ECOLOGY
OF YELLOWSTONE
THERMAL
EFFLUENT
SYSTEMS:
NET PRIMARY
PRODUCTION
AND
SPECIES DIVERSITY
OF A SUCCESSIONAL
BLUE-GREEN
ALGAL
MAT1
Richard G. Wiegert and Peter C. Fraleigh’
Department
of Zoology,
University
of Georgia,
Athens
30601
ABSTRACT
Net primary production and species diversity were studied during the successional development of a thermal blue-green
algal community
on a linear series of wooden troughs, The
1968 net primary
source spring was high in dissolved silica and CO,. During July-August
production
(minus grazing and export) was 25 kcal rnw2 days-l on board 1 and decreased
downstream
to 17 kcal m-a days-l on board 4. Linear incrcascs in standing crop were
and
observed during the first month of succession, An identical pattern of productivity
mat increase was observed during August-September
1969. Measurements
of particulate
export in the second year indicated that as much as 22% of total net production
(minus
grazing and dissolved
organic export)
may be lost via this pathway.
Efficiencies
of
conversion
of solar energy ranged from l&1.6%.
The chlorophyll
content per gram
organic matter increased and the ratio of OD 480:665 nm decreased with successional
age of the mat. Species diversity
(B) peaked after 2 weeks and increased with distance
from the source. Although
ten species of blue-green
algae were recognized,
three made
up most of the volume. Phormidium
was dominant during the first few days but declined
in importance as Mastigocladus
increased, After 3 weeks a species of Oscillatoria
increased;
its volume varying directly with distance from the source. A hypothesis naming free COO
as an important
nutrient limiting the growth rate of the mat is developed.
INTRODUCTION
The effluent streams from the hot springs
of Yellowstone
National Park arc characterized by the development of blue-green
algal mats at temperatures below 73C
(Brock 1967). Th ese mats maintain a constant standing crop as long as such environmental conditions as water flow, pH., and
temperature are unchanged. If the mat is
destroyed the algae quickly recolonize the
substrate and within a few weeks or months
reach the original steady-state value ( Brock
and Brock 1969a). Because of this rapid
succession, the small size of these ccosystems, and their relative taxonomic simplicity, they are ideal for the study of succcssional processes operating
within
a
natural community. Ncvcrthelcss, few data
exist describing either the successional del Support for this study was provided by N.S.F.
Grant GB7683 to R. G. Wiegert and by U.S.P.H.S.
Training
Grant TO1 ES-00074-03
to the Ecology
Institute, University of Georgia.
2 Present address : Department
of Electrical
Engineering
and Systems Science, Michigan
State
University,
East Lansing 48823.
LIMNOLOGY
AND
OCEANOGRAPIIY
velopment of the algal mats or their productivity.
Brock and Brock ( 1966) and
Brock (1967) measured the 14C-C02 fixation
along a thermal gradient ( Mushroom Spring
outflow) in Ycllowstone Park and found a
tcmpcraturc
optimum from 42-48@, although the highest standing crop was found
at 55C. Lenn ( 1966) reported high primary
productivity
at Drakcsbad Hot Springs,
California, whcrcas Duke (1967) found a
New Mexico hot spring in which Phormidium sp. was the only important producer
and the net primary production was extremcly low. Stockner ( 1967, 1968) used a
wooden trough as an artificial substrate in
his studies of succession and productivity
by the blue-green mats of Ohanapecosh Hot
Springs in Mount Rainier National Park.
In important respects ( temperature, pH,
net production
rate ) these previously
studied systems differ from each other and
from the thermal system studied here, Our
objectives here are to document the primary
successional process in these communities
and provide basclinc data on their net primary productivity,
developing a hypothesis
215
MARCH
1972,
V. 17(2)
216
RICHARD
G. WIEGERT
FIG. 1. Photograph
of the cxpcrimental
Each is 120 x 200 cm (2.4 m’).
AND
boards.
