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.     

Time spent in the drift by downstream-dispersing invertebrates in a Lake District stream

1. The objective was to determine the time spent in the drift by different taxa of stream invertebrates. Most data were obtained from an earlier experimental study to determine the distances travelled by drifting invertebrates of 16 taxa in Wilfin Beck. Experiments were performed at two sites: ‘site 4’ in a stony, fast‐flowing, section of stream, ‘site 3’ in a deeper stream section where macrophytes were abundant. 2. The significant relationship between the mean distance x (m) travelled in the drift and modal water velocity V (m s^?1) was described by a power function in the earlier study but, as the power was close to one, a linear relationship has now been found to provide a satisfactory model. The rate of increase in x (m) with increasing V varied considerably between taxa. The mean time [mean t (s)] spent in the drift was estimated by dividing each x (m) by the appropriate V. Mean t (s) for each taxon was usually very constant over a wide range of V at each site (0.10–0.60 m s^?1 at site 4, 0.15–0.53 m s^?1 at site 3). A simple model estimated the time spent in the drift by different percentages (e.g. 75, 50, 10 and 1%) of the drifting invertebrates. 3. The experimental taxa at site 4 were divided into three groups according to the mean time spent in the drift. Mean t (s) for the five taxa in group 1 (32.8 s) was not significantly different from that obtained in control experiments with a mixed group of dead invertebrates. A similar time (33.0 s) was obtained for the five taxa in group 2, except at water velocities less than 0.2 m s^?1 when the mean t (s) decreased to 15–21 s. Mean t (s) was constant for each of the six taxa in group 3, and significantly less than that for groups 1 and 2. Mean values ranged from 28.8 s for Ephemerella ignita to only 9.4 s for Baetis rhodani and Gammarus pulex. All mean values were lower at site 3, presumably because of the dense stands of macrophytes, with mean values of 12.9 s for the five taxa in group 1 (equalling the value for dead invertebrates). Mean values for the six remaining taxa varied from 6.4 s for Simulium spp. to only 4.9 s for Baetis rhodani and 4.8 s for Gammarus pulex. It was concluded from a discussion of this study that the time spent in the drift may provide a useful measure for comparing the downstream dispersal of invertebrates in different streams, and may be a useful addition to models for the drift feeding of salmonids.     

Day–night changes in the spatial distribution and habitat preferences of freshwater shrimps, Gammarus pulex, in a stony stream

1. As many invertebrates are nocturnal, their spatial distribution and habitat preferences may change from day to night. Both aspects are examined for Gammarus pulex by testing the hypotheses: (i) a power function was a suitable model for the spatial distribution of the shrimps in both day and night; (ii) diurnal and nocturnal spatial distributions were significantly different; (iii) diurnal and nocturnal habitat preferences were significantly different. Five different life‐stages were treated separately. To ensure that the conclusions were consistent, large samples were taken near midday and midnight in April, June and November over 4 years at two sites about 3 km apart in a stony stream: downstream (n = 30) and upstream (n = 50). 2. The first and second hypotheses were supported at both sites. A power function, relating spatial variance (s²) to mean (m), was an excellent fit in all analyses (P < 0.001, r² > 0.91), i.e. the spatial variance was density‐dependent. All five life‐stages were aggregated in the day. At night, the degree of aggregation increased for juveniles at higher densities but decreased for juveniles at lower densities, increased for immature females and males, but decreased slightly for mature females and especially mature males, the latter being close to a random distribution. There were no significant differences between sites, in spite of the lower numbers at the downstream site. 3. The third hypothesis was tested at only the upstream site and supported by comparisons between shrimp densities and 13 physical variables (distance from bank, water depth, water velocity, ten particle size‐classes), and three non‐physical variables (dry weights of bryophytes, leaf material, organic detritus). During the day, densities were strongly related to particle sizes with the following preferences: 0.5–8 mm for juveniles, 8–256 mm for the other life‐stages with a weaker relationship for males. There were no significant positive relationships with the other variables, apart from bryophytes for immature shrimps and adults. At night, densities were unrelated to particle size; juveniles and immature shrimps preferred low water velocities near the banks, often where leaf material and organic detritus accumulated, females often preferred medium water velocities slightly away from the banks, and males showed no habitat preferences. 4. Day samples do not provide a complete picture of habitat preferences and probably identify refuge habitats. Day–night changes in spatial distribution and habitat preferences are an essential part of the behavioural dynamics of the shrimps and should be investigated in other species.     

