Salt marsh tidal exchange increases residence time of silica in estuaries

Abstract

We present flux measurements of dissolved silicon (DSi), biogenic Si (BSi), and dissolved inorganic nitrogen (DIN) and phosphorus (DIP) on a temperate salt marsh in Narragansett Bay, Rhode Island, over neap, mid, and spring tide cycles during the spring and the summer seasons to determine the stoichiometry of marsh tidal export and the potential effect on Si availability in the receiving estuary. During the spring, when incoming waters were depleted in DSi (average 1.7 µmol L−1), the marsh was a net sink of BSi (132 mol h−1) and a source of DSi (31 mol h−1) to the estuary. During this period, the DIN : DSi ratio of ebbing water was more than five times lower than that of flood water. Together, these data indicate that marsh nutrient fluxes “outwell” nutrients in ratios that support spring diatom growth in Narragansett Bay. However, during the entire sampling period the marsh served as a net sink for both DSi and BSi, importing, on average, 14 mol DSi h−1 and 86 mol BSi h−1. We hypothesize that the net import and retention of Si by salt marshes provides a previously overlooked ecosystem service of increasing the residence time of Si in estuarine systems. In the absence of salt marshes, we calculate that 5.1 × 104 kmol of Si would be exported from the estuary during the growing season, decreasing the availability of Si in the system and having direct repercussions for phytoplankton species composition in nearby estuarine waters.