bluuebunny/crossref_metadata_2025 huggingface dataset:
Mixotrophic cryptophytes and their predators in the Dry Valley lakes of Antarctica
Authors: Roberts Emily C. , Laybourn‐Parry Johanna
Published: March 2003

1. The ingestion rates of planktonic, mixotrophic cryptophytes in two perennially ice‐covered Antarctic lakes in the McMurdo Dry Valleys, were investigated during the summer of 1997–1998.

2. In Lake Fryxell, which is meromictic, ingestion rates increased with depth in November and were highest in a cryptophyte maximum close to the chemocline. In Lake Hoare, which is unstratified and freshwater, there was no significant difference in ingestion rates with depth. In both lakes, the highest ingestion rates occurred in early summer, decreasing in December and January. Ingestion rates varied between 0.2 bacteria cell−1 h−1 and 3.6 bacteria cell−1 h−1.

3. During November, mixotrophic cryptophytes removed up to 13% of bacterial biomass day−1 and had a greater grazing impact than heterotrophic nanoflagellates (HNAN). As summer progressed, the grazing impact of cryptophytes and HNAN became similar.

4. The maximum depth of cryptophytes in Lake Fryxell was predated by a population of the ciliate Plagiocampa. Plagiocampa had an ingestion rate of 0.13–0.19 cryptophytes cell−1 h−1. The grazing impact on the cryptophyte community was insignificant. However, the ciliate appeared to be indulging in temporary mixotrophy, sequestering the cryptophytes for a number of weeks before digesting them.

5. It is suggested that mixotrophy is an important survival strategy in the extreme lake ecosystems of the McMurdo Dry Valleys.
Predicting the structure and diversity of young‐of‐the‐year fish assemblages in large rivers
Authors: Gozlan R. E. , Mastrorillo S. , Copp G. H. , Lek S.
Published: March 2003

1. Interactions between environmental variables and 0+ fish assemblages in the upper River Garonne (France) were quantified during late August 1995.

2. The abundance and diversity of the fish assemblages in floodplain channels were modelled using Artificial Neural Network (ANN) analysis and nine variables: the abundance of the six dominant species, fish specific richness, overall abundance of 0+ fish and the Shannon index of diversity. Multiple regression analysis was also used to assess ANN performance.

3. Using 596 samples, correlation coefficients (r adjusted) between observed and estimated values of the nine dependent parameters were all highly significant (P < 0.01). Expected values from the tested data were significantly related to the observed values. The correlation coefficient between observed and estimated values (r) varied from 0.70 to 0.85.

4. The ANN provided a high quality prediction, despite the complex nature of the relationship between microhabitat composition and fish abundance.

5. Garson’s algorithm was used to provide the explanatory power needed in ecology when using black‐box models. Parameters contained in the models (i.e. weighting) were used to determine the relative contributions of explanatory variables and thus to ascertain the structure of fish communities.
Effects of artificial ultraviolet‐B radiation on experimental aquatic microcosms
Authors: De Lange Hendrika J. , Verschoor Antonie M. , Gylstra Ronald , Cuppen Jan G. M. , Van Donk Ellen
Published: March 2003

1. The effects of prolonged ultraviolet‐B (UVB) radiation on freshwater communities were studied in indoor microcosms (600 L) with artificial light sources, simulating a clear, shallow, mesotrophic aquatic ecosystem. A range of six intensities (in duplicate) of UVB radiation, ranging from 0 (control) to 9.56 kJ m−2 day−1 at the water surface, was applied for 8 weeks. The UVB radiation levels, attenuation, shading and scattering were comparable to those in Dutch shallow freshwater systems. Physical, chemical and biological variables were monitored weekly.

2. The UVB treatment did not affect the abundance, species composition or biovolume of the phytoplankton or zooplankton communities, nor did it affect the periphyton or the macroinvertebrate community. A few species showed a significant response on some of the sampling dates, but there was no negative UVB effect at the community level. Overall, the ecosystems in the microcosms were not affected by the UVB treatment.

3. In a bio‐assay, a laboratory clone of Daphnia pulex, not subjected to UVB radiation, was fed with seston from the microcosms. Daphnia pulex feeding on seston from the control microcosms grew faster, had better survival and better reproduction than D. pulex feeding on seston from the UVB treated microcosms. The phytoplankton–zooplankton interaction may have been influenced by the UVB treatment.

4. The dissolved oxygen content (DOC) concentrations in the microcosms were around 5 mg L−1. The DOC levels in Dutch systems rarely fall below 10 mg L−1. This might provide sufficient protection against the detrimental effects of increased UVB radiation.
Foraging strategies and growth inhibition in five daphnids feeding on mixtures of a toxic cyanobacterium and a green alga
Authors: DeMott William R.
Published: March 2003

1. Laboratory experiments were used to study the feeding, growth and reproduction of five daphnids in mixtures of a toxic cyanobacterium, Microcystis aeruginosa, and a green alga, Scenedesmus acutus. The mixtures included 0%, 20%, 50%, 80% and 100% Microcystis with a total food concentration of 0.5 mg C L−−1 in each treatment. The feeding rate was measured after 1 and 24 h of acclimatization to the mixtures.