Gut passage times in Gammarus pulex (Crustacea,Amphipoda) and aspects of summer feeding in a stony stream

Gut passage times in Gammarus pulex (Crustacea, Amphipoda) were calculated by counting and measuring voided faecal pellets. Following feeding on elm leaves conditioned in a stream, or summer fine detritus from the same source, calculated gut passage times were usually short, about 2 h or less at 15 °C. Using harder oak and beech leaves, not previously conditioned in the stream, apparent gut passage times were much longer, 8–48 h, but since intermittent feeding occurred these figures are not comparable to the others.In connection with summer feeding of the animal in the stream, the microbiological status of the fine detritus there was studied. It contained fungal hyphae, which were mostly empty, viable fungal spores of terrestrial derivation, and bacteria, all largely carried on fragments of vascular plant tissues. In experiments, such fungal spores (of Aureobasidium pullulans and of Mucor sp.), incorporated into fine detritus, were not digested by the animal. The bacterium Bacillus cereus was present in the fine detritus and when the latter was consumed it survived passage through the gut of the animal. Using B. cereus as a proportion marker it was concluded that other unicellular bacteria in the fine detritus were not digested either. In other experiments, not immediately related to the summer season, nutrient extraction by the animal from hyphae of the fungus Nectria lugdunensis was examined; it probably occurs through sub-microscopic pores connecting the cells. This mode of extraction may apply when the animal consumes vascular plant tissues.     

The effect of brown trout (Salmo Trutta L.) on stream invertebrate drift,with special reference to Gammarus pulex L.

The effect of brown trout (Salmo trutta L.) on invertebrate drift density was examined in three previously fish-free springbrooks. Drift density was studied during the day following the sequential introduction of: 1) empty cages, 2) cages containing trout, and 3) empty cages. Each period lasted three whole days, and drift density was measured at daytime and night-time every day. Control drift density was determined using empty cages in two similar springbrooks.Night-time drift density of large Gammarus pulex L. (>3.9 mg DW ind^–1) decreased compared to that of smaller size-groups following trout introduction, and was significantly lower (p<0.05) than the pre-introductory level after the trout had been removed again.The daytime drift density of large G. pulex remained unchanged, as did both daytime and night-time drift density of smaller size-groups of G. pulex, and the other taxa investigated (Leuctra hippopus (Kempny), Leuctra nigra (Olivier), Amphimura sp., Nemoura flexuosa Aubert, and Baetis rhodani (Pict.)). No significant changes in drift density were observed in two control springbrooks.     

The effect of long pools on the drift of macro-invertebrates in a mountain stream

Macro-invertebrate drift was measured entering and leaving two pools on the Middle Fork of the Cosumnes River, a third order California stream. Drift rates for Baetis spp., Chironomidae, Simulium spp., Capniidae and total drift were calculated. Significant differences in the numbers of organisms entering the two pools were found for Baetis, Chironomidae, and Capniidae. Comparisons of drift rates at the upstream and downstream ends of each pool showed that the abundance of Chironomidae, Simulium, Capniidae and total drift changed in different directions across the pools. The numbers of organisms leaving the two pools, however, were not significantly different for Baetis, Simulium, Capniidae and total drift. These findings lead us to hypothesize that long pools act as barriers, not filters, to stream macro-invertebrate drift. The composition of drift leaving the pools in this experiment appeared to be controlled by the composition of the benthic habitat at the tail of the pool and not by the composition of upstream drift entering the pools.     

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.     

Effects of urban runoff from a detention pond on water quality,temperature and caged Gammarus minus (Say) (Amphipoda) in a headwater stream

