Seasonal and spatial water quality changes in the outflow plume of the Atchafalaya River, Louisiana, USA

The objective of this study was to examine the interaction between the Atchafalaya River and the Atchafalaya Delta estuarine complex. Measurements of suspended sediments, inorganic nutrients (NO3-, NH4+, PO43-), chlorophyll a (chl a),. and salinity were taken monthly from December 1996 to January 1998. These data were compiled by season, and the Atchafalaya River plume data were also analyzed using the Generalized Additive Model technique. There were significant decreases in NO3- concentrations dining summer, fall, and winter as river water passed through the estuary, that were attributable to chemical and biological processes rather than dilution with ambient water. In some regions there were higher chl a concentrations during summer and fall compared to winter and spring, when river discharge and the introduction of inorganic nutrients were highest, suggesting biological processes were active during this study. The presence of NH4+, as a percentage of available dissolved inorganic nitrogen, increased with distance from the Atchafalaya River, indicative of remineralization processes and NO3- reduction. Mean PO43- concentrations were often higher in the estuarine regions compared to the Atchafalaya River. Dining summer total suspended solid (TSS) concentrations increased with distance from the river mouth, suggesting a turbidity maximum. Highest chl a concentrations were found in the bayous and shallow water bodies of the Terrebonne marshes, as were the lowest TSS concentrations. The low chl a concentrations found in other areas of this study, despite high inorganic nutrient concentrations, suggest light limitation as the major control of phytoplankton growth. Salinity reached near seawater concentrations at the outer edge of the Atchafalaya River plume, but much lower salinities (< 10 psu) were observed at all other regions. The Atchafalaya Delta estuarine complex buffers the impact of the Atchafalaya River on the Louisiana coastal shelf zone, with a 41% to 47% decrease in Atchafalaya River NO3- concentrations before reaching Gulf waters.