| The impact of acid deposition and tree
harvesting on three lakes and their representative sub-catchments in the
Muskoka-Haliburton region of south-central Ontario was assessed using a critical
loads approach. As nitrogen dynamics in forest soils are complex and poorly
understood, for simplicity and to allow comparison among lakes and their
catchments, CLs (A) for both lakes and forest soils were calculated assuming
that nitrate leaching from catchments will not change over time (i.e. a
best case scenario). In addition, because soils in the region are shallow, base
cation weathering rates for the representative sub-catchments were calculated
for the entire soil profile and these estimates were also used to calculate
critical loads for the lakes. These results were compared with critical loads
obtained by the Steady State Water Chemistry (SSWC) model. Using the SSWC model,
critical loads for lakes were between 7 and 19 meq m-2yr-1
higher than those obtained from soil measurements. Lakes and forests are much
more sensitive to acid deposition if forests are harvested, but two
acid-sensitive lakes had much lower critical loads than their respective
forested sub-catchments implying that acceptable acid deposition levels should
be dictated by the most acid-sensitive lakes in the region. Under conditions
that assume harvesting, the CL (A) is exceeded at two of the three lakes and
five of the six sub-catchments assessed in this study. However, sulphate export
from catchments greatly exceeds input in bulk deposition and, to prevent lakes
from falling below the critical chemical limit, sulphate inputs to lakes must be
reduced by between 37% and 92% if forests are harvested. Similarly, sulphate
leaching from forested catchments that are harvested must be reduced by between
16 and 79% to prevent the ANC of water draining the rooting zone from falling
below 0 μeq l-1. These calculations assume that extremely low
calcium leaching losses (9–27 μeq l-1) from forest soils can be
maintained without any decrease in forest productivity. Calcium concentrations
in the three lakes have decreased by between ∼10 and 25%
over the past 20 years
and calculations assume that calcium concentrations in lakes can fall to around 30%
of their current values without any harmful effects on biota. Both these
assumptions require urgent investigation.
Keywords: acid deposition, calcium, critical loads, forests, harvesting, lakes |