Effects of Timber Harvesting on Dissolved Oxygen in a Northern Louisiana Headwater Stream
Many headwaters in Louisiana have low concentrations of dissolved oxygen (DO) and there is a concern that timber harvesting can further decrease these concentrations. This study was conducted to examine the direct and long-term effect of timber harvest on DO in a low-gradient headwater stream in north-central Louisiana, USA. Water quality multisensor probes were deployed at two locations along a second-order stream, approximately 500 m apart. DO concentration and saturation were recorded at 15-minute intervals from 2006 to 2010. In addition, monthly site visits were conducted for streamflow measurement and water sample collection for biochemical oxygen demand (BOD) and total carbon (TC) analyses. In late summer 2007, a 45-ha loblolly pine stand between the two locations was clearcut, which allowed data analysis in a before-after and above-below control design. At the upstream site (control), concentrations of DO, BOD, and TC averaged 2.3, 1.3, and 26.5 mg L−1, respectively. DO concentrations were mostly (83% of measurements) less than 1 mg L−1 during the summer and were frequently (33% of measurements) less than 2 mg L−1 during the winter. After the harvest, BOD and TC at the downstream site were significantly higher than those at the upstream site (paired t-tests; P < 0.01; df = 37 and 32, respectively), whereas water temperature at the downstream site increased only slightly (0.9° C). However, these changes did not lower DO under a range of flow conditions. After harvest, DO concentrations were significantly higher at the downstream site during both summer and winter (paired t-tests; P < 0.001; df = 126 and 313, respectively). The DO increase may have resulted from increased streamflow due to reduced evapotranspiration at the harvested site. Even with the harvest-induced DO increases, concentrations at both sites continued to fall below the Environmental Protection Agency-recommended 5-mg L−1 limit for greater than 70% of measurements, challenging the attainability of the current water quality standard.
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