Oxygen and nitrate respiration in a reed swamp sediment from a eutrophic lake

Seasonal measurements of the oxygen and nitrate uptake by a reed swamp sediment were carried out in a shallow, eutrophic Danish lake, Arreskov Sø. The oxidation of organic carbon in the sediment by aerobic and nitrate respiration was 290 and 188 g C m⁻² yr⁻¹ respectively. During winter, nitrate respiration amounted to 94% of the total carbon oxidation, whereas it was zero during summer. On an annual basis nitrate respiration constituted 39% of total respiration. Sediment nitrate uptake was correlated to nitrate concentration. In consequence of this the nitrate uptake rates varied during the year from zero in summer to 55 mg N m⁻² d⁻¹ in spring.
Oxygen uptake rates varied from 30 to 250 mg O₂ m⁻² h⁻¹ during the year, with a maximum uptake in August. The oxygen uptake per year was calculated to 860 g O₂ m⁻². The oxygen uptake rate was correlated to lake temperature and Kjeldahl nitrogen content of the sediment. The oxygen uptake rate, however, showed no correlation with loss on ignition of the sediment. A Q₁₀-value of 2.2 was found for lake measurements in the temperature interval of 5–15°C. The corresponding O₁₀-value in the laboratory was 2.6. A high microbial biomass indicated by the maximum content of Kjeldahl nitrogen and the lowest ratio of loss on ignition on Kjeldahl nitrogen appeared in late August, when the maximum oxygen uptake occurred. The oxygen uptake rate increased during the time interval from sampling to the start of the experiments.     

Change in CO2 balance under a series of forestry activities in a cool-temperate mixed forest with dense undergrowth

To evaluate the effects on CO₂ exchange of clearcutting a mixed forest and replacing it with a plantation, 4.5 years of continuous eddy covariance measurements of CO₂ fluxes and soil respiration measurements were conducted in a conifer-broadleaf mixed forest in Hokkaido, Japan. The mixed forest was a weak carbon sink (net ecosystem exchange, −44 g C m⁻² yr⁻¹), and it became a large carbon source (569 g C m⁻² yr⁻¹) after clearcutting. However, the large emission in the harvest year rapidly decreased in the following 2 years (495 and 153 g C m⁻² yr⁻¹, respectively) as the gross primary production (GPP) increased, while the total ecosystem respiration (RE) remained relatively stable. The rapid increase in GPP was attributed to an increase in biomass and photosynthetic activity of Sasa dwarf bamboo, an understory species. Soil respiration increased in the 3 years following clearcutting, in the first year mainly owing to the change in the gap ratio of the forest, and in the following years because of increased root respiration by the bamboo. The ratio of soil respiration to RE increased from 44% in the forest to nearly 100% after clearcutting, and aboveground parts of the vegetation contributed little to the RE although the respiration chamber measurements showed heterogeneous soil condition after clearcutting.     

Near-zero recent carbon accumulation in a bog with high nitrogen deposition in SW Sweden

We present data on the accumulation of carbon and nitrogen into an open oceanic ombrotrophic bog, SW Sweden, with high levels of anthropogenic nitrogen deposition. The aim was to investigate if this peatland currently acts as a sink for atmospheric carbon. Peat cores were sampled from the top peat layer in five different vegetation types. Small pines were used to date the cores. The cores bulk density and carbon and nitrogen content were determined. A vegetation-classified satellite image was used to estimate the areal extent of the vegetation types and to scale up these results to bog level. The rate of current carbon input into the upper oxic acrotelm was 290 g m⁻² yr⁻¹, and there were no significant differences in accumulation rates among the vegetation types. This organic matter input to the acrotelm was almost completely decomposed before it was deposited for storage in the deeper peat layers (the catotelm) and only a small fraction (≪1%) or 0.012 g m⁻² yr⁻¹ of the carbon would be left, assuming a residence time of 100 years in the acrotelm. Nitrogen accumulation rates differed between the vegetation classes, and the average input via primary production varied from 5.33 to 16.8 g m⁻² yr⁻¹. Current nitrogen input rates into the catotelm are much lower, 0–0.059 g m⁻² yr⁻¹, with the highest accumulation rates in lawn-dominated communities. We suggest that one of the main causes of the low carbon input rates is the high level of nitrogen deposition, which enhances decomposition and changes the vegetation from peat-forming Sphagnum -dominance to dominance by dwarf shrubs and graminoids.     

