bluuebunny/crossref_metadata_2025_split huggingface dataset:
Top‐down effects on pelagic heterotrophic nanoflagellates (HNF) in a large river (River Rhine): do losses to the benthos play a role?
Authors: WEITERE MARKUS , ARNDT HARTMUT 
Published: March 2003

1. The abundance, production and control of pelagic heterotrophic nanoflagellates (HNF) in the Lower River Rhine (Germany) were investigated. Field samples (live‐counting technique) were taken at least every 2 weeks at Cologne (km 685) over a period of 20 months. In addition, Lagrangian sampling was carried out 160 km downstream of Cologne (Kalkar–Grieth, km 845) over a period of 12 months. Potential HNF growth rates and loss rates caused by planktonic predators were estimated in the laboratory (size fractionation experiments) and compared with the changes in HNF‐density in a water parcel flowing downstream.2. Mean abundance (±SD) ranged from 7 ± 6 to 4890 ± 560 individuals mL–1 and was positively correlated with discharge. Heterotrophic nanoflagellates abundance increased up to 30‐fold during flood events, whereas there were only minor changes in the taxonomic composition.3. HNF growth rate ranged from 0.16 ± 0.12 to 1.98 ± 0.10 day–1. Between 0 and 95% (mean: 32%) of the gross production was lost to planktonic predators; a larger portion between 0 and 195% (mean: 64%) of the HNF gross production was lost by other means.4. There may be an important role for benthic predators in the control of pelagic HNF. First, production of HNF was high and grazing losses to planktonic predators low at times when HNF abundance was low. Secondly, high in situ loss rates (not explicable by planktonic predators), which were positively related to temperature, indicate the importance of biotic interactions. Thirdly, the dependence of HNF abundance on discharge indicated a decrease grazing intensity with rising water levels (increase in water volume/colonised river bed ratio).5. The impact of discharge on planktonic HNF mediated by the grazing impact of benthic predators was modelled, showing a good fit with the field data.
Characteristics of oligotrophic hardwater lakes in a postglacial land‐rise area in mid‐Sweden
Authors: BRUNBERG A.‐K. , NILSSON E. , BLOMQVIST P. 
Published: March 2003

1. We describe some remarkable ephemeral, oligotrophic hardwater lakes formed because of land rise in the coastal areas of the Baltic Sea, that are unique in Sweden and probably also worldwide. Two younger, coastal lakes were studied by regular sampling for 1 year and compared with an older (i.e. greater altitude) lake, that passed through the oligotrophic hardwater stage some 3–4000 years ago.2. Despite some differences in composition of the catchment, the two younger lakes were similar with regard to water chemistry and plankton community composition. The concentration of phosphorus was low while nitrogen was high, resulting in very high N/P quotients (101 and 131). Although water colour was moderate, the concentration of organic carbon was extremely high (average values of ≥ 20 μg TOC L–1), consisting mainly of dissolved compounds (DOC).3. While the plankton was poorly developed, sediments in both lakes were covered by a layer of photosynthesising micro‐organisms. This substantial `microbial mat', which has not been described in detail before, was up to 15 cm thick and dominated by cyanobacteria and purple sulphur bacteria. The concentration of sediment phosphorus was extremely low (352 μg g–1 dw) in one of the lakes and dominated by organic‐bound (residual) phosphorus.4. Deep sediments in the older lake, representing its oligotrophic hardwater period, differed in phosphorus composition from the currently oligotrophic hardwater lakes by having a strong dominance of HCl‐extractable (Ca‐bound) phosphorus. This indicates that phosphorus, initially organic‐bound within the microbial mat, is subsequently bound to calcium. We hypothesise that this is promoted by the environmental conditions created by the benthic photosynthetic activity, in combination with the prevailing hardwater conditions.5. The rich and flourishing microbial community on the sediments may also explain the high concentration of DOC in the lake.
The relative importance of lethal and non‐lethal effects of fish on insect colonisation of ponds
Authors: ÅBJÖRNSSON KAJSA , BRÖNMARK CHRISTER , HANSSON LARS‐ANDERS 
Published: March 2003

