United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development EPA/600/M-85/011 May 1985
ENVIRONMENTAL
RESEARCH BRIEF
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Experimental A cidification of a Stream Ecosystem
Initial L ong- Term A cidification
The purpose of this research was to investigate the effects
of increased acidity on a stream ecosystem. Qualitative and
quantitative effects of experimental pH manipulation in
Norris Brook within the Hubbard Brook Experimental Forest,
New Hampshire, were studied. Field experiments were
designed to test the hypothesis that lowered pH (increased
acidity) has detrimental effects on microorganisms, peri-
phyton, macroinvertebrates, and fish. A summary of the
experimental research design in Norris Brook from 1977
through 1980 is presented in Table 1.
In 1977 the pH in Norris Brook (see Figure 1) was experi-
mentally decreasedto levelsfound in incident precipitation
(pH 4) by the addition of dilute concentrations of sulfuric
acid. This experimental depression lasted for five months
(see Hall et al., 1980 for further details). Following are the
main results from this study:
1. Aluminum, calcium, magnesium, and potassium were
mobilized into the stream water during acidification.
2. Emergence of adult mayflies (Ephemeroptera), some
stoneflies (Plecoptera), and some true flies (Diptera)
decreased at the lower pH.
3. More immature aquatic invertebrates in collector,
scraper, and predator functional groups drifted out of
the acidified relative to the control area.
4. Fewer adult insects assigned to collector functional
groups were found in emergence samples from the
acidified reach.
5. Chironomids, tipulids, certopogonids, and mayflies
decreased by 75% in the benthos of the acidified
relative to the reference area.
6. Periphyton biomass increased and fungal (hyphomy-
cetes) densities decreased at low pH.
7. A basidiomycete fungus increased in the acidified area
relative to the reference section.
8. A loss of nutrients (e.g., N) occurred in the invertebrates
and organic matler but not in the water; this loss was
significant in terms of the total stream ecosystem.
9. Stream acidification altered biotic structure, metabo-
lism, and biogeochemistry of this poorly buffered
mountain stream ecosystem.
Subsequent Short- Term Acidic Episodes
In a significant result from the 1977 field work, the largest
amount of invertebrate drift occurred within the first five
days after acid addition (Figure 2), the peak occurring within
two days. Natural pH depressions in streams during spring
snowmelt are often only a few days in duration. For
example, episodic input of hydrogen ions during spring
thaw caused pH of stream water to drop to near 4.0 in first-
to third-order streams and remained at that level for three
to seven days in the Adirondack Mountains, New York
(Schofield, 1977). A similar phenomenon occurred in
southern Norway (Leivestad and Muniz, 1976). The ramifi-
Table 1. Summary of Experiments Conducted in Norris Brook Within the Hubbard Brook Experimental Forest from
1977 Through 1980
Year Season
Stress
Duration Applied
pH Values:
Reference/
Treatment
Stream
Order
Chemical
Determinations
Biological
Determinations
1977
Spring/
A utumn
5 mo
6-6.4/4.0 3rd Ca, Mg, K, Na, Al.
DOC, POC, etc., SO4~,
NH4+, NO3~, Cr, SiOi,
Pb, In, Cd, Cu, Mn, Fe
Invertebrate density in
drift, emergence, and
benthos. Standing crop
of periphyton. Aquatic
fungi species. Fish
behavior.
1978
1979
Spring/
Autumn
Spring/
Summer
1 day
each
1 -hr to
2-day
periods
NO STRESS
followed
recovery
1980 Spring 1-4 hrs HCI
Spring 1-4 hrs AIC/3
6-6.4/6-6.4
6.4/4.O
6.4/3.0
6.4/5.0
6.4/4.5
6.4/3.5
6.3/4.O
6.3/4.0
6.0/4.0
6.3/5.0-5.25
6.3/5.0-5.25
3rd
3rd
3rd
3rd
3rd
3rd
3rd
2nd
1st
3rd
2nd
Same as above
Same as above
plus TP and different
species of AI
Same as above
Same as above
Invertebrate drift and
emergence.
Invertebrate and
vertebrate drift.
Invertebrate and
vertebrate drift.
Invertebrate and
vertebrate drift.
