CAUSES OF DAILY RHYTHMS IN PHOTOSYNTHETIC RATES OF PHYTOPLANKTON. II. PHOSPHATE CONTROL OF EXPRESSION IN TUNDRA PONDS

¹ Department of Biological Sciences, State University of New York at Albany, Albany, New York 12222

Experiments were carried out to determine if photosynthetic rates of phytoplankton were nutrient-limited in a temperate lake and in an arctic pond. They were also designed to determine if daily or seasonal patterns existed.

Photosynthetic rates were measured under defined light intensities and temperatures, at various concentrations of sodium phosphate. Daily patterns were determined with 5 or 7 measurements made over a 24 to 30 hour interval.

Photosynthetic rates were stimulated by the addition of phosphate to samples of water from Lake George, New York. They were stimulated on only some of the sampling dates during one year of survey. When phosphate was not limiting, neither were other essential nutrients, with one exception. Later work, in the arctic, suggested that a negative result should not be interpreted as a non-limiting situation, however.

Arctic phytoplankton show strong daily oscillations in photosynthetic rates. The daily maximum occurred most frequently in the early evening, at 1800; that is, at the supper hour. Manipulation of the samples was necessary to evoke the response.

The addition of phosphate, or storage of the samples in low light intensity, if done early in the day, was an adequate treatment to evoke the daily maximum. The synchrony of the peak under a variety of treatments was argument for the involvement of an endogenous rhythm rather than a direct environmental reaction.

The lag in reaction to nutrient stimulation is viewed as part of a mechanism to promote stability of the phytoplankton community and an obstacle in the interpretation of negative results from short-term nutrient bioassays.