1. We hypothesised that adult insects actively monitor potential habitats for the presence of fish by means of chemical cues and avoid sites that pose significant risks. This was examined by quantifying colonisation of insects in outdoor pools with no fish (controls), fish (direct predation effect) or caged fish (chemical predator cues).2. A significant direct effect of predation was found, but no indirect effect (avoidance of chemical cue pools), on the total biomass of colonising insects. However, predatory insects avoided fish‐cue pools, thus releasing non‐predatory insects from predation. This resulted in significantly greater biomass of non‐predatory insects in fish‐cue pools than control pools.3. Fish reduced the number of species of colonising insects in pools through predation. This negative influence of fish implies that caution is necessary when stocking wetlands and ponds with fish if the goal is to maximise biodiversity.4. Our data suggest that although predatory aquatic insects may use chemical signals to assess the quality of potential habitats with respect to predation risk, direct predation is the main method by which fish affect insect assemblages in ponds. Because fish and invertebrate predators may both have strong effects on prey mortality, behavioural adjustment by insects to the actual predator regime within a habitat should be more important than avoiding colonisation of habitats with fish.
How can dragonflies discern bright and dark waters from a distance? The degree of polarisation of reflected light as a possible cue for dragonfly habitat selection
Authors: Bernáth Balázs , Szedenics Gábor , Wildermuth Hansruedi , Horváth Gábor 
Published: March 2003

SUMMARY 1. Based on the findings that some dragonflies prefer either ‘dark’ or ‘bright’ water (as perceived by the human eye viewing downwards perpendicularly to the water surface), while others choose both types of water bodies in which to lay their eggs, the question arises: How can dragonflies distinguish a bright from a dark pond from far away, before they get sufficiently close to see it is bright or dark?2. Our hypothesis is that certain dragonfly species may select their preferred breeding sites from a distance on the basis of the polarisation of reflected light. Is it that waters viewed from a distance can be classified on the basis of the polarisation of reflected light?3. Therefore we measured, at an angle of view of 20° from the horizontal, the reflection‐polarisation characteristics of several ponds differing in brightness and in their dragonfly fauna.4. We show that from a distance, at which the angle of view is 20° from the horizontal, dark water bodies cannot be distinguished from bright ones on the basis of the intensity or the angle of polarisation of reflected light. At a similar angle of view, however, dark waters reflect light with a significantly higher degree of linear polarisation than bright waters in any range of the spectrum and in any direction of view with respect to the sun.5. Thus, the degree of polarisation of reflected light may be a visual cue for the polarisation‐sensitive dragonflies to distinguish dark and bright water bodies from far away. Future experimental studies should prove if dragonflies do indeed use this cue for habitat selection.
The effects of bankside management on chalk stream invertebrate communities
Authors: Harrison Simon S. C. , Harris Iain T. 
Published: March 2003

SUMMARY 1. Communities of aquatic macroinvertebrates and the terrestrial adult phases of aquatic insects were investigated from short stretches of English chalk streams with two different bankside vegetation types: simply structured grazed grass (grazed) and structurally complex herbaceous vegetation with scattered trees (ungrazed). Macroinvertebrates were sampled in spring, summer, autumn and winter 1996–97 from three aquatic habitats: mid‐channel gravel, patches of the aquatic macrophyte Ranunculus and marginal emergent macrophytes. The terrestrial adult phases of aquatic insects were sampled in spring, summer and autumn from bankside vegetation.2. Total macroinvertebrate abundance did not differ between stretches with different bankside vegetation. Taxon richness of mid‐channel gravel was, however, significantly higher in ungrazed compared with grazed stretches and Shannon diversity (H′) of mid‐channel gravel and marginal vegetation was significantly higher in ungrazed compared with grazed stretches. Total abundance, taxon richness and Shannon diversity (H′) of the terrestrial adult phases of aquatic insect were significantly higher from the bankside vegetation of ungrazed compared with grazed stretches.3. Ordination of communities of aquatic macroinvertebrates and terrestrial adults demonstrated that individual families of both groups were generally more abundant in ungrazed stretches. Many more families were significantly associated with ungrazed stretches than with grazed stretches.4. This investigation has shown that high structural diversity of bankside vegetation along lowland chalk streams is accompanied at the reach scale by increased diversity of both aquatic macroinvertebrates and the terrestrial adult phases of aquatic insects. The conservation potential of such streams may thus be lowered by management practices that result in the removal or simplification of bankside vegetation along extensive stream stretches.
The influence of water transparency on the distribution and abundance of macrophytes among lakes of the Mackenzie Delta, Western Canadian Arctic
Authors: Squires M. M. , Lesack L. F. W. , Huebert D. 
Published: March 2003

