United States
Environmental Protection
Agency
Environmental Research
Laboratory
Corvallis OR 97333
Research and Development
EPA-600/S3-83-004 June 1983
Project Summary
Acidity, Nutrients, and
Minerals in Atmospheric
Precipitation Over Florida:
Deposition Patterns,
Mechanisms and
Ecological Effects
Patrick L Brezonik, Charles D. Hendry, Jr., Eric S. Edgerton, Randy L Schulze,
and Thomas L. Crisman
A monitoring network of 21 bulk and
4 wet/dry collectors located through-
out Florida was operated to determine
spatial and temporal trends in atmos-
pheric deposition of acidity, nutrients,
and minerals. During an intensive study
year. May 4, 1978 to April, 1979,
nitrogen (N) and phosphorus (P) depo-
sition via bulk precipitation averaged
0.77 and 0.050 g/mz-yr, respectively.
Highest deposition rates occurred in
agricultural areas and lowest deposition
rates occurred in coastal and forested
areas. Nutrient concentrations were
higher during summer (convective) rains
than in winter (frontal) events. Wet-only
input accounted for most of the deposi-
tion of inorganic N, but dryfall was
more important for organic N and
especially for total phosphorus (TP).
Inorganic forms accounted for most of
the N and P in rainfall. Statewide
deposition rates of N and P were below
the loading rates associated with eutro-
phication, but the average N loading
from bulk precipitation approached the
mesotrophic criterion of Vollenweider,
and N and P loadings exceeded meso-
trophic loading criteria at a few agri-
cultural sites.
The acidity of Florida rainfall has
increased markedly in the past 25 years,
and concentrations of nitrate and sul-
fate have risen correspondingly. Annual
average pH values of less than 4.7 occur
over the northern two-thirds of the
state. Summer rain averaged 0.2-0.3
pH units lower than winter rain, and
excess sulfate levels were higher at
most sites during summer. Sulf uric acid
accounted for about 70% of the ob-
served acidity, and nitric acid accounted
for the remainder. Local (within-state)
emissions of SO2 (and NOx) seem to
control the acidity of Florida rainfall.
The annual deposition of H+ is about
250-500 eq./ha over interior northern
Florida, or about one-third to one-half
the deposition rate for H* over the
northeastern United States.
Levels of pH in some softwater lakes
of north Florida have declined by up to
0.5 units over the past 20 years; no
changes were observed in similar lakes
of south-central Florida. Chlorophyll a
and TP decreased with pH in a survey of
20 softwater lakes. Aluminum levels
increased with decreasing pH, but ob-
served maximum levels (100-150/ug/L)
probably are not high enough to cause
fish toxicity. The major change in phyto-
plankton populations was a replace-
ment of blue-green algae by green algae
in acidic lakes. Species diversity and
abundance also declined with decreas-
ing pH, but the data exhibited much
scatter. Some trends were noted in
zooplankton and benthic invertebrates
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along a pH gradient, but both composi-
tion and abundance changes were rela-
tively subtle. Results indicate that acidic
conditions (as low as pH 4.6-4.7) do not
have major impacts on community
structure in Florida lakes.
This Project Summary was developed
by EPA's Environmental Research Lab-
oratory, Corvallis.OR. to announce key
findings of the research project that is
fully documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
Although the chemistry of rainfall has
been studied for over a century, many
questions remain about its importance in
biogeochemical cycles and in transport of
pollutants from the atmosphere to terres-
trial and aquatic ecosystems. Because
the nitrogen cycle involves several vola-
tile or gaseous compounds, the impor-
tance of atmospheric reactions and trans-
fers in this cycle has been recognized for
a long time. Much less information is
available on levels of "rock-bound" nutri-
ents (e.g., phosphorus) in rainfall. More-
over, information in the literature exhibits
a wide variation in concentrations both
spatially and temporally in rainfall.
Causes for this variability have not been
explained.
The role of rainfall as a transport
mechanism for various pollutants such as
heavy metals and acidity has been recog-
nized in recent years, and a considerable
volume of data has been assembled on
the pH of rainfall in Scandinavia and the
northeastern United States. The delete-
rious effects of acid rainfall on aquatic
systems in temperate climates also has
been documented. Previous studies on
acid rainfall in the United States have
been skewed geographically to the North-
east, where the problem apparently is
most severe. Little information is avail-
able on the extent of the problem in other
areas of the United States, the Southeast,
for example. Edaphic conditions in that
region, especially in Florida, suggest a
high susceptibility for deleterious ecolog-
ical effects; and demographic patterns
suggest that the Southeast may experi-
ence increasingly acidic precipitation in
the future.
