SIZE STRUCTURE OF PARTICULATE BIOGENIC SILICA IN LAKE-MICHIGAN

Recent data suggest that chemical estimates of biogenic silica in the Laurentian Great Lakes include large fractions of non-living fragmented diatoms. Therefore, measurements of particulate biogenic silica (BSi) in different size classes (0.2-0.4-mu-m, 0.4-10-mu-m, 10-20-mu-m, and > 20-mu-m) were made from April through July 1987 in southeastern Lake Michigan to assess the importance of diatom fragments to chemical estimates of BSi. During the spring diatom bloom, the period of maximum BSi concentrations, the greatest percentage of BSi (86%) was found in the microplankton size fraction (> 10-mu-m) associated with living diatoms. By contrast, following thermal stratification, when dissolved silica was depleted from epilimnetic waters, particles < 10-mu-m in size dominated and averaged 52% of total BSi. This fraction contained mostly fragmented diatom frustules. Our estimate of non-living diatoms is consistent with previous studies showing that, following the spring diatom bloom, 40% to 60% of total diatom abundance is non-living. Seasonal decreases in epilimnetic BSi, declining from 11.3-mu-mol L-1 in late April to an average of 1.96-mu-mol L-1 during the stratified period, were due primarily to loss of the largest size fraction (> 20-mu-m) which decreased from 76% of total BSi in April to 36% in July. BSi < 10-mu-m varied by less than a factor of two throughout the study period, averaging 3.23 +/- 2.79-mu-mol L-1. Particles 0.2-0.4-mu-m averaged 0.277 +/- 0.178-mu-mol L-1. These particles are probably produced through frustule dissolution, seetle slowly, and may lead to enhanced BSi recycling rates.