bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature‐induced changes in the life cycle of Leuctra nigra(Plecoptera: Leuctridae) from a Lake District stream
Authors: ELLIOTT J. M.
Published: May 2006

1. The life cycle of Leuctra nigra (Olivier) took 2 years in a small stream in the English Lake District and the exponential growth of the larvae was scarcely affected by variations in water temperature (range 4.2‐14.0°C). Mean growth rates for three year‐classes were 0.43±0.01, 0.42±0.01, 0.39±0.05% body length day−1. There were thirteen or fourteen larval instars for males and fourteen or fifteen for females. The ratio between successive instars was a constant 1.20 (conformed with Dyar's rule).

2. Larval growth and mortality were exponential at six constant temperatures (5.9, 8.2, 12.1, 15.8, 18.2, 19.8°C) in the laboratory. Mean growth rates (% body length day−1) increased directly with temperature from 0.37 (5.9°C) to 0.55 (19.8°C). Mean mortality rates (% day−1) increased directly with temperature from 0.20 (5.9°C) to 0.26 (12.1°C) and then markedly increased to 0.54‐0.58 at the three higher temperatures. Only 7‐10% of animals completed their life cycle at the three higher temperatures compared with 23–27% at the three lower temperatures. Egg production also decreased considerably at the higher temperatures.

3. As growth rates in the stream and laboratory were similar at similar temperatures (<14°C), the optimum conditions for growth in the laboratory were probably similar to those in the stream; therefore resources such as food and space were not restricting growth in the stream.

4. The implications of the temperature‐induced changes in growth and mortality are discussed and it is concluded that although the life cycle can change from semivoltine to univoltine with increasing temperature, the costs of a univoltine life cycle are high in terms of survival and egg production, both of which decreased markedly between 12.1 and 15.8°C. Therefore the optimum habitat for this species appears to be a summer cool stream (maximum temperature <14°C) and the optimal life cycle appears to be about 2 years from egg to adult.
Resource utilization by the freshwater deposit feeder Ptychoptera townesi (Diptera: Ptychopteridae)
Authors: MATTINGLY ROSANNA L.
Published: May 2006

SUMMARY 1. False crane fly larvae, Ptychoptera townesi (Diptera), occurred in high densities in a flow‐controlled section of stream where fine particulate organic matter (FPOM; 0.45 μm to 1 mm in diameter) had accumulated, but were quite rare both upstream and downstream from the section.

2. In laboratory studies, P. townesi grew only on FPOM less than 250 μm. Larvae consistently grew fastest when fed small particles (0.45–53 μm in diameter).

3. Ptychoptera townesi consumed relatively small amounts (0.002 mg per mg animal dry mass day−1) of FPOM (0.45–53 μm). They had long gut content passage times (greater than 19 h) and relatively high efficiencies of conversion of ingested food to body substance (20.7%). Gut content passage times were variable, and depended partially on the nature of the substrate.

4. False crane fly larvae compacted FPOM into faecal pellets considerably larger in size than particles ingested. They lost mass when allowed to feed on their own faecal material, as well as on faeces greater than 250 μm in diameter produced by shredders. However, they survived and grew on shredder faeces (53–500 μm in diameter) that contained a mixture of smaller particles and particles too large for ingestion.

5. The overall pattern of resource utilization by P. townes involved slow handling of relatively small volumes of food, which probably passed once only through a complex alimentary tract.
Population dynamics and production of five crustacean zooplankters in a subtropical reservoir during years of contrasting turbidity
Authors: HART R. C.
Published: May 2006

SUMMARY 1. The temporal dynamics and demography of Meta‐diaptomus meridianus (Van Douwe), Lovenula excellens Kiefer. Daphnia gibba Methuen, D. barbata Weltner and Moina brachiata Jurine were studied for 2 years in a small bay of Lake le Roux (Orange River, South Africa). Total zooplankton biomass and population density were 1.4–3 times higher during the less turbid conditions of 1982/83 (Secchi depth transparency around 35 cm) than they were at around 25 cm Secchi depth during 1981/82, when D. barbata was absent.

2. On average, instantaneous birth rates, rates of population change and death rates varied only slightly between years. Birth and death rates were considerably higher above 15°C than below 15°C. These rates correlated with one another and with zooplankton abundance both inter‐ and intra‐specifically suggesting that competitive interactions were important in population regulation. Mortality rates varied more strongly and consistently in a density‐dependent direction than did birth rates. In addition to depressed fecundity, the inferred survival of young was poor and population growth low, possibly because food shortage caused high post‐natal mortality.

3. Estimates of annual production derived from finite birth rate values varied consistently with annual differences in biomass, and amounted to between 6 and 10 g m−2 y−1 dry wt. Annual P/B values varied from around 20 for the daphnids to 55 for the copepods and 75 for Moina. Apart from the latter, whose annual P/B ratio virtually doubled from 45 to 75 following reductions in turbidity, annual differences in P/B ratio were slight.
The behaviour and form of Psephenus herricki (DeKay) (Coieoptera: Psephenidae) in relation to water flow*
Authors: McSHAFFREY D. , McCAFFERTY W. P.
Published: May 2006

1. Psephenus herricki (DeKay) larvae were observed in aquaria and artificial streams using videomacroscopic techniques. Dye visualization was used to determine flow around the organisms.