PETER
C. FRALEIGH
a board) 2 m long and 1.2 m wide with
sides 10 cm high was placed under the outflow. When after several days a film of
algae appeared on the board, we began
construction
of three more; these wcrc
placed in position one downstream from the
other (Fig. 1). From 22 July through 9
September 1968 and again bctwecn 12
August and 15 Septcmbcr 1969, we made
quantitative
studies of these boards (rcfcrrcd to as Bl, B2, B3, B4).
Abiotic
explaining the regulation of net primary
production rates.
WC apprcciatc the cooperation of those
responsible for the administration of Ycllowstone National Park in permitting us to conduct thcsc studies. In particular, we wish to
thank Dr. J. Douglass for his close personal
attention and assistance.
V. Mullen, S. Marshall, S. Wagner, A.
Young, and N. Collins provided valuable
technical assistance.
THE
STUDY
ARE.4
The study arca is located in a small mcadow off the Firehole Lake Drive in the
Lower Geyser Basin of Yellowstone National Park. This meadow has a number of
thermal outflows, differing in flow rate and
ranging in tcmpcraturc from 43-90C. Hot
pools in this meadow are all minor and have
been given no official names. Bccausc of
the chance “discovery” of this study area in
early 1968 by M. L. and T. D. Brock, the
meadow was named Scrcndipity Springs.
factors
WC mcasurcd pI1 in the field with a
portable meter and a microcombination
clcctrodc. Periodically water samples wcrc
transported to the laboratory; some chemical tests were complctcd within a few
hours and the remainder made on water
that had been frozen and stored. A water
analysis kit (Delta model 260) and proccdurcs described in the instruction manual
were used.
Solar radiation was measured with a
solarimeter, using a silicon photocell with
a spectral response of 0.3-1.15 pm. The
integrating amp-hr meter of the instrument
was read every few days and the values
convcrtcd to calories. In a silicon photocell,
the area under the spectral rcsponsc curve
from 0.4-0.7 pm is 0.33 of the total arca
under the curve from 0.3-1.2 ,urn (Botkin
and Malone 1968) ; each cncrgy value was
multiplied by 0.33 to yield the calories of
photosynthetically
usable solar energy.
Tcmpcratures were measured with a thermistor chcckcd periodically with a mercury
thcrmomcter.
METHODS
Experimental
board ecosystems
To reduce variability caused by slope and
flow differences and to permit complete experimcntal control over the inflow-outflow
strcams, we constructed a standardized
wooden substrate. A small spring on a
gentle slope was dug out and a wooden
pipe, about 10 x 15 cm and 6 m long, was
placed at the bottom of the spring and let
out onto the hillside; about 51 liter/min
flowed out of the end of this pipe. A flat
wooden platform (refcrrcd to hereafter as
Net primary productivity
From the time films of algae were first
obscrvcd on boards 2, 3, and 4, WC took
standing crop samples periodically with two
diffcrcnt sizes of corer (5.3 and 8.8 cm2).
Randomization was achieved by sliding a
movable T-square marked in centimeters
on top of the trough side rails to locate to
the nearest centimeter the sampling point
sclccted from a table of random numbers.
The sharp edge of the corer was pressed
into the board and the algae wcrc sucked
THERMAL
EFFLUENT
up with a mouth aspirator made of tubing
and a 250-ml flask. The arca inside the
core was then washed with stream water
from a squeeze bottle and this also was
sucked up. If algae remained on the board,
the washing was repcatcd. This procedure,
a modification
of that suggcstcd by W.
Docmcl, resulted in almost complctc rccovcry of the algal material from the arca
delimited by the corer. Material was taken
to the laboratory, filtcrcd through tared
Millipore filters, frcczc-dried, and weighed
to the nearest 0.1 mg. During later stages
of succession in 1968, when the amount of
material was too large to bc filtered rapidly,
centrifugation
at 6,000 rpm (5,836 g) for
3 min was used in place of filtration, the
clear upper layer of liquid was discarded,
and the remaining
water removed by
freczc-drying.
During the later stages of
the succession in 1969, material was removed from the boards as an intact layer
by slipping a thin spatula blade under the
corer bctwccn the algal mat and the surface of the board.
