摘要:

SUMMARY1 The diatom stratigraphy of210Pb‐dated sediment cores is used to reconstruct the recent trophic histories of four oligo/mesotrophic lakes in the lower peninsula of Michigan, U.S.A. Quantitative trends in total phosphorus concentrations are reconstructed from the cores with a calibration data set based on modem diatom assemblages and water‐chemistry variables in forty‐two Michigan lakes that span a range of nutrient concentrations and biological productivities.2 The total phosphorus reconstructions suggest that phosphorus concentrations have changed very little over the last 200 years, with modern values approximately equal to those at the time of settlement. Three of the four lakes, however, do show a trend to higher phosphorus concentrations at the time of logging and settlement and a subsequent decline.3 The quantitative inferences of trophic change are compared with interpretations based on diatom species composition and the accumulation of diatoms, biogenic silica and carbonate in the sediments, and some discrepancies occur among the various lines of evidence. These comparisons are used to discuss the sensitivity of various proxies for reconstructing the nature and magnitude of tro

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00784.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 A combination of field and laboratory experiments was used to assess the impact of chironomid grazers on taxonomic composition, abundance and dispersion of epiphytic algal assemblages.2 In the laboratory,Psectrodadiussp. reduced the biovolume of algal species preferred as food and increased the degree of clumping of non‐preferred species.Thienemanniellacf.fuscahad both positive and negative effects (depending on the algal species) on the biovolumes of algal species preferred as food and increased the degree of clumping of non‐preferred species.3 In field exclosures, no effect of removal of chironomid larvae from the grazer assemblage could be detected in autumn or winter experiments. A third, longer removal experiment, conducted in summer, resulted in increased biovolumes of edibleCosmariumspp. andAphanocapsaspp., preferred foods of chironomid larvae. Biovolumes ofLyngbyasp.,Eulbochaetespp. andOedogortiumspp., filamentous taxa used extensively in larval case construction, also increased. Chironomid larvae had no effect on total algal biovolume or biovolume of large unicellular algae.4 Chironomid larvae can influence epiphytic algal assemblages through selective grazing by reducing the biovolumes of preferred foods and through case‐building activity by reducing the biovolumes of construct

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00785.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 An experiment was conducted to investigate potential impacts of food limitation and copepod predation on juvenile survival ofChaoborus purtctipennis. We tested the hypotheses that: (i) juvenile survival ofChaoborusis influenced more by copepod predation than by starvation in a productive environment, and (ii) food limitation and predation interact to affect survival.2 Effects of food concentration (approximately 800, 1400 and 2300 microzooplankton 1‐−1) and predator density (0, 1 and 2Mesocydops edax1‐−1) onChaoborusdevelopment and survival were evaluated using a 3 × 3 factorial design. Jars containing lake water, the appropriate food and predator treatments, and twoChaoborus(<12h old) were rotated on a plankton wheel at 25°C. Survival and developmental stage were monitored daily until all individuals had either died or moulted to instar II.3 Predation byMesocydopswas the major source of mortality, causing 87.5% ofChaoborusdeaths over all treatments.Chaoborusmortality was significantly higher in treatments withMesocydops(67–100%) than in predator‐free treatment (0–13%).4 Development time was significantly longer in the low‐density food treatment than in the highest food treatment.5 No significant interaction between food limitation and predation was detected.6 These results suggest that predation by copepods may limit recruitment of juvenil

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00786.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 A simple periphyton sampler, which is cheap and easy to construct, is described. The key component is a disposable scouring disc which also serves to adsorb detached material.2 Field trials show that the scouring sampler is significantly more efficient at removing and retaining periphyton from stones in streams for biomass estimation than other commonly used techniques.3 The scouring sampler requires only one operator. Its small size and speed of use facilitate replication of samples. Samples can be collected from stones as small as 30mm dia