Respiration rate in maize roots is related to concentration of reduced nitrogen and proliferation of lateral roots

The relationship between specific rate of respiration (respiration rate per unit root dry weight) and concentration of reduced nitrogen was examined for maize ( Zea mays L.) roots. Plants with 2 primary nodal root axes were grown for 8 days in a split-root hydroponic system in which NO-₃ was supplied to both axes at 1.0 mol m⁻³, to one axis at 1.0 mol m⁻³ and the other axis at 0.0 mol m⁻³ or to both axes at 0.0 mol m⁻³ Respiration rates and root characteristics were measured at 2-day intervals. Specific rate of respiration was positively correlated in a nonlinear relationship with concentration of reduced nitrogen. The lowest specific rates of respiration occurred when neither axis received exogenous NO⁻³ and the concentration of reduced nitrogen in the axes was less than 9 mg g⁻¹. The greatest rates occurred in axes that were actively absorbing NO⁻³ and contained more than 35 mg g⁻¹ of reduced nitrogen. At 23 mg g⁻¹ of reduced nitrogen, below which initiation of lateral branches was decreased by 30–50%. specific rate of respiration was 17% greater for roots actively absorbing NO⁻³ than for roots not absorbing NO⁻³ Increases in specific rate of respiration associated with concentrations of reduced nitrogen greater than 23 mg g⁻¹ were concluded to be attributable primarily to proliferation of lateral branches.     

A limnological investigation of the tarn Syslaktjønn, W Norway

An investigation covering hydrography, chemistry, vascular and cryptogamic plants, nitrogen fixation, phytoplankton biomass and production, and zooplankton was carried out from April to November 1976 in tarn in W Norway, The volume of the tarn was 18000 m³ and the turnover time about 30 d. Temperature ranged between 3.6 and 23.4°C and pH between 4.8 and 5.5. Nuphar luteum and Carex rostrata were the two dominating vasculars-with biomasses of 117 and 97 g m⁻², respectively The biomass of the bryophytes ( Sphagnum spp.) was about 510 g m⁻² and the production of the order 0.2–2.1 μg (mg d.w.)⁻¹h⁻¹. Nitrogen fixation in association with Sphagnum spp. was estimated at 25 g yr⁻¹ for the whole tarn. Phytoplankton was dominated by diatoms, green algae and chrysophyceans. The chlorophyll a content ranged from 2 to 20 mg m⁻³ and the carbon assimilation rates from 0.03 to 20 mg C m⁻³h⁻¹ at 0–4 m depths. Production in the period was of the magnitude 22 g C m⁻². The copepod Eudiaptomus gracilis was the most common netzooplankter. Large numbers of rotationrians were found during summer.     

The effect of increased deposition of atmospheric nitrogen on Calluna vulgaris in upland Britain

Regular (monthly) additions of NH₄NO₃ (4–12 g N m⁻² yr⁻¹) were made over a period of 8 yr (1989–98) to areas of moorland in North Wales dominated by the ericaceous shrub Calluna vulgaris . Results from the early stages of the experiment (1990–94) have shown marked and dose-related increases in shoot extension and canopy height in response to the nitrogen treatments, with significantly higher shoot nitrogen contents. The nitrogen-related stimulation in the growth of the C. vulgaris canopy over this period has resulted in large accumulations of litter on the high-nitrogen-treated plots (6.6 kg m⁻² in plots treated with 12 g N m⁻² yr⁻¹, compared with 3.8 kg m⁻² for the water controls). Litter nitrogen concentrations were also significantly increased at the higher rates of nitrogen addition, leading to a doubling of the total return of nitrogen to the litter layer over the experimental period. These changes in vegetation structure were associated with large reductions in the abundance of the bryophyte and lichen species normally present under the untreated canopy. Results since 1994, however, show little increase in shoot extension in response to the nitrogen treatments, with no clear dose response to increasing levels of addition. These findings are associated with a dose-related increase in the susceptibility of the nitrogen-treated areas of the C. vulgaris canopy to late winter injury, characterized as browning of the shoot tips in early to late spring. These results indicate that deleterious effects are now accumulating as a result of the long-term addition of nitrogen to these moorland plots.     

