The role of predation in the regulation of freshwater communities is predicted to decrease along a habitat-duration gradient, from permanent to episodic waters. We tested the role of invertebrate predation in shaping the community structure in a fishless temperate intermittent pond with a three month long hydroperiod by comparing the community structure in two large field enclosures (4.2 m2) with added predators to two enclosures without added predators. The added predators reflected the density and composition of top predators in the pond and comprised weekly additions of dytiscid larvae (for three weeks) followed by weekly additions of odonate nymphs (for five weeks). Compared with the enclosure controls, the predator addition enclosures had fewer dipterans and crustaceans, higher concentrations of benthic ciliates and other protozoans, higher chlorophyll a and bacterial counts, and lower abundance of rotifers. Many treatment effects were temporally variable and this appeared to be linked to predator identity, predator size, and prey availability. Compared with the surrounding pondwater, the enclosed areas had lower abundance of molluscs, ostracods and cladocerans but higher abundance of cyclopoids and higher concentrations of phytoplankton and ciliates. Despite high productivity and seasonally variable predator and prey assemblages, which likely buffered against strong top-down control, we conclude that the top-predators regulate the dipterans and zooplankton in this intermittent pond and that the effects propagated down through the food web to lower trophic levels.

We studied assemblages of littoral chironomids from three alpine lakes in the High Tatra Mountains (Slovakia) along an altitude range of 1725–2157 m. The study sites were selected in order to emphasize differences in their thermal regimes. Littoral samples were taken quantitatively in the ice-free seasons of 2000 and 2001. In total, 3468 chironomid larvae and pupae were collected and 32 taxa were identified. Some parameters of the littoral chironomid assemblages showed clear patterns along the altitude gradient. The number of taxa and Shannon diversity increased markedly with decreasing altitude. Density, however, did not show any correlation with altitude. Even though gatherers highly dominated in all sites, their relative abundance decreased with decreasing altitude. The ratio of scrapers and predators correlated negatively with altitude. According to the results of a Detrended Correspondence Analysis, Pseudodiamesa arctica and Micropsectra radialis were chosen as indicator species for alpine lakes >2000 m a.s.l. Typical subalpine lake inhabitants are Zavrelimyia sp., Prodiamesa olivacea, Corynoneura scutellata group, Cricotopus polaris, Micropsectra bavarica and Paratanytarsus austriacus. A single species, Micropsectra junci, seemed to be characteristic of alpine lakes <2000 m a.s.l. This “gradient lake concept” could be used to predict changes in mountain lake fauna under expected climate change scenarios. Generally, an upward shift of subalpine species would be expected. The thermal regime of lakes, however, also depends on several local factors. Consequently, the schemes outlined here would not be valid for the entire Tatra lake district.

The impact of repeated hydropeaking events was assessed in the Alpine stream Noce Bianco (Trentino, NE Italy). Three stations were selected, one upstream and two at 0.25 km and 6 km downstream from a hydropower plant which causes 7-fold discharge increases. We collected hyporheic invertebrates for two years. Taxa diversity and abundance were reduced in the impacted sites, especially affecting the stygobites, which were significantly less abundant at the impacted sites, whereas stygoxene invertebrates increased exponentially. Repeated hydropeaking events alter the physical-chemical characteristics of the hyporheic habitat, resulting in the recorded faunistic pattern. The deposition of the fine sediment transported by the turbinated water downstream of the power plant and the absence of natural peak floods which remove fine sediments, probably cause a reduction of the interstitial space interstitial habitat available to stygobitic taxa. Surface water natural thermal regime is altered by the hypolimnetic discharges, and such alterations propagate into the hyporheic, affecting the stenothermic taxa. Some benthic taxa used the hyporheic habitat as a refuge to avoid catastrophic drift during hydropower production operation. The main trophic roles represented in the hyporheic assemblages do not support an important role for biological interactions such as predation in determining the fine-scale patchiness of the hyporheos.

