2.2 Where, What, and When to Sample
37
especially in the western United States, where resources are more threatened
by N inputs than by S inputs. This is at least partially because of the very low
levels of S deposition received at many western locations. There are also regions
(portions of the Northeast, West Virginia, high elevations in North Carolina
and Tennessee) where both atmospheric S and N contribute substantially to
the observed acidification in some lakes and streams.
Acidification from S and N deposition can have several important chemical
and biological effects. In particular, there are changes in the acid-base status
of surface and soil water that can cause short-term or long-term toxicity to
aquatic or terrestrial biota.
Watershed processes control the extent of ANC contribution from soils to
waters as drainage water moves through terrestrial systems. These processes
regulate the extent to which drainage waters will be acidified in response
to acidic deposition. Of particular importance is the concentration of acid
anions in solution, including sulfate (SO 4
2− ), NO 3
− , and organic acid anions.
Naturally occurring organic acid anions, produced in upper horizons of acidsensitive soils, normally are removed from solution as drainage water percolates into the deeper mineral soil horizons. In some regions, organic acids can
dominate the acid-base chemistry of a lake or stream (as indicated by color
and dissolved organic carbon [DOC] concentration) because of the occurrence of hydrologically connected wetlands. Organic acids derived from wetlands, although they acidify a lake or stream, also serve as buffers against
further pH depression from acidic deposition. Acidic atmospheric deposition
allows natural soil acidification, anion mobility, and cation-leaching processes to occur at greater depths in the soil profile, allowing water that is rich
in SO 4
2− or NO 3
− to flow from mineral soil horizons into drainage waters. If
these anions are charge balanced by H + or Al i cations, the water will have low
pH and could be toxic to aquatic biota. If they are charge balanced by base
cations (BCs), the pH of the water will be higher, but the BC reserves of the soil
can become depleted over time.
Nitrate and ammonium (NH 4
+ ) have the potential to acidify surface waters.
However, N is also a limiting nutrient for plant and microbial growth in most
terrestrial, and some aquatic, ecosystems. Therefore, atmospheric N deposition
can contribute to increased productivity, eutrophication, and N saturation in
some surface waters. This appears to most frequently be the case in estuaries
and near-coastal marine waters and in freshwaters in remote locations where
historic atmospheric N deposition has been low.
High concentrations of lake or stream water NO 3
− may be indicative of ecosystem N saturation, reflecting a condition in which the supply of N exceeds the
biological demand. Nitrogen saturation has been found at a variety of locations
throughout the United States. These have included the San Bernardino and San
Gabriel Mountains within the Los Angeles Air Basin (Fenn et al. 1996); the Front
37
especially in the western United States, where resources are more threatened
by N inputs than by S inputs. This is at least partially because of the very low
levels of S deposition received at many western locations. There are also regions
(portions of the Northeast, West Virginia, high elevations in North Carolina
and Tennessee) where both atmospheric S and N contribute substantially to
the observed acidification in some lakes and streams.
Acidification from S and N deposition can have several important chemical
and biological effects. In particular, there are changes in the acid-base status
of surface and soil water that can cause short-term or long-term toxicity to
aquatic or terrestrial biota.
Watershed processes control the extent of ANC contribution from soils to
waters as drainage water moves through terrestrial systems. These processes
regulate the extent to which drainage waters will be acidified in response
to acidic deposition. Of particular importance is the concentration of acid
anions in solution, including sulfate (SO 4
2− ), NO 3
− , and organic acid anions.
Naturally occurring organic acid anions, produced in upper horizons of acidsensitive soils, normally are removed from solution as drainage water percolates into the deeper mineral soil horizons. In some regions, organic acids can
dominate the acid-base chemistry of a lake or stream (as indicated by color
and dissolved organic carbon [DOC] concentration) because of the occurrence of hydrologically connected wetlands. Organic acids derived from wetlands, although they acidify a lake or stream, also serve as buffers against
further pH depression from acidic deposition. Acidic atmospheric deposition
allows natural soil acidification, anion mobility, and cation-leaching processes to occur at greater depths in the soil profile, allowing water that is rich
in SO 4
2− or NO 3
− to flow from mineral soil horizons into drainage waters. If
these anions are charge balanced by H + or Al i cations, the water will have low
pH and could be toxic to aquatic biota. If they are charge balanced by base
cations (BCs), the pH of the water will be higher, but the BC reserves of the soil
can become depleted over time.
Nitrate and ammonium (NH 4
+ ) have the potential to acidify surface waters.
However, N is also a limiting nutrient for plant and microbial growth in most
terrestrial, and some aquatic, ecosystems. Therefore, atmospheric N deposition
can contribute to increased productivity, eutrophication, and N saturation in
some surface waters. This appears to most frequently be the case in estuaries
and near-coastal marine waters and in freshwaters in remote locations where
historic atmospheric N deposition has been low.
High concentrations of lake or stream water NO 3
− may be indicative of ecosystem N saturation, reflecting a condition in which the supply of N exceeds the
biological demand. Nitrogen saturation has been found at a variety of locations
throughout the United States. These have included the San Bernardino and San
Gabriel Mountains within the Los Angeles Air Basin (Fenn et al. 1996); the Front