A cost-effective method to quantify biological surface sediment reworking

We propose a simple and inexpensive method to determine the rate and pattern of surface sediment reworking by benthic organisms. Unlike many existing methods commonly used in bioturbation studies, which usually require sediment sampling, our approach is fully non-destructive and is well suited for investigating non-cohesive fine sediments in streams and rivers. Optical tracer (e.g. luminophores or coloured sand) disappearance or appearance is assessed through time based on optical quantification of surfaces occupied by tracers. Data are used to calculate surface sediment reworking (SSR) coefficients depicting bioturbation intensities. Using this method, we evaluated reworking activity of stream organisms (three benthic invertebrates and a fish) in laboratory microcosms mimicking pool habitats or directly in the field within arenas set in depositional zones. Our method was sensitive enough to measure SSR as low as 0.2 cm² day^?1, such as triggered by intermediate density (774 m^?2) of Gammarus fossarum (Amphipoda) in microcosms. In contrast, complex invertebrate community in the field and a fish (Barbatula barabatula) in laboratory microcosms were found to yield to excessively high SSR (>60 cm² day^?1). Lastly, we suggest that images acquired during experiments can be used for qualitative evaluation of species-specific effects on sediment distribution.     

Production of benthic macroinvertebrate communities along a southern Appalachian river continuum

1. Annual production was estimated for macroinvertebrate communities of principle habitats along a first- to seventh-order river continuum in the southern Appalachian Mountains (U.S.A.). Annual production was relatively low in depositional habitats, pebble/gravel substrata, and on cobble devoid of plant biomass (mosses and hydrophytes). Production was greater in bedrock habitats and greatest on hydrophyte-covered cobble, with estimates reaching 364 g AFDM (ash-free dry mass)m^–2 yr^–1 in a sixth-order river reach. Annual production in depositional habitats was correlated to standing crops of benthic organic material (BOM) in low-order stream reaches but not in higher-order reaches, indicating differences in BOM availability with stream size. In cobble, pebble/gravel and bedrock habitats production was significantly correlated to standing crops of aquatic plants, which can stabilize substrata and enhance access of collector-filtering invertebrates to entrained food resources. 2. By accounting for proportional availability of habitats along the continuum, estimates of total production ranged from 5 to 154 g AFDM m^–2 yr^–1, and increased significantly with stream size. Annual production estimated for sixth- and seventh-order reaches of the continuum were amongst the highest reported thus far for lotic systems. Organization of the benthic community along the continuum, based on production estimates for individual functional feeding-groups, generally supported predictions of the River Continuum Concept (RCC): shredder contributions were greatest in low-order reaches and declined downstream; scraper percentages were greatest in the middle of the continuum; collector-filterer contributions increased with increasing stream size. Longitudinal trends for collector-gatherers and predators did not support RCC predictions; these groups appeared to be influenced by localized changes in habitat availability and occurrence of vertebrate predators along the continuum.     

Export of invertebrates and detritus from fishless headwater streams in southeastern Alaska: implications for downstream salmonid production

1. We examined the export of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters to aquatic habitats downstream in the coastal mountains of southeast Alaska, U.S.A. Fifty‐two small streams (mean discharge range: 1.2–3.6 L s^?1), representing a geographic range throughout southeast Alaska, were sampled with 250‐μm nets either seasonally (April, July, September) or every 2 weeks throughout the year. Samples were used to assess the potential subsidy of energy from fishless headwaters to downstream systems containing fish. 2. Invertebrates of aquatic and terrestrial origin were both captured, with aquatic taxa making up 65–92% of the total. Baetidae, Chironomidae and Ostracoda were most numerous of the aquatic taxa (34, 16 and 8%, respectively), although Coleoptera (mostly Amphizoidae) contributed the greatest biomass (30%). Mites (Acarina) were the most numerous terrestrial taxon, while terrestrial Coleoptera accounted for most of the terrestrial invertebrate biomass. 3. Invertebrates and detritus were exported from headwaters throughout the year, averaging 163 mg invertebrate dry mass stream^?1 day^?1 and 10.4 g detritus stream^?1 day^?1, respectively. The amount of export was highly variable among streams and seasons (5–6000 individuals stream^?1 day^?1 and <1–22 individuals m^?3 water; <1–286 g detritus stream^?1 day^?1 and <0.1–1.7 g detritus m^?3 water). Delivery of invertebrates from headwaters to habitats with fish was estimated at 0.44 g dry mass m^?2 year^?1. We estimate that every kilometre of salmonid‐bearing stream could receive enough energy (prey and detritus) from fishless headwaters to support 100–2000 young‐of‐the‐year (YOY) salmonids. These results illustrate that headwaters are source areas of aquatic and terrestrial invertebrates and detritus, linking upland ecosystems with habitats lower in the catchment.     

