Population dynamics and feeding of mayfly larvae in some acid and alkaline New Zealand streams

SUMMARY. 1. Population dynamics (density, biomass, annual production), gut contents and feeding rates of mayflies ( Deleatidium spp.; Leptophlebiidae) were compared in two naturally acid (mean pH≃4.8). brownwater streams and two alkaline (mean pH 7.5), clearwater streams in South Westland, New Zealand.
2. Mean densities of larvae (range 234–2318 m⁻²) were higher in alkaline streams on most of the six bimonthly sampling dates. Mean biomass (range 0.020–0.376 g larval dry weight (LDW) m⁻²) was always highest at the stable, spring-fed, alkaline site and was lower at the acid sites and another alkaline site where the population was always dominated by small larvae.
3. Annual production was high at the more stable, alkaline site (10.35 gLDW m⁻²) but much lower at the other sites (2.49–3.77 g m⁻²).
4. Gut contents of larvae were dominated by fine (45–75 μm widest diameter) paniculate matter (69–99%), diatoms (up to 21%) and. at one site, filamentous algae (8–13%).
5. Grazing rates of mayflies on epilithon were significantly higher on stones taken from acid than alkaline streams and material grazed from the former contained a higher proportion of inorganic material (87–93% and 61–83% inorganics, respectively).
6. Higher grazing rates may reflect lower quality of epilithic food in acid, brownwater streams, a factor that could contribute to the lower productivity of Deleatidium populations at these sites.     

Do stream macroinvertebrates use instream refugia in response to severe short-term flow reduction in New Zealand streams?

1. Demand for water is increasing and water managers need to know how much they can remove from a stream before there are significant detrimental effects on its biological integrity. Flow reduction alters a number of habitat variables known to be important to aquatic invertebrates such as depth, velocity, temperature and fine sediment accumulation. Some taxa may attempt to use instream refugia to mitigate the effects of flow reduction.
2. We experimentally manipulated flows by constructing weirs and diversions in three small New Zealand streams. Discharge was reduced by 88–96%. We tested the hypothesis that macroinvertebrates would use pools and the hyporheic zone as refugia during short-term (1-month) periods of reduced flow.
3. We sampled hyporheic invertebrates with colonization chambers and pool invertebrates with kick nets within a before-after, control-impact (BACI) experimental design. A suite of physicochemical parameters was measured concurrently including surface and hyporheic temperatures.
4. Flow reduction significantly decreased velocity (60–69%) in all streams. Depth (18–61%) and wetted width (24–31%) tended to decrease but these changes were not always significant. Sediment cover increased the most in farmland streams (10–80%). Apart from decreasing temperature range (18–26%), flow reduction had little impact on the surface water temperatures.
5. Flow reduction had no impact on the abundance of common pool macroinvertebrates or on the abundance, vertical distribution or community composition of hyporheic macroinvertebrates.
6. Our results suggest that aquatic macroinvertebrates are resistant to short-term, severe flow reduction as long as some water remains.     

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.     

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.     

Breakdown of tussock grass in streams along a gradient of agricultural development in New Zealand

Summary 1. We measured the breakdown rate of tussock grass in 12 New Zealand streams in catchments that provided a gradient of agricultural development. We also examined the microbial and invertebrate communities associated with decomposing tussock litter.
2. Pristine streams in the study had low concentrations of dissolved inorganic nitrogen (<10  μ g L⁻¹) and dissolved reactive phosphate (<3  μ g L⁻¹), whereas streams in the most developed catchments had high concentrations of nitrate (>2500  μ g L⁻¹) and phosphate (35  μ g L⁻¹), as well as greater amounts of suspended sediment and fine sediment covering the streambed.
3. Breakdown rate and microbial respiration were significantly related across the sites, and both were positively related to concentrations of nitrate and phosphate. Fungal biomass, measured as ergosterol, was positively related to microbial respiration and was also higher at sites with higher concentrations of nutrients. Total and shredding invertebrates were most abundant at the sites with high nutrient concentrations, but abundance of shredding invertebrates was not significantly related to breakdown rate. Amphipods were the most common shredding invertebrate at most sites, but probably did not contribute greatly to high rates of breakdown in streams in agricultural catchments.
4. With the exception of one site, nutrients from agricultural development appeared to have larger positive effects on litter breakdown than negative effects from sedimentation. Litter breakdown can serve as a functional measure of ecosystem health in streams, but caution should be exercised when a stress, such as land use, can have both positive (nutrients) and negative (sedimentation) effects.     