2. Toxic Microcystis inhibited feeding in all the species, but they exhibited an unexpected diversity and complexity in the pattern of feeding inhibition. Daphnia magna exhibited the strongest inhibition of feeding after 1 h of exposure to toxic food, but had substantially recovered after 24 h in the same mixtures. This pattern of inhibition and recovery may balance the benefits of reduced ingestion of toxin with the disadvantage of a reduced energy intake.

3. All five daphnids grew quickly in the Scenedesmus control, whereas growth and reproduction declined with an increasing proportion of the toxic alga in the diet. Daphnia pulicaria showed the least inhibition of growth and reproduction, D. pulex showed the strongest inhibition and the three remaining species exhibited intermediate sensitivity.

4. Estimates of gross growth efficiency (GGE; growth/ingestion) provided a means for discriminating between the effects of feeding inhibition and direct toxicity on zooplankton growth. Daphnia pulex exhibited a sharp decline in GGE, suggesting that growth inhibition was a result of both feeding inhibition and direct toxicity. In contrast, D. magna exhibited a nearly constant GGE, indicating that feeding inhibition accounted for its decline in growth. However, two Daphnia species (i.e. D. pulicaria and D. galeata) exhibited improbable increases in GGE with toxic cyanobacteria, suggesting that their feeding rates were underestimated.

Growth assays with sensitive and resistant zooplankton species are proposed for testing the potential impacts toxic cyanobacteria in lakes.
The effect of diapause emergence on the seasonal dynamics of a zooplankton assemblage
Authors: Hairston, Jr Nelson G. , Hansen Anne‐Mette , Schaffner William R.
Published: March 2003

1. The role of seasonal phenology in the emergence of zooplankton from diapause in patterns of seasonal abundance in the water column was investigated in Oneida Lake, New York. Replicate emergence traps, placed in contact with the lake sediments at two locations (one at a shallow site and one at a deep site), were monitored between May and August.  2. Although six rotifer taxa showed a clear seasonal succession in the water column throughout the study period, all but one taxon emerged exclusively in spring. Three cladoceran and three calanoid copepod species, also present in the water column throughout the study period, again showed predominantly spring emergence. In contrast, three cyclopoid copepod species had distinct seasonal periods of emergence that corresponded, at least in part, to the timing of abundance peaks in the plankton.  3. These results for a single lake are largely consistent with patterns observed or inferred by other investigators for other lakes: variable dependence of abundance in the plankton on diapause emergence for species with long‐lived diapausing eggs (i.e. rotifers, cladocerans and calanoid copepods), and much closer dependence for species with short‐lived diapausing immature stages (i.e. cyclopoid copepods).
Distribution of recreational boating across lakes: do landscape variables affect recreational use?
Authors: Reed‐Andersen TarA. , Bennett Elena M. , Jorgensen Bradley S. , Lauster George , Lewis David Bruce , Nowacek David , Riera Joan L. , Sanderson Beth L. , Stedman Richard
Published: March 2003

1. Because people impact lake ecosystems, it is important to consider factors influencing the human use of freshwater resources. We investigated the influence of the landscape position, as well as lake area, recreational facilities, and distance to highways and urban centres, on lake use by boaters in the Northern Highland Lake District of Wisconsin, U.S.A.

2. In aerial surveys of ninety‐nine randomly selected lakes, we did not see boats on over half of the lakes. Of the lakes with boats, we found a strong correlation between the number of boats and lake area. Recreational boats tended to be found on large, accessible lakes with good boating facilities. Boats were not seen on small, stained lakes with few recreational facilities.

3. Regression models showed that lake size and landscape position explained 63% of the variability in the average number of boats per lake and landscape position explained 24% of the variability in boat density on all ninety‐nine lakes. Social variables representing the quality of boating facilities and the perception of good fishing explained 70% of the variability in number of boats per lake and 54% of the variation in boat density on all lakes. A combined model using both physical and social variables increased the explanatory power for both number and density. Lake use by boaters was correlated with landscape position, the quality of fishing and the availability of recreational facilities. When the analysis was restricted to the forty‐six lakes where boats were found, only the availability of recreational facilities proved a significant predictor of boat density.

4. Our results suggest that lake choice by recreational boaters may be best predicted by a combination of the location of a lake in a regional hydrologic landscape, and considerations of available facilities and perceptions regarding fishing.
Integration of lakes and streams in a landscape perspective: the importance of material processing on spatial patterns and temporal coherence
Authors: Kling George W. , Kipphut George W. , Miller Michael M. , O'Brien W. JohN.
Published: March 2003

1. We studied the spatial and temporal patterns of change in a suite of twenty‐one chemical and biological variables in a lake district in arctic Alaska, U.S.A. The study included fourteen stream sites and ten lake sites, nine of which were in a direct series of surface drainage. All twenty‐four sites were sampled between one and five times a year from 1991 to 1997.

2. Stream sites tended to have higher values of major anions and cations than the lake sites, while the lake sites had higher values of particulate carbon, nitrogen, phosphorous and chlorophyll a. There were consistent and statistically significant differences in concentrations of variables measured at the inlet versus the outlet of lakes, and in variables measured at upstream versus downstream sites in the stream reaches which connect the lakes. In‐lake processing tended to consume alkalinity, conductivity, H+, DIC, Ca2+, Mg2+, CO2, CH4, and NO3–, and produce K+ and dissolved organic carbon (DOC). In‐stream processing resulted in the opposite trends (e.g. consumption of K+ and DOC), and the magnitudes of change were often similar to those measured in the lakes but with the opposite sign.