2. The highly flattened larvae of Psephenus herricki are able to withstand and/or avoid the force of flowing water in streams through a number of mechanisms. The legs are vital: if they cannot gain purchase the larvae can be dislodged by the lifting forces acting on the body. At the flow rates tested (up to 50 cm s−1), suction under the carapace is not needed to maintain position.

3. Active pumping of water through lateral slots of the carapace and out from under the body posteriorly greatly reduces turbulence around the body and may act to decrease drag at high Reynolds numbers.

4. An interpretation of the flattened body‐shape of Psephenus herrickilarvae as an adaptation to fast currents and/or to prevent the larvae from being removed from the substrate by a predator is of questionable ecological significance because individuals are usually found in crevices and on the undersides of stones.
Levels of protein, RNA, DNA, glycogen and lipid during growth and development of Daphnia magna Straus (Crustacea: Cladocera)
Authors: McKEE MICHAEL J. , KNOWLES CHARLES O.
Published: May 2006

1. Protein, RNA, DNA, glycogen and lipid content were determined in Daphnia magna on days 0, 2, 4, 6, 8 and 21 of growth and development. The composition of D. magna as percentage of reconstituted dry weight was similar to other zooplankton with the exception of DNA content, which was lower than values previously reported.

2. The relative content of protein, RNA, DNA and reconstituted dry weight changed during the 21‐day growth period, and these changes were related to growth rate and total growth of D. magna. RNA:protein, RNA: reconstituted dry weight, and protein:RNA : DNA ratios were highly correlated to relative growth rate and total growth as measured by protein content or reconstituted dry weight.

3. Addition of progeny biomass to adult biomass increased correlations between biochemical ratios and absolute growth rate, but had little effect on relationships involving relative growth rate or total growth.

4. The relationship between biomolecule ratios and growth established for D. magna grown under optimal conditions was not successful in predicting growth of D. magna reared under crowded conditions.

5. These data indicate that variation in biochemical ratios among life, stages of D. magna may be used to predict growth of organisms grown under similar conditions, but may not be extended to other situations. It is suggested, however, that variation in biochemical ratios in a particular life stage of a zooplankton species may be related to the productivity for that species.
Cladoceran birth and death rates estimates: experimental comparisons of egg‐ratio methods
Authors: GABRIEL WILFRIED , TAYLOR BARBARA E. , KIRSCH‐PROKOSCH SUSANNE
Published: May 2006

I. Birth and death rates of natural cladoceran populations cannot be measured directly. Estimates of these population parameters must be calculated using methods that make assumptions about the form of population growth. These methods generally assume that the population has a stable age distribution.

2. To assess the effect of variable age distributions, we tested six egg ratio methods for estimating birth and death rates with data from thirty‐seven laboratory populations of Daphnia pulicaria. The populations were grown under constant conditions, but the initial age distributions and egg ratios of the populations varied. Actual death rates were virtually zero, so the difference between the estimated and actual death rates measured the error in both birth and death rate estimates.

3. The results demonstrate that unstable population structures may produce large errors in the birth and death rates estimated by any of these methods. Among the methods tested, Taylor and Slatkin's formula and Paloheimo's formula were most reliable for the experimental data.

4. Further analyses of three of the methods were made using computer simulations of growth of age‐structured populations with initially unstable age distributions. These analyses show that the time interval between sampling strongly influences the reliability of birth and death rate estimates. At a sampling interval of 2.5 days (equal to the duration of the egg stage), Paloheimo's formula was most accurate. At longer intervals (7.5–10 days), Taylor and Slatkin's formula which includes information on population structure was most accurate.
What is cyclomorphosis?
Authors: BLACK R. W. , SLOBODKIN L. B.
Published: May 2006

1. The history of the term cyclomorphosis and of research investigating the phenomenon it describes are reviewed briefly.

2. Several distinct definitions of this term appear in the limnological literature. Some of these definitions are consistent with earlier studies; others are not and are very restrictive and reliant upon extensive laboratory analysis.

3. We argue for the adoption of a broad definition of cyclomorphosis that is consistent with the majority of previous studies and practical to apply to the field populations it attempts to describe.

4. We also urge all researchers investigating this phenomenon to provide their working definition of this term in their reports in order to avoid terminological confusion.
Spatial distribution of denitrifying activity in a stream draining an agricultural catchment
Authors: COOKE JAMES G. , WHITE ROBERT E.
Published: May 2006

SUMMARY 1. A field study was made of the spatial distribution of denitrification activity in the sediment of the River Dorn, Oxfordshire, England.

2. An assay of denitrifying enzyme activity was used to examine the distribution of denitrification with depth in cores of sediment representative of the types found in the stream. The maximum activity recorded in a predominantly silt sediment core was 5 times greater than that recorded in a sandy gravel core. In both fine sand and silt cores, peaks in denitrifier enzyme activity were shown to correspond to the limit of the nitrate diffusion front. At this depth the redox potential dropped rapidly from + 300 mV to 0 or less. Denitrifying enzyme activity in the stream water was negligible.

