Lay Abstract The exchange of gasses between water and air is important to the budgets of carbon, nutrients, and pollutants. This exchange is driven, in part, by the turbulent energy at the air–water interface. Turbulent energy at the air–water interface scales with the gas transfer velocity (k), which can be measured in streams through various methods. We performed a metadata analysis of studies that have measured k in streams using direct gas tracer releases. We evaluated models that predict k based on stream morphology. We found that models that use slope and velocity to predict k perform reasonably well and are consistent with general theory. We also used the data set to provide new stream hydraulic equations that predict stream morphology (width, depth, velocity) based on discharge.

A study of 16 streams in eastern North America shows that riparian deforestation causes channel narrowing, which reduces the total amount of stream habitat and ecosystem per unit channel length and compromises in-stream processing of pollutants. Wide forest reaches had more macroinvertebrates, total ecosystem processing of organic matter, and nitrogen uptake per unit channel length than contiguous narrow deforested reaches. Stream narrowing nullified any potential advantages of deforestation regarding abundance of fish, quality of dissolved organic matter, and pesticide degradation. These findings show that forested stream channels have a wider and more natural configuration, which significantly affects the total in-stream amount and activity of the ecosystem, including the processing of pollutants. The results reinforce both current policy of the United States that endorses riparian forest buffers as best management practice and federal and state programs that subsidize riparian reforestation for stream restoration and water quality. Not only do forest buffers prevent nonpoint source pollutants from entering small streams, they also enhance the in-stream processing of both nonpoint and point source pollutants, thereby reducing their impact on downstream rivers and estuaries.

Studies were conducted in four distinct geographic areas (biomes/sites) in northern United States to examine changes in key ecosystem parameter: benthic organic matter (BOM), transported organic matter (TOM), community production and respiration, leaf pack decomposition, and functional feeding—group composition along gradients of increasing stream size. Four stations ranging from headwaters (1st or 2nd order) to midsized rivers (5th to 7th order) were examined at each site using comparable methods. The results for each parameter are presented and discussed in light of the River Continuum Concept of Vannote et al. (1980). The postulated gradual change in a stream ecosystem's structure and function is supported by this study. However, regional and local deviations occur as a result of variations in the influence of: (1) watershed climate and geology, (2) riparian conditions, (3) tributaries, and (4) location—specific lithology and geomorphology. In particular, the continuum framework must be visualized as a sliding scale which is shifted upstream or downstream depending on macroenvironmental forces (1 and 2) or reset following the application of more localized "micro"—environmental influences (3 and 4). Analysis of interactions between BOM and TOM permitted evaluation of stream retentiveness for organic matter. Headwaters generally were most retentive and downstream reaches the least. Estimates of organic matter turnover times ranged between 0.2 and 14 yr, and commonly were 1—4 yr. Both turnover times and distances were determined primarily by the interaction between current velocity and stream retention. Biological processes played a secondary role. However, the streams varied considerably in their spiraling of organic matter due to differences in the interplay between retentiveness and biological activity. Differences in the relative importance of retention mechanisms along the continuum suggest that headwater stream ecosystems may be functionally more stable, at least to physical disturbances, than are the r intermediate river counterparts.

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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.

Diel fluctuations in stream water temperature and chemistry, microbial biomass, and bacterial activity were measured in White Clay Creek, Pennsylvania, during vernal algal blooms in three different years. DOC concentrations increased 24-37% over early morning minima and temperature increased nearly 10°C over a 7-10-h period. Total carbohydrates and monosaccharides exhibited irregular fluctuations with total carbohydrates showing concentration peaks in the morning and afternoon. Acetate concentrations were highest in midafternoon, while the concentration pattern for primary amines differed from the DOC pattern with highest values at midnight. No distinct diel patterns were found for streambed ATP, GTP : ATP, Chl a, and total or active bacteria, although significant year-to-year and between-habitat differences were observed. Bacterial activity, measured by phospholipid biosynthesis, total lipid biosynthesis, respiration, and incorporation of [3H]thymidine into DNA increased 1.4-fold to 3.0-fold from morning to afternoon. Microcosm experiments indicated that the activity of bacteria attached to sediments was more sensitive to increases in water temperature than to changes in water chemistry, whereas bacteria attached to porcelain disks responded to the influences of both temperature and water chemistry.

Diel fluctuations in dissolved organic carbon (DOC) were measured in White Clay Creek, a piedmont stream in SE Pennsylvania. DOC concentrations, measured on 16 days from late March to early June and on 2 days in early November, 1978 and 1979, showed rapid increases from predawn minima to late afternoon maxima and gradual decreases after sunset. Concentrations increased during a single day by as much as 40% of the daily minimum. These measurements were made during periods of constant discharge and were not related to volume of flow or changes in groundwater DOC concentrations. Laboratory and microcosm experiments suggested that benthic algae excreted most of the DOM and that bacterial uptake modified its concentration and composition. When the diel pulse was strongest, net algal DOC excretion accounted for 20% of the total DOC exported from the watershed that day. Additional data show that the 14C method of measuring algal excretion is quantitatively and qualitatively inaccurate for periphyton incubated for 2–4 h.

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