Most of the project results are based on
two large-scale field studies. The first, a
statewide sampling network for bulk and
wet-only precipitation, was used to eval-
uate the importance of rainfall and dryfall
as sources of nutrients, minerals, and
acidity to Florida ecosystems. The net-
work was established to allow analysis of
the influence of surrounding land-use-
patterns on deposition rates of these
substances. Samplers were located in
urban, agricultural, forested, coastal, and
in pristine areas; and transects were
established to evaluate north-south and
east-west (coastal-inland) gradients in
deposition patterns (Figure 1). The net-
work provided valuable information on
nutrient and mineral deposition patterns,
and yielded the first comprehensive anal-
ysis of the acid rainfall problem in the
state of Florida.
The second field effort involved a
sampling program on 20 softwater lakes
in north-central and south-central Flor-
ida. Routine limnological measurements
and complete chemical and biotic analy-
ses were done on each lake to evaluate
the effects of acidification. Phytoplankton,
zooplankton and benthic invertebrate
communities were analyzed for species
diversity and abundance. The results of
the project are summarized according to
the three major phases of the project: (1)
atmospheric deposition of nutrients; (2)
the spatial and temporal distribution of
rainfall acidity in Florida, and (3) the
effects of acidification on chemical anc
biological conditions in soft-water lakes
of Florida.
Atmospheric Loadings of
Nutrients
Bulk precipitation is an important
source of nitrogen for both terrestrial anc
aquatic systems. Average annual load-
ings of total nitrogen (TN) for the 21 bulk
sampling sites ranged from 0.3 to 1.3
g/m"2-yr during thetwo-year study period
with a statewide mean of 0.72 g/m2-yi
(Figure 2). About 70% of the nitrogen was
inorganic (ammonium and nitrate) anc
thus was readily available to plants
Deposition rates were highest in rura
agricultural areas (0.88 g/m2-yr) anc
lowest along the coast (0.58 g/m2-yr). Ir
comparison with critical loading rates foi
lake eutrophication, the annual depositior
of TN at all 24 collection sites (Figure 2
was below the values associated with
eutrophic conditions (assuming nitroger
was the limiting nutrient). The statewide
average deposition rate was about 75% ol
the "permissible" loading for shallow
lakes suggested by Vollenweider (1968)
KEYS TO STATIONS
AB Apopka (wet/dry)
BA Bradenton
BG Belle Glade (wet/dry)
BH Bahia Honda
BN Bronson
CH Chipley
CK Cedar Key (wet/dry)
CL Clewiston
CW Corkscrew Swamp
FM Fort Meyers
GV Gainesville (wet/dry)
HA Hastings
JA Jacksonville
JS Jasper
JY Jay
LA Lake Alfred
LP Lake Placid
LI Lisbon
MC MacArthur Farms
Ml Miami
ML Marine/and
ST Stuart
TA Tallahassee
WD Waldo
BH
Figure 1. Location of sampling stations in the Florida Atmospheric Deposition Network (FADNJ.
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deposition at several agricultural loca-
tions slightly exceeded the permissible
criterion but none approached the eutro-
phic loading criterion.
Bulk precipitation has significant levels
of total phosphorus (TP); the statewide
two-year average was 33 jug/L, and the
mean deposition rate was 48 mg P/m2-
yr. Land use had an important effect on
atmospheric deposition rates for "TP.
Rural (non-agricultural) and coastal sites
had the lowest rates (27 and 31 mg P/m2-
yr, respectively), and agricultural sites
had the highest permissible loading rate
for the lakes that are most vulnerable to
eutrophication, i.e. shallow lakes with
long hydraulic residence times. Figure 2
compares annual bulk deposition rates of
TP for the rainfall network sites to the
phosphorus loading criteria proposed by
Vollenweider (1975), according to whom
the critical (i.e., eutrophication-causing)
rogen
- 2.0
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100 §-
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BH BA CK CL FM HA JS LA LI ML ST WD
Site
Figure 2. Loadings of TN (open bars) and TP (closed bars) by bulk precipitation at each FADN
site. Statewide average loadings shown by dashed lines. Permissible loading
criteria (P) and excessive loading criteria (E; i.e., inducing eutrophy) for TP
are for two values ofareal water loading (q, - 1 and 5 m/yr), as given byVollenweider
(1975). Loading criteria for TN are from Vollenweider (1968) for lakes with mean
depth < 5 m.
loading rate is a function of the area!
water loading rate, q*. For most Florida
lakes q. is in the range of 1 -5 m/yr. Based
on Vollenweider's loading criteria appro-
priate to this range of qs, bulk precipita-
tion in Florida supplies only 12-16% of
the loading required to induce eutrophic
conditions.
