Ontogenetic changes in the drifting of four species of elmid beetles elucidate the complexity of drift-benthos relationships in a small stream in Northwest England

1. This study aimed to quantify ontogenetic changes in the drifting of Elmis aenea, Oulimnius tuberculatus, Esolus parallelepipedus and Limnius volkmari (Coleoptera: Elmidae), and to relate their drift to benthic density. Monthly samples were taken over 39 months, using three surface nets at each of two contrasting sites in a small stream: one in a deep section with abundant macrophytes, and the other in a shallow stony section. 2. Most larvae and adults were taken in the drift at night with little variation between catches in the three nets at each site. Day catches were very low, often zero. No significant relationships could be established between mean numbers in the drift catches and benthic densities. 3. When night catches were converted to drift densities (number caught per 100 m³ of water sampled), the latter were positively related to monthly losses in the benthos, but not to benthic densities. A linear regression described the relationship, and equations for the different life‐stages within each species were not significantly different from the equation for all life‐stages combined. However, drift losses were only about 0.07% of total losses in the benthos. A severe spate in October 1967 increased the number of larvae and adults in the drift, but not drift densities, except for immature adults of E. aenea, O. tuberculatus and E. parallelepipedus. 4. Key life‐stages with the highest drift density were the earliest life‐stage soon after egg hatching for E. aenea, the start of the larval overwintering period for O. tuberculatus and L. volkmari, and mature adults during the mating season for all three species. Drift density for E. parallelepipedus was too low to identify a key life‐stage. These key life‐stages corresponded with critical periods for survival in the life cycle, as identified in an earlier study in the same stream. Mortality was high during these critical periods, hence the strong relationship between drift density and benthic losses. The latter relationship was very consistent for different life‐stages within each species, and partially supported the rarely‐tested hypothesis that drift represents surplus production in the benthos.     

Intra- and interspecific dominance hierarchies and variation in foraging tactics of two species of stream-dwelling chars

Aggressive interactions, foraging behavior, habitat use and diet were studied in sympatric populations of white-sported char,Salvelinus leucomaenis, and Dolly Varden,Salvelinus malma, in a Japanese mountain stream. Underwater observations on individuals of both species revealed two distinct behavioral regimes: aggressive drift foragers and non-aggressive benthos foragers. Aggressive drift foragers defended partial territories around focal points from which they made forays to capture invertebrates drifting in the water column. Non-aggressive benthos foragers cruised around and beneath cobble in large foraging ranges that overlapped each other. Intra- and interspecific, size-dependent dominance hierarchies were recognized among aggressive drift foragers, whereas non-aggressive benthos foragers showed no such relationships. Terrestrial invertebrates were the most abundant prey in the diets of drift foragers, whereas a very small proportion of the diet of benthos foragers was made up of these taxa. Benthos foragers showed more complex diet composition than drift foragers. These results suggest that non-aggressive benthos foragers may avoid not only interference but also exploitative competition by using alternative foraging tactics. The proportion of drift foragers to benthos foragers among white-spotted char was more than 35 times that among Dolly Varden. The significant difference in the proportion of each species using the two types of foraging strategy results in interspecific food segregation in sympatric populations.     

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.     

Dynamics of Invertebrate Benthic Communities and Drift in a Regulated River of Central Spain

We document invertebrate benthic and drift dynamics in a regulated river in central Spain at two temporal scales: seasonal (for both benthos and drift) and daily (for drift). The benthic abundance of individuals and taxon richness generally increased in the summer. Drift abundance showed no seasonal or daily variation, but taxon richness of drifting individuals was higher in the spring. Both ben‐thos and drift showed clear seasonal changes in taxonomic composition. Interestingly, some benthic taxa showed their highest abundances in the spring, while others were more abundant in the summer. In contrast, most drifting taxa were more abundant in the spring. Different functional feeding groups showed different patterns of variation throughout the year, both in the benthos and the drift. Daily variations in drift were present in very few taxa and functional feeding groups, and only in some seasons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Active entry of stream benthic macroinvertebrates into the water column

Field experiments investigated the possible active entry of stream benthos into the water column. Over a 1-year period, sediment baskets were suspended for 24 hours in a stream pool so that only swimming or floating organisms, essentially unaided by current, could colonize them. A variety of benthos, including taxa characteristic of riffles, colonized the baskets, with colonization highest in late summer and negligible in winter.A modified drift net towed through the pool was used to quantitatively sample benthic animals actually in the water column. Nighttime tows captured a diverse, abundant fauna and indicated densities substantially higher than invertebrate drift densities reported in the literature. Daytime tows yielded little. Estimated percentages of the benthos in the water column at a given time were generally < 1.0%.These findings suggest that not all invertebrate drift is the result of passive mechanical removal from the substrate by current.     

A continuous study of the total drift of freshwater shrimps, Gammarus pulex, in a small stony stream in the English Lake District

1. The objective was to determine the major factors affecting the downstream dispersal (drift) of freshwater shrimps, Gammarus pulex. Sample replication and frequency are major problems in the quantification of drift. For the first time, these problems were avoided by sampling the whole stream continuously so that all the shrimps drifting downstream at the sampling point were caught in a net emptied at dusk and dawn in 1966, and every 3 days in 1967. 2. There was no consistent seasonal pattern in drift rates, but a high proportion of annual drift was taken in only a few samples. There was a nocturnal diel pattern of drift with peaks soon after dusk and just before dawn. A power function described the significant (P < 0.001) relationship between drift and flow, and was used to neutralise the dominant effects of flow by standardising total drift over 24 h, nocturnal drift and diurnal drift (drift per 50 m³). These were all significantly (P < 0.001) related to benthos density, but not to date, temperature, or length of the night or day. 3. The relationship between drift values and the independent variables, flow and benthos density, was well described (P < 0.001) by a multiple‐regression model. Adding temperature, date, and/or the length of the night or day did not improve model fit. Variations in flow and benthos density explained 94% of the variation in total drift over 24 h, 97% of the variation in nocturnal drift, but only 44% of the variation in diurnal drift. A power function described (P < 0.001) the relationship between total drift and the volume of water sampled over 3‐day periods in 1967. Flow explained 95% of this drift variation; it was unnecessary to add another variable such as benthos density. 4. The significance of this study is that it avoided the problems associated with the quantification of drift samples. Therefore, the conclusions are more robust than those of many previous studies. A high proportion of the annual drift losses would have been undetected by intermittent sampling. Temperature, season, night or day length had no significant effect on drift densities, and the relationship between drift and benthos densities was proportional, not density dependent. The nocturnal increase in drift could not be interpreted as an antipredator behaviour. The dominance of flow and benthos density was apparent but the quantitative relationships posed further questions, especially those related to drift distances at different velocities.     

Drift patterns in a high Andean stream

The drifting of invertebrates was sampled for six 24-hour periods from September 1976 to July 1977 in a small stream of the paramo of the Ecuadorian Andes. The composition of the drift is similar to that of the benthos, though percentages may differ markedly. Drift is relatively constant throughout the year, except in March when unusually high rates were noted; at this same period a marked reduction in the benthos was also observed. Diel periodicities in the drift are unclear, although on the whole drift is more important during the daytime; when individual data series are analyzed, weak patterns of day or night drifting can be recognized in some groups. The input of insects through drift from the small streams is thought to be an important source of food for the salmonid fish inhabiting the larger torrents.     

Meiofauna constitute a considerable portion of invertebrate drift among moss-rich patches within a karst hydrosystem

Aiming to establish the most frequent invertebrate taxa in drift at the small spatial scale within a moss-rich karst tufa-precipitating hydrosystem, we sampled drift among microhabitats differing in substratum type and flow conditions along a tufa barrier-cascading lotic reach. Additionally, we addressed the question of the contribution and the potential significance of meiofauna within the overall invertebrate drift at the small spatial scale. During the study period, a total of 60 invertebrate taxa were recorded in the drift. Six of these taxa belonged to the annelid/arthropod meiofauna and they represented 35% of total drift density. Macroinvertebrates found in drift were represented mainly by larval insects. The composition of the most abundant taxa in total drift was as follows: Alona spp. (Cladocera 26.7%), Riolus spp. (Coleoptera: Elmidae 13.2%), Simulium spp. (Diptera: Simuliidae 12.2%), Enchytraeidae (Oligochaeta 10.4%), Hydrachnidia (6.3%), Orthocladinae (Diptera: Chironomidae 3.9%) and Naididae (Oligochaeta 3.6%). Faunal drift densities and amounts of transported particulate matter (PM) were highest at the fast-flowing sites located at the barriers and lowest at the slow-flowing sites within pools. Similarly to the seasonal amounts of transported PM, faunal drift was lowest in winter, and peaked in autumn and in late spring/early summer. Correlation between flow velocity and PM-faunal drift densities suggested a significant effect of the dislodged PM, though a minor influence of discharge and flow velocity on faunal drift. We suggest that the small-scale habitat heterogeneity and the respective feeding and refugial strategies of the fauna, as well as faunal passive dislodgement initiated by the shear forces of the flow were the most important drivers of observed drift patterns.     

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.     

Invertebrate drift in the regulated River Tees, and an unregulated tributary Maize Beck, below Cow Green dam

SUMMARY. The Cow Green dam was completed in the summer of 1970 and invertebrate drift was sampled below the dam and in an adjacent tributary, Maize Beck, on thirty-one occasions between July 1970 and September 1973. Drift was sampled by pumping river water through a filter. The intake was placed in Maize Beck for the first sample and in the Tees for the second, and so on alternately for the rest of the sampling period. Nets were used on ten occasions, nine of these in winter months and once when the pump broke down. A total of ninety-five taxa were recognized, of which eighty-six occurred in Maize Beck and seventy-one in the Tees. The Tees fauna was dominated numerically and in terms of biomass by a large population of micro-crustaceans originating in the reservoir. Hydra and Naididae also formed a large proportion of the Tees drift but contributed little to the biomass. Ephemeroptera were most abundant in Maize Beck samples. Diptera were abundant in drift catches in both streams with simuliid larvae most numerous in Maize Beck and chironomid larvae most numerous in the Tees. The greatest drift densities of the benthic fauna were observed between April and October; the mean number of organisms per 10 m³ were seventy-three in Maize Beck and 144 in the Tees. The mean densities in winter were very low, respectively two and seventeen per 10 m³ in the two rivers. There was no significant difference between the mean levels of the total bottom fauna (numbers and biomass) in the drift in the two rivers during the period April-October, but vrtnter biomass was significantly greater in the Tees. In July 1970 micro-crustaceans represented 29% (14 per 10 m³) of total drift numbers and 3% (0.7 mg wet-weight per 10 m³) of the biomass, whereas in 1973 they represented 99% of both the numbers (37 670 per 10 m³) and weight (2.2 g wet-weight per 10 m³). The relation between benthos and drift was examined. In the drift Plecoptera and Baetidae were more abundant in Maize Beck than in the Tees. Only Chironomidae and Nais spp. were more abundant in the Tees, In the benthos the density of Plecoptera and Baetidae was not significantly different in the two rivers, but all other groups with the exception of Simuliidae occurred at greater densities in the Tees. The proportion of baetids present in the drift was greatest in Maize Beck. No such difference was demonstrated for total fauna. Diel rhythms were observed in baetids and simuhids with densities greater in night catches. Nocturnal peaks of these organisms were less pronounced in the Tees. Chironomid larvae showed no diel changes in abundance. Significant diel changes in the mean weights of individual animals were not detected in baetid nymphs or chironomids. Micro-crustaceans showed no nocturnal peaks of abundance. Preliminary observations on the quality and quantity of seston caught in drift samples between April and October showed great differences between the rivers. In the Tees the bulk ofeach sample consisted of algal filaments derived from the river and micro-crustaceans from the reservoir. In Maize Beck algae were un-common and the sample was composed of peat and mineral particles. Data are presented on seston output at different discharges.     

