Invasion of a widespread, non-native grass causes downstream reductions in bioavailable silica.
This study shows that riverine riparian (shoreline) zones - and plant management decisions made therein - can measurably influence the availability of silica delivered to lower reaches of rivers, estuaries, and some coastal oceans. The aims were to determine how much silica is sequestered by riparian vegetation in a river, and what processes govern that effect. We used the invasion and rapid spread of the non-native grass Phragmites australis (common reed) in the Platte River (central United States) to quantify the biochemical and physical mechanisms by which silica is removed from the river flow and stored in sediments. Specifically, we measured bioavailable silica in sediment, native and non-native vegetation biomass, and river water over a period of four years. The robust biomass production and high silica content of Phragmites led to biochemical production of amorphous silica particles (ASi) that was 1.5× higher than native willow and 2× higher than native Phragmites. And, the denser growth habit of non-native Phragmites contributed to more rapid physical sequestration of ASi than with other vegetation. Altogether, the rapid vegetation change due to this invasion caused sequestered silica to increase 1.5× in this reach, which is ~7% of the Platte's annual silica load. Subsequent Phragmites eradication efforts, wherein the plants were sprayed with herbicide and left physically intact, allowed rapid silica remobilization but it was highly variable.