The formation, structure and utilization of stone surface organic layers in two New Zealand streams

SUMMARY.

• 1 The development of stone surface organic layers was investigated in dark and light experimental channels at two field sites. Layer formation was monitored by measuring organic carbon, chlorophyll-a, ATP and rates of oxygen consumption, and using scanning electron microscopy.
• 2 In the darkened forest stream channel an organic layer consisting of slime, fine particles, bacteria and fungi developed and attained maximum biomass (=0.08 mg cm^-2) in about 2 months. At the second site, channels were fed by spring water low in dissolved and particulate organic matter (DOC < 0.5 g m^-3) and no organic layer developed on stones in the dark. Organic layers grown in channels subject to natural light intensities and photoperiods were dominated by diatoms and/or filamentous algae at both sites.
• 3 Laboratory experiments carried out in enclosed, recirculating stream channels demonstrated the importance of dissolved organic matter (DOM) as a prerequisite for layer formation. Also. DOM additions in the form of leaf leachates stimulated oxygen consumption by preformed layers. Uptake by microorganisms accounted for most of the reduction in water-column DOM.
• 4 Radiotracer experiments (¹⁴C and ¹⁴⁴Ce) showed that several common stream invertebrates could feed on ‘heterotrophic’ layers. Calculated assimilation efficiencies ranged from 18% to 74% and imply that nonautotrophic components of stone surface organic layers are likely to play a significant role in carbon transfer to the benthos, particularly in small, shaded streams.

     

Random patch dynamics of larval Chironomidae (Diptera) in the bed sediments of a gravel stream

• 1 The larval chironomid community of the sediment surface and the hyporheic inters titial was investigated in two longitudinal transects of an alpine gravel stream between September 1984 and August 1985. Eighty larval species and species groups were identified, most of which belonged to the subfamily Orthocladiinae. Of all larval individuals 51.1% inhabited the first 10cm of the bed sediments, and 93.2% occurred between the surface and 40cm depth.
• 2 The spatial species turnover showed marked variations between horizontally adjacent sampling sites in each of the four sediment depth layers. In both transects the species composition showed a significantly lower turnover in the upper 10cm of the bed sediments than in the deeper layers.
• 3 Spatial community stability showed an oscillating pattern between all sampling sites due to density shifts of larvae between depth layers. Temporal differences in resilience (local stability) were significantly and positively related to changes in the cumulative discharge pattern in the gravel brook, thus indicating the apparent ability of the community to recover quickly following disturbances.
• 4 The five abundant species, Corynoneura lobata, Synorthocladius semivirens, Tvetenia calvescens, Micropsectra atrofasciata and Rheotanytarsus nigricauda, exhibited significant differences in their sediment depth distribution, with density maxima shifting between depth layers. Spatial autocorrelations suggest that these larvae form patches between neighbouring sampling sites with varying sizes and inter-patch distances in each of four sediment layers. A simulation test, in which individuals of each species were randomly permuted between microhabitats of each depth layer separately, indicated that the patches might have arisen by chance.
• 5 To evaluate the significance of observed spatial resource overlap values amongst these five chironomid species, neutral models were developed based on 300 randomizations of each possible species pair-wise association of individuals and patches of species. The spatial organization of a larval chironomid assemblage in the stream Oberer Seebach seemed to be governed by coexistence due to random patch formation and dispersal patterns within the interstitial habitats, which reduce the probability of strong competitive interactions.

     

Effects of retention site on breakdown of organic matter in a mountain stream

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Spatial distribution and population dynamics of a benthic rotifer, Embata laticeps (Murray) (Rotifera, Bdelloidea) in the bed sediments of a gravel brook

