Use of perfused cores for evaluating extracellular enzyme activity in stream-bed sediments

Abstract β-Glucosidase activity was investigated in stream-bed sediments using 4-methylumbelliferyl-β- d -glucopyranoside (MUF-β-Glc) as a model substrate. In a perfused core technique, water containing MUF-β-Glc was perfused up through sediment cores. β-glucosidase activity quantified from the release of fluorescent MUF in water discharge from the cores. At low rates of perfusion, maximum β-glucosidase activity ( V _max) in perfused sediments was similar to that in suspended (unperfused) sediments. Substrate affinity( K m)was higher in the suspended sediments. V _maxand K _m both increased when the perfusion rate was raised, although naturally-low substrate concentrations could mean that variability in perfusion rates has little effect on enzyme activity in the field. V _max was uninfluenced by whether ground or stream water was perfused through the sediments, but K _m was higher in cores perfused with groundwater. Increasing concentrations of glucose in the perfusion water resulted in a progressive inhibition of β-glucosidase activity. Although natural concentrations of glucose were low, the high turnover of enzymatically-released glucose probably means that β-glucosidase activity could be regulated by product concentration.     

The effect of current velocity and sediment on the drift of the mayfly Ephemerella subvaria Mcdunnough

SUMMARY. Experiments conducted in an artificial stream showed that significantly more nymphs drifted from an inorganic substrate at a mean current velocity of 28.5 cm s⁻¹ than at 18.5 cm s⁻¹. Drift density, however, was not affected. Disproportionately large numbers of nymphs drifted while current velocities were being increased from 18.5 to 28.5 cm s⁻¹.
Both drift numbers and drift density were greater in turbid water, after the addition of large amounts of inorganic sediment, than under clear-flowing conditions during dark periods but not in the light. The interaction of increasing current velocity and sediment levels resulted in a significantly greater number of drifting nymphs under lighted conditions.
Minor spates which do not seriously disturb the stream bed may initiate significant increases in macroinvertebrate drift.     

Seasonal heterotrophic flagellate and bacterial plankton dynamics in a large oligotrophic lake– Loch Ness, Scotland

1. Bacterial production in the 0–30 m water column of Loch Ness was measured using a dual labelling procedure with [³H] thymidine and [¹⁴C] leucine between May 1993 and June 1994. In most cases the uptake of the two labels did not covary, suggesting unbalanced growth. Rates of bacterial production varied from undetectable to 46.2 μg C l^–1 day^–1. Highest production coincided with the period of highest primary production, but carbon derived from this source was insufficient to meet the bacterial carbon demand, which was met by allochthonous humic inputs to the system.
2. Heterotrophic flagellate (HNAN) grazing rates, measured using fluorescently labelled bacteria, ranged between 10.3 and 24.5 bacteria cell^–1 day^–1 at temperatures between 5 and 15 °C. They removed up to 27% of the bacterial production per day.
3. Heterotrophic flagellate specific growth rates ranged from 0.043 to 0.093 h^–1 between 5 and 15 °C, giving generation times of 7.4–16.1 h.
4. bacterial and HNAN abundances were not coupled, but the highest HNAN grazing impact related to a time of high bacterial productivity.     

A continuous-flow system for measuring in vitro oxygen and nitrogen metabolism in separated stream communities

SUMMARY. 1. This paper describes a continuous-flow system that we have used extensively for measurement of oxygen exchange and nitrogen transformations in different communities of lowland streams, i.e. the water, the sediment, and the macrophyte—biofilm community. The system, which is set up on the stream bank, consists of several macrophyte and sediment chambers equipped with a double-flow system that ensures an internal water velocity close to that in the stream and which, by continuously renewing the water, mimics diel fluctuation in stream temperature and water chemistry. Water temperature and dissolved oxygen are measured continuously and the data stored on magnetic tape. Water samples are collected in a refrigerator and analysed using standard chemical procedures.
2. The application of the system is illustrated using results obtained during a 2-day summer experiment in a shallow macrophyte—rich stream. The biological processes in the stream were mainly associated with the macrophyte—biofilm community and the sediment, those in the water being negligible. Oxygen release was confined to the macrophyte—biofilm community, the sediment consuming oxygen both by day and by night. Whole-system gross production and dark respiration occurred at similar rates (6–7g O₂ m⁻² day⁻¹), net balance being about zero. Inorganic nitrogen was consumed both by the sediment and to a greater extent by the macrophytes, the diel average consumption being 1g N m⁻² day⁻¹.
3. The sum of the activity in the macrophyte and sediment chambers corresponded to the overall activity of the stream section as determined by upstream/downstream mass balance. This indicates that the results obtained with the continuous-flow chambers realistically describe the oxygen and the nitrogen metabolism of the stream.     

