Circumneutral Streams Research Articles

Diverging effects of anthropogenic acidification and natural acidity on community structure in Swedish streams

Anthropogenic acidification caused by aerial deposition of acidifying substances is known to have detrimental effects on freshwater biota, including reductions in species diversity and ecosystem functioning. However, such impairment is not found in systems acidified to a similar extent by natural processes. A proposed explanation for this difference is that freshwater organisms have had far more time to evolve and adapt to natural than anthropogenic acidification. Thus, where acidity is natural, adaptation may account for diverse and functional communities. Here, we investigated whether adaptations – that were previously implied to occur on small spatial scales – may explain the species richness patterns on a much larger geographical scale, apply to ecological functioning, and are relevant in Sweden, where natural acidity is geologically relatively recent. Therefore, we compared differences in species diversity and ecosystem process rates between 24 acidic and circumneutral streams in northern Sweden, where acidity is natural, and southern Sweden, where acidity is largely anthropogenic. In agreement with our predictions, the difference in macroinvertebrate species richness between acidic and circumneutral streams was threefold larger in the region where acidity was anthropogenic than where it was natural, albeit marginally non-significantly. In contrast, no such trend was found for the rates of decomposition by microbes and leaf-feeding macroinvertebrates, possibly due to functional redundancy. The structure of species assemblages differed between acidic and circumneutral sites and between the regions. Our results agree with the notion that freshwater biota are adapted to natural acidity, but competing explanations including other differences in water chemistry and differences in the biogeographical colonization histories may also account for part of the observed patterns. Since naturally acidic environments similar to those in northern Sweden are widespread, we predict that diverse and functionally efficient freshwater communities that are well adapted to such conditions are more common than currently recognized.

On the ecology of the filter-feeding Neureclipsis bimaculata (Trichoptera, Polycentropodidae) in an acid and iron rich post-mining stream

Open-pit mining of lignite in East Germany has created landscapes with extreme environmental conditions. Post-mining aquatic habitats are characterized by low biodiversity and simple food webs due to the impact of acid mine drainage. In this study, the ecology of the filter-feeding caddisfly Neureclipsis bimaculata (L.) was examined, which is abundant in the acidic Flosgraben stream (pH 2.5–3.6) in Lower Lusatia, Germany. From benthic samples, we measured larval size and biomass and estimated population dynamics. The seston drift was sampled and retention efficiency of the larvae’s nets was assessed in a field tracer experiment to evaluate diet availability. Mean annual abundance was 1,380 ind m−2 with a biomass of 1,010 mg m−2. Annual secondary production of N. bimaculata was 8,450 mg m−2. The larval microdistribution reflected the preference for in-stream wood and a limitation by low flow velocity. Morphometric factors of the larvae in the acidic stream were in the range of morphometric factors found in circumneutral streams that covered a range of trophic levels. Although coverage by iron particles reduced flow, the nets account for 63% of the mean particle retention. It is suggested that the retention efficiency and the availability of drifting organisms allowed the larvae to feed on 8.4 mg m−2 per day, which revealed a ratio of biomass production to ingested food of 60%.

Climate change effects on upland stream macroinvertebrates over a 25‐year period

AbstractClimate change effects on some ecosystems are still poorly known, particularly where they interact with other climatic phenomena or stressors. We used data spanning 25 years (1981–2005) from temperate headwaters at Llyn Brianne (UK) to test three hypotheses: (1) stream macroinvertebrates vary with winter climate; (2) ecological effects attributable to directional climate change and the North Atlantic Oscillation (NAO) are distinguishable and (3) climatic effects on macroinvertebrates depend on whether streams are impacted by acidification. Positive (i.e. warmer, wetter) NAO phases were accompanied by reduced interannual stability (=similarity) in macroinvertebrate assemblage in all streams, but associated variations in composition occurred only in acid moorland. The NAO and directional climate change together explained 70% of interannual variation in temperature, but forest and moorland streams warmed respectively by 1.4 and 1.7°C (P<0.001) between 1981 and 2005 after accounting for NAO effects. Significant responses among macroinvertebrates were confined to circumneutral streams, where future thermal projections (+1, +2, +3°C) suggested considerable change. Spring macroinvertebrate abundance might decline by 21% for every 1°C rise. Although many core species could persist if temperature gain reached 3°C, 4–10 mostly scarce taxa (5–12% of the species pool) would risk local extinction. Temperature increase in Wales approaches this magnitude by the 2050s under the Hadley HadCM3 scenarios.These results support all three hypotheses and illustrate how headwater stream ecosystems are sensitive to climate change. Altered composition and abundance could affect conservation and ecological function, with the NAO compounding climate change effects during positive phases. We suggest that acidification, in impacted streams, overrides climatic effects on macroinvertebrates by simplifying assemblages and reducing richness. Climatic processes might, nevertheless, exacerbate acidification or offset biological recovery.

