Groundwater controls on nutrient cycling in a Mojave desert stream

1. Groundwater fluxes of nitrogen and dissolved organic carbon (DOC) were investigated in Grape Vine Canyon Stream in the Mojave Desert focusing on the rate of inputs and the fate of groundwater-derived nutrients in the stream. Discharge rates from different ground waters were measured using an end-member mixing model coupled with injections of a conservative solute tracer into the stream channel.
2. In surface water, nitrate concentration averaged 1.13 mg N L^–1 and DOC concentration averaged 1.82 mg C L^–1.
3. Groundwater discharge into Grape Vine Canyon Stream was derived from three sources. Nitrate concentration varied among the three groundwater sources with mean concentrations of 0.56, 0.94 and 0.08 mg N L^–1. DOC, in contrast, did not vary among ground water sources, with an overall average concentration of 2.96 mg C L^–1.
4. In the surface stream, nitrate concentration was two-fold greater than the concentration predicted from groundwater input, indicating that in-stream processes generated nitrate. Stream DOC concentration was lower than predicted based upon groundwater input rate. The production of nitrate and loss of DOC suggest that DOC is lost through mineralisation of dissolved organic matter, possibly resulting in the mineralisation of dissolved organic nitrogen to ammonium and subsequent transformation to nitrate via nitrification. In further support of this hypothesised linkage, DOC loss explained 80–89% of the variance in nitrate production in Grape Vine Canyon Stream.     

Measurement of bacterial production in stream-bed sediments via leucine incorporation

Abstract: The leucine incorporation technique was evaluated and optimised for measuring bacterial production in stream-bed sediments. The original procedure was modified in order to obtain reliable production estimates in this habitat. This included the use of higher leucine concentrations (50 μmol l⁻¹) for obtaining substrate saturation, and an enhanced protein extraction procedure after sample fixation. The leucine method was combined with a perfused core technique. Water containing ¹⁴C-labelled leucine was perfused up through sediment cores, enabling the measurement of bacterial production in an experimental situation resembling natural conditions in the stream bed. Bacterial production in the Breitenbach, a small upland stream in Central Germany, showed a high degree of spatial variability in the sandy stream bed. It was related closely to sediment organic matter content, whereas varying perfusion rates had less influence. Annual bacterial production was estimated at about 200 g C m⁻², demonstrating the potential for bacteria to act as a food resource for benthic fauna in this stream.     

Vertical distribution and abundance of invertebrates within the sandy substrate of a low-gradient headwater stream

SUMMARY. 1 The vertical distribution of invertebrates (>0.053 mm) was studied in a sandy-bottomed, first-order stream on the Coastal Plain of Virginia, U.S.A. Invertebrate species composition, abundance and bio-mass were determined monthly over one year at sediment depth intervals of 0–1, 1–5, 5–15,15–30 and 30–40 cm.
2. The subsurface community was numerically dominated by species of Chironomidae, Nematoda and Crustacea, while much of the biomass was due to early instars of several species of Trichoptera.
3. Invertebrate density and biomass decreased significantly with depth in the substrate (ANOVA; P <0.05). Annual mean density decreased from 1,346,844 individuals m⁻³ at the surface to 13,578 individuals m⁻³ at 15–30 cm. Annual mean biomass decreased from 66.30 g m⁻³ at the surface to 0.44 g m⁻³ at 15–30 cm.
4. Dissolved oxygen decreased markedly from the surface to the 5 cm depth in the substrate, anaerobic conditions often occurring below 10 cm. Density and biomass both showed a significant positive relationship with dissolved oxygen concentration (Linear regression; P <0.05).
5. Physical forces were important in structuring the subsurface invertebrate community. Besides low dissolved oxygen concentration, sediment scouring resulting from storm discharge dramatically reduced density and biomass     

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.     

Fates of dissolved free amino acids in groundwater discharged through stream bed sediments

Laboratory simulations were used to investigate the immobilization of dissolved free amino acids (DFAAs) from groundwater discharged up through cores of stream-bed sediments from a first order stream. At natural concentrations, 99% of DFAAs supplied in groundwater were immobilized, with 14–25% of this material respired and the remainder retained in the lower layers of the cores (depth = 7.5 cm). Immobilization efficiencies increased with increasing groundwater DFAA concentrations and discharge rates. Moderate enrichments (up to 1 mg l^–1) appeared to stimulate biotic immobilization of DFAAs, while abiotic processes accounted for much of the increased immobilization at higher enrichments (tested up to 100 mg l^–1). Variability in groundwater discharge rates induced no changes in the proportional contribution of biotic and abiotic immobilization processes. Thus relative contributions of DFAA concentration and groundwater discharge rate to a given DFAA load (concentration × discharge rate) influenced the degree to which immobilized DFAAs were retained (as microbial biomass or adsorbed to the biofilm) or respired. Results showed that DFAAs in groundwater discharged through the stream-bed are in a highly dynamic state of flux, suggesting that these compounds may be more significant to the transfer of organic matter to the benthic trophic structure than their normally low concentrations in groundwater would imply.     

