The distribution of Ephemerella ignita (Ephemeroptera) in streams: the role of pH and food resources

SUMMARY. 1. The mayfly Ephemerella ignita does not occur in upland streams of the River Duddon catchment which have waters with low pH values of 4.8-5.2, negative alkalinities and low ionic contents. However, it does occur in lowland streams of the catchment which have waters with higher pH values of 6.6 and above, large positive alkalinities and high ionic contents. This study addresses the question of whether the absence of the animal in the acid water streams is due directly to the water chemistry or to the absence of suitable food.
2. In comparative experiments using unfed animals of several species it was shown that E. ignita has a considerable tolerance of low pH, low ion water. In this it is similar to Amphinemura sulcicollis (Plecoptera), a stonefly which occurs in the acid water streams of the River Duddon catchment.
3. Growth rates in laboratory experiments with E. ignita were equally good whether the food supplied was that available in the low pH water streams (bryophyte Nardia compressa plus filamentous alga Hormidium subtile) or typical of that available in high pH water streams (moss Rhynchostegium riparioides with the epiphytic diatom Cocconeis placentula). However, a consideration of the natural habits of the animal suggests that it is the inaccessibility of the H. subtile algal food, rather than its unsuitability, which is relevant to the exclusion of E. ignita from the low pH streams. In addition, the very limited diversity of other algae in the low pH water streams, in the summer season, may well be influential in inhibiting the establishment of E. ignita.
4. The comparative experiments using unfed animals suggested that, contrary to the findings for E. ignita , the mayflies Baetis muticus and Beatis rhodani are probably excluded from the acid water streams of the Upper Duddon because of water chemistry.     

The role of periphyton abundance and quality in the microdistribution of a stream grazer, Helicopsyche borealis (Trichoptera: Helicopsychidae)

SUMMARY. 1. A series of laboratory and field experiments was performed to determine if the preference of larvae of Helicopsyche borealis (Trichoptera: Helicopsychidae) for exposed rock surfaces in streams was related to patchiness of periphyton food.
2. At low current velocities in the laboratory larvae preferred food-saturated as opposed to food-depleted portions of substrata and drifted off the substratum in response to low food levels.
3. When offered a choice of three potential foods—a diatom, a green filamentous or a blue-green filamentous alga—larvae showed no preference.
4. Larval densities on artificial subtrata in field experiments were highest in areas of high algal chlorophyll a .
5. In laboratory experiments more adults emerged from cultures containing green algae or diatoms than cultures with blue-green algae or bacteria alone.
6. Periphyton quality was not as important as abundance in controlling H. borealis microdistribution. By selecting exposed surfaces of rocks, H. borealis larvae place themselves in the microhabitat most likely to contain both abundant and high quality food patches.     

Micro-arthropod seasonally in streams of varying pH

SUMMARY. 1. Micro-arthropods were sampled seasonally (January, May, August and October) during 1986 from ten, stony riffle sites on streams in the Ashdown Forest of southern England, using both standard benthic and interstitial samplers.
2. Total densities peaked at most sites in summer. Species richness reached a maximum at acid sites in summer but at cireumneutral sites in autumn, when Hydrachnellae and Cladocera were particularly species rich.
3. Individual species showed no obvious differences in seasonally between sites; the majority peaking in summer or autumn, regardless of pH. However, cyclopoid copepods were particularly numerous at acid sites in summer, a pattern not observed at circumneutral sites.
4. Multivariate ordination and classification of data sets from the separate seasons, and all four seasons combined, showed that mean site pH, conductivity, and aluminium and calcium concentrations were the most important variables explaining between-site variation in species composition. This clear distinction between the community structure at acidic and circumneutral sites was evident in all seasons except winter. Species composition was also more predictable throughout the year at low-pH sites.
5. A number of species were taken consistently in interstitial samples and the cyclopoids Diacydops languidus and D. languidoides were restricted to the hyporheos at circumneutral sites. The similar faunal composition of the hyporheos and the epibenthos indicated that the separation of these communities was not well defined in Ashdown Forest streams.     

Essential fatty acid content and the phosphorus to carbon ratio in cultured algae as indicators of food quality for Daphnia

1. A series of experiments examined growth of Daphnia magna on three algal diets ( Rhodomonas minuta , Scenedesmus acutus and Synechococcus sp.) at varying physiological states [nitrogen and phosphorus (P) limitations] to test whether variation in algal fatty acid and/or elemental composition can predict Daphnia growth.
2. These algae differed widely in their essential fatty acid (EFA) composition while phosphorus (P) or nitrogen limitation had only a small influence on their ω3-polyunsaturated fatty acid (PUFA) content.
3. Individually, algal ω3-PUFA content explained 69% of the variation in the normalised growth of D. magna, while algal phosphorus content explained 11% of the variation. Quantitative models for D. magna growth used both algal ω3-PUFA content and algal C : P ratio as food quality indices. Together, algal ω3-PUFA content and C : P ratio explained 70% of the variation in the normalised growth rate of D. magna .
4. Our results indicate that EFA influenced algal food quality much more strongly than P. The EFA and mineral P impacts appear to be independent.     