The effects of low pH and palliative liming on beech litter decomposition in acid-sensitive streams

The decomposition of allochthonous leaf litter is retarded by stream acidification, but few studies have evaluated whether this effect can be offset by liming – the palliative addition of calcium carbonate either to streams or their catchments. We assessed the response of litter decomposition to pH and experimental liming in Welsh upland streams. Small-mesh (<335 μm) litter-bags containing common beech (Fagus sylvatica L.) were submerged in main river sites along the River Wye, and in replicate acid, circumneutral and experimentally limed tributaries (all n = 3) for 20 days. Beech decomposition was inhibited in acid tributaries and main river sites compared to circumneutral tributaries. Despite having only moderately increased pH relative to acid streams, limed sites had increased decomposition rates that were indistinguishable from naturally circumneutral streams. Decomposition rates increased highly significantly with pH across all 12 sites studied, and values were near identical to those in more prolonged experiments elsewhere. There were no significant variations in shredder numbers with decomposition rate, and no evidence that sites with faster decomposition had smaller shredder proportions. Although based on short-term observations and leaves from just one tree species, these results are consistent with the well-known retardation at low pH of some aspect microbial decomposition (e.g. by hyphomycete fungi). They are among the first to suggest that stream liming to combat acidification might reverse such impacts of low pH. Further data are required on the microbiological causes and ecological consequences of altered detrital processing in acid-sensitive and limed streams.

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

Summary1. 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 &gt; 5.7) remained high during both low‐ and high‐flow. By comparison, mortality increased (P &lt; 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 (&gt;80%) &gt; episodic (&gt;40%) &gt;circumneutral (&lt;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.

Assessing the short‐term response of stream diatoms to acidity using inter‐basin transplantations and chemical diffusing substrates

Summary1. Acid‐base status has major effects on diatoms, but there is little information on their short‐term response to changing acidity. This is despite the use of diatoms as bioindicators in streams where acid episodes are important during rainstorms (hours to days) or snowmelt (days to weeks). In the Llyn Brianne experimental catchments (Wales, UK), we attempted to mimic the effects of short‐term acidification by (i) reciprocally transplanting diatoms between two streams of contrasting acidity and (ii) using acid‐diffusing substrates.2. Diatom diversity decreased rapidly on substrata transplanted from the circumneutral into the acidic stream, and increased in the reciprocal transplantation. Changes in dominant taxa occurred within three days in the acidic stream because of the rapid growth of Eunotia exigua, and by nine days in the circumneutral stream because of the proliferation of Achnanthidium minutissimum. Transplants were near indistinguishable from ambient assemblages by day 12.3. There were no effects of enclosures on assemblage composition, but diatoms responded more rapidly to altered chemistry in enclosures with coarse mesh (26 × 50 mm) than finer mesh (320 μm).4. Chemical diffusing substrates comprised terracotta tiles attached to dosing reservoirs that created locally acid (using H2SO4) or metal‐rich conditions (using MnSO4) in the circumneutral stream over 26 days. Diatom responses were compared with reference substrates dosed with deionised or circumneutral stream water, and we also assessed whether effects were moderated by macroinvertebrate grazers.5. Surface pH was lower by 1–2 pH units on acid‐dosed substrates than on reference tiles or in surrounding streamwater. Grazed assemblages on acid‐dosed substrates differed significantly from ungrazed reference assemblages, acquiring significantly greater relative abundance of Eunotia spp. However, the magnitude of response was less than in the between‐stream transplantations either because (i) metal exposure and base cation concentrations differed between the transplants and dosing substrates or (ii) diatom response to reduced pH on the diffusing substrates was restricted by the scarcity of acidobiontic diatoms in the circumneutral stream. Similar filter, founder or dominance effects might also affect diatom responses to real acid episodes.6. These data show that diatom assemblages can respond rapidly and directly to changes in acid‐base status, but short‐term acidification might affect diatoms more rapidly than subsequent recovery. Because the experimental methods used were imperfect representations of episodic effects, diatom response to real acid events requires further field evaluation.

