CITY SHORTCUTS: DOCUMENTING SI EXPORT PATHWAYS IN AN URBAN ECOSYSTEM

Urban systems are hot spots of biogeochemical cycling. As such, the impact of anthropogenic activities on the cycles of carbon, nitrogen, and phosphorus in these environments is well described. Less well understood is how the cycling of another important element, silicon (Si), is altered in urban ecosystems. Identifying how urban systems alter the flux of Si to the ocean is critical to our understanding of marine and terrestrial primary productivity, higher trophic levels, and global climate. Here we describe the annual load of dissolved Si (DSi) in the influent of the 2^nd largest wastewater treatment plant (by volume) in the United States (Deer Island Wastewater Facility, Boston, MA). We partition the influent load between three sources: runoff (13%), sewage (40%), and groundwater infiltration (47%). We found that rather than being delivered to local rivers, DSi in groundwater and runoff is captured by the combined storm water-sewage infrastructure. Thus, instead of being available for biological uptake along the land-ocean continuum, Si in urban systems appears to shortcut traditional nutrient delivery processes by traversing the watershed within our subterranean pipes. The potential impact of this direct passage from watershed to sea is yet unknown but could influence coastal phytoplankton species assemblage and productivity.