Dinocras Cephalotes Research Articles

Prey switching in four species of carnivorous stoneflies

Summary1. Previous studies compared the functional responses to their prey, and both intraspecific and interspecific interference, in mature larvae of Dinocras cephalotes, Perla bipunctata, Isoperla grammatica and Perlodes microcephalus. The present study examines switching by larvae of the same species presented with different proportions of two contrasting prey types; larvae of Baetis rhodani and Chironomus sp. In each experiment, 200 prey were arranged in nine different combinations of the two prey types (20 : 180, 40 : 160, 60 : 140, 80 : 120, 100 : 100, 120 : 80, 140 : 60, 160 : 40, 180 : 20). Prey were replaced as they were eaten. A model predicted the functional response in the absence of switching and provided a null hypothesis against which any tendency for switching could be tested.2. No evidence for prey switching by Dinocras and Perla was obtained, both species showing a slight preference for Baetis over Chironomus. Prey switching occurred in Isoperla and Perlodes. As the relative abundance of one prey type increased in relation to the alternative, the proportion eaten of the former prey changed from less to more than expected from its availability, the relationship being described by an S‐shaped curve. Isoperla and Perlodes switched to a preference for Baetis when its percentage of the total available prey exceeded 57 and 42%, respectively. Equivalent values for Chironomus were 43 and 58% for Isoperla and Perlodes, respectively. Switching was strongest in Perlodes.3. Non‐switching in Dinocras and Perla was related to their feeding strategy, both species being more successful when using a non‐selective ambush strategy at dusk and dawn rather than a search strategy during the night. Both Isoperla and Perlodes used a search strategy. The smaller Isoperla fed chiefly at dusk and dawn, and preferred Chironomus larvae, whereas most of the larger Perlodes fed continuously from dusk to dawn and preferred Baetis larvae.

Seasonal Changes and Selectivity in the Diet of Brown Trout in the River Nera (Central Italy)

ABSTRACT The stomach contents of brown trout from the middle region of the Nera River, Central Italy, were compared with the composition of the macroinvertebrate community of the same river stretch. The epibenthic fauna, identified mostly at the species level, was composed mainly of Ephemeroptera (Serratella ignita, Baetis spp., Rhithrogena semicolorata), Plecoptera (Dinocras cephalotes), Trichoptera (Hydropsyche sp., Micrasema spp.) and Diptera. We found that the dietary composition of brown trout was not simply proportional to the environmental density of the prey. Ephemeroptera dominated the spring and summer diets; Trichoptera prevailed in winter. The electivity index showed a negative selection towards Baetis spp. and chironomids and positive values of selection for Trichoptera. Dimensions, mobility, degree of exposure, and visibility of prey, were important factors in shaping brown trout's diet. Nevertheless species of the genera Micrasema and Lepidostoma, despite their reduced dimensions and scarce mobility, became in winter the favorite prey. This was probably due to the wintering habitat choice of the trout, under the river banks and hidden in the vegetation.

Interspecific interference and the functional response of four species of carnivorous stoneflies

Summary1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae ofPerlodes microcephalus,Isoperla grammatica,Dinocras cephalotesandPerla bipunctata. The present study extends this work by assessing interspecific interference between pairs of these species in equal numbers (one, two or three larvae per species) to provide total predator densities of two, four or six larvae.Baetislarvae as prey were replaced as they were eaten, and their density per predator was varied between 20 and 200 larvae.2. The number of prey eaten by each competing species increased curvilinearly with prey density, the relationship being well described by a Type II model. Of the two constants in the model, handling time varied considerably between species, mean values being shortest forPerlodes,slightly higher forIsoperla,and much higher forDinocrasandPerla. It was not affected significantly either by predator density or the identity of the competing species.3. Attack rate also varied between species and decreased with predator density. This decrease was slight forPerlodes,and also forDinocrasandPerlain competition withIsoperla. The decrease inDinocrasandPerlawas similar to that for intraspecific interference.4. The decrease in attack rate was described by a convex curve forPerlodeswith the other three species and forDinocras/PerlawithIsoperla,but by a concave curve (negative power function) forIsoperlacompeting with the other three species, and for bothDinocrasandPerlain competition withPerlodes. Prey consumption also decreased with predator density, the severity of competition with different species reflecting that for attack rate.5. A comparison with previous results for intraspecific interference showed that the latter was dominant forPerlodesin all contests and forDinocrasorPerlacompeting withIsoperla,whilst interspecific interference dominated forIsoperlain all contests and forDinocrasandPerlacompeting withPerlodes. Both types of interference were applicable to competition betweenDinocrasandPerla.Isoperlawas the least, andPerlodesthe most, aggressive of the four species withDinocrasandPerlaintermediate.