Growth and population size of crayfish in headwater streams: individual‐ and higher‐level consequences of acidification

Summary1. Environmental stress may have indirect positive effects on population size through modification of food‐web interactions, despite having negative effects on individuals. Here we evaluate the individual‐ and population‐level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams.2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity.3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites.4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.

Stonefly (Plecoptera) species of an acidic intermittent stream in southwestern Pennsylvania, USA, comparison with a circumneutral perennial stream and consideration of survival strategies

The stonefly fauna of an acidic intermittent unnamed tributary to Indian Creek, southwestern Pennsylvania, is described and compared with the fauna of a nearby circumneutral, perennial stream, Camp Run. The unnamed tributary had a distinctive stonefly fauna comprised of 22 species representing seven families, with the highest species richness and abundance in the families Capniidae, Leuctridae and Nemouridae. Allocapnia nivicola (FITCH) was the dominant species. Ostrocerca albidipennis (WALKER) and Leuctra ferruginea (WALKER) were the secondary dominants. Perlids and pteronarcyids were absent. Camp Run had 22 stonefly species representing all nine North American stonefly families, with the highest species richness or abundance in the Capniidae, Leuctridae, and Peltoperlidae. Thirteen species were found in both the unnamed tributary and Camp Run, including six species of Leuctra. Univoltine and semivoltine species were present in both streams. Published accounts of life history, behavior, and acid tolerance were reviewed to explain presence or absence of stonefly species in the unnamed tributary.

Effects of stream acidification on fungal biomass in decaying beech leaves and leaf palatability

We examined the effect of surface water acidification on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and palatability to shredders of common beech leaves ( Fagus sylvatica L.) in five mountain streams (pH 4.7–7.1). Leaf decomposition was significantly faster in the circumneutral streams (pH 6.4–7.1; k⩾0.00175 d −1), when compared to acidic streams (pH 4.7–4.9; k⩽0.00100 d −1). Fungal biomass showed no particular trend along the acidification gradient except that it peaked earlier in the stream closest to neutrality. Leaf palatability, measured as the feeding activity of the leaf-shredding amphipod Gammarus fossarum Koch, varied with the exposure time in the streams. Except for the higher palatability of leaves exposed during 6 weeks at the highest pH, patterns among streams were mostly similar. These results suggest that reduced processing rates in the most acidic streams were not related to differences in fungal biomass associated with decomposing leaves and that microbial conditioning was only slightly delayed by acidification. Possible effects of low pH and related variables (Ca, Al) on microbial decomposition and detritivorous macroinvertebrates are discussed to clarify the inhibition of beech leaf decomposition in the studied systems.

Heterogeneous response of central European streams to decreased acidic atmospheric deposition

We investigated the relations between mountain streamwater chemistry and atmospheric pollution in an area of 1611 km2 of the Czech Republic by comparing concentrations of SO4, NO3, Cl, Ca and the pH at 432 localities at the time of high (1984–1986) and relatively low (1996–2000) acidic atmospheric deposition. Medians of Cl, SO4, and NO3 decreased by 17, 96 and 60 μeq l−1, or by 23, 17 and 49%, respectively, during 12±2 years. The decreased Cl corresponds to decreased emission of industrial Cl (as HCl). The decreased stream SO4 was proportionally lower than the 71% decrease in S-emissions due to leaching of previously accumulated SO4 from soils and non-zero background concentrations. Decreases of NO3 up to 60% in streamwaters with pH⩽6 was greater than the decrease of N emission in central Europe, about 35%. Extensive regional decrease of NO3 is surprising and is probably described for the first time. The difference in NO3 concentrations between the two periods was probably enhanced by (a) an increase of mineralisation of forest floor in the mid-1980s and (b) by higher uptake of N in the late-1990s. The median pH of the 432 streams did not change but the pH of the sub-population with pH<6 in the mid-1980s recovered substantially. The pH of circum-neutral streams (pH>6.5) decreased even as acidic atmospheric deposition decreased.

