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Oregon Water Science Center Hydrologic Studies

Measurement of Sediment Oxygen Demand (SOD) in Upper Klamath and Agency Lakes, Oregon (completed)

Low dissolved oxygen concentrations (less than 4 milligrams per liter) have been documented in Upper Klamath and Agency Lakes (162 KB GIF), where they may be detrimental to the survival of endangered suckers in the lakes. Data collected from 1990-1994 indicate that 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; however, low concentrations have been measured at and near the surface as well (Wood et al., 1996).

Of particular interest 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, and (or) by increasing resuspension of oxygen demanding sediment.

The Bureau of Reclamation (BOR) is responsible for managing the lake level in a way that balances the water needs of agriculture downstream of the lakes with the in-lake water requirements for maintaining a healthy ecosystem. Maintaining the health of the lake includes the protection of the lake's endangered fish species, which are of great cultural importance to the Klamath Tribes. The lack of a clear, unbiased understanding of the functioning of the lake ecosystem hampers the ability of the BOR to manage the lake in a way that will address the concerns of interests competing for the lake resourcs. The BOR wants to make management decisions with adequate supporting scientific information, in particular an understanding of the dissolved oxygen budget in the lake. SOD is only one piece of the oxygen budget, but a potentially important piece. Therefore the BOR will benefit from the information collected during this project.

1. To assess the magnitude and variability of SOD in the lakes.
2. 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.
3. To determine the correlation between measured SOD and other quantifiable sediment characteristics, in particular coarse/fine distribution, organic carbon content, and ash-free dry weight.

Eleven sites were targeted for SOD measurements. These sites were chosen to represent areas of the lake known to be of interest because of their significance to fish survival--either because of relatively poor or relatively good water quality. Most sites were established where water quality data have been collected by the Klamath Tribes or by the BOR.

SOD was measured with special open-bottomed chambers that were seated and sealed on the lake bottom by scuba divers. Triplicate measurements were made at each of the sites to provide an estimate of the mean SOD and a measure of intrasite variability. Samples of the surface sediments next to each of the chambers were collected by the divers (3 samples at each site) for the analysis of total organic carbon, percent sand (>0.063 micrometers), and ash-free dry weight.

REPORTS

WRIR 01-4080. Sediment Oxygen Demand in Upper Klamath and Agency Lakes, Oregon, 1999, by Tamara M. Wood.