3. Observed spatial patterns in the study lakes included mean concentrations of variables which increased, decreased or were constant along the lake chain from high to low altitude in the catchment (stream sites showed no spatial patterns with any variables). The strongest spatial patterns were of increasing conductivity, Ca2+, Mg2+, alkalinity, dissolved inorganic carbon and pH with lake chain number (high to low altitude in the basin). These patterns were partly determined by the effect of increasing catchment area feeding into lakes further downslope, and partly by the systematic processing of materials in lakes and in the stream segments between lakes.

4. Synchrony (the temporal coherence or correlation of response) of variables across all lakes ranged from 0.18 for particulate phosphorus to 0.90 for Mg2+ the average synchrony for all twenty‐one variables was 0.50. The synchronous behaviour of lake pairs was primarily related to the spatial location or proximity of the lakes for all variables taken together and for many individual variables, and secondarily, to the catchment to lake area ratio and the water residence time.

5. These results illustrate that, over small geographic areas, and somewhat independent of lake or stream morphometry, the consistent and directional (downslope) processing of materials helps produce spatial patterns which are coherent over time for many limnological variables. We combine concepts from stream, lake and landscape ecology, and develop a conceptual view of landscape mass balance. This view highlights that the integration of material processing in both lakes and rivers is critical for understanding the structure and function of surface waters, especially from a landscape perspective.
Costs, benefits and characteristics of mixotrophy in marine oligotrichs
Authors: Dolan John R. , PÉrez Maria Teresa
Published: March 2003

1. Oligotrich ciliates are an important part of most marine plankton communities. Mixotrophic (chloroplast‐sequestering) oligotrichs, a common component of marine oligotrich communities, obtain fixed carbon from both photosynthesis as well as the ingestion of particulate food. Mixotrophy, in general, is often considered an adaptation permitting exploitation of food‐poor environments. We examined the hypothesis that, among oligotrichs, mixotrophs may be at a disadvantage relative to heterotrophs in food‐rich conditions in a nutrient‐enrichment experiment. We compared growth responses of mixotrophic and heterotrophic oligotrichs in natural communities from the N.W. Mediterranean Sea in microcosms with daily nutrient additions resulting in increases in nanoflagellates and Synechococcus populations. The results indicated that both mixotrophic and heterotrophic oligotrichs respond to prey increases with rapid growth (μ=1.2 d−1).  2. To examine the hypothesis that the proportion of mixotrophic to heterotrophic oligotrichs changes with the trophic status of a system, increasing with oligotrophy, we examined data from a variety of marine systems. Across systems ranging in chlorophyll concentration from about 0.1 to 40 μg L−1, oligotrich cell concentrations are correlated with chlorophyll concentrations, and mixotrophs are a consistent component of oligotrich communities, averaging about 30% of oligotrich cell numbers.  3. We discuss the costs, benefits and possible uses of mixotrophy in marine oligotrichs and suggest that mixotrophy in marine oligotrichs is not closely linked to the exploitation of food‐poor environments, but probably serves a variety of purposes.
Large‐scale synchrony and inter‐annual variability in roach recruitment in the Rhône River: the relative role of climatic factors and density‐dependent processes
Authors: Grenouillet G. , Hugueny B. , Carrel G. A. , Olivier J. M. , Pont D.
Published: March 2003

1. We report patterns of temporal variation in the recruitment of roach (Rutilus rutilus). The data consist of the annual abundance of the first 2 year‐classes, 0+ and 1+ fish, at four sites in the Rhône River (France) between 1987 and 1997. Over this 11‐year period both 0+ and 1+ fish fluctuated strongly.  2. Cross‐correlation indicated high spatial synchrony in 0+ dynamics, although correlations among sites in 1+ dynamics were weaker. No clear pattern was apparent in the relationship between the level of synchrony and distance between pairs of sites.  3. The spatial synchrony in 0+ fish could be attributable to large‐scale variations in weather, influencing water temperature. Total body length of 0+ roach was correlated with water temperature (expressed in degree‐days over 12 °C), and water temperature was the main factor explaining inter‐annual variation in 0+ cohort size. Monthly variation in abiotic factors (measured by standard deviation in water temperature and discharge) did not influence 0+ fluctuations. Correlations with June water temperature suggest that year‐class strength was mainly determined by abiotic factors during the first few months of life.  4. The absence of spatial synchrony in 1+ fluctuations suggests little correlation between survival and abiotic conditions during the first year of life, other factors influencing survival.  5. Survival in the first year was density‐dependent. Intraspecific competition within the 0+ cohort could thus influence the fluctuations in recruitment to older age‐classes.  6. The implications of age‐ or stage‐dependent synchrony in temporal variation for species with complex life histories are discussed. Studying spatial synchrony for the different life history stages could enhance our understanding of the population dynamics of spatially structured species.
Contribution of terrestrial invertebrates to the annual resource budget for salmonids in forest and grassland reaches of a headwater stream
Authors: Kawaguchi Yôichi , Nakano Shigeru
Published: March 2003