Distribution and nymphal age of Dinocras cephalotes (Curtis) (Plecoptera: Perlidae) in northern Finland

The largest species of Scandinavian plecopterans, Dinocras cephalotes (Curtis), was identified in the River Inarijoki, northern Finnish Lapland in 1990, being the first record of this species in Finland. In spite of the widespread zoobenthos survey carried out in all major river systems in the northernmost Lapland, the distribution of D. cephalotes seems to be restricted to a small geographical area in the R. Inarijoki, which forms part of the border between northern Norway and Finland. The nymphal stage of D. cephalotes appears to take four years in northern Finland, and no indications of a five-year nymphal stage, earlier suggested for northernmost Scandinavia, were found.

A comparative study of the functional response of four species of carnivorous stoneflies

SUMMARY1. Comparisons were made of the functional responses of mature larvae ofPerlodes microcephalus,Isoperla grammatica,Dinocras cephalotesandPerla bipunctata. Experiments were performed in stream tanks with natural substrata and glass bottoms, so that feeding could be observed above and below the substratum. There was one stonefly per tank and one of 10 prey densities between 20 and 200 larvae of eitherChironomussp. orBaetis rhodaniper tank. Consumed prey were replaced in a first set of experiments but not in a second set. Additional experiments assessed intraspecific interference between larvae of each predator species (two to five predators per tank).2. The number of prey eaten increased curvilinearly with prey density. The relationship was described by two models, a Type II instantaneous model and its integrated equivalent, for experiments with and without prey replacement, respectively. Handling time did not change significantly with prey density, and was the same for experiments with and without prey replacement. Estimates of attack rate were similar for the two models, but varied between prey type and predator species. Handling time varied considerably but was normally distributed for each prey type and predator species. Mean handling time varied forChironomusfrom 39 s forIsoperla, which rarely ate a whole larva, to 57 s forPerlodesand forBaetisfrom 116 s forPerlodesto 167 s forPerla. All predators were more efficient at capturingBaetis, but the longer handling time forBaetisensured that moreChironomuswere eaten. It was concluded that these variations in attack rate and handling time were related to activity and growth differences between the predator species, and that experiments with and without prey replacement could both be relevant to the field, depending on how the predators searched for food.3. In the interference experiments, mean handling time did not change with increasing predator density, but attack rate decreased curvilinearly, the decrease varying from negligible forIsoperlato marked forPerlodes. Prey capture decreased with decreasing attack rate. Therefore, interference reduced prey consumption, but this effect was negligible forIsoperlaand increasingly severe in the orderDinocras,PerlaandPerlodes.

Physico-Chemical Characteristics and Macroinvertebrate Communities of the Caher River

A study of the physico-chemical characteristics of the Caher River, Co. Clare, was carried out at two sites between October 1999 and April 2000. The Caher is an alkaline system with pH values in excess of 8.0. Phosphate and nitrogen concentrations showed no evidence of eutrophication. Conductivity values were at the lower end of the range expected for a limestone region. Calcium concentrations were relatively high, and precipitation of calcium carbonate was evident for much of the lower course of the river. However, other ions such as magnesium and potassium were present in low concentrations when compared to other limestone areas. A total of 80 macroinvertebrate taxa were recorded. The upstream site (Site 1) supported a more diverse community. The compacted nature of the substratum at Site 2, located downstream of Site 1, would restrict habitat availability and preclude the establishment of some interstitial dwelling fauna. The macroinvertebrates recorded in the Caher were, for the most part, fairly typical of clean-water rivers in Ireland, being represented by taxa that are relatively common throughout the island. One noteworthy exception was the unusually high abundance of the plecopteran Dinocras cephalotes. This predatory species is uncommon in Ireland and, even where it occurs, the numbers present are generally low. The Caher River, with its excellent water quality, has high ecological status, deserving special protection as a biodiversity refuge.