Food effects on life history traits and seasonal dynamics of Ceriodaphnia pulchella

1. This paper describes the effects of differences in food quantity and quality on selected life history traits of Ceriodaphnia pulchella . Animals were fed with four concentrations of two green algae, Scenedesmus obliquus and Chlamydomonas globosa , given separately as well as in a 1 : 1 mixture.
2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase, r , increased with food concentration. Life history parameters of animals raised on S. obliquus were lowest, whereas no significant differences were found between animals fed with C. globosa and with the mixture of the two algal species.
3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics of C. pulchella in Lake Tjeukemeer showed that the effect of changes in food conditions during the growing season of this species was limited, although overall food levels seemed to be poor throughout the year.
4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predator Leptodora kindtii .     

Food effects on life history traits and seasonal dynamics of Ceriodaphnia pulchella

1. This paper describes the effects of differences in food quantity and quality on selected life history traits of Ceriodaphnia pulchella . Animals were fed with four concentrations of two green algae, Scenedesmus obliquus and Chlamydomonas globosa , given separately as well as in a 1 : 1 mixture.
2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase, r , increased with food concentration. Life history parameters of animals raised on S. obliquus were lowest, whereas no significant differences were found between animals fed with C. globosa and with the mixture of the two algal species.
3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics of C. pulchella in Lake Tjeukemeer showed that the effect of changes in food conditions during the growing season of this species was limited, although overall food levels seemed to be poor throughout the year.
4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predator Leptodora kindtii .     

Microhabitat availability in Welsh moorland and forest streams as a determinant of macroinvertebrate distribution

SUMMARY. 1 Eighteen streams in mid-Wales were sampled for macro-invertebrates in both riffles and margins in April 1985–87. Stream macro-flora, substrata and marginal habitats were surveyed in May 1988.
2. TWINSPAN classification of the macroinvertebrate data indicated three major stream groups. One was distinguished by circumneutral pH and had a flora and fauna typical of such conditions. The other two groups consisted of acidic streams with moorland and conifer afforested catchments respectively. The forest streams were the more acidic but the two groups also differed significantly in the composition of their marginal habitats.
3 The acidic moorland streams had more vegetation ('soft' features) in the margins and supported several invertebrate taxa which were relative more abundant there than in the riffles. These taxa may be excluded from forest streams because the margins are 'hard' due to greater erosiveness and shading.
4. In view of the increasing cover by conifer afforestation in Britain, it is clearly necessary to elucidate all its effects on stream ecosystems, which include changes to the physical environment.     

Does biofilm origin matter? Biofilm responses to non-flow period in permanent and temporary streams

1. In some regions, climate change is increasing the variability of rainfall and the frequency of extreme events such as drought. Consequently, non-flow periods have grown in length and frequency, both in temporary and in formerly permanent streams. Water abstraction for human use may further prolong these dry periods.
2. We analysed the resistance and resilience of biofilms from permanent and temporary streams to non-flow conditions. This was achieved by exposing cobbles (collected from permanent and temporary streams) with intact biofilm to 31 days of non-flow, followed by 20 days of stream flow in artificial stream channels. Biofilm resistance and resilience were assessed at a structural (algal biomass, pigment composition, and algae and cyanobacteria composition) and functional level (photosynthetic efficiency and community metabolism).
3. Algal taxa in biofilms from permanent and temporary streams differed throughout the experiment. Biofilms from permanent streams were less resistant to non-flow than those from temporary streams at structural level. Permanent stream biofilms also presented lower resilience at a structural level, but responded similarly to temporary stream biofilms at a functional level.
4. Our investigation shows how the non-flow period disturbed permanent stream biofilms, and suggests that temporary stream biofilms will have greater adaptive capacity as hydroperiod becomes shorter due to climate change.

