Turbidity Structures the Controls of Ecosystem Metabolism and Associated Metabolic Process Domains Along a 75-km Segment of a Semiarid Stream

Stream ecosystem metabolism contributes to global carbon cycling, yet predicting metabolism across ecosystems remains elusive. Even within stream segments, spatial variation in metabolic rates and their controls can be substantial, exhibiting sudden rather than continuous changes. We measured metabolism at 6 sites along a 75-km mainstem segment of Marsh Creek, Idaho. This agricultural stream lacks major geomorphic transitions such as tributaries or changes in valley width, but possesses patchy patterns of turbidity. We asked: (1) How variable is metabolism along this segment, and (2) How do the controls on metabolism vary along this segment? Metabolism varied several-fold along this stream segment. Average rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) among sites did not correlate with water quality or aquatic macrophyte cover. Rather, reaches along this segment appear to represent different process domains that were characterized by turbidity: More turbid reaches saw negative effects of turbidity on GPP and NEP and positive effects on ER. Turbidity was associated with increased respiration and decoupled ER from GPP at turbid sites. Less turbid sites had stronger coupling between GPP and ER, and GPP was predicted by light and temperature. Heterogeneous sediment supply and transport capacity along Marsh Creek create patchy patterns of turbidity, which affects the controls on metabolism. Geomorphic controls on ecosystem processes are complex but not random. Understanding how metabolism and its controls vary across process domains is needed to scale up process rates and understand how process rates might respond to future changes.