shrimp ( Lepidurus arcticus). Ordi- 
narily, this organism undergoes a rapid 
series of molts as it progresses from 
the egg to maturity. My studies show 
that a pH of 6, which is only slightly 
acidic, delays the normal rates of pro- 
gression through this series of distinct 
life stages and more and more of the 
animals die before becoming adult. 
As the pH level decreases, interfer- 
ence with the progression of life stages 
and premature death both increase. 
Another common invertebrate, the 
freshwater scud gammarus ( Gamma - 
rus lacustris), is also absent from wa- 
ters with a pH level below 6, and 
my research has shown that even gam- 
marus adults cannot tolerate a de- 
crease in pH much below the 6 level. 
Immature stages of this scud would 
probably be even more sensitive to 
high acidity. 
Animal life of all kinds in lakes 
and streams, from fish down to the 
smallest invertebrates, depends upon 
energy from food, which is derived 
either from plants growing in the 
water or from organic litter such as 
leaves and twigs transported by the 
wind or the flow of streams. In streams 
and small lakes or ponds, plant litter 
is the most important food source for 
animal life. Ecologists have shown, 
however, that bacteria and fungi are 
very important in making these 
sources of food available to animals, 
and many invertebrates obtain more 
energy from eating these microdecom- 
posers than from eating the leaves or 
other detritus with which the microbes 
are associated. Decreasing this micro- 
bial activity may therefore have pro- 
found effects throughout the food web 
of a stream or lake. 
Investigators working in the 1970s 
observed abnormal accumulations of 
organic litter in Swedish lakes. The 
suspected cause was inhibited decom- 
position due to low pH levels. Several 
other concurrent studies in North 
America and Scandinavia, including 
experiments in stream acidification 
and lake neutralization, indicate that 
microbial decomposition is retarded 
by acidification. 
Small glgae that live suspended in 
the water of a lake are called phy- 
toplankton. By means of photosynthe- 
sis, these plants reproduce themselves, 
creating new organic material in the 
water, a process known as primary 
production. This process is one of the 
basic steps in providing energy from 
food for the animals of a lake. We 
have already seen that the number 
of species of phytoplankton is reduced 
by acidification, but we do not know 
whether acidification also alters the 
process of phytoplankton production. 
The production of phytoplankton is 
regulated by the availability of both 
sunlight and nutrients. In lakes that 
are sensitive to acidification, the criti- 
cal nutrient is phosphorus, a key sub- 
stance regulating plant growth. When 
the effects of acidification on phy- 
toplankton production in different 
lakes are compared, it is difficult to 
tell whether the differences are due 
to the acid itself, to the presence of 
other material, such as aluminum, that 
is increased due to the acid, or to 
the low availability of nutrients. For 
most lakes that are susceptible to acid- 
ification, the susceptibility arises from 
their low concentrations of essential 
nutrients and of various chemicals, 
such as calcium, magnesium, and bi- 
carbonate. Acidification appears to re- 
tard the exchange of nutrients be- 
tween sediments, decomposing litter, 
and the water, however, and so in the 
presence of acidification, nutrients are 
probably even less readily available. 
The acidification of watersheds also 
releases aluminum into the ground 
water, and aluminum can bind to phos- 
phorus, preventing it from moving 
through soils. By this process, the sup- 
ply of phosphorus in streams and lakes 
may possibly be reduced in acidified 
watersheds, which would reduce phy- 
toplankton growth. If acidification 
does reduce phytoplankton produc- 
tion, then part of the foundation of 
animal life and, consequently, the sup- 
ply of food for fish, would be reduced. 
A side effect of lake acidification 
is to make lake water clearer. This 
may result from the acid reducing the 
brownish color of natural humus ma- 
terials in the water, from a reduction 
in the density of algae in the water, 
or from some combination of these 
two factors. Algal density would be 
reduced if the availability of phos- 
phorus was decreased, but whether 
that decrease actually takes place is 
not yet known. 
Large aquatic plants also seem to 
be altered by acidification. In some 
acidified lakes, particularly those 
studied in Sweden, abnormal and very 
dense growths of sphagnum moss are 
found. In Lake Colden, where pH val- 
ues declined from about 5.5 in the 
mid-1950s to 4.7 in the summer of 
1979, sphagnum, not reported in a bo- 
tanical survey conducted in the 1930s, 
is now abundant on the lake bottom. 
Another development seen in the Swed- 
ish lakes, Lake Colden, and many 
other acidified lakes, is the extensive 
growth of long, threadlike algae. 
Both the sphagnum and the algae 
form mats that become a tight barrier, 
sealing off the bottom sediments from 
the overlying water. Since normal mi- 
crobial decomposition seems to be re- 
tarded by acidification, the underside 
of these mats may constitute a rich 
source of organic materials. Low rates 
of microbial activity eventually con- 
sume all the oxygen in this zone, and 
a new oxygen-free environment devel- 
ops. Anaerobic bacteria (those that 
live without oxygen) can thrive there, 
and although they are slower and less 
efficient in decomposing material than 
aerobic bacteria (those that require 
oxygen), they too decompose the dead 
plant matter, producing the gases car- 
bon dioxide, methane, hydrogen sul- 
fide, and some more complicated and 
smelly organic acids and sulfur com- 
pounds as their waste products. These 
wastes erupt from below the algal mat 
in great bubbles. I suspect that this 
is the cause of the garbage-dump-like 
odor that wafts over the surface of 
some acidified Adirondack lakes dur- 
ing the warmest parts of the year. 
In summary, lakes and streams sen- 
sitive to acidification are located in 
regions where the bedrock and over- 
lying soils are deficient in easily 
weathered minerals and thus cannot 
provide much buffering capacity to 
surface waters. Mountainous regions 
are particularly susceptible to acidi- 
fication. Many lakes and streams in 
North America from Florida to New- 
foundland have already been acidi- 
fied. In extreme cases, the biological 
consequences include the complete 
elimination of fish and many other 
species of animals and plants. At in- 
termediate pH levels, the relationships 
among organisms are likely to be 
changed in ways that are not yet 
known but that may exacerbate the 
problems of freshwater fish life. □ 
Plant litter in lakes, such as 
these autumn leaves, is an 
important source of food for 
animal life. If the microbial activity 
that makes this food available 
to animals is inhibited by 
acidification, the lake’s entire 
food web may be affected. 
Bruce D Thom 
64 
