Drift Density Research Articles

Effects of aquatic bryophytes and long-term fertilization on arctic stream insects

The effect of bryophytes on stream macroinvertebrates has gained attention in many temperate systems, but is not well understood relative to the role of mineral substrates. Long-term fertilization of the Kuparuk River in arctic Alaska has resulted in dense growths of bryophytes (the moss Hygrohypnum), and a recovery zone created in 1996 left an unfertilized reach of moss-dominated riffles. We examined aquatic insect density, drift, and growth during 1996 and fine and coarse particulate organic matter (FPOM and CPOM) in transport in the reference, recovery, and fertilized zones of the Kuparuk River, to quantify the impact of moss on invertebrate assemblages, epiphytic diatoms, and detrital export. We also used enclosures to examine the effects of fertilization, position in the streambed, and substrate type on insect densities. Moss increased densities of Ephemerella aurivilla and Chironomidae, but had no effect on Baetis spp. or Simuliidae densities. Drift densities of Baetis spp. and Ephemerella were similar in the 3 fertilization regimes, yet ratios of Ephemerella drift densities to benthic densities were highest in the unfertilized zone. Both Baetis and Ephemerella grew larger in the fertilized zone, likely the result of increased ephiphytic diatom densities rather than the presence of moss. Enclosure experiments showed that substrate type (i.e., bare rock, natural and artificial moss) had no effect on any taxa except for Ephemerella, which had higher densities in complex (moss) microhabitats compared to bare-rock microhabitats. The presence of moss beds did not significantly alter FPOM concentrations, but CPOM in transport was greater downstream of moss beds in the recovery zone compared to the fertilized zone. Nutrient-induced moss beds affect arctic stream invertebrate communities by enhancing selected taxa through increased habitat complexity (e.g., Ephemerella, Chironomidae), and by enhancing growth of selected taxa through increased periphyton abundance (e.g., Ephemerella, Baetis). However, taxa that showed positive density responses to mosses did so because of increased habitat complexity, rather than enhanced food resources.

Disturbance of Small-Stream Habitat by Spawning Sockeye Salmon in Alaska

Abstract We examined how the spatially and temporally predictable disturbance caused by a spawning run of native sockeye salmon Oncorhynchus nerka temporarily affected benthic and invertebrate drift communities of two small streams in southwestern Alaska. Surface gravel size temporarily increased as a consequence of this mechanical disturbance, and periphyton biomass decreased in one stream. The postspawning rise in periphyton biomass in Bear Creek (larger sockeye salmon run) likely reflects nutrient release by decomposing sockeye salmon. Total benthic invertebrate and larval chironomid density decreased during spawning in one stream, but most invertebrate taxa in both streams did not exhibit clear spawning-related impacts. In contrast, invertebrate drift almost always increased during daylight hours when sockeye salmon were present, but it was difficult to separate mechanical effects of sockeye salmon acting via redd construction versus swimming. Total daytime drift density during spawning was four times...

Development and Test of a Whole-Lifetime Foraging and Bioenergetics Growth Model for Drift-Feeding Brown Trout

We developed and tested a combined foraging and bioenergetics model for predicting growth over the lifetime of drift-feeding brown trout. The foraging component estimates gross energy intake within a fish- and prey size-dependent semicircular foraging area that is perpendicular to the flow, with options for fish feeding across velocity differentials. The bioenergetics component predicts how energy is allocated to growth, reproduction, foraging costs, and basal metabolism. The model can reveal the degree to which growth is limited by the density and size structure of invertebrate drift within the physiological constraints set by water temperature. We tested the model by predicting growth based on water temperature and on drift density and size structure data from postemergence to age 12, and we compared the predictions with observed size at age as determined from otoliths and scales for a New Zealand river brown trout population. The model produced realistically shaped growth curves in relation to the observed data, accurately predicting mean size at age over the lifetime of the trout, assuming 24-h maximum rations and including diurnal drift-foraging costs (predicted versus observed weight r2 = 0.94; length r2 = 0.97). The model predicted that, within a given water-temperature regime, growth is limited primarily by reproduction costs but also by increasing foraging costs as trout grow (a phenomenon that is associated with the increasing foraging time that is required in order to feed to satiation on small invertebrate drift prey). Invertebrate drift size structure significantly influenced predicted growth, especially maximum size, through its effect on foraging time. The model has potential in terms of the exploration of growth-limiting factors and has associated use as an environmental-impact tool and as an aid for hypothesis generation in studies of salmonid growth processes.