1. The annual input, contribution to the diet of salmonids, and quantitative input of terrestrial invertebrates to four reaches with contrasting forest (n=2) and grassland riparian vegetation (n=2) were compared in a Japanese headwater stream.  2. The annual input of terrestrial invertebrates falling into the forest reaches (mean±1 SE=8.7×103±0.3×103 mg m−2 year−1) was 1.7 times greater than that in the grassland reaches (5.1×103±0.8×103 mg m−2 year−1), with clear seasonality in the daily input of invertebrates in both vegetation types. The daily input, however, differed between the vegetation types only in summer, when it rose to a maximum in both vegetation types.  3. Fish biomass also differed among the seasons in both vegetation types, being less in the grassland reaches. The contribution of terrestrial invertebrates to the salmonid diet in the forest and grassland reaches was 11 and 7% in spring, 68 and 77% in summer, 48 and 33% in autumn, and 1 and 1% in winter, respectively. The prey consumption rate of fish, which was similar between the vegetation types, increased with stream temperature and was highest in summer. Terrestrial invertebrates supported 49% (mean±1 SE=5.3×103±0.4×103 mg m−2 year−1) of the annual, total prey consumption (10.9×103±1.7×103 mg m−2 year−1) by salmonids in the forest and 53% (2.0×103±0.3×103 mg m−2 year−1) (3.8×103±0.6×103 mg m−2 year−1) in the grassland reaches.  4. Salmonids were estimated to consume 51 and 35% of the annual total (falling plus drift) input of terrestrial invertebrates in the forest and grassland reaches, respectively. The input of terrestrial invertebrates by drift, however, was almost equal to the output in both vegetation types, suggesting that the reach‐based, in‐stream retention of terrestrial invertebrates almost balanced these falling in.  5. Difference in the riparian vegetation, which caused spatial heterogeneity in the input of terrestrial invertebrates, could play an important role in determining the local distribution of salmonids.
Oxygen measurements in the burrows of freshwater insects
Authors: Wang Feiyue , Tessier André , Hare Landis
Published: March 2003

1. Thin‐tipped micro‐electrodes were used to measure oxygen concentrations in the burrows of two common aquatic insects, the mayfly Hexagenia limbata and the alderfly Sialis velata. Both species maintain their surroundings oxygenated by drawing water from above the sediment surface into their tubes.  2. The temporal pattern of oxygen in the burrows differed between the species. The constant high oxygen concentration (>75% of air saturation) measured in the tubes of the mayfly suggest that this animal pumps water almost continuously, which is consistent with its high oxygen requirements. In contrast, oxygen concentration in burrows of the alderfly fluctuated widely over time, suggesting that this animal irrigates only irregularly, probably because it can tolerate short periods of low oxygen concentration in its burrow.  3. The interval between pumping episodes by the alderfly decreased with increasing temperature, a result of increased oxygen consumption by the animal and by sediment at high temperature.  4. Based on the tube dimensions, oxygen penetration depth and animal density in lakes, we estimate that Hexagenia could create an oxic micro‐environment equivalent to 3–35% of the volume of the surface oxidized sediment layer created by molecular diffusion. The mosaic of oxic micro‐environments created by the burrowing and irrigation of freshwater animals could influence chemical and biological processes in sediments, the fluxes of materials between the sediment and the overlying water column, and the exposure of benthic animals to sedimentary contaminants.
Food web quantification using secondary production analysis: predaceous invertebrates of the snag habitat in a subtropical river
Authors: Benke Arthur C. , Wallace J. Bruce , Harrison James W. , Koebel Joseph W.
Published: March 2003

1. Secondary production was estimated for Plecoptera, Odonata and Megaloptera (mostly large predators) occurring on the snag habitat of a subtropical blackwater river in the southeastern U.S.A. Coastal Plain for 2 years. Production estimates and gut analyses were used in estimating species‐specific ingestion to construct a quantitative food web of the predator portion of the invertebrate assemblage. Neither basal resources (e.g. detritus) nor predaceous vertebrates (e.g. fishes) were considered in this analysis. A discharge‐specific model of snag‐habitat availability was used to convert values per m2 of snag surface to values per m2 of river bed.  2. These three orders included the major large predators on the snag habitat, as well as two detritivorous stoneflies. The major predators were the hellgrammite (Corydaluscornutus), five perlid stoneflies (Paragnetinakansensis, Perlestaplacida, Neoperlaclymene, Acroneuriaevoluta and Acroneuriaabnormis) and two dragonflies (Neurocorduliamolesta and Boyeriavinosa). The detritivores were Pteronarcysdorsata and Taeniopteryxlita.  3. Total predator production was high, but varied from only 7.1 to 7.4 g dry mass (DM) m−2 y−1 of snag surface (2.4–2.7 g DM m−2 y−1 of river bed) over two years. Corydalus was the largest predator and had the highest production (2.8–3.1 g DM m−2 of snag surface). The most productive stoneflies were Perlesta (0.7–1.0 g DM m−2 of snag surface) and Paragnetina (1.0–1.3 g DM m−2 of snag surface). The most productive dragonfly was Neurocordulia (0.7–1.9 g DM m−2 of snag surface). Production of the non‐predaceous stoneflies was 1.0–2.3 g DM m−2 of snag surface. Production values per m2 of river bed were 2–3.5 times lower than the values per m2 snag surface.  4. Measurement of ingestion fluxes within the predator portion of the food web showed that predaceous invertebrates were primarily supported by chironomid and mayfly prey. However, the greatest consumption of chironomids and mayflies was by omnivorous hydropsychid caddisflies, which had a considerably higher production than the larger predators. There was a hierarchy of feeding with Corydalus as top predator consuming all other groups, followed in order by dragonflies, stoneflies and hydropsychids. Although the feeding hierarchy suggested the presence of four predatory trophic levels within the invertebrate assemblage, calculations of trophic position indicated there were less than two. With primary consumers (e.g. midges) having a trophic position of 2, Corydalus had a trophic position of only 3.5.  5. A relatively high fraction of invertebrate production was consumed by predaceous invertebrates, ranging from 9 to >100% for various primary consumer groups, with total consumption representing 52% of total production. Because these estimates do not include vertebrate consumption or emergence, it means that a high fraction of larval mortality is due to predation.
Problems with the use of terracotta clay saucers as phosphorus‐diffusing substrata to assess nutrient limitation of epilithic algae
Authors: Brown Glen G. , Maher William A. , Norris Richard H. , Mathieu
Published: March 2003