3.In situdenitrification activity (I DA) measurements were carried out in an 800 m reach of the Dorn using the acetylene inhibition technique on small sediment cores. Concurrent measurements were also made of stream depth and velocity, nitrate concentration in the interstitial water, and the wet bulk density, loss on ignition, mineraliz‐ able carbon and total nitrogen contents of the sediment. Mineralizable carbon was the variable which showed the best correlation with I DA. Highest IDAs were associated with accumulations of fine‐grained sediment at meander bends. Mean IDAs measured under flood conditions were significantly higher (P<0.05) than those measured under baseflow. It was estimated that denitrification reduced the nitrate load in the River Dorn by 15% under summer baseflow conditions
The relation between wave exposure and distribution of emergent vegetation in a eutrophic lake
Authors: WEISNER STEFAN E. B.
Published: May 2006

SUMMARY 1. Maximum water depth penetration and changes in horizontal distribution during 39 years of the emergent vegetation in a eutrophic lake in southern Sweden were investigated.

2. The capacity of the emergent vegetation to penetrate into deeper water areas was found to be higher at wave exposed than al sheltered sites.

3. Differences in biomass and biomass allocation of the dominant speeies. Phragmites australis, between an exposed and a sheltered site suggest that horizontal expansion towards deeper water at sheltered sites is limited by unfavourable substrate conditions.
Basis of selective predation by the aquatic larvae of the salamander,Ambystoma tigrinum
Authors: LEFF LAURA G. , BACHMANN MARILYN D.
Published: May 2006

SUMMARY.1. The effect of several factors on feeding selectivity by three size classes of larvae ofAmbystoma tigrinum(Green) was examined. Five types of invertebrate prey were used:Chaoborus, Daphnia, damselflies,Hyalellaand water boatmen.

2. Five variables were examined in relation to the electivity indices associated with each prey type: ease of prey consumption, ease of prey capture, relative prey size, prey activity, and microhabitat overlap. No single variable explained the observed electivity.

3. For small larvae (snout‐vent length, SVL, <2.5cm) relative prey size was correlated with electivity. For large larvae (SVL >6.5cm), ease of prey consumption, prey activity, and relative prey size predicted electivity values. However, none of the measured variables was related to electivity values found for medium sized larvae (SVL >3.5 and <5.5cm).
Aspects of mating, reproduction, and co‐occurrence in three freshwater calanoid copepods
Authors: CHOW‐FRASER PATRICIA , MALY EDWARD J.
Published: May 2006

SUMMARY. 1. We examined the seasonal distributions and mating behaviour of Epischura lacustris, Diaptomus minutus and Diaptomus oregonensis to evaluate several proposed mechanisms for reproductive isolation. There was no divergence in onset of maturity or timing of breeding periods between sympatric diaptomid populations; size displacement between co‐occurring diaptomid species did not result in concomitant reduction in mating frequency between heterospecific pairs in the laboratory.

2. Divergence in mating behaviour was supported by observations that, in conspecific mating experiments, males of both diaptomid species discriminated between females bearing ripened ovaries (gravid) and those that did not (non‐gravid), and that, in heterospecific trials, males either mated indiscriminantly with all females, or engaged in mating very infrequently.

3. In mating experiments with Epischura. males did not discriminate between gravid and non‐gravid females, but mated exclusively with virgins of a specific relative size. Divergence in timing of breeding seasons between Epischura and the two diaptomid species suggests that even if mating between genera is possible, it does not occur often in nature.

4. We discuss the adaptive significance of different reproductive strategies of these three copepods and speculate on mechanisms that allow for coexistence of the closely related taxa in nature.
Zooplankton feeding rates in relation to suspended sediment content: potential influences on community structure in a turbid reservoir
Authors: HART R. C.
Published: May 2006

SUMMARY. 1. Changes in zoopiankton composition and abundance in Lake le Roux, a turbid subtropical reservoir on the Orange River in South Africa, were correlated with changes in water transparency (related to suspended sediment levels) during a 7 year field study. Results of radiotracer studies of the effect of mineral turbidity on zooplankton feeding rates which potentially influence competitive ability, and thus community structure, are reported here.

2. Feeding rates of five zooplankters were very variable, but consistently declined with rising turbidity: rates of decline differed between species. A regression estimate of the critical turbidity threshold at which food intake matched the estimated respiratory need was derived for each species. This yielded the following “turbidity‐tolerance” ranking: Moina brachiata Jurine > Metadiaptomus meridianus (van Douwe) ×Daphnia gibba Methuen > D. barbata Weltner > D. longispina O. F. Muller. The consistency between this ranking and one based upon abundance‐transparency relationships in the field study suggests that community structure is related to differential feeding capabilities, although other influences are not excluded.

3. Tests on D. gibba and M. meridianus failed to reveal any detectable feeding rate saturation (incipient limiting food level) below 1.2mg 1−1 C. The relative reduction in feeding rates at elevated turbidity was nearly 3 times greater for the daphnid than the copepod over a range of food concentrations, and considerably reduces the competitive ability of this (and other) daphnids. The turbidity tolerance disparity between Moina and the daphnids demonstrates a more complex situation than a simple copepod/cladoceran dichotomy. These findings and their implieations are discussed in relation to wider features of zooplankton ecology.
Relative vulnerability of six freshwater gastropods to the leech Nephelopsis obscura (Verrill)
Authors: BROWN KENNETH M. , STROUSE BRIAN H.
Published: May 2006

SUMMARY. 1. The vulnerability of six species of freshwater snails to the leech Nephelopsis obscura was determined in laboratory predation experiments.