Thompson Run, a headwater stream in central Pennsylvania (U.S.A.), supports an impaired macroinvertebrate community downstream of the outlet of a detention pond that receives urban runoff. To determine if toxicity from the metals or other pollutants in urban runoff contributed to impairment, we exposed adult, male Gammarus minus to urban runoff during a 42-day in situ bioassay that included 12 rain events. Test animals were collected from a site upstream of the detention pond outlet using two methods: precopula pair separation and sieving. Water quality, temperature and tissue metal concentrations were measured during the bioassay. The survival of precopula G. minus was lower (p=0.048) at a site downstream of the detention pond outlet compared to a site upstream of it, but the survival of sieved G. minus was not different between sites (p=0.803). Large hourly increases in temperature (up to 6.6 °C) and major reductions in water quality including order of magnitude increases in suspended materials (measured as turbidity) and the concentrations of copper, zinc and lead occurred downstream of the detention pond outlet during stormflow (i.e. following rain events). In contrast, changes in temperature and water quality were minor upstream of the pond outlet throughout the bioassay. Copper and cadmium concentrations in leaf samples and copper, zinc and lead concentrations in G. minus samples were significantly higher downstream of the pond outlet than they were upstream of it. Despite harsh conditions downstream of the pond outlet (i.e. metal contamination, inputs of suspended materials and rapid temperature increases), the in situ bioassay did not convincingly demonstrate that urban runoff was toxic to adult, male G. minus.     

A comparison of the fatty acid composition of Gammarus lacustris and its food sources from a freshwater reservoir, Bugach, and the saline Lake Shira in Siberia, Russia

We studied fatty acid (FA) composition in samples from bodies and intestinal contents of the littoral amphipod Gammarus lacustris Sars, from the Bugach freshwater reservoir. Simultaneously, samples of seston and bottom sediments were also collected from the reservoir during early August. There were no differences in FA composition of gut contents, seston and sediments of pebbly bottom. Seston was the main food source of Gammarus but some FAs Gammarus got from sediments. The FA composition of G. lacustris and seston from the Bugach freshwater reservoir were compared with those of the animals from the saltwater Lake Shira (Siberia). While FA composition of the two Gammarus populations differed significantly, those of seston were practically similar: the composition of long-chain unsaturated fatty acids, 20:53, 22:63 and 20:46, were significantly higher in animals from saline Shira Lake, whereas 16:1 and 16:0 were higher in the freshwater populations of amphipods from the Bugach freshwater reservoir. Taking into account the relevant literature data, we hypothesise that this difference in C16 acid might be a distinguishing characteristic of FA composition of freshwater and saltwater crustaceans.     

The relationship of floods, drying, flow and light to primary production and producer biomass in a prairie stream

Factors related to autochthonous production were investigated at several sites along a prairie stream at Konza Prairie Research Natural Area. Primary production, algal biomass, litter input, and ability of floods to move native substrate were measured. Additional experiments were conducted to establish the influence of light and water velocity on primary production rates and recovery of biomass following dry periods. The study period encompassed two extreme (> 50 year calculated return time) floods, thus we were able to analyze the effects of scour on periphyton biomass and productivity. Biomass of sedimentary algae was reduced greatly by flooding and did not reach preflood amounts during the 2 months following the first flood. Rates of primary production associated with sediments recovered to levels above preflood rates within 2 weeks. Biomass of epilithic periphyton was not affected as severely as that of sedimentary algae. Little relationship was observed between water velocity and photosythetic rates. Production reached maximum rates at 25% of full sun light. Epilithic chlorophyll levels recovered within eight days following a dry period, and chl a was an order of magnitude greater on rocks than sediments 51 days after re-wetting. Estimated annual rates of primary production were 2.6 times greater in the prairie than in the forest reaches of the stream. The ratio of annual autochthonous:allochthonous carbon input was 4.81 for prairie and 0.32 for the forest. Periphyton production in prairie streams is resilient with regard to flooding and drought and represents a primary carbon source for the system.     

The relative influence of diffuse- and point-source disturbances on a small upland stream in East Java,Indonesia: a preliminary investigation

The relative effects of a diffuse disturbance (alteration of land use from forest to plantation) and a point-source disturbance (a village and its associated coffee-processing plant, within the plantation) on longitudinal variation in water chemistry and macroinvertebrate community composition were assessed in Kali Dinoyo, a small upland stream in East Java, Indonesia. Four sites were sampled. The catchments of the two most-upstream sites were covered primarily by rainforest, while the two lower sites fell within a coffee plantation. The lowest site was downstream of a small village and its associated coffee-processing plant. Most of the variance in all water quality variables and in several community composition measures was explained by the difference between plantation and forest sites. Comparatively small differences in total suspended solids and macroinvertebrate community composition were observed downstream of the village. Diffuse disturbances associated with land clearance and plantation agriculture therefore appear to have a larger impact on the ecology of Kali Dinoyo than the point source impacts associated with the village. More robust and powerful study designs to formally test these findings are discussed.     