     

Land use impacts on stream community composition and concordance along a natural stress gradient

Most ecosystems are subjected to multiple stressors derived from natural and anthropogenic sources and community responses to human disturbance in naturally stressful habitats may differ from those in more benign habitats. We examined the influence of a natural (geology-driven acidity) vs. human-induced stress (land drainage) and their interaction on the composition and concordance of stream diatom, bryophyte and invertebrate communities. To account for differing drainage impacts in circumneutral (sedimentation) and naturally acid (reduced pH and increased metal concentrations) streams we investigated concordance in three groups of streams: reference (circumneutral and naturally acidic reference), circumneutral (reference and drained) and naturally acidic (reference and drained) streams. We expected diatoms to respond more strongly to anthropogenic acidification and more weakly to sedimentation compared to bryophytes and invertebrates. We expected overall strong concordance among the three taxonomic groups, but especially so in reference streams. All three organism groups had distinct species composition in naturally acidic vs. circumneutral streams. Concordance between communities was overall strong, especially so in the reference streams. All groups responded to drainage disturbance in both types of streams. Invertebrates were slightly less responsive to increased acidification in the naturally acidic streams but were more affected by sedimentation in the circumneutral streams than were the other two groups. The natural stressor affected communities more than the anthropogenic stressors. Naturally stressed communities were affected by an anthropogenic stressor as much as those in more benign habitats, although the additional stressor was similar to the initial stress (further reduction of stream pH). Naturally acid streams may need special concern in bioassessment because models based on circumneutral reference sites will likely produce biased predictions for these streams.     

In situ light and phosphorus manipulations reveal potential role of biofilm algae in enhancing enzyme‐mediated decomposition of organic matter in streams

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Effects of acidic pH and phytoplankton on survival and condition of Bosmina longirostris and Daphnia pulex

The effects of pH, algal composition and algal biomass on abundance, size, reproduction and condition of Daphnia pulex and Bosmina longirostris were tested in a field experiment using water and natural phytoplankton assemblages from a circumneutral (pH 6.43) and a moderately acidic (pH 5.75) lake in south-central Ontario. Both species were affected by pH and phytoplankton composition, with decreased egg production, lipid reserves, body size or abundance in treatments containing algae and/or water from the more acidic lake compared to treatments containing water and phytoplankton from the circumneutral lake. This result was unexpected for Bosmina, which often increases in relative and/or absolute abundance in acidified lakes. The negative effect of acidic conditions on Bosmina suggests that the population increase observed in most acidified lakes is not due to a positive response to low pH or ambient phytoplankton, but to altered biotic interactions possibly involving reduced competition.     

Cascading effects of predatory fish on the composition of benthic algae in high‐altitude streams

Cascading effects of predators can affect ecosystem properties by changing plant biomass, distribution and assemblage composition. Using data from field surveys and whole‐stream experiments we tested the hypothesis that predatory trout change assemblage composition of benthic algae in high‐elevation streams mediated by grazer behavior. Field surveys revealed that the taxonomic composition of algal assemblages differed significantly between streams that contained trout and those that were fishless; but comparisons of palatable versus unpalatable algal taxa between fish and fishless streams were equivocal because of high natural variability. Therefore, we tested for a behavioral (non‐consumptive) trophic cascade experimentally by adding brook trout chemical cues to six naturally fishless streams for 25 days and compared responses of grazers and algae to six reference streams without fish cues added. Algal response variables included rates of change in the abundance of three physiognomic categories, from most palatable (attached erect and prostrate diatoms) to least palatable (non‐diatoms), as determined from food selectivity analyses of the most common grazers (mayflies and caddisflies). Fish cues did not affect the mean densities or changes in densities of total grazers or any individual grazer species. However, in streams where fish cues were added, rates of accrual of attached erect diatoms, which was the preferred algal type for the grazer most vulnerable to trout predation (Baetis), were higher and their densities increased significantly faster with increasing densities of this grazer species than in reference streams. Results of his experiment support the hypothesis that predator induced suppression of grazer foraging behavior, rather than cascading effects of top predators on grazer density, may contribute to variation in the composition of algal assemblages among streams by allowing proliferation of most palatable algal species.     

Bacteria and algae in stream periphyton along a nutrient gradient

1. Stream riffles in southern Ontario and western Quèbec were sampled for biomass (58 stations from 51 streams) and production (22 stations from 21 streams) of algae and bacteria in periphyton to test the hypothesis that bacteria in benthic biofilms compete with algae for nutrients. 2. Algal and bacterial biomass were positively correlated, as were algal and bacterial production. Bacterial production was also positively correlated to algal and bacterial biomass, but the relationship was not significant. The ratio of algal to bacterial biomass did not vary with nutrients whereas algal production tended to increase with nutrients more rapidly than bacterial production. 3. Instream nitrogen concentrations explained 38–58% of the variability in algal biomass and production. Bacterial abundance explained an additional 9–29% of the residual variance in algal production and biomass. However, the relationship between bacterial abundance and algal production and biomass, once nutrients were taken into account, was positive, in contrast to the predicted effect of competition. 4. Hence, we reject our original hypothesis that bacteria in biofilms compete with algae for nutrients and instead suggest that bacteria and algae in biofilms coexist in an association that offers space and resources to sustain production of both groups of organisms.     

