bluuebunny/crossref_metadata_2025 huggingface dataset:
Epibionts of	Asellus aquaticus(L.) (Crustacea, Isopoda): an SEM study
Authors: Cook Jane A. , Chubb James C. , Veltkamp
Published: March 2003

1. This study presents a qualitative and quantitative survey of epibionts infesting two populations of the freshwater isopod Asellus aquaticus (L.). Using scanning electron microscopy, the prevalence, mean intensity, abundance, spatial preferences and distribution of organisms on various external surfaces were ascertained.

2. A diverse community, consisting primarily of protozoa and rotifers, utilised A. aquaticus as a substrate organism. Every individual A. aquaticus supported some form of epizoic life. Peritrich species comprised 89.2% of all epibionts. Carchesium polypinum, Pseudocarchesium aselli, Pseudocarchesium asellicola, Pseudocarchesium simulans and Opercularia hebes dominated the epifauna.

3. Highest epibiont burdens occurred on the mouthparts, the first few ventral segments and the gills and largely comprised the above peritrich species. Most epibionts exhibited some degree of site preference on the host. A distinct gill epifauna existed. Apart from Acineta tuberosa and Vorticella rotunda, which had equal prevalences on dorsal and ventral surfaces in one culture, most species largely avoided the dorsal surface.

4. Epibionts exhibited varying degrees of adaptation to life on a living substrate. They ranged from facultative species such as A. tuberosa, which are capable of colonising abiotic substrata, to Gymnodinoides aselli whose life cycle relies on interaction with A. aquaticus. Highly specialised species tended to exhibit the most pronounced and consistent site restriction.

5. Asellus aquaticus offers a highly tractable system for further study of substrate species–epibiont relationships.
A new functional model for estimating the maximum amount of invertebrate food consumed per day by brown trout, Salmo trutta
Authors: Elliott J. M. , Hurley M. A.
Published: March 2003

1. A model developed over 20 years ago has been used to estimate daily food intake in brown trout living in streams and lakes over a wide geographical range. The chief disadvantages of this early model are that it is not continuous and requires twelve parameters, not all of which can be interpreted biologically. A new model, using a larger data set, was therefore developed to overcome these problems and estimate the mean daily energy intake.

2. The two data sets used to develop the original model were also used to develop the general form of the new one, but a third data set was used to specify the model more precisely and to estimate the parameters. This third data set originated from experiments in which 185 trout (live weight range 1–350 g) were kept individually at 19 constant temperatures (range 3.8–21.7 °C) usually for 5–6 weeks. They were fed freshly killed shrimps (Gammarus pulex) and their food consumption was recorded throughout each experiment.

3. Five, six and eight parameter versions of the new model were all excellent fits to the data (P < 0.001, R2 > 0.99), with the eight parameter version being slightly the best. All parameters can be interpreted in biological terms; three define threshold temperatures, three define the curvilinear slopes in the model over different temperature ranges, one is a weight exponent and one is the maximum daily energy intake of a 1 g trout. The simpler six parameter model was adequate at temperatures above 7 °C.

4. An additional experiment with twenty‐eight trout feeding on six different invertebrate foods provided estimates of energy intake that were very similar to those predicted from the model. However, when daily intake was converted to dry weight, agreement with values from the model (also as dry weight) was poor. Possible reasons for this are discussed, as are other studies using the earlier model, and it is shown that different conclusions can be reached depending upon whether comparisons are based on units of energy, dry weight or wet weight.
Sediment microbial respiration in a synoptic survey of mid‐Atlantic region streams
Authors: Hill  , Herlihy  , KauFmann  , SinsabauGh
Published: March 2003

1. The rate of microbial respiration on fine‐grained stream sediments was measured at 196 first‐ to third‐order sites in the mid‐Atlantic region of the United States.

2. Sample collection took place between April and July in 1993, 1994 and 1995.

3. Study streams were randomly selected from the United States Environmental Protection Agency’s (USEPA) River Reach File (RF3) using the sample design developed by USEPAs Environmental Monitoring and Assessment Programme (EMAP).

4. Respiration rate ranged from 0 to 0.622 g O2 g–1 ash‐free dry mass (AFDM) h–1, and was significantly correlated with stream temperature, water chemistry and physical habitat.

5. Respiration rate was significantly higher in streams from the Chesapeake drainage area compared to those from the Ohio drainage area, and in streams from the Northern Appalachian Plateau and Uplands and Central Appalachian Valleys ecoregions compared with streams from the Blue Ridge and Central Appalachian Plateau ecoregions.