• 1 The spatial and temporal distributions of the most abundant bdelloid rotifer, Embata laticeps (Murray), inhabiting the sediment surface and the hyporheic interstitial of a gravel stream were investigated between October 1991 and October 1992. Three temporal peaks of population density occurred during the year at the sediment surface differing in their riming from the density peaks detected within the bed sediments.
• 2 The depth distributions of E. laticeps in pool and riffle areas differed significantly. Higher abundances were found between 10 and 20 cm depth in the pool and between 20 and 30 cm in the riffle area. In these two sites a significant effect of surface discharge (estimated 1 week before sampling) on rotifer densities was found. Discharge greater than 0.424 m³s^?1 significantly reduced the densities in the riffle, while only at values greater than 1.660m³s^?1 were abundances influenced in the pool area.
• 3 The percentages of egg-bearing females were higher within the bed sediment than in the surface layer, and in the pool area than in the riffle. Thus, overall mean values of population increase were higher in the pool habitat (x?= 0.015 ± SE 0.029 day^?1) than in the riffle and at the sediment surface (x?= 0.004 ± 0.025 and 0.005 ± 0.026 day^?1, respectively). In addition, population growth fluctuated more strongly at the sediment surface than within the hyporheic intersiritial. Comparison of the observed values of population increase with a randomization test, where observed densities were randomly permuted, revealed no significant differences between those observed and values from a random model. Thus, a clear trend in population growth of E. laticeps over time could not be demonstrated in this gravel stream.

     

Herbivory in an acid stream

• 1 Spatial and temporal variation in the distribution and feeding of non‐predatory macroinvertebrates was investigated in a first‐order, acid stream in the Ashdown Forest, southern England.
• 2 Stonefly (Nemouridae) and chironomid (Orthocladiinae) larvae were abundant on the upper surfaces of mineral substrata of three sizes (small stones, large stones, bedrock). The density of larvae in each taxonomic group did not vary among substrata of different sizes, although strong seasonal variation existed.
• 3 Nemourids and chironomids (H. marcidus) collected from the upper surfaces of substrata exhibited generalist feeding habits, consuming algae (diatoms, coccoid and filamentous green algae), detritus (biofilm matrix material and fine particulate organic matter (FPOM)) and inorganic debris.
• 4 There was spatial variation in the gut contents of nemourids. The proportion of algae in the guts of larvae often increased with the size of the substratum from which they were collected. Strong temporal variation in the composition of the diet also existed. Nemourids ingested a large quantity of attached algae and biofilm matrix from the biofilm in spring and winter, but consumed loose FPOM and associated microflora in summer and autumn.
• 5 We conclude that, in this acid stream, the trophic linkage between algae and grazers is maintained by ‘detritivorous’ stonefly and chironomid species. The relationship between the feeding habits of these larvae and other life‐history attributes, such as mouthpart morphology and mobility, is discussed.

     

Species richness and assemblage structure of Trichoptera in Danish streams

• 1 Species richness and assemblages of Trichoptera from 157 ‘pristine’ Danish lowland stream sites were analyzed, for dependence on geographical position of the sites and simple physical variables, using two way indicator species analysis and detrended correspondence analysis.
• 2 A total of 106 species were recorded, representing ≈ 90% of the species pool known from Danish streams. Only seven species occurred at more than half the sites, whereas an additional 11 species were found at more than a quarter of the sites.
• 3 Although sites showed significant regional differences in environmental variables (stream order, width, slope and presence/absence of riparian forest), species richness and assemblages were primarily correlated with stream order, width and slope. Maximum richness was found at the largest (5th order) stream sites.
• 4 Regional differences in species assemblages were found, with several species being absent from the islands Funen and Bornholm. Species assemblages also differed between forested and non‐forested stream sites.
• 5 We found evidence that stream temperature may be of only minor importance in determining Trichoptera species richness and assemblage composition in Danish streams compared to other size‐related physical factors.

     

Spatial uptake of dissolved organic carbon in river beds

The uptake of dissolved organic carbon by three stream bed components; surface stones, underlying gravel and organic floc was measured in three rivers in North Wales, U.K. Overall, surface stones and underlying gravel appeared to be the major sites of uptake but the relative importance of these two components varied both temporally and spatially. Organic floc was found to be relatively unimportant as a site of dissolved organic carbon uptake.     