The relative importance of shading and nutrients on algal production in subtropical streams

1. We used artificial substrata in forested and open streams in South-East Queensland, Australia, to determine the relative importance of shading from riparian vegetation and of nutrients on periphyton growth, and whether nitrogen and/or phosphorus limited algal productivity.
2. Nutrient-diffusing substrata consisting of agar enriched with N, P and N + P, and controls without nutrients, were deployed in duplicate at 15 sites in headwater streams with riparian canopy cover ranging from 0 to 88%.
3. Shading was the over-riding factor controlling periphyton biomass accrual on the artificial substrata, with nutrients playing a relatively minor role. Microscopic examination of periphyton scrapings taken after 7 weeks revealed that diatoms dominated on the artificial substrata in shaded streams, whereas filamentous green algae dominated the algal assemblage in the more open canopy streams.
4. Whilst nutrients had little effect on the accrual of algal biomass compared with riparian shading, there was evidence that nitrogen, and not phosphorus, stimulated periphyton production in streams with sufficient light.     

Pathways for algal recolonization in seasonally‐flowing streams

1. In semi‐arid climates, seasonally‐flowing streams provide most of the water required for human use, but knowledge of how water extraction affects ecological processes is limited. Predicted alterations in stream flows associated with the impacts of climate change further emphasize the need to understand these processes. Benthic algae are an important base for stream food webs, but we have little knowledge of how algae survive dry periods or respond to altered flow regimes. 2. We sampled 19 streams within the Grampians National Park, south‐eastern Australia and included four components: a survey of different drought refuges (e.g. permanent pools, dry biofilm on stones and dry leaf packs) and associated algal taxa; a survey of algal regrowth on stones after flows recommenced to determine which refuges contributed to regrowth; reciprocal transplant experiments to determine the relative importance of algal drift and regrowth from dry biofilm in recolonization; direct measurement of algal drift to determine taxonomic composition in relation to benthic assemblage composition. 3. Algae showed little specificity for drought refuges but did depend on them; no species were found that were not present in at least one of the perennial pool, dry biofilm or leaf pack refuges. Perennial pools were most closely correlated with the composition of algal assemblages once flows resumed, but the loss or gain of perennial pools that might arise from stream regulation is unlikely to affect the composition of algal regrowth. However, regulated streams were associated with strong increases in algal density in dry biofilm, including increased densities of Cyanobacteria. 4. A model for algal recolonization in seasonally‐flowing streams identified three pathways for algal recolonization (drift‐dependent, dry biofilm‐dependent and contributions from both), depending on whether streams are diatom‐dominated or dominated by filamentous algae. The model predicted the effects of changes to stream flow regimes on benthic algal recolonization and provides a basis for hypotheses testable in streams elsewhere.     

Effects of acidification on the breeding ecology of a stream‐dependent songbird,the Louisiana waterthrush (Seiurus motacilla)

1. We compared breeding ecology of the Louisiana waterthrush (Seiurus motacilla) on acidified and circumneutral streams in the Appalachian Highlands of Southwestern Pennsylvania from 1996 to 2005. 2. Headwater streams impacted by acid mine drainage and/or acidic precipitation showed reduced pH (range 4.5–5.5) compared to four circumneutral streams (pH c. 7). Acid‐sensitive taxa, including most mayflies (Ephemeroptera), were almost completely absent from acidified streams, whereas several acid‐tolerant taxa, especially stonefly (Plecoptera) genera Leuctra and Amphinemura, were abundant. 3. Louisiana waterthrush breeding density (c. 1 territory km^?1) was significantly reduced on acidified streams compared to circumneutral streams (>2 territories km^?1). Territories on acidified streams were almost twice as long as on circumneutral streams. Territories usually were contiguous on circumneutral streams, but they were often disjunct on acidified streams. Breeding density declined on one acidified stream that we studied over a 10‐year period. 4. Clutch initiation was significantly delayed on acidified streams, on average by 9 days in comparison to circumneutral streams, and first‐egg dates were inversely related to breeding density. Birds nesting along acidified streams laid smaller clutches, and nestlings had shorter age‐adjusted wing lengths. Stream acidity had no effect on nest success or annual fecundity (fledglings/female). However, the number of young fledged km^?1 was nearly twice as high on circumneutral streams as on acidified streams. 5. Acidified streams were characterized by a younger, less site‐faithful breeding population. Individuals were less likely to return multiple years to breed, allowing inexperienced breeders to settle on acidified streams. Pairing success was lower on acidified streams, and we observed four cases of waterthrushes emigrating from territories on acidified streams to nearby circumneutral streams in the following year. 6. We conclude that acidified headwaters constitute lower quality habitat for breeding Louisiana waterthrush. However, breeding birds can apparently compensate for reduced prey resources to fledge young on acidified streams by increasing territory size, foraging in peripheral non‐acidified areas, and by provisioning young with novel prey.     

