The Survival of Freshwater Algae During Dry Periods: Part II. Drying Experiments: Part III. Stratification of Algae in Pond Margin Litter and Mud

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Abstract
For most habitats the number of species of algae found and the frequency of individuals observed decreased during drought, particularly below a moisture content of about 50% (of the wet weight). Nitzschia palea and Oscillatoria tenuis, however, showed an increase during the initial stages of drought. Only 2 species, Pandorina morum and Euglena mutabilis, did not appear to survive severe artificial drying but even these are able to survive some exposure in nature. Although survival of some species depends upon earlier-formed zygospores, certain algae, normally aquatic (e.g. Cosmarium cucurbitinum), may survive drought by some modification of the vegetative stage usually including one or more of the following: accumulation of oil in the protoplast; thickening of the cell wall; the protective action of a mucilage sheath. The germination of a resting stage may be delayed longer after a prolonged resting period than after a short one. Germination may also be delayed by resistance to rewetting by the litter or mud, or, conversely, by a sudden complete immersion of the resting stage. A buried slide technique and a simple micro-sampling method were used to study the micro-stratification of algae in pond margin litter and mud. Many algae may avoid the consequences of drought by descent into the deeper layers of litter at the pond margin.