Average concentrations of TP in sum-
mer rainfall were about 1.5 times as high
as those in winter rainfall. Whereas most
of the atmospheric deposition of inorganic
N and TN occurs via rainfall rather than
dryfall, the opposite is true for phospho-
rus. At the four sites with wet-only/dry-
fall collectors, wet deposition accounted
for an average of only 20% of the TP.
Thus, most of the phosphorus in bulk
precipitation is dryfall, presumably of
large wind-blown particles of dust and
soil that are not transported large dis-
tances.
Average concentrations of total sulfate
in rainfall ranged from about 0.2 to 1.2
mg/L (as S); about two-thirds of the total
deposition of sulfate was by rainfall.
Deposition rates (Figure 3) ranged from 3
to 23 kg S/ha-yr (for all sites over the
two-year study), indicating that the at-
mosphere is a significant source of sulfur
to soils in Florida. Except at sites very
close (a few km) to the coast, most of the
sulfate in Florida rain is excess SO2" i.e.,
sulfate derived from SOz. Sea salt aero-
sols contributed only about 0.5 kg/ha-yr
of sulfate-S to bulk deposition at inland
••"3.2
2.03
Figure 3. Annual deposition (kg/ha) of (A) excess sulfate-S and (B) sea-salt sulfate-S across Florida for period May 1978- April 1979.
3
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sites. Because formation of excess sulfate
from 862 in the atmosphere concomitant-
ly results in stoichiometric production of
hydrogen ion (SOZ + YiQz + H20 — 2H* +
SOD, the occurrence of excess sulfate in
Florida rainfall also has important implica-
tions regarding the acidity of the rain.
Acidity of Precipitation
Rainfall throughout Florida is acidic
(Figure 4), with average values for all but
a few stations in south Florida being less
than geochemical neutrality (pH~5.7).
Single events as low as pH 3.9 were
measured at Gainesville, and the lowest
pH of a bulk precipitation sample (col-
lected weekly or biweekly) was 3.73 (at
Jay in the western panhandle during
August 1978). A definite geographic
pattern exists for acid deposition in bulk
precipitation around the state; mean
annual pH values (volume-weighted) for
1978-1979 were around 4.6-4.7 through-
out the panhandle and the northern two-
thirds of the peninsula. Mean annual
values south of Lake Okeechobee were
around 5.0 or above.
Neutralization of bulk precipitation was
found for coastal sites, but wet-only
precipitation collected near both the Gulf
and Atlantic coasts was approximately as
acidic as inland stations of comparable
latitude. Partial neutralization of acidity in
coastal rain apparently results from dry
deposition of alkaline particles containing
calcium carbonate of local (terrestrial)
origin. Analysis of the ionic composition
of coastal bulk precipitation indicates that
sea spray is not the agent of neutraliza-
tion. Sea-salt sulfate levels were only
modestly elevated compared to inland
bulk precipitation, and the calculated
amount of sea-salt calcium carbonate is
too low to account for the neutralization.
A seasonal pattern was found in precip-
itation acidity throughout the state with
summertime pH averaging 0.2-0.3 units
lower than wintertime values. Possible
reasons for the difference include (1)
increased summertime emissions of S02
and NOx (caused in part by seasonal
demands for air conditioning); (2) greater
thunderstorm activity in summer, result-
ing in greater fixation of N0« by lightning;
(3) enhanced scavenging efficiency of
summer convective showers compared to
winter frontal storms; and (4) differences
in the frequency and size of individual
rain events between summer and winter.
Further studies are needed to evaluate
the importance of these factors.
(4.7)
(4.7)-
(5.0)
•-••5.45
Figure 4. Volume-weighted mean pH of precipitation throughout Florida, May 1978-April
1979.
In spite of the north-south gradient of
decreasing rainfall acidity, long-range
(interstate) transport of acid precursors is
not a wholly satisfactory explanation for
acid precipitation in Florida. A substantial
portion of the H2S04 and HNOs must be
derived from in-state emissions of SO:
and NOx, which are widespread and
substantial. These conclusions are sup-
ported by several lines of evidence, includ-
ing the fact that summer rainfall through-
out the entire state is more acidic than
winter rainfall. From a meteorological
viewpoint, peninsular Florida is isolated
from the rest of the United States during
summer. Large-scale weather patterns
for the peninsula come from the south-
east (Caribbean) or southwest (Gulf of
Mexico) during summer, and cold fronts
from the north rarely penetrate the state
during this period.
