The drift of zooplankton and microzoobenthos in the river Strandaelva,western Norway

The drift of zooplankton (rotifers, cladocerans, cyclopoid copepods) and microscopical zoobenthos (mainly bdelloid rotifers and small chironomid larvae) was investigated by filtering samples of river water. The number of drifting benthic rotifers varied between 1 000 and 6 000 ind. m^–3 in the lake inlet, and between 30 and 500 ind. m^–3 in the lake outlet, without any seasonal trend. The number of drifting insect larvae was approx. equal in the lake inlet and outlet, with a maximum in summer (250–300 ind. m^–3) and minimum in winter (ca. 10 ind. m^–3). Increasing water flow resulted in an increasing number of drifting zoobenthos. Downstream from the lake, the number of drifting benthic rotifers was increasing from approx. 300 ind. m^–3 in the outlet to 6 500 ind. m^–3 3.4 km downstream, while the number of insect larvae was ca. 100 ind. m^–3 in the outlet and leveled off at approx. 300 ind. m^–3 after 200 m. The number of drifting zooplankton in the lake outlet varied between 20 and 2 000 ind. m^–3 for crustaceans, and between 300 and 20 000 ind. m^–3 for rotifers, both with a maximum in late summer/autumn and a minimum in winter. The number of drifting zooplankton decreased by some 45% in the first 200 m from the lake outlet, but some zooplankton was still found in the drift 3.4 km downstream. The largest species was removed first from the drift. The diurnal variation in the number of drifting zooplankton in lake outlets appear to be related to the vertical migration in the lake, i.e. the largest number drifting when most animals are in the upper water layers.Contribution from the Voss Project, University of OsloContribution from the Voss Project, University of Oslo     

Biological consequences of stream routing through a final-cut Strip mine pit: Benthic macroinvertebrates

Benthic macroinvertebrates were collected from the Plachek Pit, a final-cut coal strip mine pit at the Big Horn Mine near Sheridan, Wyoming, USA. Goose Creek was routed through the pit to allow mining under the original creek bed. The benthic community in the pit was dominated by the worm Limnodrilus hoffmeisteri and various midge genera, including Chironomus sp., Procladius sp. and Tanypus stellatus. Insects other than chironomids were minor components of the benthic fauna, and were collected only in the upstream portions of the pit; they were concluded to have originated from drift deposition. The pit functions as a sink for drifting macroinvertebrates. Based on community composition and supplemental water quality information it was concluded that the Plachek Pit best exemplified an organically-enriched, lake-like ecosystem. Overall, rerouting of Goose Creek through the Plachek Pit was not viewed as deleterious to Goose Creek, but rather benefited the creek by serving as a sink for inputs from upstream municipal and agricultural effluents.     

An assessment of short-term depletion of stream macroinvertebrate benthos by drift

A study was conducted to determine if emigration of drifting macroinvertebrates from a stream riffle which was blocked for one week from immigration by upstream colonists significantly reduced the abundance of drift collected from the tail of the riffle. The head of a 9 m long riffle of a 2nd order stream in Maryland (USA) was blocked from incoming drift by a 250 m mesh weir. Upstream immigration of invertebrates into the riffle was largely prevented by a partition placed at the tail of the riffle which held the drift nets. Benthos and drift samples were collected from the riffle prior to weir placement and following its removal, and drift was collected at dusk on each day. No difference in drift or in benthic abundance between the beginning and end of the study was observed. This is largely attributed to recruitment of immature insects (primarily hatching of eggs present at the outset), particularly of Dolophilodes distinctus and species of Tanytarsini, from within the riffle. Results suggest that recruitment of riffle species is of sufficient magnitude to more than compensate for short-term riffle depletion due to drift. Samples of drifting and non-drifting (benthic) animals were held without food for 12 h after collection and mortality within each group was determined. The mortality of drifting animals was three-fold that of benthic animals.     

