This project will evaluate the role of riparian (riverbank) vegetation in the transfer of bioavailable forms of silica (Si) from land to sea via rivers. Plants absorb dissolved silica (DSi) from water and transform it into amorphous particles (ASi) that accumulate in soils. Therefore, one factor that can alter the flux of Si in rivers is a change in vegetation cover in the active riverbed. Changes in riparian vegetation may cause significant reductions in riverine Si transport for two reasons. First, plants sequester Si by transforming DSi into ASi. Second, vegetation leads to reduced river flow velocity and increased deposition of suspended sediment, including ASi. Two highly-impacted rivers, the Platte River in Nebraska and the Green River in Utah, will serve as case studies to quantify the effect that riparian vegetation in general, and the invasive species of grass Phragmites australis in particular, have on Si transport to the ocean. Measurement of three pools of ASi ? in riparian sediment, riparian vegetation, and suspended sediment in the river ? and DSi in the river will be used to determine the magnitude of the vegetation effect. This project will consider silicon geochemistry, plant identification and characterization, and particle transport and deposition to develop a holistic understanding of the physical-biological-biogeochemical interactions controlling riverine Si transport.
Rivers are the primary source of silicon to coastal ocean ecosystems, where it is often a limiting nutrient for important groups of phytoplankton. An array of human activities has decreased the delivery of Si from land to sea, which is a significant concern for marine ecosystems already under pressure from a variety of environmental changes. Human modifications to river flows may lead to conditions favoring the expansion of riparian vegetation, which may reduce Si transported by the river. Our research will be the first isolate the effects of riparian vegetation on Si transported by rivers, and to measure the total Si flux in two typical western U.S. rivers. Therefore, the work will improve our understanding of the land-river-sea continuum, and may inform policy decisions regarding rivers and coastal ocean resource management. This collaborative project brings together the expertise of the PI and six undergraduate researchers from Gustavus Adolphus College, a primarily undergraduate institution, and the co-PIs from Utah State University and the University of Aix-Marseille. The undergraduate researchers will be involved in all aspects of the project, ultimately including dissemination of results via publications, conference presentations and development of a college museum exhibit.
