Paleocology of Hall Lake, Washington: a history of meromixis and disturbance

Hall Lake, Washington, has been meromictic for known periods in the recent past (1950-1980). Information on the factors initiating biogenic meromixis in Hall Lake, Washington, was sought using an analysis of the recent sediment in conjunction with known historical events. X-rays of six 50-cm cores showed cryptic laminations that proved to be varies resulting from winter rains, which concentrated silt and clay input in that season. Precise estimation of annual sedimentation rates over the past 350 yr permitted calculations of fluxes of total mineral matter, organic matter, Fe, Ca, Na, K, Pb, Si, Al, Mg, Ti, S, P, chlorophyll degradation products, and cladoceran microfossils. We also constructed relative abundance diagrams for pollen and major diatom taxa. The sedimentary record left during the known period of meromixis (1950-1962) was characterized by mesotrophy, stable mineral flux rates, moderate algal production, stable organic fluxes, and relatively low sedimentation rates, implying insignificant influx of silt-laden waters during winter when meromictic stability is minimal. The absence of sapropel in the sediments during he meromictic era suggests that sapropel is more characteristic of anoxia and adequate Fe and S than of incomplete winter circulation. Whereas we were able to document a change from Daphnia rosea to Daphnia pulex associated with the destruction of meromixis in 1963, we cannot explain this shift. Similarly, major changes in the flux of Daphnia rosea and Bosmina longirostris microfossils to the sediments in the lower portion of the core are as yet unexplained. An attempt to study the population dynamics of Daphnia rosea based on size-frequency distributions and using a computer model demonstrated that there is a systematic underrepresentation of the smaller instars preserved in the sediments, making such studies impossible. These data show the lake has been eutrophic most of the time since 1900 and perhaps at various times in the preceding 230 yr, including the period since European settlers arrived in 1856. The lake has experienced major changes in productivity in the past 350 yr due to disturbance in the watershed from lumbering, road construction, and probably periodic utilizations by beaver colonies. There is no unequivocal evidence of meromixis at times prior to 1915 when the sawmill left the lake.