Daytime Variations in Phytoplankton Photosynthesis

A study of COS-removal during July and August, 1954, under natural conditions in western Lake Erie demonstrated a maximal rate during the hours of 0700-1000 (10 rmoles CO2 absorbed per liter of water per hour), a reduced rate during the hours 1000-1600 (Gpmol/L/ hr), a slightly negative rate during the daylight hours 1600-1900 (1 pmol/L/hr), and nighttime negative rates similar to the day-time positive rates (-6pmol/L/hr). Doty and Oguri (1957) have reported evidence of daily periodicity in photosynthetic rates exhibited by marine phytoplankton. They demonstrated a 5.7-fold higher rate for collections taken at 0800 than for collections taken at 1900, and showed that a sharp decline in photosynthetic capacity was evident during the late daylight hours 16002000. In my report on photosynthesis of phytoplankton in western Lake Erie (Verduin 1956a) I described a method of measuring photosynthesis under completely natural conditions. This is accomplished by measuring the pH of water samples collected at intervals during the day, using a Beckman pII meter, The pH change between any two intervals can then be converted to CO2 change by referring to a curve prepared by differential titration of the lake water with O.OlN NaOH. During July and August, 1954, surface water pII values were determined at 3-hour intervals with the exception of the 2200-0700 interval, during which no determinations were made. These determinations were not planned to study periodicity in photosynthesis, but simply to measure the magnitude of CO2 changes under completely natural conditions. After reading the paper by Doty and Oguri I recognized that our data contained an interesting trend which might represent the effect of this phenomenon. The observed pH changes during each time interval (Eastern Standard time) were converted to pmoles COz absorbed per liter of water per hour and are shown in Figure 1. A wide scatter of values is evident (caused by variations in plankton population, meteorologic conditions, etc.) including some negative values during the hours of 1000-1600. These were associated with dense cloud cover and rain during the interval between pH determinations. The data show a maximal rate of CO2 removal during the 0700-1000 interval. The mean value for that interval is about 10 pmoles of CO, removed per liter of water per hour. The photosynthetic rates were lower during the 1000-1300 and 1300-1600 intervals, yielding a mean of about 6, if the negative values are excluded. During the 1600-1900 interval negative values were observed more often than positive values, and the mean rate computed from the data is about 1, indicating that the respiration rate in the aquatic community during these hours usually exceeded the photosynthesis of the autotrophic component, although the mean light supplies during this time interval (submarine photometer data, Weston photronic cell) were about one-third the mean intensity during the 0700-1000 interval. On the basis of photosynthesis vs. light intensity curves (Verduin 1956a) a rate about one-half as high as the 0700-1000 rate would be expected, rather than the negative average rate appearing in Figure 1. During the hours of darkness negative values were observed, indicating a mean respiration rate of about G pmoles of COz evolved per liter per hour. The low rates of photosynthesis observed during the late afternoon are similar to the decline in activity observed by Doty and Oguri in samples collected during this time of day. Their methods were quite different,