Performance of the prescribed gear of the European Standard (i.e. 1.5 m deep Nordic gillnet) was compared with gillnets deep enough to cover the whole water column (WWCG) for characterizing fish assemblages in a large and shallow lake (Lake Balaton, Hungary). Differences in species number, relative abundance and biomass, and length distribution of fish were examined in both inshore and offshore habitats of the lake. Nordic gillnet samples provided comparable information to that of WWCG for both species richness and abundance and biomass of benthic species. However, substantial differences were found between the two net types in the representation of pelagic fish species (i.e. bleak, razor fish and asp). Overall, the WWCG provided different information with more precise (less variable) data on fish assemblage structure than the Nordic gillnet. Results suggest that identification of neither horizontal nor vertical gradients in within lake fish assemblage structure can be guaranteed with the Nordic gillnet alone even in such shallow lakes, like Lake Balaton. It was concluded that benthic Nordic gillnet samples should be complemented with information on fish occurring in the upper water layers (i.e. pelagic species) even in very shallow lakes either by using pelagic or floating gillnets or by whole water column gillnetting.

We studied the spatiotemporal trends of Eragrostis curvula (Schrad.) Nees in a flood meadow of a regulated river. The response variables, including the abundance of colonies, colony sizes, flowering spikes, biomass and root structure were checked against a set of environmental variables. These variables included distance from the river, substrate conditions (stony to sandy), soil nutrients (total nitrogen, phosphorous and calcium) and soil moisture. Relationships were evidenced using redundancy analysis. Response variables were found to be distinctive and based on substrate conditions (i.e. stony or sandy). The variables ‘distance from the river’ and ‘soil phosphorous’ contributed significantly to the overall variance. Stony habitats were observed to have lower soil nutrient levels and were characterised by monospecific stands of E. curvula, while the high nutrient levels of sandy habitats promoted heterogeneous herbaceous vegetation growth. Primary production, especially belowground, was measured to be significantly higher (t-test, P<0.05) in stony habitats. Spatiotemporal trends suggested that, irrespective of the number of colonies, E. curvula demonstrated significant levels of adaptation to micro-environments through biomass allocation and the morphological plasticity of its roots. The study results suggest that river regulation and the subsequent reduction in the frequency of inundation, in addition to the reduction of nutrients and fine sediment supply downstream, give a competitive edge to E. curvula over other herbaceous species.

The aim of the study was to design and put together a compact, easy-to-assemble and cost-efficient incubation system for aquatic plant photosynthesis measurements. Incubation tank consisting of glass sidewalls and mirror inner walls was constructed. The tank was split into six incubation cells and two water collecting cells. Each incubation cell was built of mirror with reflective side turned into the incubation cells to prevent the self shading of plants and minimized the variance of light intensity within the cell. The wall of each incubation cell facing the source of light was made of 3 mm glass and was covered by light absorbing film. To produce different light intensities single light source (fluorescent tubes) and light absorbing film (3M Scotchtint™ Sun Control RE50NEARL) were chosen. The sidewall of each incubation cell was covered with metal-coated neutral sun control film to produce six distinct light intensities. The different transparency of the sidewalls was achieved by lamination of the chosen film in increasing numbers of layers. The effect of the lamination on optical properties of the film was also studied. The variation of photon flux density within a cell was 3.9%. Continuous use of the system and occasional repetitive measurements of film's transparency showed that it maintained its neutral optical properties over a long period of time.

Colonial Microcystis aeruginosa were obtained when the unicellular algae were exposed to flagellate Ochromonas sp. filtrate. To investigate the benefit of this morphological change, flagellates were added into cultures of unicellular and colonial M. aeruginosa, respectively. The clearance rates of flagellates on algae were markedly decreased when they were cultivated with induced colonial M. aeruginosa. This result indicated that colony formation in M. aeruginosa was a predator-induced defense, which could reduce predation risk from flagellate. The increased content of soluble extracellular polysaccharide (sEPS) and bound extracellular polysaccharide (bEPS) may play an important role in adhering M. aeruginosa cells together to form colonies. The decrease of ΦPS II and the increase of sinking rates of induced colonial M. aeruginosa showed that the costs of grazed-induced colony formation in M. aeruginosa may reflect in the photosystem II efficiency, and in the sinking rates.