Nutrient stoichiometry, freshwater residence time, and nutrient retention in a river-dominated estuary in the Mississippi Delta

A 3-month field study was conducted to examine the effects of Atchafalaya River discharge on nitrogen and phosphorus concentrations in the Fourleague Bay system, to document patterns with salinity variation, to evaluate stoichiometric nutrient ratios of nitrogen and phosphorus in the river and bay, and to examine the relationship between estuarine freshwater residence time and export of total nitrogen (TN) and total phosphorus (TP) to the Gulf of Mexico. During spring peak discharge of the Atchafalaya River, nutrient ratios in lower Fourleague Bay indicate potential phosphorus limitation with an average dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) ratio of 32:1, primarily a result of high concentrations of nitrogen entering the northern bay from the Atchafalaya River and of fairly stable phosphorus concentrations. Ratios of DIN to phosphorus in the river were much higher (54:1), indicating a significant loss of nitrogen within the Fourleague Bay system. Freshwater residence time averaged approximately 7 days during the study and ranged from 2 to 100 days. TN export averaged 57% over the study and ranged from less than 3% at long residence times to greater than 80% at short residence times. TN export to the coastal ocean with respect to residence time is considerably less than has been shown in other studies. Nitrate + nitrite export averaged 49% for the 3-month study. Percentages of TP export were greater than TN, averaging 82% for the study period. By examining the Atchafalaya River delta as a natural analog for controlled river diversions, which are currently being used as coastal restoration tools, this study shows that discharging river water into highly productive shallow coastal estuarine and wetland systems can significantly reduce the amount of nitrogen exported to the Gulf of Mexico.