Local and ecoregion effects on fish assemblage structure in tributaries of the Rio Paraíba do Sul, Brazil

1.  We examined the effects of physical and chemical habitat variables and ecoregions on species occurrence and fish assemblage structure in streams of the Paraíba do Sul basin, in southeast Brazil.
2.  Fish and environmental data were collected from 42 sites on 26 first to fourth order streams (1 : 50 000 map scale) in three ecoregions. The sites occurred in one valley and two plateau ecoregions at altitudes of 40–1080 m and distances of 0.1–188 km from the main channel of the Rio Paraíba do Sul. Physical habitat (substratum, riparian cover, habitat types) and water quality (dissolved oxygen, pH, temperature and conductivity) variables were measured at each sampling site.
3.  A total of 2684 individuals in 16 families and 59 species were recorded.
4.  Ecoregion was a better predictor of the fish assemblage than the other environmental variables, according to the differences between the mean within-class and mean between-class similarities in assemblage data.
5.  Differing landscape characteristics were associated with differing local variables and thereby with differing fish assemblage structures. Riffles, shrub, grass, dissolved oxygen, conductivity and temperature were closely related to fish assemblage structure.
6.  Fish assemblages in sites far from the main river and at higher altitudes also differed from those near the Paraíba do Sul main channel, presumably as a result of differences in connectivity, covarying environmental factors and anthropogenic influence.
7.  These results reinforce the importance of understanding how stream communities are influenced by processes and patterns operating at local and regional scales, which will aid water resource managers to target those factors in their management and rehabilitation efforts.     

Inferring the minimum pH of streams from macroinvertebrates using weighted averaging regression and calibration

1. Data on macroinvertebrates and stream chemistry were collected from sixty-four streams in Finland. Weighted averaging (WA) regression and calibration models were constructed to infer the minimum pH of streams from their invertebrate assemblages. The purpose was to develop an instrument for biological assessment and monitoring of stream acidification. The WA method was compared with simpler approaches, based on qualitative invertebrate data and pH tolerance limits, that are widely used.
2. Performance of the two approaches was assessed in terms of correlation between the inferred and observed minimum pH within the 'training set', and in terms of root mean squared differences (predicted – observed) (RMSEP) estimated by cross-validation or bootstrap resampling techniques. The models were further tested using independent data from the literature representative of a wide geographical range.
3. The predictive power of the WA models was reasonable (RMSEP 0.40–0.44 pH units) in the training set and consistently better than that of the tolerance limit method. In contrast to the latter, the WA models were able to infer a minimum pH above 5.5, suggesting they could detect the early stages of acidification.
4. The WA models performed better than the tolerance limit method in inferring pH from the independent literature, further demonstrating the superiority and generality of the WA approach.
5. The weighted averaging technique could be an effective and widely applicable tool for contemporary biological monitoring and assessment using aquatic invertebrates.     

Inferring the minimum pH of streams from macroinvertebrates using weighted averaging regression and calibration

1. Data on macroinvertebrates and stream chemistry were collected from sixty-four streams in Finland. Weighted averaging (WA) regression and calibration models were constructed to infer the minimum pH of streams from their invertebrate assemblages. The purpose was to develop an instrument for biological assessment and monitoring of stream acidification. The WA method was compared with simpler approaches, based on qualitative invertebrate data and pH tolerance limits, that are widely used.
2. Performance of the two approaches was assessed in terms of correlation between the inferred and observed minimum pH within the 'training set', and in terms of root mean squared differences (predicted – observed) (RMSEP) estimated by cross-validation or bootstrap resampling techniques. The models were further tested using independent data from the literature representative of a wide geographical range.
3. The predictive power of the WA models was reasonable (RMSEP 0.40–0.44 pH units) in the training set and consistently better than that of the tolerance limit method. In contrast to the latter, the WA models were able to infer a minimum pH above 5.5, suggesting they could detect the early stages of acidification.
4. The WA models performed better than the tolerance limit method in inferring pH from the independent literature, further demonstrating the superiority and generality of the WA approach.
5. The weighted averaging technique could be an effective and widely applicable tool for contemporary biological monitoring and assessment using aquatic invertebrates.     

