The method of measuring photosynthesis described by OSTERHOUT and HAAS (2) was modified by using a Beckman pH meter instead of colored indicators for pH determination. The equilibrium equations published by MOORE (1) in his graphic method of determining free carbon dioxide, bicarbonate, and carbonate concentrations in natural waters form a basis for determining carbon dioxide removal during photosynthesis (or evolution during respiration) by making pH measurements at the beginning and end of an experiment. The graph in figure 1 shows the relation between carbon dioxide removal and pH-change in water of a given total alkalinity (90 p.p.m.) between pH values of 7.7 to 9.5. When a Beckman pH meter is used the associated C02-change can be measured with an error of less than 10%, if the pH-change amounts to 0.30 pH units or more.
The phytoplankton was collected with a 10-liter Juday trap, the samples were transferred to bottles having a volume of about 30 ml. and placed in a dark compartment until the collecting trip was completed (two to four hours). On arriving at the laboratory the samples were diluted with lake water which had been filtered through silk bolting cloth of the same type used on the trap (0.03-0.04 mm.-mesh). The degree of concentration to which the natural populations were subjected varied with their density in the lake. The highest populations observed, 2.5 x 106 standard volumetric units per liter (8x 103 cubic micra equals 1 unit, see WELCH (4)), were made up to concentrations of five times their natural density. The lowest populations observed were made up to 400-fold concentrations. The experimental concentrations, however, were so low that light transmission through a test flask was never more than a few per cent. lower than for a control flask containing filtered lake water. No attempt was made to exclude zooplankton.
Subsamples of the phytoplankton concentrate were enclosed in 125-ml. Erlenmeyers for the photosynthesis test. The flasks were placed in a tray through which lake water was circulated to maintain them near lake temperature, and were illuminated with fluorescent light of about 400 footcandle intensity. Tests showed this intensity to be close to the optimum for photosynthesis of the Tabellaria community which developed in the spring of 1949. The pH was determined at the beginning of an experiment, and again after two hours of illumination. If the pH-change was less than 0.3 units the experiment was continued, but was usually ended within four hours because sharp reductions in rate occurred when tests were prolonged beyond four hours; the rates approaching zero after eight hours of illumination. Such samples, however, recovered their initial rate after being stored in the dark overnight.
Phytoplankton counts were made, on fresh (unformalinized) samples, soon after the photosynthesis tests were completed. Ten fields were counted for each sample; see WELCH (4) for details of counting procedure. Threedimensional measurements were made of the major components of the populations studied providing volumetric units for expressing photosynthetic rates.
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