Life history consequences for Daphnia pulex feeding on nutrient-limited phytoplankton

1. The growth and feeding of Daphnia pulex De Geer on different algal species was examined. The green algae Chlamydomonas reinhardtii Dangeard and Scenedesmus acutus Meyen, the diatom Synedra tenuissima Kützing, the cryptophyte Cryptomonas pyrenoidifera Geitler and the cyanobacterium Microcystis aeruginosa Kützing were cultured in non-limiting and in N- or P-limiting medium and used as food for D. pulex.
2. Growth limitations were reflected in the elemental and biochemical composition and the morphological characteristics of the algal resources.
3. The clearance rates of D. pulex feeding on nutrient-limited algae were reduced. This was not observed when nutrient-limited mutant Chlamydomonas cells without cell walls were used as food, indicating that the cell wall may play an important part.
4. In comparison with animals grown on nutrient-sufficient cells, nutrient-limited algae resulted in smaller body length, reduced brood sizes, reduced size at maturity, increased age at first reproduction and, consequently, in reduced Daphnia population growth rates.
5. Daphnia population growth rates ( r ) were negatively correlated with the C : P ratio and the carbohydrate content of the food. Moreover, significant correlations between r and clearance rates were found.
6. The observed differences in the grazing and the life history parameters of Daphnia feeding on non-limited and nutrient-limited algae may be the result of both reduced nutritional value and reduced digestibility of nutrient-limited algae.     

Effects of light and nutrients on the net accumulation and elemental composition of epilithon in boreal lakes

1. Two experiments in the Experimental Lakes Area (ELA) in north-western Ontario, Canada examined the effects of light and two key elements on the net accumulation and elemental composition of epilithon. In Lake (L) 224, benthic algae were grown under different light intensity and phosphorus supply, while in L302S we provided three levels of two different carbon sources (bicarbonate and glucose) to algae colonizing nutrient-diffusing substrata. After 1 month of accumulation, we sampled biofilms for chlorophyll (chl), carbon (C), phosphorus (P) and algal C.
2. Increased C supply did not significantly affect algal growth (C or chl) or elemental composition (C/P ratios) in L302S. However, P enrichment increased chl and algal C, dramatically reduced the C/P ratio of epilithon, and did not affect total organic C in L224. Phosphorus enrichment also increased the proportion of algal material in the total particulate organic matter and altered the taxonomic composition of algae in L224 biofilms. Shading had no significant effect on the C/P ratio and total organic C in epilithon from the L224 experiment.
3. Our results demonstrate that P supply affects the elemental composition of organic matter that collects on rock substrata. It thus appears that low availability of P relative to C and light drives the formation and retention of high C/P organic matter on rock surfaces in oligotrophic boreal lakes.     

Effect of inhibitors on calcium carbonate deposition mediated by freshwater algae

Nine green algae, a diatom and three cyanobacteria were shown to precipitate CaCO₃ in batch culture, when grown in the light in a hard water medium containing 68 mg L⁻¹ soluble calcium. The composition of the medium was based on that found in a natural hardwater marina where precipitation of CaCO₃ within algal biofilms occurred. Deposition occurred as a direct result of photosynthesis which caused an increase in the pH of the medium. Once a critical pH had been reached, typically approximately pH 9.0, precipitation began evidenced by a fall in the concentration of soluble calcium in the medium. Certain characteristics of the precipitation process displayed by the diatom Navicula sp. were different to those of the other algae. All algae produced extracellular crystals of irregular morphology. Using a standardized protocol employing the green algae Chlorococcum sp. and Stigeoclonium variabile, the effects of various inhibitors of CaCO₃ nucleation or growth of crystals were studied. Fifteen compounds were screened and assessed for their performance in this context. Most materials effectively delayed deposition of CaCO₃, many decreased precipitation rates and all had a marked effect on crystal morphology. The most effective compound was HEDP (1-hydroxyethylene 1,1 diphosphonic acid), which inhibited precipitation completely at a concentration of 2.5 mg L⁻¹ The use of such compounds to reduce the precipitation of calcium salts within algal biofilms in natural hard waters is discussed.