Urban infrastructure influences dissolved organic matter quality and bacterial metabolism in an urban stream network

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Physicochemical stream bed characteristics and recruitment of the freshwater pearl mussel (Margaritifera margaritifera)

1. The freshwater pearl mussel (Margaritifera margaritifera) is endangered and of conservation importance. We used its survival/mortality during the critical post‐parasitic phase as a biological indicator for the habitat quality of the stream substratum. 2. We established and tested biological, physical and chemical methods of assessing the stream bed in 26 streams from seven European countries. We analysed penetration resistance, texture, the concentrations and ratios of C, N, S, P, Fe, Mn in fine material <100 μm, and redox, pH and electric conductivity at the surface and at 5 and 10 cm into the substratum. 3. Sites with high stream bed quality (promoting pearl mussel populations with good juvenile recruitment) had coarser and better sorted substrata with significantly lower quantities of fines, and a higher Mn concentration in the fines, than poor quality sites. Redox potential (Eh) at sites without recruitment differed markedly between the free‐flowing water at the surface and at 5 and 10 cm in the bed, whereas no differences were detectable at good quality sites. This was also true of electric conductivity and, to a lesser extent, pH. The stream bed at sites lacking pearl mussel recruitment had a more variable and higher penetration resistance, indicating clogging of the interstitial macropore system by the deposition of mud and compaction of the stream bed. 4. Our results show that habitat quality for pearl mussels depends strongly on the exchange between the surface and the interstices, which is governed by physicochemical characteristics of the stream substratum. Combined measurements of penetration resistance, depth gradients of Eh and texture were most suitable for assessing stream bed quality, while water chemistry was insufficient because of the decoupling of interstitial and free‐flowing water at poor quality sites.     

Macroinvertebrate communities in streams in the Himalaya, Nepal

• 1 Macroinvertebrates were sampled in the riffles of fifty-eight streams from three regions of the Himalaya (Anapurna, Langtang and Everest) in Nepal. A semi-quantitative method with identification to family level was used to describe communities on-site.
• 2 Stream physicochemistry was assessed and the community structure of macroinvertebrates was related to chemistry, physiography (substratum composition, altitude and size), geographical location and the dominant land use in each catchment (terraced agriculture, forest or scrub). Community data were analysed by ordination (DECORANA) and classification (TWINSPAN).
• 3 The concentration of cations in stream water decreased significantly with altitude. Chemistry also differed between regions; sites from Anapurna had a higher pH and conductivity than those in the other two areas.
• 4 Communities were dominated by aquatic insect larvae, with Ephemeroptera, in particular the Baetidae, most numerous across sites.
• 5 There were, nevertheless, differences in community structure between sites, which were related closely to stream physicochemistry. Ordination scores were strongly correlated with altitude, magnesium concentration and substratum composition. Classification was also linked to altitude and chemistry, differentiating high-altitude sites with low silica concentrations from others. Sites from the Anapurna and Everest regions, with their contrasting chemistry, were also separated.
• 6 Community structure was also related to land use: streams draining catchments dominated by terraced agriculture had different communities from those in scrub or forest. This result was confounded, however, by the strong relationship between land use, altitude and chemistry; sites in terracing were at lower altitude, had higher concentrations of silica and a higher proportion of fine sediments than those in the other land-use types.
• 7 Overall, our data indicate that natural features of the relief and geology in the Himalaya create strong gradients in their invertebrate faunas, but that activities of man may have an effect on stream structure and ecology through catchment management.

     

Biofilm development and invertebrate colonization of wood in four New Zealand streams of contrasting pH

1. Biofilm development and activity on wood substrata (Nothofagus menziesii) were examined at four forested sites in a South Island, New Zealand, river catchment over a period of 6 months. Two of the sites had brown waters and mean pH of 3.7 and 4.5, whereas the other two had clear waters and mean pH of 6.3 and 6.8. 2. Fungi and other filamentous heterotrophs were the dominant colonizers of wood at all sites; few algal cells were present. Incorporation of ¹⁴C-glucose by biofilms was greatest in all four streams after 3 months, whereas endocellulase activity fluctuated over time and temporal patterns differed among streams. 3. No clear relationship was found between the incorporation of ¹⁴C-glucose or endocellulase activity of biofilms and pH, although at one near-neutral pH site ¹⁴C-glucose uptake increased in response to nutrient (N + P) additions. 4. After 6 months, incorporation of ¹⁴C-glucose and endocellulase activity of biofilms on Pinus radiata dowels buried vertically in the stream beds did not differ at depths of 3–9 cm and 19–25 cm in each stream. 5. Radiotracer experiments with a grazing amphipod (Paraleptamphopus sp.) demonstrated that biofilms on wood from all four sites could be ingested and at least partially assimilated. Chironomid larvae and harpacricoid copepods were the most abundant invertebrates colonizing wood substrata at all sites. Different chironomid species dominated at acidic and near-neutral pH sites. 6. Overall, our findings provide little support for the hypothesis that microbial activity on organic substrata is necessarily lower in streams of low pH.     