Drift and benthic invertebrate responses to stepwise and abrupt increases in non-scouring flow

We conducted two experiments to assess drift and benthic invertebrate responses to stepwise and abrupt changes in non-scouring flow in gravel-bed experimental streams. Intuitively, a stepwise flow increase should allow aquatic invertebrates more time to seek refuges than would an abrupt increase. We hypothesized that abrupt flow increases would result in larger increases in taxon richness and in the number of invertebrates in the drift, and a larger decrease in benthic density than would stepwise flow increases. Two kinds of drift response to flow increases were observed in the stepwise experiment: (1) no response (e.g. Caenissp. [Ephemeroptera] and Sphaeriidae [Pelecypoda]); (2) threshold response of some aquatic (e.g. Crangonyx pseudogracilis[Amphipoda]) and semiaquatic (e.g. Ormosiasp. [Diptera]) taxa. Drift richness and drift density in both experiments declined after reaching a peak. The peak was reached almost immediately in the abrupt treatment and later in the stepwise treatment. Maximum richness of taxa and taxon composition in the drift were similar in both experiments. Despite significant increases in drift, stepwise and abrupt increases in flow did not have a significant effect on benthic density. However, relative to reference streams, the percentage of total benthic invertebrates in the drift increased 10× in the stepwise experiment and 33× in the abrupt experiment. These non-scouring increases in flow were non-trivial. Our results suggest that several high flow events of the same magnitude (i.e. 2.5–3.0 fold increases) may cause considerable losses of benthic populations to the drift. The rate of increases in flow appears to be important: abrupt increases in flow had a stronger effect on invertebrate drift than did stepwise increases.

Larval fish movement in the River Lohajang,Tangail, Bangladesh

AbstractLarval fish drift and distribution patterns were studied in the River Lohajang, a tributary of the River Jamuna, Bangladesh, during the monsoon seasons (June ‐ October) of 1992, 1993 and 1994. Larval fish drift peaks with the water levels of the River Jamuna. In all three years, the Indian carp, Catla catla, Labeo rohita L. and Cirrhinus mrigala, were associated with the first pulses, but were absent from drift samples after September. Hilsa species and other non‐identified fish species were found throughout the monsoon season. The highest larval fish densities were found in the upper strata of the river, near the shore. The drift densities of these fish became evenly distributed through time. Water turbidity probably influenced the vertical and horizontal distribution, and the absence of diel patterns. The most important species in the larval drift were Hilsa spp., followed by Labeo rohita L., Glossogobius giuris L. and Rasbora daniconius. Among the major carp, Labeo rohita was the dominant species, contributing 80‐90%. The impact of flood control programmes on the reproduction of riverine and larval fish drift is discussed.

Rapid size-specific changes in the drift of Baetis bicaudatus (Ephemeroptera) caused by alterations in fish odour concentration.

Chemical cues from fish can alter the behaviour of stream invertebrates in experimental tanks but their effect in natural streams has received little attention. By adding brook trout (Salvelinus fontinalis) odour to a trout stream in the Rocky Mountains of Colorado, USA, we tested whether changes in the concentration of chemical cues from visually feeding predatory fish would alter the drift of mayfly nymphs (Ephemeroptera). Stream water was piped from stream-side tanks with (odour) and without (control) three brook trout to two locations in the stream 3.5 m upstream of drift nets at six replicate sites. Five-minute drift samples were collected downstream from odour and control pipes before, during and after the release of water from the tanks into the stream during both the day and night. Almost all drift occurred at night and consisted predominantly of Baetis bicaudatus nymphs. The odour manipulation had no measurable effect on Baetis drift during the day but statistical power was low. During the night, however, the drift of large (>0.65 mm head capsule width, HCW) Baetis nymphs decreased significantly during the odour addition compared to control drift. In contrast, the drift of small nymphs (≤0.65 mm HCW) increased both during and after the odour addition in comparison to control drift. Since the stream contains brook trout (0.04-0.18 m-2), and water from the stream (presumably containing fish odour) altered the behaviour of fishless-stream Baetis nymphs in another experiment, we conclude that the changes in Baetis drift density were a response to an increase in the concentration of fish odour in the stream. Furthermore, we were able to detect the effect within 5 min. of odour addition, indicating that mayfly behavioural response to trout odour was rapid. These results suggest that mayflies can distinguish different concentrations of trout odour in natural streams and that the response is size-specific, according to the relative risk of predation of large and small Baetis.

