Streams are constantly subject to downstream movement of materials. The role of fallen leaves in resisting downstream transport of particulates is largely unquantified. The litter exclusion study at Coweeta presented the opportunity to study a stream without litter input. I expected removal of leaf litter to reduce the capacity of the stream to retain fine particulate organic matter, FPOM. However, leaves are also a major source of FPOM. I studied the effect of leaf exclusion on FPOM transport by field sampling and by generating computer simulations of particle transport in the stream. Sampling of suspended particles during storms showed that although litter inputs and subsequent particle generation were greatly decreased (Wallace et al. 1997), storm exports did not differ significantly from those of the reference stream. This suggested that the effect of litter exclusion was to reduce FPOM retention. Although there was no new organic matter entering the stream during the exclusion period, entrainment of stored material compensated for it. The computer simulations predicted higher concentrations of FPOM for storms after litter exclusion than I actually measured except during heavy rains that greatly increased discharge. These results suggested that after litter exclusion, low-intensity storms exported lower concentrations than before exclusion. However, after exclusion, intense storms that greatly increased discharge entrained higher concentrations of FPOM. Both field studies and computer models indicated that stability of the litter-excluded streambed was lower compared to the reference and pre-treatment streams, and stability was further reduced with increased discharge. / Master of Science

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. / Master of Science

Seston is the complex of organic and inorganic particles suspended in the water column in stream ecosystems. This detrital pool is a significant vector of energy and nutrients from headwaters to the ocean. Many of the processes involved in seston generation in streams involve the terrestrial ecosystems they drain. My objective was to determine how land use influences seston quality. Seston was collected from 9 streams along a gradient of catchment forest cover, draining 3 land-use categories: forested, agricultural, and residential. Quality variations were determined through the physical composition (size classes and concentration), chemical composition (carbon, nitrogen, and phosphorous) and organic content of seston, as well as microbial respiration of associated assemblages and growth rates of Chironomus tentans fed seston. There was more seston in streams draining less forested catchments but proportionally that material was less organic. Ultrafine particles dominated seston size classes at all sites and was higher in streams with less catchment forest cover. Phosphorous content of seston, as a % of ash free dry mass, and dissolved P in the water were higher in more disturbed streams, and C:N and N:P ratios of seston were higher in forested streams. Microbial respiration and instantaneous growth rates of C. tentans were highest on residential seston, indicating seston from those streams was more bioavailable. These results indicate that the quality of seston in stream ecosystems is linked to terrestrial processes and influenced by land use. / Master of Science