Granat-type analysis indicates that
about 70% of the rainfall acidity in Florida
is derived from sulfuric acid and 30%
from nitric acid. A multiple regression
equation with [H+] as the dependent
variable and [S042lxs and [Ca2+]*s as
independent variables explained about
75% of the variance in hydrogen ion
concentration over the statewide net-
work: [Hi = 6.1 + 0.54[SO42~]«s - 0.35
[Ca2+]xs, where xs refers to the fraction ol
the ions of non-marine origin. Thus the
pH of rainfall in Florida primarily reflects
the degree to which sulfuric acid has
been neutralized by terrestrial calcium
carbonate.
Bulk precipitation throughout northerr
and central Florida deposited 250-500
equiv HVha-yr during 1978-1979, which
is about one-third to one-half of the
annual deposition of H+ in the heavily
impacted northeastern United States
Comparable values for excess sulfate are
7-11 kg/ha-vr in northern Florida anc
~13 kg/ha-yr in the northeastern U.S
(the 10-year average for Hubbard Brook,
N.H.). Thus Florida ecosystems receive
50-90% of the excess sulfate from the
atmosphere as their northern counter-
parts. Although historical data are lacking
on the pH of Florida rainfall, calculatec
values for rainfall pH during the mid-
1950's indicate wet-only precipitatior
was not acidic at that time (Table 1)
Moreover, present values of sulfate depo-
sition in northern Florida are up to foui
times higher than values for the earl\
1950's. Scattered information for the pl-
of rainfall at Gainesville from 1973 tc
the present, however, do not show an^
long-term trends.
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Table 1. Comparison of Parameters Related to the Acidity of Florida Rainfall in 1955-56 and
1978-79
Weighted-Mean Concentration (ueq/L)
Location
Mobile
Tallahassee
Jacksonville
Tampa/Bradenton
W. Palm Bch. /Stuart
Mean
1978-9/1956
1956
<2.5
<2.5
<2.5
<2.5
<2.5
<2.5
rf
1978-9^
24.0
17.4
18.3
20.1
6.9
17.3
>8.4
Excess-SOl
1956*
16.0
18.8
27.9
28.8
13.5
21.0
1978-91
34.7
33.0
4^.5
36-4
20.1
33.5
7.6ft
7956
2.6
2.9
2.9
2.7
4.1
3.1
A/Oa
* 7978-9f
13.9
13.9
16.2
14.3
12.1
14.1
4.511
* Data from Junge and Werby 11958) and Junge (1958); proton concentration inferred via
anion/cation balance. ,
t This study; May 1978-April 1979.
tt Present data are for bulk precipitation; 1956 data are for rainfall-only. Adjacent wet-only (W)
and bulk (B) collectors at Gainesville in this study yielded the following volume-weighted
average concentrations fin fjieq/IJ: excess sulfate, 35.11B) and26.6 (W), B/W=1.3; nitrate, 16.9
(B) and 13.6 (W). B/W1.24. Differences in collector type thus do not wholly explain the increases
in concentrations.
Effects of Acid
Precipitation on
Florida Lakes
A large number of soft-water lakes
occur in the sand-hill highlands region of
peninsular Florida. Based on comparison
of historical and current data for 13 such
lakes in northern Florida and 7 soft-water
lakes in south-central Florida, pH has
decreased by up to about 0.5 units in
some of the northern soft-water lakes,
whereas no temporal trends could be
discerned for the southern group. Corre-
sponding decreases in alkalinity and
increases in sulfate concentration were
observed in the northern lakes. The
northern (Trail Ridge) lakes lie about 40-
50 km east of Gainesville in a region
receiving rainfall with a (volume-weight-
ed) average annual pH of 4.5-4.7. The
southern (Highlands Ridge) lakes lie
northwest of Lake Okeechobee, near the
current southern terminus of pronounced
rainfall acidity. The 20 lakes surveyed had
annual average pH levels ranging from
4.72 to 6.80, but otherwise had generally
similar characteristics (soft water, oligo-
trophic to mesotrophic nutritional condi-
tions). The group thus served as a good
data base to evaluate the effects of acid
precipitation on vulnerable aquatic eco-
systems in Florida.
Ageneraltrend of increasing aluminum
with decreasing pH was found in the 20
Florida lakes (Figure 5). However, maxi-
mum values (100-150 ug/L) were below
the levels associated with fish toxicity;
this may explain the occurrence of large-
mouth bass and several other common
game fish species in lakes with pH values
below 5.0. Aluminum concentrations are
generally low in Florida's sandy soils, but
further studies are needed to evaluate the
impact of acid precipitation on leaching of
aluminum from these soils to Florida's
soft-water lakes.