Modelling of lightly doped collector of a bipolar transistor operating in quasi-saturation region

A physical model taking into consideration the effects of finite collector minority carrier lifetime and minority carrier blocking property of the low-high (n−n+) junction is proposed for predicting the bipolar transistor dc characteristics covering saturation and quasi-saturation operations. Solution for the electron and hole carrier profiles, recombination, drift and diffusion current densities, electric field distributions and voltage drops as a function of the collector current density is presented. The low-level injection model neglecting collector minority carrier recombination current within the injection region is also present. The validity of the complete model is verified by comparing the results obtained by the present analysis with experimental results available in the literature.

Responses of Aquatic Invertebrates to a Linear Alcohol Ethoxylate Surfactant in Stream Mesocosms

Responses of aquatic invertebrates to 30-day exposures to a nonionic linear alcohol ethoxylate (LAE) surfactant (C12–13AE-6.5) were evaluated in 10 outdoor stream mesocosms. Responses were measured as changes in invertebrate densities and invertebrate drift densities during a 14-day pretreatment period, a 30-day treatment period, and a 14-day posttreatment period relative to untreated control stream densities. Mean measured surfactant concentrations in duplicate streams were 0.32, 0.88, 1.99, and 5.15 mg LAE/liter, with two streams serving as untreated controls. Statistically significant effects were observed on simulid, copepod, and cladoceran population densities during the treatment period. Based on these effects, the no-observed-effect concentration (NOEC) for invertebrate densities was <0.32 mg LAE/liter and the lowest-observed-effect concentration (LOEC) was 0.32 mg LAE/liter. No statistically significant differences due to treatment were detected in drifting invertebrates; however, a trend was observed with increased numbers of invertebrates drifting in the streams treated with 5.15 mg LAE/liter as compared with controls.

An update on the quantification of stream drift

The reliable quantification of stream drift is of vital importance for all drift studies. Although variation between drift samples is smaller than between benthic samples, some six to seven replicates have been considered necessary by ALLAN & RUSSEK (1985) to obtain a precision of 95 %-confidence limits within 50 % of the mean. While a review of drift studies published during the last twelve years showed that many researchers have probably used too few replicates to allow a reliable quantification of stream drift, it may be often unnecessary to use as many as six or seven. In the present study conducted in a Swiss river, we analysed data from 24 drift samples taken on six different occasions during three studies conducted in winter, spring, and summer. In each season, drift was sampled twice. The 95 %-confidence limits obtained with just four replicates were within the 50 %-boundary on all six sampling occasions for the total number of invertebrates and the five relatively most common taxa in spring and summer. In the two winter collections, taxon-specific confidence limits were outside 50% of the mean in five cases out of ten. However, overall drift densities in winter were low, and all but one of the taxa with high variation between replicates were quite rare in actual numbers. A similar or even better statistical precision may be possible in other rivers, because the higher number of replicates recommended by ALLAN & RUSSEK (1985) may have been partly due to collecting some of their replicate samples during periods of low overall drift. When studying the drift of rare taxa, however, actually more than six to seven replicates may be required to generate reasonable estimates of their drift densities. We suggest that researchers conduct more pilot studies in their individual study systems to determine the appropriate number of replicates needed, since this has been rarely done in the past.

Colonization of a new man-made river (Marchfeldcanal, Lower Austria) by benthic copepods

Abstract The role of copepods in the colonization and emigration processes of benthos and drift of an irrigation canal were studied. During the 3 first years of existence of the canal, fifteen species of copepods were recorded (1 diaptomid, 11 cyclopoids, 3 harpacticoids). Copepod succession in the canal was initially represented by lentic species, then by predatory species like Macrocyclops albidus that developed with the proliferation of filamentous algae and epibionts during the first two years. Primary consumers appeared from the beginning of the flowing conditions but started to dominate only from the third year. Smaller species associated with the bed sediments like the harpacticoid species Nitocra hibernica , were late colonizers. The latter as well as Paracyclops fimbriatus and Eucyclops serrulatus were the dominant copepod species in the canal. Total copepod density in the benthos ranged from 15,800 to 68,000 ind./m 2 , with a mean of 7.6% of the total zoobenthos. Copepods constituted up to 25% of the total faunal drift density, ranging from 50 to 140 ind./m 3 . Their highest abundance was observed around midnight. A three-fold increase of the discharge temporarily affected the drift density, but did not change the diel rhythm of the drift. Cyclopoid copepods namely P. fimbriatus (54.77%), copepodid cyclopoids (20.76%) and E. serrulatus (16.11%) formed the main part of the drift samples. The harpacticoid Nitocra hibernica was a minor participant in the drift processes.

