Effects of discharge and substrate characteristics on FPOM retention

I released chloride and corn pollen into artificial streams to study the effects of discharge, substrate size, leaf packs, and pools on the retention and transport of fine particulate organic matter (FPOM). I found that doubling discharge significantly decreased both hydraulic and FPOM retention (measured as pollen uptake length). However, quadrupling discharge returned both hydraulic and FPOM retention to original or higher values. There was a strong positive relationship between FPOM depositional velocity and discharge (r² = 0.846, p < 0.0001), and between FPOM depositional velocity and turbulence (r² = 0.831, p < 0.0001). Depositional velocity for all experiments was considerably less than predicted by the “fall-velocity model”. Substrate size controlled hydraulic retention through the size of interstitial spaces in the bed and FPOM retention through substrate-created turbulence. Small gravel substrate had the largest transient storage zone relative to stream cross-sectional area. Large gravel substrate had the highest depositional velocity of pollen and the most turbulence. Cobble had the least hydraulic and FPOM retention. Adding leaf packs significantly decreased hydraulic and FPOM retention. However, the number of leaf packs made little difference. Adding pools significantly increased hydraulic retention and FROM depositional velocity.