NO2‐induced nitrate reductase activity in needles of Norway spruce (Picea abies) under laboratory and field conditions

The induction of activity of the enzyme nitrate reductase (NR, EC 1.6.6.1, 1.6.6.2) in needles of Norway spruce ( Picea abies [L.] Karst.) by nitrogen dioxide (NO₂) was studied under laboratory and field conditions. In fumigation chambers an increase in nitrate reductase activity (NRA) was detected 4 h after the start of the NO₂ treatment. During the first 2 days with 100 µg NO₂ m⁻³, NRA reached a constant level and did not change during the following 4 days. At the same level of NO₂, NRA was lower in needles from trees grown on NPK‐fertilized soil than on non‐fertilized soil. After the transfer of spruce trees from fertilized soil to NPK‐rich nutrient solution, NRA was transiently increased. This effect was assigned to root injuries causing nitrate transport to the shoot and subsequent induction of NRA. Neither trees on fertilized soil nor trees transferred to NPK‐poor nutrient solution had increased NRA unless NO₂ was provided. The NO₂ gradient in the vicinity of a highway was used to test the long‐term effect of elevated levels of NO₂ on needle NRA of potted and field‐grown spruce trees. Compared with less polluted sites, permanently increased NRAs were detected when NO₂ concentrations were above 20 µg m⁻³. Controls of field measurements some 10 years after the introduction of catalytic converters in cars showed no significant change neither in NO₂ levels nor in the decreasing NRA of spruce needles with the distance from the highway.     

Carbon stored in human settlements: the conterminous United States

Urban areas are home to more than half of the world's people, responsible for >70% of anthropogenic release of carbon dioxide and 76% of wood used for industrial purposes. By 2050 the proportion of the urban population is expected to increase to 70% worldwide. Despite fast rates of change and potential value for mitigation of carbon dioxide emissions, the organic carbon storage in human settlements has not been well quantified. Here, we show that human settlements can store as much carbon per unit area (23–42 kg C m⁻² urban areas and 7–16 kg C m⁻²exurban areas) as tropical forests, which have the highest carbon density of natural ecosystems (4–25 kg C m⁻²). By the year 2000 carbon storage attributed to human settlements of the conterminous United States was 18 Pg of carbon or 10% of its total land carbon storage. Sixty-four percent of this carbon was attributed to soil, 20% to vegetation, 11% to landfills, and 5% to buildings. To offset rising urban emissions of carbon, regional and national governments should consider how to protect or even to increase carbon storage of human-dominated landscapes. Rigorous studies addressing carbon budgets of human settlements and vulnerability of their carbon storage are needed.     

Effect of plant succession, ploughing, and fertilization on the microbiological oxidation of atmospheric methane in a heathland soil

Abstract: The effect of plant succession on methane uptake was measured on intact soil cores collected from seven heathland sites. Six of the sites had undergone either secondary succession with grass or oak, ammonium fertilization or ploughing, while the seventh site was located in the native heathland. There was a positive relationship between methane uptake rate and time elapsed since the plant invasion had taken place in the native heathland. The native heathland site showed an insignificant atmospheric methane uptake of 0.01 mg CH₄ m⁻² d⁻¹, whereas the established oak brushwood (70 years old) and the grass invaded heathland (13 years old) showed rates of 1.36 mg CH₄ m⁻² d⁻¹ and 0.73 mg CH₄ m⁻² d⁻¹, respectively. In the fertilized heathland plot (112 kg N ha⁻¹ six years prior to this study) grass had become the dominating species and showed a methane oxidation rate of 0.28 mg CH₄ m⁻² d⁻¹. Ploughing of the heathland resulted in methane oxidation rates seven times the rates measured in the native heathland. The results suggested that an increased future atmospheric nitrogen deposition in heathlands and other nutrient poor ecosystems may have a stimulating effect on the soil sink for atmospheric methane.     