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00787.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 Freshwater fish assemblages were sampled quantitatively before, during, and after naturally produced conditions of elevated pH and dissolved oxygen (DO) that occurred for 2–6h in two dense macrophyte beds. Field observations were followed by 3–4‐h laboratory behavioural experiments in which banded killifish (Fundulus diaphanus), bluegill (Lepomis macrochirus), and striped bass (Morone saxatilis) were exposed to gradually increasing levels of: (i) pH; (ii) DO; and (iii) pH and DO.2 Field results indicated that the macrophyte bed dominated byHydrilla verticillatahad a higher fish density than the mixed‐species (H. verticillataandVailisneria americana) bed, but beds did not differ significantly in fish species richness or biomass. Within beds, diel changes in fish species composition, richness, density and biomass were minor.3 In the laboratory, avoidance of pH levels exceeding 9.5 was exhibited by all species tested. No significant response was observed to either DO increase treatments, or to pH and DO increase treatments.4 Results suggest fishes do not avoid areas, such as dense macrophyte beds, when high pH levels (9.5–10.0) are accompanied by high DO levels (200

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00788.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 Greenhouse experiments tested the effect of pH 5vpH 7.5 on reproductive success for the freshwater macrophytesNajas flexilis, an annual, andVallisneria americana, which perennates as a tuber.2 Seed production by the smallNajasplants grown at pH 5 averaged 0.25 seeds/plant, in contrast to 95.5 seeds/plant at pH 7.5. At low pH,Vallisneriagrown from seeds produced no flowers and too few tubers to replace themselves, so that sexual reproduction failed nearly completely for both species.3 Vallisneriagrown from tubers produced 97% less total tuber mass at pH 5 (0.4v14.9 g), the compounded result of producing, on average, 89% fewer and 82% smaller tubers. The smaller tubers developed at pH 5 were less likely to overwinter in the field, and those surviving tubers subsequently grew into smaller plants.4 These findings generated the hypothesis of a closing spiral: growth at low pH (and relatively low CO2concentrations) results in small plants that produce a smaller number of progressively smaller tubers each autumn, which in turn develop into progressively smaller plants each summer. This hypothesis was supported by field transplant experiments in two acidic Adirondack Mountain (NY

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00789.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 Fecundity ofGammarus fossarumpopulations at six stream sites (S1‐S6) andG. roeselipopulations at two sites (S5 and S6) was studied monthly during 1985–1988, distinguishing seven morphological stages of embryonic development. Mean survival ofG. fossarumeggs was 65% (60–70%) in nature and 60% (57–63%) in the laboratory; egg survival ofG. roeseliwas 52% (46–58%) in nature and 41% (35–47%) in the laboratory. Regressions of egg numbers per clutch against body wet weight (WWT) decreased significantly for egg developmental stages 2 to 7 (hatching).2 In stream populations, ovigerous females ofG. fossarumwere absent in October (six sites) and November (five sites); ovigerousG. roeseliwere usually absent from October until March.3 Overall mean egg volume (EV) increased significantly from 0,08 ± 0.001 mm3and 0.075 ± 0.001 mm3for stage 2 eggs, to 0.174 ± 0.012mm3and 0.160 ± 0.013 mm3for stage 6 eggs ofG. fossarumand G.roeseli, respectively. The dry weights of stage 2 eggs, stage 6 eggs and neonates (stage 7) were not significantly different, within and between species; mean dry weight was 36 ± 3 μg.4 Egg volumes and fecundity indices (FI, weight‐specific number of eggs per clutch) were not significantly different between the same months of four successive years, so the data were pooled to give a single representative ‘year’. At most sites there were marked seasonal fluctuations in these variables. In both species, mean EV was largest for ‘winter’ eggs in December/January and smallest for ‘summer’ eggs in May, EV decreased during some months of the year when FI increased, and vice versa. Mean reproductive effort (RE = EV × FI) declined from high ‘winter’ values to low ‘summer’ values, and this difference was most marked at sites where the major foodstuff available was seasonally limited (autumn‐shed tree leaves). Overall (‘annual’) mean RE was also lowest at these sites (S3 and S4) but was some 13% greater at sites where nutrients and food were present in the greatest quantifies (S5 and S6). Specific maximum potential body sizes and hence maximum potential fecundities (fifty‐nine eggs forG. fossarumand eighty‐seven eggs for G.roeseli) were attained only at S5 and 56. Mean RE was nearly identical for G.fossarumandG. roeselifrom the same sites.5 At experimental temperatures (T) in the range 3.8–20.2°C, females of the same body weight carried significantly different egg numbers per clutch. The relationship between FI andTwas described by a second‐order polynomial. The calculated optimumTwas 12.1°C forG. fossarumand 16.3°C forG. roeseli. Interspecific differences were highly significant.6 Theoretical

