摘要:

SUMMARY1.Ophrydium versatileis a symbiotic ciliate which forms gelatinous colonies up to several centimetres in diameter in transparent temperate lakes. The ciliates are evenly spaced at the colony surface and constitute a small proportion of the surface area (7%) and volume (3.1%) of the colony, but a large proportion of organic carbon (74%) and nitrogen content (82%) (exemplified for 1 cm3colonies). The majority of the colony volume is formed by the jelly. The biomass proportion of ciliates scales inversely with colony size, following the decline of surface area to colony volume. The largest colonies found in Danish lakes in early summer contain almost 1 million ciliates, and assuming they derive from a single ciliate undergoing exponential division, they need twenty generations and, presumably, almost a year to reach maximum size.2. The ciliates contain numerous symbiotic zoochlorellae that constitute about 10% of ciliate volume and more than half of the carbon content. Zoochlorellae dominate oxygen metabolism of the assemblage, resulting in low light compensation points, a large diel photosynthetic surplus, and a marked dependence on light for sustained growth and ciliate metabolism. Estimated gross photosynthesis (7ng C ciliate−1day−1) ofOphnrydiumfrom shallow, clear waters in June greatly exceeded the estimated carbon contained in filtered bacteria and small algae (1.9ng C cilicate−1day−1). Nitrogen and phosphorus content of the prey, however, may provide the main nutrient source consistent with the correspondence between mass‐specific rates of nutrient uptake and measured relative growth rates (average 0.067 day−1, generation time 10 days).3. The largeOphrydiumcolonies require increased allocation of photosynthetic carbohydrates with increasing colony size to maintain the jelly. The large colonies tend to become gas‐filled, floating, mechanically destroyed and their ciliate inhabitants abandon them as swarmers. Colony formation, however, should offer protection against predators which may be more important for the natural abundance than the costs of growin

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00846.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. Monthly variation in patterns of invertebrate density was examined in two streams in the Venezuelan Andes that differed in elevation, geomorphology, and disturbance regime.2. At one stream, located in the Andean piedmont, invertebrate densities displayed large fluctuations and between‐month variation differed by as many as four orders of magnitude. Major declines in invertebrate numbers were generally associated with frequent disasters, defined as channel‐shifting spates and drought.3. At the other stream, a montane site located at higher elevation, invertebrate densities varied by tittle more than an order of magnitude over the duration of the study. Reductions in invertebrate numbers between successive months generally occurred during the rainy season.4. Monthly invertebrate densities were inversely related to rainfall at each site. In addition, a highly significant positive relationship was observed between invertebrate density and the number of days elapsed since the previous major rainstorm event. Relationships between rainfall and invertebrate densities differed between streams, however, with significantly steeper regression slopes being found at the piedmont than the montane site.5. We suggest that disturbance plays a seasonally important role in structuring neotropical stream communities in which changes in discharge are severe and unpredictable. Our data contrast with studies from tropical floodplain rivers in which rainy‐season floods are predictable events that cannot be readily characterized as disturb

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00847.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. The fits of exponential, square root, surface‐dependent and power exponential polynomials to gastric evacuation curves in alpine newts were compared. Both the more complex power exponential function and the exponential model provided good descriptions of gastric evacuation in adult and larval alpine newts.2. Gastric evacuation rates of adult female and male alpine newts did not differ significantly. Temperature‐dependent gastric evacuation rates were strongly affected by the composition of the meals eaten in the natural habitat. The time for total gastric evacuation in adults was generally longer than reported for teleost fishes and was within the range reported for some elasmobranchs.3. Gastric evacuation rates of newt larvae were low compared to fish larvae. Time for total emptying of the stomach increased exponentially with increasing body dry weight of lar

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00848.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. Field experiments were conducted to examine interactions between larvae of the caddisflyGumaga nigriculaand the algae (primarily diatoms) located on their sand grain cases. Floating experimental enclosures were placed in sunlit pools of a California stream. Treatments applied to occupied cases, or to those from which caddisflies had been removed, included variations in grazer density, addition of nutrients, and modification of illumination using shade cloth.2. Neither faecal‐enriched agar in empty cases nor injections of faecal material into plugged cases affected case chlorophyllaconcentration; injected nutrients (N and P), however, did increase chlorophyllaconcentration. Faecal material may be nutrient poor, or nutrients may not be readily released. In addition, the relatively impermeable case wall may reduce the likelihood that nutrients from caddisflies reach the algae on the outer case surface.3. Although aGumagalarva may graze algae from its own case, grazing by conspecifics and the caddisflyHelicopsyche borealiscauses a greater reduction in the chlorophyllacontent of cases.4. Chlorophyllacontent of empty cases was decreased by experimental reduction of light. Because of shading associated with burrowing, chlorophyllacontent of cases occupied byGumagawas low and was unaffected by experimental light reduction.5. Removal ofGumagafrom its case results in rapid accrual of algae. Thus, the net effect of larval presence is to diminish the algal content of its cas

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00849.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. We performed computer simulations and a field experiment to determine the effect that sample duration and, thus, sample volume had on estimates of drift density and sample variance.2. In computer simulations, when the spatial arrangement of individuals in the water column approximated a random and a contagious‐random distribution, estimated mean drift density was not significantly affected by sample duration, but sample variance decreased curvilinearly as sample duration increased.3. Similar results were obtained in field experiments in habitats of high and low water velocity.4. Our findings from an Albertan stream indicate that the relationship between sample variance (i.e. coefficient of variation) and duration of drift samples is curvilinear. This relationship affected the number of samples required to achieve a specific level of precision (i.e. a standard error within 10% of the mean). For estimates in low and high current velocities, sample variation was halved by increasing the duration of sample collections from 10 to 20 min. The increased precision obtained with samples of 20 min duration reduced the amount of drift material that needed to be processed by approximately 50% compared with an equivalent 10% level of precision for samples of 10 min duration. This reduction in the number of samples required to obtain a given level of precision has important consequences to the cost of processing drift samples.5. Thus to optimize studies of stream invertebrate drift, both in terms of sample precision and processing effort, researchers must consider the effect that sample volume has on the variance of drift density estimates. Because researchers generally use drift nets with similar‐sized apertures (>300cm2), the problem for specific field applications becomes one of optimizing sample duration relative to variance estimates for drift dens

