Sediment oxygen demand in Upper Klamath and Agency Lakes, Oregon, 1999

Low dissolved oxygen concentrations (less than 4 milligrams per liter) have been documented in Upper Klamath and Agency Lakes, where they are detrimental to the survival of endangered sucker species in the lakes. The Bureau of Reclamation and the Klamath Tribes have been collecting water-quality data in Upper Klamath and Agency Lakes since 1988. These data indicate that dissolved oxygen concentrations low enough to be of concern are most likely to occur in late summer, after large algal blooms have started to decline. The lowest dissolved oxygen concentrations are most likely to occur near the bottom of the water column; however, low concentrations have been measured at and near the surface as well (Wood et al., 1996; also Wood, unpub. data). Upper Klamath Lake (fig. 1) is a large (360 km2 [square kilometer]), shallow (mean depth about 2.4 m [meter]) lake in south-central Oregon that the historical record indicates has been eutrophic since at least the beginning of the 20th century. In recent decades, however, the lake has had annual occurrences of nearmonocultural blooms of the blue-green alga Aphani-zomenon flos-aquae. In 1988, the Lost River sucker and the shortnose sucker, which live primarily in the lake and spawn in its tributaries, were listed as endangered by the U.S. Fish and Wildlife Service under the Endangered Species Act. The poor water quality associated with the extremely long and productive blooms is contributing to the decline of those fish species (Perkins et al., 2000). The water-quality monitoring data collected to date have not been adequate to quantify the various mechanisms that influence dissolved oxygen concentrations -most notably photosynthetic production, respiration, sediment oxygen demand (SOD), carbonaceous biological oxygen demand (CBOD) in the water column, reaeration, and resuspension of oxygen demanding bed sediment. The scope of this work is the quantification of one piece of the oxygen budget- SOD. SOD operates on a longer time scale than the highly dynamic processes of algal photosynthesis and respiration. It is to be expected that daily fluctuations in dissolved oxygen would be dominated by those algal processes. SOD is nonetheless important because it provides a "background" oxygen demand onto which the other more dynamic oxygen demands are superimposed, thereby enhancing, for example, the oxygendemanding effects of algal respiration and CBOD, and limiting the effectiveness of oxygen-producing photosynthesis. The SOD rate is also an important rate required for efforts to model dissolved oxygen, nutrients, and algae in the lake. While the SOD is stable when compared to the highly dynamic processes associated with algal metabolism, it might be expected to vary on seasonal time scales-because SOD is determined by sediment characteristics, the settling of new sediments on an annual cycle could change the SOD associated with those sediments. Newer sediments, having undergone less decay than older sediments, have the potential to create a higher oxygen demand as they are richer in unmetabolized organic nutrients. In a eutrophic lake like Upper Klamath, the sediments are largely composed of algal detritus; fresh sediments are produced on an annual cycle when the bloom crashes in the late summer and fall. Of particular interest in Upper Klamath Lake is the role that lake management might play in producing low dissolved oxygen concentrations. Lowering lake levels through the summer can potentially enhance the net effect of sediment oxygen demand (SOD) on water column dissolved oxygen by increasing the bed sediment area to lake volume ratio. Lake managers at the Bureau of Reclamation (BOR) can establish minimum lake levels in the hope of reducing sediment resuspension and/or providing more water volume to dilute the effects of the SOD. Given a measured value of SOD and realistic assumptions about reaeration rates and the frequency of lake mixing, the BOR will be better able to assess the likelihood that controlling lake levels will reduce the frequency of occurrence of very low dissolved oxygen concentrations. The objectives of this study were: • To determine the magnitude and variability of SOD in Upper Klamath and Agency lakes. • To determine the change in SOD from before the development of summertime algal blooms to late summer, after or during the decline of the blooms. • To attempt some correlation between measured SOD and other quantifiable sediment characteristics, in particular: coarse/fine distribution, organic carbon content, and the residue lost on ignition.