Ecosystem Metabolism in Piedmont Streams: Reach Geomorphology Modulates the Influence of Riparian Vegetation

We measured the impact of riparian zone vegetation on ecosystem metabolism in paired forested and meadow reaches on 13 streams in southeastern Pennsylvania and Maryland, USA. Metabolism estimates were based on open-system measurements of dissolved oxygen changes, with reaeration determined from propane evasion. Daily gross primary productivity (GPP) in meadow and forested reaches averaged 2.85 and 0.86 g O₂ m⁻² d⁻¹, respectively, at water temperatures of 12°C or greater when the forest canopy was developed and 1.74 and 1.09 g O₂ m⁻² d⁻¹, respectively, at temperatures below 12°C when the canopy was bare. Community respiration (CR₂₄) also was greater in meadow reaches than in forested reaches, averaging 5.58 and 3.57 g O₂ m⁻² d⁻¹, respectively, in the warm season and 4.87 and 2.88 g O₂ m⁻² d⁻¹, respectively, during the cold season. Thus, both meadow and forested reaches were heterotrophic. Forested reaches were always wider and nearly always shallower than companion meadow reaches. When ecosystem function was assessed per unit of stream length, the difference in average GPP between meadow and forested reaches was reduced from three-fold to 1.9-fold in the warm season, and mean GPP was greater in the forested reaches during the cold season. Mean CR₂₄ per meter stream length was greater in forested reaches during both seasons. Even though riparian shading reduced primary productivity per unit area of streambed, the greater stream width of the forested reaches counteracted that reduction in part. Thus, when rates of ecosystem function were expressed per length of stream, differences between reaches were always smaller than when expressed per area, and activity per unit stream length was sometimes greater in forested reaches than in meadow reaches.     

The expansion of woody riparian vegetation,and subsequent stream restoration,influences the metabolism of prairie streams

1. Tallgrass prairies and their streams are highly endangered ecosystems, and many remaining streams are threatened by the encroachment of woody riparian vegetation. An increase in riparian vegetation converts the naturally open‐canopy prairie streams to closed‐canopy systems. The effects of a change in canopy cover on stream metabolism are unknown. 2. Our goal was to determine the effects of canopy cover on prairie stream metabolism during a 4‐year period in Kings Creek, KS, U.S.A. Metabolic rates from forested reaches were compared to rates in naturally open‐canopy reaches and restoration reaches, the latter having closed canopies in 2006 and 2007 and open canopies in 2008 and 2009. Whole‐stream metabolism was estimated using the two‐station diurnal method. Chlorophyll a concentrations and mass of filamentous algae were measured after riparian removal to assess potential differences in algal biomass between reaches with open or closed canopies. 3. Metabolic rates were spatially and temporally variable even though the sites were on very similar streams or adjacent to each other within streams. Before riparian vegetation removal, whole‐stream community respiration (CR) and net ecosystem production were greater with greater canopy cover. In the vegetation removal reaches, gross primary production was slightly greater after removal. 4. Chlorophyll a concentrations were marginally significantly greater in open (naturally open and removal reaches) than in closed canopy and differed significantly between seasons. Filamentous algal biomass was greater in open than in closed‐canopy reaches. 5. Overall, the restoration allowed recovery of some features of open‐canopy prairie streams. Woody expansion apparently increases CR and moves prairie stream metabolism towards a more net heterotrophic state. An increase in canopy cover decreases benthic chlorophyll, decreases dominance of filamentous algae and potentially alters resources available to the stream food web. The results of this study provide insights for land managers and conservationists interested in preserving prairie streams in their native open‐canopy state.     

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

相似文献   

Inter-biome comparison of factors controlling stream metabolism

1. We studied whole-ecosystem metabolism in eight streams from several biomes in North America to identify controls on the rate of stream metabolism over a large geographic range. The streams studied had climates ranging from tropical to cool-temperate and from humid to arid and were all relatively uninfluenced by human disturbances.
2. Rates of gross primary production (GPP), ecosystem respiration (R) and net ecosystem production (NEP) were determined using the open-system, two-station diurnal oxygen change method.
3. Three general patterns in metabolism were evident among streams: (1) relatively high GPP with positive NEP (i.e. net oxygen production) in early afternoon, (2) moderate primary production with a distinct peak in GPP during daylight but negative NEP at all times and (3) little or no evidence of GPP during daylight and a relatively constant and negative NEP over the entire day.
4. Gross primary production was most strongly correlated with photosynthetically active radiation (PAR). A multiple regression model that included log PAR and stream water soluble reactive phosphorus (SRP) concentration explained 90% of the variation in log GPP.
5. Ecosystem respiration was significantly correlated with SRP concentration and size of the transient storage zone and, together, these factors explained 73% of the variation in R. The rate of R was poorly correlated with the rate of GPP.
6. Net ecosystem production was significantly correlated only with PAR, with 53% of the variation in log NEP explained by log PAR. Only Sycamore Creek, a desert stream in Arizona, had positive NEP (GPP: R  > 1), supporting the idea that streams are generally net sinks rather than net sources of organic matter.
7. Our results suggest that light, phosphorus concentration and channel hydraulics are important controls on the rate of ecosystem metabolism in streams over very extensive geographic areas.     

