Mississippi-Atchafalaya River System Research Articles

Isotopic signature of nitrate in river waters of the lower Mississippi and its distributary, the Atchafalaya

AbstractPeriodic summer hypoxia occurring in the Northern Gulf of Mexico has been attributed to large nutrient inputs, especially nitrate‐nitrogen, from the Mississippi–Atchafalaya River system. The 2008 Gulf Hypoxia Action Plan calls for river corridor wetland restoration to reduce nitrate loads, but it is largely unknown how effective riverine wetland systems in the lower Mississippi River are for nitrate removal. We carried out an intensive isotope study to address this question by comparing nitrate isotopic signatures of the well‐channelized Mississippi River with those of the Atchafalaya River, which has extensive floodplains and backwater swamps. We investigated changes in δ15NNO3 and δ18ONO3 for water samples collected biweekly to monthly over a 2‐year period at the Atchafalaya River outlets (Morgan City and Wax Lake) and on the Mississippi River at Baton Rouge. In addition, in situ water quality parameters including temperature, dissolved oxygen and pH were recorded for each sampling date. Waters from both rivers showed moderately high nitrate concentration (>1 mg l−1) and undetectable (< 0.01 mg l−1) nitrite throughout the study period. The Mississippi River had slightly, but significantly higher (p=0.01) mean nitrate concentrations (1.5 mg l−1) and higher δ15NNO3 (7.7‰) than the Atchafalaya (1.1 mg l−1, 7.0‰); while no difference in δ18ONO3 (4.6‰) was found between the rivers. Flux‐weighted mean isotope values were overall lower than mean values for both the Mississippi and Atchafalaya Rivers, with a greater difference between the two rivers (7.4‰ versus 6.5‰, respectively). River flooding and hurricane storm surge also appeared to affect nitrate isotopic values. The lack of large difference in isotopic values between the Atchafalaya and Mississippi Rivers suggests that the majority of nitrate is transported through the Atchafalaya River with relatively little processing, and that riverine floodplains and wetlands are not effective sinks for nitrate, as previously assumed, because of insufficient residence time and well‐oxygenated river waters. Copyright © 2012 John Wiley & Sons, Ltd.

Texas Coastal Hypoxia Linked to Brazos River Discharge as Revealed by Oxygen Isotopes

Hypoxic conditions in the coastal waters off Texas (USA) were observed since the late 1970s, but little is known about the causes of stratification that contribute to hypoxia formation. Typically, this hypoxia is attributed to downcoast (southwestward) advection of waters from the Mississippi–Atchafalaya River system. Here, we present evidence for a hypoxic event on the inner shelf of Texas coincident with the presence of freshwater linked to high flow of the Brazos River in Texas. These conclusions are based on hydrographic observations and isotopic measurements of waters on the inner shelf near the Brazos River mouth. These data characterize the development, breakdown, and dispersal of a hypoxic event lasting from June through September 2007 off the Texas coast. Oxygen isotope compositions of shelf water indicate that (1) discharge from the Brazos River was the principal source of freshwater and water column stratification during the 2007 event, and (2) during low Brazos River discharge in 2008, freshwater on the Texas shelf was derived mainly from the Mississippi–Atchafalaya River System. Based on these findings, we conclude that the Mississippi–Atchafalaya River System is not the sole cause of hypoxia in the northern Gulf of Mexico; however, more data are needed to determine the relative influence of the Texas versus Mississippi rivers during normal and low flow conditions of Texas rivers.

