphotype-genotype relation it can be assumed that the composition of Microcystis genotypes at the onset of pelagic growth results from a selective recruitment of mcy(+) genotypes from the sediment to the water column. (C) 2010 Elsevier B.V. All rights reserved.

A mechanistic model was applied to study the influence of diurnal vertical migration (DVM) of planktonic crustaceans on the succession and composition of the phytoplankton community. While zooplankton was restricted to only one functional group, the phytoplankton community was divided into two functional groups which are distinguished by their maximum growth rates and vulnerability to zooplankton grazing. DVM causes a pulsed grazing regime and may also entail a corresponding reduction of the cumulative daily rates of ingestion and losses of zooplankton. To study the relative importance of these two mechanisms of DVM to phytoplankton we performed a scenario analysis consisting of 5 different scenarios. The results show that DVM has a strong influence on the phytoplankton community. Well edible algae benefit during the first 3–4 weeks of summer stratification by reduced daily grazing. The typical shift from small, well edible algae to larger, poorly or non-edible phytoplankton is distinctly delayed. Under the assumption of unchanged daily grazing, however, a pulsed grazing regime has nearly no influence on the resulting phytoplankton composition. As similar effects are also found for completely non-edible phytoplankton, indirect effects via phosphorus availability must be assumed. Thus, the scenario analysis reveals that the observed effects of DVM on phytoplankton can be explained by a combination of two mechanisms: (1) reduction of the daily zooplankton grazing, and (2) changed assimilation and remineralisation of phosphorus. Surprisingly and in contradiction to earlier reports there is almost no DVM effect on phytoplankton due to the sole action of a pulsed grazing regime.

Beyond the traditional paradigm of reducing the loading of water-bodies by pollutants, during the last four decades the new paradigm of ecotechnology developed in water pollution control. Ecotechnology focuses on minimising the consequences of pollution by intentionally manipulating ecosystem-internal structures and processes. To promote the new paradigm a new "ecotechnological immission concept" is proposed. The key idea of this new concept states that for non-toxic, bioconvertible and non bioaccumulating pollutants higher emissions may be granted, if the physical, chemical, and biological structure of the ecosystem has been optimised by ecotechnology to achieve the natural degradation or immobilisation of these substances. All types of pollutants, however, must not exceed ecotechnologically critical limits. Thus, the new "ecotechnological immission concept" integrates emission control and ecotechnology as mutually promoting strategies.

Water level fluctuations in a drinking water reservoir (Saidenbach Reservoir, Saxony, Germany) and length–frequency data of roach (Rutilus rutilus) derived from gillnet catches were studied over a period of eight succeeding years and subsequently analysed with respect to level change and cohort establishment. The results indicate a strong impact of water level on the year class strength of roach. Constantly high water level with large areas of inundated plants led to a very strong roach year class. Conversely, decreasing water level shortly after the spawning period was found to result in a total loss of the new roach year class. Any other possible factors influencing year class strength of roach (such as food, temperature, winter duration) may only play a role if the water level after spawning remained relatively stable. Consequently, water quantity management in reservoirs can be used as a biomanipulation tool to reduce the planktivorous fish stock and to improve water quality, which should be an interesting option especially in drinking water reservoirs.

Long-term (1976-1999) biomanipulation in Bautzen Reservoir (BR) revealed that a combination of piscivore stocking and catch restrictions for piscivores led to the desired effects of low planktivorous fish biomass and enhanced biomass of large filter feeders (Daphnia galeata). Despite the hypertrophic status of BR, fisheries management shifted the planktivore-dominated fish community into a piscivore-dominated community. High winter (Jan-Mar) Daphnia biomass was a sensitive indicator of reduced planktivory. Although edible phytoplankton was suppressed by elevated Daphnia biomass, mean seasonal (May-Oct) total phytoplankton biomass remained unchanged due to growth of large inedible algae and cyanobacteria. Inedible and total phytoplankton biomass was primarily controlled by phosphorus availability. However, during clear water periods a reduction of total phytoplankton was achieved with drastically increased Secchi readings. In Feldberger Haussee (FH), despite intensive long-term manual removal of cyprinids (1985-2002) and stocking of piscivorous fish (1988-2002), biomanipulation only had restricted, delayed, or transient effects on the ecosystem. Mean proportion of piscivores within total yield increased but was below 20% in most years. Planktivore cyprinid yields dropped until 1990 and remained constant thereafter. Daphnia biomass slightly increased after biomanipulation became effective (1987-1989) but declined to pre-biomanipulation levels later in the experiment (1996-2005). Beginning in 1997, both edible and inedible phytoplankton biomass started to decrease. Finally, when biomanipulation had been implemented after a delay of 8 years water clarity increased significantly. Although external loading reductions and biomanipulation in both lakes resulted in moderate phytoplankton biomass reduction and Secchi depth enhancement, the reasons for the observed changes were different. Even though strong cascading effects were detected at the top of the food web in BR, the trophic cascade was largely decoupled between phyto- and zooplankton. External plus internal phosphorus loading still exceeded a critical threshold below which a top-down-induced indirect effect of phosphorus sedimentation and finally limitation could have reduced phytoplankton biomass. In constrast to BR, the critical phosphorus loading threshold in FH has probably been approached. Nonetheless, cascading effects were weak due to insufficient reduction of planktivorous cyprinids. Improved water quality was primarily a result of resource-related effects. Thus, the 2 long-term experiments reveal that (1) biomanipulation cannot be applied successfully without reducing nutrient loading below a critical threshold (BR), and (2) for successful biomanipulation, 30-40% piscivores within total fish standing stocks are required (BR and FH). Without optimum piscivory, manual removal of planktivores will hardly produce sustained cascading effects (FH).