Physico–Chemical Characteristics and macroinvertebrate Communities of the Caher River

A study of the physico-chemical characteristics of the Caher River, Co. Clare, was carried out at two sites between October 1999 and April 2000. The Caher is an alkaline system with pH values in excess of 8.0. Phosphate and nitrogen concentrations showed no evidence of eutrophication. Conductivity values were at the lower end of the range expected for a limestone region. Calcium concentrations were relatively high, and precipitation of calcium carbonate was evident for much of the lower course of the river. However, other ions such as magnesium and potassium were present in low concentrations when compared to other limestone areas. A total of 80 macroinvertebrate taxa were recorded. The upstream site (Site 1) supported a more diverse community. The compacted nature of the substratum at Site 2, located downstream of Site 1, would restrict habitat availability and preclude the establishment of some interstitial dwelling fauna. The macroinvertebrates recorded in the Caher were, for the most part, fairly typical of clean-water rivers in Ireland, being represented by taxa that are relatively common throughout the island. One noteworthy exception was the unusually high abundance of the plecopteran Dinocras cephalotes. This predatory species is uncommon in Ireland and, even where it occurs, the numbers present are generally low. The Caher River, with its excellent water quality, has high ecological status, deserving special protection as a biodiversity refuge. M. Kelly-Quinn

Comparative spermatology in Plecoptera (Insecta). II. An ultrastructural investigation on four species of Systellognatha

Sperm structure of four Systellognatha species, Dinocras cephalotes (Curtis, 1827) and Perla grandis (Rambur, 1841), family Perlidae, Siphonoperla torrentium (Pictet, 1841), family Chloroperlidae, and Isoperla grammatica (Poda, 1761), family Perlodidae, was investigated by light microscopy, scanning and transmission electron microscopy, and immunofluorescence microscopy. The spermatozoa of all the species are filiform and flagellate; they have an elongated compact nucleus, topped by either a bi-layered or three-layered acrosomal complex, and a ‘9+9+2’ axoneme flanked by two mitochondrial derivatives. However, some variations have been observed, particularly regarding the morphology of the acrosomal complex, the shape of the nucleus, two more or less evident accessory bodies flanking the axoneme and the degree of crystallisation of the mitochondrial derivatives. Our results support the suggestion that Systellognatha is a monophyletic group, showing a single sperm pattern. Chloroperlidae and Perlodidae retain the plesiomorphic condition of some sperm characters while Perlidae show some autoapomorphies. A phylogenetic affinity between Plecoptera and the other orders of Polyneoptera has been confirmed.

Patterns of genetic differentiation and gene flow in Central Italian populations of Dinocras cephalotes (Curtis, 1827) (Insecta, Plecoptera)

Dispersal is a crucial phenomenon which has a strong influence on the ecology and evolution of species, determining the extent of gene flow among conspecific populations. In this paper, we present data on the degree of genetic structuring in 11 populations of the stonefly Dinocras cephalotes, focusing our attention on three Central Italian rivers: Aniene, Nera and Velino. We studied the genetic variation at 28 enzymatic loci by means of starch gel electrophoresis: 19 loci were polymorphic in one population at least. The degree of genetic structuring and levels of gene flow were estimated by F-statistics using WEIR & COCKERHAM'S estimators. Relatively high levels of genetic differentiation were revealed, considerably higher than those obtained in other studies of stonefly species. Neighbour-joining analysis provided a geographic scheme of relationships of the study populations, grouping them according to the river of origin. This analysis was supported by bootstrap values. We were not able to detect an unequivocal relationship between gene flow and geographic distances between sites within each river; this led us to hypothesise that equilibrium has not been reached between genetic drift and gene flow.