Effects of electroshocking on macroinvertebrate drift in three cold water streams

The effect of electroshocking and walking on the substrate on macroinvertebrate drift was evaluated in three streams located in southwestern Oregon, USA. A randomized block experimental design was used to determine treatment (electroshocking and walking, electroshocking-only, walking-only) and drift distance effects on the number, biomass, and length of macroinvertebrates drifting up to 30 m downstream. In all streams, electroshocking caused significantly (p < 0.05) greater number of macroinvertebrates to drift compared to merely walking on the substrate. The differences among treatments decreased the farther downstream the macroinvertebrates drifted. No significant difference (p > 0.05) was observed in mean biomass between electroshocking and walking on the substrate among the drift distances. The longest macroinvertebrates were collected from the electroshocking treatment at the shortest drift distance (2.5 m) in all of the streams. The length of macroinvertebrates collected between electroshocking and walking on the substrate were similar at drift distances of 10 m and greater and represented predominately the smaller, poor swimming taxa.     

Flood drift and propagule bank of aquatic macrophytes in a riverine wetland

Abstract. Drift of aquatic macrophyte propagules was investigated in a wetland along the River Rhône, during the first flood after the growing season (i.e. in the winter of 1995–1996). Input and output drift were studied at the beginning, around the top, and at the end of the river overflow in the upper reach of a cut-off channel. The soil propagule bank was sampled along the study area before and after the flood. The amount and composition of viable propagule drift and bank were determined, analysed and compared. Drift densities and richness were on average higher at the outlet of the channel than at the inlet (respectively: 23.2 vs 13.1 viable propagules/100 m³ of water and 8.7 vs 2.6 taxa per sample). Immigrating taxa were mostly in the form of helophyte seeds, whereas numerous resident hydrophyte species left the disturbed area rather as vegetative propagules. Temporal variability in propagule bank structure was weak, and mean bank densities did not change before and after the flood (respectively: 33 047 ± 10 510 vs 35 653 ± 15 070 viable propagules/m² of ground, including Chara). However, the density of Elodea canadensis significantly increased after the flood while that of Eleocharis acicularis decreased. This contrast suggests that flood responses vary among species. Despite a broad overlap in the taxa (18 out of 25 were common both to drift and bank collections), no significant relationship occurred in composition or structural changes between flood drift and propagule bank. Flood acted as a means of distribution of existing propagules and also as a provider of new vegetative dispersal units.     

Ontogenetic shifts in drift periodicity and benthic dispersal in elmid beetles

1. There is a paucity of information on ontogenetic changes in the dispersal of benthic invertebrates, which is an important aspect of their ecology. This study quantifies ontogenetic changes in diel periodicity in drift, and in upstream–downstream dispersal on the substratum for Elmis aenea, Oulimnius tuberculatus, Esolus parallelepipedus and Limnius volkmari (Coleoptera: Elmidae). Three drift nets were emptied every 3 h over 24 h in each month (October 1965–December 1968) at two contrasting sites: one in a deep section with abundant macrophytes, the other in a shallow stony riffle. Comparisons of periodicity between life‐stages of the same species were limited to months when numbers in the drift were highest. Dispersal was evaluated in six experimental stream channels, placed above the stream, with initial numbers of each life‐stage varying from 20 to 80. 2. Drift numbers were always highest at night with few or no animals in the day samples. Ontogenetic shifts in diel periodicity were similar for all four species. Drift catches were similar throughout the night for the early and intermediate larval instars and for mature adults, but were highest in the early hours of the night with a gradual decline thereafter for later larval instars and immature adults. These patterns were unaffected by a severe spate, even though drift numbers increased considerably. 3. Dispersal was density‐independent; the number of dispersing animals was a constant proportion of the initial number for each life‐stage. The relationship between dispersal distance and the number of animals travelling that distance was well described by an inverse power function. Median and maximum distances (m day^?1) were estimated for each life‐stage. 4. Ontogenetic shifts in dispersal in the stream channels matched those shown in diel drift periodicity. For all four species, the later larval instars and immature adults showed little movement in either direction, whereas early and intermediate larval instars and mature adults dispersed predominantly upstream, adults travelling further than any other life‐stage. 5. Ontogenetic shifts in diel drift periodicity and dispersal were related to seasonal changes in drift density and critical periods in the life cycle. Such shifts have not been quantified in other stream invertebrates, but should be considered when evaluating the role of dispersal in their population dynamics and their colonization ability.     