1. We examined the diffusion properties of terracotta clay saucers, of types often used as components in phosphorus‐diffusing substrata for investigating nutrient limitation of epilithic algae.

2. Laboratory experiments showed that phosphorus diffusion was low and inconsistent (0.06–2.6 mg P day−1) through clay saucers filled with agar containing orthophosphate. Similarly, in situ release of phosphorus from two types of terracotta clay saucers (Australian and Italian) was variable (2–8 mg P day−1; 5–25%) under three flow regimes (0, 0.1 and 0.3 m s−1) over 30 days, with most phosphorus being released during the first day. Clogging of pores by agar appears to prevent the diffusion of phosphorus through the terracotta clay saucers. However, the two types of terracotta clay saucer also irreversibly sorbed large quantities of phosphorus (40–140 mg P).

3. Individual saucers can have markedly different physical and chemical properties both within and among terracotta types. Terracotta saucers also contain large quantities of iron, calcium and aluminium that are known binding agents for phosphorus. Variability in saucer composition and diffusion properties makes treatments difficult to replicate and weakens comparisons between studies that have used terracotta with different clay composition and manufacture.

4. We recommend that phosphorus‐diffusing substrata should not be constructed from terracotta clay components.
Perch or plankton: top‐down control of Daphnia by yellow perch (Perca flavescens) or Bythotrephes cederstroemi in an inland lake?
Authors: Hoffman Joel C. , Smith Meghan E. , Lehman John T.
Published: March 2003

1. Seasonal termination of the vernal clear‐water phase in Long Lake, Grand Traverse Co., Michigan coincided with severe size‐selective predation on juvenile Daphnia pulicaria from 0.8 to 1.8 mm in length. This could be caused by predation by age‐0 yellow perch (Perca flavescens) or by the exotic predatory zooplankter Bythotrephes cederstroemi.

2. During the initial decline of Daphnia, Ivlev’s electivity coefficient for yellow perch from 15.0 to 20.0 mm in length was 0.50 for copepods and −0.75 for D. pulicaria.

3. Bioenergetics modelling of both yellow perch and Bythotrephes demonstrates that, during the initial Daphnia decline, Bythotrephes consumed 1.5–5 times greater total mass than yellow perch. Furthermore, models in which Bythotrephes consumed juvenile Daphnia were more consistent with the timing of the Daphnia decline than those in which yellow perch consumed juvenile Daphnia.

4. The invasion of Bythotrephes into Long Lake seems to be a significant perturbation, introducing effects that propagate throughout the food chain. Bythotrephes created a possible bottleneck for age‐0 yellow perch in late June by suppressing Daphnia.
Distribution and diel migration of macroinvertebrates within dense submerged vegetation
Authors: Marklund Ola , Blindow Irmgard , Hargeby Anders
Published: March 2003

1. We studied vertical and horizontal distribution of macroinvertebrates within a dense stand of Chara spp. in Lake Krankesjön, southern Sweden. Invertebrates were sampled at three depths within the vegetation and at three distances from the vegetation edge during day and night in July and August. Corresponding samples of oxygen content of the water were taken.

2. The densities (number of invertebrates per unit plant biomass) of most invertebrate taxa were generally lower in the upper layers of the vegetation than in the layers close to the sediment. The densities of several taxa (Asellus aquaticus, Cloëon sp. and Polycentropodidae), as well as total density of invertebrates, were higher at the edge than in the innermost parts of the macrophyte stand, whereas snail densities generally were highest at the innermost sites.

3. Densities of A. aquaticus, Chironomidae larvae and Helobdella stagnalis generally increased at night. These taxa appear to undertake a diel vertical migration within the vegetation, towards, or even down to the sediment in daytime and up into the vegetation, in some cases to the vegetation surface, at night. Factors underlying the diel vertical migration are discussed, as are their ecological consequences.
Implications of Chaoborus pupation and ecdysis in cold water
Authors: Ouimet Chantal
Published: March 2003

1. Cold water acted differently to delay and lengthen the pupation period for the larvae of two species of the zooplankton predator Chaoborus (Diptera: Chaoboridae). During Chaoborus pupation, the zooplankton community is released from predation, while the dark‐coloured Chaoborus pupae are more susceptible to their own predators.

2. Fourth instar larvae of Chaoborus americanus and C. trivittatus, collected from an oligotrophic lake, were reared individually at 5 °C in the dark. Chaoborus americanus was also reared at 9 and 12 °C under spring photoperiod conditions (L : D, 16 : 8 h). Individuals were observed through pupation to emergence (ecdysis) or death.