Effect of temporary changes in light intensity on carbon transport, partitioning and respiratory loss in young tomato seedlings raised under different light intensities

Tomato plants were grown under light intensities of 36 or 90 W m⁻² [photosynthetically active radiation (PAR)], and then the light intensity was changed to 36, 90 or 180 W m⁻² for 8 h to investigate the effect of temporary changes in light intensity on the carbon budget of photoassimilates from the third leaf using a ¹⁴CO₂ steady-state feeding method. In the plants that were raised under 90 W m⁻², the photosynthetic rate increased when the light intensity was increased to 180 W m⁻², whereas no increase occurred in the plants that were raised under 36 W m⁻². Although the total amount of carbon fixed during the 8-h light period showed a large difference between plants grown at the two initial light intensities, the proportion of carbon exported during the light period did not differ apparently, irrespective of the change in light intensity. However, the amount of carbon exported during the time course was higher in plants that were raised under 90 W m⁻² than those raised under 36 W m⁻², irrespective of the change in light intensity. The partitioning pattern of ¹⁴C-photoassimilates was not changed by the change in light intensity, irrespective of whether the light intensity was increased or not. However, the amount of ¹⁴C-photoassimilates accumulated in each part differed according to the two initial light intensities. The carbon transport from a source leaf was also investigated through a quantitative analysis of carbon balance.     

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

Nutrient acquisition in the mature root zone is under systemic control by the shoot and the root tip. In maize, exposure of the shoot to light induces short-term (within 1–2 min) effects on net K⁺ and H⁺ transport at the root surface. H⁺ efflux decreased (from −18 to −12 nmol m⁻² s⁻¹) and K⁺ uptake (∼2 nmol m⁻² s⁻¹) reverted to efflux (∼−3 nmol m⁻² s⁻¹). Xylem probing revealed that the trans-root (electrical) potential drop between xylem vessels and an external electrode responded within seconds to a stepwise increase in light intensity; xylem pressure started to decrease after a ∼3 min delay, favouring electrical as opposed to hydraulic signalling. Cutting of maize and barley roots at the base reduced H⁺ efflux and stopped K⁺ influx in low-salt medium; xylem pressure rapidly increased to atmospheric levels. With 100 m m NaCl added to the bath, the pressure jump upon cutting was more dramatic, but fluxes remained unaffected, providing further evidence against hydraulic regulation of ion uptake. Following excision of the apical part of barley roots, influx changed to large efflux (−50 nmol m⁻² s⁻¹). Kinetin (2–4  µ m ), a synthetic cytokinin, reversed this effect. Regulation of ion transport by root-tip-synthesized cytokinins is discussed.     

Simulated chronic nitrogen deposition increases carbon storage in Northern Temperate forests

High levels of atmospheric nitrogen (N) deposition in Europe and North America were maintained throughout the 1990s, and global N deposition is expected to increase by a factor of 2.5 over the next century. Available soil N limits primary production in many terrestrial ecosystems, and some computer simulation models have predicted that increasing atmospheric N deposition may result in greater terrestrial carbon (C) storage in woody biomass. However, empirical evidence demonstrating widespread increases in woody biomass C storage due to atmospheric N deposition is uncommon. Increased C storage in soil organic matter due to chronic N inputs has rarely been reported and is often not considered in computer simulation models of N deposition effects. Since 1994, we have experimentally simulated chronic N deposition by adding 3 g N m⁻² yr⁻¹ to four different northern hardwood forests, which span a 500 km geographic gradient in Michigan. Each year we measured tree growth. In 2004, we also examined soil C content to a depth of 70 cm. When we compared the control treatment with the NO₃⁻ deposition treatment after a decade of experimentation, ecosystem C storage had significantly increased in both woody biomass (500 g C m⁻²) and surface soil (0–10 cm) organic matter (690 g C m⁻²). The increase in surface soil C storage was apparently driven by altered rates of organic matter decomposition, rather than an increase in detrital inputs to soil. Our results, for study locations stretching across hundreds of kilometers, support the hypothesis that chronic N deposition may increase C storage in northern forests, potentially contributing to a sink for anthropogenic CO₂ in the northern Hemisphere.     

