Master of Science (MS)


Renewable Natural Resources

Document Type



One of the dominant themes of stream water quality research has been the effect of nutrients and organic materials on eutrophication of coastal waters. Despite this prevalence in water quality research, few studies have been conducted on water quality changes in low-gradient watersheds under a humid, warm subtropical climate, such as those in the coastal plains of the Northern Gulf of Mexico. This study addresses: (1) the nutrient conditions in headwater streams of a low-gradient, subtropical watershed, especially as it relates to the suggested criteria by the Environtmental Protection Agency (EPA), (2) organic and inorganic carbon dynamics in the headwaters and how they affects nutrient concentrations, (3) dissolved oxygen (DO) conditions in the headwaters and its dependence on temperature, streamflow, and carbon, and (4) nutrient and carbon transport from the headwater catchment. Monthly in-stream measurements of DO, water temperature, pH and conductivity were conducted at 15 locations within the Flat Creek watershed, a 3rd-order watershed in central Louisiana over a 22-month period spanning December 2005 to September 2007. Monthly and storm event water samples were collected from these locations for chemical analyses of nitrogen, phosphorus, and carbon. The results reveal a seasonal trend of increased inorganic carbon in the dry, summer months, while increased organic carbon was found during the wet winter/early spring months. There was a wide range of monthly DO levels (0.4 to 9.0 mg L-1) with the lowest levels generally occurring from May to July, a period with decreased organic carbon and increased inorganic carbon. Localized conditions were more indicative of dissolved oxygen than stream order in the watershed. Nutrient levels, especially nitrate/nitrite (0.127-1.378 mg L-1) were not meeting EPA’s suggested criteria (P25 for nitrate/nitrite is 0.067 mg L-1). There were no spatial relationships in nitrogen, but there was an increasing trend in total phosphorus downstream until a reduction downstream of beaver dam impacted sites. Annually, the Flat Creek watershed exported 15.36 kg carbon ha-1, 0.0087 kg nitrate/nitrite ha-1, and 0.0022 kg phosphorus ha-1. Nutrient fluxes were largely affected by storm runoff and discharge and showed a decreasing trend with increasing drainage size. This study shows that in the forest-dominated landscape of central Louisiana, it may not be possible to reduce nutrient concentrations sufficiently to limit dissolved oxygen consumption, implying that existing water quality standards may not adequately address natural conditions.



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Committee Chair

Y. Jun Xu