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Norris Brook
Acid
Addition
0 3
meters
Legend
II Drift nets
l~~l Plastic house
^ Black plastic house
4 Direction of flow
Hubbard Brook
Experimental Forest
West Thornton, New Hampshire
/ • Weather station
' A Operating weir
2 Gaged watershed
Mile
0 5 1
Figure 1. Norris Brook study area located in the Hubbard Brook Experimental Forest, New Hampshire. Reference area (site A) was
located 5 m above the acidification point. Sampling sites in the treatment area were located at 15 m (site B), 50 m (site CJ, 75
m (site Dj, and 100 m (site E) below the point of acid addition.
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Acid addition
800
eoo
•Q
I 400]
g 200
Q>
Q
15
Figure 2.
20 25
April
30
10
May
15
20
Drift of invertebrates in relation to experimental
acidification of Norris Brook, New Hampshire. Total
number of organisms collected in drift nets per 24 '-h
period from 16 April through 17 May 1977. Closed
circles represent mean for two drift nets; vertical
bars represent range.
cation of the results of the first five days of our 1977
experimental acidification, then, becomes very clear. The
following questions were formulated after analyses of that
work:
1. Does episodic input of hydrogen ions during the initial
phase of the snowmelt period significantly alter
community structure and metabolism of poorly buffered
mountain streams?
2. What is the effect of short-term pulses of acidity and
aluminum on community structure and metabolism
and on nutrient cycling and energy flow in streams?
3. How does the impact of acidity alter nutrient flux during
high (spring) and low (summer) discharge, as well as
during storm events?
To answer the above questions, additional research was
completed. The results are as follows:
1. Hall, R. J. and G. E. Likens. 1 980. Ecological effects of
experimental acidification on a stream ecosystem, pp.
375-376. In: D. Drablefs and A. Tollan (eds.). Proceed-
ings of the International Conference on Ecological
Impact of Acid Precipitation. SNSF Project, Sandefjord,
Norway, March 11-14, 1980.
Experimental acidification to pH 4.0 in a small mountain
stream in the Hubbard Brook Experimental Forest, New
Hampshire, stressed the biotic and abiotic components
of that system. The biotic structure, metabolism, and
biogeochemistry of this stream ecosystem underlain by
siliceous bedrock were quantitatively altered at pH
levels (4.0) commonly found during snowmelt as well
as in ambient precipitation.
2. Hall, R. J. and G. E. Likens. 1 981. Chemical flux in an
acid-stressed stream. Nature, 292:329-331.
A third-order section of a small mountain stream in the
Hubbard Brook Experimental Forest, New Hampshire,
was experimentally acidified to pH 4.0 to measure the
effects of increased acidity on chemical and biological
export in the stream. The most significant inorganic
component affected by the experiment was aluminum.
A significant net flux of organic carbon and nitrogen
occurred in the biologically bound forms but not in the
dissolved substances. The net flux of phosphorus was
significant only in the biologically bound forms. The
increased loss of nutrients in the particulate organic
fraction was also important, particularly if scaled to the
total stream ecosystem.
3. Pratt, J. M. and R. J. Hall. 1981. Acute effects of stream
acidification on the diversity of macroinvertebrate drift.
pp. 77-95. In: R. Singer (ed.), Effects of Acidic Precipita-
tion on Benthos. Proceedings of a Symposium on the
Effects of Acidic Precipitation on Benthos. NABS,
Hamilton, N.Y.
A third-order reach of Norris Brook, a small stream in
the Hubbard Brook Experimental Forest, was experi-
mentally acidified for five months to determine what
effects acid precipitation may have on the ecology of a
poorly buffered lotic ecosystem. The initial six days of
stream acidification simulated a lowpH regimethatcan
occur in a small mountain stream receiving meltwater
from a rapidly thawing snowpack contaminated with
acidic deposition.
The first two to three days of acid addition constituted a
period of acute I-T stress (pH 4) that elicited a tenfold
increase in the daily drift rate of benthic macroinverte-
brates. This increased drift leaving the acidified reach
was also more diverse overall in terms of major taxa
(orders), trophic functional groups, and behavioral
groups but less diverse at the generic level than the
drift entering. In addition, the macrofauna abandoning
the acidified area compared to that entering was
particularly more diverse generically in mayflies and
midges, collector-gatherers, and clingers and swim-
mers.
Ecological implications of the acute changes in drift
rate and diversity extrapolated throughout the low-
order (1 -3) tributaries of a weakly buffered stream are
discussed. Recommendations for monitoring the effects
of acid-sensitive lotic ecosystems are offered.
4. Hall, R. J., J. M. Pratt, and G. E. Likens. 1 982. Effects of
experimental acidification on macroinvertebrate drift
diversity in a mountain stream. Water, Air, and Soil
Pollution, 17:1-15(1982).