Denitrification in sediment determined from nitrogen isotope pairing

Abstract A new method for accurate and easy measurement of denitrification in sediments is presented. The water overlying intact sediment cores was enriched with ¹⁵NO₃⁻ which mixed with the ¹⁴NO₃⁻ of the natural sources of NO₃⁻. The formation by denitrification of single-labeled (¹⁴N¹⁵N) and double-labeled (¹⁵N¹⁵N) dinitrogen pairs was measured by mass spectrometry after a few hours incubation. Total denitrification including the formation of unlabeled (¹⁴N¹⁴N) dinitrogen could be calculated assuming random isotope pairing by denitrification of the uniformly mixed NO₃⁻ species. In contrast to previous approaches, by this method it is possible to measure denitrification of both NO₃⁻ diffusing from the overlying water and NO₃⁻ from nitrification within the sediment.     

The Abundance of Different Peritrich Ciliates on Stone Surfaces in Contrasting Lowland Streams Throughout the Year

ABSTRACT. Pentrich ciliates attached to small stones from the beds of two streams, one large with hard water, the other small with soft water, were enumerated throughout an annual cycle. Throughout the year, Platycola was the dominant peritrich in both streams, except for a brief period during the spring when Vorticella and Carchesium predominated. Vorticella reached peak levels of 89 ciliates cm² of stone surface, and up to 102 Platycola per cm² of stone surface were found. Mean volumes of samples of the main species were calculated, and used to estimate the standing stock biomasses. using a standard value of dry weight per unit volume. Published values of the growth rates of representatives of the main genera were used to estimate production values, which totalled about 6.5 g dry weight of peritrich cytoplasm/m² of stream bed per annum in the large stream (mean annual density = 8.3 peritrichs/cm² of stone surface), and 33 g dry weight/m² of stream bed per annum in the small stream (mean annual density = 47 peritrichs/cm² of stone surface). Food supply, temperature and predation were the primary factors determining peritrich abundance     

Production of a stream shredder, Peltoperla maria (Plecoptera: Peltoperlidae) in disturbed and undisturbed hardwood catchments

SUMMARY. (1) The average benthic density of Peltoperla maria in an undisturbed southern Appalachian stream was more than twice that of a nearby stream draining a previously clear-cul catchment in its tenth year of natural secondary succession.
(2) Peltoperla production estimates, using three methods, do not show a significant difference in production between streams draining the two catchments. We attribute these results to quicker growth and slightly higher densities of larger nymphs in the disturbed stream. Production estimates for the disturbed stream ranged from 498 to 560 mg (ash free dry weight) m⁻²y⁻¹ while those for the undisturbed stream were 41–4–515 mg m⁻² y⁻¹.
(3) Our results reinforce the view that conclusions based solely upon numerical densities may lead to erroneus interpretations about the roles organisms play in ecosystems.
(4) Annual frass production by this shredder is about 20 times (10 g m⁻² y⁻¹) the secondary production of P. maria.     

