The carbon uptake rate of phytoplankton was found to increase from poorly buffered, headwater lakes to better buffered, downstream lakes in the Turkey Lakes Watershed near Sault Ste. Marie, Ontario, Canada. While the nutrient, sunlight, and temperature conditions remained fairly uniform, the pH, alkalinity, and dissolved inorganic carbon concentration increased in a similar manner as the primary production. Discussions are presented on the relationships of these spatial gradients and their probable causes in such a small watershed (area = 10.5 km2). In particular, results from a hydrological model show that more groundwater flowed into downstream lakes than headwater lakes. The greater soil-water contact time and the higher concentration of calcium carbonate (CaCO3) present in the till at low-lying areas cause the introduction of relatively greater quantities of Ca2+ and alkalinity into the downstream lakes. Thus, the development of acidification-eutrophication models must incorporate the physical, chemical, and biological processes for both soil and water.