Life history and production of Paranephrops zealandicus in a forest stream, with comments about the sustainable harvest of a freshwater crayfish

1. Life history and production were assessed for the crayfish Paranephrops zealandicus in three reaches of a headwater stream with a catchment of regenerating coniferous-broadleaf forest in the south-east of the South Island of New Zealand.
2. Crayfish density ranged from 3 to 4 m⁻² in riffles and 4–12 m⁻² in pools, depending on reach. Crayfish biomass (4–33 g AFDW m⁻²) and annual production (2–11 g AFDW m⁻²) were high compared with values reported elsewhere, while P:B ratio was low (0.33–0.43). This substantial production was dependent primarily upon high biomass rather than high growth rate.
3. The crayfish of this population rank amongst the longest lived and slowest growing ever recorded. Individuals estimated to be 16+ year of age were not uncommon. Females became reproductively active at 6+ year. Fewer than 4% of females carried eggs, and young remained attached to females for at least 15 months.
4. We propose that characteristics of this population are the consequence of a cool thermal regime (mean daily stream temperature = 7.0 °C, range 1.8–11.9 °C), and that low biomass turnover and poor reproductive rate precludes any sustainable commercial harvest of crayfish from streams in New Zealand with similar thermal regimes.     

Dissolved organic carbon concentrations and fluxes along the Moisie River, Quebec

SUMMARY. 1. The main stream and tributaries of a 145 km reach of the Moisie River, Quebec, were examined for temperature, conductivity, pH, dissolved organic carbon (DOC),%DOC>100,000 nominal molecular weight (NMW), optical density (OD₃₅₀), and the ratio of OD₄₀₀ to OD₆₀₀ (E4:E6).
2. Dissolved organic carbon concentrations correlated closely with OD₃₅₀ ( r ²=0.92, P <0.001). However,%DOC>100,000 NMW did not correlate with the E4:E6 ratio.
3. Except for a slight increase in%DOC> 100,000 NMW ( r ²=0.37, P <0.05), no change in any characteristic occurred down the length of the Moisie River, despite consistently higher levels of DOC in the tributaries.
4. Results suggest that high concentrations of DOC in tributary waters are rapidly removed within the main river channel. These results are discussed in terms of both biotic and abiotic models of in-strcam processing.     

Grazing by oligochaetes and snails on epiphytes

SUMMARY. 1. The isotope ³³P was used to assess the effect of grazitig by oligochaetes (mainly Stylaria lacttstris L.) and the snail, Lymnaea peregra (Muller), on epiphytes within an Equisetum fluviatile L. stand.
2. Two 1 m² polystyrene enclosures were set up within the emergent macrophyte zone of the lake. At the start of the experiment ³³P-solution was mixed with the water in both tanks. Algal and animal samples for ³³P- analysis were collected during the peak occurrence of epiphytes in June.
3. Phosphorus release rates from the animals through defaecation and excretion were measured in the laboratory. The grazing rate of oligochaetes was 2.2–4.1 mg P m⁻² (of bottom) d⁻¹ of which about two - thirds was released and recycled. The oligochaete density ranged from 13,400 to 20,900 m⁻². The snails (25 ind. m⁻²) grazed 1.2-1.5 mg P m⁻²d⁻¹of whieh about a quarter was released through defaecation and excretion.
4. Daily consumption by the oligochaetes and snails corresponded to 22–45% of the average phosphorus uptake by the epiphytes.     

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.     