The forces on microorganisms at surfaces in flowing water

SUMMARY.

• 1 Surfaces in flowing water are covered by boundary layers within which the velocity of flow ranges between zero at the surface and the velocity of the main stream at the limits of the boundary layer.
• 2 Many organisms are small enough to live entirely within the boundary layer and so are subjected to much smaller drag forces than if they were exposed to the main stream velocity.
• 3 The form and extent of boundary layers at different stream velocities around selected structures, representing leaves and stems, have been calculated and presented graphically.
• 4 These data provide a basis for the estimation of shear stresses on substrate surfaces and of forces exerted upon organisms of different sizes that live within the boundary layers around such surfaces, when exposed to different current velocities. Values for shear stresses on bacteria calculated in this way lie within the range of shear stresses required to detach bacteria from surfaces experimentally.

     

Quantifying the direct and indirect effects of flow‐related disturbance on stream fish assemblages

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Distribution of solutes, microbes and invertebrates in river sediments along a riffle-pool-riffle sequence

• 1 Unlike riffles, research has focused rarely on the hyporheic zone of pools. To highlight the functioning of a pool, field investigations were performed in a riffle‐pool‐riffle sequence by integrating simultaneously physico‐chemistry, microbes and invertebrates. The study was conducted in a channel characterised by strong downwelling of surface water.
• 2 To include the downstream flux of water within the sediment, a longitudinal profile was studied along six stations situated: at the centre (Station 1) and at the lower end (Station 2) of the first riffle, at the upstream part (Station 3), at the centre (Station 4, at the inflection point) and at the lower end (Station 5) of the pool, and at the centre of the second riffle (Station 6). At each station, three replicate samples were taken and three sample depths were investigated (0.2, 0.5, and 1.0 m below the stream bed) on two dates.
• 3 Physico‐chemical parameters (vertical hydraulic gradient, oxygen concentration and specific conductance) differed between stations depending on infiltration rates. In contrast, organic matter and microbial parameters presented patchy distributions linked with factors other than the geomorphological pattern. Despite not very pronounced geomorphologic features, the slope variation at the centre of the pool (at the inflection point) affects the distribution of epigean and hypogean invertebrates.
• 4 Based upon faunal parameters, the pool could be divided into an upstream and a downstream part, the latter being more strongly influenced by surface water.
• 5 The pool should be considered as a heterogeneous area. In that respect, the inflection point of a pool may be as important as the top of a riffle in the functioning of river sediments.

     

Strong land‐use effects on the dispersal patterns of adult stream insects: implications for transfers of aquatic subsidies to terrestrial consumers

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Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

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Can current velocity mediate trophic cascades in a mountain stream?

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Invertebrate colonization of a new, man-made stream in southern Sweden

SUMMARY.

• 1 The invertebrate colonization of a man-made stream, Flugströmmen, in southern Sweden was monitored for 18 months in 1988 and 1989. Benthic samples were taken on twelve occasions from three sites (upstream, middle and downstream) and community structure was compared with that at ten natural, permanent reference sites nearby.
• 2 The number of species colonizing increased rapidly during the first 3 months. The increase was most rapid upstream but levelled off during the second year at this site, while numbers continued to increase downstream.
• 3 Simuliid species were the earliest colonizers and reached high densities at upstream and middle sites during the first year. Ephemeropteran and plecopteran species also occurred early on, whereas Coleoptera, Odonata and Trichoptera were, on average, slower to colonize. Blackfly densities decreased upstream after the first year and hydropsychids became numerically dominant.
• 4 The colonization order of functional feeding groups was as predicted: filter feeders first, grazers/collectors intermediate, predators and shredders last.
• 5 After a year, the community structure in Flugströmmen closely resembled that in lake-outlet streams situated in the area, although communities at the three sites within the stream were most similar to one another.
• 6 The possible role of competitive and predatory processes in determining the observed successional patterns are discussed.