6. Respiration was not significantly different among stream orders or between years.
δ13C variability of benthic algae: effects of water colour via modulation by stream current
Authors: France Robert , Cattaneo And Antonella
Published: March 2003

1. Increased water motion is expected to reduce boundary layer diffusion resistance of autotrophs, thereby enabling greater isotopic discrimination against 13C such that lower δ13C values (ratio of 13C : 12C) should ensue. A field test of this hypothesis was undertaken by sampling benthic algae in streams of differing current speed.

2. Contrary to the expected negative relationship between δ13C and water motion, filamentous benthic algae were found to exhibit higher δ13C values in rapid water.

3. Under conditions of low current in the streams studied, concentrations of dissolved organic carbon as measured by water colour are elevated through the microbial decomposition of largely terrestrial organic matter. Photoassimilation of this respired carbon by benthic filamentous algae generates 13C‐depletion and lower δ13C values, and appears to be substantial enough in the streams used in the present study to override the competing influence of water motion on boundary layer thickness.
Fighting the flow: downstream–upstream linkages in the ecology of diadromous fish faunas in West Coast New Zealand rivers
Authors: Mcdowall
Published: March 2003

1. Diadromy is a dominating behavioural characteristic of fish faunas in New Zealand rivers, with amphidromy and catadromy being the most common strategies.

2. Juvenile life stages of amphidromous and catadromous species migrate from the sea, through river systems, to find habitats for feeding, growth, maturation and reproduction.

3. Studies of fish distributions in rivers of the West Coast of the South Island of New Zealand show that, in most species, these migrations result in more or less continuous distributions from the lower reaches to the upstream limits of each species’ range.

4. Upstream penetration of rivers varies widely between species and this generates downstream–upstream trajectories of declining species richness.

5. Parallel trajectories of declining downstream–upstream abundance are likely in each species.

6. These patterns demonstrate the presence of downstream–upstream linkages in the community ecology of freshwater fishes in New Zealand rivers.
Odour‐induced antipredator behaviour of the water flea, Ceriodaphnia reticulata, in varying predator and prey densities
Authors: Seely  , Lutnesky
Published: March 2003

1. A new marking technique was developed and used to track individual water flea behaviour. A single clone of Ceriodaphnia reticulata exhibited no significant differences in mean depth, mean activity (antennae beats used in swimming), and mean travel (total vertical displacement) for fast‐green dyed and control animals (n = 30, each treatment).

2. Individual and group antipredator responses were recorded in three vertebrate predator densities (three odour concentrations of green sunfish Lepomis cyanellus; 0.00, 0.03 and 0.10 fish L–1) and four water flea group sizes (1, 5, 15 and 25), with n = 18 for each of the twelve treatment combinations. Water flea groups were probably mixtures of clones. Differences in behaviour after addition of predator odour showed that increasing predator density increased mean depth of marked individuals and mean proportion of water fleas descending. Additionally, increasing prey density decreased both mean depth of marked individuals and mean proportion of water fleas descending.

3. Predator density and prey density had opposing affects on activity and travel for marked individuals. Higher predator density decreased mean activity (antennae beats), yet increased mean travel. Higher prey density increased mean activity, but decreased mean travel. Descending animals altered their swimming pattern by using fewer swimming beats to travel greater vertical distances in the presence of predator odour.
Ingestion and digestion of epilithic algae by larval insects in a heavily grazed montane stream
Authors: Peterson Christopher G. , Vormittag Kara A. , Valett H. Maurice
Published: March 2003

1. Epilithic algae grown on elevated or non‐elevated ceramic tiles were exposed (to produce assemblages with different grazing histories) in a heavily grazed, montane stream in New Mexico, U.S.A. to Ameletus nymphs (Ephemeroptera) and Ecclisomyia larvae (Trichoptera) and the algal composition in insect faeces was compared to that on the tiles. Differences in grazing and digestion efficiency between grazers were then assessed and also differences in susceptibility to ingestion and digestibility among common algae.

2. Ordination of tile and faecal samples, using the relative abundance of common algae, revealed that: (i) algal assemblages on elevated vs. non‐elevated tiles differed only slightly; (ii) the taxonomic composition of algae in faeces of both caddis and mayflies differed substantially from that on the tiles, indicating low grazing efficiency for some algal taxa; and (iii) the algal composition of faeces produced by caddis larvae and mayflies was similar, indicating little difference in grazing efficiency between them. However, some algal taxa were more susceptible to ingestion by caddisfly larvae when occurring on elevated tiles than on non‐elevated tiles, suggesting that previous exposure to caddis grazing influenced assemblage attributes.

3. Although Ameletus and Ecclisomyia differed little in grazing efficiency, the percentage of diatoms that were dead after passage through the gut was greatest in the mayfly treatment, suggesting that mayflies digested diatoms more efficiently than the caddis. Analyses of differences in the condition of chloroplasts within diatoms in tile and faecal samples showed that losses of ’live‘ diatom cells (i.e. those containing full chloroplasts) during gut passage through mayflies equalled the increase, in faeces, of ’dead‘ (empty frustules) cells of all common diatoms. In contrast, some diatoms were digested inefficiently by caddis larvae.

