History of the acid precipitation problem
Interest for acid precipitation in Québec arose at the end of the 70's, after scientific studies showed that southwestern Québec was receiving highly acidic precipitation. These observations added to the fact that this territory was also very sensitive to acidification urged governments, interest groups, scientists and the population in general to look at possible consequences of such pollution on the environment. The scientific community does recognize that acid precipitation has adverse effects on aquatic, terrestrial, and forest ecosystems, as on human health.
In the early 80's, the Ministry of the Environment has put emphasis on the evaluation of the impact of acid precipitation on surface waters, biological organisms and precipitation quality. From 1986 to 1990, the Ministry conducted the first phase of the water quality and fish population monitoring programs as part of the Québec Lake Survey (QLS).
The chemical portion of the QLS survey represents a statistical census of the surface water quality of Canadian Shield lakes located north of the St.Lawrence River. Water samples were gathered for 1253 lakes selected randomly from 160 000 lakes scattered throughout five hydrographic regions. Each lake sample was analysed for 19 variables (pH, color, etc.) for a grand total of 24 000 laboratory analyses. These samples enabled us to describe the acidity, sensitivity and acid deposition levels, along with the origin of acidity for Canadian Shield lakes. This same survey network enabled us to assess the health status of fish populations of more than 253 lakes based on the acidity and sensitivity levels of surface waters.
Sensitivity of surface waters generaly depends on the soil and bedrock capacity to neutralize the precipitation acidity. This implies that swift examination of geological maps helps to rapidly identify the potentially vulnerable areas of Québec. In general, the Canadian Shield located north of the St.Lawrence River constitutes the most sensitive area to acid deposition. Water sensitivity is also very high in mountainous regions where the layer of soil overlying the bedrock is shallow (Laurentian Wildlife Preserve and Côte-Nord). On the other hand, the St.Lawrence Southshore, Anticosti Island, Lake St.Jean area, the area north of Ottawa-Hull, and some sectors of the Québec Northwest are less sensitive to acidification because of the presence of limestone and carbonate-rich soils within the watersheds.
In Québec, acidic lakes are mostly found on the Canadian Shield. They are mostly located east and south-east of Rouyn-Noranda in the Abitibi-Témiscamingue area, in Mauricie near the Laurentian Wildlife Preserve, and on the Moyenne Côte-Nord area. The number and proportion of acidic lakes and those ongoing acidification (in transition) vary greatly from one hydrographic region to another. These differences can be explained by an exposure more or less important to acidic deposition or by differences in environment sensitivities. The Côte-Nord and Outaouais regions thus present acidic lake proportions greater than the overall mean with 33 % and 23 % respectively, while proportions are 15,9 % in Abitibi, 11,8 % in Mauricie, and 6,9 % in Saguenay - Lake St.Jean. The number of acidic lakes also varies greatly form one region to another. It varies from 2 447 in Abitibi to more than 13 000 in Côte-Nord. This variability is also observed for pH<=6 lakes (acidic and transition lakes). The percentage of such lakes goes from 29 % in Saguenay - Lake St.Jean to 66 % in Côte-Nord, with a total lake number going from 6 171 (Abitibi) to more than 26 000 in the Côte-Nord area.
There are more than 450 000 lakes of all sizes in Québec. Recent studies conducted south of the 51rst northern parallel (latitude of Mistassini Lake and Manic 5 reservoir) show that there are at least 29 432 acidic lakes out of the 159 842 census lakes having an area larger than one hectare. This represents a acid lake/total lake ratio of 18 %. The combined number of acidic and transition lakes should be well over 81 000, or 52 % of all census lakes.
The origin of lake acidity in Québec comes from different sources. For most of the lakes from the Outaouais, Mauricie, and Abitibi regions, surface waters have been recently acidified because of high acidic sulphate and nitrate deposition originating mostly from human activities, while acidity of surface waters from Côte-Nord lakes is essentially natural (brownwater lakes).
As for water quality, sulphates are often considered as a good indicator of the acid deposition intensity. A high sulphate concentration measured in a lake often implies high deposition loads. Bedrock and soils within the watershed may also contribute sulphates. This natural contribution is generally minimal and varies very little within the Canadian Shield area. The highest sulphate concentrations are observed in southwest Québec and around Rouyn-Noranda, and decrease toward the north and northeast. Sulphate concentrations in lake goes from more than 6 miligrams per liter in the Québec's southwest to less than 2 miligrams per liter on the Côte-Nord. This spatial pattern of concentrations is almost identical to the one for sulphate deposition, where annual wet deposition decrease from more than 30 kilograms per hectare per year in southern Québec to less than 10 in northern and northeastern Québec.
Close observation of the presence, population structure, and biological species richness in relation to water quality helps us to evaluate their health status, along with the damage intensity caused by acidity. This obervation of the organisms is possible through field sampling based on different capture methods (nets, hoop nets and minnow's traps for fish sampling, benthos' traps for sampling of aquatic insects, etc.). As for fish populations, 253 Canadian Shield lakes were visited during the summers of 1985 to 1990, at a rate of 40 to 70 lakes for each hydrographic region.
Scientific studies clearly show that species richness for fish and other aquatic organisms (plankton, aquatic insects, plants, etc.) decreases with an increase in lake acidity. Most organisms do not suffer adverse effects when lake pH remains higher than 6 units. Between 6 and 5.5, lake acidity can get rid of the most sensitive fish species such as minnows and walleye. Damages increase as pH goes from 5.5 to 5. At pH 5, 75% of all fish species will have disapeared. When lake pH goes under 5, only the most resistant fish species can survive, although their reproduction capacity becomes highly compromised. In the Outaouais hydrographic region alone, we evaluate that more than 10 000 fish populations (distributed along 30 or some species) may have disapeared since the start of the century.
With addition to the diversity losses, acidification is also responsible of various direct and indirect effects on biological organisms, such as: decrease in population density, disapearance of certain species, increase of egg's and fry's mortality rate, increase of environmental stress, appearance of respiratory problems at the gill level, decrease of capture rates, increase toxicity of aluminum and other micro-pollutants, impoverishment of the food web, impoverishment of the species genetic pool, etc.
The dammages and stress levels related to lake acidity vary among hydrographic regions. The highest dammages were observed in those sectors receiving the highest acidic depositions. This is the case for acidic lakes from the Outaouais, Mauricie and Abitibi regions, where fish populations where particularly impacted. Fish populations have disapeared from several of these lakes in these areas. As opposed to these, few effects were observed for fish populations from the Saguenay - Lake St.John area, where the proportion of acidic lakes is lower. The Côte-Nord area shows a very different picture. Although it hosts a very high proportion of acidic lakes, this area was not much affected by anthropogenic acidification (human origin). A large proportion of lakes devoided of fish populations are in fact "virgin lakes" where fish has never been introduced. Also, the naturally occurring acidity conditions observed for this area have probably been existing since several millenias, thus implying that fish populations may have adapted to such acidic environments.
Acidification also has an impact on aquatic vegetation. When the pH goes from 6 to 5 units, aquatic plants are replaced by mosses, which eventually cover the entire bottom of the lake. This invasion, combined to the ambient acidity, lowers the quality of the fish habitat by covering and destroying the spawning sites. This phenomenum also accelerate the disapearance of aquatic insects on the lake bothom (benthos), which provide the major food source for many fish species.