Short communicationThe potential for multiple signatures of invasive species in the geologic record

During the Anthropocene, the rapid spread of invasive species into new habitats has repeatedly led to major ecosystem change. Invasive species succeed by outcompeting native species or significantly altering the physical or chemical properties of a habitat. Here we propose that invasive species, which by definition have a large impact on an ecosystem, will leave multiple signatures in sediments. As a case study, a reach of the Platte River (Nebraska, USA) where a Eurasian lineage of the grass Phragmites australis invaded the river channel in 2003, was investigated. This study focuses on the effects of this invasion on silica, a micronutrient that can limit productivity of phytoplankton like diatoms. Dissolved silica (DSi) and particles of amorphous silica (ASi), which are produced by phytoplankton and terrestrial plants, comprise the pool of bioavailable silica in aquatic ecosystems. ASi concentrations were measured in sediments accumulated under Phragmites stands. These values were compared to ASi concentrations in unvegetated sediments and sediments accumulating under native Salix spp. (willows). Non-native Phragmites sediments had more than twice as much ASi as Salix sediments, and a first-order estimate revealed that in 8 years Phragmites sequestered approximately half a year's worth of the DSi load carried by the Platte. Given the importance of ASi in driving aquatic food webs, increased storage due to Phragmites is likely having a substantial effect on downstream ecosystems. Furthermore, because Phragmites accumulated thicker layers of fine-grained sediments, we demonstrated that this invasive species is leaving a record of both physical and biochemical ecosystem change.