THE PHOSPHORUS CYCLE IN A VOLCANIC BASIN LAKE IS AFFECTED BY ELEVATED SILICA CONCENTRATIONS: UPPER KLAMATH LAKE, OREGON

The bottom sediment of Upper Klamath Lake, OR, is considered a primary source of phosphorus to the overlying water column. The average concentration of phosphorus in Upper Klamath Lake bottom sediments is 0.4 mg per gram dry weight of sediment. This study examined the mechanisms for phosphorus release from the sediment to the overlying water column and retention of phosphorus by sediment solids. The average concentration of silica in the sediment is approximately 50 percent by weight. The large concentration of silica in the sediment effects the phosphorus cycle in the lake because large concentrations of silica in the bottom sediments lead to elevated concentrations of silica in sediment interstitial water. Competition for sorption sites between phosphate and silica dissolved in interstitial water potentially increases release of phosphate from the sediment solids. This limits the amount of phosphorus sorbed to solids, adds to phosphate in the interstitial water, and favors transfer of phosphate from the sediment to the overlying water column via diffusion processes or mixing of sediment with the overlying water column. In addition to sediment processes that contribute to the release of phosphorus from the sediment, there are sediment processes, one of which involves silica, that result in retention of phosphate in the sediment. We have evaluated three forms of phosphorus in the sediment which undergo degradation pathways with very slow kinetics. These forms of phosphorus in Upper Klamath Lake sediments should not be considered bioavailable due to their long half-lives. (1) Diatom frustules which are abundant in the sediment contain phorylated proteins (silaffins) which are enclosed in a silicate shell. Encased in glass, silaffins are not subject to microbial degradation. (2) The mineral apatite in anaerobic conditions would be expected to have limited interaction with microbial populations. There is evidence that, in addition to detrital apatite, there is authigenic Ca₃(PO₄)₂ formed in Upper Klamath Lake sediments. (3) Upper Klamath Lake sediments contain detrital minerals including basalts which have concentrations of phosphorus in the mg per gram dry weight range and slow dissolution kinetics.