Inter‐regional comparison of land‐use effects on stream metabolism

1. Rates of whole‐system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first‐order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban‐influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land‐use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land‐use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land‐use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land‐use categories showed reduced inter‐regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.     

Metabolic balance of streams draining urban and agricultural watersheds in central Japan

Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coefficient (K ₂) determined from seven empirical depth-velocity equations. Because the predicted values of K ₂ showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.     

Factors driving the metabolism of two north temperate ponds

Two beaver ponds in central New York State, one in a forested and one in an agricultural setting, were studied to determine the influence of nutrient enrichment on metabolic activity in small, shallow north temperate lentic systems with extensive littoral zones. Metabolic activity was determined from every 15-min oxygen measurements during the growing season utilizing a sonde and wind, temperature, and depth data. The agricultural Timmerman Pond was more enriched in phosphorus and nitrogen than the forested Hoxie Gorge Pond, a factor likely driving the greater gross primary production (GPP) and more positive net ecosystem production (NEP) measured in Timmerman. Average daily GPP was over 3X greater for the more enriched pond and seasonal average NEP was positive in the enriched pond while negative for the forested pond. Daily GPP and NEP were positively correlated (P < 0.001) with daily solar radiation at both ponds. The active metabolism of small ponds, the most abundant size class of lentic systems, further confirms the important role of inland waters in the transformation of carbon on a global scale.     

The Metabolic Regimes at the Scale of an Entire Stream Network Unveiled Through Sensor Data and Machine Learning

Streams and rivers form dense networks that drain the terrestrial landscape and are relevant for biodiversity dynamics, ecosystem functioning, and transport and transformation of carbon. Yet, resolving in both space and time gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP) at the scale of entire stream networks has been elusive so far. Here, combining Random Forest (RF) with time series of sensor data in 12 reach sites, we predicted annual regimes of GPP, ER, and NEP in 292 individual stream reaches and disclosed properties emerging from the network they form. We further predicted available light and thermal regimes for the entire network and expanded the library of stream metabolism predictors. We found that the annual network-scale metabolism was heterotrophic yet with a clear peak of autotrophy in spring. In agreement with the River Continuum Concept, small headwaters and larger downstream reaches contributed 16% and 60%, respectively, to the annual network-scale GPP. Our results suggest that ER rather than GPP drives the metabolic stability at the network scale, which is likely attributable to the buffering function of the streambed for ER, while GPP is more susceptible to flow-induced disturbance and fluctuations in light availability. Furthermore, we found large terrestrial subsidies fueling ER, pointing to an unexpectedly high network-scale level of heterotrophy, otherwise masked by simply considering reach-scale NEP estimations. Our machine learning approach sheds new light on the spatiotemporal dynamics of ecosystem metabolism at the network scale, which is a prerequisite to integrate aquatic and terrestrial carbon cycling at relevant scales.

     

Climate-Induced Changes in Spring Snowmelt Impact Ecosystem Metabolism and Carbon Fluxes in an Alpine Stream Network

Although stream ecosystems are recognized as an important component of the global carbon cycle, the impacts of climate-induced hydrological extremes on carbon fluxes in stream networks remain unclear. Using continuous measurements of ecosystem metabolism, we report on the effects of changes in snowmelt hydrology during the anomalously warm winter 2013/2014 on gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) in an Alpine stream network. We estimated ecosystem metabolism across 12 study reaches of the 254 km² subalpine Ybbs River Network (YRN), Austria, for 18 months. During spring snowmelt, GPP peaked in 10 of our 12 study reaches, which appeared to be driven by PAR and catchment area. In contrast, the winter precipitation shift from snow to rain following the low-snow winter in 2013/2014 increased spring ER in upper elevation catchments, causing spring NEP to shift from autotrophy to heterotrophy. Our findings suggest that the YRN transitioned from a transient sink to a source of carbon dioxide (CO₂) in spring as snowmelt hydrology differed following the high-snow versus low-snow winter. This shift toward increased heterotrophy during spring snowmelt following a warm winter has potential consequences for annual ecosystem metabolism, as spring GPP contributed on average 33% to annual GPP fluxes compared to spring ER, which averaged 21% of annual ER fluxes. We propose that Alpine headwaters will emit more within-stream respiratory CO₂ to the atmosphere while providing less autochthonous organic energy to downstream ecosystems as the climate gets warmer.     

