We performed a set of nutrient-enrichment bioassay experiments using metal-clean protocols, testing for macronutrient and micronutrient limitation of algae and bacteria in western Lake Superior. Our analysis involved a novel approach to estimating the strength of nutrient limitation in factorial enrichment designs. We found that phytoplankton and bacterioplankton growth both increased with added phosphorus alone but not with iron, manganese, or zinc alone. However, even very small increases in growth rates following P supplementation (on the order of an additional 0.1 d-1) induced iron limitation in the phytoplankton, indicating that the algae in this large lake may be on the cusp of micronutrient and macronutrient limitation. In contrast, and contrary to expectations, bacteria responded only to P and not to metals. Spatial and temporal variability in degree and quality of nutrient limitation was considerable.