3. Chaoborus americanus pupated at 5, 9 and 12 °C with substantial emergence only at 12 °C. In comparison, C. trivittatus emerged at 5 °C. Light was not a necessary cue for pupation and ecdysis, contrary to previous reports. Cold water delayed the onset and lengthened and increased the variability of the duration of pupation.

4. In Shirley Lake, C. americanus pupated in late June–early July while C. trivittatus pupated first in April and again in June–July.

5. Chaoborus americanus pupae needed a temperature cue to complete ecdysis. The ecdysis temperature threshold helps to explain the difference in pupation timing, and the geographical distribution, of C. americanus and its relatively inflexible life history contrasted with C. trivittatus. Delayed predator pupation in years with low spring temperature can affect the community dynamics of the prey.
Colonization, riparian habitat selection and home range size in a reintroduced population of European beavers in the Loire
Authors: Fustec J. , Lode T. , Le Jacques D. , Cormier J. P.
Published: March 2003

1. Colonization in a reintroduced population of European beavers in the Loire Valley was studied between 1974 and 1999. It followed a discontinuous remoteness model and a scattered distribution, beavers occupying only 25% of the river system over the 2800 km explored.

2. After 5 years, the colonization rate reached 104.2% year–1 of new sites occupied (SD 75% year–1), before dropping over the next 20 years. Nevertheless, the number of new colonies per km (0.125) remained stable throughout the years.

3. Populus nigra, Salix alba and Fraxinus angustifolia were the dominant woody species in beaver sites, often associated with some herbaceous species.

4. The length of willow grove dominated by S. alba and P. nigra (x) was the best predictor of beaver home range (y), fitting the equation y=−0.742x + 5.9. Long‐term maintenance of the population requires a minimum of 1.79‐km of willows per colony.

5. In cutting tree trunks, beavers stimulate shoot development from the remaining stumps. They rejuvenate riparian forests, increase the number of tree stems and help stabilize the banks. Their effect on woody plant morphogenesis may have consequences for the helophyte communities used as food or habitat by other aquatic species.
Macroinvertebrate stream communities along regional and physico‐chemical gradients in Western Greenland
Authors: Friberg Nikolai , Milner Alexander M. , Svendsen Lars M. , Lindegaard Claus , Larsen Søren Erik
Published: March 2003

1. Macroinvertebrates were collected and physico‐chemical variables measured at 16 stream sites in Western Greenland during July 1999. Eight sites were located on Disko Island in an arctic oceanic climate and eight sites in the Kangerlussuaq area close to the icecap where the climate is arctic continental. The streams had different water sources (glacial, groundwater, snowmelt and lake water).

2. The streams showed pronounced differences in water temperature (2.2–17.3 °C), concentrations of suspended solids (0–2400 mg L−1), and conductivity (10–109 μS cm−1). Principal component analysis (PCA) analysis of the physico‐chemical variables separated the Disko Island sites into a distinct group, whereas the sites in the Kangerlussuaq area were more dispersed.

3. A total of 56 macroinvertebrate species were found, including 31 species of Chironomidae, the most abundant of which was Orthocladius thienemanni. Diamesa sp. was only the sixth most abundant chironomid taxon. Species composition varied between sites, and abundance varied from about 20 individuals m−2 in a glacier fed stream to more than 16 000 m−2 in a lake outlet.

4. The macroinvertebrate communities of the 16 streams were separated into five TWINSPAN groups reflecting water source, irrespective of region. Lake outlets and ground‐water‐fed streams had the highest species richness and abundance, temperature and bed stability, while glacier‐fed streams were characterized by low species richness, abundance, temperature, bed stability and high concentrations of suspended solids. Macroinvertebrate species richness was positively correlated with water temperature and negatively with bed stability. Conductivity was positively correlated with invertebrate abundance.

5. The results of this study suggest that the source of stream water can be used to predict invertebrate community composition in Greenlandic streams and thus the effects of changes in water balance and flow regime, and to identify sites of special conservation interest.
Trends of macroinvertebrate community structure in glacier‐fed rivers in relation to environmental conditions: a synthesis
Authors: Milner Alexander M. , Brittain John E. , Castella Emmanuel , Petts Geoffrey E.
Published: March 2003

1. Generalized additive models (GAMs) were used to predict macroinvertebrate taxonomic richness and individual taxon diversity at the reach level across seven European glacier‐fed river sites from a set of 11 environmental variables. Maximum water temperature and channel stability were found to explain the most deviance in these models.

2. Using this information, and data from other recent studies of glacier‐fed rivers, a modified conceptual model based on Milner & Petts (1994) is presented which predicts the occurrence of macroinvertebrate families and subfamilies as determined by maximum water temperature (Tmax) and channel stability. This deterministic model only applies to the summer meltwater period when abiotic variables drive community structure.

3. Where maximum water temperature is below 2 °C, Diamesinae chironomids are typically the sole inhabitants, but where Tmax >2 °C but <4 °C Orthocladiinae are found and, where channels are more stable, Tipulidae and Oligochaeta also occur. Above 4 °C Perlodidae, Taeniopterygidae, Baetidae, Simuliidae and Empididae can be expected to be part of the glacier‐fed river community, particularly in Europe.