Applying the light : nutrient hypothesis to stream periphyton

1. The light : nutrient hypothesis (LNH) states that algal nutrient content is determined by the balance of light and dissolved nutrients available to algae during growth. Light and phosphorus gradients in both laboratory and natural streams were used to examine the relevance of the LNH to stream periphyton. Controlled gradients of light (12–426 μmol photons m^?2 s^?1) and dissolved reactive phosphorus (DRP, 3–344 μg L^?1) were applied experimentally to large flow‐through laboratory streams, and natural variability in canopy cover and discharge from a wastewater treatment facility created gradients of light (0.4–35 mol photons m^?2 day^?1) and DRP (10–1766 μg L^?1) in a natural stream. 2. Periphyton phosphorus content was strongly influenced by the light and DRP gradients, ranging from 1.8 to 10.7 μg mg AFDM^?1 in the laboratory streams and from 2.3 to 36.9 μg mg AFDM^?1 in the natural stream. Phosphorus content decreased with increasing light and increased with increasing water column phosphorus. The simultaneous effects of light and phosphorus were consistent with the LNH that the balance between light and nutrients determines algal nutrient content. 3. In experiments in the laboratory streams, periphyton phosphorus increased hyperbolically with increasing DRP. Uptake then began levelling off around 50 μg L^?1. 4. The relationship between periphyton phosphorus and the light : phosphorus ratio was highly nonlinear in both the laboratory and natural streams, with phosphorus content declining sharply with initial increases in the light : phosphorus ratio, then leveling off at higher values of the ratio. 5. Although light and DRP both affected periphyton phosphorus content, the effects of DRP were much stronger than those of light in both the laboratory and natural streams. DRP explained substantially more of the overall variability in periphyton phosphorus than did light, and light effects were evident only at lower phosphorus concentrations (≤25 μg L^?1) in the laboratory streams. These results suggest that light has a significant negative effect on the food quality of grazers in streams only under a limited set of conditions.     

Flow extremes and benthic organic matter shape the metabolism of a headwater Mediterranean stream

1. Single‐station diel oxygen curves were used to monitor the oxygen metabolism of an intermittent, forested third‐order stream (Fuirosos) in the Mediterranean area, over a period of 22 months. Ecosystem respiration (ER) and gross primary production (GPP) were estimated and related to organic matter inputs and photosynthetically active radiation (PAR) in order to understand the effect of the riparian forest on stream metabolism. 2. Annual ER was 1690 g O₂ m^?2 year^?1 and annual GPP was 275 g O₂ m^?2 year^?1. Fuirosos was therefore a heterotrophic stream, with P : R ratios averaging 0.16. 3. GPP rates were relatively low, ranging from 0.05 to 1.9 g O₂ m^?2 day^?1. The maximum values of GPP occurred during a few weeks in spring, and ended when the riparian canopy was fully closed. The phenology of the riparian vegetation was an important determinant of light availability, and consequently, of GPP. 4. On a daily scale, light and temperature were the most important factors governing the shape of photosynthesis–irradiance (P–I) curves. Several patterns could be generalised in the P–I relationships. Hysteresis‐type curves were characteristic of late autumn and winter. Light saturation responses (that occurred at irradiances higher than 90 μE m^?2 s^?1) were characteristic of early spring. Linear responses occurred during late spring, summer and early autumn when there was no evidence of light saturation. 5. Rates of ER were high when compared with analogous streams, ranging from 0.4 to 32 g O₂ m^?2 day^?1. ER was highest in autumn 2001, when organic matter accumulations on the streambed were extremely high. By contrast, the higher discharge in autumn 2002 prevented these accumulations and caused lower ER. The Mediterranean climate, and in its effect the hydrological regime, were mainly responsible for the temporal variation in benthic organic matter, and consequently of ER.     