Seasonal Variation, Export Dynamics and Consumption of Freshwater Invertebrates in an Estuarine Environment

In the Aber Estuary, North Wales, significant numbers of freshwater benthic invertebrates occurred in the tidal freshwater area. Distinct seasonal patterns were observed in their longitudinal zonation which appeared to be unrelated to variations in tidal inundation. The December extension downstream of freshwater taxa is hypothesized to be in response to decreasing water temperatures. In April, larvae/nymphs of the Trichoptera (caddisflies), Ephemeroptera (mayflies) and Plecoptera (stoneflies) ranged as far as a site inundated by 80·9% of all high tides, and larval Elmidae and Chironomidae (midges) occurred at the most marine site (inundated twice daily by all high tides). In July, with the exception of the Chironomidae, the range of most aquatic insects had contracted to the upper estuary. Although, in general, densities of aquatic insects decreased towards the lower estuary, significant densities persisted there. For example, maxima of 3514 chironomid larvae and 48 caddisfly larvae m−2were recorded at the 80·9% inundation site. An estimated 31×106freshwater invertebrates (weighing 62·6kg), per annum, passed from fresh water into salt water across any given transect along the estuary. In comparison, the annual influx of invertebrates carried upstream by incoming tides was estimated to be 1·9×106(6·2%; weighing 2·5kg). Predominant in the downstream drift were the larvae/nymphs and/or pupae of chironomids, mayflies, stoneflies and caddisflies. The ‘reverse’ drift comprised mainly copepods, ostracods, amphipods and oligochaetes. Mites and the brackishwater amphipodGammarus zaddachicommonly moved in both directions. Highest drift densities occurred in July, whereas the lowest densities occurred in late autumn and winter. Multiple regression analysis showed no relationship between total drift or ‘reverse’ drift densities and any of the measured environmental variables. Many of the freshwater invertebrates appeared not to die upon passing into tidal sections but resumed a benthic existence by virtue of varying degrees of salt tolerance. Of the three fish species common in the estuary, eel, common goby and flounder, the last two preyed measurably on freshwater taxa. Whereas gobies tended to be opportunistic feeders, depending on the section of estuary that they occupied, flounder were more restricted to the upper estuary where they fed selectively on chironomid larvae. On the latter diet, between March and September, the mean wet weight of flounders increased by more than 100 times (from 5 to 540mg). Gobies were more numerous in the estuary from September to February, and although they ate insects their primary prey wasG. zaddachi.

Use of both benthic and drift sampling techniques to assess tropical stream invertebrate communities along an altitudinal gradient, Costa Rica

1. Two sampling techniques were used to characterize invertebrate communities in eight, low‐order streams along an altitudinal gradient in Costa Rica that represents the last continuous tract of primary forest spanning such extremes in elevation (i.e. near sea level to 2900 m a.s.l.) along the Caribbean Slope of Central America. A standard Surber sampler was used to sample invertebrates on the stream bottom, and drift sampling nets were used to sample invertebrates drifting in the stream flow.2. Sites were established at 30, 50, 700 1800 and 2700 m a.s.l. In one to two streams per site, six Surber samples were collected, and drift was sampled every 3 h over one 24‐h period between April and August 1994. All sites were in primary forest, with the exception of the lowest elevation site (30 m) which was located in banana plantations.3. Both sampling techniques indicated that Diptera (Chironomidae) and Ephemeroptera were the dominant insect groups at all sites. Disturbed streams draining banana plantations were dominated by Chironomidae and had lower taxon richness and diversity than other sites.4. While data from benthic samples indicated that insects were the major faunal component (&gt; 90%) at all sites, drift samples were dominated by larval shrimps (&gt; 50%) at the 30 m and 50 m sites.5. Drift periodicity of invertebrates was observed at those sites characterized by predaceous fishes: nocturnal drift densities were higher than diurnal densities at 30, 50 and 700 m a.s.l., however, no periodicity was observed at 1800 and 2700 m a.s.l. where fish were absent.6. This study shows the importance of measuring invertebrate drift, in addition to directly sampling the benthos. Drift sampling provided data on a major community component (shrimps) of lowland tropical streams, that would have been overlooked using traditional benthic sampling techniques, and in some cases provided additional information on taxon richness.7. Based on results of the present study, it is recommended that drift sampling be included as a standard complementary tool to benthic sampling in biological assessments (e.g. bioassessment protocols) of tropical streams, which are often characterized by migratory invertebrate species such as shrimps. Drift samples provide critical information on the presence or absence of shrimps and also on the timing and magnitude of their migration which is an important link between many tropical rivers and their estuaries.