Validation and implications of a growth model for brown trout, Salmo trutta, using long-term data from a small stream in north-west England

Summary 1. The objectives were: (i) to check the validity of a new growth model; (ii) to examine the relationship between population density and both mean mass and mean growth rate and (iii) to discover if compensatory growth occurred. First (0+) and second (1+) year‐old juvenile sea‐trout were sampled by electrofishing at the beginning and end of the summer from 1967 to 2000. Additional samples were taken in some years in winter and in the critical period for survival when the fry first emerge from the gravel. The trout left the stream as pre‐smolts in May, soon after their second birthday. 2. A growth model ( Elliott, Hurley & Fryer, 1995 ) estimated the mean mass of the trout over the 2 years spent in fresh water. The date and mean mass at the start of the growth period were defined as the median date for fry emerging from the gravel and their mean mass at emergence, both being estimated from individual‐based models ( Elliott & Hurley, 1998a, b ). 3. The variation in mean mass among year‐classes was small for newly‐emerged fry (CV = 6.2%), maximum at the start of the first summer of the life cycle (CV = 38.1%), and then decreased gradually for successive life‐stages to a low value for pre‐smolts (CV = 10.8%). Mean mass was not related to population density and, therefore, mean growth rate was density‐independent. Growth in the first, but not the second, winter of the life cycle was lower than model prediction, but when it was assumed in the model that there was no first‐winter growth, there was good agreement in most year‐classes between model estimated values and observed mean mass. Exceptions were that mean masses and growth rates for 0+ trout after four summer droughts were lower than expected, but compensatory growth followed, so that observed and expected masses were similar for 1+ trout. 4. Pre‐smolt mean mass on 30 April measured total growth achieved in the freshwater phase of the life cycle. This was significantly related to mean mass at the end of the first and second summers of the life cycle, but not to the emergence date and mean mass of emerging fry. 5. These juvenile sea‐trout were growing at their maximum potential in most year‐classes but when this was not achieved, compensatory growth soon restored their mass to values expected from the model. This ensured a low variation in the mean mass of pre‐smolts just before they migrated to the sea. However, the latter mass was higher in more recent year‐classes (1987–98) than in previous ones (1967–86), demonstrating the effect of slightly higher stream temperature. This study has shown the importance of developing realistic growth models in order to detect departure from maximum potential growth, and the more subtle effects of temperature change, possibly due to the effects of climate change.     

Estimation of the retention times and distances of seed dispersed by two monkey species, Alouatta seniculus and Lagothrix lagotricha, in a Colombian forest

Isolated-bout method to estimate the retention times and dispersal distances was applied to the seed dispersal by red howler monkeys (Alouatta seniculus) and Humboldts woolly monkeys (Lagothrix lagotricha) in a lowland tropical forest at La Macarena, on the border of the Macarena and Tinigua National Parks, the Department of Meta, Colombia. Continuous observations were made on the feeding and ranging behavior of well-habituated troops of howler monkeys and woolly monkeys as well as continuous collection of monkeys feces. We selected out the isolated-bout as a feeding bout on the specific species that was only once recorded within 48 h before the seeds of that species appeared in the feces of monkeys. In that case, the seeds were strongly suggested to come from that isolated bout. Then retention times, route seed dispersal distances and direct seed dispersal distances were estimated. Howler monkeys, which are regarded as generalist herbivores, showed longer retention times and dispersal distances along monkeys route than did woolly monkeys, which are specialist frugivores. However, the direct distances that seeds were carried from the mother tree were not significantly greater for howler monkeys than for woolly monkeys. This shows that both retention time and movement patterns by the monkeys, especially the total ranging area, influence the direct distance that seeds are carried from the mother tree.     