The influence of predatory fish on mayfly drift: extrapolating from experiments to nature

1. A knowledge of how individual behaviour affects populations in nature is needed to understand many ecologically important processes, such as the dispersal of larval insects in streams. The influence of chemical cues from drift‐feeding fish on the drift dispersal of mayflies has been documented in small experimental channels (i.e. < 3 m), but their influence on dispersal in natural systems (e.g. 30 m stream reaches) is unclear. 2. Using surveys in 10 Rocky Mountain streams in Western Colorado we examined whether the effects of predatory brook trout (Salvelinus fontinalis) on mayfly drift, that were apparent in stream‐side channels, could also be detected in natural streams. 3. In channel experiments, the drift of Baetis bicaudatus (Baetidae) was more responsive to variation in the concentration of chemical cues from brook trout than that of another mayfly, Epeorus deceptivus (Heptageniidae). The rate of brook trout predation on drifting mayflies of both species in a 2‐m long observation tank was higher during the day (60–75%) but still measurable at night (5–10%). Epeorus individuals released into the water column were more vulnerable to trout predation by both day and night than were Baetis larvae treated similarly. 4. Drift of all mayfly taxa in five fishless streams was aperiodic, whereas their drift was nocturnal in five trout streams. The propensity of mayflies to drift was decreased during the day and increased during the night in trout streams compared with fishless streams. In contrast to the channel experiments, fish biomass and density did not alter the nocturnal nature nor magnitude of mayfly drift in natural streams. 5. In combination, these results indicate that mayflies respond to subtle differences in concentration of fish cues in experimental channels. However, temporal and spatial variation in fish cues available to mayflies in natural streams may have obscured our ability to detect responses at larger scales.     

Time spent in the drift by downstream-dispersing invertebrates in a Lake District stream

1. The objective was to determine the time spent in the drift by different taxa of stream invertebrates. Most data were obtained from an earlier experimental study to determine the distances travelled by drifting invertebrates of 16 taxa in Wilfin Beck. Experiments were performed at two sites: ‘site 4’ in a stony, fast‐flowing, section of stream, ‘site 3’ in a deeper stream section where macrophytes were abundant. 2. The significant relationship between the mean distance x (m) travelled in the drift and modal water velocity V (m s^?1) was described by a power function in the earlier study but, as the power was close to one, a linear relationship has now been found to provide a satisfactory model. The rate of increase in x (m) with increasing V varied considerably between taxa. The mean time [mean t (s)] spent in the drift was estimated by dividing each x (m) by the appropriate V. Mean t (s) for each taxon was usually very constant over a wide range of V at each site (0.10–0.60 m s^?1 at site 4, 0.15–0.53 m s^?1 at site 3). A simple model estimated the time spent in the drift by different percentages (e.g. 75, 50, 10 and 1%) of the drifting invertebrates. 3. The experimental taxa at site 4 were divided into three groups according to the mean time spent in the drift. Mean t (s) for the five taxa in group 1 (32.8 s) was not significantly different from that obtained in control experiments with a mixed group of dead invertebrates. A similar time (33.0 s) was obtained for the five taxa in group 2, except at water velocities less than 0.2 m s^?1 when the mean t (s) decreased to 15–21 s. Mean t (s) was constant for each of the six taxa in group 3, and significantly less than that for groups 1 and 2. Mean values ranged from 28.8 s for Ephemerella ignita to only 9.4 s for Baetis rhodani and Gammarus pulex. All mean values were lower at site 3, presumably because of the dense stands of macrophytes, with mean values of 12.9 s for the five taxa in group 1 (equalling the value for dead invertebrates). Mean values for the six remaining taxa varied from 6.4 s for Simulium spp. to only 4.9 s for Baetis rhodani and 4.8 s for Gammarus pulex. It was concluded from a discussion of this study that the time spent in the drift may provide a useful measure for comparing the downstream dispersal of invertebrates in different streams, and may be a useful addition to models for the drift feeding of salmonids.     