Nutrients, algae and grazers in some British streams of contrasting pH

1. The relationship between algal biomass accumulation, invertebrate colonization, and stream-water pH was investigated in seven streams in three regions of England and Wales. Possible nutrient limitation of algal production at all sites was examined with diffusion substrata. 2. Periphyton assemblages on experimental substrata after 30 days were dominated by diatoms, notably Eunotia spp., at all sites. Algal pigment concentration (chlorophyll a and phaeopigments) was not correlated with stream-water pH, and mean concentrations on control (unenriched) substrata ranged from 0.08 to 1.94 μg cm^?2. 3. The growth response of periphyton to nutrient additions was site specific. Algal production was stimulated by nutrient additions at sites in the English Lake District and Llyn Brianne (south-west Wales), but not in the Ashdown Forest (southern England). 4. Larval Chironomidae were the main invertebrates retrieved from substrata at all sites. Within all three regions, larval abundance was positively related to algal pigment concentration (biomass). Abundance of the stonefly Nemurella pictetii was also positively correlated with algal biomass at the one site where it occurred. 5. Our results indicate that epilithic algal production in small, oligotrophic streams is unlikely to be determined primarily by pH. Neither do they support the view that an absence of grazers from acid streams is necessarily due to an inadequate food supply.     

Role of transported particulate organic matter in the macroinvertebrate colonization of litter bags in streams

1. The exposure of mesh litter bags has been widely used to investigate the role of benthic macroinvertebrates in leaf processing in freshwaters. In this sense, several studies have related litter bag breakdown rates to the presence of colonizing invertebrates. A possible confounding factor in such experiments is that the litter bags trap suspended particulate organic matter that can itself attract invertebrate colonists, irrespective of the intended experimental treatment.
2. We attempted to quantify the accumulation of particulate organic matter (POM) within litter bags and to investigate its possible impact on macroinvertebrate density and richness. In seven headwater forested streams we exposed mesh bags filled either with beech leaves ( Fagus sylvatica ) or with plastic strips of an equal surface area.
3. Principal component analysis (PCA) showed that bag type and stream were the main explanatory variables for invertebrate colonization and POM accumulation within the bags. In contrast, there was little variation among sampling dates (6.4% of the total inertia).
4. The accumulated POM within the bags was substantial (up to 8.83 g ash-free dry mass (AFDM)) but highly variable among sites (mean from 0.32 to 4.58 g AFDM). At each of the seven sites, both richness and abundance of invertebrates were positively correlated with the mass of accumulated POM in bags. Macroinvertebrate colonization (notably taxon richness) was directly linked with the quantity of POM accumulated.
5. Our findings provide evidence of a potential pitfall in linking invertebrates to litter processing in mesh bags, particularly when large amounts of POM, entrained in stream flow, accumulate within the bags. An evaluation of the POM mass trapped in litter bags could account for the erratic patterns sometimes observed in their colonization by invertebrates.     

Invertebrate responses to short-term water abstraction in small New Zealand streams

1. Small permanent streams are coming under increasing pressure for water abstraction. Although these abstractions might only be required on a short‐term basis (e.g. summer time irrigation), the highest demand for water often coincides with seasonal low flows. 2. We constructed weirs and diversions that reduced discharge in three small streams (<4 m width) to test the hypotheses that short‐term water abstractions would decrease habitat availability and suitability for invertebrates, resulting in increased invertebrate drift, reduced taxonomic richness and decreased benthic invertebrate densities. 3. We sampled benthic invertebrates, invertebrate drift and periphyton at control (upstream) and impact (downstream) sites on each stream before and during 1 month of discharge reduction. 4. Discharge decreased by an average of 89–98% at impact sites and wetted width decreased by 24–30%. Water depth decreased by 28–64% while velocity decreased by 50–62%. Water conductivity, temperature and dissolved oxygen showed varying responses to flow reduction among the three streams, whereas algal biomass and pH were unaffected in all streams. 5. The densities of invertebrate taxa tended to increase in the impact reaches of these streams, even though invertebrate drift increased at impact sites in the first few days following discharge reduction. There were a higher proportion of mayflies, stoneflies and caddisflies at the impact site on one stream after flow reduction. There were no changes to the number of taxa or species evenness at impact sites. 6. Our results suggest that for these small streams, the response of invertebrates to short‐term discharge reduction was to accumulate in the decreased available area, increasing local invertebrate density.     