Corn-based ethanol production compromises goal of reducing nitrogen export by the Mississippi River

Corn cultivation in the United States is expected to increase to meet demand for ethanol. Nitrogen leaching from fertilized corn fields to the Mississippi-Atchafalaya River system is a primary cause of the bottom-water hypoxia that develops on the continental shelf of the northern Gulf of Mexico each summer. In this study, we combine agricultural land use scenarios with physically based models of terrestrial and aquatic nitrogen to examine the effect of present and future expansion of corn-based ethanol production on nitrogen export by the Mississippi and Atchafalaya Rivers to the Gulf of Mexico. The results show that the increase in corn cultivation required to meet the goal of 15-36 billion gallons of renewable fuels by the year 2022 suggested by a recent U.S. Senate energy policy would increase the annual average flux of dissolved inorganic nitrogen (DIN) export by the Mississippi and Atchafalaya Rivers by 10-34%. Generating 15 billion gallons of corn-based ethanol by the year 2022 will increase the odds that annual DIN export exceeds the target set for reducing hypoxia in the Gulf of Mexico to >95%. Examination of extreme mitigation options shows that expanding corn-based ethanol production would make the already difficult challenges of reducing nitrogen export to the Gulf of Mexico and the extent of hypoxia practically impossible without large shifts in food production and agricultural management.

Organic Nitrogen Retention in the Atchafalaya River Swamp

Freshwater diversions from the lower Mississippi River into the region’s wetlands have been considered an alternative means for reducing nitrogen loading. The Atchafalaya River Swamp, the largest freshwater swamp in North America, carries the entire discharge of the Red River and 30% of the discharge of the Mississippi River, but it is largely unknown how much nitrogen actually can be retained from the overflowing waters of the Mississippi–Atchafalaya River system. Nitrogen discharge from the upper Mississippi River Basin has been implicated as the major cause for the hypoxia in the Northern Gulf of Mexico, which threatens not only the aquatic ecosystem health, but also Louisiana’s fishery industry, among other problems. This study was conducted to determine the change in organic nitrogen mass as water flows through the Atchafalaya River Swamp and into the Gulf of Mexico. By utilizing the river’s long-term discharge and water quality data (1978–2002), monthly and annual organic nitrogen fluxes were quantified, and their relationships with the basin’s hydrologic conditions were investigated. A total Kjeldahl nitrogen (TKN) mass input–output balance between the upstream (Simmesport) and downstream (Morgan City and Wax Lake Outlet) locations was established to examine the organic nitrogen removal potential for this large swamp. The results showed that on average, TKN input into the Atchafalaya was 200 323 tons year−1 and TKN output leaving the basin was 145 917 tons year−1, resulting in a 27% removal rate of organic nitrogen. Monthly TKN input and output in the basin were highest from March to June (input vs. output: 25 000 vs. 18 000 tons month−1) and lowest from August to November (8000 vs. 6000 tons month−1). There was a large variation in both annual and inter-annual organic nitrogen removals. The variability was positively correlated with the amount of inflow water at Simmesport, suggesting that regulating the river’s inflow at the Old River flood control structures may help reduce nitrogen loading of the Mississippi River to the Gulf of Mexico. Furthermore, the in-stream loss of organic nitrogen indicates that previous studies may have overestimated nitrogen discharge from the Mississippi–Atchafalaya River system.

On the seasonal correlation of surface particle fields with wind stress and Mississippi discharge in the northern Gulf of Mexico

Abstract Spatio-temporal correlation analyses were performed on time series of daily freshwater discharge, wind fields, and SeaWiFS-derived surface particle concentrations in the northern Gulf of Mexico. The influences of discharge and winds on surface particle concentrations were investigated by mapping temporal correlation coefficients at each pixel for the whole time series (1997–2000) and for each season during 1999 and 2000. Maps of the correlation between suspended particulate matter concentration (SPM) and river discharge suggest regions that are fluvially influenced. The particulate matter may be sediments carried by the river plume or biogenic particles (e.g., detritus) stimulated by the river discharge. The algorithm used to estimate SPM concentrations does not differentiate between sediment and detritus. Maps of the correlation between wind stress and SPM suggest regions where wind mixing accounts for particulate resuspension and subsequent transport. Regions of significant positive wind-SPM correlation were independent, and often spatially separated, from regions of strong positive discharge-SPM correlation. Thus, the influences of winds and discharge on particle distributions can be investigated independently. Regions of high wind-SPM correlation were associated with shallow shelf areas, as correlation contours generally followed the bathymetric contours, and expanded in size under offshore wind regimes. These areas exhibited less spatial and temporal variability than the regions of high discharge-SPM correlation associated with the Mississippi-Atchafalaya river system. There was no apparent relationship between the magnitude of Mississippi- Atchafalaya discharge and the spatial extent of the region of high discharge-SPM correlation during seasonal analyses. Instead, the spatial extent and orientation of the discharge-SPM correlation field appeared to be a function of winds (both their direction and speed) and the buoyancy of the plume.