Influence of hypolimnetic hydropeaking on the distribution and population dynamics of Ephemeroptera in a mountain stream

1. The Ephemeroptera assemblage of the River Oriège (Pyrenees, France) was studied up- and downstream of a hydroelectric power plant with hypolimnetic releases from a nearby high altitude reservoir. The life histories and larval growth of the six dominant species (Rhithrogena semicolorata, R. sp. gr. hercynia, R. kimminsi, Baetis alpinus, B. rhodani, Ephemerella ignita) were studied 700 m upstream (site A) and 700 and 3500 m downstream (sites B and C, respectively) of the power plant. Their drift patterns were studied at sites A and B. 2. The natural flow of the river was preserved when the plant was inoperative. During power generation, flow and temperature were the two main environmental factors modified. The natural flow in the river below the outlet may be enhanced several times a day from 1 to 11 m³ s^–1 in summer and winter, and from 5 to 15 m³ s^–1 during spring spates. During hydropeaking, the water was cooled in summer and slightly warmed in winter, but this was attenuated 3500 m downstream from the plant. 3. The density and biomass of the species studied at the three sites reflected both the impact of hydropeaking and the natural longitudinal zonation of the fauna. The lowest density and biomass was estimated at 700 m downstream from the plant (site B), consistent with a prominent role for hydropeaking. Site A showed constant mayfly drift caused by accidental dislodgement plus behavioural drift that was mainly nocturnal. Below the plant, the flushing action of peaking flows added to this a catastrophic drift, which was highest in autumn when the difference between natural and peak flows was greatest. 4. Environmental change caused only slight modifications in the life history patterns, affecting the hatching and/or emergence period and growth of several species. For instance, R. semicolorata grew through the winter below the outlet, when growth was slight upstream from the power plant. Under this kind of river regulation (natural discharge and temperature except during periods of power generation, and intermittent hydropeaking from a separate reservoir) modifications of the thermal regime had a minor effect on the population dynamics of mayflies, unlike hydraulic disturbances which strongly influenced the abundance and structure of the benthic community. Frequent (at least daily) high flow disturbances caused depletion of mayfly populations, despite daily renewal and colonization (drift) from undisturbed upstream locations.     

Genetic drift during the spread phase of a biological invasion

Recent theoretical and experimental models have revealed the role played by evolution during species spread, and in particular have questioned the influence of genetic drift at range edges. By investigating the spread of an aquatic invader in patchy habitats, we quantified genetic drift and explored its consequences for genetic diversity and fitness. We examined the interplay of gene flow and genetic drift in 36 populations of the red swamp crayfish, Procambarus clarkii, in a relatively recently invaded wetland area (30 years, Brière, northwest France). Despite the small spatial scale of our study (15 km²), populations were highly structured according to the strong barrier of land surfaces and revealed a clear pattern of colonization through watercourses. Isolated populations exhibited small effective sizes and low dispersal rates that depended on water connectivity, suggesting that genetic drift dominated in the evolution of allele frequencies in these populations. We also observed a significant decrease in the genetic diversity of isolated populations over only a 2‐year period, but failed to demonstrate an associated fitness cost using fluctuating asymmetry. This study documents the possible strong influence of genetic drift during the spread of a species, and such findings provide critical insights into the current context of profound rearrangements in species distributions due to global change.     

Seasonal fluctuations of macroinvertebrate drift in a South Carolina Piedmont stream

The seasonal fluctuations of larval macroinvertebrate drift, exuvial drift and larval benthic density were quantitatively examined over a 1-year period in a fourth order, spring-fed stream in the Piedmont area of South Carolina. The drift was dominated by the mayfly Baetis spp. and by two species of blackfly (Prosimulium mixtum and Simulium jenningsi). Peak drift densities were noted during early spring and especially late summer. Strong correlations were noted between larval drift densities and exuvial drift, indicating a relationship between drift and seasonal growth and emergence patterns. Seasonal trends in drift and benthic densities, though less strongly correlated, were also generally similar.     

