We assembled data from a global network of automated lake observatories to test hypotheses regarding the drivers of ecosystem metabolism. We estimated daily rates of respiration and gross primary production (GPP) for up to a full year in each lake, via maximum likelihood fits of a free-water metabolism model to continuous highfrequency measurements of dissolved oxygen concentrations. Uncertainties were determined by a bootstrap analysis, allowing lake-days with poorly constrained rate estimates to be down-weighted in subsequent analyses. GPP and respiration varied considerably among lakes and at seasonal and daily timescales. Mean annual GPP and respiration ranged from 0.1 to 5.0 mg O 2 L 21 d 21 and were positively related to total phosphorus but not dissolved organic carbon concentration. Within lakes, significant day-to-day differences in respiration were common despite large uncertainties in estimated rates on some lake-days. Daily variation in GPP explained 5% to 85% of the daily variation in respiration after temperature correction. Respiration was tightly coupled to GPP at a daily scale in oligotrophic and dystrophic lakes, and more weakly coupled in mesotrophic and eutrophic lakes. Background respiration ranged from 0.017 to 2.1 mg O 2 L 21 d 21 and was positively related to indicators of recalcitrant allochthonous and autochthonous organic matter loads, but was not clearly related to an indicator of the quality of allochthonous organic matter inputs.Gross primary production (GPP) and respiration are perhaps the two most fundamental processes in ecosystems. At the cellular or organismal level, they describe biochemical pathways that make organic carbon molecules and energy available to cells. When these cellular processes are integrated across an entire ecosystem, the result-ecosystemlevel GPP, ecosystem respiration, or collectively ecosystem metabolism-describes biogeochemical and trophic processes occurring at the system level.There is substantial interest in understanding the controls on ecosystem metabolism in aquatic (Mulholland et al.

We used nondisruptive, whole-stream methods to measure hydraulic characteristics, ecosystem metabolism, and phosphorus cycling in the west fork of Walker Branch (WB), Tennessee and in Hugh White Creek (HWC), North Carolina. Although similar in many of their hydrological and chemical characteristics, transient storage zone volume in HWC was relatively large (-1.5 times that of the flowing water zone), whereas transient storage zone volume in WB was small (-0.1 times that of the flowing water zone). Both streams were highly heterotrophic (gross primary production : total respiration ratios

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SUMMARY 1. We monitored streamwater and streambed sediment porewaters from White Clay Creek (WCC), SE Pennsylvania, for dissolved organic carbon (DOC), dissolved oxygen (DO) and conductivity to investigate organic matter processing within the hyporheic zone. Dissolved organic carbon and DO concentrations were higher in the streamwater than in the porewaters and, in many cases, concentrations continued to diminish with increasing depth into the streambed. 2. Hydrological exchange data demonstrated that the permeability of the stream bed declines with depth and constrains downwelling, effectively isolating porewaters >30 cm from streamwater. 3. End‐member mixing analysis (EMMA) based on conductivity documented a DOC source and DO sink in the hyporheic zone. We calculated hyporheic streambed DOC fluxes and respiration from the EMMA results and estimates of water flux. Based upon our calculations of biodegradable DOC entering the hyporheic zone, we estimate that DOC supports 39% of the hyporheic zone respiration, with the remaining 61% presumably being supported by entrained particulate organic carbon. Hyporheic respiration averaged 0.38 g C m−2 d−1, accounted for 41% of whole ecosystem respiration, and increased baseflow ecosystem efficiency from 46 to 59%. 4. Advective transport of labile organic molecules into the streambed concentrates microbial activity in near‐surface regions of the hyporheic zone. Steep gradients in biogeochemical activity could explain how a shallow and hydrologically constrained hyporheic zone can dramatically influence organic matter processing at the ecosystem scale.

Interstitial temperature and chemistry were examined longitudinally and with depth in a hyporheic zone beneath a riffle–pool sequence of a third-order, sand-bottom river in northern Michigan (USA). Longitudinal and depth patterns were compared with surface and groundwater chemistries at the site and with surface chemistry over a 10-km length of the river. Interstitial water was more characteristic of surface water at the upstream end of the hyporheic zone and of groundwater at the downstream end. Hyporheic longitudinal and depth patterns occurred for temperature, chloride, silica, soluble reactive phosphorus, and winter dissolved oxygen. Dissolved organic carbon decreased with depth, but not with longitudinal distance downstream within the hyporheic zone. Hyporheic nitrate and ammonium patterns were variable, but concentrations generally increased downstream. Hyporheic specific conductance and alkalinity decreased with distance downstream, but not with depth. Gradients for temperature, chloride, silica, nitrate, and phosphate over the 10-km surface water reach were similar to those observed within the single hyporheic zone. Hyporheic chemistry patterns were consistent with models of underflow through porous substrata implying ground water/surface water interaction. Importance of the hyporheic zone should not be overlooked in solute transport or stream nutrient budgets.

Abstract:Twenty conservative tracer injections were carried out in the same reach of a small woodland stream in order to determine how variation in discharge and leaf accumulation aect stream hydraulic parameters. The injections were made at various discharge rates ranging from 2 . 6 to 40 l/s. Five of the injections were made during late autumn, when there were large accumulations of leaves in the stream. Estimates of hydraulic parameters were made by ®tting a transient storage solute transport model to conservative tracer concentration pro®les. Velocity increased almost linearly with increasing discharge, indicating a decline in the Darcy friction factor. Dispersion also increased with increasing discharge, especially for the lower¯ow injections. The relative size of the storage zone was small ($0 . 1). There was no de®nable relationship between discharge and the relative storage zone size, but the rates of exchange between the storage zone and the main channel increased markedly with increasing discharge. The presence of large accumulations of leaves had a clear eect on the hydraulic characteristics of the stream, producing much higher friction factors, larger storage zone sizes and lower velocity than would have been predicted by discharge alone.

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