SUCCESSION OF DIATOM ASSEMBLAGES IN RECENT SEDIMENTS OF LAKE WASHINGTON

Examination of the diatom remains in the recent sediment of Lake Washington reveals striking correlations with the pattern of sewage enrichment over the past 80 years. In the deeper sediment, which was deposited prior to cultural enrichment, the relative composition of the diatoms was constant. Diversity and redundancy values also reflect this constancy and near steady-state conditions. An approximate time sequence, based on estimates of sedimentation rates, was derived for the sediments. Correlated with the pattern of sewage discharge into the lake, many of the species changed in proportion in accordance with their ascribed trophic behavior as indicator species. Statistical treatment revealed significant shifts in species ranking, including a change in dominance from Melosira italica to Fragilaria crotonensis between 35 and 20 cm that is associated with the establishment of Seattle. The reversal of this relationship from 9 to 6 cm and its re-establishment from 5 cm to the surface of the sediment correspond to a diversion of sewage from the lake about 1930 and a gradual increase from 1941 to 1965. The considerable changes in species ranking, diversity, and redundancy values in the upper 30 cm of sediment is interpreted as the result of the interaction between the various species and the degree of cultural enrichment. The percentages of Araphidineae and Centrales followed this pattern very closely and seem to be much more reliable indicators than the fluctuations of individual indicator species.