Stream drift as a consequence of disturbance by invertebrate predators

Summary We carried out an experimental field study in a Swedish stream in order to determine whether mobile predators enhance the drift of stream insects. We increased the density of nymphs of the predaceous perlid stonefly, Dinocras cephalotes, in an experimental section of a stream up to densities in another more densely populated part of the same stream. The drift of several benthic species increased significantly compared to a control section where D. cephalotes were rare. Experiments carried out in September showed a strongly elevated drift response in nymphs of the mayfly Baetis rhodani only, whereas May experiments resulted in increased drift in B. rhodani as well as the amphipod Gammarus pulex, the stonefly Leuctra fusca, chironomids, and the total number of drifting animals. In September, we found that the drift response of Baetis rhodani to predator disturbance was dependent on the size of mayfly nymphs; small nymphs appeared in greater numbers in the drift nets than did large nymphs. A subsequent laboratory analysis of drift lengths of B. rhodani nymphs supported the hypothesis that small nymphs travel in the drift for longer than do large nymphs, particularly in darkness. We suggest that morphological constraints in vision or swimming performance, or both, cause small nymphs to drift longer. In May, size-dependent drift was less obvious, probably because the size of the nymphs was considerably greater than in September.     

Intensive sampling and transplantation experiments reveal continued effects of episodic acidification on sensitive stream invertebrates

1. Although mean pH is increasing in acidified streams throughout Europe, benthic invertebrates are recovering only slowly. One developing hypothesis is that acid episodes continue to restrict recolonisation and recovery. Here, we used intra‐annual sampling in replicate circumneutral, acid and episodic streams at the Llyn Brianne experimental catchments to assess possible mayfly exposure to episodic effects quantified from a risk index based on long‐term discharge. Episodic effects were then tested using short‐term transplantations of animals in enclosures. 2. No mayflies occurred in acid streams while Baetis rhodani occurred in episodic streams but declined to zero density following low pH in autumn. Ephemerella ignita, Baetis vernus, B. muticus, Rhithrogena semicolorata, Ecdyonurus spp. and Heptagenia lateralis occurred only in the circumneutral streams. The first two species were present as nymphs only during July and August so that episodic exposure risk was minimal, but the remaining species occurred as nymphs in all months. 3. Baetis rhodani transplanted during base‐flow (September 2003) and high‐flow (April 2004) were exposed to either; (i) continual (chronic) exposure in the acid streams over 16 days or (ii) repeated short‐term (episodic) exposure to acid conditions for 2 × 4 day periods interspersed with 4‐day recovery periods in the circumneutral streams. Baetis survival in the circumneutral streams (always pH > 5.7) remained high during both low‐ and high‐flow. By comparison, mortality increased (P < 0.01) during chronic and episodic exposure, but only during high‐flow (mean pH 3.8–3.9, cf. 5.5–5.8 at low flow) when mortality varied significantly in the order chronic (>80%) > episodic (>40%) >circumneutral (<10%). 4. We conclude that, despite Europe‐wide trends towards chemical recovery from acidification, even short exposures to high‐flow events at Llyn Brianne are still sufficiently acid to reduce the survival of B. rhodani. Most mayfly species absent from acid and episodic streams have life cycles that would increase acid exposure risks during autumn and winter, and this may be sufficient to explain their current distribution.     

Maternal behaviours may explain riffle‐scale variations in some stream insect populations

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Life cycles and growth rates of Baetis spp. (Ephemeroptera: Baetidae) in the laboratory and in two stony streams in Austria