2. Nephelopsis was unable to prey on prosobranch snails with an operculum, but did consume certain pulmonate species though predation rates were low, ranging from one to two snails per leech per night at 20°C. Apparently, Nephelopsis does not actively select prey, but merely consumes those species most easily handled.

3. Among pulmonates, leeches captured species in the following order: Physa gyrina (Say), Helisoma anceps (Menke). Lymnaea emarginata (Say) and Helisoma trivolvis (Say). Susceptibihty of size classes varied among snail species.

4. Neither environmental structure, such as cobble or macrophytes, nor gastropod escape behaviors, such as clamping to substrates or vertical migration away from bottom‐feeding leeches, lowered capture rates by Nephelopsis.

5. In 2 years sampling, only one of seven lakes, however, had appreciable abundances of Nephelopsis. It is therefore suggested that the rarity, low feeding rates, and lack of strong gastropod preference limit the effect of Nephelopsis on field gastropod distributions.
The chemical ecology of some British freshwater gastropod molluses: behavioural responses to short chain carboxylic acids and maltose
Authors: DALDORPH P. W. G. , THOMAS J. D.
Published: May 2006

SUMMARY.1. The behavioural responses of six species of British freshwater gastropods to chemical gradients of short chain carboxylic acids (C‐2 to C‐5, C‐8) and maltose were investigated by means of diffusion olfactometers.

2. The species were ranked as follows on the basis of the number of significant behavioural responses to the test chemicals:Lymnaea peregra>Planorbis contortus > Physa fontinalis > Planorbis planorbis > Bithynia tentaculata > Planorbis vortex.These differences were not a result of differences in activity but reflect real differences in ehemoreception.

3. Propanoate (C‐3) was a significant attractant or arrestant to five snail species, C‐4, C‐8 and maltose to four species and C‐5 to three species. Acetate (C‐2) was a significant attractant toL. peregraandP. fontinalisbut it was the only acid to act as a repellent, toP. planorbisandB. tentaculata.

4.Lymnaea peregracontinued to respond to repeated butanoate application. Response levels could be enhanced by increasing concentration (minimum thresholds 10‐4M to 10−5M), pH (above 6) and food deprivation.

5. The ecological relevance of the results are discussed with particular reference to the distributional patterns of the chemicals and snails and the feeding niches of the latter.
Effects of low pH and humus on the survivorship, growth and feeding of Gammarus pulex (L.) (Amphipoda)
Authors: HARGEBY A. , PETERSEN R. C.
Published: May 2006

SUMMARY. 1. The toxicity to the freshwater amphipod Gammarus pulex (L.) of soft water (pH6.0) with three concentrations of XAD‐extracted aquatic humus was treated in the laboratory.

2. Exposure to pH6.0 water without humus added resulted in 92% mortality after 3 weeks. Humus, added at concentrations of 7 and 20 g Cm 3, decreased the mortality to 80% and 64% in the same period.

3. Surviving animals kept at pH6.0 had a lower growth rate, lower food conversion efficiency and higher body water content than animals kept at pH 7.3.

4. Humus had no significant effects on growth, food conversion or body composition of G. pulex kept at pH 6.0. However, there was a tendency for growth and food conversion to increase, and for body water content to decrease with increased humus concentration. The effects of humus on growth and food conversion of G. pulex observed in this study do not support the contention that humus acts as a free coupler to lower metabolic efficiency. On the contrary, humus tended to benefit food conversion.

5. It is suggested that low concentrations of humus can be directly beneficial to organisms in acidified water in ways other than by complexing toxic metals.
The relative importance of algae and bacteria as food for Daphnia longispina (Cladocera) in a polyhumic lake
Authors: KANKAALA PAULA
Published: May 2006

SUMMARY. 1. Grazing on algae and bacteria by the planktonic cladoceran, Daphnia longispina, was studied in a small polyhumic lake with low phytoplankton primary production in southern Finland.

2. D. longispina filtered algae at average rates of 0.09‐0.82 ml ind−1 h−1. The filtering rates on bacteria were 26‐92% of those on algae in parallel experiments.

3. From June to August algae, including mixotrophic and heterotrophic forms, comprised 56‐93% of the food ingested by D. longispina. In mid‐September and early October, when the Daphnia population was declining and the algal biomass was low, bacteria comprised 73% and 55% respectively of the food of Daphnia.

4. For D. longispina, an energy pathway via bacteria and bactivorous flagellates is probably a more important link to allochthonous organic matter than direct utilization of epilimnetic bacteria.
Dissolved organic matter in Cumbrian lakes and streams
Authors: TIPPING E. , HILTON J. , JAMES B.
Published: May 2006

SUMMARY. 1. A survey of dissolved organic carbon concentration [DOC] and optical absorbance at 340 nm (A340) was carried out for thirteen lakes and twenty‐three streams in Cumbria over the period May 1986 to April 1987.