Contrasting diel activity and feeding patterns of four species of carnivorous stoneflies

Summary1. The hypothesis under test was that larvae of Dinocras cephalotes (Curtis), Perla bipunctata Pictet, Isoperla grammatica (Poda), and Perlodes microcephalus (Pictet) differed markedly in their diel activity and feeding patterns. Mature larvae collected about 1 month prior to adult emergence were used either for gut analyses or for observations of their activity and feeding patterns in three stream tanks with natural substrata and glass bottoms, so that activity could be observed above and below the substratum. A dull red light was used for observations in the dark. Food (larvae of Ephemeroptera, Simuliidae, and Chironomidae) was provided in excess.2. Larvae for gut analyses were taken 1 h before dusk or dawn (n = 30 larvae per species for each day or night sample). The only species with food in the gut for the day samples was P. microcephalus. All species fed at night, the mean number of prey per larva being very similar for D. cephalotes, P. bipunctata, and I. grammatica but significantly higher for P. microcephalus. Most prey were insect larvae, especially Simuliidae and Chironomidae.3. Diel activity patterns of single larvae differed interspecifically but not intraspecifically. Larvae of D. cephalotes and P. bipunctata were rarely active during the day; their activity increased at dusk and decreased at dawn, and was highest during the night; their success at prey capture was highest at dusk and dawn, with an ambush rather than a search strategy. Isoperla grammatica was rarely active during the day, most active at dusk and dawn when prey capture was highest, using a search strategy, and less active for the rest of the night. Perlodes microcephalus was active during the day, but only below the substratum, and very active from dusk to dawn with a high prey‐capture success, using a search strategy. These experiments provided support for the hypothesis under test. The discussion concludes that the results could also help to explain known differences in growth rate and the length of the life‐cycle in these four species.

Aquatic insects in an estuarine environment: densities, distribution and salinity tolerance

1. Aquatic insects were quantitatively surveyed at five sites along the tidally influenced section of a river‐dominated estuary in North Wales. Site 1 was the furthest upstream and was established as a reference site as it was never inundated by salt water. Site 5 was the furthest downstream and was inundated by all incoming tides. Numerically, insects made up 32% of the estuarine invertebrate fauna.2. Although the densities of most insect taxa decreased towards the estuary mouth, there were significant numbers present downstream for much of the year; for example, in April at site 4 (which was inundated by 81% of all high tides), a mean of 3514 chironomid larvae were recorded per m2 of estuary bed. Even at site 5, which was inundated twice daily, there were 747 larvae per m2. Among the larger aquatic insects, caddisfly and elmid beetle larvae, together with stonefly nymphs, were consistently taken at site 4 (e.g. maxima of forty‐eight caddisfly larvae m–2 in December and seventy elmids m–2 in April), although their densities were lower than upstream.3. There were seasonal shifts in the longitudinal distribution of several taxa, most notably the extension of chironomids down the estuary in April and July, and the concentration of simuliid larvae and mayfly nymphs at site 2 in July. The total freshwater benthos showed a downstream shift between September and December, which was maintained through April and into the summer. The latter was despite peak saltwater inundation (highest tides) in October, November and April. In June and July, when saltwater intrusion was lowest, the ranges of many aquatic insects had contracted to sites 1 and 2.4. Laboratory experiments showed that virtually all individuals of nineteen species of insects collected from site 1 (freshwater) survived a 4‐h immersion in 8.75‰ saltwater (25% strength seawater). Immersion in progressively more saline solutions reduced the survivorship of first the mayflies, followed by the caddisflies Glossosoma conformis and Hydropsyche instabilis. After 4 h in full strength seawater, all specimens of the stonefly Dinocras cephalotes, over half of the Perla bipunctata, and some individuals of nine species of caddisfly were alive. Four species of caddisfly (Sericostoma personatum, Odontocerum albicorne, Potamophylax cingulatus and Adicella reducta) survived a 24‐h simulated tidal cycle of immersion. With the exception of P. cingulatus, a few individuals of these caddisfly species survived immersion in full‐strength seawater for 24 h. For some individual species there was good agreement between their observed longitudinal distribution in the estuary and laboratory‐measured salinity tolerance; however, there was no significant correlation, overall, for the fauna.