The ecological effect of acid conditions and precipitation of hydrous metal oxides in a Rocky Mountain stream

Periphyton and benthic invertebrates assemblages were studied at the confluence of two Rocky Mountain streams, Deer Creek and the Snake River near Montezuma, Colorado. Upstream from the confluence the Snake River is acidic and enriched in dissolved trace metals, while Deer Creek is a typical Rocky Mountain stream. In the Snake River, downstream from the confluence, the pH increases and hydrous metal oxides precipitate and cover the streambed. The algal and benthic invertebrate communities in the upstream reaches of the Snake River and in Deer Creek were very different. A liverwort, Scapania undulata var. undulata, was abundant in the Snake River, and although periphyton were very sparse, there were as many benthic invertebrates as in Deer Creek. Downstream from the confleunce, the precipitation of hydrous metal oxides greatly decreased the abundance of periphyton and benthic invertebrates. This study shows that in streams metal precipitates covering the streambed may have a more deleterious effect on stream communities than high metal-ion activities.     

Organic matter breakdown as a measure of stream health in New Zealand streams affected by acid mine drainage

Functional indicators of stream health have the potential to provide insights into stream condition that cannot be gained by traditional structural indices. We examined breakdown of leaves, wood, and cotton cloth strips at 18 sites along a gradient of effects of drainage from coal mines in New Zealand to determine the usefulness of these methods as functional indicators of stream health. The pH varied from 2.7 to neutral across the streams, and the more acidic streams typically had higher concentrations of aluminum, iron, zinc, and other metal ions. Precipitates of metal (mainly iron) hydroxides were present in most streams affected by mine drainage, especially in those with a pH of 4–5. Breakdown rates of all organic matter types were highest in several reference streams with neutral pH and lowest in sites with high rates of metal hydroxide deposition. Breakdown was relatively fast in the most acidic streams (pH < 3), in some cases as fast as at reference sites; these sites also had elevated nutrient concentrations. Shredding invertebrates were absent in litterbags from acidic streams and common at only 2 reference sites; their presence contributed to fast breakdown of leaves in the field and in lab microcosms. Microbial respiration was closely related to breakdown rates of leaves and wood; it was high at neutral and highly acidic streams, but lower at sites with pH 4–5, where metal hydroxides were precipitating onto solid surfaces. In these metal hydroxide-stressed streams, leaf and wood breakdown was slower, and associated biota, including microbes, were more affected than by water chemistry stressors (pH, dissolved metals) associated with mine drainage. Litter breakdown and microbial respiration provide insight into the functioning of streams, yielding different responses than traditional structural measures based on macroinvertebrates, which did not accurately distinguish impacts from acid mine drainage.     

Density of red alder (Alnus rubra) in headwaters influences invertebrate and detritus subsidies to downstream fish habitats in Alaska

We investigated the influence of red alder (Alnus rubra) stand density in upland, riparian forests on invertebrate and detritus transport from fishless headwater streams to downstream, salmonid habitats in southeastern Alaska. Red alder commonly regenerates after soil disturbance (such as from natural landsliding or timber harvesting), and is common along streams in varying densities, but its effect on food delivery from headwater channels to downstream salmonid habitats is not clear. Fluvial transport of invertebrates and detritus was measured at 13 sites in spring, summer and fall during two years (2000–2001). The 13 streams encompassed a riparian red alder density gradient (1–82% canopy cover or 0–53% basal area) growing amongst young-growth conifer (45-yr-old stands that regenerated after forest clearcutting). Sites with more riparian red alder exported significantly more invertebrates than did sites with little alder (mean range across 1–82% alder gradient was about 1–4 invertebrates m^?3 water, and 0.1–1 mg invertebrates m^?3 water, respectively). Three-quarters of the invertebrates were of aquatic origin; the remainder was of terrestrial origin. Aquatic taxa were positively related to the alder density gradient, while terrestrially-derived taxa were not. Streams with more riparian alder also exported significantly more detritus than streams with less alder (mean range across 1–82% alder gradient was 0.01–0.06 g detritus m^?3 water). Based on these data, we predict that headwater streams with more riparian alder will provide more invertebrates and support more downstream fish biomass than those basins with little or no riparian alder, provided these downstream food webs fully utilize this resource subsidy.     