2. Values of [DOC] were in the range 0‐6.2 mg 1‐1‐1, no water having a mean [DOC] greater than 3.6mgI‐1. Streamwaters showed no gross seasonal dependence in [DOC], but there was a peak in [DOC] associated with the first major rain event of autumn 1986. Summer values of [DOC] in lakes were greater than winter values, probably because of the production of DOC by phytoplankton.

3. In streams, [DOC] was weakly positively correlated (r2 <0.3) with discharge (as estimated from rainfall); A340 was more strongly correlated (0.4 < r2 < 0.6). Linear regression of mean A340 against mean [DOC] for streams gave a slope consistent with the absorptivities of aquatic humic substances, and a positive intercept on the abscissa that suggests the presence of, on average 0.6 mg 1‐1 of DOC that does not absorb at 340 nm. Lake samples taken in winter gave a similar A.340/[DOC] relationship to that for streams, but in summer A340/[DOC] ratios were lower, suggesting that phytoplankton‐derived [DOC] absorbs weakly or not at all at 340nm.
Competition for space by predators in streams: field experiments on a net‐spinning caddisfly
Authors: LANCASTER JILL , HILDREW ALAN G. , TOWNSEND COLIN R.
Published: May 2006

SUMMARY. 1. Field experiments in a fishless stream were carried out on an abundant caddisfly with a predatory, net‐spinning larva, Plectrocnemia conspersa (Curtis), to assess whether net site availability affects their microdistribution.

2. Net sites were supplemented by adding nought, one or four artificial structures to replicated patches on the stream bed. In each of three experiments at different seasons (summer, autumn and late winter), caddis densities increased significantly in patches with extra net sites.

3. The response of caddis to supplemented net sites could be affected by the subsidiary effects of food and offish. These potential interactions were assessed in each experiment by varying net site density in two additional treatment stretches in which (1) prey abundance was increased by releasing Daphnia, and (2) brown trout (Salmo trutta L.) were enclosed. The responses of caddis in these two treatments were compared to that in the reference stretch, where only net site density varied.

4. Increased food abundance enhanced the response of caddis to net site supplementation in winter, when natural prey was least abundant, but not in summer or autumn. We suggest that extra food affects the mechanism determining net building only when prey availability is below some threshold.

5. The presence of fish precluded any effect of extra net sites in summer, but had no effect in autumn (the winter fish treatment was lost). We suggest fish predation reduced the densities of caddis in summer, so that net sites no longer limited local densities. In autumn, fallen leaves provided refugia from fish, which consequently were less effective predators of P. conspersa.
Beluga whale ecology: a concern of freshwater biologists?
Authors: WATTS P. D. , DRAPER B. A.
Published: May 2006

SUMMARY.1. Churchill estuary beluga whales displayed their highest densities in freshwater habitat, suggesting preferential use.

2. Whale densities in freshwater were as high as 350 animals km−2and represented over one tenth of the estimated Hudson Bay population.

3. Future environmental assessments of hydroelectric development in northern Manitoba, Canada, must consider the effects on the beluga whales.
Very high secondary production at a lake outlet
Authors: WOTTON R. S.
Published: May 2006

SUMMARY. 1. Larvae and pupae of Simulium noelleri Fried, coated the concrete of parts of an artificial lake outlet in southern England.

2. In the first two (of three) summer generations, development was synchronous and this allowed the calculation of their secondary production by the instantaneous growth method. The production of the two summer generations was, respectively, 229.1 g C m−2 (7.4 g C m−2day−1) and 185.5 g C m−2 (8.8 g C m−2 day−1) The contribution of the third summer generation, and the overwintering generation, to annual production would be less than that of the first two summer generations. Nevertheless, annual production will have exceeded 500 g C m−2 at this site.

3. Larvae are suspension feeders and they captured the rich supply of particulate and dissolved organic material which passed over them after export from the lake. As food is brought to the larvae they only require space for attachment and can thus build up very high population densities (which exceeded 1 × 106 m−2 on some occasions during the summer). The high population densities result in a high biomass and hence in the high levels of production.
The abundance of phytophilous invertebrates on different species of submerged macrophytes*
Authors: CYR HÉLÈNE , DOWNING JOHN A.
Published: May 2006

SUMMARY. 1. We tested Krecker's model (1939) which states that the abundance of invertebrates per unit macrophyte biomass varies with plant species and is higher on plants with finely dissected leaves than on plants with broad leaves. The abundance of invertebrates was measured in thirteen lacustrine macrophyte beds in southern Québec, Canada. The model was tested for the total abundance of invertebrates and for the abundances of Chironomidae, Cladocera, Cyclopoida, Gastropoda, Hydracarina, Ostracoda and Trichoptera.

2. More epiphytic invertebrates were found on the dissected Myriophyllum spp. than on the broad‐leaved Potamogeton amplifolius Tuckerm, P. robbinsii Oakes and Vallisneria americana Michx. (P<0.01). More invertebrates were also found on P. amplifolius than on P. robbinsii or V. americana (P<0.01). The total abundance of invertebrates was not systematically related to the degree of plant dissection.