Impact of acid mine drainage on benthic communities in streams: the relative roles of substratum vs. aqueous effects

Restoration of streams impacted by acid mine drainage (AMD) focuses on improving water quality, however precipitates of metals on the substrata can remain and adversely affect the benthos. To examine the effects of AMD precipitates independently of aqueous effects, four substrata treatments, clean sandstone, clean limestone, AMD precipitate-coated sandstone and coated limestone, were placed in a circumneutral stream of high water quality for 4 weeks. Iron and Al were the most abundant metals on rocks with AMD precipitate, and significantly decreased after the exposure. Precipitate on the substrata did not significantly affect macroinvertebrate or periphyton density and species composition. In an additional experiment, percent survival of caged live caddisflies was significantly lower when exposed in situ for 5 days in an AMD affected stream than in a reference stream. Caddisfly whole-body concentrations of all combined metals and Fe alone were significantly higher in the AMD stream. Whole-body metal concentrations were higher in killed caddisflies than in live, indicating the importance of passive uptake. The results suggest the aqueous chemical environment of AMD had a greater affect on organisms than a coating of recent AMD precipitate on the substrata (ca. 0.5 mm thick), and treatment that improves water quality in AMD impacted streams has the potential to aid in recovery of the abiotic and biotic benthic environment.

Long‐term effects of catchment liming on invertebrates in upland streams

1. Catchment liming to mitigate acidification causes major chemical change in freshwaters but longer‐term effects are poorly understood. Using a replicated basin‐scale experiment with a multiple BACI design (= before‐after‐control‐impact), we assessed chemical and biological effects for 10 years after the catchments of three acidified Welsh streams at Llyn Brianne were limed in 1987/88.2. Stream chemistry was measured weekly to monthly, and macroinvertebrates monitored annually, between 1985 and 1998. Biological change through time was assessed from the abundance and taxon richness of invertebrates. We paid particular attention to 18 species known to be acid‐sensitive. The effects of liming were assessed by comparing chemical and biological trends among the three replicate limed streams, three acid reference streams and two naturally circumneutral streams.3. Following single lime applications, acid‐base chemistry in treated streams changed significantly. High mean pH (&gt; 6), increased calcium (&gt; 2.5 mg L−1) and low aluminium (&lt; 0.1 mg L−1) persisted throughout the 10 years following liming.4. The effects of liming on invertebrates were modest. Acid sensitive taxa increased significantly in abundance in limed streams, but only during 2 years following treatment. Significant effects on richness were more sustained, but on average added only 2–3 acid‐sensitive species to the treated streams, roughly one‐third of their average richness in adjacent circumneutral streams. Only the mayfly Baetis rhodani and the stonefly Brachyptera risi occurred significantly more often in limed streams after treatment than before it.5. Despite these modest long‐term effects on invertebrates, nearly 80% of the total pool of acid‐sensitive species has occurred at least once in the limed streams in the 10 years since treatment. This pattern of occurrence suggests that the colonization of limed streams by acid‐sensitive taxa reflects limited persistence rather than restricted dispersal. We present evidence to show that episodes of low pH continued to affect acid‐sensitive taxa even after liming. We highlight the importance of extending the time‐periods over which the effects of large‐scale ecological experiments are assessed.

Growth of an acid‐tolerant stonefly on epilithic biofilms from streams of contrasting pH

1. The apparent absence of a specialist herbivorous grazer guild from many acid streams suggests that algae‐grazer linkages in acid‐stream food webs are weak or absent. It has been hypothesized that the absence of herbivores is a consequence of the low quality and/or quantity of biofilms in acid streams.2. We compared the taxonomic composition, biomass and potential nutritional quality of epilithic biofilms from four acid and four circumneutral streams, and examined whether nymphs of a herbivore–detritivore, the stonefly Nemurella pictetii (Plecoptera: Nemouridae), could grow equally well when fed on eight biofilms from four acid and four circumneutral streams.3. Biofilms from acid and circumneutral streams differed strongly in algal composition, the former having relatively more coccoid green algae but fewer diatoms and filamentous green algae. Diatom floras differed with stream water pH.4. The quantity (i.e. area‐specific chlorophyll content, algal numbers and AFDM) and quality (biomass‐specific protein and soluble carbohydrate content) of biofilms differed significantly, both among sites of similar pH, and overall between the groups of acid and circumneutral streams.5. Nymphs of N. pictetii grew successfully on biofilms for 8–10 weeks up to emergence. However, no systematic differences in growth rate were found between the two groups of acid and circumneutral streams. Differences in the digestibility of benthic algae from different sources, and the adjustment of nymphal feeding rates, are discussed in the light of a lack of a clear relationship between growth and food quality.