广东横石水河流域溪流大型底栖动物漂流的昼夜节律

比较了广东横石水河流域二条相邻的3级溪流大型底栖动物的漂流种类组成及昼夜节律,其中一条溪流受广东大宝山矿外排的酸性矿山废水严重污染,水体pH值仅为3.45且重金属严重超标,而另一条为相邻的清洁溪流.结果表明：清洁溪流中大型底栖动物的数量和种类远比受酸性矿山废水污染的溪流丰富.在清洁溪流中共采获漂流底栖动物6 871头,隶属10目52类群,其中水生昆虫的数量和种类占绝对优势 (99.5%).但总漂流密度占优势的类群（相对多度超过5%）不多,主要集中在以下几种水生昆虫：七鳃假二翅蜉 (28.5%)、宜兴似动蜉 (13.8%)、短脉纹石蛾(13.2%)、白背锯形蜉（7.5%)、摇蚊科(6.5%) 和肖扁泥甲(5.0%).蜉蝣目种类和数量最多,占全部漂流底栖动物总个体数的65%,其中又以四节蜉科居多,占蜉蝣目总个体数的63%.毛翅目昆虫的数量（18%）仅次于蜉蝣目.大型底栖动物的漂流表现出明显的昼夜节律,漂流主要在夜间进行,未发现有日漂者.漂流密度高峰出现在21:00和2:00,漂流密度分别为（70.3±10.8）和（289.0±124.6） 头·100 m^-3.大多数优势种类漂流高峰出现的时段略有不同,但有些优势种类（如摇蚊科和肖扁泥甲）并未表现出明显的漂流昼夜差异.在受酸性矿山废水污染的溪流中,漂流动物只有1种嗜酸性的多足摇蚊,其漂流活动也在夜间进行,并有3个明显的漂流高峰,分别出现在19:00、0:00和4:00,最高漂流密度仅为（6.7±5.2)头·100m^-3.说明酸性矿山废水不仅降低了溪流中漂流底栖动物的物种多样性和数量,也改变了其漂流模式.     

Diel drift of Chironomidae in a large lowland river (Central Poland)

The diel drift patterns of Chironomidae larvae were investigated in a seventh order section of the Warta River (Central Poland) over two diel cycles during May 1989. Three nets (mesh size 400 m) were installed in a cross section of the Warta River.The estimated drift density was low, but was comparable to that calculated for other large rivers. Spatio-temporal fluctuations in abundance and composition of macroinvertebrate drift, including Chironomidae, were observed with the highest density of drifting macrobenthos recorded near the depositional bank of this river. The ratio benthosdrift indicated differing propensities for of the older instars of a given chironomid taxon to drift. Orthocladiinae larvae were the most abundant subfamily of Chironomidae in drift but not in benthos, reaching up to 73% of the total drifting chironomid larvae. More taxa but fewer individuals (about 20% of the chironomid larvae collected) belonged to the tribe Chironomini, the dominant group in benthos.A major part of chironomid drift collection may represent behavioural drift because the net mesh size used in the Warta River was insufficient to catch the earliest instars (distributional drift). Both at the family and subfamily level chironomid larvae exhibited a distinct nocturnal drift periodicity. Nocturnal periodicity was documented for the dominant species, but due to the low density of many chironomid species, it was impossible to determine their diel drift pattern. Some Chironomidae appeared to be aperiodic.     

The drift distances and time spent in the drift by freshwater shrimps, Gammarus pulex, in a small stony stream, and their implications for the interpretation of downstream dispersal