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.     

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 Floßgraben 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%.     

CO2 dynamics along Danish lowland streams: water–air gradients, piston velocities and evasion rates

We measured CO₂ concentration and determined evasion rate and piston velocity across the water–air interface in flow-through chambers at eight stations along two 20 km long streams in agricultural landscapes in Zealand, Denmark. Both streams were 9–18-fold supersaturated in CO₂ with daily means of 240 and 340 μM in January–March and 130 and 180 μM in June–August. Annual CO₂ medians were 212 μM in six other streams and 460 μM in four groundwater wells, while seven lakes were weakly supersaturated (29 μM). Air concentrations immediately above stream surfaces were close to mean atmospheric conditions except during calm summer nights. Piston velocity from 0.4 to 21.6 cm h^?1 was closely related to current velocity permitting calculation of evasion rates for entire streams. CO₂ evasion rates were highest in midstream reaches (170–1,200 mmol m^?2 day^?1) where CO₂-rich soil water entered fast stream flow, while rates were tenfold lower (25–100 mmol m^?2 day^?1) in slow-flowing lower reaches. CO₂ evasion mainly derived from the input of CO₂ in soil water. The variability of CO₂ evasion along the two lowland streams covered much of the range in sub-Arctic and temperate streams reported previously. In budgets for the two stream catchments, loss of carbon from soils via the hydrological cycle was substantial (3.2–5.7 mmol m^?2 day^?1) and dominated by CO₂ consumed to form HCO₃ ^? by mineral dissolution (69–76%) and export of organic carbon (15–23%) relative to dissolved CO₂ export (7–9%).     

Activities of benthic nitrifiers in streams and their role in oxygen consumption

The in situ rates of oxygen consumption by benthic nitrifiers were estimated at 11 study sites in 4 streams. Two methods were used: an in situ respiration chamber method and a method involving conversion of nitrifying potential measurements to in situ rates. Estimates of benthic nitrogenous oxygen consumption (BNOC) rate ranged from 0–380 mmol of O₂ m^–2·day^–1, and BNOC contributed between 0–85% of the total benthic oxygen consumption rate. The activity of nitrifiers residing in the sediments was influenced by O₂ availability, temperature, pH, and substrate. Depending upon site, nitrification could approximate either first-order or zero-order kinetics with respect to ammonium concentration. The source of ammonium for benthic nitrifiers could be either totally from within the sediment or totally from the overlying water. Nitrate produced in the sediments could flux to the water above or be lost within the sediment. The sediments could act as a source (positive flux) or sink (negative flux) for both ammonium (–185 mmol·m^–2·day^–1 to +195 mmol·m^–2·day^–1) and nitrate (–135 mmol·m^–2·day^–1 to +185 mmol·m^–2·day^–1).This study provides evidence to suggest that measurements of down-stream mass flow changes in inorganic nitrogen forms may give poor estimates of in situ rates of nitrification in flowing waters.     

Plant—herbivore interactions in streams near Mount St Helens

1. In four separate field experiments near Mount St Helens (Washington, U.S.A.) during 1986, the grazing effects of two large benthic herbivores, tadpoles of the tailed frog Ascaphus truei and larvae of the caddisfly Dicosmoecus gilvipes, were investigated using streamside channels and in-stream manipulations. In the experimental channels, abundances of periphyton and small benthic invertebrates declined significantly with increasing density of these larger herbivores. 2. In eleven small, high-gradient streams affected to varying degrees by the May 1980 eruption, in-stream platforms were used to reduce grazing by A, truei tadpoles on tile substrates. Single platforms erected in each tributary and compared to grazed controls revealed only minor grazing effects, and no significant differences among streams varying in disturbance intensity (and, consequently, tadpole density). However, results probably were confounded by high variability among streams in factors other than tadpole abundance. 3. Grazing effects were further examined in two unshaded streams with different tadpole densities, using five platforms per stream. In the stream with five tadpoles m^?2, grazing reduced periphyton biomass by 98% and chlorophyll a by 82%. In the stream lacking tadpoles, no significant grazing effects were revealed. Low algal abundance on both platforms and controls, and high invertebrate density in that stream (c. 30000m^?2) suggests that grazing by small, vagile invertebrates was approximately equivalent to that of tadpoles. 4. The influence of large benthic herbivores on algal and invertebrate communities in streams of Mount St Helens can be important, but reponses vary spatially in relation to stream disturbance history, local environmental factors, and herbivore distributional patterns and abundance.     