Influence of shredders in mediating breakdown rates of beech leaves in circumneutral and acidic forest streams

From December 1997 to July 1998, the breakdown of beech leaves (Fagus sylvatica) and associated macroinvertebrate assemblages was investigated in seven headwater forested streams that differed in mean pH (4.64 to 7.20) due to acidification. Comparisons were made between leaves held in 0.3 mm, and 5 mm mesh bags in order to assess the contribution of macroinvertebrate shredders in litter breakdown. In addition, shredding efficiencies of five acid-tolerant and acid-sensitive invertebrate taxa (Protonemura sp., Leuctra sp., Sericostoma personatum, Potamophylax cingulatus, Gammarus fossarum) were investigated under laboratory conditions. Results showed that breakdown of leaves enclosed in fine-mesh bags was very low in all the streams, indicating that direct microbial contribution to the mass loss of beech leaves was small. On the contrary, breakdown of leaves enclosed in coarse-mesh bags was drastically reduced under acidic conditions (mass loss = 15 to 23 % after 270 days) when compared to circumneutral conditions (mass loss = 91.7 to 97.7 % after 270 days). Shredder assemblages associated with bags in acidic streams were dominated by Nemouroidea stonefly larvae, which showed low shredding efficiencies in laboratory experiments, particularly the early-instar larvae. Beech leaf breakdown rates rapidly increased at a mean stream pH>5.5, apparently due to the increasing abundance of the acid-sensitive shredders, S. personatum and G. fossarum, which shredded beech leaves more efficiently in laboratory feeding trials (even the first instars in the case of G. fossarum). The results of this study suggest that invertebrate shredders play an important role in controlling rates of beech leaf breakdown in forested Vosgian streams that differ in levels of anthropogenic acidification.

Resource depression by a trophic generalist in an acid stream

The occurrence and impact of algal grazing by invertebrates is widespread and well known in circumneutral streams but poorly understood in acidified streams, where specialist grazing taxa are absent. We examined the ability of the generalist stonefly, Nemurella pictetii (Plecoptera: Nemouridae), maintained at different densities using in-stream experimental enclosures, to depress the biofilm resources coating substrata in an acid stream. Nymphs were reared on the epilithic biofilm coating cobbles in clear aquaria in Lone Oak at 0, 0.5, 1 and 4 times ambient density and their growth rate compared. The biomass and algal taxonomic composition of the biofilm was also compared among treatments and contrasted with stones from the stream. After 23 d, biofilm resources, evidenced by descriptors such as diatom abundance, chlorophyll, carbohydrate and protein content, had been depressed by the nymphs. The acidobiontic diatom Eunotia exigua was harvested more efficiently than other algal taxa when nymphal densities were high. Growth of N. pictetii nymphs in the experimental channels was density dependent and the decline in growth of the nymphs with increasing density was probably due to food limitation. The ability of N. pictetii, often classed as a shredder of leafy detritus, to affect the biomass and composition of epilithic biofilm suggests that this abundant species is an important consumer of algae in acidified systems, where specialist herbivores are often absent. Grazing can clearly be important in acidified streams.

Impact of Headwater Stream Acidification on the Trophic Structure of Macroinvertebrate Communities

AbstractIn order to investigate the impact of freshwater acidification on the trophic structure of macroinvertebrate communities, we performed a study on 22 forested headwater streams characterised by different degrees of acidification (mean pH = 4.49 to 6.98). Results showed that in acidic streams all functional feeding groups were affected in terms of taxonomic richness. As far as the population density was concerned, only a few acid‐tolerant taxa of shredders and predators showed an increasing abundance under acidic conditions. Trophic structure of acidified streams appeared to be deeply impacted with a large contribution of shredders and a complete disappearance of scrapers. In contrast, in circumneutral streams, we found that each functional feeding group had an almost equal share of the trophic web.

A comparative study of beech leaf breakdown, energetic content, and associated fauna in acidic and non-acidic streams

We performed an eight-month study on beech litter breakdown and caloric content in two headwater streams characterized by different degrees of acidification (mean pH = 4.5 and 7.2). We also compared macroinvertebrate colonization of the leaves in the two streams. Beech leaf breakdown rate was 9 times slower under acidic conditions, which is a decrease greater than in other records from the literature. In both streams leaf caloric content decreased as litter decomposition proceeded but was always lower in the non-acidic one. Furthermore, results of energy loss per litter basket showed that the acidified stream was energetically inefficient when compared with the circumneutral stream. Gammarus fossarum dominated the shredder assemblage in leaf packs placed in the circumneutral stream. Gammarus was lacking in the acidified stream where Leuctra assumed greatest importance. This suggests that the absence of an efficient shredder, G. fossarum may have partly accounted for the slow leaf breakdown and the resulting accumulation of leaf litter in the acidified stream.