SUMMARY 1. Macrophyte abundance and distribution was assessed in a chain of six interconnected lakes (all with the same flooding frequency) in the Arctic, where increasing distance from the Mackenzie River channel resulted in a gradient of water transparency (‘chain‐set’ lakes), and in a group of 26 spatially discrete lakes where increasing frequency and duration of lake flooding with river water (controlled by sill height) also resulted in a transparency gradient (‘sill‐set’ lakes).2. Among the chain‐set lakes, above‐ground macrophyte biomass increased from 0 to 1000 g m−2 with increasing water transparency. Among the sill‐set lakes, the transparency gradient among the lakes was less well defined and the relations with biomass were more varied. A decrease in flooding was associated with increasing water transparency and an increasing biomass of macrophytes from about 0 to over 2000 g m−2. For a specific flood frequency, however, the effect of flooding was much greater when lakes were directly connected to a river channel than when floodwaters flowed first through an intervening lake. Among infrequently flooded lakes the effect of flooding on water transparency and biomass was negligible.3. Among relatively clear lakes in both sets of lakes, biomass increased with increasing water transparency and decreasing lake depth. Among relatively turbid lakes, however, biomass increased with the combined effect of increasing water colour (decreasing water transparency) and increasing lake water depth. The increases in biomass with increasing water colour (coloured dissolved organic matter) and increasing depth, which together result in reduced light at the bed, may be explained by reduced exposure to ultra violet light.4. An average light attenuation of 1.3 m−1 (Secchi depth about 1 m) over the growing season appears to represent a threshold water transparency which, in combination with water depths early in the growing season, is consistent with a light supply on the bed required for growth of the common macrophytes in lakes of the Mackenzie Delta. However, a comparison with other systems indicates that macrophytes among lakes of the Mackenzie Delta grow deeper, for a given level of transparency, than is reported in lakes at lower latitude, despite the lower sun angles and increased reflectivity of water surfaces in the arctic.5. A complete accounting of water transparency (at PAR and UV wavelengths), lake depth, summer sun angle and duration of sunlight may be necessary to explain patterns of macrophyte growth among lakes across a full range of latitudes.
Nutrient limitation of phytoplankton and periphyton growth in upland lakes
Authors: Maberly S. C. , King L. , Dent M. M. , Jones R. I. , Gibson C. E. 
Published: March 2003

SUMMARY 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were visited three times between April and August 2000. On each occasion water chemistry was measured and phytoplankton bioassays were performed in the laboratory to assess growth‐rate and yield limitation by phosphorus and nitrogen. In addition, yield limitation of periphyton growth was investigated twice, in situ, using nutrient‐diffusing substrata.2. Over the whole season the percentage frequency of P, N and co‐limitation was 24, 13 and 63%, respectively, for phytoplankton rate limitation and 20, 22 and 58%, respectively, for phytoplankton yield limitation.3. A clear response of periphyton yield to nutrient additions was found in 75% of all cases and of these, co‐limitation was most common (54%). Average percentage frequency for P and N limitation was 26 and 20%, respectively.4. Phytoplankton and periphyton showed seasonal changes in nutrient limitation within sites. In particular, co‐limitation became progressively more common as the season progressed.5. The response of phytoplankton growth rate to ammonium and nitrate addition was identical, but ammonium was a slightly better source of nitrogen than nitrate for phytoplankton yield on 7% and for periphyton yield on 10% of the occasions. However, the magnitude of the effect was small.6. The concentration of dissolved inorganic nitrogen (DIN) and the molar ratio of DIN to total dissolved phosphorus (TDP), appeared to be the main environmental factors controlling the extent of nitrogen or phosphorus limitation at a given site. Nitrogen limitation was more likely than phosphorus limitation where the DIN was <6.5 mmol m−3 and the ratio of DIN : TDP was <53. Co‐limitation was the most likely outcome at a DIN concentration <13 mmol m−3 and at a DIN : TDP molar ratio <250. Above these values phosphorus limitation was most likely.7. The relatively high frequency of nitrogen limitation and co‐limitation at higher N : P ratios than previously reported, may result from the inability of nitrogen‐fixing cyanobacteria to thrive in these upland lakes where pH and the concentration of phosphorus tended to be low and where flushing rates tended to be high.
Breakdown of wood in the Agüera stream
Authors: Díez Joserra , Elosegi Arturo , Chauvet Eric , Pozo Jesús 
Published: March 2003