Influence of invasive macrophytes on channel morphology and hydrology in an open tropical lowland stream, and potential control by riparian shading

1. The catchments of many tropical lowland streams in far north Queensland have been extensively cleared for the cultivation of sugar cane to the extent where very little of the native riparian vegetation remains. Stream channels are often choked by a matrix of introduced pasture grass ( Brachiaria mutica , or para grass) and accumulated sediment from cropland erosion.
2. Detailed transects across Bamboo Creek, a fourth order cane-land stream, revealed an estimated sediment load of 20 000 t km^–1. This has resulted in an estimated 85% reduction in the predicted bankful discharge of the original stream channel. Channel capacity has been reduced from 2.3 times to 0.3 times the predicted Q₅₀ flood discharge of 140 m³ s^–1.
3. Shade cloth treatments of 50% and 90% across the stream were used to mimic the effect of shading by riparian vegetation. Three months of shading resulted in a substantial reduction in the height and standing biomass of para grass in both shade treatments, compared to open plots (0% shade). The most dramatic effect was in the 90% treatment, where a mean reduction of 63% in height and 52% in total biomass was recorded. This was despite high net primary production of para grass in the open plots of 2.8 g dry wt m^–2 day^–1, which resulted in a overall increase of 11% and 28% in plant height and total biomass, respectively.
4. These data suggest that restoration of native riparian vegetation will be an effective long-term means of controlling invasive macrophytes in disturbed cane-land stream channels. Reduction of excessive macrophyte growth and the mobilisation of accumulated sediment are essential to the restoration of natural hydrological and ecological processes.     

Abundance and distribution of anaerobic protozoa and their contribution to methane production in Lake Cadagno (Switzerland)

Abstract In the uupermost layers of the anoxic sediment in Lake Cadagno, 9 different species of anaerobic protozoa were identified. The total number of these organisms was about 580 cells·ml⁻¹ sediment. Most pf these protozoa contained endosymbiotic methanogenic bacteria which in total amounted to 10⁶ methanogens·ml⁻¹ sediment. In addition to the methanogenic endosymbionts, cells of Metopus setosus and Caenomorpha lata also contained a non-fluorescent bacterial rod inside the cytoplasm. In some individual cells of C. lata this second type of endosymbiotic bacterium was sometimes the only endosymbiont observed. Contrary to earlier suggestions, anaerobic protozoa do not seem to play a major role in methane production at least in Lake Cadagno. No significant methane production due to the anaerobic protozoa and their methanogenic endosymbionts was found in situ. Isolated ciliates and amoebae produced methane at 12°C, but not at 6°C, probably as a result of temperature limitation. In the sediment of Lake Cadagno sulfate reduction seemed to be the dominant terminal degradation process.     

Denitrification in river sediments: relationship between process rate and properties of water and sediment

1. A survey was made of denitrification and nitrous oxide (N₂O) production in river sediments at fifty sites in north‐east England during one season in order to investigate the relationship between rates and environmental factors likely to influence these processes. The sites were chosen to represent a wide range of physical and chemical conditions. Denitrification rate and N₂O production were measured within 5 h of sampling using the slurry acetylene blockage technique.
2. Denitrification rate ranged from less than 0.005–260 nmol N g^–1 DW h^–1, tending to increase in a downstream direction. N₂O production ranged from negative values (net consumption) to 13 nmol N₂O‐N g^–1 DW h^–1 and accounted for 0–115% of the N gases produced.
3. Denitrification rate and N₂O concentration in the sediment were correlated positively with nitrate concentration in the water column, water content of the sediment and percentage of fine (< 100 μm) particles in the sediment.
4. The variation in denitrification rate was satisfactorily explained (64% total variance) by a model employing measurements of water nitrate and water content of sediments. No simple or multiple relationship was found for N₂O production.     

Immobilization of dissolved organic matter from groundwater discharging through the stream bed

SUMMARY. 1. In laboratory experiments, 9.7–25.7% of dissolved organic carbon (DOC) in groundwater (at concentrations of 18.7–24.8 mg 1-⁻¹) was immobilized after perfusion through 8-cm-deep (22-cm-diameter) cores of stony stream-bed substratum.
2. This represented immobilization rates of 7.1–23.5 mg m⁻² h⁻¹ or, extrapolated across the year, potential immobilization rates within the stream bed of 62.2–205.9g m⁻² yr⁻¹. Actual rates in the entire stream bed were probably higher because perfusion through the experimental cores did not reduce groundwater DOC concentrations to levels measured in the adjacent stream.
3. Natural concentrations of dissolved free amino acids (DFAAs) in groundwater were generally unchanged following perfusion through the cores, suggesting the maintenance of a dynamic equilibrium in their concentrations.
4. Selective enrichments of amino acids in groundwater (up to 20-fold) were entirely immobilized following perfusion, indicating their rapid retention and flux in this environment. Thus, immobilization of the bulk DOC in stream-bed cores probably did not reflect net reductions in dissolved free, low-molecular-weight material, with higher molecular weight, more 'refractory' material being immobilized instead.
5. We conclude that groundwater can contribute substantial amounts of DOC, both high and low molecular weight, to a stream ecosystem. The stream bed is the site at which much of this material could be initially immobilized and made available to the stream trophic structure.     

