The light : nutrient ratio in lakes: a test of hypothesized trends in bacterial nutrient limitation

Thomas H. Chrzanowski and James P. Grover Department of Biology, The University of Texas at Arlington, PO Box 19498, Arlington, TX 76019, U.S.A. E-mail: [email protected] Abstract The light-to-nutrient hypothesis explores how the balance between energy (as light energy) and nutrients (as total phosphorus) shapes aquatic ecosystem structure and process. The balance of energy and nutrients is thought to regulate ecosystem structure and process such that, in a ``high'' light-to-nutrient environment, bacteria would probably be driven towards phosphorus (P) limitation, whereas, in a ``low'' lightto-nutrient environment, bacteria would be driven towards carbon (C) limitation. We assessed the growth limitation of bacteria in two reservoirs of the southern U.S.A. using a mortality-corrected dilution-growth approach. We compared the frequency of P and C growth limitation with the intralake variation in the light-to-nutrient environment. As a metric of the light-to-nutrient environment, we used the ratio of the mean light in the surface mixed layer (Im) to the total phosphorus concentration (TP ). In each lake, bacterial growth was more often P-limited when the Im : TP ratio was above the median ratio than below. We believe our data provide the ®rst evidence supporting this aspect of the light-to-nutrient hypothesis.