Studies on Asterionella Formosa Hass: II. Nutrient Depletion and the Spring Maximum

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Abstract
Observations have been made during five years on the fluctuations in the numbers of Asterionella formosa and other plankton diatoms, and in the concentrations of dissolved silica, nitrates and phosphates in four bodies of water in the English Lake District. This account refers mainly to the period from winter to summer. The close of the spring period of increase of Asterionella is not directly due to light or temperature. Loss of cells by floods is more or less compensated for by the replenishment of the lake with nutrients in the inflow water. Grazing by animals has no appreciable effect on the fluctuations in numbers. Fungal parasitism may affect the course of the spring increase in numbers, but is rarely the cause of its end. Very rarely, depression of the numbers of Asterionella by parasitism may lead to other diatoms becoming dominant and utilizing the available nutrients (e.g. silica). The close of the spring period of increasing numbers of Asterionella is frequently due to depletion of the available silica. Confirmation is deduced for Pearsall's (1932) view that diatoms cannot multiply to any marked extent when the concentration of silica is less than 0.5 mg. per litre. The supply of available silica does not provide a complete explanation of the fluctuations in diatom numbers in all the lakes considered or at all times of year. The mean silica content of the cells of Asterionella formosa is 140μg. per million. Confirmation has been obtained for the view of Einsele & Grim (1938) that the amount of silica per unit area of cell is constant under all conditions. The nitrogen content in natural and cultural populations varied from 6 to 12μg. per million cells (mean value 8μg.). The phosphorus content, by contrast with silica and nitrogen, varied widely in natural and cultural populations (0.06-4.2μg. per million cells). The cells of Asterionella can store phosphorus in excess of immediate requirements and when the concentration in the water is 1μg. per litre or less. Asterionella can utilize calcium at a concentration of 400μg. per litre, a lower concentration than occurs in any lake in the English Lake District. Reasons are given for the view that the supplies of nitrogen, phosphorus, carbon and calcium appear to be sufficient to support larger populations of Asterionella than are observed during the period under review. Little or nothing is known concerning the importance of any other substances in the lake waters. Limitation of growth due to lack of silica shows a difference from that due to lack of light. The present observations and conclusions are discussed in relation to those for other lakes made by the author and other workers. It is emphasized that every algal species and lake must be considered separately.