Ecosystem measures of river health and their response to riparian and catchment degradation

1. Measurements of ecological patterns are often used as primary biological indicators of river health. However, these patterns provide little information about important stream ecosystem processes (e.g. the sources and fate of energy and nutrients). The direct measurement of these processes is considered fundamental to the determination of the health of stream and river ecosystems. 2. In this paper we used two basic approaches to assess stream ecosystem response to catchment disturbance and, particularly, to the loss of riparian vegetation in different forested biomes across Australia. Benthic gross primary production (GPP) and respiration (R₂₄) provided measures of the amounts of organic carbon produced and consumed within the system, respectively. Stable isotope analysis was used to trace the fate of terrestrial and instream sources of organic matter in the aquatic food web. In a focal catchment in SE Queensland, additional measurements were taken of riparian attributes, catchment features and water quality. 3. Baseline measurements of GPP and R₂₄ from undisturbed forest streams provided reference values for healthy streams for comparison with sites where the catchment or riparian vegetation had been disturbed. These values of metabolism were low by world standards in all biomes examined. Preliminary data from the Mary River catchment in SE Queensland indicated that these parameters were sensitive to variations in riparian canopy cover and, to a lesser extent, catchment clearing, and predictive models were developed. The ratio P : R (GPP : R₂₄) was used to determine whether sites were net consumers (P < R) or producers (P > R) of carbon but this was not considered a reliable indicator of stream health on its own. 4. Although forest streams were typically net consumers of carbon (P < < R), stable isotope analysis of metazoan food webs indicated a high dependence on inconspicuous epilithic algae in some biomes. 5. A dramatic decline in the health of forest streams was observed when GPP substantially exceeded R₂₄, especially when instream primary producers shifted from palatable unicellular algae to prolific filamentous green algae and macrophytes. These sources of instream production do not appear to enter aquatic food webs, either directly through grazing or indirectly through a detrital loop. Accumulation of these plants has led to changes in channel morphology, loss of aquatic habitat and often a major decline in water quality in some of the streams studied.     

An approach for the identification of exemplar sites for scaling up targeted field observations of benthic biogeochemistry in heterogeneous environments

Headwater streams influence the biogeochemical characteristics of large rivers and play important roles in regional and global carbon budgets. The combined effects of seasonality and land use change on the biogeochemistry of headwater streams, however, are not well understood. In this study we assessed the influence of catchment land use and seasonality on the composition of dissolved organic matter (DOM) and ecosystem metabolism in headwater streams of a Kenyan river. Fifty sites in 34 streams draining a gradient of catchment land use from 100% natural forest to 100% agriculture were sampled to determine temporal and spatial variation in DOM composition. Gross primary production (GPP) and ecosystem respiration (ER) were determined in 10 streams draining primarily forest or agricultural catchments. Absorbance and fluorescence spectrophotometry of DOM reflected notable shifts in composition along the land use gradient and with season. During the dry season, forest streams contained higher molecular weight and terrestrially derived DOM, whereas agricultural streams were dominated by autochthonous production and low molecular weight DOM. During the rainy season, aromaticity and high molecular weight DOM increased in agricultural streams, coinciding with seasonal erosion of soils and inputs of organic matter from farmlands. Most of the streams were heterotrophic. However, GPP and ER were generally greater in agricultural streams, driven by higher dissolved nutrient (mainly TDN) concentrations, light availability (open canopy) and temperature compared with forest streams. There were correlations between freshly and autochthonously produced DOM, GPP and ER during both the dry and wet seasons. This is one of the few studies to link land-use with organic carbon dynamics and DOM composition. Measures of ecosystem metabolism in these streams help to affirm the role of tropical streams and rivers as important components of the global carbon cycle and demonstrate that even semi-intensive, smallholder agriculture can have measurable effects on riverine ecosystem functioning.     