4. Algae on elevated tiles contained a higher proportion of dead diatoms than those on non‐elevated tiles, possibly because mayflies visited raised tiles more often and, consequently, ingested and defaecated cells at a higher rate in the absence of caddis larvae. Moreover, diatom taxa differed in the percentage of cells that were dead within tile assemblages, with populations of typically grazer‐resistant taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum and Cocconeis placentula var. euglypta) containing significantly more dead cells than grazer‐susceptible taxa [e.g. small, chain‐forming Fragilaria (= Staurosirella)]. This result suggests that a trade‐off exists between ingestion vs. digestion resistance of microalgae. Both the ingestion and digestion efficiency of algivorous macroinvertebrates could influence the structure and function of algal assemblages. In heavily grazed systems, where algal cells are probably processed through grazer guts repeatedly, differential resistance to digestion among algae may be particularly important.
Development, maintenance and role of riparian vegetation in the river landscape
Authors: Tabacchi Eric , Correll David L. , Hauer Richard , Pinay Gilles , Planty‐Tabacchi Anne‐Marie , Wissmar Robert C.
Published: March 2003

1. Riparian structure and function were considered from a longitudinal perspective in order to identify multiscale couplings with adjacent ecosystems and to identify research needs.

2. We characterized functional zones (with respect to vegetation development in association with various biogeochemical processes) within geomorphological settings using a delineation based upon erosional, transitional and depositional properties.

3. Vegetation dynamics within the riparian corridor are clearly influenced substantially by hydrological disturbance regimes. In turn, we suggest that vegetation productivity and diversity may widely influence riverine biogeochemical processes, especially as related to the consequences of changing redox conditions occurring from upstream to downstream.

4. However, surface and groundwater linkages are the predominant controls of landscape connectivity within riparian systems.

5. The importance of riparian zones as sources and sinks of matter and energy was examined in context of structural and functional attributes, such as sequestering or cycling of nutrients in sediments, retention of water in vegetation, and retention, diffusion or dispersal of biota.

6. The consequences of interactions between different communities (e.g. animals and plants, micro‐organisms and plants) on biogeochemical processes are notably in need of research, especially with respect to control of landscape features. Multiscale approaches, coupling regional and local factors in all three spatial dimensions, are needed in order to understand more synthetically and to model biogeochemical and community processes within the river‐riparian‐upland landscape of catchments.
The effects of biofilms on chemical processes in surficial sediments
Authors: Woodruff S. L. , House W. A. , Callow M. E. , Leadbeater B. S. C.
Published: March 2003

1. The objectives of the present work were: (a) to evaluate the effects of the development and the presence of a photosynthetically active algal biofilm on chemical fluxes and processes at the sediment–water interface; (b) to measure the effects of the biofilm on chemical concentration gradients in the bulk sediment; and (c) to monitor pH and dissolved oxygen concentration in the biofilm, and through the sediment–water interface using microelectrodes.

2. Two experiments were performed over a period of 8 weeks using a recirculating fluvarium channel containing river sediments with an exposed surface of 0.2 m2 and 20 dm3 of overlying solution. The first experiment was in darkness with minimal effects of a photosynthetically active biofilm. The second experiment in natural light produced a complex photosynthetic biofilm involving a succession of diatoms, green algae and cyanobacteria.

3. The solution overlying the biofilm was monitored continuously for dissolved calcium, silicon, phosphorus, alkalinity and oxygen, as well as conductivity, temperature and pH. The surface of the sediment was also monitored for biological and physical changes as the biofilm developed. The overlying solution was analysed over a period of 48 h at 2‐h intervals to examine the effects of a well‐developed algal biofilm. At the end of the 48 h, pH and oxygen microelectrodes were used to measure gradients above and through the biofilm, and porewaters were analysed from sediments which had been longitudinally sectioned at a maximum depth resolution of 0.5 mm.

4. Biofilm development had a large influence on the composition of the overlying solution and the development of vertical concentration gradients of solutes in the porewater. Once a diatom community was established, the concentration of dissolved silicon was low (< 40 μm), with all the silicon diffusing from the underlying sediment being consumed in the biofilm (≈ 0.026 μmol m−2 s−1 at the end of the experiment).

5. The concentrations of calcium, alkalinity and phosphorus in digested sediment increased near the sediment–water interface. X‐ray diffraction analysis showed that calcite was formed at the surface. Estimation of the fluxes of alkalinity and calcium in the overlying solution were consistent with calcite formation during daylight and possible dissolution in darkness. The maximum precipitation flux of calcium was 0.87 μmol m−2 s−1 and the maximum net release flux was 0.89 μmol m−2 s−1.