Capacity of discontinuous egg development and its importance for the geographic distribution of the warm water stenotherm,Dinocras cephalotes(Insecta: Plecoptera: Perlidae)

Aspects of the egg biology of the perlid stonefly, Dinocras cephalotes (Curtis), were studied experimentally. Development under constant laboratory conditions did not differ from development under variable field conditions. Exposure to 4°C, the lower threshold temperature for egg development, caused abrupt interruptions of embryonic development. Development was resumed after return to favourable temperatures, with an approximate two-week-delay. Up to six interruptions of egg development did not negatively affect hatching success, regardless of developmental stage at which disturbances occurred. However, the older embryos grew, the more rapidly development was resumed after a cold disturbance; embryos from fully developed eggs hatched even at 4°C, although this is too cold to support larval growth. While eggs at 4°C remain dormant and vital for long periods of time, eggs exposed to suboptimal temperatures (10°C) remain only temporarily dormant and eventually develop and hatch. D. cephalotes is assumed to satisfy its high thermal demand for egg development under harsh conditions, for example in arctic Scandinavia, by opportunistically using summer warmth and spreading embryogenesis over more than one year. I suggest that high cue temperatures required to initiate egg development in Scandinavian populations are adaptive and ensure that larvae in the Arctic hatch under favourable seasonal conditions.

Sand Erosion by Mobile Predaceous Stream Insects: Implications for Ecology and Hydrology

Despite increasing knowledge of the ability of keystone animal species (“ecosystem engineers”) to change their physical environment, there is little and inconsistent evidence that benthic invertebrates affect the erosion of bottom material in streams. Therefore we designed field stream experiments and observations to investigate the effect of mobile predaceous stonefly (Dinocras cephalotes) larvae on sand erosion. When short of prey, the stoneflies erode sand from stream riffles thereby deepening the interstices between cobbles. On the basis of reasonable assumptions, we speculate that Dinocras has an erosion potential of about 200–400 kg sand m−2 yr−1 at natural population densities under favorable flow conditions. We consider the possible implications of the bioturbation potential of stream invertebrates for ecology (habitat quality and disturbance) and hydrology (sand transport and stability of coarse stream bottoms), which call for joint research on this novel role invertebrates play in the functioning of stream ecosystems.

Variable egg development of Dinocras spp. (Plecoptera, Perlidae) and the stonefly seed bank theory

1. Temperature dependence of egg development of Dinocras cephalotes (Curtis) (three German and one Norwegian population) and Slovenian D. megacephala (Klapálek) was studied under a constant 14 : 10 light : dark photoperiod and constant temperature ranges of 4–24 °C and 4–18 °C, respectively. D. cephalotes was also incubated under seasonal field conditions; natural daylength and fluctuating temperatures had no modifying effect.2. Both species have very similar lower threshold temperatures (4 and 3.5 °C, respectively), thermal demand for development (c. 600 degree days) and high dependence of mean incubation period on temperature (exponents of regressions near 1.5). Present data on D. cephalotes agree with the literature on British and Norwegian material of the same species.3. Development occurs only at cue temperatures above the lower threshold. Cue temperatures range from 6 °C (some D. megacephala) to 14 °C (some D. cephalotes) and vary strongly within and between egg masses of D. cephalotes. Variation is not random, but seems to be genetically determined.4. The variable temperature response renders study of effects of particular experimental regimes, and comparisons between local populations, difficult.5. A latitudinal gradient in cue temperatures for development from 6 °C at c. 46 °N to 12 or even 14 °C at c. 61 °N seems to reflect reduced diversity at high latitudes.6. Average success of spontaneous hatching exceeded 90% between 12 and 20 °C, but declined towards higher and lower temperatures.7. Unhatched eggs were not dead but in parapause; development at other, higher or lower, temperatures was induced. Spontaneous plus induced hatching success approached 90%. Developing eggs rarely died; most dead eggs were apparently unfertilized.8. Dormant plecopteran eggs are proposed to form a seed bank in stream bed sediments. Highly successful development after up to 220 days of dormancy was ascertained in Dinocras, and survival for up to 3 years is reported for other Perloidea.9. Only systellognathan egg morphology provides options for long dormancy; the other plecopteran superfamilies, notably Nemouroidea, follow different strategies.