1. The objectives were (i) to determine experimentally and to model the relationship between mean water velocity and both the mean distance travelled, and the mean time spent, in the drift by freshwater shrimps, Gammarus pulex; (ii) to develop a drift distance–water velocity model from the experimental study, and validate it with field data; (iii) to examine the relationship between drift rate, water velocity and benthic density with the latter expressed as a mean value for the whole stream and a mean value corrected for the distance travelled in the drift. 2. In field experiments at 10 water velocities (0.032–0.962 m s^?1), the significant relationship between the mean drift distance and mean water velocity was described both by a power function (power, 0.96) and a linear relationship. The mean drift time was fairly constant at 8.3 s (95% CL ± 0.4). A simple model estimated the drift distance and time spent in the drift by different percentages of the drifting invertebrates. This model predicted correctly the positive relationship between drift rate and water velocity for field data over a year. 3. The relationship between drift rate per hour and the independent variables, water velocity and benthic density, was well described by a multiple‐regression model. Adding temperature and date did not improve model fit. Variations in water velocity and benthic density explained 96% of the variation in nocturnal drift rate (65% to velocity, 31% to benthic density), but only 40% of the variation in diurnal drift rate (29% to velocity, 11% to benthic density). Correcting benthic density for the drift distances did not improve model fit. 4. The significance of this study is that it developed models to predict drift distances and time, values being similar to those obtained in another, larger stream. It also illustrated the importance of spatial scale in the interpretation of drift by showing that when drift distances were taken into account, the impact of drift on the population was higher (4–10% lost day^?1) than when drift distances were ignored (usually < 3% lost day^?1), especially at a local level.     

Species traits and channel architecture mediate flow disturbance impacts on invertebrate drift

相似文献   

Longitudinal epigenetic drift in mice perinatally exposed to lead

An understanding of the natural change in DNA methylation over time, defined as “epigenetic drift,” will inform the study of environmental effects on the epigenome. This study investigates epigenetic drift in isogenic mice exposed perinatally to lead (Pb) acetate at four concentrations, 0 ppm (control), 2.1 ppm (low), 16 ppm (medium), and 32 ppm (high) prior to conception through weaning, then followed until 10 months of age. Absolute values of DNA methylation in a transposon-associated metastable locus, Cdk5-activator binding protein (Cabp^IAP), and three imprinted loci (Igf2, Igf2r, and H19) were obtained from tail tissue in paired samples. DNA methylation levels in the controls increased over time at the imprinted Igf2 and Igf2r loci (both P = 0.0001), but not at the imprinted H19 locus or the Cabp^IAP metastable epiallele. Pb exposure was associated with accelerated DNA hypermethylation in Cabp^IAP (P = 0.0209) and moderated hypermethylation in Igf2r (P = 0.0447), and with marginally accelerated hypermethylation at H19 (P = 0.0847). In summary, the presence and magnitude of epigenetic drift was locus-dependent, and enhancement of drift was mediated by perinatal Pb exposure, in some, but not all, loci.     

Benthivorous fish reduce stream invertebrate drift in a large-scale field experiment

Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or to escape after encountering a predator. While the control of the diurnal patterns of invertebrate drift activity by fish predators has received considerable interest, it remains unclear whether benthivorous fish reduce or increase drift activity. We performed a large-scale field experiment in a second-order stream to test if invertebrate drift was controlled by two benthivorous fish species (gudgeon Gobio gobio and stone loach Barbatula barbatula). An almost fishless reference reach was compared with a reach stocked with gudgeon and loach, and density and structure of the invertebrate communities in the benthos and in the drift were quantified in both reaches. The presence of gudgeon and stone loach reduced the nocturnal drift of larvae of the mayfly Baetis rhodani significantly, in contrast to the findings of most previous studies that fish predators induced higher night-time drift. Both drift density and relative drift activity of B. rhodani were lower at the fish reach during the study period that spanned 3 years. Total invertebrate drift was not reduced, by contrast, possibly due to differences in vulnerability to predation or mobility between the common invertebrate taxa. For instance, Chironomidae only showed a slight reduction in drift activity at the fish reach, and Oligochaeta showed no reduction at all. Although benthic community composition was similar at both reaches, drift composition differed significantly between reaches, implying that these differences were caused by behavioural changes of the invertebrates rather than by preferential fish consumption. The direction and intensity of changes in the drift activity of stream invertebrates in response to the presence of benthivorous fish may depend on the extent to which invertebrate taxa can control their drifting behaviour (i.e. active versus passive drift). We conclude that invertebrate drift is not always a mechanism of active escape from fish predators in natural streams, especially when benthos-feeding fish are present.     

