OSCILLAXANTHIN IN THE SEDIMENT OF LAKE WASHINGTON1

Abstract

Oscillaxanthin was determined quantitatively in portions of five cores cut in slices 1 cm thick. The distribution varied with depth in a manner qualitatively consistent in all cores, although there were quantitative differences. The pigment was undetectable below 9 cm in two cores, below 8 in another, and below 13 in the remaining two. The amount of pigment per unit dry weight of sediment was highest at the surface and had two subsurface maxima, at 4 cm for the shallow maximum, with a deep maximum at 7 cm for three cores, 8 cm for the fourth, and 9 cm for the fifth. The total accumulated pigment under unit area of bottom mud varied from 27.10 to 6.66 µ g/cm 2 . To some extent these differences also relate to accumulation of organic matter and to other pigments. The oscillaxanthin‐producing algae evidently are somewhat differently distributed during sedimentation, for the differences are not related to differences in distribution of the plankton. The vertical distribution of oscillaxanthin within the sediments is considered in relation to the recent history of sewage enrichment of the lake. Oscillaxanthin is known to be produced by Oscillatoria rubescens and O. agardhii, both of which have occurred in abundance in Lake Washington since 1955 when O. rubescens was observed for the first time, in a relatively dense population. Oscillatoria agardhii occurred in small quantities in 1950. Cultured Anabaena circinalis, Microcystis aeruginosa, and Aphanizomenon flos‐aquae had negligible amounts of oscillaxanthin. On the basis of available evidence about sedimentation rates, oscillaxanthin must have been produced in the lake before the appearance of O. rubescens in 1955 and also before 1950. The first appearance of oscillaxanthin probably came early in the period of sewage enrichment that led to the recent diversion of sewage from Lake Washington. The reasons for the two maxima in concentration are not known. The changes apparently cannot be attributed to destruction of pigment or dilution with other components of sediment in varying amounts over the years and therefore seem to reflect changes in the production of Oscillatoria.