About half of the dry material on each of
five filters from a board on each date was
removed and combined, ground in a small
electric mortar, pellets formed, and caloric
values determined
with
a commercial
version of the Phillipson microbomb calorimctcr. Corrections wcrc made for nitric
acid formation (Parr Instr. Co. 1960). The
remaining material from each of the five
samples was combined, wcighcd, ashcd in
a muffle furnace at 550C for 45 min, removed to a desiccator, cooled, and weighed
again. The ash values wcrc used to correct
the dry weights to total organic matter per
square meter.
In 1969 the loss of particulate matter was
measured twice during the succession. The
entire outflow from B4 was passed through
a No. 325 standard U.S. sieve (0.044-mm
opening) for 5 min, or until the sicvc would
no longer pass the full flow, The particulate material was washed into a container,
frcczc-dried,
corrcctcd for inorganic content, and cxprcssed as milligrams of organic
matter.
217
SYSTEMS
Species determinations
A small ( 1.2 cm2) core was taken immcdiatcly adjacent to each of the larger
biomass cores and the five samples from
each board wcrc combined and preserved
in 40% Formalin.
After the material was
ground in a Teflon homogenizer, an aliquot was placed on the counting grid of a
Pctroff-Hausser
cell, and random microscope fields wcrc observed. Total Icngths of
filaments of each spccics were calculated
from the horizontal
and vertical intersections with the grid lines (method 5:
Olson 1950). Using the mcasurcd filament
diameter, WC calculated total volume for
each spccics. In the cast of rare species,
those with very short mean filament length,
or chains of ovoid or spherical cells, direct
measurements
and counts were made.
Thcsc volumes were rcfcrrcd back to the
area from which the core was taken and a
volume per square mctcr was calculated.
Bccausc of great differences in size and
frcqucncy of occurrcncc between some speties, . various microscope field sizes were
used. WC used the nomcnclaturc of Copcland ( 1936).
Chlorophyll
We used a modified version of the method
of Strickland and Parsons ( 1968) to mcasure chlorophyll.
Immediately
after sampling, the core of algae was placed in acctone buffcrcd with MgCOn, then placed on
ice in the dark. The samples were ground
with a Teflon homogenizer and left in the
refrigerator overnight. After centrifugation
the optical densities wcrc measured on a
Optical density readspectrophotometer.
ings wcrc convcrtcd to milligrams of chlorophyll from the equations of Parsons and
Strickland ( 1963).
RESULTS
AND
DISCUSSION
Chemical composition
Source tempcraturc was 43 ? 0.5C. At
the outflow of board 4 the tempcraturcs
ranged from 36-4OC, dcpcnding
on air
tcmpcrature and wind conditions. Gcncrally
there was a drop of about 1C per board,
These systems did not exhibit a large
218
RICHARD
G. WIEGERT
AND
PETER
C. FRALEIGII
TABLE
1. Chemical composition
of water flowing from the four experimental
boards; silica and hardness were measured only in the source water.
Values (except for pH) in mg/liter.
Carbon dioxicle
was computed according
to Rainwater
and Thatcher
(1960); NO8 values are means of three replicates; remaining values represent 1-2 determinations
Date
POc-ortho
NO,-N
CaC03 hardness
MgCO, hardness
Silica
Alkalinity
(HCOZ-)
PH
COTfree
27
16
23
23
12
17
17
17
Jnl
Aug
Jul
Jul
Aug
Aug
Aug
Aug
Source
0.17
< 0.05
26.4
4.8
164
240
6.6
95
thermal gradient of the kind studied by
Brock (1967) but rescmblcd the trough
temperature gradient (37-36 -I: 2.2C) reported by Stockncr ( 1968).
