INVASION! HOW THE SEDIMENTOLOGICAL EFFECT OF ONE PLANT SPECIES IS CHANGING THE GEOCHEMISTRY OF THE PLATTE RIVER, NEBRASKA

Rivers are the primary source of bioavailable silica (Si) to coastal oceans. Silt-sized amorphous particulate silica (ASi; diatoms, sponge spicules and phytoliths), transported as suspended sediment, is a significant part of that flux because it is more soluble than mineral silicates. The goal of this study was to determine if increased fine sediment deposition due to riparian vegetation could have an unexpected consequence for downstream water quality by increasing ASi sequestration in riverine sediments and therefore decreasing the total amount of ASi reaching marine ecosystems.

Riparian vegetation sequesters ASi in two ways: (1) physical trapping of silt-sized ASi transported from upstream sources and (2) in situ ASi production by plants, followed by ASi accumulation in soils owing to the reduced flow velocities in vegetated stands. Our research was carried out on the Platte River, NE, a classic case study of biogeomorphologic change. Flow reduction in the 20th century allowed vegetation to colonize extensive areas of the riverbed that had been abandoned by flow. Beginning in 2002, the introduced European lineage of Phragmites australis (subsp. australis), colonized more than 500 km of river bed, equivalent to 75 km² of riparian area.

We hypothesized that Phragmites had also sequestered a significant amount of ASi in new wetland soils. In 2010, sediment cores were collected from riparian areas of the Platte River from stands of Phragmites, native willow and from unvegetated areas currently kept free of vegetation by herbicide application and tillage. The average ASi concentration in Phragmites soils was found to be 5 times higher than willow soils and more than 14 times higher than in unvegetated sediments. Multiplying ASi concentration by the total area occupied by Phragmites since it was introduced suggests that the species is responsible for sequestering ~17,000 t of Si in Platte River sediments, approximately equivalent to one year of the river’s dissolved silica load. This study suggests that the sedimentological changes in the river wrought by Phragmites have had a significant impact on geochemical cycles, as well.