SUMMARY. Larvae of Baetis alpinus, B. lutheri and B. rhodani were reared in a stream channel (water temperature range 4.2–11.4°C) in the laboratory. The larval growth was exponential and the mean specific growth rate varied from 1.93 to 2.24% day^?1 for B. alpinus, 1.49 to 3.41% day^?1 for B. lutheri and 0.79 to 3.11% day^?1 for B. rhodani. These variations in growth rate were related to variations in mean temperature and this was the major factor affecting growth in the laboratory. Non-quantitative samples of the benthos in the Seebach and Unterseebach, two stony streams near Lunz, Austria, were taken at approximately monthly intervals from November 1965 to May 1968. In each year, there were two winter and three summer cohorts for B. alpinus from Seebach and two or three winter and one to three summer cohorts for B. lutheri and B. rhodani from Unterseebach. Over the study period of 30 months, eleven cohorts were recorded for B. alpinus and B. lutheri, and ten cohorts for B. rhodani. The life cycle of a cohort varied from 3 to 8 months in B. alpinus, from 2.5 to 9 months in B. lutheri and from 2.5 to 8 months in B. rhodani. Mean specific growth rate in length varied from 0.82 to 2.97% day^?1 for B. alpinus, 0.96 to 3.33% day^?1 for B. lutheri and 0.65 to 3.01% day^?1 for B. rhodani. The percentage of the variability in growth rate accounted for by variations in mean temperature was 63% for B. alpinus, 91% for B. lutheri and 82% for B. rhodani. Therefore mean temperature was clearly the major factor affecting the growth rates in the field. An agreement was found between the growth rates of B. alpinus in the field and the laboratory. The growth rates of B. lutheri and B. rhodani were slower in the field than in the laboratory at higher temperatures. The possible reasons for this latter discrepancy are discussed, and the growth rates of the three Baetis spp. are compared with those of other species of Ephemeroptera.     

Egg envelopes of Baetis rhodani and Cloeon dipterum (Ephemeroptera, Baetidae): a comparative analysis between an oviparous and an ovoviviparous species

A comparative ultrastructural investigation on the eggshell (vitelline and chorionic envelopes) has been carried out in the nymphs of two mayfly species encompassed into the Baetidae, namely Baetis rhodani and Cloeon dipterum. During oocyte differentiation in the meroistic telotrophic ovarioles, gametes are connected to discrete nurse cells by trophic cords. In B. rhodani, each ovariole contains several eggs arranged in sequence, whereas in C. dipterum each contains a single egg. Follicle cells are competent for vitelline and chorionic envelope synthesis. Baetis rhodani is an oviparous species and the chorion is fairly thick, formed by an alveolate endochorion and a fibrillar exochorion delimited by a honey‐comb roof. Cloeon dipterum stands out among Ephemeroptera for being ovoviviparous. In B. rhodani, ovulation starts in the older nymphs with dark wing‐pads, whereas in C. dipterum choriogenesis ends in the imaginal stage. Here the chorion is very thin and laid eggs hatch almost immediately, allowing the larvulae to move out. The maturation of a single egg per ovariole is synchronized with the achievement of the adult stage. The acquisition of ovoviviparity has led to remarkable changes in the ovariole organization along with a simplification of the eggshell structure.     

Ecosystem engineering by beavers affects mayfly life histories

1. The North American beaver has been studied as a model ecosystem engineer for many decades. Previous studies have documented physical, chemical and biological impacts attributed to beaver engineering in both aquatic and terrestrial environments. This study focused on the effects of ecosystem engineering by beavers on life histories of a common mayfly and on the potential consequences for mayfly populations. 2. We studied 18 montane beaver ponds of varying size and shape in western Colorado near the Rocky Mountain Biological Laboratory. Our goal was to test whether variation in beaver pond morphology (pond size and shape) explains downstream changes in stream temperature, mayfly size and timing of emergence. 3. Downstream water temperatures varied predictably with pond morphology, being colder downstream of high‐head dams and warmer downstream of low‐head dams. Pond morphology was also a significant predictor of variation in the size of mature female Baetis bicaudatus (the most abundant mayfly), with larger females emerging downstream of high‐head dams and smaller females downstream of low‐head dams. The size of male B. bicaudatus was not significantly related to pond morphology or stream temperature. There was no relationship between pond morphology and variation in the timing of emergence of Baetis (males or females) between upstream and downstream reaches. 4. Our results have implications for the effects of beaver ponds on Baetis individual fitness because large Baetis females are more fecund. Therefore, predictable female size variation associated with beaver pond morphology makes it possible to model the effects of beaver activity on local contributions of Baetis to the regional pool of reproductive adults at the catchment scale. Additionally, predictable changes in the size of emerging mayflies may have important consequences for the magnitude of aquatic to terrestrial resource subsidies in beaver‐modified systems.     