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.

A Model of Habitat Selection by Drift-Feeding Stream Salmonids at Different Scales

Whole-stream size gradients of drift-feeding stream salmonids have received practically no attention, perhaps because the smaller-fish-upstream pattern that is thought to prevail is consistent with knowledge of habitat selection by other stream fish at the local scale. However, a rather counterintuitive larger-fish-upstream pattern has recently been documented for Arctic grayling (Thymallus arcticus) in Alaska and brown trout (Salmo trutta) in New Zealand, and habitat selection theory cannot explain these observations. My goal in this paper is to improve this situation by developing a model that predicts the distribution of a population of fish both within single pools and over the length of the entire river, as well as the behavioral mechanism responsible. The model uses information on invertebrate drift density and water temperature to predict the growth rate of different sizes of fish at the positions available in the stream. Fish then distribute themselves so that each individual occupies the most profitable position it can defend, with the largest fish winning any disputes. The model suggests that streams can be classified into categories based on the way temperature and drift density vary with the passage downstream. Each of these categories favors a different combination of size gradient and behavioral mechanism. A larger-fish-upstream pattern due to size-dependent habitat preference is predicted for streams with cool temperatures and low drift densities, conditions found in Arctic grayling streams. A larger-fish-upstream pattern due to competition between fish of different size is predicted in warm streams, irrespective of drift density, conditions found in many New Zealand trout streams. A smaller-fish-upstream pattern due to competition between fish of different size is expected in cool streams with high drift abundance; currently no data are available to test this prediction.

A MODEL OF HABITAT SELECTION BY DRIFT-FEEDING STREAM SALMONIDS AT DIFFERENT SCALES

Whole-stream size gradients of drift-feeding stream salmonids have received practically no attention, perhaps because the smaller-fish-upstream pattern that is thought to prevail is consistent with knowledge of habitat selection by other stream fish at the local scale. However, a rather counterintuitive larger-fish-upstream pattern has recently been documented for Arctic grayling (Thymallus arcticus) in Alaska and brown trout (Salmo trutta) in New Zealand, and habitat selection theory cannot explain these observations. My goal in this paper is to improve this situation by developing a model that predicts the distribution of a population of fish both within single pools and over the length of the entire river, as well as the behavioral mechanism responsible. The model uses information on invertebrate drift density and water temperature to predict the growth rate of different sizes of fish at the positions available in the stream. Fish then distribute themselves so that each individual occupies the most profitable position it can defend, with the largest fish winning any disputes. The model suggests that streams can be classified into categories based on the way temperature and drift density vary with the passage downstream. Each of these categories favors a different combination of size gradient and behavioral mechanism. A larger-fish-upstream pattern due to size-dependent habitat preference is predicted for streams with cool temperatures and low drift densities, conditions found in Arctic grayling streams. A larger-fish-upstream pattern due to competition between fish of different size is predicted in warm streams, irrespective of drift density, conditions found in many New Zealand trout streams. A smaller-fish-upstream pattern due to competition between fish of different size is expected in cool streams with high drift abundance; currently no data are available to test this prediction.

Invertebrate drift and benthic community dynamics in a lowland neotropical stream, Costa Rica

In this study we quantified invertebrate drift and related it to the structure of the benthic community, over a 6–8 month period, in a 4th-order tropical stream in Costa Rica. Relative to reports from similar-sized temperate and tropical streams, drift densities were high (2-fold greater: mean 11.2 m−3; range 2.5–25 m−3), and benthic insect densities were relatively low (>3-fold lower: mean 890 m−2; range 228–1504 m−2). Drift was dominated by larval shrimps that represented more than 70% of total drift on any given date; the remaining 30% was composed of 54 insect taxa. Among insects, Simuliidae and Chironomidae (Diptera) and Baetidae, Leptohyphes and Tricorythodes (Ephemeroptera) comprised 24% of total drift. Drift periodicity was strongly nocturnal, with peaks at 18:00 h (sunset) and 03:00 h. Our results, and those of previous experiments in the study stream, suggest that nighttime drift is driven by the presence of predatory diurnal drift-feeding fishes and nocturnal adult shrimps. There were no clear seasonal patterns over both ‘dry’ and wet seasons, suggesting that benthic communities are subject to similar stresses throughout the year, and that populations grow and reproduce continuously.

