ASSESSMENT OF THE EFFECT OF ACIDIC DEPOSITION ON CANADIAN LAKES - DETERMINATION OF CRITICAL LOADS FOR SULFATE DEPOSITION

The operational definition of the ''critical'' load for wet SO4 deposi tion is that which does not increase the regional percent of lake havi ng p less-than-or-equal-to 6 for the portion of the population that hi storically had pH>6. Reviews of the effects of acidic deposition on aq uatic biota show that the pH=6 threshold used to determine the critica l load is sufficient to protect most organisms. It is inappropriate to specify a single critical load for the whole of eastern Canada due to variability in terrain sensitivity. The Atlantic provinces and easter n Quebec generally have very low critical loads (<8 kg. ha-1.yr-1). Th e terrain characteristics of this area dictate a critical load very cl ose to background deposition. Lakes in southwestern Quebec and Ontario have critical loads of <8 to >20 kg.ha-1.yr-1 reflecting the wide ran ge in terrain sensitivities that exist in each province.