3. The abundances of Chironomidae, Cladocera, Cyclopoida, Gastropoda, Hydracarina, Ostracoda and Trichoptera varied on different plant species (P<0.01). Contrary to Krecker's hypothesis, however, macrophytes with finely dissected leaves (Ceratophyllum demersum and Myriophyllum spp.) did not in general support more invertebrates per unit plant biomass than plants with large leaves (Potamogeton amplifolius, P. robbinsii and Vallisneria americana).
Limnological imbalances: an antipodean viewpoint
Authors: WILLIAMS W. D.
Published: May 2006

SUMMARY. 1. It is argued that modern limnology is excessively concerned with the study of fresh waters in the northern temperate region. Limnology has been and is unbalanced in its interests and emphases and, as a result, many widely held limnological concepts need revision.

2. The genesis of the argument is outlined. It involved the initial assumption that Australia was limnologically distinctive. Later, it was realized that since most of the world is arid, semi‐arid, tropical or semi‐tropical, Australia was much less distinctive than originally thought.

3. Other limnological assumptions considered in the paper concern saline lakes and salinization, the importance and nature of temporary bodies of standing fresh water, biological diversity in tropical fresh waters, the evolutionary importance of permanent freshwater lakes, and the global applicability of the River Continuum Concept. Consideration suggests that some widely‐held views on these matters are erroneous.

4. Thus, saline lakes are more important (scientifically, commercially), widespread and numerous environments than is generally thought. Salinization is a significant environmental hazard for many rivers in semiarid regions. Temporary bodies of standing fresh water are geographically widespread and abundant aquatic environments with high species diversity, and in which many species have restricted distributions and low dispersal abilities. Biological diversity appears to be low in tropical fresh waters. Permanent freshwater lakes are perhaps less important loci for the evolution of the freshwater biota than generally thought, temporary fresh waters more so. Finally, the River Continuum does not seem to be applicable to many running waters outside the north temperate region.

5. A number of suggestions are made of how a better balance in limnology may be arrived at: how a more globally comprehensive intellectual framework may be built. Suggestions include: an increased awareness of the frequently different natures of waters within and beyond the northern temperate region; recognition of the potential benefits to be gained from the study of waters outside the northern temperate region;
Prey selection by the dobsonfly larva, Protohermes grandis (Megaloptera: Corydalidae)
Authors: HAYASHI FUMIO
Published: May 2006

SUMMARY 1. Prey selection by the dobsonfly larva, Protohermes grandis (Thunberg), was studied in stony riffles of the Yataro River, central Japan. The density, size distribution and taxonomic composition of available prey were assessed for 2 years. In order to know the encounter rate between prey and this ambush predator, prey mobility was also estimated from patterns of colonization of experimentally detiuded stones.

2. Foregut analyses revealed that maximum size of prey eaten increased with larval size, and large larvae did not take the smallest prey in spite of high availability in all seasons.

3. Charnov's (1976) optimal diet model quantitatively predicted such size‐selective feeding under seasonally fluctuating conditions of water temperature and prey availability. Larvae maximized the feeding rate by selecting prey.

4. Maximum width of prey eaten coincided approximately with larval mandible length. Mandible size seemed to play an important role in the selection of prey in the optimal size range.
Life histories of some species of Chydoridae (Cladocera: Crustacea)
Authors: ROBERTSON ANNE L.
Published: May 2006

SUMMARY 1. Life history parameters of two species of Chydoridae (Leydigia leydigi and Disparalona rostrata) were studied in the field and in the laboratory.

2. For both species the number of juvenile instars was variable and tentatively related to initial size of neonate. Egg volume increased with increasing parent length and with decreasing temperature; possible advantages gained by the offspring are discussed in relation to invertebrate predation.

3. The life history strategy employed by the littoral Chydoridae is compared to that of large and small planktonic cladocerans. Small planktonic cladocerans and the Chydoridae (except the Eurycercinae and the Saycinae) produce large young relative to their size at maturity and mature early. However, unlike the small planktonic cladocerans, growth in the Chydoridae is curtailed after the onset of reproduction and in this they resemble the large planktonic cladocerans. This strategy may be related to vertebrate predation and the presence of vegetation in the habitat.
The loss of submerged plants with eutrophication II. Relationships between fish and zooplankton in a set of experimental ponds, and conclusions
Authors: IRVINE KENNETH , MOSS BRIAN , BALLS HILARY
Published: May 2006

SUMMARY. 1. In 1982 and 1983 sets of experimental ponds were left with their submerged plant communities intact (plant ponds) or were cleared manually of them (cleared ponds). The ponds were all fertilized with ammonium nitrate and with variable amounts of phosphate. In 1982 fish were removed from the ponds. Zooplankton communities were dominated by large Cladocera with Daphnia prominent in the cleared ponds and Simocephalus in the plant ponds. There was no detectable effect of differential phosphorus additions on zooplankton communities or populations.

2. In 1983 zooplanktivorous fish (mainly roach) were stocked in the ponds. In the plant ponds the fish did not survive, probably through severe deoxygenation and the zooplankton community again included large‐bodied Simocephalus. Fish survival was variable in the cleared ponds. Where fish stocks were absent or low (0.5–1 g m−2) a Daphnia‐ dominated community persisted; at intermediate fish stocks (18.1 g m−2) Eudiaptomus gracilis was predominant and where fish stock was high (22.8–29.1 g m−2) Bosmina longirostris, and cyclopoid copepods dominated the communities. Mean biomass of the zooplankton community declined with increase in fish stock to between 5.1 and 18.1 g m−2 then increased.

