The dependence of substrate saturated uptake of 15NH4+, 15NO3−, 32PO43−, and 14CO2 on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO43− uptake was not dependent upon PPFD at any time of the year, whereas NH4+, NO3−, and CO2 uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH4+, NO3− and CO2 uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH4+, NO3− and PO43− were incorporated in the dark at measurable rates most of the year, whereas dark CO2 uptake was always near 0 relative to light uptake. CO2 and NO3− uptake were more strongly influenced by PPFD than was NH43− uptake. The PPFD dependence of PO43−, NH4+, NO3− and CO2 uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.