Changes in stoichiometric Si, N and P ratios of Mississippi River water diverted through coastal wetlands to the Gulf of Mexico

Abstract

During the spring of 2001, we monitored nutrient concentrations and stoichiometry of diverted Mississippi River water as it flowed through the Breton Sound estuary, Louisiana, USA. River water was discharged through a diversion structure at Caernarvon as a two-week pulse that peaked at 220 m 3 s −1. There were reductions in observed concentrations of TN, TP, DIN, DIP and DSi, of up to 44%, 62%, 57%, 23%, and 38%, respectively, as water flowed through the estuary. TN, TP, DIN, DIP and DSi concentrations in the river were 137–140, 5.0–5.1, 104–153, 1.1–1.3 and 114–121 μM, respectively, and 36–122, 1.8–3.6, 13–119, 0.3–1.8 and 29–110 μM, respectively, at the Gulfward end member stations. The DSi:DIN ratio rose from 0.9 at the Caernarvon diversion to 2.6 at the Gulf end member station, while the DIN:DIP ratio fell from 107 to 26. This study shows that freshwater diversions can significantly alter riverine nutrient concentrations and ratios and reduce the overall amount of exported nitrogen.