Linda F. Benedict, Xu, Yi Jun
April Bryant-Mason and Y. Jun Xu
The Mississippi-Atchafalaya River carries a large quantity of nutrients, making Louisiana’s estuarine and coastal waters highly productive for commercial fisheries. However, excessive riverine nutrients, especially nitrate nitrogen, have caused algal growth and the consequent oxygen depletion in the summer over the past two decades. This condition, also known as hypoxic dead zone, exerts not only long-term ecological impacts, but also economic damage from lost fisheries income.
As the hypoxic dead zone in the northern Gulf of Mexico continues to grow, alternative means to reduce nutrient loads from the Mississippi River to the Gulf are being sought. One of the options is to divert the nutrient-rich water to the river corridor and floodplain wetlands in the lower Mississippi-Atchafalaya River Basin. It is unclear how much nitrate reduction this would cause.
The Mississippi River flood of 2011 required the opening of the Morganza Spillway to divert the flood water into the Atchafalaya River Basin. This event offered a unique opportunity for investigating nitrate transport and retention in this large river swamp basin. During this flood event, LSU AgCenter scientists collected water samples for nitrate analysis at three locations on the Atchafalaya River, using Simmesport as the input and Wax Lake Outlet and Morgan City as the output. In addition, measurements of water temperature and dissolved oxygen were taken at these locations. This data collection design was to quantify mass inflow, outflow and the retention rate of nitrate in the Atchafalaya River Basin during the extreme flood event.
Nitrate Concentration and Transport
From May 15 to July 20, nitrate concentration in the Atchafalaya River was lower at the outlets than at the input. The concentration change was inversely correlated with the flood discharge, with the lowest nitrate concentration occurring at the peak flow and the highest concentration occurring approximately one month later as the river flow receded. While the concentration fluctuation at Simmesport is mainly dependent on the Mississippi River water, the concentration change at Morgan City represents additional influences from the basin.
During the 10 weeks of the 2011 flood, a total of 107,034 tons of nitrate entered and 103,066 tons of nitrate left the Atchafalaya River. The nitrate transport during such a short period of time represents approximately half of the average annual nitrate load in the Atchafalaya. Because the Atchafalaya River discharge represents about 32 percent of the total Mississippi River’s flow during the flood, the Mississippi River main channel would have discharged 219,029 tons of nitrate, assuming the concentration at Simmesport represents that in the Mississippi River. The total nitrate transported from the Mississippi-Atchafalaya system during this flood would amount to 322,095 tons of nitrate, which is about 31 percent of the annual nitrate input to the Gulf.
Atchafalaya River Nitrate Retention
The Atchafalaya River Basin acted as a small sink for nitrate, retaining nearly 4 percent of the nitrate entering the basin during the 2011 flood. Although there were some weeks in early June that the basin released nitrate, nitrate was retained in the basin during the flood recession (Figure 1). Timing of this flood later in the season may have allowed for higher denitrification or permanent removal. Because denitrification is dependent both on lack of oxygen and presence of the required biological community, warmer water favors these conditions in holding less oxygen while being high enough to support a biological community.
Flow paths in the Atchafalaya Basin are not fully understood and are apt to become even more convoluted in large floods. Intensive sampling results show that the flood diverted an extraordinarily large quantity of nitrate to the Atchafalaya River, and that this swamp river basin with high connectivity to its floodplains functioned as a nitrate sink, which is unlike the findings of previous studies. Despite the magnitude of the flood and the potential for the Atchafalaya Basin to reduce nitrate load, only 4 percent of nitrate was retained. Further testing will help determine if the nitrate change is from temporary storage (assimilation) or permanent removal (denitrification), which will be helpful for predicting riverine nitrate transport and transformation in floods of this magnitude.
The authors acknowledge funding from McIntire-Stennis and thank the U.S. Geological Survey Louisiana Water Science Center for providing river discharge data. The authors also thank Abram DaSilva for field assistance.
(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)