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00790.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 Leaf litter processing rates and macroinvertebrate shredder assemblages in leaf packs were compared in four streams on the Allegheny plateau in the central Appalachian Mountains, U.S.A.; these streams were characterized by different bedrock geology and streamwater pH.2 Leaf litter processing rates were fastest in the neutral streams, slowest in the acidic stream, and intermediate in the most alkaline stream.3 Slower processing rates in the acidic stream were associated with lower total shredder biomass, made up predominantly by small leuctrid and nemourid stoneflies.4 The differences in processing rates between the more alkaline stream and the neutral streams were not associated with differences in shredder biomass, but appeared to be related to taxonomic differences in the shredder assembiages. Insects were dominant in the neutral streams, and amphipods were dominant in the more alkaline

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00791.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 Adjacent populations of the copepodCyclops scutiferSars living in lakes in southern Norway exhibit remarkably different life cycles. A series of laboratory‐common environment experiments were used to partition variance in one life cycle trait—time to metamorphosis—among and within five populations ofC. scutifer, to examine the extent to which variation in this trait has a generic basis and to test whether populations are polymorphic for development rates. The experimental populations exhibit a variety of life cycles in the field and occupy environments that represent a broad range of conditions.2 Populations with different life cycles in the field continued to express differences in time to metamorphosis in the laboratory, indicating a genetic contribution to life cycle variation. Differences in developmental rates in the laboratory were smaller, however, than differences observed in the field, suggesting that environmental conditions also contribute to observed life cycle variation.3 In addition to interpopulation variation, each population maintained substantial intrapopulation variation in time to metamorphosis; differences between individuals from the same population were often as large as or larger than differences between populations.4 Individual females in most populations produced highly variable offspring, and there often was little difference in time to metamorphosis among families within a population.5 Cyclops scutiferexhibits a hierarchy of variation in time to metamorphosis, with a major portion of this variation expressed among siblings. Intrapopulation life history variation important to natural selection may be maintained by different processes among the major groups of freshwa

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00792.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARY1 Variation in abundances of stream fauna across a range of spatial scales has been neglected, yet such variation can have important implications for experimental and descriptive work. We tested the hypothesis that the abundances of stream animals living on stones vary between sites within the same stream order, riffles within the same site and groups of stones within the same riffle. We also examined how epilithon and water depth and velocity varied over these spatial scales.2 Thirty stones were sampled from each of three sites in a fourth‐order section of the Taggerty and Steavenson Rivers, south‐eastern Australia. At each site, fifteen stones were taken from each of two riffles; within each riffle, the fifteen stones were collected as five groups of three stones within 0.5m of each other. For each stone, we measured water depth and velocity, took samples of epilithon to estimate chlorophyllaconcentrations, organic biomass and bacterial and algal cell densities, and collected all macroinvertebrates.3 Hierarchical analyses of variance revealed that species richness did not vary over any of the spatial scales, but densities of individuals varied substantially between riffles and groups of stones. Of the thirty‐five most abundant taxa, three‐quarters (twenty‐seven) varied over one or more of the spatial scales, with variation between riffles being particularly common. DECORANA analyses confirmed that two of the three sites had riffle faunas that were dissimilar to each other, whereas the third site had riffle faunas that were very similar. Significant spatial variation was observed also in water velocities and depths and chlorophyllaand organic biomass concentrations.4 Overall, we have demonstrated that significant variation in faunal densities may occur over small spatial scales, such as those represented by groups of stones. Moreover, such variation was not consistent between taxonomically or functionally related species within the assemblage.5 The results emphasize the need for stream ecologists to be aware of the ramifications of such variation. Neglect of small‐scale variation has produced spatially confounded designs in both surveys and experimental studies. Additionally, the assumption that variation over small scales does not occur or is trivial seems to have been responsible for the virtual enshrinement of certain large‐scale models of

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1993.tb00793.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY