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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.

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