Streamflow and nutrient dependence of temperature effects on dissolved oxygen in low-order forest streams

Low dissolved oxygen (DO) concentrations in streams can be linked to both natural conditions and human activities. In Louisiana, natural stream conditions such as low flow, high temperature and high organic content, often result in DO levels already below current water quality criteria, making it difficult to develop standards for Best Management Practices (BMPs). Along three low-order streams within a West Gulf Coastal Plain watershed in central Louisiana, streamflow conditions, temperature, organic carbon and DO were measured for one year in order to: (1) investigate spatial and seasonal differences in dissolved oxygen, (2) determine factors influencing the temperature dependence of DO concentrations, and (3) assess consequences of high temperature and organic content on dissolved oxygen levels downstream at higher order streams. Streamflow was measured and monthly water samples were collected at 11 locations. Water samples were analyzed for total and dissolved organic carbon. Stream temperature and dissolved oxygen were measured monthly in-situ at each location. The results show overall oxygen depletion in most of the sampled streams. 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. On average, DO levels at the sites varied from 2.0 to 6.1 mg L-1. The stream site with the lowest average total organic carbon (7.74 mg L-1) also showed DO meeting TMDL standards (=5 mg L-1). This site, located near the headwaters of Spring Creek, is impacted the least by organic material transported downstream. There was a close relationship between organic carbon and dissolved oxygen, which appeared to be further affected by stream hydrologic conditions. 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.