LAKES that have become acidified by airborne pollutants typically have low biological productivity and support an impoverished flora and fauna(1). In their natural state, these lakes were poorly buffered, and had a pre-industrial pH on the acid side of neutral(2). Although they can be neutralized by adding base, the resulting calcium-rich water supports plant and animal communities that are unlike those found in natural softwater lakes(1). IL is known, however, that the long-term buffering of soft waters can be appreciably influenced by their biological productivity(3-5). Here we report field-study results that show that by adding phosphate fertilizer to stimulate primary productivity, it is possible to generate sufficient : base by the assimilation of nitrate to raise the pH of acid lake waters without drastically altering their community structure. Owing to the high efficiency of base production, only modest additions of phosphate are required, phytoplankton growth is not excessive and there is a marked increase in biological productivity at all trophic levels. In the longer term, additional quantities of base should be generated by the anoxic decomposition of organic material accumulating on the lake bed.