SUMMARY 1. Breakdown of wood was compared at three sites of the Agüera catchment (Iberian Peninsula): two oligotrophic first‐order reaches (one under deciduous forest, the other under Eucalyptus globulus plantations) and one third‐order reach under mixed forest, where concentration of dissolved nutrients was higher.2. Branches (diameter = 3 cm, length = 10 cm) of oak (Quercus robur), alder (Alnus glutinosa), pine (Pinus radiata) and eucalyptus, plus prisms (2.5 × 2.5 × 10 cm) of alder heartwood were enclosed in mesh bags (1 cm mesh size) and placed in the streams. Mass loss was determined over 4.5 years, whereas nutrient, lignin and ergosterol were determined over 3 years. In order to describe fungal dynamics, ergosterol was also determined separately on the outer and inner parts of some branches.3. Breakdown rates ranged from 0.0159 to 0.2706 year−1 with the third‐order reach having the highest values whatever the species considered. The most rapid breakdown occurred in alder heartwood and the slowest in pine branches; breakdown rates of oak, eucalyptus and alder branches did not differ significantly.4. The highest nitrogen and phosphorus contents were found in alder, followed by oak, while pine and eucalyptus had low values. During breakdown, all materials rapidly lost phosphorus, but nitrogen content remained constant or slightly increased. Lignin content remained similar.5. Peaks of ergosterol ranged from 0.023 to 0.139 mg g−1 and were higher in alder than in other species in two of the three sites. The third‐order reach generally had the greatest increase in ergosterol, especially in alder branches, eucalyptus and alder heartwood. The overall species/site pattern of fungal biomass was thus consistent with the observed differences in breakdown.6. When compared with leaves of the same species decomposing at these sites, wood breakdown appeared to be less sensitive to the tree species but more sensitive to stream water chemistry. Although wood breakdown is slower and its inputs are lower than those of leaf litter, its higher resistance to downstream transport results in a relatively high standing stock and a significant contribution to the energy flux.
Drag and reconfiguration of freshwater macrophytes
Authors: Sand‐Jensen Kaj 
Published: March 2003

SUMMARY 1. Submerged freshwater macrophytes face large hydrodynamic forces in flowing waters in streams and on wave‐swept lake shores and require morphological adaptations to reduce the drag and the physical damage. This experiment studied five species of freshwater macrophytes and strap‐formed plastic leaves to test the predictions that: (i) increasing flexibility leads to greater reconfiguration and lower drag coefficients, (ii) flexible plants experience a steeper decline of drag coefficients with increasing water velocity than unflexible plants and (iii) plants mounted vertically on a horizontal substratum bend over in fast flow attaining a shielded position of low drag.2. The results confirmed all three predictions. In fast flow, plants mounted upright on a horizontal platform gradually approached a position aligned with the flow, depending on their flexibility. In the range 8–50 cm s−1 the deflection followed an interspecific negative linear relationship between log (tangent Φ) and velocity, where Φ represents the shoot angle normal to the horizontal level. Above 50 cm s−1, further deflection was reduced perhaps by a combination of the elasticity and packing of shoots and the increasing lift generated by fast flow.3. Drag coefficients of plants ranged between 0.01 and 0.1, typical of moderately to very streamlined objects. Drag coefficients declined log‐log linearly at increasing velocity, following negative slopes between −0.67 and −1.24 (median: −1.0) because of reconfiguration and formation of a shielding canopy. Drag coefficients declined much less (median: −0.55) for plants floating freely in the streaming water and which were capable of changing their shape but unable to form a shielding canopy. Drag coefficients declined even less for relatively unflexible plastic leaves (−0.30 to −0.40), and they remained constant for stiff, bluff objects.4. The experiments suggest that flow resistance of flexible, submerged macrophytes in natural streams may increase in direct proportion to water velocity because they form a shielding submerged canopy, and high water stages at peak flow may result in greater proportions of the water passing unimpeded above the canopy. In contrast, stiff amphibious and emergent reed plants should experience an increase of flow resistance with at least the square of velocity as reconfiguration is small and former aerial plant surfaces come into contact with the streaming water at higher water stages. Field experiments to test these predictions are urgently needed.
Longevity of the freshwater anostracan Streptocephalus mackini (Crustacean: Anostraca) in relation to food (Chlorella vulgaris) concentration
Authors: Anaya‐Soto Alejandro , Sarma S. S. S. , Nandini S. 
Published: March 2003