Ecosystem metabolism and nutrient uptake in an urban,piped headwater stream

Piped streams, or streams that run underground, are often associated with urbanization. Despite the fact that they are ubiquitous in many urban watersheds, there is little empirical evidence regarding the ecological structure and function of piped stream reaches. This study measured ecosystem metabolism, nutrient uptake, and related characteristics of Pettee Brook—an urban stream that flows through several piped sections in Durham, New Hampshire, USA. Pettee Brook had high chloride and nutrient concentrations, low benthic biomass, and low rates of gross primary productivity (GPP), ecosystem respiration (ER), and nutrient uptake along its entire length during summer. Spring was a period of elevated biological activity, as increased light availability in the un-piped sections of the stream led to substantially higher GPP, ER, NH₄ uptake, and PO₄ uptake in these open reaches. Piped reaches of Pettee Brook were similar to open reaches in terms of water quality, dissolved O₂ concentration, temperature, and discharge. Piped reaches did, however, have significantly less light, shallower sediments, and no debris dams. The absence of light inhibited autotrophic activity in piped reaches, resulting in the complete loss of GPP as well as a significant reduction in benthic AFDM and chlorophyll a biomass. Heterotrophic activity in piped reaches was not impaired to the same extent as autotrophic activity. Reduced ER was observed in piped reaches during the summer, but we failed to find significantly lower DOC or nutrient uptake rates in piped reaches than in open reaches. Carbon consumption in piped reaches, which do not have significant autochthonous or allochthonous carbon replenishment, must rely primarily on upstream inputs of organic matter. These results suggest that although ecological conditions in piped streams may be degraded beyond the extent of other urban stream reaches, piped reaches may still sustain some measurable ecosystem function.     

Multi‐scaled drivers of rural prairie stream metabolism along human activity gradients

1. Stream metabolism is increasingly used for monitoring and assessment of the biological condition of aquatic ecosystems. However, distal environmental drivers, such as land use, are typically not well connected to the proximate controls, such as stream chemistry, that are usually invoked as driving metabolism. This is particularly true for North American prairie streams and for grassland streams worldwide. 2. Stream metabolism was measured at the outflow of 19 subcatchments of the Red River in southern Manitoba, Canada. Subcatchments represented gradients of nutrient‐producing human activities present in the region, that is, wastewater treatment (WWT), livestock production and crop cultivation. Stream metabolism was estimated at all sites using diel changes in dissolved oxygen (DO) concentration over 1 week in the middle of summer. Environmental parameters hypothesised to control stream metabolism were sampled across three spatial scales (stream reach, stream segment and catchment). Model selection using Akaike’s information criterion (AIC) was used to determine linkages between environmental parameters and measures of stream metabolism. 3. Estimated rates of metabolism were within the range of past studies of metabolism in prairie streams, although most streams had negative values of net ecosystem metabolism. However, production‐to‐respiration ratios were >0.5, at all but three sites suggesting that autochthonous production was an important source of organic matter. 4. The a priori model that best predicted gross primary production (GPP) was the intensity of nutrient‐producing human activities (i.e. WWT, livestock and crop cultivation) measured at the catchment scale. Ecosystem respiration (ER) was best predicted by the a priori model comprised of GPP, total nitrogen (TN) and total phosphorus (TP). However, model averaging revealed that prediction of ER could be improved by including riparian cover and removing TP from the model. The positive association between GPP and ER suggested that heterotrophic compartments of the ecosystem were modest contributors to variation in respiration rates. 5. Overall, this study suggests that variation of metabolism in prairie streams of southern Manitoba is controlled by human activities occurring at the catchment scale, a finding consistent with current hierarchically structured riverine paradigms. Moreover, increased understanding of the hierarchical structure of stream metabolism drivers will help to ensure that assessment results can be used more effectively to inform management strategies for prairie ecosystems.     

Land use controls stream ecosystem metabolism by shifting dissolved organic matter and nutrient regimes

相似文献   

Effects of urban stream burial on nitrogen uptake and ecosystem metabolism: implications for watershed nitrogen and carbon fluxes

