Controls on Patterns of Coarse Organic Particle Retention in Headwater Streams

Abstract

Organic matter retention is an integral ecosystem process affecting C and nutrient dynamics and biota in streams. Influences of discharge (Q), reach-scale channel form, and riparian vegetation on coarse particulate organic matter (CPOM) retention were analyzed in 2 headwater streams in northeastern Oregon. Ginkgo biloba leaves were released in coniferous forest reaches and downstream floodplain meadow reaches during spring high flow and summer baseflow. Transitional reaches were also analyzed during summer baseflow. Paper strips, simulating sedge blade retention, were released in meadow reaches during high flow. Mean transport distances (S_P) were calculated as the inverse of the longitudinal loss rate (k) of leaves in transport. The metrics S_P, width-specific discharge (Q_W = Q/stream width), and the mass transfer coefficient (v_dep=Q_W/S_P) were used to investigate retention. Values of S_P (0.9-97 m) were 2 to 11 times longer during high flow than baseflow. Mean S_P in forest reaches (29.3 m) was significantly shorter than in meadow reaches (68.9 m) during high flow but not during baseflow. Standardizing k for the scaling effects of Q by analyzing the relationship between Q_W and S_P, in which the slope equaled the inverse of mean v_dep of all Ginkgo releases, indicated times when v_dep was higher or lower than predicted by Q. Values of S_P were driven largely by Q, yet most experiments in which values of v_dep exceeded those predicted by Q_W occurred during high flow. Values of v_dep (0.3-32 mm/s) across experiments were generally inversely related to S_P but did not differ between forest and meadow reaches during high flow. Unlike meadow reaches, mean v_dep in forest reaches was higher during high flow (5.2 mm/s) than baseflow (1.1 mm/s). Values of v_dep were positively related to large wood volume and negatively related to the extent of floodplain inundation during high flow. Yet, in the meadow reach that had lower relative channel constraint, paper strips were transported farther onto the floodplain as Q rose, resulting in long-term (∼1.5 mo) retention. Despite downstream increases in Q, there were no differences in mean baseflow S_P or v_dep among reaches in either stream, indicating some longitudinal compensation in retention. Alternating associations between retention metrics and structural elements of the stream channels between flow periods suggests dynamic reach-scale hydrologic-retention thresholds in response to changes in Q. Analysis of v_dep across experiments indicated that channel morphology, stream wood, and riparian vegetation are major controls on CPOM retention.