SUMMARY 1. Temporary ponds are inhabited by a variety of invertebrates, of which anostracans are an important group. We studied the lifetables of male and female anostracan Streptocephalus mackini at 3 algal concentrations (0.5 × 106, 1.0 × 106 and 1.5 × 106 cells mL−1).2. Regardless of sex, S. mackini showed better survivorship at lower food levels. The longest average lifespan observed was 85 ± 2 days for males fed Chlorella at 0.5 × 106 cells mL−1.3. Both net reproductive rate and generation time decreased with increasing food level. The highest net reproductive rate was about 120 cysts per female. The longest generation time of about 40 days, observed at 0.5 × 106 cells mL−1, was more than three times that at 1.5 × 106 cells mL−1.4. The rate of population increase (r) was nearly the same (0.31 ± 0.06) at high (1.5 × 106 cells mL−1) and intermediate (1.0 × 106 cells mL−1) food levels. The r‐value at low food level (0.5 × 106 cells mL−1 of Chlorella) was 0.20 ± 0.01 per day.
Estimation of the in‐situ settling velocity of particles in lakes using a time series sediment trap
Authors: Douglas Richard W. , Rippey Brian , Gibson Christopher E. 
Published: March 2003

SUMMARY 1. We outline a simple method for estimating the in‐situ settling velocity of particles in wind disturbed lakes. The total sedimentation rate (primary and secondary sedimentation) was measured during storm events in two lakes using a time series sediment trap.2. The sediment trap had 12 sample bottles which were programmed to open for periods of 3–4 h, giving total deployment times ranging from 36 to 48 h.3. Wave mixed layer theory was used to infer if and when sediment resuspension occurred and the first order rate equation was used to estimate the settling velocity of resuspended particles.4. The settling velocity during three resuspension events in Lough Neagh and one in Lough Macnean were 34.3, 29.5, 24.1 and 77.3 m day−1, respectively. These are similar to values obtained using recent in‐situ video imaging in marine environments, but much larger than previous lake sediment trap studies.
Response of bee‐flies to the shape and pattern of model flowers: implications for floral evolution in a Mediterranean herb
Authors: Johnson S. D. , Dafni A. 
Published: March 2003

1. The functional significance of floral traits in Linum pubescens (Linaceae), a Mediterranean annual that is pollinated almost exclusively by the bee‐fly Usia bicolor (Bombyliidae), was investigated. The flies feed on both pollen and nectar, and use the flowers as mating rendezvous sites in the afternoon. 2. Choice experiments with model flowers were used to determine the response of U. bicolor to visual cues, such as shape, size, colour and pattern. 3. Bee‐flies strongly preferred models with a dissected outline over models with a simple outline. They also preferred pink models over other colours, and larger models over smaller models. 5. Flies landing on models with converging lines (‘nectar guides’) tended to follow the lines to the point where they meet in the centre of the model, while flies landing on plain models showed undirected behaviour, often moving to the edge of the model. 6. Flies were strongly attracted to flowers of L. pubescens which had a fly glued on to one of the petals, as well as flowers with a dark spot painted onto one of the petals. In addition, models with a dark spot were strongly preferred over plain models during the afternoon when flies exhibit mating behaviour. This evidence suggests that the dark centre of the L. pubescens flower may function as an attractant to mate‐seeking bee‐flies.