RECENT PALEOLIMNOLOGY OF DEVILS LAKE, OREGON

A study of the sediments in an eutrophic Oregon coastal lake, Devils L ake, was initiated to assist in management decisions regarding lake re storation. Recent limnologic changes in the lake were determined throu gh examination of the sediments and measurement of lake bathymetry. A 1.5 m long core, collected near the lake center, was analyzed for perc ent water, loss-on-ignition, nitrogen, phosphorus, and diatoms. The ag e of the sediments was measured using Pb-210 for the upper sediments a nd C-14 for the lower sediments on a total of three cores. The upper 4 0 cm of sediment represented 140 years of accumulation. The age of sed iments at the base of three cores ranged in age from 2030 (+/- 80) b.p . (core D 1.5 m) to 2870 (+/- 80) b.p. (core A 1.0 m) in the lake cent er. Marine sand was present at the base of core A, which was most like ly derived from a tsunami. The analysis of diatoms in the sediments sh owed a very diverse flora with almost 250 taxa identified. The dominan t diatom taxa changed relatively little throughout most of the last 14 0 yrs, suggesting that the lake was productive prior to recent develop ment. Most of the changes in diatom taxa occurred in the last several decades and involved subtle changes in the less abundant taxa. A major erosional event occurred about 1912 (+/- 8 yr) causing a 5-fold incre ase in sedimentation rate based on Pb-210 dating. Sediment accumulatio n rates returned to pre-1900 rates in the 1920's and 1930's, but incre ased again in the last several decades. The major increase in sediment ation rare in the early part of the century corresponded to periods of intense storms which coincided with early development activities. The more recent increases in sedimentation rate which caused a loss in la ke volume apparently resulted from a combination of urbanization and l ogging.