A comparison of the acidification efficiencies of nitric and sulfuric acids by two whole‐lake addition experiments

The acidification efficiencies of nitric and sulfuric acids were compared for 5 yr by separate experimental additions of the acids to the partitioned north and south basins of Lake 302. In the epilimnion, nitric acid was 70% as efficient as sulfuric acid in reducing alkalinity. On a whole‐lake basis, acidification by nitric acid was about half as efficient as by sulfuric acid. In‐lake processes removed 70 and 57% of the added nitric and sulfuric acids. Before acidification, algal uptake and sediment SO42− uptake were of about equal importance in terms of in‐lake SO42− removal. NO3− was primarily removed by algal uptake and sedimentation. After acidification, algal uptake of NO3− and SO42− did not change, but rates of uptake in the sediments by bacterial processes increased and dominated in‐lake removal. Nitric acid acidified the north basin because the acid additions exceeded the capacity of all biological removal processes. Algal uptake and sedimentation of N could not increase in proportion to increases in NO3− because of P limitation. Sediment‐denitrification rates increased tremendously (∼40‐fold) but only removed a portion of the incoming NO3− because denitrification was limited by the rate of transport to the sediment‐water interface.