PHYSIOLOGICAL RESPONSES TO TEMPERATURE AND IRRADIANCE IN SPIROGYRA (ZYGNEMATALES, CHAROPHYCEAE)1

Spirogyra Link (1820) is an anabranched filamentous green alga that forms free-floating mats in shallow waters. It occurs widely in static waters such as ponds and ditches, sheltered littoral areas of lakes, and stow-flowing streams. Field observations of its seasonal distribution suggest that the 70-μm-wide filament form of Spirogyra should have a cool temperature and high irradiance optimum for net photosynthesis. Measurements of net photosynthesis and respiration were marie at 58 combinations of tight and temperature in a controlled environment facility. Optimum conditions were 25°C and 1500 μmol photons m⁻² s⁻¹, at which net photosynthesis averaged 75.7 mg O₂ gdm⁻¹ h⁻¹. Net photosynthesis was positive at temperatures from 5° to 35°C at most irradiances except at combinations of extremely low irradiances and high temperatures (7 and 23 μmol photons m⁻² s⁻¹ at 30°C and 7, 23, and 35 μmol photons m⁻² s⁻¹ at 35°C). Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiances of 750 μmol photons m⁻² s⁻¹ or greater. Polynomials were fitted to the data to generate response surfaces; such response surfaces can be used to represent net photosynthesis and respiration in ecological models. The data indicate that the alga can tolerate the cool water and high irradiances characteristic of early spring but cannot maintain positive net photosynthesis under conditions of high temperature and low light (e.g. when exposed to self-shading ).     

Effects of boundary layer conductance on substomatal pressures of carbon dioxide

Abstract. Gas exchange measurements were made on single leaves of three C₃ and one C₄ species at air speeds of 0.4 and 4.0 m s⁻¹ to determine if boundary layer conductance substantially affected the substomatal pressure of carbon dioxide. Boundary layer conductances to water vapour were 0.4 to 0.5 mol m⁻² s⁻¹ at the lower air speed, and 1.2 to 1.5 mol m⁻² s⁻¹ at the higher air speed. Substomatal carbon dioxide pressures were about 5 Pa lower at low boundary layer conductance in the C₃ species, and about 3 Pa lower in the C₄ species when measurements were made at high and moderate photosynthetic photon flux densities. No evidence of stomatal adjustment to altered boundary layer conductance was found. Photosynthetic rates at high photon flux densities were reduced by about 20% at the low air speed in the C₃ species. The commonly reported values of substomatal carbon dioxide pressure for C₃ and C₄ species were found to occur only when measurements were made at the higher air speed.     

Polar deserts, their plant cover and plant production in the Canadian High Arctic

A total of 41 stands was sampled for species composition and 29 of these stands for plant standing crop and net annual production at 7 sites on 6 arctic islands. Fourteen additional sites on 10 islands were studied in less detail.
Through polar ordination, three groupings were recognized: polar barrens with an average species richness of 6, a phytomass of 24 g m⁻², and a net annual production of 0.8 g m⁻². Comparable data for the cushion plant and snowflush communities were 9, 120, 3 and 13 species, 400 g m⁻², phytomass and 41 g m⁻² net production respectively. Cryptogams are minor except within showflush communities.
The soils show no horizon development, arc alkaline, and are very tow in organic matter, nitrogen, and phosphorus. It is believed that the combination of limited soil moisture in mid-summer and very low nutrient levels are the primary reason for such low plant cover and plant production in these predominantly polar barren landscapes. Geologic substrate with an abundance of frost-shattered rock and topographic position are factors that control the limited availability of water.     

Changes in topsoil carbon stock in the Tibetan grasslands between the 1980s and 2004

Climate warming is likely inducing carbon loss from soils of northern ecosystems, but little evidence comes from large-scale observations. Here we used data from a repeated soil survey and remote sensing vegetation index to explore changes in soil organic carbon (SOC) stock on the Tibetan Plateau during the past two decades. Our results showed that SOC stock in the top 30 cm depth in alpine grasslands on the plateau amounted to 4.4 Pg C (1 Pg=10¹⁵ g), with an overall average of 3.9 kg C m⁻². SOC changes during 1980s–2004 were estimated at −0.6 g C m⁻² yr⁻¹, ranging from −36.5 to 35.8 g C m⁻² yr⁻¹ at 95% confidence, indicating that SOC stock in the Tibetan alpine grasslands remained relatively stable over the sampling periods. Our findings are nonconsistent with previous reports of loss of soil C in grassland ecosystems due to the accelerated decomposition with warming. In the case of the alpine grasslands on the Tibetan Plateau studied here, we speculate that increased rates of decomposition as soils warmed during the last two decades may have been compensated by increased soil C inputs due to increased grass productivity. These results suggest that soil C stock in terrestrial ecosystems may respond differently to climate change depending on ecosystem type, regional climate pattern, and intensity of human disturbance.     

