Anna Harrison's Dissertation Defense - "Investigations into the Hyporheic Zone: Assessing the Influence of Ground-Water Surface Water Interactions on Metal Exposure and Effects to Aquatic Ecosystems"
Rackham Graduate School
Title: Investigations into the Hyporheic Zone: Assessing the Influence of Ground-water Surface Water Interactions on Metal Exposure and Effects to Aquatic Ecosystems
Metal contaminated sediments are a common stressor on biological communities in freshwater ecosystems. Sediments have the capacity to store metals (via various chemical binding ligands in sediments), rendering them unavailable for uptake by biological communities. Physical processes in streams, however, can influence the chemistry of sediments and ultimately control how sediments store metals. Groundwater-surface water interactions (hyporheic flows) are an example of a natural physical process that influences sediment chemistry and potentially the exposure of metals to aquatic biota. Hyporheic flows are generally characterized as either ‘downwelling’ (surface water entering streambed sediments) or ‘upwelling’ (from sediments into surface water). Using multiple lines of evidence (field, mesocosm, and laboratory studies), this dissertation investigates the role of hyporheic flows on metal exposure and effects to aquatic organisms. Biological assessments included: test organism (Hyalella azteca) survival, benthic macroinvertebrate community composition, and biofilm structure and function. Oxygenated (oxic) hyporheic flows (downwelling) increased the concentration of bioavailable metals, and subsequent effects on biota were observed. In experiments with upwelling hyporheic flows, sediments had less oxygen (more reduced) and metals were less bioavailable. This research demonstrates the importance of hyporheic flows on redox sensitive binding ligands and the subsequent effects on aquatic biological communities. Inclusion of hyporheic flows in ecological risk assessments could more accurately characterize metal exposure pathways to aquatic biota.