Chemical analyses of the water are given
in Table 1. The phosphate content of the
water did not decrease with distance from
the source. That this phosphate was not
contained in particulate matter was shown
by later analyses of water filtered through
acid-washed HA 0.45-pm Millipore filters,
which gave results similar to those of Table
1. Unfortunately we could determine nitrate
with no more than 0.05 ppm sensitivity-a
serious deficiency in view of the low levels
of nitrate given by Brock (1967) (O.OOl0.008 ppm) and Stockner (1968) (0.070.05 ppm ) . However, our source obviously
did not have as much nitrate-N as did the
Mount Rainier springs, or WC wouId have
detected at least a trace amount. In the
same studies, Brock reported an increase in
nitrate-N
downstream
whereas Stockncr
found that the algal mat took up nitrate-N.
The mat studied by Stockner contained no
N-fixing species of blue-green algae; our
boards and the Brock’s stream did contain
nitrogen-fixers.
The high silica content is typica
of
thermal waters in the Lower Geyser Basin
and results in extensive deposition of siliceous sinter ( SiOa ) in and around the algal
material.
The pH always increases downstream
(‘Table 1). Calculations of the free-CO2
changes that would produce such a difference were made using the method of
Rainwater and Thatcher ( 1960).
B2
Bl
0.16
< 0.05
242
6.8
61
0.17
< 0.05
23;
7.0
37
Standing
B3
0.17
< 0.05
232
7.2
23
B4
0.17
< 0.05
242
7.4
15
crop and ash content
The standing crop of algae quickly developcd considerable spatial heterogeneity
( Table 2). Even early in succession, when
the standing crop was less than 15-20 g/m2,
the coefficient of variation (s/Z) was greater
than 10%. Nevertheless, these sampling
errors, even with the low number of samples
(5) per stratum, are no higher than those
commonly
encountered
when sampling
terrestrial
vegetation
( see Wiegcrt and
Evans 1964).
The data in Table 2 indicate considerable
differences in standing crop, not only with
time, but bctwcen boards, For comparison
of thcsc changes in standing crop, the raw
data wcrc converted first to organic biomass
by correcting for the ash content ( Table 2).
We made no chemical analyses of the inorganic material, but the predominant hotspring deposit in the Lower Geyser Basin
of Yellowstone Park is siliceous sinter, and
we assume this material may be deposited
both as a result of cooling and algal activity
and as particulate material washed downstream from the source. Shortly after Bl
was set up, its surface had thin deposits of
sintcr particles heavy enough to settle out
in the current. The trapping of sintcr particles would be increased by the presence
of a filamentous algal mat. We think that
this phcnomcnon
accounts for the high
mean ash content of Bl samples in 1968
(43.2%) as contrasted with the means from
B2 (25.6%), B3 (27.6%), and B4 (23.5%)
for the same year. The percent ash on B2B4 generally decreased with time, consistent
with the hypothesis that a large share of
THERMAL
TABLE
2. Successional
gha
B2
% ash
g/ma
19"
130.6(58)
50
22"
211.2 (53)
49
19.2 26.2(8)
28.0 (2)
35.8(13)
27"
30"
34"
37"
41"
44"
51"
60*
303.1(93)
329.1(98)
519.7 (250)
368.2 (78)
39
35
44
37
70.5 (66)
108.5 (18)
146.0(72)
141.1(31)
190.8 (25)
13 August
3
1:
21
28
34
* Corrected
97.0 (29)
198.9 (40)
180.8 (49)
225.9(78)
13
12
18
11
ash on four
22
24
ii
35
21
22
22
19
1969-Day
8.9 (3)
66
26.8(12)
95.6 (20)
27
14
175.5 (78)
12
g/l+
board
experimental
B4
B3
% ash
18 July 1968-Day
5
6
7
9
12
14
17
21
24
28
31
38
47
219
SYSTEMS
changes in standing crop of algae and percent
substrates (1 SD given in parentheses)
Bl
Day
EFFLUENT
70 ash
g/ma
VO
ash
1
13.1 18.6(5)
23.3(2)
32.4(6)
42.8(7)
55.6(14)
138.3 (56)
117.9 (51)
159.7(44)
48
38
32
34
22
30
19
22
20
l&iy
13.8(2)
23.4(6)
35.8 (10)
46.0 (8)
71.6 (21)
109.6 (15)
113.9 (19)
33
35
44
30
32
ii
14
13
1
47.1(7)
100.4 (36)
104.7(18)
137.0 (9)
16
10
12
12
7.1 (2)
65
67.1 (20)
84.1(15)
106.1(23)
13
17
13
day.
