No abstract

Increased external nutrient loads of anthropogenic origin, especially those of phosphorus (P), were one of the major causes of eutrophication during the first half of the twentieth century in Europe. They led to deterioration of lake ecosystems, particularly including noxious blooms of (potentially toxic) cyanobacteria. From the 1970-1980s, strategies to decrease the phosphorus loads from sewage were increasingly implemented, among them are the ban of phosphates in detergents, the expansion of sewer systems and improvement in wastewater treatment to remove nutrients. Case studies of eight lakes, whose response to point source reduction of phosphorus was observed over decades, show that a pronounced reduction of the phosphorus load from point sources can be achieved either by the diversion of inflows carrying high loads, by upgraded sewage treatment, or by phosphorus precipitation in the major tributary directly before its inflow into the water body. Outcomes demonstrate that in order to effectively control cyanobacterial blooms, the measures taken need to reduce in-lake concentrations of total phosphorus below 20-50 lg L -1 , with this threshold varying somewhat between lakes depending in particular on hydromorphological and biological conditions. Whether and when load reduction succeeds in controlling cyanobacteria depends primarily on the load

Preserved diatom assemblages are found in deep water sediments in Lake Washington above and below an ash layer originating from an eruption of Mt. Mazama 6800 B.P. Total abundance of frustules did not change after the ash event, but a significant shift in community structure occurred. Melosira italica var. subarctic increased after the ash fall, and Fragilaria crotonensis decreased. .The changes most likely indicate an increase in the Si: P ratio and decreased light penetration in the euphotic zone during winter and spring.

The hypothesis of Edmondson and Litt concerning the inhibition of Daphnia by Oscillatoria in Lake Washington was tested in experiments with cultures. Growth and reproduction of Daphnia pulicaria and Daphnia thorata were reduced by increasing concentrations of Oscillatoria agardhii from 0 to 400 filaments ml-' in the presence of adequate amounts of Cryptomonas as food. Cryptomonas reproduction also was inhibited by Oscillatoria, by an undetermined mechanism. The results are consistent with the idea that the scarcity of Daphnia in Lake Washington during the 1 O-year period before 1976 was related to high concentrations of 0. agardhii.

Holocene sediments in Lake Washington contain a series of turbidites that were episodically deposited throughout the lake. The magnetic signatures of these terrigenous layers are temporally and areally correlatable. Large earthquakes appear to have triggered slumping on the steep basin walls and landslides in the drainage area, resulting in turbidite deposition. One prominent turbidite appears to have been deposited about 1100 years ago as the result of a large earthquake. Downcore susceptibility patterns suggest that near-simultaneous slumping occurred in at least three separate locations, two of which now contain submerged forests. Several other large earthquakes may have occurred in the last 3000 years.