Hydraulic Habitat and the Assemblage Structure of Stream Benthic Microcrustacea

The relationship between microcrustacean assemblage structure and flow habitat was investigated in 9 streams with differing hydraulic characteristics. A series of partial canonical correspondence analyses was used to relate species abundances of microcrustacea to geographical, chemical, hydraulic, and seasonal variables. Once the influence of geography (latitude) on species distributions was removed, the environmental variables accounted for almost 38% of the variation in the combined (all seasons) data set. Circumneutral streams had greater species richness and higher abundance of cosmopolitan species than more acidic streams. The microcrustacean assemblages were distinguished further on the basis of stream hydraulic character. Previous research had categorised the 9 streams into Type I, II, or III depending on the fraction of stream bed retaining low shear stress at high discharge (declining from Types I to III). Taxa that are largely epibenthic in habit tended to be most abundant in Type I streams. These taxa may use as habitat the extensive low shear stress areas present in these streams, enabling their survival even at high discharge. Conversely, taxa that are largely interstitial in habit tended to be most abundant in Type II and III streams. The distribution of interstitial taxa may reflect conditions within the substratum rather than at the surface. The differences in microcrustacean assemblage structure were largely species-specific; the Harpacticoida were the only group to show a consistent pattern in relation to stream hydraulics.

The autumn communities of freshwater hyphomycetes in the tributaries of the River Neelum

Freshwater hyphomycete communities of autumn, based on data generated by three techniques used simultaneously, were richer than those detected by these techniques used singly. The fungal communities in the acidic streams were poorer (28–32 species) than in the circumneutral streams (39–64 species). No significant correlation was found between number of fungal species and pH of the stream water. Combining the data from this study with those of studies by other authors using the same techniques, linear correlation and regression analyses revealed the significant negative correlation between species composition of communities in a stream and its pH. The number of fungal species in assemblages detected by different techniques used singly (e.g., filtration of stream water or examination of foam concentrate) or communities based on data generated by these techniques used simultaneously versus pH values formed a unimodal curve with a maximum at a pH between 6 and 7, mostly at 6.7. The conidial dynamics were influenced by the riparian vegetation. Conidium production from submerged deciduous and conifer litter was determined under laboratory conditions. Generally conidium production per disc was higher for Anguillospora longissima (Sacc. &amp; Syd) Ingold, Flagellospora cumula Ingold, Lunulospora curvula Ingold, and Tetracladium marchalianum de Wildeman in deciduous leaves than conifer leaves. Heliscus lugdunensis Sacc. &amp; Therry, Alatospora acuminata Ingold, and Articulospora tetracladia Ingold produced more conidia per disc in conifer than in deciduous leaves. Percentage similarity between spora produced by submerged substrate and stream spora was higher for deciduous substratum than for coniferous substratum in five circumneutral streams with a riparian vegetation dominated by deciduous trees. This index was higher for coniferous substratum in three acidic streams in which coniferous and deciduous trees were evenly distributed. The fungal communities in the Nosehri, Pathika, and Jabbar Camp streams showed higher values of species diversity than in other streams. Key words: freshwater, hyphomycetes, species diversity.

Effects of aluminium in acid streams on growth and sporulation of aquatic hyphomycetes

We investigated, by field and laboratory experiments, the effects of aluminium in an acid stream (pH 5.0) on the growth and sporulation of aquatic hyphomycete fungi which degrade organic litter. The stream water had monomeric aluminium (Al m) concentrations of 9.1–13.4 μ m — fifty times higher than a nearby circumneutral stream. Alder leaves submersed in the stream accumulated Al, most of which was tightly bound. Growth rates of four species of aquatic hyphomycetes were altered by inclusion of Al m in the culture medium. On a polypectate substrate, and on low-phosphate medium with glucose, growth rates increased significantly. On a low-nutrient substrate of homogenized alder leaves, growth rates were inhibited by aluminium. The pattern of mycelial growth was found to be different on a polypectate medium including Al m, compared with a control without aluminium. There was a significant increase in hyphal radial growth and a decrease in the hyphal growth unit. The effect resembled the growth of a starved fungal colony. Treatment with Al m decreased pectinase production by the four fungal species tested. The capacity of these species to sporulate was reduced by flooding culture plates with Al m solution. These deleterious metabolic effects were most severe in isolates taken from circumneutral streams and less marked, though significant, in species originating from acid streams.