Y. Jun Xu, Abram DaSilva and April Bryant-Mason

Dissolved oxygen (DO) is an important indicator for water quality in surface water bodies. Like most terrestrial organisms, fish and many other aquatic organisms are adapted to a narrow range of DO concentrations.

While oxygen concentration in the atmosphere stays relatively constant, DO concentration in water can vary because of a number of physical, chemical and biological factors.

Stream water temperature, turbulence and the amount of organic matter in the water all affect how much DO is present. The solubility of oxygen in water is inversely related to water temperature. Turbulence increases the rate of reaeration, which is the rate that oxygen moves from the atmosphere into solution when the DO concentration is below saturation. Respiration by aerobic organisms, decomposition of organic matter and chemical oxidation are all processes that consume oxygen from water.

In Louisiana, high average temperatures, sluggish streamflow resulting from the flat landscape and high organic content found in the majority of streams combine to cause low DO concentrations. Currently, about half of the state’s watersheds are listed as impaired for the low DO levels in their water bodies. While the current acceptable DO minimum is 5 milligrams per liter, LSU AgCenter researchers found that many streams in forested areas of northern Louisiana were below this standard. Stream oxygen depletion is a pervasive problem in many Louisiana streams.

Because of the low DO levels found in many Louisiana’s forested streams, there is a concern that timber harvest in these areas can further exacerbate this condition. Although forestry best management practices (BMPs) are widely implemented in Louisiana, it is not clear if the practices can prevent further DO degradation in the streams that are already under DO stress. To fill this knowledge gap, AgCenter researchers began an intensive monitoring study on stream DO changes in 2006 in the Flat Creek watershed, an area representative of the flat forested landscape in much of Louisiana.

From 2006 to 2010, AgCenter researchers monitored changes in stream DO in two monitoring intensities – monthly measurements at three harvested sites and 15-minute continuous measurements at one harvested site. At each harvested site, researchers installed two monitoring locations – one upstream above the harvested area and another downstream below the harvested area. Timber harvest was conducted during the summer and fall of 2007.

The five-year study shows that Louisiana’s current forestry BMPs can maintain existing stream DO levels through minimizing stream temperature increases and excessive intrusion of harvest residue. Neither water temperature nor organic carbon concentrations were changed by the timber harvest. At two of the harvested sites, no change in DO levels was found between the upstream and downstream sites. At one harvested site, DO increased slightly downstream of the harvested site, which can be attributed to increased surface runoff and the resulting increased turbulence.

Large seasonal variation in DO levels exists in the studied streams. Based on the findings, an attainment of 5 milligrams per liter DO standard seems to be unrealistic for many forested streams that are already classified as DO-impaired in this region (Figure 1). The goal of the Louisiana Department of Environmental Quality is to determine the best attainable criteria. Therefore, the attainment standard applied to this region should be adjusted accordingly to account for seasonal load allocations for DO. A more appropriate goal is 3 milligrams per liter for the summer months and 5 milligrams per liter for the remaining months.