Sulfide-induced dissimilatory nitrate reduction to ammonia in anaerobic freshwater sediments

Abstract: Different reduced sulfur compounds (H₂S, FeS, S₂O₃²⁻) were tested as electron donors for dissimilatory nitrate reduction in nitrate-amended sediment slurries. Only in the free sulfide-enriched slurries was nitrate appreciably reduced to ammonia (     ), with concomitant oxidation of sulfide to S⁰ (     ). The initial concentration of free sulfide appears as a factor determining the type of nitrate reduction. At extremely low concentrations of free S²⁻ (metal sulfides) nitrate was reduced via denitrification whereas at higher S²⁻ concentrations, dissimilatory nitrate reduction to ammonia (DNRA) and incomplete denitrification to gaseous nitrogen oxides took place. Sulfide inhibition of NO- and N₂O- reductases is proposed as being responsible for the driving part of the electron flow from S²⁻ to NH₄⁺.     

Stimulation of marine free-living and epibiotic bacterial activity by copepod excretions

Abstract The influence of marine copepod exudates (amino acids and NH₄⁺) on activity by free-living bacteria, and the relative activity of epibiotic bacteria living on copepods were examined. Samples that contained copepods (100 copepods 1⁻¹) were estimated to have leucine concentrations (K+S) that were higher by factor of 4.4, and leucine V_max's that were higher by a factor of 1.8 relative to samples without copepods. NH₄⁺ additions stimulated bacterial activity in samples that did not contain copepods, but had no effect in samples that contained copepods, suggesting that free-living bacteria were N-limited. Epibiotic bacteria accounted for about 20% of total bacterial activity. These results suggested that high-density copepod aggregations may liberate nutrients in sufficient quantity to significantly stimulate bacterial activity, and that epibiotic bacteria may be optimally positioned to exploit these resources.     

Egg-to-fry survival of the sea trout in some streams of Gotland

The quality of the surface water of Gotland (Sweden) streams was good, and did not limit sea trout egg-to-fry (ETF) survival. The oxygen concentration of the interstitial water was positively correlated to the permeability of the stream bed, and to the geometric mean diameter of the substratum. When the oxygen at 15 cm inside the stream bed was undiminished from surface values, the permeability of the stream bed was at least 6000 cm h^–1 and the geometric mean diameter of the substratum at least 15.0 mm. When the interstitial oxygen concentration increased, the interstitial ammonium concentration decreased, the NO₂ concentration remained stable and the NO₃ concentration increased. The ETF survival up to hatching was highly dependent upon the oxygen concentration of the interstitial water. For survival from hatching to emergence, the geometric mean diameter of the substratum was the most important factor. ETF survival >50% was observed where the interstitial oxygen concentration averaged at least 10mg I-¹, the substratum permeability at least 2000cm h-¹ and the substratum geometric mean diameter at least 15 mm. Other factors, such as overdigging and drying out of the spawning grounds could increase the ETF mortality.     

Contribution of virus-induced lysis and protozoan grazing to benthic bacterial mortality estimated simultaneously in microcosms