4. At other times of the year when environmental conditions ameloriate, glacial rivers support higher macroinvertebrate abundance and diversity, with a number of taxa present that are not found during the summer melt period.

5. Dispersal constraints influence macroinvertebrate assemblages of many glacier‐fed rivers located on islands and in some alpine areas.
Resource competition among saline‐lake diatoms under varying N/P ratio, salinity and anion composition
Authors: Saros Jasmine E. , Fritz Sherilyn C.
Published: March 2003

1. A series of resource competition experiments were performed with four diatom taxa isolated from saline lakes in the Northern Great Plains of North America. Three variables were tested in a factorial design: N/P ratio (6 : 1, 12 : 1, 24 : 1, 48 : 1), salinity (5, 8, 11‰), and anion composition (sulphate versus bicarbonate‐dominated medium).

2. Coexistence of Chaetoceros elmorei, Cyclotella quillensis, and Cymbella pusilla occurred at the two lowest N/P ratios. These three taxa have similar N requirements and thus competitive exclusion by the taxon with the lowest N requirement may have required more time.

3. Anomoeoneis costata, which had the highest nitrogen requirements and slowest growth rate of these four taxa, declined in all cases. Cyclotella biovolume concentration consistently declined at the two highest N/P ratios, indicating a higher P requirement, which is consistent with its distribution pattern.

4. In general, we found opposite trends with respect to all variables for Chaetoceros, a taxon that mainly inhabits sulphate systems, versus Cymbella, a bicarbonate‐associated taxon. Our results suggest that Cymbella may be found mainly in bicarbonate systems because of a higher C requirement, whereas Chaetoceros may be less abundant in these systems, in part, because of competitive interactions with bicarbonate‐associated taxa.
Predicting the spatial dominance of phytoplankton in a light limited and incompletely mixed eutrophic water column using the PROTECH model
Authors: ELLIOTT J. A. , IRISH A. E. , REYNOLDS C. S.
Published: March 2003

1. A phytoplankton community model [Phytoplankton RespOnses To Environmental CHange (PROTECH)] was used to examine the effect of a wide range of varied light intensities and mixed depths upon simulated phytoplankton populations. Two different column lengths of the simulated water body were examined (the upper 5 m and the whole 14.5 m water column) for each scenario.

2. The hypotheses tested were that: (i) under low light intensity and/or deep mixing the simulated community will be dominated by a phytoplankter with a low critical light intensity; (ii) at high light intensity and shallow mixing the simulated community will be dominated by small, fast‐growing phytoplankters; (iii) under all conditions, except deep mixing, the largest proportion of phytoplankton biomass will be found near the surface.

3. It was found under most conditions that, although there was a bloom in the upper column (dominated by algae such as Chlorella, Ceratium or Rhodomonas), the largest phytoplankton biomass in the water column was located 9 m below the surface and consisted of solely Asterionella. This bloom was missed by the 5‐m samples. Thus, using the whole column sample lengths, hypothesis (i) was not rejected but hypotheses (ii) and (iii) were refuted.

4. The inclusion of specific movement characteristics of phytoplankton in the model allowed the possibility of the dominance of multiple spaces within the water column and should be included in any model‐based investigation of this topic. Further, the results from the model suggest that a reduced depth of mixing creates greater environmental heterogeneity, allowing more species to persist.
Mortality and growth responses of Daphnia carinata to increases in temperature and salinity
Authors: HALL CATHERINE J. , BURNS CAROLYN W.
Published: March 2003

1. Daphnia carinata King, a freshwater cladoceran, occurs in Lake Waihola, a tidal lake with seasonal fluctuations in temperature (4–21 °C), and salinity (30–2000 mg L–1 Cl). We hypothesise that these fluctuations influence the seasonal changes in D. carinata abundance.

2. To test this hypothesis, adults and juveniles were exposed to combinations of temperature and salinity. We measured mortality of adults and juveniles, growth of juveniles over 20 days, and their age at first reproduction.

3. The salinity tolerance of adult D. carinata was temperature‐dependent. Juveniles were more sensitive to salinity, but more tolerant of temperature increases.

4. Growth rates were higher at elevated temperatures, but reduced by elevated salinities. The onset of reproduction was earlier at elevated temperatures.

5. We conclude that seasonal changes in temperature and salinity contribute to seasonal population changes of D. carinata.
Detritivores in Kenyan highland streams: more evidence for the paucity of shredders in the tropics?
Authors: DOBSON MIKE , MAGANA ADIEL , MATHOOKO JUDE M. , NDEGWA FIDENSIO K.
Published: March 2003

1. The relationship between coarse particulate organic matter (CPOM) standing stock and benthic macroinvertebrate assemblages in Kenyan highland streams was determined by sampling seven sites on three rivers (2000–2700 m a.s.l.). Taxa recorded were allocated to functional feeding groups using published literature, mouthpart analysis and examination of gut contents. Patterns were compared with five structurally similar streams in three areas of Europe (south‐west France, south‐east England, north‐east England).

2. Number of individuals and proportion of detritivores in Kenyan streams were equivalent to, or greater than, those in European sites. Shredders were, however, almost completely absent from Kenyan sites, despite high standing stocks of CPOM. Shredders were abundant in all European sites.

3. The phenomenon of low shredder abundance has been observed in other tropical streams in south‐east Asia and Central and South America but, in contrast to these regions, the African rivers studied were devoid of shrimps or fish which may occupy the shredding niche elsewhere.

