Stream Community Metabolism and Dissolved-Oxygen Dynamics: Where Did the Oxygen Come From?

Stream metabolism is traditionally defined as the combined metabolism of all aerobic organisms in a stream. Its component processes of oxygenic photosynthesis and aerobic respiration create and consume dissolved oxygen (DO) and therefore can be measured using time series of DO concentration, solar radiation, and water temperature, in conjunction with a model of DO dynamics that includes photosynthesis, respiration, and oxygen exchange with the atmosphere. A complication is that stream communities typically exhibit pronounced longitudinal heterogeneity in habitat type (e.g., shaded versus unshaded reaches) and species composition and abundance. The influence of a given stream reach and associated community on DO concentration propagates downstream with the current, gradually being replaced, over a transition zone, by the influence of the next downstream reach. Knowing the approximate length of this transition zone is important when estimating stream metabolism with one-station DO monitoring, since it indicates which stream reach and associated community the metabolism estimates apply to. We propose new methods for estimating the transition-zone length and for estimating the proportions of DO at a given location in a stream reach that entered the reach from upstream, from photosynthesis within the reach, and from atmospheric uptake within the reach. We also propose methods for estimating the residence-time distribution of DO present at a given stream location, and the corresponding distribution of upstream distances at which the DO entered the stream. Both distributions are approximately exponential. Thus, habitat immediately upstream of the sonde has the greatest influence on metabolism estimates with one-station monitoring, and it is therefore important to place the sonde so this habitat is representative of the study reach.