Nutrient enhanced primary productivity of the Texas-Louisiana continental shelf

Abstract Light bottle and dark bottle 14C uptake was measured on deck in 4–6 h shipboard incubations at 12 locations on the NW continental margin of the Gulf of Mexico in July and at 9 locations in October 1990. In July, rates of Pmax m−3 h−1 were higher than previously reported for the Texas-Louisiana shelf, and daily production calculated frm these 4–6 h incubations was 1–1.4 g C m−2 d−1 at most inner and middle shelf locations. However, in May–June 1990 freshwater discharge from the nitrate-rich Mississippi-Atchafalaya river system and from the Trinity River had reached highest recorded outflows in many years; near-surface nitrate concentrations over the Texas-Louisiana shelf remained well above the 0.05 μM 1−1 limit of detection into July. In contrast, near-surface nitrate concentrations were close to or at the limit of detection in October. Reflecting this variation in nitrate inventory, in July the production index (P/B ratio) at stations near riverine and estuarine nutrient sources and over the inner shelf reached 40 mg C fixed m−3 h−1 · (mg chl m−3)−1, whereas in October this index generally was 2− to 3-fold lower. Thus, primary productivity of the Texas-Louisiana continental margin appears to be moderate when not enhanced by the high “new” nitrogen nutrient loads that enter via riverine/estuarine outflows. Since the volume of freshwater discharge varies markedly on seasonal and interannual time scales, we suggest that “discharge driven” is a more appropriate description of the primary productivity of this subtropical continental margin than is its spatial partition into regions of high (250–500), medium (150–250) and low (100–150 mg C m−2 d−1) mean production.

Transport of Particulate Organic Carbon by the Mississippi River and Its Fate in the Gulf of Mexico

This study was designed to determine the amount of particulate organic carbon (POC) introduced to the Gulf of Mexico by the Mississippi River and assess the influence of POC inputs on the development of hypoxia and burial of organic carbon on the Louisiana continental shelf. Samples of suspended sediment and supporting hydrographic data were collected from the river and >50 sites on the adjacent shelf. Suspended particles collected in the river averaged 1.8±0.3% organic carbon. Because of this uniformity, POC values (in μmol l−1) correlated well with concentrations of total suspended matter. Net transport of total organic carbon by the Mississippi-Atchafalaya River system averaged 0.48×1012 moles y−1 with 66% of the total organic carbon carried as POC. Concentrations of POC decreased from as high as 600 μmol l−1 in the river to 35% along the shelf at ≥10 km from the river mouth. River flow was a dominant factor in controlling particle and POC distributions; however, time-series data showed that tides and weather fronts can influence particle movement and POC concentrations. Values for apparent oxygen utilization (AOU) increased from ∼60 μmol l−1 to >200 μmol l−1 along the shelf on approach to the region of chronic hypoxia. Short-term increases in AOU were related to transport of more particle-rich waters. Sediments buried on the shelf contained less organic carbon than incoming river particles. Orgamic carbon and δ13C values for shelf sediments indicated 3 that large amounts of both terrigenous and marine organic carbon are being decomposed in shelf waters and sediments to fuel observed hypoxia.

Notes on Invasion of Fresh Water by Fishes of the Gulf of Mexico, with Special Reference to the Mississippi-Atchafalaya River System

Notes on Invasion of Fresh Water by Fishes of the Gulf of Mexico, with Special Reference to the Mississippi-Atchafalaya River System