The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the relative balance of light and phosphorus in a set of freshwater pelagic ecosystems. We calculated a ratio of light : phosphorus by putting mixed-layer mean light in the numerator and total P concentration in the denominator. This light: phosphorus ratio was a good predictor of the C: P ratio of particulate matter (seston), with a positive correlation demonstrated between these two ratios. We argue that the balance between light and nutrients controls 'nutrient use efficiency' at the base of the food web in lakes. Thus, when light energy is high relative to nutrient availability, the base of the food web is carbon rich and phosphorus poor. In the opposite case, where light is relatively less available compared to nutrients, the base of the food web is relatively P rich. The significance of this relationship lies in the fact that the composition of sestonic material is known to influence a large number of ecosystem processes such as secondary production, nutrient cycling, and (we hypothesize) the relative strength of microbial versus grazing processes. Using the central result of increased C:P ratio with an increased light:phosphorus ratio, we make specific predictions of how ecosystem structure and process should vary with light and nutrient balance. Among these predictions, we suggest that lake ecosystems with low light:phosphorus ratios should have several trophic levels simultaneously carbon or energy limited, while ecosystems with high light:phosphorus ratios should have several trophic levels simultaneously limited by phosphorus. Our results provide an alternative perspective to the question of what determines nutrient use efficiency in ecosystems.