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Drift Density (drift + density)
Selected AbstractsOntogenetic changes in the drifting of four species of elmid beetles elucidate the complexity of drift-benthos relationships in a small stream in Northwest EnglandFRESHWATER BIOLOGY, Issue 1 2008J. M. ELLIOTT Summary 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. [source] A comparison of the relative contributions of temporal and spatial variation in the density of drifting invertebrates in a Dorset (U.K.) chalk streamFRESHWATER BIOLOGY, Issue 8 2008MARTIN W. NEALE Summary 1. Invertebrate drift is commonly investigated in streams, with the majority of studies focussed on temporal (typically diel) variation. In comparison, few studies have investigated spatial variation in drift and there is little consensus among them. We tested the hypothesis that spatial variation in invertebrate drift is as important as temporal variation. 2. The density of drifting invertebrates in a chalk stream was sampled using an array of nets arranged to determine vertical, lateral and longitudinal variation. Samples were collected at dawn, during the day, at dusk and by night, on four separate monthly occasions. Insecta and Crustacea were analysed separately to identify the effect of differing life history strategies. The density of drifting debris was also recorded, to act as a null model. 3. Time of day and vertical position together explained the majority of the variance in invertebrate drift (79% for Insecta and 97% for Crustacea), with drift densities higher at dusk and night, and nearer the stream bed. Independently, time of day (38%, Insecta; 52%, Crustacea) and vertical position (41%, Insecta; 45%, Crustacea) explained a similar amount of the observed variance. Month explained some of the variance in insect drift (9%) but none for Crustacea. 4. Variation in the density of drifting debris showed little in common with invertebrate drift. There was little variation associated with time of day and only 27% of the observed variation in debris could be explained by the factors investigated here, with month explaining the largest proportion (20%). We suggest the difference in drifting debris and invertebrates provides further evidence for a strong behavioural component in invertebrate drift. 5. Spatial variation in invertebrate drift can be of the same order of magnitude as the much-described diel temporal variation. The extent of this spatial variation poses problems when attempting to quantify invertebrate drift and we recommend that spatial replication should be incorporated into drift studies. [source] Land-use influences macroinvertebrate community response following a pulse disturbanceFRESHWATER BIOLOGY, Issue 8 2003Kevin J. Collier Summary 1.,We tested the hypothesis that interactions between disturbance types can influence invertebrate community response and recovery in two streams draining pasture (press-pulse disturbance) and native forest (pulse disturbance) catchments before and after a one-in-28-year flood. We also sampled drift and adult insects to gain insights into the relative importance of these two postdisturbance recolonisation pathways. 2.,Taxa numbers and total density declined markedly at the forested site after the flood, but there was a delayed response at the pasture site, reflecting greater initial resistance to this pulse disturbance among taxa adapted to the underlying press disturbance. 3.,Community composition was less stable at the pasture site where per cent abundance of taxa was highly variable prior to the flood and over the 2-year postflood sampling period. After the flood, the pasture stream fauna was more heavily dominated by vagile taxa, including several chironomid species and hydroptilid caddisflies. 4.,Taxa numbers and densities recovered to preflood levels within 5,7 months at both sites, but a range of taxa-specific responses was observed that took up to 18 months to recover to preflood densities. Community stability at the pasture site had not returned to preflood composition by 2 years postflood. 5.,Changes in drift densities of several common stream invertebrates at the pasture site reflected postflood changes in benthic densities and seasonally low drift in winter. Terrestrial invertebrates dominated drift at the pasture site for 3 months postflood whereas Ephemeroptera were most common at the native forest site. 6.,Flight patterns of selected adult aquatic insects showed a strongly seasonal pattern. Abundance of adults at the pasture site in the second year following the flood increased in line with the recovery of the non-Dipteran benthic fauna. Significant upstream flight occurred for several caddisfly species at the native forest site, and weakly directional or downstream flight was evident for most common Plecoptera and Ephemeroptera. 7.,This study indicates that the magnitude and duration of responses to major pulse disturbances can depend on the presence or absence of an underlying press disturbance. This finding has implications for monitoring, and suggests that a knowledge of disturbance history beyond 2 years may be required to interpret mechanisms contributing to observed land-use impacts. [source] Scale-dependence of movement rates in stream invertebratesOIKOS, Issue 1 2004Göran Englund 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. [source] Combined effects of discharge, turbidity, and pesticides on mayfly behavior: Experimental evaluation of spray-drift and runoff scenariosENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2005James M. Dabrowski Abstract The effects of the pyrethroid-insecticide cypermethrin (CYP), increased flow speed (Flow), and increased suspended particles (Part) on drift behavior and activity of mayfly nymphs (Baetis harrisoni) were investigated both individually and in combination in a laboratory stream microcosm. Spray-drift trials were performed by exposing the nymphs to 1 ,g/L of CYP. During runoff trials (CYP × Part), contaminated sediment containing 2,000 ,g/kg of CYP was introduced to the microcosm at a concentration of 500 mg/L. Both trials were carried out under high-flow (CYP × Flow and CYP × Part × Flow) and low-flow (CYP and CYP × Part) conditions, and for all cases, control experiments were performed. Drift rate, drift density (for any treatments with increased flow), and activity were used as behavioral endpoints. Multifactorial analysis of variance shows that CYP exposure significantly increased the drift, whereas Part and Flow trials significantly decreased the drift (p < 0.05). In addition, activity decreased significantly under high-flow conditions. The CYP × Part and CYP × Flow treatments resulted in increased drift rate and drift density, respectively, whereas Part × Flow and CYP × Part × Flow treatments resulted in decreased drift density. The CYP × Part and CYP × Flow trials had a significant antagonistic, interactive effect on drift rate and drift density, respectively, with measured levels being lower than expected levels. The reduction in bioavailability of CYP in the presence of increased flow and sediment levels suggests that mayflies are more likely to be affected by spray-drift exposure (CYP) than by runoff exposure (CYP × Part × Flow). Results indicate that mayflies reacted actively in response to flow conditions and passively in response to pesticide exposure. [source] Brook trout, Salvelinus fontinalis, microhabitat selection and diet under low summer stream flowsFISHERIES MANAGEMENT & ECOLOGY, Issue 3 2006J. C. SOTIROPOULOS Abstract, This study investigated the effects of low summer discharge on habitat, prey use and prey availability for age 1 brook trout, Salvelinus fontinalis (Mitchill), in two small streams in Massachusetts, USA. Stream discharge declined substantially from June to August, with corresponding decreases in microhabitat depth and velocity; but fish habitat preferences were consistent throughout the summer, with fish selecting deep, low current velocity locations. Invertebrate drift rate, drift density and trout stomach fullness were significantly greater in June than August samples. Diets were dominated by aquatic-derived prey (chironomid larvae and adult blackflies) in June, but terrestrial invertebrates were the most frequent diet items in August. Consistent occupancy of low-velocity, deep microhabitats with low invertebrate flux rates indicated that, despite variation in habitat and prey conditions, trout adopted a habitat-use strategy of minimising risks and energy costs rather than maximising forage gain. This observation is consistent with, and provides a potential explanation for, the low summer growth rates of brook trout observed in small streams. [source] Ontogenetic changes in the drifting of four species of elmid beetles elucidate the complexity of drift-benthos relationships in a small stream in Northwest EnglandFRESHWATER BIOLOGY, Issue 1 2008J. M. ELLIOTT Summary 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. [source] | ||||||||||