Genetic structure of the stonefly, Yoraperla brevis, populations: the extent of gene flow among adjacent montane streams

1. Gene flow in populations of stream insects is expected to depend on the distance between and the connectedness of sites in stream networks, and on dispersal ability (i.e. larval drift and adult flight).
2. Yoraperla brevis (Banks) is an abundant and characteristic stonefly of smaller streams in the northern Rocky Mountains. The present authors analysed genetic structure at 27 sites in sevenz streams flowing into the Bitterroot River in western Montana, USA. Cellulose acetate electrophoresis identified five variable loci with 16 alleles.
3. Genotype frequencies conformed to Hardy–Weinberg expectations. Within-stream differentiation was low and among-stream variation ( F _st) was an order of magnitude higher.
4. UPGMA grouped sites within streams and also grouped adjacent streams. The tree produced by the Neighbour Joining Method was similar although not quite so clear cut.
5. This orderly pattern (i.e. Hardy–Weinberg proportions, homogeneity within streams and geographical structure) contrasts strongly with patterns observed in invertebrates from subtropical streams in Australia. Yoraperla brevis maintains large populations in predictable environments, has a long life-cycle with a likelihood of cohort mixing, emerges synchronously in large breeding populations and occupies streams separated by areas of high relief; the Australian situation is the opposite in most respects.
6. Further analysis of a range of species is required to determine whether the different genetic structure in Y. brevis compared to the Australian species occurs more generally in North American stream insects.     

Breeding density of Dippers Cinclus cinclus' Grey Wagtails Motacilla cinerea and Common Sandpipers Actitis hypoleucos in relation to the acidity of streams in south-west Scotland

The distribution of breeding pairs of Dippers Cinclus cinclus , Grey Wagtails Motacilla cinerea and Common Sandpipers Actitis hypoleucos was assessed along 18 streams in south-west Scotland during summer 1987, and the lengths of territories, occupied by 5 5 breeding pairs of Dippers, were measured. In each stream the water chemistry, physical structure (e.g. gradient, altitude, nature of bankside vegetation) and density of aquatic invertebrates was determined. The density of breeding pairs of dippers was significantly lower along streams of low pH (high acidity) and steep gradients, and territories were significantly longer at these sites compared with those of higher pH. Many of the invertebrate families that are important prey for Dippers were scarce at sites of low pH, particularly caddisfly larvae (Trichoptera) and mayfly nymphs (Ephemeroptera). The low numbers and large territories of breeding pairs of Dippers on acidic streams may be due to reduced density and diversity of macroinvertebrate prey. Stream pH was not an important correlate of the distribution or density of Grey Wagtails or Common Sandpipers. It is suggested that the lack of any similar effect of pH on these other two species compared with Dippers is attributable to differences in their feeding ecology.     

Faunal and chemical dynamics of some acid and alkaline New Zealand streams

SUMMARY 1. Water from acid (pH 4.3–5.7), brown water streams was low in alkalinity (0–2.3 g m^?3 CaCO₃) and conductivity (2.5–4.1 mS m^?1) but contained relatively high concentrations of dissolved organic carbon (6.6–16.3 gm^?3). In contrast, alkaline (pH 6.6–8.0), clearwater streams had high CaCO₃ (12.6–57.6 g m^?3) and conductivity (3.7–22.3 mS m^?1) but low dissolved organic carbon concentrations (0.3–4.7 g m^?3). 2. Total reactive aluminium (Al) concentrations were high in acid streams (123–363 mg m?³) but never exceeded 84 mg m?³ in alkaline streams. Acid-soluble and organic monomeric Al were the major Al species in the acid streams (31–168 and 84–178mg m?³, respectively). Concentrations of toxic inorganic monomeric Al were low in all streams (<50mg m?³). 3. Sixty-four invertebrate taxa were collected from the alkaline streams compared to forty-seven from the acid sites. Numbers of taxa in specific insect orders were similar at all sites, however. Benthic faunas at most sites were dominated by the mayfly Deleatidium sp. and chironomids. 4. Overall, mean densities of benthic invertebrates were 2.4–4.8 times higher in alkaline streams than acid streams. No seasonal patterns of abundance were evident at any site. 5. Temporal variability of invertebrate densities was correlated with stream channel stability such that fluctuations in densities declined as stability increased. 6. Sources of dissolved organic carbon and aluminium in acid, brown water streams are discussed. We suggest that changes in the food supply available in acid streams account for the depauperate faunas found there.     

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.     

Response of rainbow smelt, Osmerus mordax (Mitchill), eggs to low pH

The eggs of rainbow smelt were incubated at pH c . 4.5, c . 5.5 and c ontrol ( c . 6.8) for durations of 2–14 days to assess their vulnerability to acid precipitation. These eggs are likely to be exposed to marked pH depressions during their incubation in small streams owing to run-off from snow melt and spring rains. Survival after 48 h exposure was not related to pH, although longer exposure to low pH significantly decreased survival. The developmental stage at initial exposure was also important in determining survival. Overall egg survival varied from 0–87% at pH c . 4.5,0–89% at pH c . 5.5, and 44–100% at control pH levels, (c. 6.8).