Paleocology of Hall Lake, Washington: A History of Meromixis and Disturbance

Abstract

Hall Lake, Washington, has been meromictic and holomictic for known periods in the recent past (1950 to 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 varves 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. The relative abundance diagrams for pollen and major diatom taxa were also constructed. The sedimentary record left during the known period of meromixis (1950 to 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 the meromictic era suggests that sapropel is more characteristic of anoxia and adequate Fe and S than of incomplete winter circulation. A change was documented frommore » Daphnia rosea to Daphnia pulex associated with the destruction of meromixis in 1963.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.« less