Salt Marshes as Sources and Sinks of Silica

Abstract:

The role of salt marshes in controlling silica exchange between terrestrial and marine environments is unclear. In some studies, large quantities of dissolved silica (DSi) appear to be exported from marshes via tidal exchange, potentially fueling future diatom production in adjacent waters. In contrast, other studies report insignificant DSi export and found instead that salt marshes appeared to be Si sinks. Further, few studies examine salt marsh Si export in relation to inorganic nitrogen (DIN) and phosphorus (DIP). We address these uncertainties by quantifying net fluxes of DSi and biogenic Si (BSi), as well as DIN and DIP during the spring and summer in a relatively undisturbed southern New England salt marsh (Narragansett Bay, USA). Our data demonstrates that during the spring, when estuarine waters are deplete in DSi, the marsh serves as a net sink of BSi (132 mol h^-1) and a source of DSi (31 mol h^-1) to the estuary. The spring DIN:DSi ratios of ebbing water were more than five times lower than flood waters. Most importantly, the DSi export rates (6.5 x10³ mol d^-1 km^-2) are an order of magnitude larger than the export by rivers in the region (115 mol d^-1 km^-2), indicating the marsh tidal exchange is vital in supplying the Si necessary for spring diatom blooms in the estuary. Conversely, during the summer the marsh served as a net Si sink, importing on average 59 mol DSi h^-1 and 39 mol BSi h^-1. These data highlight that the role of salt marshes in silica cycling appears to have a strong seasonality. We hypothesize that net import of Si increases the residence time of Si in estuarine systems, providing an important and previously over-looked ecosystem service. In the absence of salt marshes, ~5.1 x 10⁴ kmol of Si would be exported from this system during the growing season, possibly decreasing Si availability and altering phytoplankton species composition in the estuary.