Monitoring of behavioral patterns of aquatic organisms with an impedance conversion technique

An impedance converter, based onthe tetrapole electrode system, was used to record the behavioral patterns of a wide range of aquatic organisms, such as Daphnia magna, Gammarus pulex, Sialis lutaria, Leptophlebia vespertina, Baetis niger, Simuliidae, Dinocras cephalotes, Hydropsyche siltalai, and tadpoles of Rana temporaria. The method proved to be sensitive for different kinds of behavior, e.g., ventilation, grazing, filter feeding, net spinning, and locomotion (swimming, creeping, and looping), which makes it a promising tool for continuous biomonitoring purposes.

The effects of prey profitability and habitat complexity on the foraging success and growth of stonefly (Plecoptera) nymphs

SUMMARY The feeding behaviour of nymphs of the perlid stonefly Dinocras cephalotes when presented with two different mayfly prey types on different substrates was examined in the laboratory. Most of the evidence suggested that Dinocras cephalotes did not choose between Baetis rhodani and Rhithrogena semicolomta as, under identical circumstances: (i) the survival of both prey types was the same; (ii) the predator's overall searching behaviour was similar in the presence of either mayfly, although B. rhodani was pursued more, especially on complex substrates; (iii) the number of attacks on both prey types was the same; and (iv) the intake of dry weight prey biomass over 24 h remained the same, irrespective of prey species. The predation efficiency (no. of prey captured/no, of encounters) of D. cephalotes decreased with increasing substrate complexity—but only with R. semicolorata as prey, and also when more than one stonefly was present—but only when B. rhodani was the prey. Predator efficiency was greater at low densities of R. semicolorata but highest at a medium density of B. rhodani. Increase in the body weight of D. cephalotes was greater when nymphs were fed a monospecific diet of R. semicolorata compared with one of B. rhodani. This was despite the findings that: (i) nymphs of B. rhodani contained 31.3% more protein (per mg dry body weight) than nymphs of R. semicolorata, whereas the latter contained 81.0% more chitin; and (ii) nymphs of R. semicolorata took 2.7 times as long to consume and almost twice as long to pass through the predator's gut. The longer gut passage time may have allowed a greater assimilation efficiency which may have led to the observed better growth. Dinocras cephalotes is thought to feed opportunistically, with a certain daily food biomass intake as its primary need rather than any specific prey preference. Foraging decisions, or opportunities, affect the fitness of individual nymphs and this may be reflected in the wide size range of contemporary nymphs found in field populations. Variation in foraging efficiency by predators and evasion success by prey, across substrate types, is thought to contribute to the well‐known microdistribution patterns of species observed in lotic communities.

Predators and planariid competitors of the triclad Phagocata vitta (Dugés)

When Phagocata vitta, Crenobia alpina and Polycelis felina were exposed separately to each of seventeen potential invertebrate predators in the laboratory, only two stonefly species, Dinocras cephalotes and Perlodes microcephala, fed on the three triclad species, whilst the trichopteran Rhyacophila dorsalis ate the last two triclads. On exposing pairs of triclad species to D. cephalotes, significantly more P. felina than Ph. vitta were consumed, whereas similar numbers were eaten in each of the other two triclad combinations. Cannibalism and interspecific predation by triclad species were not observed. It is concluded that predation is unlikely to have a major influence in determining the observed distribution and abundance of triclad species in a Welsh study stream which harbours low numbers of effective predators.

