“…As previously mentioned, "acid" rain or SO 2 fumes may reduce the surface pH of soils, waters, and tree barks. In oligotrophic lakes in western Sweden, there was a significant reduction in the pH of surface waters, 339 and soils and waterways in Canada have shown drastic pH reductions due to fumes from nearby smelters. 340 " 343 Lowered environmental pH adversely affects a number of microorganisms, in particular those involved in the nitrogen cycle, e.g., nitrogen fixation by blue-green algae ceases at pH values less than 5.5.…”