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6029979 
Journal Article 
Formation of Sub-Surface Maxima of a Diatom Within a Stratified Lake and in a Laboratory Column 
Brooks, AS; Davey, MC; Heaney, SI 
1989 
The development of a sub-surface layer of the diatom Asterionella formosa Hass. was studied during the final phase of its spring growth and subsequent decline in a moderately productive, 60 m deep lake. The layer was short-lived (about 2 weeks), had a maximum cell concentration four times greater than in the waters above and below, and formed within the pycnocline at the silicon gradient and the base of the euphotic zone. The mechanisms governing the formation of sub-surface maxima were examined in a 4 m tall stratified laboratory column using live and formaldehyde-killed cultures of A. formosa. Dead cells added to the top of the column formed a broad layer as they sedimented through the density gradient. This was in contrast to a more discrete maximum with a sharp lower boundary obtained using silicon-depleted cells as they encountered a silicacline within the pycnocline. There were marked changes in the chemical composition of live cells during the phase of growth and settlement in the experimental column. Silicon-depleted cells entering the deep silicacline showed a decrease in sinking rate within a period of 1 day. It is clear from the laboratory experiments that changes in the biological characteristics of the population, in particular its sinking velocity, are required for the formation of mid-water maxima of the form observed in natural populations. These changes intensify accumulations of settling cells within vertical density gradients. (Author 's abstract) 
Water Resources Abstracts; Diatoms; Lake stratification; Primary productivity; Limnology; Seasonal variation; Asterionella; Silicon; Buoyancy; Chemical composition; Population studies