Biometric studies of some stoneflies and a mayfly (Plecoptera and Ephemeroptera)

Aspects of the nymphal/adult developmental change were investigated in biometric studies of several species of Plecoptera: Nemouridae near Schlitz, Hesse, Germany. Preliminary information on the mayfly, Baetis vernus Curtis, is also provided. Nemourid nymphs pass through 3 wing bearing stages before reaching adulthood. Instars can be identified by their characteristic shapes, as expressed by the wing length/head width (WL/HW) ratio. Size does not allow instar discrimination, mainly due to sexual size differences. HW is ca 10% larger in last instar female than in male nemourid nymphs; exuviae shed at the moult to adult represent about 14% of nymphal ash free dry weight (AFDW). Biomass lost with exuviae during the many larval moults should be accounted for in estimates of production. Freshly emerged nemourid females are about 6% larger and 30% heavier than males. The HW/AFDW relationship is the same in both sexes. Through terrestrial feeding during adult life, males double their weight on average. Mature females are up to three times heavier than freshly emerged ones. They invest about 30% of their final AFDW in reproduction.Shape of last instar nymphal Baetis was expressed as the ratio wing length/mesonotum length. It is size-dependent, a characteristic, instar-specific shape may not occur in this mayfly. Nymphal and subimaginal exuviae together represent about 14% of last instar nymphal dry weight. Females of Baetis are about 55% heavier than males. Unlike in Plecoptera, the size/weight (ML/AFDW) relationship differs between sexes.     

How river structure and biological traits influence gene flow: a population genetic study of two stream invertebrates with differing dispersal abilities

1. Determined by landscape structure as well as dispersal‐related traits of species, connectivity influences various key aspects of population biology, ranging from population persistence to genetic structure and diversity. Here, we investigated differences in small‐scale connectivity in terms of gene flow between populations of two ecologically important invertebrates with contrasting dispersal‐related traits: an amphipod (Gammarus fossarum) with a purely aquatic life cycle and a mayfly (Baetis rhodani) with a terrestrial adult stage. 2. We used highly polymorphic markers to estimate genetic differentiation between populations of both species within a Swiss pre‐alpine catchment and compared these results to the broader‐scale genetic structure within the Rhine drainage. Landscape genetic approaches were used to test for correlations of genetic and geographical structures and in‐stream barrier effects. 3. We found overall very weak genetic structure in populations of B. rhodani. In contrast, G. fossarum showed strong genetic differentiation, even at spatial scales of a few kilometres, and a clear pattern of isolation by distance. Genetic diversity decreased from downstream towards upstream populations of G. fossarum, suggesting asymmetric gene flow. Correlation of genetic structure with landscape topography was more pronounced in the amphipod. Our study also indicates that G. fossarum might be capable of dispersing overland in headwater regions and of crossing small in‐stream barriers. 4. We speculate that differences in dispersal capacity but also habitat specialisation and potentially the extent of local adaptation could be responsible for the differences in genetic differentiation found between the two species. These results highlight the importance of taking into account dispersal‐related traits when planning management and conservation strategies.     

Microsatellite loci for the mayfly Baetis rhodani (Baetidae,Ephemeroptera)

The first microsatellite primers are described for the mayfly family Baetidae (order Ephemeroptera). Seven polymorphic loci were isolated and characterized for the species Baetis rhodani from two enriched genomic libraries. A total of 183 individuals were genotyped using these loci. Allelic diversity was high for all loci, and observed heterozygosities ranged between 0.382 and 0.772. A heterozygote deficiency was detected in some loci, suggesting the presence of ‘null’ alleles.     

The effect of partial and total drought on the macroinvertebrate communities of three small Danish streams

Three cases of partial or total drought were studied. A two weeks' stop of water flow with reduced water level and stagnant water was survived by most stream species. Only the population of Baetis rhodani Pict. was almost eliminated. An unprecedented drought of 2-3 months reduced numbers of stream species. The differential effects are discussed in relationship to the behaviour, life cycle and physiology of the individual species. After the drought many invaders were found, but most disappeared rapidly. Only Asellus aquaticus L. maintained a population in the stream. In an intermittent stream with 3–4 months' drought no changes were observed, and many species were the same as those which survived in the second stream. It is concluded that the consequences of an increasing frequency of drought, for example due to increasing ground water use, will depend on the species normally present and on the season and duration of the drought.