|Abstract or Summary
- There is evidence that recent ecological changes in lakes and their watersheds can be detected from analysis of lake sediments. This study was initiated to correlate
changes in some sediment characteristics of four Oregon lakes with past cultural developments of the lakes and their watersheds. The lakes chosen for this study were
Waldo, Odell, and Diamond Lakes located in the Cascade mountains and Devils Lake located on the Oregon coast. Percentage dry weight, percentage organic matter, sedimentary chlorophyll degradation products (SCDP), total phosphorus, total organic nitrogen, and diatom assemblages were used as indices of sediment changes through time.
To evaluate the usefulness of the sediment record for showing changes in lake productivity, sediment cores were taken from lakes that exhibit present day differences in trophic status. Profiles of selected characteristics of the sediment
cores reflected the events unique to each lake and its watershed. The characteristics measured in cores from Waldo Lake sediments indicated that the lake has always
been ultra-oligotrophic. The sedimentation rate was very low and the diatom assemblages revealed the unchanging water quality of the lake. Odell Lake responded to enrichment of its waters by shifting from its original oligotrophic condition to a more productive status. When the enrichment was ended, the lake responded by returning to a new intermediate productivity level. This was revealed by changes in profiles of percentage dry weight, percentage organic matter, SCDP, and diatom assemblages. Diamond Lake appears to have increased in productivity as a result of human activities in its watershed. This was shown also by shifts in SCDP, percentage organic matter, percentage dry weight and diatom assemblages. Although many changes occurred in the watershed of Devils Lake, sediment analysis showed changes only in profiles of total phosphorus and total organic nitrogen. Many factors in this lake may have combined to destroy the fine layer stratigraphy of the sediments. Meaningful successions of events were described with the aid of the sediment characteristics. The most useful sediment characteristics measured in this study were SCDP, diatom assemblages, percentage dry
weights, percentage organic matter, and total phosphorus. Increases in SCDP and the percentage of organic matter seemed to indicate increases in the productivity of the
lakes. Large decreases in the percentage of dry weight seemed to represent increases in rates of sedimentation. Changes in the diatom community undoubtedly indicated
shifts in the trophic status of the lakes. Shifts in total phosphorus concentrations probably represented disturbances in the watersheds. Total organic nitrogen was the only sediment characteristic that was not very useful.