The water velocity preferences of Austrosimulium furiosum and Simulium ornatipes (Diptera: Simuliidae) larvae,and the implications for micro-habitat partitioning

The aquatic larvae of two simuliid species, Austrosimulium furiosum (Skuse) and Simulium ornatipes Skuse, which often occur together in Victorian streams, were shown in laboratory experiments to have preferences for different water velocities: larvae of A. furiosum preferred water velocities of 0.2–0.3 m s^–1, and S. ornatipes preferred water velocities of 0.9–1.3 m s ^–1 . Final instar larvae of both species selected slow water speeds of less than 0.25 m s ^–1 prior to pupation. Flow patterns around a cylinder in a laboratory stream were mapped, and the distribution of A. furiosum larvae within the wake, paired vortices and horseshoe vortex was recorded. Larvae selected areas with suitable water velocities and aligned with the flow, providing flow visualization of micro-current speed and direction. The potential for micro-habitat partitioning is discussed in relation to benthic sampling strategies.     

The distribution and abundance of wheat roots in a dense,structured subsoil – implications for water uptake

We analysed the abundance, spatial distribution and soil contact of wheat roots in dense, structured subsoil to determine whether incomplete extraction of subsoil water was due to root system limitations. Intact soil cores were collected to 1.6 m below wheat crops at maturity on a red Kandosol in southern Australia. Wheat roots, remnant roots, soil pores and root–soil contact were quantified at fresh breaks in the soil cores. In surface soil layers (<0.6 m) 30–40% of roots were clumped within pores and cracks in the soil, increasing to 85–100% in the subsoil (>0.6 m), where 44% of roots were in pores with at least three other roots. Most pores contained no roots, with occupancy declining from 20% in surface layers to 5% in subsoil. Wheat roots clumped into pores contacted the surrounding soil via numerous root hairs, whereas roots in cracks were appressed to the soil surface and had very few root hairs. Calculations assuming good root–soil contact indicated that root density was sufficient to extract available subsoil water, suggesting that uptake is constrained at the root–soil interface. To increase extraction of subsoil water, genetic targets could include increasing root–soil contact with denser root hairs, and increasing root proliferation to utilize existing soil pores.     

The primate origins of female sexuality,and their implications for the role of nonconceptive sex in the reproductive strategies of women

In many species of monkeys and apes, sexual solicitations of males by females are more facultative and opportunistic than generally realized. Although female sexual solicitations peak at midcyle, solicitations and copulations are not necessarily confined to the days just around ovulation. Human female sexuality, and the physiological underpinnings of this sexuality evolved in prehominid contexts in which female primates solicited and copulated with multiple males on a situation-dependent basis. Such sexual behavior became increasingly costly to females in the course of hominid evolution, and women's sexuality today must be viewed as an imperfect compromise between formerly adaptive organs (such as the female clitoris) and the chronic challenges mothers face in eliciting and insuring male protection and investment in offspring.     

Variation in highbush blueberry floral volatile profiles as a function of pollination status, cultivar, time of day and flower part: implications for flower visitation by bees

Background and Aims

Studies of the effects of pollination on floral scent and bee visitation remain rare, particularly in agricultural crops. To fill this gap, the hypothesis that bee visitation to flowers decreases after pollination through reduced floral volatile emissions in highbush blueberries, Vaccinium corymbosum, was tested. Other sources of variation in floral emissions and the role of floral volatiles in bee attraction were also examined.

Methods

Pollinator visitation to blueberry flowers was manipulated by bagging all flowers within a bush (pollinator excluded) or leaving them unbagged (open pollinated), and then the effect on floral volatile emissions and future bee visitation were measured. Floral volatiles were also measured from different blueberry cultivars, times of the day and flower parts, and a study was conducted to test the attraction of bees to floral volatiles.

Key Results

Open-pollinated blueberry flowers had 32 % lower volatile emissions than pollinator-excluded flowers. In particular, cinnamyl alcohol, a major component of the floral blend that is emitted exclusively from petals, was emitted in lower quantities from open-pollinated flowers. Although, no differences in cinnamyl alcohol emissions were detected among three blueberry cultivars or at different times of day, some components of the blueberry floral blend were emitted in higher amounts from certain cultivars and at mid-day. Field observations showed that more bees visited bushes with pollinator-excluded flowers. Also, more honey bees were caught in traps baited with a synthetic blueberry floral blend than in unbaited traps.

Conclusions

Greater volatile emissions may help guide bees to unpollinated flowers, and thus increase plant fitness and bee energetic return when foraging in blueberries. Furthermore, the variation in volatile emissions from blueberry flowers depending on pollination status, plant cultivar and time of day suggests an adaptive role of floral signals in increasing pollination of flowers.