Diel,size-differential drift patterns of three macroinvertebrate species in the lower Mississippi River,Louisiana (USA)

We explored macroinvertebrate size-differential drift in the lower Mississippi River (a 9th order system). Because this river system is highly turbid, we hypothesized that visually-dependent vertebrate predators feeding on drifting organisms would be at a disadvantage. Thus, size-differential drift should not occur. For one 24-hour period in both January and April, six drift nets were used to sample surface drift. Nets were emptied once every four hours. Individual intra-ocular distances of three macroinvertebrate species (Hydropsyche orris: Trichoptera, Hexagenia limbata: Ephemeroptera, Macrobrachium ohione: Crustacea) were measured. Percentages of size classes in the drift were determined. In both months, large individuals of H. orris and H. limbata were prevalent in the nocturnal but scarce in the diurnal drift. In January, large M. ohione drifted regardless of time. In April, large M. ohione predominated the nocturnal drift. Our results could not be attributed solely to vertebrate predator avoidance. Other mechanisms such as diel microhabitat migration and current velocity may have accounted for the results.     

Measuring drift during a receding flood: Results from an Austrian Mountain Brook (Ritrodat-Lunz)

The drift of organisms and large particulate organic matter >200 μm (LPOM) was investigated during a single receding flood event from 16 to 23 June 1989 in a second order, calcareous, alpine, gravel brook. Starting with the peak level of the hydrograph, which was well above bankfull level, sampling lasted for five days (= 8 sampling dates). Between four and eight replicates were taken at each sampling date. No significant differences (P < 0.05) could be detected in the proportion of the main aquatic taxa (excluding miscellaneous taxa) drifting during above versus below bankfull water levels. However, terrestrial taxa were significantly (P < 0.05) over-represented (23–25% of the total) at flood peak and a small secondary flood peak four days later. In addition, aquatic taxa which normally are scarce in drift samples at the Seebach (e.g. oligochaetes, ostracods) were abundant during the receding main flood event. Above bankfull stage (water level ⩾ 70 cm), animal drift densities were significantly (P < 0.01) and up to 22-times higher (e.g. 45.6 individuals m⁻³) than during baseflow (e.g. 2.1 individuals m⁻³). Below bankfull stage, drift densities remain constant, independent of water discharge (Student-Newman-Keuls test; P < 0.01). In LPOM drift, this ratio was nearly 100: 1, with drift values ranging from 1.83 g dry weight m⁻³ at flood peak to 0.02 g dry weight m⁻³ at baseflow. Drift densities of animals and LPOM exhibited a positive exponential relationship with water level. Drift rates of anmimals and LPOM ranged from 3200700 individuals and 148.9 kg dry weight per hour at flood peak to 17440 individuals and 0.2 kg dry weight per hour at baseflow. During a single receding flood (water level ⩾ bankfull) significantly more organisms and LPOM were transported than during a whole year at baseflow discharge.     

Mesh-size effects on drift sample composition as determined with a triple net sampler

Nested nets of three different mesh apertures were used to study mesh-size effects on drift collected in a small mountain stream. The innermost, middle, and outermost nets had, respectively, 425 µm, 209 µm and 106 µm openings, a design that reduced clogging while partitioning collections into three size groups. The open area of mesh in each net, from largest to smallest mesh opening, was 3.7, 5.7 and 8.0 times the area of the net mouth. Volumes of filtered water were determined with a flowmeter. The results are expressed as (1) drift retained by each net, (2) drift that would have been collected by a single net of given mesh size, and (3) the percentage of total drift (the sum of the catches from all three nets) that passed through the 425 µm and 209 µm nets. During a two day period in August 1986, Chironomidae larvae were dominant numerically in all 209 µm and 106 µm samples and midday 425 µm samples. Large drifters (Ephemerellidae) occurred only in 425 µm or 209 µm nets, but the general pattern was an increase in abundance and number of taxa with decreasing mesh size. Relatively more individuals occurred in the larger mesh nets at night than during the day. The two larger mesh sizes retained 70% of the total sediment/detritus in the drift collections, and this decreased the rate of clogging of the 106 µm net. If an objective of a sampling program is to compare drift density or drift rate between areas or sampling dates, the same mesh size should be used for all sample collection and processing. The mesh aperture used for drift collection should retain all species and life stages of significance in a study. The nested net design enables an investigator to test the adequacy of drift samples.