Effects of experimental flow regulation on invertebrate drift and stranding in the Flathead and Kootenai Rivers,Montana, USA

Studies were conducted to determine the effects of experimental manipulations of discharge on invertebrate drift in two regulated rivers in northwestern Montana, USA. During these studies the discharge regime in the Flathead River was characterized by frequent flow fluctuations, while in the Kootenai River high discharge was maintained for much longer periods before flow was reduced to minimum discharge. The magnitude of the response of invertebrates to disturbance was different in the two rivers, in part because of the different frequencies of flow changes. Midstream invertebrate drift increased an order of magnitude during increasing discharges in the Flathead River but was not substantially increased during decreasing discharges. When the prior discharge regime had been sustained at high levels in the Kootenai River, invertebrate drift densities as high as 300 000/100 m³ were measured along the shoreline following reductions in discharge, both immediately after flow began to decrease and after dark on the same day. There was also more recolonization of shoreline areas and more stranding of insects following dewatering of nearshore regions when there had been sustained high discharge levels prior to the flow reduction. More insect stranding occurred during a faster rate of decrease in discharge (50 000 to 100 000 organisms m⁻²).     

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 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     

Correlations between species richness and exposure: Freshwater molluscs and macrophytes

The present paper describes changes in the chemical composition of water and the animal communities below three dam reservoirs lying in ecologically different river sectors and differing in the degree of water pollution. The reservoirs transform the chemical composition of the water in various ways, depending on the quality of the inflowing water and the type of reservoir. Changes in the zoocenoses are caused by various factors. In clean or only slightly polluted rivers changes in physico-chemical properties of the water are responsible for the changes in the zoocenoses. In reservoirs from which only a small amount of water flows out reduction inflow is most important, while in polluted rivers it is pollution which determines the composition of the zoocenosis.     

A study on the faeces of some chalk stream invertebrates

An account is given of the size, form, texture, colour, cohesion and composition of the faeces of 41 species of invertebrates found in chalk streams. The relationships between the character of the faeces and the taxonomic positions and habits of the producers are considered.The importance of faeces production relative to the bulk of stream bed sediments is discussed. Faeces of different origins accumulate in different areas of the stream bed and these differences may be associated in part with their form and structure and in part with the distribution of the species from which they originate. In the summer months tubificid worms alone may be responsible for reworking between 0.3% and 0.5% of the fine particulate material in sediments every day.     

The influence of abiotic factors and density-dependent mechanisms on between-year variations in a stream invertebrate community

Samples of invertebrates were taken in two seasons (October and April) over 5 years in a trout stream in Belgium. Diversity, total benthic invertebrate density, and densities of 27 of 34 taxa tested showed significant differences between years. Significant seasonal effects and interactions were observed in 18 and 7 taxa respectively. The most common pattern of between-year variation was related to hydrological events (flooding) which caused low densities for many species. Most species recovered rapidly, but the recovery of some predators seemed to show a delay of one year. The amphipod Gammarus fossarum appeared to respond positively to flooding as well as to certain post-flood conditions. A significant positive correlation between mortality and the logarithm of initial density was obtained for 6 of 11 species tested. The relationships between mortality and abiotic factors such as water temperature, mean and maximum water level, were very poor. A hierarchy of environmental factors in the regulation of stream macroinvertebrate populations is suggested with: (1) exceptional abiotic events acting in a density-independent manner; and (2) biotic factors that may account for density-dependent regulation.     

Effects of substratum stability on diversity of stream invertebrates during baseflow at two spatial scales

1. The effects of substratum stability on the diversity of stream invertebrates were assessed at two spatial scales in a Japanese stream during baseflow, from May to June 1998. Deposition and erosion were examined separately as distinct elements of substratum stability by a newly developed method using small steel pins. Stream invertebrates were sampled after 28 days of measurement of substratum stability. We also measured physical environmental variables, current velocity and depth, and food resource parameters including periphyton and particulate organic matter (POM) standing crops.
2. At the scale of the habitat patch, the effects of substratum stability on invertebrates were overwhelmed by those of POM standing crop. Moreover, higher taxon richness at high abundance may simply result from a higher likelihood of more taxa being included in samples. Therefore, this small scale revealed no role for substratum stability in explaining spatial pattern of community diversity.
3. At the reach scale ( n =10), taxon richness and evenness peaked at an intermediate level of deposition, whereas invertebrate abundance did not show any significant relationship. This result, and the pattern of relative abundance of common taxa, implies that the diversity of stream invertebrates was determined by subtle substratum movements and by the habitat preference of each taxon.
4. The difference in the determinant of community parameters between the habitat patch and the reach affirm the importance of a cross-scale analysis to choose an appropriate spatial scale for investigating the community structure of stream invertebrates. Prominent effects of substratum stability, particularly the deposition of substratum particles, during baseflow suggest that subtle and constant movement of small substratum particles can contribute to determine the diversity of stream invertebrates.     

