Abstract:
As a result of increased acid precipitation, the pH of a large
number of Canadian Shield lakes has been falling. Prior to this study
there was no documentation available to explain the history of lake
acidification for the Algoma area lakes. In order to obtain this
information the diatom inferred pH technique was developed in this
study.
During two field seasons, July 1981 and July 1982, short sediment
cores (circa 25-30 cm) were collected from 28 study lakes located north
of Lake Superior, District Algoma, Ontario. The surface sediment
diatoms (0-1 cm) from each of these lakes were carefully identified,
enumerated, and classified in terms of their pH indicator status.
The surface sediment diatom analysis indicated that lake pH is one of
the most important factors affecting the species composition and
relative abundance of diatom populations. Thus diatom assemblages can
be sensitive indicators of lake acidification. When Nygaard's index
alpha was plotted against observed lake pH, a statistically
significant relationship resulted (r=-0.89; p=<O.OI).
The index alpha regression equation was used to construct the pH
histories of 4 lakes (lakes X4, CS, U3, and WI). The repeatability of
this technique was confirmed by comparing two downcore paleo-pH
profiles of Lake WI. These two paleo-pH profiles represented almost
identical paleo-pH patterns for Lake WI. The paleo-pH study of Lake X4
revealed that the lake has been rather acidic (pH <5.6) for the last
200 years. It appears that the recent increase in acid precipitation
3
over the last 30 years has not altered the water pH compared to the
lake's pH history. However, the paleo-pH study of another acidic lake
(Lake CS) indicated that its pH has significantl}* dropped over the last
30 years . During this time the Lake CS pH has dropped almost 2 pH units
(7.1 to 5.2). The other two lakes studied for downcore pH were
circumneutral in nature . One of these lakes (Lake U3) displayed a
relatively stable pH history while the other lake (Lake WI) displayed
significant pH fluctuations over post-Ambrosia time. The variable pH
history of Lake WI was probably associated with the Algoma sintering
plant plume and forest fires.
A significant relationship between surface sediment diatoms and
observed lake pH and secondly a statistically significant relationship
between index alpha and observed pH suggested that diatoms are one of
the best indicators of lake pH. Thus diatom inferred pH technique has
great potential in explaining the rate of lake acidification.