3. On the basis of results from the experimental ponds and elsewhere, a new hypothesis is put forward to account for the switch from aquatic plant to phytoplankton dominance in eutrophicated shallow lakes. It envisages dominance by either group to be possible as alternative states over a wide range of high nutrient loadings. It suggests that each state is buffered against increased loading by mechanisms involving plant and algal physiology and zooplankton grazer populations. The nature of the buffers and the reasons by which one state may be switched to the other are, discussed.
The loss of submerged plants with eutrophication III. Potential role of organochlorine pesticides: a palaeoecological study
Authors: STANSFIELD JULIA , MOSS BRIAN , IRVINE KENNETH
Published: May 2006

SUMMARY. 1 Mechanisms which might have caused a switch from submerged plant dominance to phytoplankton dominance in a series of shallow lakes, the Norfolk Broads, during the 1950s and 1960s, are reviewed. It is argued that a likely mechanism was a poisoning of the community of Cladocera which graze on algae and are associated with the plants. This allowed phytoplankton to take advantage of the increased nutrient loadings and to increase. The 1950s and 1960s were periods of liberal organochlorine pesticide use. Cladocera are particularly susceptible to organochlorine toxicity.

2. Sediment cores were taken from Hoveton Great Broad, in which the switch to phytoplankton has occurred, and from two Broads (Upton and Martham South Broads) in which submerged plants are still dominant. Analyses were made for cladoceran remains, diatoms and molluscs. Cross dating with previously taken dated cores allowed time‐scales to be applied.

3. In Hoveton Great Broad, a switch from clear‐water‐associated chydorids to turbid‐water forms was found to coincide with the loss of aquatic plants in the 1950s. No Daphnia remains were found. In the other two Broads, clear‐water chydorids, mostly plant‐associated, were found throughout the cores.

4. Residues of dieldrin (HEOD), DDD and TDE were found in the Hoveton Great Broad cores, with the DDT derivatives particularly associated with the end of the phase of submerged plant dominance and the beginning of that of phytoplankton dominance. Bosmina remains became more abundant after this point. Pesticide residues were scarce in the cores from the other two Broads and did not form any particular pattern. Residues of polychlorinated biphenyls were widespread in all cores.

5. Calculation of DDT concentration in the water at the time the sediments with greatest concentrations of residue were laid down suggests that concentrations above typical LC50 values for Daphnia species could have been present.
Energy maximization and foraging strategies in Potamon fluviatile (Decapoda, Brachyura)
Authors: GHERARDI FRANCESCA , TARDUCCl FEDERICA , MICHELI FIORENZA
Published: May 2006

SUMMARY. 1 Foraging of the freshwater crab Potamon fluviatile was studied by recording the activity of seventy‐eight specimens in a 50 m2 pool in a Tuscan stream during early summer. Foraging was related both to the organic content of the substrate and to the crabs’ oxygen consumption. During this period, adult females underwent “second vitellogenesis”. with abundant deposition of yolk in oocytes.

2. A dispotic distribution (not accompanied by agonistic interactions, but ‘peacefully’ based on size) was observed within the foraging area. Larger animals (mostly males) fed on the rare patches of vegetable debris, which presented the highest organic content. Conversely, smaller specimens were relegated lo the poorer substrates, such as the stream banks.

3. The females extended and diversified their foraging areas by also venturing into terrestrial habitats, in contrast to the more sedentary and aquatic males. This behaviour (which was not accompanied by a different energetic output) resulted in a more proteinaceous diet (even when the N‐content of vegetable debris fell drastically), and in a significant increase in fats and the hepatopancreas index.

4. The reserves of energetic substances are presumed to sustain the highly expensive vitellogenesis, with the production of macrolecithal eggs. The females’ behaviour as energy maximizers’ seemed to be under a strong selective pressure, since their reproductive success is directly related to the efficient harvesting of food.
The effect of growth pattern, sampling interval and number of size classes on benthic invertebrate production estimated by the size‐frequency method
Authors: MOTHIVERSEN TORBEN , DALL PETER
Published: May 2006

SUMMARY. 1. A hypothetical leech population with known initial density, initial weight, final weight and cohort production interval (CPI) was established. Production estimated by the size‐frequency method for various growth patterns, mortalities, number of samples per CPI and number of size classes was compared with actual production estimated from daily growth and mortality by the increment‐summation method. The population had either perfectly continuous reproduction or a perfectly synchronous cohort.

2. When size‐classes were delimited in order to equalize the time spent in each size class, the deviations from actual production increased with decreasing number of size‐classes and increasing mortality. For a population with perfectly continuous reproduction, production was only overestimated by 32% with an extreme mortality of 2.0% day−1 and three size‐classes. For a perfectly synchronous cohort, production was either underestimated or overestimated, depending on the first day of sampling. The deviations from actual production increased considerably with decreasing number of size‐classes, increasing mortality and decreasing number of samples per CPI.

