2010 Progress Report - International Joint Commission

Reduction in surface water calcium concentrations
has several ecological implications, although many
impacts are not well quantifi ed yet. Populations of
calcium-rich zooplankton (e.g. Daphnia species)
occur in many Canadian Shield lakes. In the Muskoka
region of Ontario, the mean calcium concentration
of 36 lakes declined 13% between 1985 and 2005.
Contemporaneous evaluation of the relative abundance
of calcium-rich daphniids in 43 Muskoka lakes showed
that they have declined in 60% of the lakes having a
present-day calcium concentration less than 1.5 mg L -1
(the level at which reproduction is delayed), and in
Recovery of Acidified Lakes
and Streams
Acid rain, resulting from SO 2
and NO x
emissions, is one of many large-scale
anthropogenic effects that negatively
affect the health of lakes and streams
in the United States and Canada.
Surface water chemistry provides direct indicators of
the potential effects of acidic deposition on the overall
health of aquatic ecosystems.
UNITED STATES
Three indicators of acidity in surface waters provide
information regarding both sensitivity to surface water
acidifi cation and the level of acidifi cation that has
occurred today and in the past. These indicators are
sulphate ions (SO 4
2-
), nitrate ions and acid-neutralizing
capacity (ANC). Sulphate and nitrate are negatively
charged ions with the potential to acidify drainage
waters and leach acidic aluminum cations from
watershed soils. Aluminium cations are known to be
toxic to aquatic life. Assessments of acidic deposition
effects dating from the 1970s to the present have
shown sulphate to be the primary negatively charged
ion in most acid-sensitive waters.
Long-term monitoring networks, such as the U.S.
EPA’s Long-Term Monitoring (LTM) program, provide
information on the chemistry of lakes and streams,
which allow us to look at how water bodies respond
to changing emissions. The LTM program monitors a
total of 170 lakes and streams, representing the major
acid-sensitive regions of the northern and eastern
67% of the lakes having present-day calcium between
1.5 and 2.0 mg L -1 . Because calcium-rich daphniids
are often the most abundant zooplankton in the lake
environment, the population decline may affect the
entire food web. Hence, declining calcium levels are
expected to affect fi sh and other aquatic species as
well, and the effects may even extend outside of the
aquatic environment to the birds and animals that
depend on the lakes for food. Studies indicate that
even after recovery of lake pH, continued low levels of
calcium could prevent full population recovery of the
daphniids to pre-impact levels.
• Sulphate ion concentrations in surface waters
provide important information on the extent
of base cation (i.e. calcium, magnesium,
potassium and sodium) leaching in soils and
offer insight on how sulphate concentrations
relate to the levels of ambient atmospheric
sulphur and atmospheric deposition.
• Nitrogen is an important nutrient for plant
growth and, therefore, most nitrogen inputs
by deposition are quickly incorporated
into biomass during the growing season as
organic nitrogen, with little leaching of nitrate
into surface waters during the growing season.
As atmospheric nitrogen deposition increases,
there is greater potential for increased leaching
of nitrate into surface waters.
• ANC is a measure of the acid-buffering
capacity of water and an important
indicator of the sensitivity and the degree of
surface water acidifi cation or recovery that
occurs over time. Acidifi cation results in a
diminishing ability of water in the lake or
stream to neutralize strong acids that enter
aquatic ecosystems.
United States (New England, Adirondack Mountains,
northern Appalachian Plateau, and Ridge/Blue Ridge
provinces of Virginia).
Scientific and Technical Cooperation and Research
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