THE LIGHT-NUTRIENT RATIO IN LAKES - THE BALANCE OF ENERGY AND MATERIALS AFFECTS ECOSYSTEM STRUCTURE AND PROCESS

The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the rel ative balance of light and phosphorus in a set of freshwater pelagic e cosystems. We calculated a ratio of light:phosphorus by putting mixed- layer mean light in the numerator and total P concentration in the den ominator. This light: phosphorus ratio was a good predictor of the C:P ratio of particulate matter (seston), with a positive correlation dem onstrated between these two ratios. We argue that the balance between light and nutrients controls ''nutrient use efficiency'' at the base o f the food web in lakes. Thus, when light energy is high relative to n utrient availability, the base of the food web is carbon rich and phos phorus poor. In the opposite case, where light is relatively less avai lable 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 grazin g processes. Using the central result of increased C:P ratio with an i ncreased light:phosphorus ratio, we make specific predictions of how e cosystem structure and process should vary with light and nutrient bal ance. Among these predictions, we suggest that lake ecosystems with lo w light:phosphorus ratios should have several trophic levels simultane ously carbon or energy limited, while ecosystems with high light:phosp horus 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.