Fine Particle Transport and Retention in Streams: Particulate Organic Carbon Dynamics and Pathogen Transmission

Rivers transport water and associated dissolved and suspended constituents long distances providing ample opportunity for in-stream processing before reaching the oceans. This thesis specifically focuses on the transport of particulate organic carbon and pathogenic microorganisms. Fine organic particle dynamics are important to stream biogeochemistry and ecology. Both metabolism of particulate organic carbon and transmission of pathogens in river networks are governed by hydrodynamic transport and retention. The majority of particulate organic matter standing stock in streams is less than one millimeter in diameter, and the mobile phase is primarily very fine particles. Such fine particles migrate downstream through a series of deposition and resuspension events, which results in a wide range of residence times. This retention influences biogeochemical processing of particulate organic carbon in streams. Microorganisms also migrate downstream through a series of deposition and resuspension events. The resulting wide range of microbial residence times in the streambed influences the persistence of harmful microbes in the environment. I conducted experiments to gain insights into fine organic particle and microbial dynamics in streams using fluorescent tracer particles. Particle and microbial deposition and filtration in streambeds were observed directly in situ and in sediment column experiments. I developed a stochastic model to describe the transport and retention of fine organic particles and microbes in rivers, including advective delivery to the streambed, transport through 4 porewaters, and reversible filtration within the streambed. I used this model to upscale information on deposition and retention of organic fine particles and microbes in sediments to overall migration through streams. This work advances the understanding of the mechanisms controlling transport of fine organic particles and microbes in streams and rivers under baseflow and floodflow conditions. This work improves the prediction of particulate organic carbon and pathogen transport and retention to aid risk assessment and water management.