6. The net loss of soluble reactive phosphorus from the overlying solution measured over the intensive sampling period of 48 h is consistent with a coprecipitation mechanism and a surface density of phosphorus included in the lattice of calcite of 0.097 μmol m−2. Processes in the biofilm rather than diffusion from the sediment porewater control chemical fluxes of calcium, alkalinity and phosphorus from the sediment to the overlying water.
Importance of transient storage zones for ammonium and phosphate retention in a sandy‐bottom Mediterranean stream
Authors: Butturini Andrea , Sabater Francesc
Published: March 2003

1. The ability of hyporheic sediments to exchange water and retain ammonium and phosphate in the Riera Major stream ,North‐East Spain, under different discharge conditions was measured by conducting short‐term nutrient and chloride additions.

2. The mean exchange coefficients from free‐flowing water to the storage zone (k1) and vice versa (k2) were 0.82 × 10–4 s−−1 and 7 × 10−−3 s−−1, respectively. The ratio of storage zone cross‐sectional area to stream cross‐sectional area (AS/A) averaged 2.8 × 10–2 and was negatively correlated with discharge (r = –0.85, d.f. = 13, P < 0.001).

3. The percentage of hyporheic zone water which came from surface water varied as a function of discharge and hyporheic depth, ranging between 33% and 95% at 25 cm depth, and between 78% and 100% at 10 cm depth.

4. The nutrient retention efficiency in the hyporheic zone at 10 cm depth measured as uptake length (Swh) was less than 3.3 cm for ammonium and 37 cm for phosphate. Higher nutrient retentions were measured in the sediments at 10 cm depth than at 25 cm, indicating that near‐surface sediments were involved more actively in phosphate retention than the deeper hyporheic sediments. The lack of ammonium at any depth of the hyporheic zone showed that ammonium was very rapidly taken up in the surfacial sediments.
AusRivAS: using macroinvertebrates to assess ecological condition of rivers in Western Australia
Authors: Smith M. J. , Kay W. R. , Edward D. H. D. , Papas P. J. , Richardson K. St J. , Simpson J. C. , Pinder A. M. , Cale D. J. , Horwitz P. H. J. , Davis J. A. , Yung F. H. , Norris R. H. , Halse S. A.
Published: March 2003

1. AusRivAS (Australian River Assessment Scheme) models were developed, using macroinvertebrates as indicators, to assess the ecological condition of rivers in Western Australia as part of an Australia‐wide program. The models were based on data from 188 minimally disturbed reference sites and are similar to RIVPACS models used in Britain. The major habitats in the rivers (macrophyte, channel) were sampled separately and macroinvertebrates collected were identified to family level.

2. Laboratory sorting of preserved macroinvertebrate samples recovered about 90% of families present when 150 animals were collected, whereas live picking in the field recovered only 76%.

3. Reference sites clustered into five groups on the basis of macroinvertebrate families present. Using seven physical variables, a discriminant function allocated 73% of sites to the correct classification group. A discriminant function based on seven physical and two chemical variables allocated 81% of sites to the correct group. However, when the same reference sites were re‐sampled the following year, the nine variable discriminant function misallocated more sites than the seven variable function, owing to annual fluctuations in water chemistry that were not accompanied by changes in fauna.

4. In preliminary testing, the wet season channel model correctly assessed 80% of reference sites as undisturbed in the year subsequent to model building (10% of sites were expected to rate as disturbed because the 10th percentile was used as the threshold for disturbance). Nine sites from an independent data set, all thought to be disturbed, were assessed as such by the model. Results from twenty test sites, chosen because they represented a wide range of ecological condition, were less clear‐cut. In its current state the model reliably distinguishes undisturbed and severely disturbed sites. Subtle impacts are either detected inconsistently or do not affect ecological condition.
The effects of fish on assemblages of amphibians in ponds: a field experiment
Authors: Smith G. R. , Rettig J. E. , Mittelbach G. G. , Valiulis J. L. , Schaack S. R.
Published: March 2003

1. Bluegill sunfish (Lepomis macrochirus) dominate fish assemblages of small lakes and ponds throughout the eastern United States and may play a major role in structuring aquatic communities. We examined the impact of adult bluegill on amphibian density by stocking bluegill at a range of densities into partitions of an experimental pond in which amphibians were free to colonize.

2. Adult bluegill had a major impact on the amphibian assemblage. By the end of the experiment, gray treefrog (Hyla versicolor) tadpoles were nine times less abundant, and red‐spotted newt (Notophthalmus viridescens) adults were three times less abundant in the presence of adult bluegill than in their absence. In contrast, bullfrog (Rana catesbeiana) tadpoles tended to increase in the presence of adult bluegill. Adult bluegill also had a negative effect on the abundance of predaceous aquatic insects.