3. Differences between actual and assumed growth patterns may give underestimates or overestimates of more than one order of magnitude at high mortalities and few size‐classes. It is concluded that knowing the actual growth pattern, the size frequency method will give realistic estimates of production in cases when normal cohort methods cannot be used. The estimate can be improved significantly by increasing the number of size classes and the number of samples per CPI.
Life‐history patterns of the mosquito‐fish, Gambusia affinis, in the estuary of the Guadalquivir river of south‐west Spain
Authors: FERNÁNDEZ‐DELGADO C.
Published: May 2006

SUMMARY. 1. The age, growth and reproduction of the small, introduced fish Gambusia affinis (Baird & Girard, 1853) were studied in the estuary of the Guadalquivir river.

2. The life‐span was very short, the stock contained only two age groups: with annulus (1+ group; 10–12 months old) and without annulus (0+ group).

3. In both sexes growth restarted in April when the annulus appeared, but whilst 1+ males stopped growth, 1+ females grew steadily to June. Growth of 0+ spawners was only evident in September, the last month of the reproductive period. A differential growth rate between sexes was also evident.

4. 1+ specimens reproduced during May and June and their offspring from July to September. In both age groups somatic condition progressively declined during the spawning period.

5. The loss of condition and the disappearance of 1 + and the larger 04‐specimens after reproduction may indicate the cost of a prolonged high level of reproductive effort.

6. The total fecundity (taken as the number of embryos) of 1 + females was represented by the formula: Fec=5.08 T.L. (mm) ‐170.07 and that of 0+ specimens by: Fec=2.23 T.L. (mm) ‐42.92. The maximum average monthly fecundity was reached in June when the length of the mother was at its greatest.

7. Length at first maturity was smaller in 0+ group than in the 1 + group; the difference between the two groups was greater in males (≅5 mm, T.L.) than in females (≅3 mm, T.L.). Also the average total length of 14‐spawners was greater than 0+ spawners. There were significant differences in the overall sex ratio of 956 males to 2057 females.

8. The differences found in growth and reproduction between the two age groups suggest that life‐history tactics may vary not only between different stocks but also within the same stock among its different components.
Comparisons of the diets and reproductive performances of two sympatric rotifers, Asplanchna girodi and Asplanchna priodonta
Authors: SALT GEORGE W.
Published: May 2006

SUMMARY. 1. Diet composition and the production of embryos were measured in Asplanchna girodi and Asplanchna priodonta collected through 3 years in the San Joaquin‐Sacramento delta of California. The two species are sympatric in low to moderate densities for 5 months of the year.

2. Despite marked differences in the structure of the trophi, both species consumed the same array of prey species. Percentage composition of the diet varied both intra‐ and interspecifically. The food niches of the two species appear to differ little from one another.

3. Individuals of Asplanchna girodi had a larger mean number of prey in the gut per individual than did those of A.priodonta, due to their larger size. Per unit volume, each species prey gathering performance was the same. Asplanchna priodonta produced more embryos per calculated unit volume of prey ingested than did A.girodi.

4. During spring, when both species were present in the plankton, A.priodonta did not capture Size Class 2 and 5 animals (volumes 0.09–1.0 μm3×106 and 3.0–3.5 μm3X106 respectively) nor large Synchaeta. Both these prey classes were in the diet during the summer and autumn months. Several hypotheses are examined as possible explanations for this phenomenon.
Periphyton responses to invertebrate grazing and riparian canopy in three northern California coastal streams
Authors: FEMINELLA JACK W. , POWER MARY E. , RESH VINCENT H.
Published: May 2006

SUMMARY. 1. Field experiments were conducted to examine the impact of grazing invertebrates on periphyton biomass in twenty‐one pools across three northern California coastal streams (U.S.A.): Big Sulphur Creek, the Rice Fork of the Eel River, and Big Canyon Creek. Periphyton accrual on artificial substrate tiles was compared in each stream between two treatments: those elevated slightly above the stream bottom to reduce access by grazers (= platforms) and those placed directly on the stream bottom to allow access by grazers (=controls).

2. Crawling invertebrate grazers (cased caddisflies and snails) were numerically dominant in each stream (86% of all grazers in Big Sulphur Creek, 61% in the Rice Fork, 84% in Big Canyon Creek). Platforms effectively excluded crawling grazers, but were less effective in excluding swimming mayfly grazers (Baetidae).

3. Periphyton biomass (as AFDM) on tiles was significantly lower on controls compared to platforms for the Rice Fork, an open‐canopy stream, and Big Sulphur Creek, a stream with a heterogeneous canopy. In contrast, no grazer impact was found for Big Canyon Creek, a densely shaded stream. Here, extremely low periphyton biomass occurred for both treatments throughout the 60 day study.

4. The influence of riparian canopy on periphyton growth (i.e. accrual on platforms), grazer impact on periphyton, and grazer abundance was examined for Big Sulphur Creek. As canopy increased (15–98% cover), periphyton biomass on platforms decreased. In contrast, canopy had little influence on periphyton accrual on controls; apparently, grazers could maintain low periphyton standing crops across the full range of canopy levels. The abundance of one grazer species, the caddisfly Gumaga nigricula, was highest in open, sunlit stream pools; abundance of two other prominent grazers, Helicopsyche borealis (Trichoptera) and Centroptilum convexum (Ephemeroptera), however, was unrelated to canopy.