Temperature and the metabolic balance of streams

1. It is becoming increasingly clear that fresh waters play a major role in the global C cycle. Stream ecosystem respiration (ER) and gross primary productivity (GPP) exert a significant control on organic carbon fluxes in fluvial networks. However, little is known about how climate change will influence these fluxes. 2. Here, we used a ‘natural experiment’ to demonstrate the role of temperature and nutrient cycling in whole‐system metabolism (ER, GPP and net ecosystem production – NEP), in naturally heated geothermal (5–25 °C) Icelandic streams. 3. We calculated ER and GPP with a new, more accurate method, which enabled us to take into account the additional uncertainties owing to stream spatial heterogeneity in oxygen concentrations within a reach. ER ranged 1–25 g C m^?2 day^?1 and GPP 1–10 g C m^?2 day^?1. The median uncertainties (based on 1 SD) in ER and GPP were 50% and 20%, respectively. 4. Despite extremely low water nutrient concentrations, high metabolic rates in the warm streams were supported by fast cycling rates of nutrients, as revealed from inorganic nutrient (N, P) addition experiments. 5. ER exceeded GPP in all streams (with average GPP/ER = 0.6) and was more strongly related to temperature than GPP, resulting in elevated negative NEP with warming. We show that, as a first approximation based on summer investigations, global stream carbon emission to the atmosphere would nearly double from 0.12 Pg C year^?1 at 13 °C to 0.21 (0.15–0.33) Pg C year^?1 with a 5 °C warming. 6. Compared to previous studies from natural systems (including terrestrial ecosystems), the temperature dependence of stream metabolism was not confounded by latitude or altitude, seasonality, light and nutrient availability, water chemistry, space availability (water transient storage), and water availability. 7. Consequently, stream nutrient processing is likely to increase with warming, protecting downstream ecosystems (rivers, estuaries, coastal marine systems) during the summer low flows from nutrient enrichment, but at the cost of increased CO₂ flux back to the atmosphere.     

Bryophytes as invertebrate habitat in two New Zealand alpine streams

SUMMARY. 1. Quantitative samples of lotic benthic invertebrates and associated organic material were taken monthly for 18 months from bryophyte and gravel habitats in two New Zealand alpine streams in Arthur's Pass National Park, South Island; an unshaded site above the tree-line and a shaded stream flowing through mountain beech forest. This was to assess the ecological role of bryophytes in these habitats and to determine the relative effects on the fauna of algal and detrital resources. 2. Bryophytes were confined to stable bedrock substrata in shallow (mean depth of 4.5 and 10.5 cm), fast water (mean velocity of 0.21 ms^?1 and 0.30ms^?1) at the unshaded and shaded sites. Mean ash-free dry weight (AFDW) of organic material from bryophyte areas was similar at the unshaded and shaded sites (335.6 and 272.4 gm^?2). Low stream-bed stability of the gravel areas resulted in poor retention of organic matter, with average AFDW of 59.1 and 77.9 gm^?2 at the unshaded and shaded sites, respectively. Periphyton biomass, however, was much higher at the unshaded site. 3. Within bryophytes, water velocity is reduced and large quantities of periphyton and detritus accumulate. This explains the greatly enhanced invertebrate densities within bryophytes (218400 and 53500 individuals m^?2), compared with gravel areas (20900 and 7400 individuals m^?2) in the unshaded and shaded sites, respectively. The higher invertebrate densities at the unshaded site reflect greater periphyton biomass there. 4. Invertebrate densities followed clear seasonal patterns, attributable to changes in chironomid densities. These reflected larval chironomid hatching following adult oviposition. This illustrates the importance of these plants as oviposition sites, and as protective nurseries for small larvae where they can obtain shelter during floods and consume the abundant food sources available to them. 5. DECORANA ordination revealed taxonomically distinct invertebrate communities in each stream, and in bryophytes and gravels. Several taxa appeared restricted to either the unshaded site (e.g. Acroperla spiniger and Hydrobiosis silvicola) or the shaded site (e.g. Zelolessica     

Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland

1. Two acidic peatland upland streams in north‐east Scotland draining catchments of 1.3 and 41.4 km² were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation. 2. Free CO₂ showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over‐riding diurnal patterns of free CO₂, alkalinity and pH. 3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO₂, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in‐stream processes caused changes in CO₂ concentrations and temperature affected both the solubility of CO₂ and changes in up‐stream CO₂ inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied. 4. The mean GPP (as CO₂) was 0.81 g CO₂ m^?2 day^?1 and mean CR 2.67 g CO₂ m^?2 day^?1. The mean primary production/respiration (P/R) ratio was 0.26 ± 0.09 and 0.33 ± 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months. 5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO₂ and HCO₃^? than samples taken during darkness. During spring, concentrations of free CO₂ were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under‐estimate the annual flux of these determinands in streamwater by as much as 40%.     

Increases in benthic community production and metabolism in response to marine‐derived nutrients from spawning Atlantic salmon (Salmo salar)

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Benthic resources are the key to Daphnia middendorffiana survival in a high arctic pond

1. Shallow arctic lakes and ponds have simple and short food webs, but large uncertainties remain about benthic–pelagic links in these systems. We tested whether organic matter of benthic origin supports zooplankton biomass in a pond in NE Greenland, using stable isotope analysis of carbon and nitrogen in the pond itself and in a ¹³C‐enrichment enclosure experiment. In the latter, we manipulated the carbon isotope signature of benthic algae to enhance its isotopic discrimination from other potential food sources for zooplankton. 2. The cladoceran Daphnia middendorffiana responded to the ¹³C‐enrichment of benthic mats with progressively increasing δ¹³C values, suggesting benthic feeding. Stable isotope analysis also pointed towards a negligible contribution of terrestrial carbon to the diet of D. middendorffiana. This agreed with the apparent dominance of autochthonous dissolved organic matter in the pond revealed by analysis of coloured dissolved organic matter. 3. Daily net production by phytoplankton in the pond (18 mg C m^?2 day^?1) could satisfy only up to half of the calculated minimum energy requirements of D. middendorffiana (35 mg C m^?2 day^?1), whereas benthic primary production alone (145 mg C m^?2 day^?1) was more than sufficient. 4. Our findings highlight benthic primary production as a major dietary source for D. middendorffiana in this system and suggest that benthic organic matter may play a key role in sustaining pelagic secondary production in such nutrient‐limited high arctic ponds.     

Fluctuating water levels control water chemistry and metabolism of a charophyte‐dominated pond

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The influence of native replanting on stream ecosystem metabolism in a degraded landscape: can a little vegetation go a long way?

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Do tadpoles affect leaf decomposition in neotropical streams?