3. There was no indication that interactions among amphibians were significant in determining the above patterns. We suggest that the strong impact of adult bluegill resulted from a combination of direct and indirect effects on amphibian larvae and predaceous aquatic insects.
A new energetics model for brown trout, Salmo trutta
Authors: Elliott J. M. , Hurley M. A.
Published: March 2003

1. The chief objective of the present study was to develop a functional model for the daily change in the total energy content of a brown trout, Salmo trutta , (equivalent to growth when positive) in relation to the difference between energy intake (energy content of food) and energy losses (metabolism + losses in faeces and excretory products). Energy budgets for individual fish were obtained in earlier experiments with 210 hatchery trout (live weight = 11–270 g) kept at fairly constant temperatures (mean values ranging from 3.6 to 20.4 °C), but without strict control of temperature or oxygen, and in later experiments, with 252 trout (1–300 g) bred from wild parents and kept at five constant temperatures (5, 10, 13, 15 and 18 °C) and 100% oxygen saturation. Each trout was fed a fixed ration of shrimps, Gammarus pulex, the ration level varying between zero and maximum.

2. Energy intake (CIN, cal day−−1) was measured directly and expressed as a proportion (p) of the maximum energy intake (C, cal day−−1), the latter being estimated from a model developed earlier. In a new model, energy losses (CQ, cal day−−1) were expressed as a function of temperature, fish weight and ration level. This model was continuous over the 3.6–20.4 °C range, had twelve fitted parameters and was an excellent fit to the data for the 462 trout (P < 0.001, R2 = 0.9970). In an extended model, the weight exponent for energy losses was not assumed equal to that for energy intake, the difference between the two exponents being very small, but significant, with a slight improvement in the fit of the model (R2 increased to 0.9972).

3. The limits of model use were discussed. An example of its utility was to elucidate the complex relationships between both positive (growth) and negative daily changes in the total energy content of the trout, and temperature, fish size and variable energy intake. The model has raised several questions for future work, including the effect of increasing energy intake by a change of diet from invertebrates to fish or fish pellets, and a comparison of growth models based on weight or energy changes.
Stratification and dynamics of microbial loop communities in Lake Fryxell, Antarctica
Authors: Roberts Emily C. , Laybourn‐Parry Johanna , McKnight Diane M. , Novarino Gianfranco
Published: March 2003

1. Lake Fryxell, situated in the McMurdo Dry Valleys, Antarctica, offers the opportunity to study microbial loop processes in the absence of crustacean zooplankton and other higher organisms. This is the first study of Lake Fryxell to provide detailed temporal and vertical variations of microbial loop organisms.  2. Protozoan communities are concentrated around the chemocline (9–10 m) in Lake Fryxell. Phototrophic nanoflagellates (PNAN), heterotrophic nanoflagellates (HNAN) and ciliates formed deep maxima of 14 580, 694 and 58 cells mL−1 respectively. Although abundance and biomass at the chemocline was high, diversity of protozoa was low, Plagiocampa accounting for> 80% of the total ciliate biomass.  3. In the mixolimnion (4.5–8 m), protozoa were less abundant, but more diverse, with 24 ciliate morphotypes being identified within this region of the water column. Inter‐annual variability of protozoan biomass and abundance was greater in the mixolimnion than at the chemocline due to more variable nutrient and prey concentrations.  4. Physicochemical gradients in Lake Fryxell were very stable because the perennial ice cover reduced wind driven currents. As a consequence, ciliate species occurred in distinct depth strata, Monodinium being most abundant directly beneath the ice cover, Askenasia having maximum abundance at 8 m and Plagiocampa dominating ciliate biomass at the chemocline. The lack of vertical mixing reduced seasonal successions of PNAN and ciliate species. Three cryptophyte species dominated the PNAN community at all times (>79% of total biomass).
The different primary producers in a small African tropical reservoir during a drought: temporal changes and interactions
Authors: Thomas Serge , Cecchi Philippe , Corbin Daniel , Lemoalle Jacques
Published: March 2003