The production of brown trout, Salmo trutta in tributaries of the Upper Wye, Wales

The population density, age structure, biomass, growth and production of brown trout were investigated in four tributaries of the upper River Wye. The populations at each site were largely maintained by immigration from nursery areas. Abundance of separate year classes at sites on the three largest tributaries reached a peak at age 2+. On the smallest stream numbers reached a peak at 1+. Recruitment occurred throughout the year but decreased with age of year classes. Maximum O+ densities ranged from 0.04 to 0.89 m⁻², and >0+ densities from 0.13 to 0.59 m⁻². Average total biomass in 1975 ranged from 2.6 to 14.2 g m⁻². Within the study sites annual trout production in 1975 ranged from 2.9–19.7 g m⁻². Production values were dependent on age structure and population mobility at the study sites. In the three largest streams 2+ and 3+ fish contributed 66.3–88.3% of total production whilst 1+ and 2+ fish contributed 74.5–84.5 % of the total in the smallest stream. The mobile (non-resident) component of the population accounted for up to 60–70% of production at certain times of the year, but over the year (1976) accounted for =30 % of total production. The resident component of the highest annual production value (19.7 g m⁻²) was estimated to be between 15.0 and 18.2 g m⁻².     

Production and litter processing by crayfish in an Appalachian mountain stream

SUMMARY 1. Mean annual density and biomass () of Cambarus bartonii in an Appalachian mountain stream (U.S.A.) was 12 individuals m⁻² and 1669 mg (ash-free dry weight) m⁻².
2. Annual production ( P ) of C bartonii was 961 mg AFDW m⁻². Despite high biomass, low growth rates resulted in low production and a low P/ ratio of 0.58.
3. While C bartonii constituted 61% of the total macroinvertebrate biomass, it contributed only 13% of annual community secondary production.
4. Litter processing was positively related to temperature and crayfish size. Cambariis bartotnii was estimated to comminute 36 g m⁻² y⁻¹ of leaf litter (>1 mm²) to 24 g m−^>2 y⁻¹ fine particulate material (<1 mm²). The annual pattern of litter comminution by crayfish was regulated by temperature. As a result, >5()% of shredding activity by crayfish occurred from June to September which was also the period of lowest litter standing crops and activity of other shredding macroinvertebrates.
5. We speculate that during summer crayfish play an important role in temperate woodland streams by converting slowly processed leaf litter species (e.g. Rhododendron ) to fine particles which are then available to collector-gatherers (e.g. Chironomidae, Oligochaeta).     

Terrestrial export of highly bioavailable carbon from small boreal catchments in spring floods

1. We assessed the terrestrial export of organic carbon, which effectively supported aquatic bacterial production (BP), from small boreal catchments during spring flood. We analysed stream runoff from nine small catchments with different proportions of peat mires and coniferous forests by monitoring the dissolved organic carbon (DOC) flux in combination with conducting bacterial bioassays.
2. Multiple linear regression analysis showed that BP during 7-day-dark bioassays (BP₇; μg C L⁻¹day⁻¹) was explained by both the quantity and quality (low-molecular weight fractions) of the DOC. BP₇ can be used as a measure of export of terrestrial organic carbon that is highly bioavailable.
3. Total export of DOC during spring flood from the different catchments ranged from 20 to 27 kg ha⁻¹ and was negatively correlated to forest cover (%). However, the export of BP₇ carbon was positively correlated to forest cover and varied from about 0.1 kg ha⁻¹ in mire-dominated streams to about 0.2 kg ha⁻¹ in forest-dominated streams.
4. The high bioavailability of forest carbon suggests that forests are the main contributors of BP-supporting carbon in boreal streams although mires have higher area-specific export of DOC.     

Input, movement and exchange of organic matter within a subtropical coastal black water river-flood plain system

SUMMARY. 1. Inputs, movements and exchanges of particulate organic matter were measured on two contrasting floodplains of the Ogeechee River, Georgia, U.S.A. A model, which incorporated measurements of standing crop, respiration, litterfall, inundation, and litter processing rates, was used to estimate annual exchanges of organic matter between the river and floodplains.
2. Annual litterfall was higher on the East floodplain than on the lower elevation West floodplain (902 v. 784 g ash-free-dry-mass [AFDM] m⁻²).
3. Experiments with tagged leaves and sticks demonstrated that litter was readily displaced during floods. The distance and direction of displacement varied within and between floodplains but tended to be higher closer to the river and was generally parallel to the river.
4. The model indicated that both floodplains lost organic matter to the river. The lower elevation floodplain (East) lost more organic matter to the river (208 g AFDM m⁻² year⁻¹) than did the higher elevation (West) floodplain (79g AFDM m⁻² year⁻¹).
5. Inputs of organic matter from the floodplain to the river exceeded the amount of litterfall typically entering heavily forested high gradient headwater streams (5.5 v. 0.4-0.6 kg AFDM m⁻² year⁻¹).
6. Floodplain organic matter inputs may exert a greater influence upon structure and function within these streams than do upstream inputs or primary production. Consequently, current conceptualizations of stream structure and function need to be modified to account for the effects of floodplain inputs on stream channel processes within large, low-gradient rivers.     