Restricted movement by mottled sculpin (pisces: cottidae) in a southern Appalachian stream

1. We used direct observation and mark‐recapture techniques to quantify movements by mottled sculpins (Cottus bairdi) in a 1 km segment of Shope Fork in western North Carolina. Our objectives were to: (i) quantify the overall rate of sculpin movement, (ii) assess variation in movement among years, individuals, and sculpin size classes, (iii) relate movement to variation in stream flow and population size structure, and (iv) quantify relationships between movement and individual growth rates. 2. Movements were very restricted: median and mean movement distances for all sculpin size classes over a 45 day period were 1.3 and 4.4 m respectively. Nevertheless, there was a high degree of intrapopulation and temporal variation in sculpin movement. Movement of juveniles increased with discharge and with the density of large adults. Movement by small and large adults was not influenced by stream flow, but large adults where more mobile when their own density was high. Finally, there were differences in the growth rates of mobile and sedentary sculpins. Mobile juveniles grew faster than sedentary individuals under conditions of low flow and high density of large adults, whereas adults exhibited the opposite pattern. 3. Our results support the hypothesis that juvenile movement and growth is influenced by both intraspecific interactions with adults and stream flow. In contrast, adult movement appears to be influenced by competitive interactions among residents for suitable space. The relationship between movement and growth may provide a negative feedback mechanism regulating mottled sculpin populations in this system.     

Inconsistency in the experimentally derived relationship between epilithon abundance and the micro-distribution of Agapetus monticolus (Trichoptera)

Abstract Studies of the micro-distribution of herbivorous lotic macroinvertebrates have tended to rely on short-term experiments. While these experiments have provided information on the factors that determine the distributions of herbivorous invertebrates, they ignore the complexity of the epilithon as a food source, any interaction between physical and biological factors and the possibility that the factors that determine an invertebrate species’ distribution may vary through time. Five colonization experiments were performed in an Australian upland stream to determine the role of epilithon abundance in the distribution of an abundant invertebrate (Agapetus monticolus: Trichoptera). Artificial cobbles with different types and amounts of epilithon were placed in the Taggerty River at two sites and the response of A. monticolus recorded. The experiments revealed that, although A. monticolus expressed a preference for patches of epilithon with abundant periphyton, the response varied among experiments performed at different times and was always secondary to the flow preferences of A. monticolus. Studies that fail to acknowledge the presence of temporal variation in the response of animals to their environment run the risk that they will over- or underestimate the importance of particular factors because of the timing of their experiment.     

Ecoregion and land‐use influence invertebrate and detritus transport from headwater streams

Summary 1. Habitats are often connected by fluxes of energy and nutrients across their boundaries. For example, headwater streams are linked to surrounding riparian vegetation through invertebrate and leaf litter inputs, and there is evidence that consumers in downstream habitats are subsidised by resources flowing from headwater systems. However, the strength of these linkages and the manner in which potential headwater subsidies vary along climatic and disturbance gradients are unknown. 2. We quantified the downstream transport of invertebrates, organic matter and inorganic sediment from 60 fishless headwater streams in the Wenatchee River Basin located on the eastern slope of the Cascade Range in Washington, U.S.A. Streams were classified into four groups (each n = 15) based on their position within two ecological subregions (wet and dry) and the extent of past timber harvest and road development (logged and unlogged). 3. Time and ecoregion were significant for all response variables as transport varied across sampling periods, and dry ecoregion streams displayed significantly higher mean values. Logged sites also generally showed higher mean transport, but only inorganic sediment transport was significantly higher in logged sites. Both ecoregion and land‐use interacted significantly with time depending on the response variable. Differences among stream categories were driven by relatively low levels of transport in unlogged drainages of the wet ecoregion. Interestingly, unlogged dry ecoregion streams showed comparable transport rates to logged sites in the wet ecoregion. Dominance by deciduous riparian vegetation in all but unlogged streams in the wet ecoregion is a primary hypothesised mechanism determining transport dynamics in our study streams. 4. Understanding the quantity and variation of headwater subsidies across climate and disturbance gradients is needed to appreciate the significance of ecological linkages between headwaters and associated downstream habitats. This will enable the accurate assessment of resource management impacts on stream ecosystems. Predicting the consequences of natural and anthropogenic disturbances on headwater stream transport rates will require knowledge of how both local and regional factors influence these potential subsidies. Our results suggest that resources transported from headwater streams reflect both the meso‐scale land‐use surrounding these areas and the constraints imposed by the ecoregion in which they are embedded.