CHANGES IN THE CHEMICAL COMPOSITION OF SEDIMENTS OF LAKE WASHINGTON, 1958–1970

Abstract

Comparisons are made among three cores taken in deep water in different parts of Lake Washington in 1958 and 1959 and two cores taken at one of the same locations in 1968 and 1970. During the interval between the two sets of cores, all the sewage effluent that had been causing eutrophication of the lake was diverted, and the lake began to regress to a relatively unproductive condition.All the cores show higher concentrations of organic matter, phosphorus, and nitrogen (expressed as a fraction of dry weight) in the top few centimeters than deeper, but in general, the higher surface values of the earlier material have decreased somewhat now that they lie below the surface in the recent cores. Nevertheless, it appears that the high phosphorus content of the sediment laid down in the years of maximum enrichment will continue to be indicated by a clear maximum, which was at a depth of 4.0 cm in 1970. Nitrogen, although enriched in present surface sediments, does not show a corresponding older maximum.In parts of the lake, erratic, very high concentrations of phosphorus are associated with several types of particles containing much iron. Fractionation of sedimentary phosphorus by a biochemical procedure showed that a large part (70–80%) of the phosphorus content of the mud is acid‐soluble, but that the fraction releasable by reduction decreases with depth.These conditions may be related to events in the hypolimnion; during each summer, a dense population of an undescribed iron‐depositing microorganism has been observed to form.