1. Of the relatively few studies that have examined consequences of amphibian declines on stream ecosystems, virtually all have focused on changes in algae (or algal‐based food webs) and little is known about the potential effects of tadpoles on leaf decomposition. We compared leaf litter decomposition dynamics in two neotropical streams: one with an intact community of tadpoles (with frogs) and one where tadpoles were absent (frogless) as a result of a fungal pathogen that had driven amphibians locally extinct. The stream with tadpoles contained a diverse assemblage (23 species) of larval anurans, and we identified five species of glass frog (Centrolenidae) tadpoles that were patchily distributed but commonly associated with leaf detritus and organic sediments in pools. The latter reached total densities of 0–318 tadpoles m^?2. 2. We experimentally excluded tadpoles from single‐species leaf packs incubated over a 40‐day period in streams with and without frogs. We predicted that decomposition rates would be higher in control (allowing access of tadpoles) treatments in the study stream with frogs than in the frogless stream and, in the stream with frogs, in the control than in the tadpole exclusion treatment. 3. In the stream with frogs, Centrolene prosoblepon and Cochranella albomaculata tadpoles were patchily distributed in leaf packs (0.0–33.3 m^?2). In contrast to our predictions, leaf mass loss and temperature‐corrected leaf decomposition rates in control treatments were almost identical in our stream with frogs (41.01% AFDM lost, k_{degree day} = ?0.028 day^?1) and in the frogless stream (41.81% AFDM lost, k_{degree day} = ?0.027 day^?1) and between control and tadpole exclusion treatments within each stream. Similarly, there were no significant differences in leaf pack bacterial biomass, microbial respiration rates or macroinvertebrate abundance between treatments or streams. Invertebrate assemblages on leaf packs were similar between treatments (SIMI = 0.97) and streams (SIMI = 0.95) and were dominated by larval Chironomidae, Simuliidae (Diptera) and larval Anchytarsus spp. (Coleoptera). 4. In contrast to dramatic effects of grazing tadpoles on algal communities observed previously, tadpoles had no major effects on decomposition. While centrolenid tadpoles were common in the stream with frogs, their patchy distribution in both experimental and natural leaf packs suggests that their effects on detrital dynamics and microbes are probably more localised than those of grazing tadpoles on algae.     

Respiration and annual fungal production associated with decomposing leaf litter in two streams

1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch). 2. Both streams received similar annual leaf litter fall (478–492 g m^?2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m^?2; mean ± SE) than Payne Creek (39 ± 9 g m^?2). 3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g^?1) than those from Payne Creek (54 ± 8 mg g^?1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day^?1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek. 4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop. 5. Annual fungal production was 16 ± 6 g m^?2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m^?2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter‐year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m^?2 in Payne Creek and 7–27 g m^?2 in Lindsey Spring Branch. 6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.     

Comparison of bacterial production in sediments, epiphyton and the pelagic zone of a lowland river

1. The microbial metabolism of organic matter in rivers has received little study compared with that of small streams. Therefore, we investigated the rate and location of bacterial production in a sixth‐order lowland river (Spree, Germany). To estimate the contribution of various habitats (sediments, epiphyton, and the pelagic zone) to total bacterial production, we quantified the contribution of these habitats to areal production by bacteria. 2. Large areas of the river bottom were characterized by loose and shifting sands of relatively homogenous particle size distribution. Aquatic macrophytes grew on 40% of the river bottom. Leaf areas of 2.8 m² m^?2 river bottom were found in a 6.6 km river stretch. 3. The epiphyton supported a bacterial production of 5–58 ng C cm^?2 h^?1. Bacterial production in the pelagic zone was 0.9–3.9 μg C L^?1 h^?1, and abundance was 4.0–7.8 × 10⁹ cells L^?1. Bacterial production in the uppermost 2 cm of sediments ranged from 1 to 8 μg C cm^?3 h^?1, and abundance from 0.84 to 6.7 × 10⁹ cells cm^?3. Bacteria were larger and more active in sediments than in the pelagic zone. 4. In spite of relatively low macrophyte abundance, areal production by bacteria in the pelagic zone was only slightly higher than in the epiphyton. Bacterial biomass in the uppermost 2 cm of sediments exceeded pelagic biomass by factors of 6–22, and sedimentary bacterial production was 17–35 times higher than in the overlying water column. 5. On a square meter basis, total bacterial production in the Spree was clearly higher than primary productivity. Thus, the lowland river Spree is a heterotrophic system with benthic processes dominating. Therefore, sedimentary and epiphytic bacterial productivity form important components of ecosystem carbon metabolism in rivers and shallow lakes. 6. The sediments are focal sites of microbial degradation of organic carbon in a sand‐bottomed lowland river. The presence of a lowland river section within a river continuum probably greatly changes the geochemical fluxes within the river network. This implies that current concepts of longitudinal biogeochemical relationships within river systems have to be revised.