Stonefly predation along a hydraulic gradient: a field test of the harsh—benign hypothesis

SUMMARY. 1. Microhabitat preferences of predatory stoneflies and four prey taxa were assessed by taking benthic samples along a hydraulic gradient in a Black Forest stream in West Germany. Densities of predator and prey species were estimated at twenty‐one hydraulic regimes.2. Enclosures containing the stonefly, Dinocras cephalotes, and control cages with no predators were placed in the substrate at hydraulic regimes favourable and unfavourable to predators. Cages received initial prey communities that were obtained from benthic samples taken at hydraulic regimes matching those intended for each cage.3. Population densities of the two most numerically important prey taxa, the mayfly. Baetis rhodani, and the Chironomidae, were reduced in the presence of Dinocras, but only when enclosures were placed in the hydraulic regimes favourable to the predator. Thus, predation effects increased as the hydraulic regime became more benign to the predators.4. Densities of two other prey species rare in the diets of Dinocras (Hydropsyche instabilis and Gammarus fossarum) were generally unaffected by predators regardless of the hydraulic regime.5. These data provide support for the hypothesis that perception of the abiotic regime as harsh or benign to predators is a good predictor of predator impact on densities of preferred prey species. In harsher abiotic regimes, impact will be low, while impact will be high in benign abiotic regimes.

Diet ofDinocras cephalotesandperla bipunctata(Plecoptera, Perlidae) in a South‐West Irish stream

This study provides the first, although limited, examination of the diets of the only two Irish perlids, Dinocras cephalotes (Curtis) and Perla bipunctata (Pictet). D. cephalotes was found to include a wide range of prey items in its diet (including plant matter) but the major components (Baetis rhodani and chironomids) varied little despite considerable variation in benthic invertebrate taxa at different sites. Marked positive selection of certain prey taxa was also apparent. The diet of P. bipunctata appeared narrower but relatively low numbers of individuals were available. Although there were some differences in the composition of the diet (prey types and size) amongst three age classes of D. cephalotes there was still considerable overlap between them. An expansion in the size range of mayfly prey was also evident with increase in predator age.

Short‐term experimental acidification of a Welsh stream: comparing the biological effects of hydrogen ions and aluminium

SUMMARY. 1. A soft‐water stream iti upland Wales was dosed with sulphuric acid and aluminiutn sulphate at two successive points to create sitnultaneous episodes of low pH, and low pH with increased aluminiutn. Chemical atid biological responses were measured before, during and after the episode and were compared with a reference zone.2. The pH fell frotn ∼7.0 to 4.28 (±0.18 SD) and 5.02 (±0.10) respectively in the acid and aluminium zones. Corresponding aluminium concentrations during the episode were 0.052 g Al m−3 (±0.008) and 0.347 g Al nr3 (±0.047), the former not differing significantly from the reference zone. The concentration of cadmium rose to 0.002‐ 0.011 g Cd m−3in both treated areas, but the concentrations of other metals were unchanged.3. In situ toxicity tests were performed with macroinvertebrates and fish. Chironomus riparius. Hydropsyche angustipennis and Dinocras cephalotes suffered no mortality. Ecdyonurus venosus, Baetis rhodani and Gammarus pulex showed up to 25% mortality in both treatment zones and further mortalities occurred after the episode. Brown trout Salmo trutta and salmon Valmo salar s howed 7–10% mortality in the acid zone, but 50–87% in the aluminium zone, where salmon had a significantly shorter LT50than trout.4. The drift of Simuliidae increased during treatment in both acid and aluminium zones. Drift densities of Dixa puherula, Protonemura meyeri, Ephemeralla ignita and Dicranota sp. increased in the aluminium zone. The most pronounced response was by Baetis rhodani in the aluminium zone where drift density increased by ×8.4 during the episode.5. Baetis rhodani was the only taxon to show a significant decline in benthic density during the treatment, and then only in the aluminium zone. Drift could account for most of the losses.6. The depth distribution of invertebrates in the substratum differed between zones following treatment. More individuals were present at the surface of the reference zone (1287 m−2±747) than at the surface of the other zones (<400 m−2); however, densities at greater depths were similar. These patterns probably reflected differences prior to the treatments.