ABSTRACT Batch culture experiments with the cyanobacterium Microcystis aeruginosa PCC 7806 were performed in order to test the hypothesis that microcystins (MCYSTs) are produced in response to a relative deficiency of intracellular inorganic carbon (Ci,i). In the first experiment, MCYST production was studied under increased Ci,i deficiency conditions, achieved by restricting sodium-dependent bicarbonate uptake through replacement of sodium bicarbonate in the medium with its potassium analog. The same experimental approach was used in a second experiment to compare the response of the wild-type strain M. aeruginosa PCC 7806 with its mcyB mutant, which lacks the ability to produce MCYSTs. In a third experiment, the impact of varying the Ci,i status on MCYST production was examined without suppressing the sodium-dependent bicarbonate transporter; instead, a detailed investigation of a dark-light cycle was performed. In all experiments, a relative Ci,i deficiency was indicated by an elevated variable fluorescence signal and led to enhanced phycocyanin cell quotas. Higher MCYST cell quotas (in the first and third experiments) and increased total (intracellular plus extracellular) MCYST production (in the first experiment) were detected with increased Ci,i deficiency. Furthermore, the MCYST-producing wild-type strain and its mcyB mutant showed basically the same response to restrained inorganic carbon uptake, with elevated variable fluorescence and phycocyanin cell quotas with increased Ci,i deficiency. The response of the wild type, however, was distinctly stronger and also included elevated chlorophyll a cell quotas. These differences indicate the limited ability of the mutant to adapt to low-Ci,i conditions. We concluded that MCYSTs may be involved in enhancing the efficiency of the adaptation of the photosynthetic apparatus to fluctuating inorganic carbon conditions in cyanobacterial cells.

Summary1. We present data with a high spatio‐temporal resolution from a 72‐h field survey in Bautzen Reservoir (Saxony, Germany). The aims of this survey were to observe hydrophysical processes during a period of unstable stratification in spring and investigate the effect of wind‐induced internal waves on the vertical distribution of zooplankton.2. Wind velocities up to 10 m s−1 caused a strong downwelling event of warm water at the sampling site and led to the generation of internal waves with an amplitude of 4 m.3. The zooplankton community, which was dominated by Daphnia galeata, inhabited epilimnetic waters. Downwelling enlarged the thickness of the epilimnetic layer and, hence, led to high zooplankton abundances down to relatively deep water strata indicating lateral transport of zooplankton. As a consequence, area‐specific zooplankton abundances increased considerably (max. fourfold) during downwelling.4. We conclude that classical limnological field sampling, such as for monitoring purposes, can lead to severely biased estimates of zooplankton abundance due to the interfering effects of hydrophysical processes like internal waves.5. Backscattering strengths measured by a simultaneously deployed Acoustic Doppler Current Profiler (600 kHz) were found to be correlated with estimated zooplankton abundances based on plankton samples.

Defining the reference condition is one of the most critical aspects of ecosystem investigations since it describes the baseline against which the experimental sites will be evaluated and compared. In large-scale ecosystem experiments, this reference is ideally another ecosystem which is similar to the experimental system. We investigated two streams for their potential as experimental sites for a full-size pairwise ecosystem experiment. Temporal (2 years) and spatial (pool, riffle) variabilities of abiotic factors and as biotic element the structure of the macroinvertebrate communities were investigated. Criteria of similarity that we applied at the two streams were: (1) high similarity in abiotic factors, (2) only small differences in the faunal assemblages (abundance structures, composition, feeding types), and (3) that the differences between the two systems should not exceed the temporal and spatial differences within each system. Among the abiotic factors investigated, only the inorganic nutrients (nitrate and soluble reactive phosphorus), major ions (magnesium, calcium), electric conductivity, and pH showed significant differences between the two streams. Discharge rate, current velocity, temperature, and oxygen concentrations did not significantly differ between the streams. Also, the community structure did not differ in species richness, abundance, and biomass; and only small differences in dominance structure and feeding-type composition were observed. The differences between habitats within each stream were always higher than those between the streams. Thus, both the streams are characterized by a similar structure of the macroinvertebrate community, a main component of the stream food-web, which make them suitable for a full size pairwise ecosystem experiment. The present case study can form a basis for other full-size field experiments.