J. Great Lakes Res. 22(4):803-809 Internat. Assoc. Great Lakes Res., 1996 A Comparison of Photosynthate Allocation in Lakes Bruce C. Wainman! and David R.S. Lean 2 1McMaster University Midwifery Education Programme St. Joseph's Hospital Hamilton, Ontario LBN 4A6 2National Water Research Institute Canada Centre for Inland Waters P. O. Box 5050 Burlington, Ontario L7R 4A6 ABSTRACT. We compared the relationships between photosynthate allocation to protein, carbohy- drate, lipid and low molecular weight (LMW) fractions and the variables daylength and water tempera- ture in Lakes Huron, Michigan, and Ontario as well as three smaller headwater lakes in the Lake Ontario drainage. In all lakes investigated the allocation of recently produced photosynthate to carbohydrate was strongly related to daylength (% carbohydrate = -3.5 * daylength (hr) + 72.8; n = 59, ,-2 = 0.56). The percentage of photosynthate allocated to protein was a function of water temperature in all lakes although the y-intercept for the protein-temperature relationship was much lower in the three headwater lakes and Lake Ontario (% protein = 0.50 * temperature (DC) + 6.1; n = 37, ,-2 = 0.52) than in Lake Huron and Lake Michigan (% protein = 0.68 * temperature (DC) + 24.2; n = 23, ,-2 = 0.49). The increase in allocation to protein was related to a decrease in allocation to low molecular weight material (% LMW = -1.1 * % protein + 57.13; n = 60, ,-2 = 0.72). The percentages of photosynthate in lipid and LMW material were not related to any of the environmental variables measured. Assuming that photosyn- thate allocation is related to biochemical composition, the phytoplankton in Lakes Huron and Michigan were more protein rich for a given temperature than those in Lake Ontario and in the smaller inland lakes. The protein deficit was due to an increase in allocation to LMW material. INDEX WORDS: Phytoplankton, Great Lakes, photosynthesis, photosynthate. INTRODUCTION The majority of carbon fixed by phytoplankton is rapidly allocated to the biomolecules protein, lipid, and carbohydrate, and to low molecular weight (LMW) intermediate biosynthetic materials (e.g., Cuhel and Lean 1987a). The allocation of photosyn- thate is the first step in determining the biochemical composition and, therefore, the food quality of phy- toplankton for planktivores. Adequate levels of pro- tein and lipid in the phytoplankton are absolutely necessary for the growth and reproduction of zoo- plankton (Scott 1980, Ahlgren et al. 1990). Morris et al. (1974) suggested that the instanta- neous physiological state of phytoplankton could be assayed by measuring the relative rate of carbon al- located to protein, lipid, and carbohydrate. They showed that for cultured marine phytoplankton pro- tein production was maintained under low light or 803 low nitrogen and low temperature conditions while lipid and carbohydrate synthesis decreased. Later researchers working with both cultured and natural populations of algae have shown that the allocation of photosynthate is a function of algal species and growth stage as well as a number of environmental factors such as light intensity, daylength, tempera- ture, and nutrient availability. Interpretation of photosynthate allocation data, particularly in natural populations of algae, is diffi- cult. For example, the current view is that the frac- tion of recently produced photosynthate allocated to protein is highest with high temperatures (Cuhel and Lean 1987b) and adequate nitrogen supply (Madariaga and Joint 1992) which is opposite to the conclusions of Morris et al. (1974). Allocation of photosynthate to lipid appears to be indicative of nitrogen stress for many algae (Shifrin and Chisolm 1981, Suen et al. 1987) and silica