THE REDUCTION OF RIVERINE SILICA TRANSPORT DUE TO INVASIVE RIPARIAN VEGETATION

The plant Phragmites australis subs. australis, also known as the common reed, has spread widely across the United States after its introduction on the eastern coast. On the Platte River in central Nebraska, one effect of Phragmites invasion is that previously unvegetated or lightly vegetated banks and islands have become stabilized by the vegetation. This has allowed sediment to accumulate where it might otherwise have been transported downstream. Phragmites may additionally reduce the amount of dissolved silica in rivers with riparian zones dominated by it due to the production of silica phytoliths by the plant. Also, the dense growth habit of Phragmites may locally slow river velocity and cause deposition of the river’s suspended load, which contains some biogenic silica like diatoms, freshwater sponge spicules and phytoliths that have been washed into the river. The combination of the biological and physical effects could significantly decrease the total load of silica flowing down the river. This study explored the differences in biogenic silica concentration in Platte River sediments occupied by live Phragmites, recently killed Phragmites, native Salix (willow), and unvegetated tracts. A sodium hydroxide digestion was used to dissolve biogenic silica in sediment, the silica was then quantified using a molybdate colormetric test UV visualization and also inductively coupled plasma mass spectrometry (ICP-MS). The study showed that biogenic silica concentrations are higher in places dominated by live Phragmites, indicating a loss of bioavailable silica in the river system. Biogenic silica concentrations in sediments underlying killed Phragmites are lower, suggesting that when the vegetation has died the silica is re-released back to the river system on a relatively short time scale.