1. The biomass of the different primary producers, and their relative contributions to the total, were measured quarterly from July 1997 to September 1998 in a tropical reservoir (Brobo, Côte‐d'Ivoire). The study period was marked by an exceptional decrease in the water level as a result of a long drought with only one significant rainfall in June 1998.  2. In July 1997, at the beginning of the usual rainy season, the level of the lake was normal and the ecosystem was in a moderately clear water state dominated by littoral macrophytes, mostly Potamogeton octandrus (more than 55% of the 6.4 tons of carbon of the total primary biomass). The microphytobenthos (19%) and the phytoplankton (25%) were secondary contributors, whereas the periphyton on macrophytes (0.5%) and the epixylon (periphyton on dead flooded trees, 0.3%) were negligible.  3. As a result of the steady decline in water level due to a lack of rain, the macrophytes were progressively stranded on the shores and had disappeared by March 1998. From this time, microalgae (microphytobenthos and phytoplankton) became dominant. Their combined biomass increased slowly from 2.8 tons of carbon in July 1997 to 3.7 tons in September 1998. The microphytobenthic biomass contributed 78% to the total in March, 55% in June and 60% in September, while other contributions were mainly due to the phytoplankton. The epixylon remained negligible throughout the study (≤0.5% of the whole primary biomass).  4. The distribution and temporal changes in the biomass during each survey were mainly linked to hydrology and to interactions between primary producers.  5. The effect of water level changes on free (planktonic) and fixed primary producers (periphytic microalgae and rooted macrophytes) is discussed.
Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long‐term data from the biomanipulated Bautzen Reservoir (Germany)
Authors: Benndorf JÜrgen , Kranich Johannes , Mehner Thomas , Wagner Annekatrin
Published: March 2003

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.  2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).  3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.  4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.  5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.  6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.
A model for resolving the plankton paradox: coexistence in open flows
Authors: Scheuring IstvÁn , KÁrolyi GyÖrgy , PÉntek Áron , TÉl TamÁs , Toroczkai ZoltÁn
Published: March 2003

1. Recent developments in the field of chaotic advection in hydrodynamical/environmental flows encourage us to revisit the population dynamics of competing species in open aquatic systems.  2. We assume that these species are in competition for a common limiting resource in open flows with chaotic advection dynamics. As an illustrative example, we consider a time periodic two‐dimensional flow of viscous fluid (water) around a cylindrical obstacle.  3. Individuals accumulate along a fractal set in the wake of the cylinder, which acts as a catalyst for the biological reproduction process. While in homogeneous, well mixed environments only one species could survive this competition, coexistence of competitors is typical in our hydrodynamical system.  4. It is shown that a steady state sets in after sufficiently long times. In this state, the relative density of competitors is determined rather by the fractal nature of the spatial distribution of the advected species, and by their initial conditions, than by their competitive abilities. We argue that two factors, the strong chaotic mixing along a fractal set and the boundary layer around the obstacle, are responsible for the coexistence.
A probabilistic model characterizing fish assemblages of French rivers: a framework for environmental assessment
Authors: Oberdorff Thierry , Pont Didier , Hugueny Bernard , Chessel Daniel
Published: March 2003

1. Management of running waters and assessment of water quality trends require the use of biological methods. Among potential indicators, fish assemblages are of particular interest because of their ability to integrate environmental variability at different spatial scales.  2. The French Water Agencies and the Ministry of the Environment initiated a research programme to develop a fish‐based index that would be applicable nation‐wide. Such an index should encompass the relative importance of geographic, ecoregional and local factors influencing the distribution of riverine fishes.  3. An effective way of using the information available from fish assemblages to establish such an index is through the use of the ‘reference condition approach’ which involves testing a fish assemblage exposed to a potential stress against a reference condition that is unexposed to such a stress.  4. Logistic regression procedures were applied, using a fish data set of 650 reference sites fairly evenly distributed across French rivers and defined by some easily measured regional and local characteristics, to elaborate the simplest possible response model that adequately explains the observed patterns of occurrence for each species of a fish assemblage at a given site of any given river. This allows us to predict a ‘theoretical’ assemblage at a site.  5. The models were validated using a second independent data set of 88 reference sites. Using a third data set of 88 disturbed sites, the observed assemblages were then compared against the reference condition as defined by the ‘theoretical’ assemblages. The amount of deviation between the expected and observed assemblages within these sites is used as a measure of degradation.  6. This approach could be used as a framework for adapting and calibrating a multimetric index, thereby serving as a practical technical reference for conducting cost‐effective biological assessments of lotic systems.
Exploring patterns of population subdivision in the net‐winged midge, Elporia barnardi (Diptera: Blephariceridae), in mountain streams of the south‐western Cape, South Africa
Authors: Wishart  , Hughes
Published: March 2003

1. The net‐winged midges (Diptera: Blephariceridae) are a highly specialized group whose morphological characteristics and specific habitat requirements suggest a limited potential for dispersal. Levels of genetic variation were examined within streams, between streams in the same range and between mountain ranges in larval populations of Elporia barnardi in the south‐western Cape of South Africa. The aim was to examine the hypothesis that population structure would reflect the poor potential for dispersal.

2. Significant deviations from Hardy–Weinberg equilibrium in 17 of the 57 individual comparisons indicate a non‐random mating population. Given the swarming behaviour and life history traits, larvae sampled may reflect the product of limited matings.

