The problem of phytoplankton production in tropical waters differs from its counterpart in temperate regions in several important respects. The higher temperature and stronger radiation that are usually found within the tropics are the primary factors responsible. It is well known that certain species of planktonic alg,e are limited to cold water. The wellmarked seasonal changes that occur in temperate latitudes afford an opportunity for the development of both stenothermal and eurythermal organisms :~t different times of the year in the same lake. There is a good deal of data available to show that fluctuations in the population of certain planktonic algre are controlled by limiting concentratio'ns of various chemical nutrients, e.g. phosphorus and nitrogen [2J, silica [3J. However, Ruttner [I J in a recent review of limnological problems, states that the overall production of phytoplankton is uSl!ally limited by low temperature in temperate lakes although, in winter. light is freqllently a limiting factor. Hutchinson's conclusions, from a study of phytoplankton growth in Linsey Pond , were that fluctuations in growth could not be explained merely in terms of chemical nutrient deficiencies.