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.     

A comparative study of the effects of NaCl salinity on respiration, photosynthesis, and leaf extension growth in Beta vulgaris L. (sugar beet)

Abstract. Three parameters influencing the capacity for carbon accumulation, i.e. photosynthesis, respiration, and leaf extension growth, were studied in Beta vulgaris L. (sugar beet) cultured in nutrient solution containing 0.5 to 500 mol m⁻³ NaCl. Leaf extension growth was the parameter most sensitive to salinity: the initial rate of leaf extension and final leaf length each declined linearly with increase in external NaCl concentration. Photosynthetic O₂ evolution of thin leaf slices did not decline until salinity levels reached 350 to 500 mol m⁻³ NaCl, while respiratory O₂ consumption was not affected by salinity throughout the range. The results suggest that the influence of salinity on the capacity for carbon accumulation in B. vulgaris occurs primarily through reduction in the area of photosynthetic surface.     

Changes in organic carbon distribution with depth in agricultural soils in northern Belgium, 1960–2006

In most studies concerning the carbon (C) exchange between soil and atmosphere only the topsoil (0–0.3 m) is taken into account. However, it has been shown that important amounts of stable soil organic carbon (SOC) are also stored at greater depth. Here, we developed a quantitative model to estimate the evolution of the distribution of SOC with depth between 1960 (database 'Aardewerk') and 2006 in northern Belgium. This temporal analysis was conducted under different land use, texture and drainage conditions. The results indicate that intensified land management practices seriously affect the SOC status of the soil. The increase in plough depth and a change in crop rotation result in a significant decrease of C near the surface for dry silt loam cropland soils, (i.e. 1.02 ± 0.23 kg C m⁻² in the top 0.3 m between 1960 and 2006). In wet to extremely wet grasslands, topsoil SOC decreased significantly, indicating a negative influence of intensive soil drainage on SOC stock. This resulted in a decline of SOC between 1960 and 2006 in the top 1 m, ranging from 3.99 ± 2.57 kg C m⁻² in extremely wet silt loam soils to 2.04 ± 2.08 kg C m⁻² in wet sandy soils. A slight increase of SOC stock is observed under dry to moderately wet grasslands at greater depths corresponding to increased livestock densities in the region. The increase of SOC in the top 1 m under grassland ranges from 0.65 ± 1.39 kg C m⁻² in well drained silt loam soils to 2.59 ± 6.49 kg C m⁻² in moderately drained silt loam soils over entire period.     

Metabolism of a desert stream

SUMMARY. Rates of photosynthesis and community respiration were determined for benthic assemblages in Sycamore Creek, a Sonoran Desert stream in Arizona. Benthos in this stream can be separated into (1) mats of Cladophora glomerata and associated epiphytes and (2) assemblages of epipelic diatoms and blue-green algae. Community respiration and net photosynthesis were measured for these assemblages using submerged light-dark chambers in situ . Multiple regression analysis was used to predict (1) gross photosynthesis as a function of photosynthetically active radiation, temperature and chlorophyll-α concentration; and (2) community respiration as a function of temperature and biomass.
Calculations suggest that Sycamore Creek is autotrophic during the summer ( P/R = 1.7) and that the rates of gross photosynthesis ( P =8.5 g O₂ m⁻² day⁻¹) and community respiration ( R = 5.1 g O₂ m⁻² day⁻¹) are high for a small stream. Considerable difference exists between the Cladophora mat assemblages, in which mean P is 12.5gO₂m⁻² day⁻¹and the P/R ratio is 2.3, and the epipelic assemblages in which mean P is 4.4 g O₂m⁻² day⁻¹ and P/R is 0.96. The high rate of gross photosynthesis, low litter inputs, high biomass of algae and the intermittent but severe floods that characterize Sycamore Creek indicate that this stream and other similar desert streams are net exporters of organic matter and are, thereby, truly autotrophic stream ecosystems.