Primary Production in Lakes

The mean photosynthetic rate of lake phytoplankton under optimal light (natural conditions) is about 0.5 micromoles of O2 evolved or CO2 consumed per microliter of organisms per hour; 1–2 micromoles per milligram ash‐free dry weight per hour; and 0.2 micromoles per microgram of chlorophyll per hour. Simple equations are presented for computing photosynthetic yield per square meter of water surface. Computations for several lakes yielded values mostly lying between 150–200 millimoles per square meter per day. A year‐round study under completely natural conditions in western Lake Erie showed winter yields of about 40 millimoles per square meter per day and summer maxima of about 300. The annual curve followed the solar radiation curve closely. It is estimated that 28 days are required for a gram of phytoplankton (ash‐free dry weight) to produce an additional gram of glucose available for reproduction; 80 per cent of the photosynthate produced is required to support phytoplankton respiration.