A small stream (Norris Brook) within the Hubbard Brook
Experimental Forest was acidified to determine what
effect elevated H+ stress may have on the ecology of a
mountain stream. The experiment was designed to
simulate a pH level (4.0) that can occur during initial
snowmelt (acute period) and during long-term (chronic
period) acidification. Daily macroinvertebrate drift
samples were collected from treatment and reference
areas of Norris Brook. Drift diversity at the generic level
was calculated using Brillouin's formula and partitioned
hierarchically following macroinvertebrate classifica-
tions based on taxonomy (orders) and feeding strategies
(functional groups or guilds).
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The rate of movement of individuals and genera was
significantly greater for those organisms leaving the
acid-stressed area during the first five days than for
those entering, whereas no difference between the
rate of macroinvertebrates entering or leaving the acid-
stressed area was apparent for either numbers or
genera over the remaining 25-day study period. For the
acute period (first five days), the increased macro-
invertebrate drift leaving the acidified area was signifi-
cantly more diverse at the levels of aquatic insect
orders and functional groups but less diverse at the
generic level than the drift entering. For the chronic
period (25-day period) no significant differences were
detected in major taxa, functional group (with the
exception of collectors), or generic diversity between
the drift entering and leaving the treatment reach.
Mayflies and chironomids leaving the acid-stressed
area during the acute period were generically more
diverse than those drifting into the area. The overall
change in the normal pattern of spatial and temporal
variation in drift rate and diversity provides quantitative
evidence that H+ stress significantly altered the struc-
ture and metabolism of the macrobenthic community.
5. Hall, R. J. and G. E. Likens. 1984. Effect of discharge
rate on biotic and abiotic chemical flux in an acidified
stream. Can. J. Fish. Aquat. Sci., 41, in press.
Experimental manipulation of hydrogen ion concentra-
tion was conducted in a mountain stream ecosystem to
estimate the effects on biotic and abiotic chemical flux
during high (spring) and low (summer) discharge
periods. Dilute concentrations of sulfuric acid were
added to Norris Brook, a stream in the Hubbard Brook
Experimental Forest. The streamwater was maintained
near pH 4.0 from April to September 1977.
During acidification, Al, Ca, Mg, and K were mobilized
in the streamwater with the concentrations progres-
sively increasing downstream. Na and NOa were not
affected. DOC concentrations decreased downstream
during high discharge but did not change during low
flow. During storms these elements increased in
concentration below acid addition as well as in the
reference section.
The net flux of dissolved aluminum in the streamwater
was significant during both the high and low discharge
periods. The net flux of aluminum in invertebrate
biomass was significant only during high flow but was
insignificant when compared to the amount in the
dissolved state. On the other hand, the net flux of
nitrogen was significant only in the biomass of inverte-
brates at high discharge. This latter amount, however,
was totally overshadowed by the transport of dissolved
nitrogen during storms.
6. Hall, R. J., C. T. Driscoll, G. E. Likens, and J. M. Pratt.
1984. Physical, chemical, and biological consequences
of episodic aluminum additions to a stream ecosystem.
Limno. and Oceanogr., in press.
Experimental addition of AlClato a second-order stream
was conducted to simulate episodic increase* in
aluminum concentration during acidic snowmelt. With
astepwise increase in aluminum in the stream water a
significant decrease in pH and dissolved organic carbon
(DOC) and an increase in foam accumulations at the
stream surface were observed. It was hypothesized
that foam production was visible evidence of reduced
surface tension of the water. Controlled laboratory
experiments with stream water corroborated the field
results and produced a 20% reduction in surface
tension. Electrophoretic mobility studies showed that
increased aluminum concentration produced a surface
charge reversal on suspended colloidal particles. The
mechanism for a change in surface tension, it is
postulated, is the binding of aluminum to functional
groups (e.g., a carbonyl group) on DOC, thus rendering
DOC more hydrophobic and less soluble. Some of these
organo-aluminum complexes subsequently accumu-
late at the air-water interface.
Concomitant with physical-chemical changes in stream
water following elevated aluminum levels were altera-
tions in terrestrial and aquatic invertebrate drift
behavior in the field. These changes in drift behavior
could be attributed to responses either to (1) chemical
changes in the stream related to potential aluminum
and H+ toxicity or (2) a physical change in the surface
film.
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U. S. GOVERNMENT PRINTING OFFICE: 1985/559 111/10830
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