4. These preliminary data suggest that shredder‐mediated detritus processing, which is a key functional component of streams in the North Temperate Zone, does not operate in East African streams. There are three possible reasons for this. The first is that tropical African rivers are functionally different to those in temperate regions. This could be because of enhanced microbial activity replacing shredder activity at high temperatures. Alternatively, it could be a result of low palatability of detrital inputs from dominant riparian trees in the region. The second and third are methodological: that our allocation to functional feeding groups is incorrect, and that our sampling methods missed a potentially key shredding taxon – the freshwater crab Potamonautes sp.
Groundwater controls on nutrient cycling in a Mojave desert stream
Authors: JONES JEREMY B.
Published: March 2003

1. Groundwater fluxes of nitrogen and dissolved organic carbon (DOC) were investigated in Grape Vine Canyon Stream in the Mojave Desert focusing on the rate of inputs and the fate of groundwater‐derived nutrients in the stream. Discharge rates from different ground waters were measured using an end‐member mixing model coupled with injections of a conservative solute tracer into the stream channel.

2. In surface water, nitrate concentration averaged 1.13 mg N L–1 and DOC concentration averaged 1.82 mg C L–1.

3. Groundwater discharge into Grape Vine Canyon Stream was derived from three sources. Nitrate concentration varied among the three groundwater sources with mean concentrations of 0.56, 0.94 and 0.08 mg N L–1. DOC, in contrast, did not vary among ground water sources, with an overall average concentration of 2.96 mg C L–1.

4. In the surface stream, nitrate concentration was two‐fold greater than the concentration predicted from groundwater input, indicating that in‐stream processes generated nitrate. Stream DOC concentration was lower than predicted based upon groundwater input rate. The production of nitrate and loss of DOC suggest that DOC is lost through mineralisation of dissolved organic matter, possibly resulting in the mineralisation of dissolved organic nitrogen to ammonium and subsequent transformation to nitrate via nitrification. In further support of this hypothesised linkage, DOC loss explained 80–89% of the variance in nitrate production in Grape Vine Canyon Stream.
Effects of riparian leaf dynamics on periphyton photosynthesis and light utilisation efficiency
Authors: HILL WALTER R. , DIMICK SARAH M.
Published: March 2003

1. Streambed light regimes change dramatically when riparian trees gain leaves in spring and lose them in autumn. This study examined the effect of these changes on periphyton photosynthetic characteristics, primary production, and light utilisation efficiency in two eastern Tennessee streams.

2. Photosynthesis–irradiance responses were measured at intervals covering leaf emergence and abscission in spring and autumn. Photosynthetic efficiency (αchl) increased with declining streambed irradiances during spring leaf emergence, but returned to pre‐emergence values after autumn leaf fall. The onset of photosaturation (Ik) displayed the opposite pattern, decreasing during leaf emergence and increasing after leaf fall. Both αchl and Ik were closely associated (P < 0.01) with daily integrated streambed irradiance, as were periphyton carotenoids. Internal shading by photoprotective carotenoids is hypothesised to account for lower αchl when streambed irradiances are high.

3. An in situ shading experiment confirmed that the temporal changes observed in periphyton photosynthetic characteristics and carotenoids were primarily the result of changing light levels and not other environmental factors (e.g. nutrients, temperature).

4. Daily chlorophyll‐specific primary production (PPchl) was calculated with P–I models and recorded streambed irradiances. In both streams, PPchl was the highest in early spring when trees were leafless, and then declined markedly as leaves emerged, reaching a minimum in summer. PPchl increased after leaf abscission, but was still lower than it was in early spring, when the sun was higher and daylength was longer. A hyperbolic tangent equation fit to PPchl and daily integrated irradiance (r2=0. 85) suggested that primary production was light saturated at 4–8 mol m–2 d–1.

5. Light utilisation efficiency (Ψ) increased 10‐fold during leaf emergence. Photosaturation at high irradiances and photoacclimation at lower irradiances were responsible for a negative hyperbolic relationship between Ψ and daily integrated irradiance.
Essential fatty acid content and the phosphorus to carbon ratio in cultured algae as indicators of food quality for Daphnia
Authors: PARK SANGKYU , BRETT MICHAEL T. , MÜLLER‐NAVARRA DÖRTHE C. , GOLDMAN CHARLES R.
Published: March 2003

1. A series of experiments examined growth of Daphnia magna on three algal diets (Rhodomonas minuta, Scenedesmus acutus and Synechococcus sp.) at varying physiological states [nitrogen and phosphorus (P) limitations] to test whether variation in algal fatty acid and/or elemental composition can predict Daphnia growth.

2. These algae differed widely in their essential fatty acid (EFA) composition while phosphorus (P) or nitrogen limitation had only a small influence on their ω3‐polyunsaturated fatty acid (PUFA) content.

3. Individually, algal ω3‐PUFA content explained 69% of the variation in the normalised growth of D. magna, while algal phosphorus content explained 11% of the variation. Quantitative models for D. magna growth used both algal ω3‐PUFA content and algal C : P ratio as food quality indices. Together, algal ω3‐PUFA content and C : P ratio explained 70% of the variation in the normalised growth rate of D. magna.

4. Our results indicate that EFA influenced algal food quality much more strongly than P. The EFA and mineral P impacts appear to be independent.