A general trend of increasing chloro-
phyll a concentration with increasing pH
was found. However, total phosphate
concentration also tended to rise with pH
(Figure 6). The trend of greater oligo-
trophic conditions in more acidic lakes
may be caused by lower rates of nutrient
cycling at lower pH, or it may reflect
watershed nutrient loading factors that
just happened to correlate with lake pH.
Further studies are needed on this point.
The number of phytoplankton species
and their abundance in a lake decreased
with increasing acidity, but much scatter
occurred for both parameters. Although
the data are limited, a trend of increasing
phytoplankton abundance with increas-
ing pH was found for a series of lakes with
similar levels of phosphate, the lake
survey also indicated that species compo-
sition varied along a pH gradient, with
green algae replacing blue-greens at low
pH. (Figure 7). In lakes with pH values of
4.5-5.0, 60% of the algae were green
(Chlorophyta), and 25% were blue-green
(Cyanophyta). Corresponding values for
lakes in the pH range 6.5-7.0 were 31 %
green algae, 63% blue-green algae.
Similar trends were found in the zoo-
plankton; i.e., slight decreases in the
numbers of species and individuals with
pH, but the trends exhibited considerable
scatter. In general, the number of zoo-
plankton species found at a given pH was
greater than the number found in temper-
ate lakes of comparable pH. Six species of
zooplankton were dominant at all pH
levels, and five other species were always
present but never dominant. Two types of
multivariate analysis (principal compo-
nent and cluster analysis) showed that
the zooplankton populations could be
grouped along pH gradients, but the
population differences with pH are rela-
tively subtle. Rare species showed greater
differences with pH than did common
species. Some of the observed changes in
zooplankton community structure may
not have been directly related to changes
in pH but rather to changes in overall
trophic conditions that varied somewhat
along the pH gradient. Acidic lakes tended
to be less productive, were more nutrient-
depauperate, and had low standing crops
of phytoplankton. Whether these trends
were caused by or merely correlated with
pH cannot be determined from the survey
data, and further experimental studies
are needed to evaluate this matter.
No clear trends were seen in either the
diversity of the abundance of benthic
invertebrates with pH, and the differences
that were found among the lakes may
reflect differences in trophic conditions
and substrate type more than direct
effects of pH.
Overall, pH appears to have relatively
small effects (in the range 4.7-6.8) on
community structure in soft-water Florida
lakes that otherwise have similar chemi-
cal composition. More dramatic effects
may occur, of course, under more acidic
conditions (pH < 4.7), and significant
changes may occur in community metab-
olism, productivity, and nutrient cycling
processes within the pH range of the
lakes included in the survey. Further
studies on the biological effects of acidi-
fication on Florida lakes should be direct-
ed at these processes.
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750 |-
125 -
100 -
I
a
75 -
50 -
25 -
-
-
McCloud t—
1
1
1
Acidic Lake Group \ Nonacidic Lake Group
i
••
Cowpen
flaliloo I—
"*
Brooklyn » [
1 , 1 i! lilt
| O -j^L^t^etj
5.00
5.50
6.00
6.50
Figure 5. A verage and range of labile aluminum in the 20 survey lakes vs. pH.
References
Junge, C.E. 1958. The distribution of
ammonia and nitrate in rain water over
the United States. Trans. Amer. Geo-
phys. Union 39:241-248.
Junge, C. E. and R. T. Werby. 1958. The
concentration of chloride, sodium, po-
tassium, calcium, and sulfate in rain-
water over the United States. J. Mete-
orol. 15:417-425.
Vollenweider, R. A. 1968. Scientific fun-
damentals of the eutrophication of
lakes and flowing waters, with partic-
ular reference to nitrogen and phos-
phorus as factors in eutrophication.
Org. Econ. Cooperation and Develop-
ment, Paris. Rept. DAS/CSL/68.27.
Vollenweider, R. A. 1975. Input-output
models with special reference to the
phosphorus loading concept in limnol-
ogy. Schweiz. Z. Hydrol. 37:53-84.
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Figure 6. Three-dimensional plot of chlorophyll a as function of pH and TP in the 20 survey
lakes.
3
I
3
•Q
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Patrick L Brezonik, Charles D. Hendry. Jr., EricS. Edgerton, Randy L Schulze, and
Thomas L. Crisman are with the University of Florida, Gainesville, FL 32611.
Charles F. Powers is the EPA Project Officer (see below).
The complete report, entitled "Acidity, Nutrients, and Minerals in Atmospheric
Precipitation Over Florida: Deposition Patterns, Mechanisms and Ecological
Effects," (Order No. PB 83-165 837; Cost: $16.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
200 SW 35th Street
Corvallis, OR 97333
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
SSS
HICAGO IL 60601
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