Benthic community metabolism was studied on four stream systems located in different biomes in the United States: the eastern deciduous forest (Pennsylvania, PA, and Michigan, MI), the high desert (Idaho, ID), and the coniferous forest (Oregon, OR). Studies were designed to test the hypothesis advanced within the River Continuum Concept that a transition in community metabolism will occur from a predominance of heterotrophy in headwaters to a predominance of autotrophy in mid-sized reaches, with a return to heterotrophy further downstream. Both gross primary productivity (GPP) and community respiration (CR 24 ) increased with downstream direction on all systems. Net daily metabolism (NDM, or GPP -CR 2 4 ) shifted from heterotrophy (-NDM, GPP < CR 24 ) to autotrophy (+NDM, GPP > CR 2 4 ) with downstream direction at all sites, supporting the hypothesis. Annual metabolism in the most upstream reach of all sites was dominated by respiration; however, the farthest downstream reach was not necessarily the most autotrophic. Site-specific factors affected manifestation of the trend. Photosynthesis predominated annual metabolism in reaches (designated 1-4 in order of increasing size) 2-4 in ID, 3 and 4 in OR, and 4 in MI. In PA annual photosynthesis was slightly greater than respiration only at Station 3. Photosynthesis was predominant most consistently in ID and respiration most often in PA. About half the reaches that were heterotrophic annually were autotrophic at one or more seasons. Annual means of benthic GPP, CR 24 and NDM ranged from 0.16 to 3.37, 0.36 to 2.88 and -0.73 to 0.50 g 02 m 2 d 1, respectively. Metabolic rates were usually high in PA and Ml (and sometimes ID) and almost always lowest in OR. Parameters accounting for most variance in multiple linear regression analyses of the combined metabolism data from all sites were indicators of stream size, photosynthetically active radiation, temperature, and chlorophyll a concentration.

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Carbon dioxide and oxygen exchange procedures for measuring community metabolism (two open stream methods and three chamber methods) were compared on the same reach of a thirdorder stream . Open stream methods were complicated by high diffusion rates and yielded net community primary productivity estimates lower than those obtained with chamber methods . Chamber methods yielded variable productivity and respiration data . However, when normalized for chlorophyll a, productivity estimates from the chamber methods were within an expected range for the system . Balances of photosynthesis and respiration from the chamber methods were similar between methods and indicated that autotrophic or heterotrophic processes could dominate the system . Considerations in applying the various procedures are discussed .

Groundwater-surface-water (GW-SW) interactions in streams are difficult to quantify because of heterogeneity in hydraulic and reactive processes across a range of spatial and temporal scales. The challenge of quantifying these interactions has led to the development of several techniques, from centimeter-scale probes to whole-system tracers, including chemical, thermal, and electrical methods. We co-applied conservative and smart reactive solute-tracer tests, measurement of hydraulic heads, distributed temperature sensing, vertical profiles of solute tracer and temperature in the stream bed, and electrical resistivity imaging in a 450-m reach of a 3 rd -order stream. GW-SW interactions were not spatially expansive, but were high in flux through a shallow hyporheic zone surrounding the reach. NaCl and resazurin tracers suggested different surface-subsurface exchange patterns in the upper ⅔ and lower ⅓ of the reach. Subsurface sampling of tracers and vertical thermal profiles quantified relatively high fluxes through a 10-to 20-cm deep hyporheic zone with chemical reactivity of the resazurin tracer indicated at 3-, 6-, and 9-cm sampling depths. Monitoring of hydraulic gradients along transects with MINI-POINT streambed samplers starting ∼40 m from the stream indicated that groundwater discharge prevented development of a larger hyporheic zone, which progressively decreased from the stream thalweg toward the banks. Distributed temperature sensing did not detect extensive inflow of ground water to the stream, and electrical resistivity imaging showed limited large-scale hyporheic exchange. We recommend choosing technique(s) based on: 1) clear definition of the questions to be addressed (physical, biological, or chemical processes), 2) explicit identification of the spatial and temporal scales to be covered and those required to provide an appropriate context for interpretation, and 3) maximizing generation of mechanistic understanding and reducing costs of implementing multiple techniques through collaborative research.

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e t a l .S t r e a m m a c r o i n v e r t e b r a t e s o v e r 1 0 y e a r s f o l l o wi n g w i l d f i r e W F 0 1 0 1 8 G . W a y n e M i n s h a l l , C h r i s t o p h e r T . R o b i n s o n , D e r o n E . L a w r e n c e , D o u g l a s A . A n d r e w s a n d J a m e s T . B r o c kAbstract. The effects of wildfire on benthic macroinvertebrate assemblages of streams in mixed-conifer forest were examined for 10 successive years following the Mortar Creek Fire of 1979. Changes in burned-catchment streams were evaluated relative to a paired set of reference-catchment streams. Taxa richness and total abundance tended to be lower in burn than in reference streams but to converge near the end of the study; increases in the final years in both burn and reference streams were associated with reduced flows due to drought. Total biomass and that of the scraper, filterer, and miner functional groups usually were greater in the burn streams. Lack of a strong relationship of macroinvertebrate metrics with weather conditions showed that factors specific to each stream also were influencing the biotic community. Mean among-year Jaccard similarity was lower for burn than for reference streams. Specific taxa responded differently to the effects of fire. Densities of disturbance-adapted forms (e.g. Chironomidae, Baetis) increased after the fire but not during the drought period of more stable flows at the end of the study; many other taxa showed the opposite response. Adverse effects of wildfire on the biotic community were largely the result of physical changes in habitat due to increased runoff. Timing and magnitude of effects differed widely among streams as a result of differences in stream size, burn severity, and specific storm or snowmelt events. Though major effects of the fire on the macroinvertebrates dissipated within 7 years, adjustment in the habitat and biotic conditions still were taking place at the end of 10 year and normal recovery patterns may have been obscured by the drought.