3. Analysis of population substructuring revealed significant levels of differentiation among geographically proximate populations. Large differences between streams within Table Mountain, similar in magnitude to those between mountain ranges, suggest that movement of individuals out of the stream catchment is rare. Observed FST values are more similar to those of fully aquatic species than other lotic insects with winged adult stages.

4. Results suggest that mountain ridges provided effective physical barriers to the dispersal of E. barnardi, with the catchment representing the effective population unit.
Coupling the microbial food web with fish: are bacteria attached to cyanobacteria an important food source for underyearling roach?
Authors: Kamjunke Norbert , Mehner Thomas
Published: March 2003

1. After observing that juvenile roach fed intensively on cyanobacteria and that cyanobacteria were densely colonized by heterotrophic bacteria, we tested whether the bacteria are used by underyearling roach and the extent to which they contribute to the energy requirements of the fish.

2. We radiolabelled attached bacteria in a natural cyanobacterial suspension, fed the fish with these particles, and estimated their assimilation by roach. Biomass of attached bacteria on cyanobacteria increased with the proportion of the cyanobacterium Microcystis in total cyanobacteria. Biomass‐specific thymidine incorporation of attached bacteria was higher than that of free bacteria.

3. In feeding experiments, we detected assimilation of bacterial biomass into muscle tissue of underyearling roach. Fish consumed Microcystis to a lesser extent compared with Aphanizomenon but assimilation of attached bacteria was higher when roach fed on Microcystis because of the higher biomass of epibacteria on this cyanobacterium. However, biomass of attached bacteria was too low to be an important food source for underyearling roach.

4. We conclude that assimilation of epibacteria from cyanobacteria cannot explain the success of roach in eutrophic lakes.
Effects of logging on macroinvertebrate production in a sand‐bottomed, low‐gradient stream
Authors: Kedzierski Wendy M. , Smock Leonard A.
Published: March 2003

1. Macroinvertebrate production and macrophyte growth were studied in logged and unlogged sections of a sand‐bottomed, low‐gradient, blackwater stream on the Coastal Plain of Virginia, U.S.A. A section of the catchment had been clear‐cut 3 years prior to sampling. No logging occurred in the upstream area of the catchment, which had experienced almost no land disturbance by humans for over 100 years.

2. A primary difference among the logged and unlogged sections of the stream was in the abundance of macrophytes. The combined biomass of Sparganium americanum and of Chara sp. was over 300‐times greater in the logged than the unlogged section.

3. Annual macroinvertebrate production in the sediment was higher in the unlogged section (41 g dry mass m–2) than in the logged section (25 g m–2).

4. Annual macroinvertebrate production on Sparganium was higher in the logged section (10 g m–2 of plant surface area) than in the unlogged section (6 g m–2). Annual production associated with Chara, which occurred only in the logged section, was 196 g m–2 of stream bottom covered by this plant.

5. Whole‐stream annual macroinvertebrate production, calculated by summing habitat‐specific production that was weighted by habitat availability, was greater in the logged section (103 g m–2) than in the unlogged section (41 g m–2). Sediments supported 99% of the annual production in the unlogged section, whereas macrophytes supported 76% in the logged section.

6. Much of the additional macroinvertebrate production in the logged section was by collector‐filterers living on macrophytes. Production by collector‐gatherers was also greater in the logged section, whereas production by other functional feeding groups changed little with logging.

7. Although logging along high‐gradient, rocky streams also results in increased macroinvertebrate production, that increase often is stimulated by greater periphyton growth rather than the macrophyte growth observed in this low‐gradient stream.
Effects of an introduced piscivorous fish on native benthic fishes in a coastal river
Authors: White Jason L. , Harvey Bret C.
Published: March 2003

1. We used field surveys to compare the density and mesohabitat‐scale distribution of the native coastrange sculpin (Cottus aleuticus) and the prickly sculpin (C. asper) in coastal rivers in north‐western California, U.S.A., with and without an introduced piscivorous fish, the Sacramento pikeminnow, Ptychocheilus grandis. We also measured mortality of tethered prickly sculpin in a field experiment including river, habitat type (pools versus riffles) and cover as factors.

2. Average sculpin density (C. aleuticus and C. asper combined) in two rivers without pikeminnow was 21 times higher than the average density in two rivers in a drainage with introduced pikeminnow. In riffles, differences in the density of sculpins among rivers could be linked to differences in cover. However, riffles in rivers without pikeminnow had an average sculpin density 77 times higher than rivers with pikeminnow, yet only nine times more cover. In pools, cover availability did not differ among rivers, but the density of sculpins in rivers without pikeminnow was 11 times higher than rivers with pikeminnow.