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 seasonal biomass and productivity of the submerged macrophytes in a polluted Wisconsin stream

SUMMARY 1. We measured biomass and light/dark bottle productivity of macrophytes in a Wisconsin stream throughout one growing season. Except for a brief period in early spring when a Cladophora glomerata -filamentous algal community was dominant, Potamogeton pectinatus was the dominant macrophyte species in Badfish Creek.
2. Maximum community biomass was 710 g DW m⁻², with a maximum above ground biomass of 620 g DW m⁻² and a maximum below ground biomass of 120 g DW m⁻². Annual productivity was estimated at 1435 g DW m⁻² year⁻¹, with a calculated P/B of 2.01.
3. In situ net production averaged 2.83g C g AFDW⁻¹ h⁻¹ Net positive carbon gain by the P. pectinatus community occurred when water temperatures were above 15°C, and daylength at least 12h. This is correlated to the onset of tuber germination in spring, and the point of maximal biomass decline in autumn.     

The invertebrate fauna of four macrophytes in a lotic system

SUMMARY. 1. Invertebrates associated with four species of aquatic plants and on the adjacent substrate were investigated in a riffle/run section of a hard-water stream.
2. Invertebrates were more numerous on stones than on any species of plant (> 17,000m⁻² versus 1000–4000 m⁻²). Gatherers were more abundant on Ranunculus longirostris , where proportionally more fine detritus was trapped in the leaf bundles than on other plants. Scrapers were most numerous on Charu vulgaris , where attached diatoms were also most prevalent.
3. Invertebrate community structure on three plant speeies was quite different from that on stones. This was not the case for Potamogeton amplifolius which appeared to be used by the organisms as a mere extension of the substrate.     

Plasma steroid concentrations in relation to the reproductive cycle of cultured male Rhamdia quelen

Seepage water through a lake bottom and the layering of tight and high-permeable layers in the aquifer produce groundwater under greater pressure than the water column of the lake. This results in under-water springs and seepages. When spawning substrata were available, there was a spatial correlation between the degree of groundwater influx and the redd density of brown trout Salmo trutta . In high density spawning areas with > 100 observed redds ha⁻¹, the mean seepage water influx during spring was c . 1200 ml m⁻² min⁻¹ compared to a mean flux of 113 ml m⁻² min⁻¹ in low density spawning areas with 5–10 redds ha⁻¹.     

The influence of drought on the fish and invertebrate populations of an upland stream in Wales

SUMMARY. 1. The exceptionally dry summer of 1976 presented an opportunity to assess the impact of drought on the fauna of an unregulated stream in Wales.
2. Total number of macroinvertebrates varied from 1808 m⁻² in September 1976 to 7412m⁻² in August 1977. Two main effects of drought were observed: an initial reduction in abundance during the drought and a change in community structure in the following year, probably through impaired reproduction.
3. Trout density ranged from 0.075±0.028 to 0.402±0.093 m⁻² and salmon parr density varied between 0.014±0.004 and 0.414±0.119 m⁻². The only detrimental effect of the drought on the fish fauna was the elimination of the 1976 year class of young salmon. probably due to prolonged high water temperature.     

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.     

Heterotrophic bacterioplankton production and grazing mortality rates in an Ethiopian rift-valley lake (Awassa)

SUMMARY. 1. Heterotrophic bacterioplankton growth and production rates were estimated in a tropical lake by various methods. Mean growth rates, determined by tritiated thymidine incoporation into DNA, frequency of dividing cells and increase in cell density varied between 0.013 and 0.014 (with a range of 0.006–0.026) h⁻¹ corresponding to bacterial production of 1.16–1.22 (0.34–3.63) mg C m⁻³ h⁻¹.
2. Heterotrophic bacterial production estimated from oxygen and inorganic carbon consumption in the dark were compared with these values. The oxygen method gave similar results, while values from dark carbon uptake were as much as 2.5 times higher.
3. Although the different estimates of rates of bacterial production showed different patterns, the existence of spatial (vertical) and temporal (diel and seasonal) variation was demonstrated. Bacterial production was 13–41% of the net primary production and 10–30% of gross primary production.
4. Bacterial grazing mortality rate was estimated from size-fractionation and metablic inhibitor experiments. Average grazing rates were between 0.34 and 3.77 mg C m⁻³ h⁻¹ corresponding to 76–120% of the mean bacterial production rate. Organisms 1–12 μm in size, possibly mainly ciliates. were implicated as important bacterial grazers.