The Seasonal Cycle of the Plankton Diatom, Melosira Italica (EHR.) Kutz. Subsp. Subarctica O. Mull

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Abstract
The periodicity of this diatom was followed in some lakes during 2 and others during 7 1/2 consecutive years. It is most abundant between autumn and late spring, and the spring maxima are generally earlier in the shallow than in the deep lakes. A decrease in numbers usually occurs in spring before thermal stratification of the water begins, though, when it does, the alga disappears rapidly and almost completely or wholly from the water. Many filaments pass to the deposits in summer and there remain alive till they are resuspended in the water through the resumption of isothermal turbulent conditions in autumn. On and within the deposits the cells pass into a "resting stage" which can be recognized by the changes occurring in the size and shape of the chromatophores. Some cells, at least, can exist for 3 years in the dark under anaerobic conditions, but growth never occurs. It sinks fast relative to some other plankton algae which commonly accompany it. Parasitism by fungi is common but rarely causes appreciable changes in the growth of the population. Grazing by animals is also unimportant. Floods, in some lakes, lead to marked losses by outflow. Ice-cover leads to a rapid loss from the surface to the deeper layers and, eventually, to the deposits, during inverse thermal stratification. Practically nothing is known about its nutrition. Observations suggest that its growth is stopped if the concentration of available silica in the water falls to approx. 0.6 mg./l. The periodicity of the diatom is considered to be due to its relatively high sinking-rate, its inability to grow appreciably at high light intensities (and perhaps high temperatures) and its ability to remain alive on and in the deposits in the dark and under anaerobic conditions. The changes in population occurring in one season or a series of seasons may influence the size of the maxima in the next because of the importance of the resting stage in providing a reserve to live cells during summer which serve as an "inoculum" for renewed growth when thermal stratification is lost in the autumn. The ecology of other planktonic Melosirae is considered.