In contrast to the water column, the fate of bacterial production in freshwater sediments is still a matter of debate. Thus, the importance of virus-induced lysis and protozoan grazing of bacteria was investigated for the first time simultaneously in a silty sediment layer of a mesotrophic oxbow lake. Microcosms were installed in the laboratory in order to study the dynamics of these processes over 15 days. All microbial and physicochemical parameters showed acceptable resemblance to field data observed during a concomitant in situ study, and similar conclusions can be drawn with respect to the quantitative impact of viruses and protozoa on the bacterial compartment. Viral decay rates ranged from undetectable to 0.078 h⁻¹ (average, 0.033 h⁻¹), and the control of bacterial production from below the detection limit to 36% (average, 12%). The contribution of virus-induced lysis of bacteria to the dissolved organic matter pool as well as to benthic bacterial nutrition was low. Ingestion rates of protozoan grazers ranged from undetectable to 24.7 bacteria per heterotrophic nanoflagellate (HNF) per hour (average, 4.8 bacteria HNF⁻¹ h⁻¹) and from undetectable to 73.3 bacteria per ciliate per hour (average, 11.2 bacteria ciliate⁻¹ h⁻¹). Heterotrophic nanoflagellate and ciliates together cropped up to 5% (average, 1%) of bacterial production. The viral impact on bacteria prevailed over protozoan grazing by a factor of 2.5–19.9 (average, 9.5). In sum, these factors together removed up to 36% (average, 12%) of bacterial production. The high number of correlations between viral and protozoan parameters is discussed in view of a possible relationship between virus removal and the presence of protozoan grazers.     

Specific monitoring by PCR amplification and bioluminescence of firefly luciferase gene-tagged bacteria added to environmental samples

Abstract The firefly luciferase gene, luc , was demonstrated to hold promise as a specific marker for monitoring of genetically modified bacteria in the environment. PCR amplification and bioluminescence procedures were modified and compared for environmental monitoring of luc -tagged bacteria, using Escherichia coli as a model. The methods were used to track luc -tagged bacterial cells added to intact sediment core microcosms. Detection limits for the luc -tagged cells were the following, expressed as cells per 0.5 g of sediment: 10², by PCR amplification; 10³, by whole cell luminescence; and 10³−10⁴, by measurement of luminescence in cell extracts.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

A rapid and sensitive ion chromatographic technique for the determination of sulfate and sulfate reduction rates in freshwater lake sediments

Abstract Newly developed low capacity columns were used in suppressed ion chromatography for rapid and highly reproducible determination of SO₄²⁻ in porewater samples from freshwater sediments without preconcentration of samples. With a 50 μl injection the detection limit for SO₄²⁻ was ca. 50 pmol (= 1 μ M) with a precision of 1–3% at the 10–200 μM level and <1% at concentrations above 200 μM. SO₄²⁻ could be measured in 4–5 min with the routinely used eluent (3.0 mM NaHCO₃/0.8 mM Na₂CO₃). When the strength of the eluent was increased to 3.0 mM NaHCO₃/2.0 mM Na₂CO₃, sulfate analysis was possible in less than 3 min, provided that samples were nitrate-free. Under these conditions S₂O₃²⁻ could also be sensitively determined in about 6 min. Examples of application of the method are given for measurements of sulfate reduction rates in freshwater sediment samples from Lake Constance.     

The energetic importance of terrestrial arthropod inputs to three warm-water streams

1. Inputs of terrestrial arthropods (number and mass m^–2 d^–1) from riparian corridors to three streams representing different orders were highly variable among seasons and sites, with significantly greater ( P < 0.05) inputs at the headwater stream during summer months, compared with other sites and seasons.
2. No significant differences in estimates of stream retention of terrestrial arthropods (number and mass m^–2 d^–1) were observed among sites; however, retention of terrestrial arthropods at all sites was significantly greater during summer months, compared with other periods.
3. The gravimetric proportion of terrestrial arthropods present in the stomachs of redbreast sunfish ( Lepomis auritus ) and bluegill ( L. macrochirus ) was equivalent among sites. However, estimates of the dietary importance of terrestrial arthropods at all study sites were significantly greater in the summer, compared with other seasons.
4. Estimates of the potential annual energetic contribution (kJ m^–2 d^–1) of terrestrial arthropod inputs to the stream system were comparable with published rates of total annual production of aquatic macroinvertebrates in other Virginia headwater streams.
5. Results of this study supported the hypothesis that terrestrial arthropods represented an important energetic subsidy to stream fish during periods of low aquatic macroinvertebrate availability, and suggest that this component of allochthonous input is a potentially significant, but poorly understood energetic linkage between riparian landscapes and stream ecosystems.