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An annual investigation of rates of photolithotrophy, photoheterotrophy, and chemoheterotrophy utilizing glucose and bicarbonate was made within the pelagic zone of a small, hardwater, southwestern Michigan lake. Sampling proceeded on a monthly, diurnal, and depth-wise basis. Annual mean photoheterotrophic uptake was estimated at 2.6μg C m(-3)h(-1). Two periods of relatively high activity were observed: one during spring overturn and the second during the late summer period. In general, greatest contributions to overall carbon cycling occurred during morning to midday incubation periods and at intermediate depths within the water column. Rates of chemoheterotrophy averaged 6.9μg C m(-3)h(-1) and were relatively uniform throughout the annual period. Greatest overall chemoheterotrophic activity was associated with periods of overturn. In general, this activity increased throughout the day and with increasing depth within the water column. The annual mean for photolithotrophic fixation was 1.33 mg C m(-3)h(-1). Greatest contributions to rates of photosynthesis were associated with epilimnetic waters during early morning and midday incubations. Relatively minor contributions to inorganic fixation were made by waters below the 6-meter contour. Spring overturn and late summer represented periods of particularly great photolithotrophic activity. Quantitative comparisons among carbon pathways indicate that rates of pelagic heterotrophy, both photo- and chemoheterotrophy combined, contribute small quantities of carbon to overall carbon metabolism in this oligotrophic system. Qualitative comparisons among pathways indicate strong spatial and temporal separation. The late summer period showed greatest seasonal separation of the three pathways. Spring values represented a period of relatively high activity for all three pathways. On a depth-wise basis, photolithotrophic activity was greatest near the surface and chemolithotrophic activity greatest near the bottom. Photoheterotrophy took an intermediate position between the two. Diurnally, photoheterotrophy and photolithotrophy showed greatest activity during midday and early morning periods, whereas chemoheterotrophy increased throughout the daylight period and reached maximal values in sunset incubations.

This study examines the occurrence of photoheterotrophy, the light—mediated assimilation of organic compounds at or near natural substrate concentrations, in the phytoplankton of a lake. The pelagic zone of Lawrence Lake, an oligotrophic, dimictic, hard—water lake in southwestern Michigan, was selected as the study site because extensive information is already available on its carbon cycle. The uptake of glucose by the plankton was monitored in both light and dark bottles over an annual period. Uptake was measured during 3 sampling periods throughput the daylight hours (i.e., sunrise, midday, and sunset) and at 3 depths within the water column (i.e., 2, 6, and 10 metres). The patterns of dark heterotrophic activity, chemoheterotrophy, and light—mediated heterotrophic activity, photoheterotrophy, were significantly related to the variables of months, depths, and time of day. Chemoheterotrophic activity generally increased throughout the daylight period and with greater depth within the water column. Maximal values were generally observed during the sunset—incubation series and at 10 metres depth. Generally high and uniform activities with respect to depth were observed during periods of water circulation. Increasing activity at greater depth during the stratified summer period was also observed. Maximal values for photoheterotrophy were observed during spring circulation and late summer stratification. Activity was generally greatest below 2 metres and during morning and midday incubation periods. There was an apparent shift during the daylight period in the area of maximal uptake from 2 and 6 metres in the morning to 6 and 10 metres as the day progressed. Chemoheterotrophy and photoheterotrophy are both temporally and spatially separated with respect to activity within the water colum on a diurnal as well as a seasonal basis. The study revealed that dark bottle measurements of heterotrophic activity undrestimate total activity and can be justified as a means of assessing chemoheterotropich activity only. Light bottle uptake values averaged 146% of dark bottle estimates during the light period. Photocheterotrophy was shown to be quantitatively comparable to chemoheterotrophy within the pelagic zone. Photoassimilation averaged 67.6% of chemoheterotrophic activity based on individual comparisons (N = 360) over an annual period. Photoheterotrophy represents an important step in our further understanding of the cycling of carbon within aquatic systems and of biological interactions within the planktonic community.