Riparian ground beetles (Coeloptera, Carabidae) preying on aquatic invertebrates: a feeding strategy in alpine floodplains.

The food and feeding habits of riparian ground beetles were studied in four alpine floodplains (Bavaria, Germany): a 5th-order stream (the Isar) and three 3rd-order streams. The riparian fauna along the streams mainly consists of predaceous species. Riparian ground beetle densities were much higher along the Isar than along the small streams. Aquatic invertebrates composed 89% of the potential prey for carnivorous terrestrial insects along the Isar. Besides aquatic organisms washed ashore, stoneflies emerging on land are of considerable importance as potential prey for terrestrial predators. In contrast, only 34% of the potential prey organisms collected along the small streams were of aquatic origin. Food abundance was 9 times higher in the shore region of the Isar compared to the small streams. Surface drift in the Isar, a potentially important food source for riparian organisms, was about 106 organisms and exuviae per meter stream width in 24 h. The drift density in the Isar was 59 times higher than that in a small stream. Terrestrial organisms provided only 3% of the drifting particles in the Isar, but 50% in the small stream. Gut content analysis reveals, that riparian ground beetles in the Isar floodplain mainly feed on aquatic organisms washed ashore or emerging on land. While small Bembidion species prefer chironomids (larvae and adults) the larger species Nebria picicornis feeds on emerging stoneflies, terrestrial riparian organisms and aquatic organisms accumulating along the shoreline. The prey of riparian ground beetles in the floodplain of the three small streams mainly consists of terrestrial species some of which may have been washed ashore.

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

AbstractThe drift of organisms and large particulate organic matter &gt;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 &lt; 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 &lt; 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 &lt; 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 &lt; 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.

Influence of side‐arms on aquatic macroinvertebrate drift in the main channel of a large river

1. Drift from lotic and lentic side‐arms to the main channel was studied over 2 years in different sections of the Upper Rhône River (France). The hypothesis of an influence of drift on the main channel was tested by measuring drift in the main river at sites downstream and upstream from the confluences with two side‐arms, as well as in the two side‐arms themselves.2. Side‐arms were differentiated from the main channel by having a typical composition of macrofauna with a high propensity to drift, particularly in spring–summer and during spates. Drift densities in side‐arms averaged more than twice those measured in main channel sites, but these inputs did not significantly affect taxon richness and total drift density in the main channel.3. Nevertheless, some taxa found in the more lotic of the two side‐arms were more abundant downstream of the confluence than upstream, indicating that they had drifted into the main channel; no such direct effect was found throughout the sampling period for the more lentic side‐arm. The densities of two competitive filter feeders (Hydropsyche and Simuliidae) were significantly higher below the confluences than above, suggesting that side‐arms were also contributing substantial amounts of seston to the main channel.4. Therefore, side‐arms influence the community structure of drifting macroinvertebrates in the Upper Rhône River, both directly through immigration—depending on the degree of connectivity to the river—and indirectly via food supply.

SOME ASPECTS OF THE INTERACTION OF BLOWING SNOW WITH THE ATMOSPHERIC BOUNDARY LAYER

Several possible effects of blowing snow on the atmospheric boundary layer are investigated, mostly within the general framework of the Prairie Blowing Snow Model (PBSM). The processes of snow saltation and suspension are first described. Variations to the drift density profile are tested and the effects of stratification and density variation calculations are evaluated. Despite high density gradients of blowing snow, stratification effects on turbulence and the velocity profiles can generally be neglected. However, with saltating or suspended snow in a constant shear stress layer, part of the shear stress is carried by the particles. A highly simplified, single-phase approach, based on the density variation of the air-snow mixture coupled to a simple turbulent stress-strain relationship, is used to illustrate this. Sublimation rates in a column of blowing snow are calculated using the PBSM and results are compared with those obtained with a modified formulation which incorporates a spectrum of sublimating particles of varying sizes at each height in a steady-state surface boundary layer and different specifications of the ventilation velocity.