The Mortar Creek Fire burned 26 000 ha of mixed-conifer Rocky Mountain forest in July-August 1979. Changes in burn stream conditions were examined relative to reference streams for various ecological factors on two to six occasions, from October 1979 to August 1980. Factors included major ions and nutrients, suspended and benthic particulate matter, periphyton (algae), and macroinvertebrates. Elevated levels of most dissolved chemicals in the burn streams were evident soon after the fire and again during spring runoff. However, there were no major disruptions in the relative composition of cations (and presumably of anions also) in the burn streams during the study. Concentration (mg/L) and load (g/s) of some constituents were higher (e.g. NO 3 -N) and of others (e.g. Ca) were lower in the burn than in the reference streams during spring runoff, depending on whether they were normally under biological or geological control, respectively. Suspended sediment and particulate organic matter generally were higher in burn streams, especially during snow-melt runoff or following heavy rain storms. Benthic organic matter was higher in burn streams and was mainly charcoal, compared to the usual leaf litter found in the reference streams. Fine sediments increased and periphyton decreased in the burn streams. The fire increased the sensitivity of the burn streams to more routine smaller-scale disturbances, such as rainstorms, which had major impacts on the burn streams but not on the reference streams. The macroinvertebrate assemblage showed little direct effect from the fire but was severely altered in composition and abundance by the subsequent runoff, scouring, and channel alteration initiated by spring runoff. The burn streams showed considerable individuality in their response to fire depending on the particular set of conditions to which they were exposed. s o f I d a h o s t r e a ms t h e f i r s t y e a r a f t e r w i l d f i r e WF 0 1 0 1 7 G . W a y n e M i n s h a l lA , J a m e s T . B r o c k , D o u g l a s A . A n d r e w s , a n d C h r i s t o p h e r T . R o b i n s o n

Experiments were performed to compare 14 C uptake, dissolved O 2 and CO 2 change, and microelectrode techniques for measuring benthic primary productivity and community metabolism at a site on the South Saskatchewan River, Saskatchewan. The highest estimates of primary productivity were obtained with O 2 microelectrodes (two to four times higher than simultaneous estimates from bulk-water dissolved O 2 measurements), presumably because measurements are biased toward active communities under optimal conditions for photosynthesis. In daytime experiments lasting~1.3 and 2.4 h, estimates from 14 C uptake were higher than those measured simultaneously from dissolved O 2 and CO 2 change, suggesting that 14 C uptake measured gross primary production (GPP) in these short-term incubations. Differences in experimental water velocity ranging from 0.2 to >6.5 cm⋅s -1 had less of an effect than chlorophyll a concentration on metabolic rates in diel experiments with rocks taken from a site where velocities were from 5.6 to 7.8 cm⋅s -1 . There was a potential for phosphorus and possibly NH 4 limitation of metabolic activity during long-term incubations. Results highlight areas in which further research is needed when using chambers to determine periphyton metabolism.Résumé : Nos expériences visaient à comparer l'assimilation du 14 C, les changements dans l'O 2 dissous et le CO 2 et les techniques faisant appel à des microélectrodes pour mesurer la productivité primaire benthique et le métabolisme des communautés sur un site de la rivière Saskatchewan sud (Saskatchewan). Ce sont les microélectrodes à O 2 qui ont donné les estimations les plus élevées de la productivité primaire (deux à quatre fois plus élevées que les estimations simultanées des mesures de l'O 2 dissous dans l'eau libre), ce qui est probablement dû au fait que les mesures sont biaisées en faveur des communautés actives dans les conditions idéales de la photosynthèse. Dans des expériences en lumière du jour d'une durée dẽ 1,3 et 2,4 h, les estimations établies à partir de l'assimilation du 14 C étaient plus élevées que celles fournies par les mesures simultanées du changement de l'O 2 dissous et du CO 2 , ce qui permet de penser que l'assimilation du 14 C mesurait la production primaire brute dans ces incubations de courte durée. Les différences dans la vitesse de l'eau pendant les expériences, de l'ordre de 0,2 à >6,5 cm⋅s -1 , avaient moins d'effet que la concentration de chlorophylle a sur le métabolisme dans les expériences diurnes réalisées avec des roches prélevées sur un site où les vitesses étaient de l'ordre de 5,6 à 7,8 cm⋅s -1 . Les incubations de longue durée présentaient un potentiel de limitation du métabolisme du phosphore et peut-être du NH 4 . Les résultats font ressortir les domaines où il est nécessaire de poursuivre les recherches quand on utilise des enceintes pour mesurer le métabolisme du périphyton. [Traduit par la Rédaction]Can.