Environmental Engineering Reference
In-Depth Information
2010
). Moreover, the acidification of surface waters can stress, or even kill, individual
organisms such as mayflies, which may be sensitive to low pH. The acidification also can
impact food webs by selective impact on different levels of the food chain, for example, on
sensitive cladoceran zooplankton in lakes (
e.g., Confer et al. 1983; Schindler et al. 1985
).
Our long-term studies of forest, lake, and stream ecosystems during the past four dec-
ades have revealed findings about this complexity that are important to the understanding
and management of acid rain as a major environmental problem:
Changes in emissions of sulfur dioxide, SO
2
, as a result of federal and state legislation,
are strongly correlated with changes in sulfate concentrations in precipitation and in
stream water at HBEF (
Likens et al. 2002, 2005
).
Eighteen years of continuous study was required to verify that the acidity of
precipitation had decreased significantly at HBEF (
Figure 15.2
). The volume-weighted,
average, annual pH of precipitation has increased from about 4.1 in the mid-1960s to
about 4.9 today (
Figure 15.2
).
Nitric acid is increasing in importance in precipitation at HBEF and is predicted to be
the dominant acid in precipitation in the near future without further changes in the
controls on emissions of both SO
2
and NO
X
.
Calcium and other plant nutrients have been markedly depleted in the soils of the HBEF
as a result of leaching by inputs of acid rain (
Likens et al. 1996, 1998; Likens 2010
).
As much as one-half of the pool of exchangeable calcium in the soil of the HBEF has
been depleted during the past 50 years by acid rain (
Likens et al. 1998
).
As a result of losses in soil buffering, the forest ecosystem is currently much more
sensitive to acid rain inputs than previously predicted (
Likens et al. 1996, 1998;
Likens 2010
).
So, how did the ecosystem approach help to understand and manage the problem of acid
rain? Understanding the sources and diverse effects of acid rain required me and other scien-
tists to consider the quantitative flux of chemicals across ecosystem boundaries, as well as to
evaluate the complicated exchanges and interactions among air, land, and water (e.g., meteo-
rological analysis of air-mass trajectories from source to deposition, reactions in the atmo-
sphere that made the pH less than 5.6, biogeochemical interactions in the watershed that led
to accelerated weathering, and leaching of calcium from the soil and the dissolution of alumi-
num minerals). The expertise of biologists, geologists, hydrologists, meteorologists, chemists,
foresters, limnologists, and humans (including their social institutions) was and is intricately
entwined in understanding and managing the effects of acid rain on natural ecosystems and
human-made structures; this problem is not just about chemistry! Understanding and manag-
ing acid rain requires careful consideration of boundaries. The low pH of precipitation in the
White Mountains was not a unique consequence of local conditions, nor could it be solved
solely by local action. Boundaries for airsheds, watersheds, lakes, and streams needed to be
determined and evaluated, and political boundaries, although not recognized by pollutants
moving in the atmosphere or water, were important to the political solutions.
Unfortunately, in spite of legislative controls and progress made in reducing emissions
of SO
2
and NO
X
, the acid rain problem persists in the White Mountains and in many other
locations throughout the world (
Rodhe et al. 2002; Likens et al. 2012
). Because of the relent-
less impact of acid rain on sensitive forest soils, this environmental problem arguably is