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00850.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. A new photometric method for determining the surface area of plants, which relies on the adsorption of Coomassie Brilliant Blue G‐250 on the plant surface, is described and compared with other methods.2. The method was applied to a variety of aquatic plants and found to provide a routine procedure to determine plant surface are

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00851.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. Laboratory experiments were conducted to test the effect of nutrient enrichment on bacterioplankton growth in the presence and absence of phytoplankton.2. In one series of experiments, bacterioplankton growth in terms of specific activity [3H‐thymidine incorporation (cell number)−1] was greater in whole lake water samples than in samples from which phytoplankton had been removed by filtration (1.0 μm), regardless of the nutrient enrichments (control, NH+4plus PO3‐4and mannitol). Organic C enhanced bacterioplankton growth in both whole and filtered lake water.3. In another series of experiments (with the same nutrient enrichments as in the first experiment except that glucose replaced mannitol), bacterioplankton growth in whole lake water enriched with PO3‐4plus NH+4and incubated in the light was greater than in two treatments designed to inhibit photosynthetic activity (+DCMU and dark). Bacterioplankton response to nutrient addition was greatest in the PO3‐4plus NH+4enrichment under all three conditions (light +DCMU, and dark).4. These results indicate that bacterioplankton growth could be directly limited by inorganic P and N when these elements are in short supply. Enhancement of bacterioplankton growth by phytoplankton occurs only under PO3‐4and NH+4replete

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00852.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. Data on sample means (x|) and variances (s2) of propagule abundance for five typos of aquatic plants growing in a variety of habitats were collected. The data set consisted of 379 values for x| and s2, based on 4942 individual sample units (sediment samples), drawn from the literature or from data collected as part of this study.2. Regression analysis relating log s2to log x| yielded an equation which was solved for the number of sample units (n) required to achieve a given degree of precision. For precise estimates [standard error (SE) = 20% of x|], values ofnbetween twenty‐seven and 234 are required when propagule density is low (<200m−2), whereas eight to twenty‐six sample units are required when propaguie density is high (200–1000 m−2). For larger SE (i.e. 30% of x|),ndecreased by about 50%. Sampler area (20 to 1600cm2) did not affect estimates ofn.3. Time required to collect and process sediment cores (collected with either a 5, 10, or 15cm corer) was approximately 7, 10, or 13min, respectively. Combining this information with the required number of samples is useful in planning studies designed to measure changes in the abundance of underground propagules of submersed aquat

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00853.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1. The longitudinal effects of herbivory on stream periphyton assemblages were examined in laboratory stream channels, each of which consisted of an upstream chamber, which either contained snail grazers or not, and downstream chambers, none of which contained grazers. Periphyton assemblages of two ages (0–21 days old and 21–42 days old) were sampled in both upstream and downstream chambers to detect proximate (i.e. localized) and longitudinal (i.e. downstream) effects of herbivory.2. Both proximate and longitudinal effects were detected, although they differed in their impact on the periphyton assemblage. Periphyton biomass and cell accumulation were lower in grazed than in ungrazed upstream chambers throughout the experimental period. Accumulation rates on initially bare tiles were substantially higher downstream of grazed than of ungrazed chambers, but grazing had no effect on cell densities in established (21–42 day old) assemblages downstream.3. Longitudinal effects of herbivory were not due to quantitative differences in the flux of propagules or nutrients from grazed and ungrazed chambers. Although not tested in this study, it is hypothesized that differences in the physiological condition of exported propagules may have contributed to differences in downstream colonization rates in grazed and ungrazed streams.4. The magnitude of longitudinal impacts of herbivory and the importance of different causal mechanisms are predicted to vary depending on the standing crop and productive capacity of the periphyton assemblages as well as the consumptive demand of the herbivore

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00854.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1.Cladophora glomeratais the dominant filamentous green alga in the tailwaters of the Colorado River, U.S.A., below Glen Canyon Dam, but becomes co‐dominant with filamentous cyanobacteria,Oscillatoriaspp., below the confluence of the Paria River (26km below the dam) where suspended sediments are elevated.2. Benthic algal assemblages played an important role in the distribution of the amphipod,Gammarus lacustris, in the dam‐controlled Colorado River through Grand Canyon National Park, Arizona.Cladopharaand G.lacustrisshowed a weak positive relationship at ten cobble‐riffle habitats in the Colorado River from Lees Ferry (25km below the dam) to Diamond Creek (362km downstream), while no relationship was found betweenOscillatoriaand G.lacustris.3. The relationship between algal substrata and G.lacustriswas tested by a series ofin situhabitat choice experiments. G.lacustrisshowed a significant preference forCladophora(with epiphytes) overOscillatoriaspp., detritus and gravel in treatment pans at Lees Ferry.4. Epiphytic diatoms (i.e. food) were the overriding determinant of subtratum choice by G.lacustrisin laboratory experiments.Gammaruschose theCladophora/epiphytic diatom community over sonicatedCladophorawith few diatoms. The amphipods also chose string soaked in diatom extract over string without diatom extract.5. Importance of mutualistic interactions in aquatic benthic community structure is disc

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00855.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY