Observations on the vertical distribution of the benthos in a Malaysian stream

An apparatus is described wbich is capable of sampling tbe bentbic fauna to a depth of 50 cm by providing a natural substrate for colonization. Tbe results obtained from an area of visually uniform sediments demonstrate that stream animals occur in significant numbers deep in tbe bottom sediments and that, at most, only about half tbe bentbos lives in tbe upper 10 cm usually ‘quantitatively’ sampled in stream surveys. The significance of a deep-living fauna in regulating community density, in recolonizing denuded areas and in optimizing utilization of available food resources is discussed.     

Quantitative hydrological preferences of benthic stream invertebrates in Germany

Current knowledge regarding the flow preferences of benthic stream invertebrates is mostly based on qualitative data or expert knowledge and literature analysis. These established flow preferences are difficult to use in predictions of the effects of global change on aquatic biota. To complement the existing categories, we performed a large-scale analysis on the distribution of stream invertebrates at stream monitoring sites in order to determine their responses to various hydrological conditions.We used 325 invertebrate surveys from environmental agencies at 238 sites paired to 217 gauges across Germany covering a broad range of hydrological conditions. Based on these data, we modelled the respective probabilities of occurrences for 120 benthic invertebrate taxa within this hydrological range using hierarchical logistic regression models.Our analyses revealed that more than one-third of the taxa (18–40%) can be considered as ubiquitous and having a broad hydrological tolerance. Furthermore, 22–41% of the taxa responded to specific ranges of flow conditions with detectable optima. “Duration high flow event” represented the flow parameter that correlated best with the abundance of individual taxa, followed by “rate of change average event”, with 41 and 38% of the taxa showing a peak in their probability of occurrence at specific ranges of these metrics, respectively. The habitat suitability for these taxa may be potentially affected by global change-induced hydrological changes.Quantified hydrological traits of individual taxa might therefore support stream management and enable the prediction of taxa responses to flow alteration. The hydrological traits of stream benthic invertebrates may be used in forecasting studies in central Europe, and the methods used in this study are suitable for application in other regions with different flow regimes.     

Size-mass relationships of stream invertebrates in a northern Spain stream

We show the regression equations that relate the head width of 10 freshwater insect species or the length of the first thoracic segment of 2 amphipods with their dry mass in 3 reaches along a small stream in northern Spain. Spatial differences in size–mass relationships were found. Organisms stored in alcohol showed lower dry mass than those specimens that were dried immediately after collection. The use of both size measures for dry mass estimation in field samples is discussed. It is recommended that the size–mass relationships be constructed using non-preserved organisms. Furthermore, great care is required when such allometric equations are taken from the literature.     

The occurrence of benthos deep in the substratum of a stream

• 1 The vertical distribution of the benthic fauna of the Speed River, Ontario, was studied over a 13-month period from October 1970 to October 1971. Various physical and chemical parameters of this interstitial environment were also measured.
• 2 Several new techniques for sampling the interstitial environment of rivers wert devised. These methods and their relative efficiencies are considered.
• 3 The validity of the terms ‘hyporheal’ and ‘hyporheic’ are discussed and the term ‘hyporheos’ is offered to replace the former.
• 4 A brief resume of interstitial sampling methods is given with comments on their limitations for sampling deep heterogeneous substrates.
• 5 Chemical parameters are thought to be more important in the control and distribution ofthe fauna than physical parameters.
• 6 It is suggested that many larvae of stream-dwelling chironomids have over-wintering stages when they penetrate deep into the substrate to: (a) actively feed on the trapped organic detritus; (b) follow an optimum temperature for development.
• 7 It is suggested that the shape of an organism determines its success as a hypo-rheic form and examples are given.
• 8 The numbers of animals oecurring in the sub-benthic populations are shown to be very large indeed. For the Speed River, estimates of between 184,760 and 797,960 animals/m^ are made for different times of the year. Dry weight biomass is estimated o t vary between 30 9 g and 253-2 g/m^ throughout the year.
• 9 Sub-benthic or hyporheic populations are shown to exist in at least three other Canadian rivers. Some ofthe animals found are shown to be common to two or more of these rivers.
• 10 The inefficiencies of many conventional benthic samplers in sampling the total biomass of certain streams with hyporheic populations is discussed.

     

Vertical distribution and abundance of invertebrates within the sandy substrate of a low-gradient headwater stream

SUMMARY. 1 The vertical distribution of invertebrates (>0.053 mm) was studied in a sandy-bottomed, first-order stream on the Coastal Plain of Virginia, U.S.A. Invertebrate species composition, abundance and bio-mass were determined monthly over one year at sediment depth intervals of 0–1, 1–5, 5–15,15–30 and 30–40 cm.
2. The subsurface community was numerically dominated by species of Chironomidae, Nematoda and Crustacea, while much of the biomass was due to early instars of several species of Trichoptera.
3. Invertebrate density and biomass decreased significantly with depth in the substrate (ANOVA; P <0.05). Annual mean density decreased from 1,346,844 individuals m⁻³ at the surface to 13,578 individuals m⁻³ at 15–30 cm. Annual mean biomass decreased from 66.30 g m⁻³ at the surface to 0.44 g m⁻³ at 15–30 cm.
4. Dissolved oxygen decreased markedly from the surface to the 5 cm depth in the substrate, anaerobic conditions often occurring below 10 cm. Density and biomass both showed a significant positive relationship with dissolved oxygen concentration (Linear regression; P <0.05).
5. Physical forces were important in structuring the subsurface invertebrate community. Besides low dissolved oxygen concentration, sediment scouring resulting from storm discharge dramatically reduced density and biomass     

All creatures great and small: patterns in the stream benthos across a wide range of metazoan body size

SUMMARY 1. The whole metazoan community (i.e. including the meiofauna) of an acidic, fishless stream in south-east England was surveyed over 14 months between March 1999 and April 2000. Invertebrate density, biomass and taxonomic richness were assessed on each sampling occasion in relation to physico-chemical variables.
2. The meiofauna were more numerous and diverse than the macrofauna, while their total biomass occasionally equalled that of the macrofauna.
3. The meiofaunal and macrofaunal assemblages appeared to respond to different environmental factors. The meiofauna showed genuine species turnover through the year, while the macrofauna varied less in taxonomic composition though there were substantial variations in density.
4. These data suggest that the meiofauna and macrofauna exist at different temporal and spatial scales and perceive their environment with a different 'grain'.     

Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment

1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood‐prone German stream. 2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m²) to a depth of 15–20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near‐bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM). 3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study. 4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near‐bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters. 5. Our results suggest that several ‘direct’ initial effects of local disturbance history on the invertebrates were subsequently replaced by ‘indirect’ effects of disturbance history (via disturbance‐induced changes in habitat parameters such as current velocity or food).     

Drift of stream invertebrates below a cave source

Stream invertebrate drift below the cave source of South Branch Creek, Minnesota, generally increased rapidly. Ephemeroptera and Trichoptera increased rapidly and then decreased at the lower stations (to 430 m) below the cave. Chironomidae drift, in high numbers but low biomass because of their small size, increased fairly rapidly and leveled out at the lowermost stations. Gastropods increased slowly below the cave, reached a maximum, then decreased somewhat at the lowermost station. Drifting oligochaetes, small in size but in very large numbers, increased more slowly below the cave and appeared not to have reached a maximum at the lowermost station. It was concluded that, in general, drift increases fairly rapidly below the stream origin and fluctuates in the upper reaches, probably reflecting benthic population abundance and local ecological conditions, before attaining equilibria downstream, rather than increasing linearly due to cumulative effects. Paper no. 11,172 Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108 Paper no. 11,172 Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108     

Gradients in the distribution of riverine benthos

SUMMARY. 1. Numerous hypotheses regarding the controls on species distributions in streams depend on longitudinal analyses of collections of stream organisms. Techniques for the detection of trends and for the detection of station groupings among biological collections have been widely reported, but rarely compared with each other. We have conducted an analysis of ten or eleven samples from each of thirteen stations along a stream abiotic gradient, and have compared several techniques.
2. An analysis of longitudinal distribution must first assess the existence of a gradient in biotic composition. Random skewers on species proportions were the most sensitive in demonstrating such a gradient. Q and M statistics were most useful in further describing the trend due to their additive properties; i.e. the importance of one or more stations in dictating the trend may be determined. Cluster analysis, as well as percent faunal similarity, was most efficient at isolating station groupings. Techniques such as faunal replacement and species loss calculations are generally more useful in testing hypotheses about controls on species distributions, and should be employed where potential causal factors may be isolated.     

Upstream-downstream movements of aquatic invertebrates in a Rocky Mountain stream

Simultaneous collections of drift and organisms moving either upstream or downstream in association with the substrate were made using a specially designed sampler. Samples were taken in a diel series along a transect across the study riffle of a Colorado foothills stream on six dates over an annual cycle. In addition to longitudinal movements, taxonomic composition and diel periodicity were evaluated. The insect-dominated fauna showed a net downstream displacement. Only the caddisflies Helicopsyche borealis and Hesperophylax occidentalis exhibited net upstream movement, primarily a result of low drift frequencies. The taxonomic composition of moving invertebrates differed from that of the benthos. Drift resembled downstream moving substrate-associated invertebrates in composition, but differed from that of the upstream directed fauna. Taxa collectively exhibited four types of diel patterns: 1) similar downstream (drift and substrate-associated movements) patterns, which generally differed from the upstream pattern; 2) similar benthic (upstream and downstream) patterns, which differed from that of drift; 3) aperiodic patterns; and 4) independent patterns for each type of directional movement. Analysis of size classes based on head capsule width for the mayfly Baetis tricaudatus showed significantly smaller size in stationary individuals compared with moving individuals in the population and revealed that nymphs moving during the day were smaller than those moving at night.     

Scale-dependence of movement rates in stream invertebrates

We used analytical models and random walk simulations in a one-dimensional habitat to study the scale-dependence of migration rates in stream invertebrates. Our models predict that per capita migration rate is inversely proportional to patch length when patches are large compared to the scale of movements. When patches are small the scale-dependence is weaker and primarily determined by the length of individual movements (steps) relative to patch size. Laboratory experiments using isopods ( Asellus aquaticus L . ) and mayfly nymphs ( Baetis sp.) confirmed that the strength of the scale-dependence decreased with increasing step length.
For the case when step length distributions follow an exponential probability distribution, which is often the case for stream organisms, we provide a simple model that allows the scale-dependence to be predicted from the mean step length. We fitted this model to published field data on drift densities at different downstream distances from a net that blocks the drift from upstream areas. Agreement between model and data was excellent in most cases. We then used already published data on the length of induced drift movements to predict the scale-dependence that was observed in block experiments performed in the same system. Predicted and observed scale-dependence showed very close agreement.
We conclude that our models and published data on drift distances can be used to calculate the expected scale-dependence of per capita emigration rates for a large number of taxa under a wide range of environmental conditions.     

Patchy bed disturbance and fish predation independently influence the distribution of stream invertebrates and algae

1. The identification of factors determining the patchy distribution of organisms in space and time is a central concern of ecology. Predation and abiotic disturbance are both well-known drivers of this patchiness, but their interplay is still poorly understood, especially for communities dominated by mobile organisms in frequently disturbed ecosystems. 2. We investigated the separate and interactive influences of bed disturbance by floods and predation by fish on the benthic community in a flood-prone stream. Electric fields excluded fish predators from half of 48 stream bed patches (area 0·49 m(2) ) with contrasting disturbance treatments. Three types of bed disturbance were created by either scouring or filling patches to a depth of 15-20 cm or by leaving the patches undisturbed, thus mimicking the mosaic of scour and fill caused by a moderate flood. Benthic invertebrates and algae were sampled repeatedly until 57 days after the disturbance. 3. Disturbance influenced all ten investigated biological response variables, whereas predation affected four variables. Averaged across time, invertebrate taxon richness and total abundance were highest in stable patches. Algal biomass and densities of five of the seven most common invertebrate taxa (most of which were highly mobile) were higher in fill than in scour patches, whereas two taxa were more abundant in scour and stable than in fill patches. Furthermore, two common invertebrate grazers were more abundant and algal biomass tended to be reduced in fish exclusion patches, suggesting a patch-scale trophic cascade from fish to algae. 4. Our results highlight the importance of patchy physical disturbance for the microdistribution of mobile stream organisms and indicate a notable, but less prevalent, influence of fish predation at the patch scale in this frequently disturbed environment. Disturbance and predation treatments interacted only once, suggesting that the observed predation effects were largely independent of local bed disturbance patterns.     

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.     

Seasonal variations in the benthos of a chalk trout stream,the River Samson,Belgium

Seasonal variations in the benthos of a Belgian chalk stream were investigated. Total biomass varies extensively over the year (30 to 100 g wet wt. m^–2). In summer, however, it is smaller than fish biomass. Densities range from 30 000 to 50 000 individuals m^–2, but the actual number of taxa is always near 60. Diversity indices (Shannon-Weaver and Pielou) exhibit slight fluctuations. Numerical indices are minimal in winter when the stream is in flood. Gravimetric indices are minimal in spring when biomass is maximal. Species-abundance curves almost always indicate a fauna with many rare species, with, also, some indication of a second maximum of moderately abundant species. Six taxa dominate: Oligochaeta, Gammarus fossarum, Ephemeroptera, Coleoptera, Trichoptera and Diptera. There are great fluctuations in their relative importance over a year. Thus, there seems to be a succession of peaks in densities of individual taxa permitting more efficient utilization of resources. However, biomass variations depend on the species involved. As a general rule, maximum biomass precedes maximum density.     

Nonindigenous invertebrates in the periphyton and benthos of upper Volga reservoirs: A review

The mechanisms, ways, and traits of acclimatization of aquatic animals within and outside their historical ranges are reviewed. It is shown on the example of the upper Volga basin that formation still continues of the fauna in reservoirs that began after their impoundment during 1930s–1950s. It is also shown that Ponto-Caspian invaders and intentionally introduced Baikal amphipods play a great role in this process. The data on the traits of biological cycles of mass nonindigenous invertebrates in upper Volga reservoirs are presented. The main ecological factors facilitating expansion of new species in reservoirs are demonstrated. It is shown that the assessment of the functional role of dreissena in waterbodies should include determination of the abundance of the larvae of this species in plankton.     

Field experiments on the mobility of benthic invertebrates in a southern English stream

1. In each of twenty-six week-long experiments, the colonization by macroinvertebrates of boxes of natural sediment in a stony stream was measured. The experiments took place between February and November 1992 and environmental conditions prevailing during the weeks (particularly discharge and temperature) differed widely.
2. Colonization rate also varied widely between the weekly experiments and was sensitive to discharge, temperature and background benthic density, depending on the taxa considered.
3. A 'mobility index' measured colonization rate independently of background benthic density. This index was most strongly and positively correlated with discharge for the abundant stonefly Leuctra nigra, the net-spinning caddis Plectrocnemia conspersa and the Chironomidae, perhaps indicating that their mobility is physically driven by the influence of flow on drift.
4. Mobility in a second stonefly ( Nemurella pictetii ) was greatest in summer, when flows were low but temperature was high. Peak mobility in this species occurred during an ochreous bloom on the stream bed. It is likely the mobility of N. pictetii is more active than the other taxa.
5. There were thus differences among taxa in their mobility at baseflow and in their susceptibility to flow fluctuations. The population consequences of differences in mobility among taxa are discussed.     

The role of fungi in the nutrition of stream invertebrates

Dead leaves falling into streams are an important food source for many invertebrates. They are generally made more palatable and more nutritious if they are first colonized by aquatic hyphomycetes and other micro-organisms. At least two mechanisms appear to be responsible for this conditioning effect: microbial production (addition of easily digested microbial compounds to the nutritionally poor leaf substrate), and microbial catalysis (conversion of indigestible leaf substances into digestible subunits by microbial enzymes). Different invertebrate species vary in their ability to take advantage of microbial conditioning. This appears to be influenced by their mobility, the range of their food resources and their ability to overcome defense mechanisms of leaf-colonizing microorganisms.