Planktonic diatoms dominate the Holocene varved-sediment record of Elk Lake, Minnesota. For the past ~10,400 yr, the lake never became shallow enough to allow large numbers of benthic and epiphytic diatoms to become deposited in the center of the lake. The relatively great depth of Elk Lake throughout this time is consistent with the continuous presence of varves in the record and the predominantly autochthonous character of sediment in the profundal part of the lake. The planktonic diatom assemblages are dominated by two species, Fragilaria crotonensis and Stephanodiscus minutulus. They alternate in dominance on scales of hundreds to thousands of years and indicate shifting limnological conditions under subtle climatic control. Fragilaria crotonensis is typically a summer and early-fall diatom that prospers when supplies of silicon are high compared to those of phosphorus. Stephanodiscus minutulus blooms in the early spring when circulation provides abundant phosphorus. The alternation of these two diatoms reflects principally the climatic conditions that drive spring circulation and summer stagnation and thereby control the fluxes of silicon and phosphorus to and within the lake. Cold and dry climates in late spring and early summer promote blooms of S. minutulus, and hot summers with some frontal storm activity provide conditions suitable for F. crotonensis. The mid-Holocene prairie period (8.2 to 4.0 ka) is characterized by a greatly increased diatom accumulation rate and a general dominance of S. minutulus. Between 6.4 and 4.0 ka Aulacoseira ambigua became prominent, implying increased nutrient fluxes and summer turbulence, probably related to winds and storms in that season. Lake levels were probably lower at times during this period. This part of the Holocene, however, was interrupted by a 600 yr interval of moister and warmer climates (5.4 to 4.8 ka), with low diatom influx and a dominance of F. crotonensis. After 4.0 ka diatom productivity fell, and F. crotonensis tended to dominate in response to reduced spring circulation and probably increased precipitation. The Little Ice Age, between A.D. 1450 and 1850, is documented by increased abundances of S. minutulus, indicating cooler late spring conditions. Logging activities in the vicinity of Elk Lake in the early twentieth century allowed Aulacoseira ambigua to return by increasing turbulence and nutrient fluxes to the lake.

Fossil diatoms from a 177-m core (CL-80-1) taken near the center of the main basin (Upper Arm) of Clear Lake, California, provide evidence about the stratigraphie relationships, age, and environmental history of these lacustrine deposits. In general, diatom assemblages from the core are dominated by planktonic genera such as Stephanodiscus , Cyclotella , and Melosira . Shallow-water species of Fragilaria and Amphora are common and sometimes abundant. Several planktonic diatoms from the core are also found in the Kelseyville Formation, which is exposed on the southern margin of Clear Lake and is inferred to underlie the modern lacustrine deposits. The presence of these taxa in the same stratigraphie order in both the Clear Lake core and the Kelseyville Formation suggests partial correlation between the two and implies a relationship between the Kelseyville Formation and the lacustrine sediments beneath Clear Lake. In the upper 50 m of core CL-80-1, diatom assemblages apparently reflect late Pleistocene and Holocene paleoenvironmental changes, although their environmental significance may be obscured by reworking of diatoms from older sediments, by tectonically caused changes in patterns and rates of sedimentation, and by the impact of volcanism. Nevertheless, the diatoms indicate that lacustrine environments have been characterized by fresh, moderately deep, nutrient-rich water throughout much of their sedimentary history. Cooler climatic and lacustrine environments of the late Pleistocene were characterized by a codominance of Stephanodiscus and Melosira species, implying a mesotrophic to eutrophic, stratified lake. After the change from Pleistocene to Holocene climates, Clear Lake became yet more eutrophic and turbid. Stratification was short-term and irregular, and warm-water conditions extended throughout a greater portion of the growing season although there is evidence for a middle Holocene return to cooler and moister conditions. The modern limnology of Clear Lake, which is characterized by massive blooms of blue-green algae and by the abundance of Melosira granulata , apparently began about 15,000 years ago.