Size characteristics and diet of emergent chinook salmon in a small, stable, New Zealand stream

The emergence timing, quality and diet of chinook salmon, Oncorhynchus tshawytscha , fry were studied as they emerged from ten natural redds. The impact of selected environmental factors on emergence was studied also. Samples of fry were obtained daily using redd traps.
The mean period of total emergence for redds with more than 450 fry emerging was 44.8 days (range 25–57 days), but for 80% of the fry in each redd to emerge the mean time elapsed was 10.8 days (range 6–21 days). No fry sampled had a visible yolk sac. The effect of temperature and light on emergence timing was weak, but stream flow had a strong influence. Fry length was constant for each redd throughout emergence, but fry weight tended to decline; consequently condition factor declined also. There was evidence that fry fed before emergence, but they did not grow. The major prey item, Deleatidium spp. (47.4% of the contents of all stomachs sampled), was the most abundant invertebrate in stream benthos and drift. Empty stomachs were prevalent at the start of emergence, with full stomachs becoming more common as emergence progressed.
The time distribution of numbers of fry emerging was usually unimodal, but occasionally bimodal. Over one third of fry emerged during the best 2 days of sampling.     

Drift dynamics of Chironomidae larvae: I. Preliminary results and discussion of importance of mesh size and level of taxonomic identification in resolving Chironomidae diel drift patterns

Diel drift samples utilizing nets with mesh size less than 200 microns were taken in Linesville Creek, Pennsylvania, an eastern deciduous forest stream, and Inlet Run, Wyoming, an alpine snow melt stream. Identification of drifting Chironomidae larvae to lowest level taxonomic categories indicated 51 species or species group categories representing 51.95% of the total insect drift in Linesville Creek and 18 species or species group categories representing 70.47% of the total insect drift in Inlet Run. Orthocladiinae were the predominant larvae in the drift in Linesville Creek, with 19 species comprising 43.84% of the Chironomidae drift. In decreasing abundance were Chironomini (12 species, 40.36% of Chironomidae drift), Tanytarsini (10 species, 8.89%), and Tanypodinae (10 species, 6.91%). By contrast, Diamesinae were the predominant larvae in the drift in Inlet Run, (5 species, 71.43%) followed by Orthocladiinae (10 species, 27.25%), Tanytarsini (2 species, 1.20%), and Podonominae (1 species, 0.12%). Comparison of drift composition with substrate samples and/ or emergence data indicated a close relationship between relative abundance in drift and relative abundance in the benthos. Behavioral drift patterns with nocturnal peaks were seen for 3 species or species groups in Linesville Creek. Four species with diurnal drift peaks were present in Inlet Run. Analysis of the size distribution of drifting larvae indicates that a mesh size as small as 200 microns is required to resolve diel drift patterns. It is postulated that random factors greatly influence the apparent diel drift pattern of Chironomidae when nets with mesh size in excess of 400 microns are employed in drift studies. Conflicting literature reports of behavioral drift for Chironomidae may be due to differing species composition of drifting larvae and net mesh size related artifacts.     

