This observation confirms measurements of sediment deposition made by Pollen-Bankhead et al. (2012). And, the invasive Phragmites sequesters substantially more ASi in the top 10-cm of sediments than does native willow, while any difference between native willow and unvegetated sediments is not detectable with this common analytical method. ASi is typically in the silt-size range, so the river’s suspended load of ASi was deposited along with fine particles of Anti-cancer Compound Library mineralogic sediment in low velocity stands of Phragmites. However,

because Phragmites is a relatively prolific producer of ASi particles, it is likely that in situ production of ASi accounts at least in part for the high Z-VAD-FMK ASi content of these sediments.

In other words, two different processes – physical sequestration and biogenic production – are likely at work, and future studies will need to disentangle the two effects on ASi accumulation in river sediments. In this study, the top 10 cm of sediment at each site were analyzed because field observations indicated that most fine-grain deposition occurred within that depth, and laboratory analyses confirmed that sediments at 10–20 cm depth had negligible ASi. However, it is important to note that sediment erosion and deposition in rivers, and in particular in anabranching rivers like the Platte, is complex and spatially heterogeneous. It is possible that for any given site, a recent high flow buried an ASi-rich sediment layer under a thick deposit of sand or eroded a former ASi-rich deposit. Indeed, four cores contained buried organic-rich layers containing Phragmites rhizomes, suggesting that some burial occurred within the previous 8 years (when Phragmites first invaded this river). In other words, these data represent a snapshot of the riverbed at the time the samples were PAK5 collected with no guarantee that sediment has been deposited and preserved in a spatially and temporally continuous manner. Nevertheless, flow and sediment dynamics during high flows at any given site are not independent

of vegetation type: Phragmites has a denser stem network than native willows and therefore its presence will diminish flow velocity and transport capacity through the patch. We expect this local and temporal variability to be less pronounced in longer-term geologic records or in studies of more spatially extensive environments. The rough estimate of 9500 t of additional ASi sequestered in Phragmites sediments can be contextualized by calculating the annual silica load being transported by the Platte. Unfortunately, few measurements of silica in the Platte exist. The calculated river load of 18,000 t DSi yr−1 reported here, based on 3 years of DSi monitoring in the mid-1990s, serves as a pre-Phragmites baseline.