SummaryFrom a series of experiments and measurements covering one 24‐h period in Lake George the diurnal patterns of photosynthesis, nitrogen‐fixation and stratification are described. The thermal cycle shows three distinct phases of isothermy, intense stratification and mixing. During stratification the surface water temperature reached 36°C, whereas the bottom temperature remained at 25°C. The phytoplankton are evenly distributed at dawn, sink out during the day and tend towards an even redistribution as thermal stratification breaks down. Depth‐time profiles of oxygen concentration and pH values showed that intense photosynthetic and respiratory activity occurred. The shortness of the predominant limnological cycle is thought to be instrumental in restricting species fluctuations.Changes of total CO2 concentration in situ within the euphotic zone indicated that 2.25 g C/m2.12 h were photosynthetically fixed during daytime. From changes of in situ oxygen concentration, gross photosynthesis was estimated to be 12 g O2/m2.12 h and net particulate production over 24 h as zero.Photosynthetic activity was also measured with the i^C and oxygen techniques by enclosing algal samples in light and dark bottles. The patterns of activity recorded by the two methods were similar, although quantitatively the oxygen method gave rather higher values than did the method. Daily gross photosynthesis was estimated as 15.56 g O2/m2.12 h and 4.5 gC/m2.12 h. These figures are compatible with one another and with the production estimates calculated from Tailing's model of integral daily photosynthesis.The balance between respiration and photosynthesis is described as a function of the underwater light climate and the relative rate of respiration. Both the experimental and theoretical approaches suggest that the column 24‐h net particulate production is an extremely small percentage of gross photosynthesis.Nitrogen‐fixation was measured using both the15N2and the acetylene reduction techniques. The day‐long incubation of the15N2experiment showed that 11 mg N/m2.day were fixed. The series of shorter acetylene reduction experiments gave a value of 58 mg N/m2.day. The acetylene technique showed relatively high nitro‐genase activity at very low light intensities. The15N2results indicated little N2‐fixation at low light intensities. The differences between the two techniques are discussed.Gross photosynthesis estimates calculated from seven different methods run simultaneously are compared. The figures show reasonable agreement, but since the gaseous environment in the euphotic zone is particularly suitable for photo respiration the values are not considered reliable estimates of phytoplankt