The feeding relationships of two invertebrate predators in a New Zealand river

The size frequency distributions and foods of the larvae of Archichauliodes diversus (Megaloptera: Corydalidae) and Stenoperla prasina (Plecoptera: Eustheniidae) were studied for a year in the Glentui River, South Island, New Zealand. These two species are the largest invertebrate predators inhabiting streams and rivers in New Zealand, where they are the only carnivorous members of their respective orders. Both species occupied the same habitat with A. diversus being slightly more abundant in most months. Many small larvae occurred within the sediments of the stream bed, whereas more larger larvae were found on stones lying on the surface. A wide size range of larvae was present throughout the year and the median size (in terms of head width) of both species was similar in most months. Larval growth could not be determined from field data and the life cycles of both species can be described as non-seasonal. Quantitative sampling in 5 months provided estimates of population densities. These were maximal in December when values of 136/m² for A. diversus and 114/m² for S. prasina were obtained. Larval Chironomidae and mayflies of the genus Deleatidium were the most frequently taken prey of both predators in all months, and no strong relationship between size or species of predator and size of prey was found. Caseless trichopteran larvae formed a less important component of the diets of both species and detritus was ingested by some individuals. No instances of cannibalism and only one example of cross-predation by each predator was seen. Diel sampling in November showed that large Deleatidium larvae were relatively more abundant in the guts of insect predators and in nocturnal drift samples than in the benthos. This suggests that their greater activity at night may increase their susceptibility to predation by nocturnal feeders. No clear ecological segregation of the two species with respect to habitat or prey utilization was found and there was no obvious interspecific competition for food, which appeared to be abundant at all times. Finally, the prevalence of non-seasonal life cycles in New Zealand aquatic insects is discussed in relation to the low degree of speciation in several groups.     

Diel changes in the benthos of stones and of drift in a southern Australian upland stream

Diel changes in the density of the fauna of stones and of drift were investigated in the Toorongo River, an upland river of southern Australia. The densities of the ten most common taxa and of the total fauna in the drift and on the stones were negatively correlated, with 16 out of 33 cases being significant (p < 0.05). Five of the ten most common taxa displayed a general trend of reaching day-time peaks in the benthos (11 out of 15 cases) and night-time peaks in the drift (10 out of 15 cases). The total density on the stones reached a significant peak in the day-time while the total drift density peaked at night.     

Oligochaete drift in a large river (French Upper Rhône): the effect of life cycle and discharge

In the framework of a macroinvertebrate drift study, the oligochaete community was analysed over one year (1989) at six sites located in a partly impounded reach of the Upper Rhône River (France). Drift composition between sites was somewhat affected by the environmental conditions. These differences were caused by significant deviations in drift rates (P < 0.05) of endobenthic oligochaetes and non-swimming Naididae. There was no significant difference (P > 0.05) among the swimming Naididae. As the latter strongly dominated, global spatial differences in the drift were weak. Temporal changes constituted the main source of variability in drifting oligochaetes. Life cycles and reproductive ecology of four major swimming Naidid species, Nais elinguis, Stylaria lacustris, Nais christinae, and Nais barbata, explained most of the drift structure over the year. Hydrological fluctuations also affected drift structure, both for swimming and non-swimming species. Taxon richness and relative abundance in the drift were very similar to those obtained in the river benthos from a reference reach previously studied. There was a lack of stygobiont and stygophilous species that are frequent in area of groundwater seepage. Biological and environmental implications of oligochaete drift are stressed. The drift phenomenon is appropriated to provide ecological informations on oligochaete communities in fast flowing and deep, large rivers.     

Fish drift in a Danube sidearm‐system: I. Site‐, inter‐ and intraspecific patterns