Urbanization has resulted in the extensive burial and channelization of headwater streams, yet little is known about the impacts of stream burial on ecosystem functions critical for reducing downstream nitrogen (N) and carbon (C) exports. In order to characterize the biogeochemical effects of stream burial on N and C, we measured NO₃ ^? uptake (using ¹⁵N-NO₃ ^? isotope tracer releases) and gross primary productivity (GPP) and ecosystem respiration (ER) (using whole stream metabolism measurements). Experiments were carried out during four seasons, in three paired buried and open stream reaches, within the Baltimore Ecosystem Study Long-term Ecological Research site. Stream burial increased NO₃ ^? uptake lengths by a factor of 7.5 (p < 0.01) and decreased NO₃ ^? uptake velocity and areal NO₃ ^? uptake rate by factors of 8.2 (p < 0.05) and 9.6 (p < 0.001), respectively. Stream burial decreased GPP by a factor of 11.0 (p < 0.01) and decreased ER by a factor of 5.0 (p < 0.05). From fluorescence Excitation Emissions Matrices analysis, buried streams were found to have significantly altered C quality, showing less labile dissolved organic matter. Furthermore, buried streams had significantly lower transient storage (TS) and water temperatures. Differences in NO₃ ^? uptake, GPP, and ER in buried streams, were primarily explained by decreased TS, light availability, and C quality, respectively. At the watershed scale, we estimate that stream burial decreases NO₃ ^? uptake by 39 % and C production by 194 %. Overall, our results suggest that stream burial significantly impacts NO₃ ^? uptake, stream metabolism, and the quality of organic C exported from watersheds. Given the large impacts of stream burial on stream ecosystem processes, daylighting or de-channelization of streams, through hydrologic floodplain reconnection, may have the potential to alter ecosystem functions in urban watersheds, when used appropriately.     

Experimental increases and reductions of light to streams: effects on periphyton and macroinvertebrate assemblages in a coniferous forest landscape

Increased light reaching streams as a result of riparian vegetation management is often thought to be responsible for enhanced algal productivity. However, concomitant changes in nutrients and other physical processes confound that interpretation. We manipulated light in two separate experiments to test the role of light as a controlling factor for periphyton productivity and biomass, and to observe invertebrate responses in small streams in central British Columbia, Canada. We did this by adding artificial light to reaches of three forested streams, and in a second experiment we used shadecloth to cover reaches of two streams flowing through clearcuts. Periphyton growth, productivity and composition, and macroinvertebrate benthic densities were contrasted with control reaches within the same streams. Gross primary production (GPP) was increased at least 31% by light addition to forested streams. Periphyton biomass was higher under light additions, but only significantly so in one of the streams. In one stream grazers increased along with the periphyton response, whilst in the other two lit streams invertebrates, including grazers, decreased with increased light. The shading significantly reduced GPP to about 11% of that in clearcut sections, but failed to produce any significant responses in either periphyton standing stock or invertebrates in the clearcut streams. Measures of algal production and biomass responded as predicted; however, invertebrate responses to increased and decreased light were idiosyncratic amongst streams, perhaps indicating lagged responses and limitation by other resources.     

Longitudinal patterns in carbon and nitrogen fluxes and stream metabolism along an urban watershed continuum

An urban watershed continuum framework hypothesizes that there are coupled changes in (1) carbon and nitrogen cycling, (2) groundwater-surface water interactions, and (3) ecosystem metabolism along broader hydrologic flowpaths. It expands our understanding of urban streams beyond a reach scale. We evaluated this framework by analyzing longitudinal patterns in: C and N concentrations and mass balances, groundwater-surface interactions, and stream metabolism and carbon quality from headwaters to larger order streams. 52 monitoring sites were sampled seasonally and monthly along the Gwynns Falls watershed, which drains 170 km² of the Baltimore Long-Term Ecological Research site. Regarding our first hypothesis of coupled C and N cycles, there were significant inverse linear relationships between nitrate and dissolved organic carbon (DOC) and nitrogen longitudinally (P < 0.05). Regarding our second hypothesis of coupled groundwater-surface water interactions, groundwater seepage and leaky piped infrastructure contributed significant inputs of water and N to stream reaches based on mass balance and chloride/fluoride tracer data. Regarding our third hypothesis of coupled ecosystem metabolism and carbon quality, stream metabolism increased downstream and showed potential to enhance DOC lability (e.g., ~4 times higher mean monthly primary production in urban streams than forest streams). DOC lability also increased with distance downstream and watershed urbanization based on protein and humic-like fractions, with major implications for ecosystem metabolism, biological oxygen demand, and CO₂ production and alkalinity. Overall, our results showed significant in-stream retention and release (0–100 %) of watershed C and N loads over the scale of kilometers, seldom considered when evaluating monitoring, management, and restoration effectiveness. Given dynamic transport and retention across evolving spatial scales, there is a strong need to longitudinally and synoptically expand studies of hydrologic and biogeochemical processes beyond a stream reach scale along the urban watershed continuum.     

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.     

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

相似文献   

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.