3. In the field experiment, mortality of tethered sculpin varied substantially among treatments and ANOVA indicated a significant River × Habitat × Cover interaction (P < 0.001). Overall, tethered prickly sculpin suffered 40% mortality over 24 h in rivers with pikeminnow and 2% mortality in rivers without pikeminnow, suggesting that predation is the mechanism by which the pikeminnow affects sculpins.

4. The apparent reduction in sculpin abundance by introduced pikeminnow has probably significantly altered food webs and nutrient transport processes, and increased the probability of extinction of coastrange and prickly sculpins in the Eel River drainage.
Morphological differences between subitaneous and diapause eggs of Boeckella triarticulata (Copepoda: Calanoida)
Authors: Couch Kyleigh M. , Downes Matthew , Burns Carolyn W.
Published: March 2003

1. Many calanoid copepods produce subitaneous eggs that hatch immediately and diapause eggs that remain dormant for long periods of time. Boeckella triarticulata (Copepoda: Calanoida) produces diapause eggs that hatch asynchronously over an extended period of time and appear identical under light microscopy to those that hatch immediately.

2. We compared the internal morphology of subitaneous and diapause eggs of B. triarticulata that had been prepared for transmission electron microscopy by four methods.

3. Subitaneous eggs had a thin, single‐layered shell whereas diapause eggs had a thick, three‐layered shell that appeared consistent over 3 months of diapause. Boeckella triarticulata appears to enter diapause at the multicellular stage.

4. Consistent morphological differences between subitaneous and diapause eggs of B. triarticulata suggest that these two egg types also have distinct differences in physiology, and are not merely extremes of a continuum from subitaneous to diapause eggs.
Validation of a diatom–phosphorus calibration set for Sweden
Authors: Bradshaw Emily G. , Anderson N. John
Published: March 2003

1. A weighted averaging (WA) regression and calibration model for diatoms and total phosphorus (TP) was developed from a dataset of 45 surface‐sediment samples from Swedish lakes. Jack‐knifed error statistics were comparable with those for similar diatom–TP datasets: r2jack=0.47, root mean squared error of prediction (RMSEP)=0.24 log10μg TP L–1 and mean bias=–0.002 log10 μg TP L–1 for the simple WA model; r2jack=0.36, RMSEP=0.27 log10 μg TP L–1 and mean bias=0.017 log10 μg TP L–1 for WA with tolerance downweighting.

2. The model was used to estimate TP concentrations for the Ekoln basin of Lake Mälaren using a 210Pb‐dated sediment core record. Highly eutrophic conditions developed in the basin in the 1960s as a result of nutrient inputs from cultivated land and the city of Uppsala. A reduction in the supply of phosphorus from sewage outlets in the late 1960s resulted in less eutrophic conditions.

3. The model results indicated levels of 50–60 μg TP L–1 prior to 1900. The rapid eutrophication of the lake basin after the 1950s and the subsequent recovery were evident from the diatom data.

4. Diatom‐inferred TP (DI–TP) values were validated by comparison with monitored data for the period 1966–95. The diatom model tended to underestimate TP at high levels (> 80 μg L–1) but overestimate at lower concentrations.

5. A good agreement was observed between the trends in TP concentration and the DI–TP concentration and the timing of the maximum was well reflected by the diatom‐based reconstruction. A significant correlation (r2=0.69, P < 0.01) was found between DI–TP and measured TP at this site.
The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake
Authors: Horppila Jukka , Nurminen Leena
Published: March 2003

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia.

2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m−2 of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m−2, respectively.

3. With the resuspended sediment, 39.4 mg P m−2 day−1 was brought to the water column outside the stand, 22.4 mg P m−2 day−1 at the edge and 13.4 mg P m−2 day−1 within the stand.

4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.
Effects of single Ca(OH)2 doses on phosphorus concentration and macrophyte biomass of two boreal eutrophic lakes over 2 years
Authors: Reedyk Sharon , Prepas Ellie E. , Chambers Patricia A.
Published: March 2003

1. Two hardwater eutrophic lakes of central Alberta were subjected to single doses of Ca(OH)2 (74 or 107 mg L–1). The effects of lime treatment on phosphorus (P) precipitation, sediment P release, and macrophyte biomass were assessed for up to 2 years.

2. In both lakes, sediment P release was reduced to 16 and 27%, respectively, of pre‐treatment values by the first winter following treatment. However, sediment P release returned to pre‐treatment values during the following year.

3. In contrast to these short‐term effects, macrophyte biomass decreased by as much as 80% after lime application and remained there for at least 2 years.

4. Our results indicate that a single dose of Ca(OH)2 may give short‐term (< 1 year) control of P and long‐term control (> 1 year) of macrophytes in hardwater eutrophic lakes of Alberta.