A total of 10 649 larval and juvenile fishes of 24 species were caught in the drift at Marchfeldkanal, a man‐made side branch of the Danube River near Vienna, Austria, with tubenose goby Proterorhinus marmoratus being the dominant species. Distinct differences in inter‐ and intraspecific drift patterns among different sampling stations along the course of the channel were found. The percentage of the rheophilic and rheoparous ecological guild was highest at the inlet of the channel, where it is directly fed with water from the Danube. For individual species, significant differences in drift densities among sites were found for tubenose goby, barbel Barbus barbus , ide Leuciscus idus , roach Rutilus rutilus and for the two most abundant percids, the pike‐perch Sander lucioperca and zingel Zingel zingel combined. The occurrence of larval and juvenile fishes in the drift was related to certain developmental stages and differed between species and sites. Most species (common bream Abramis brama , bleak Alburnus alburnus , gudgeon species Gobio spp., chub Leuciscus cephalus , ide and roach) occurred with highest densities at the earliest developmental larval stage, but some species ( e.g . common bream and roach) were also found abundantly in drift at later developmental stages. Application of Ivlev's index of electivity as a drift index describing the propensity of the different species to drift, yielded the highest indices for the gudgeon species, common bream and bleak and the lowest for perch Perca fluviatilis , Prussian carp Carassius auratus gibelio and rudd Scardinius erythrophthalmus .     

Dispersal in drift-prone macroinvertebrates: a case for density-independence

1. Studies of dispersal of macroinvertebrates in streams and rivers tend to be focused on drift, whilst benthic movements are usually considered to be less important.
2. Field-enclosure experiments with the mayfly Baetis rhodani indicate that net dispersal in this species is simply a proportional loss of individuals from the benthos.
3. Neither net upstream or downstream movements exhibited evidence of density-dependence in the form of curvilinear relationships between initial and final densities.
4. The net number of animals moving upstream did not differ significantly from the net number moving downstream.
5. The probable mechanisms behind density-independent dispersal are discussed, as are the implications for our understanding of population dynamics in relation to invertebrate drift.     

Sponges (porifera) distribution along a depth gradient in a coral reef, Parque Nacional San Esteban, Carabobo, Venezuela

Sponges constitute one of the most diverse and abundant animal groups in the marine tropical benthos especially in coral reefs, though poorly studied to species level. The aim of this study is to characterize the sponge community along a depth gradient at Isla Larga (Parque Nacional San Esteban, Venezuela) fringe reef. Net and total sedimentation, roughness index, sponge species richness, density and proportion of the bottom covered by sponges, were evaluated at seven depths (1, 3, 6, 9, 12, 15, 18 m), 17 species were identified grouped in 10 demosponges families. The highest densities and coverage corresponded to 6 m of depth (6.03ind/m2; 11%), that coincides with the lowest net sedimentation and highest substrate heterogeneity. Most abundant species were Desmapsamma anchorata, Amphimedon erina and Scopalina rueztleri. Principal component analysis divided this community in three zones according to depth. The shallow zone of the reef (1 and 3 m), where wave force and high irradiance exert a constant stress sponges, shows the lowest density and coverage by sponges. In contrast, medium depth (6, 9 y 12 m) and deep zone (15 y 18 m) with lower light and sedimentation levels seem to enhance sponge growth and survival that are reflected on the higher densities and coverage of sponges.     

Crowded waters: short-term response of invertebrate drift to water abstraction

Water abstraction modifies the environmental conditions of stream ecosystems, which can affect invertebrate assemblages by altering drift. We examined this issue with a before–after–control–impact design experiment in which we diverted 90% of the natural flow from a headwater stream. We measured flow-reduction effects on drift densities (animals/m³), total drift rates (animals leaving the reach per hour) and net balance of invertebrates entering or leaving the Impact reach. We also identified the specific taxa and traits that drove these responses. The sudden decrease in flow promoted a 12-fold increase in overall drift density at the Impact reach, which was primarily driven by filterers, shredders and taxa associated with fast velocities, such as simulids. By contrast, drift densities of other abundant taxa, such as Baetis, Esolus and chironomids, increased less than what could be expected from the magnitude of flow reduction. While drift rates remained unchanged after water abstraction, the Impact reach became a net invertebrate exporter indicating that many taxa drifted actively as a response to stressful conditions rather than passively, which would be reduced by water abstraction. Therefore, our results suggest that water abstraction influences drift, with potentially important consequences for the invertebrate assemblages and ecosystem processes further downstream.