the inorganic material is particulate material coming from the newly disturbed
source. The mat on Bl continued to trap
material, but little material was lost by it,
with the result that the growth of mat on
boards B2-B4 began to dilute the effect of
the initial inorganic deposit. A Friedman
nonparametric
2-way ANOVA performed
on all ash values from B2-B4 showed great
significance for the means between boards
( p < 0.001) as well as the decline in ash
content with time on these three boards
(p < 0.01).
By 1969 the disturbance
caused by
digging had subsided; the source water had
little inorganic particulate material and the
pattern of ash content of the samples
differed from that of 1968 (Table 2). The
ash content was highest on all boards on
day 3, when the standing crop was very
low ( < 9 g/m2), and decreased progressively with time until day 21. A Friedman
e-way ANOVA showed a highly significant
effect of time (‘p < 0.01) but a significant
diffcrcnce between boards was not dcmon-
strated ( p < 0.05). The high ash content
of the early samples was caused by inclusion with the samples of bits of sinter
deposited on the board during its year of
exposure. This deposition gives old boards
a furry appearance and pieces are sucked
up into the collecting vessel when the inside
of the corer is washed. Quantitatively
this
material is unimportant,
thus accounting
for the rapid decline in sample ash content
as the standing crop of algae increases
(Table 2).
Calorific content and stancling crop changes
The calorific analysts of the algal material ( expressed on an ash-free basis)
varied little with time and board. Only the
1968 sample material was used. A Friedman 2-way ANOVA showed no significant
differcncc bctwecn boards (p > 0.1) or
with time ( p > 0.2). Therefore, all calorific
mcasurcmcnts were pooled and a single
mean value of 4.92 kcal/g ash-free dry wt
was used to compute the energy content of
the standing crop data. These values are
RICHARD
G. WJEGERT
AND
PETER
C. FRALEIGH
10 days bcforc a well-established
mat
appcarcd.
To
make
the
2
years
more
comI \
parable in the cast of Bl, WC have moved
the origin back 7 days ( Fig. 2).
The negative values for c, the coefficient
of x2, provides the growth-slowing
fccdback necessary to fit the data properly. Of
course, fitting such a model to the data is
only valid over short periods, since the
curve would begin to turn downward as x
(age) increased. The mats, however, arc
cxpccted to reach a steady-state value and
fluctuate about the mean.
Figure 2 shows a marked diffcrencc in
the magnitude
of the standing
crop
achieved on the four boards after 47 days.
In general the steady-state standing crop
appears to decline with distance from the
source, although B3 and B4 differ little. The
good agrecmcnt between both the form
and the magnitude of the time-rclatcd biomass changes in July-August
1968 and in
August-September
1969 suggests a rather
definite pattern of successional change from
a bare substrate in these mats.
A minimum estimate of the net primary
30
production of an ecosystem is provided by
DAYS
the net change in standing crop of autotrophs during an interval (Wicgert and
FIG. 2. Successional
change in standing stock
Evans 1964). From the polynomial equa(kcal)
of blue-green
algae on four experimental
boards.
In 1968 succession on board 1 began
tions fitted to the 1968 data of Fig. 2, WC
on 5 July; boards 2, 3, and 4 began on 18 July.
calculated the day on which the mat began
In 1969 succession on all boards began on 12
to grow and the day on which peak standing
August.
the successional interval
crop (within
studied) was reached. Table 3 lists total
net production, Bccausc the growth periods
shown in Fig. 2 as a function of successional
vary slightly (from 4245 days) we have
age in days.
In 1968 the initial relationship between
expressed the data as a rate of production
time and increase of the algal standing crop
( kcal m-2 day-l ) .
This harvest method of measuring net priappcarcd to be linear, but as succession
requires some assumpprocccdcd
growth slowed.
Several re- mary production
grcssion models wcrc compared with these tions, bccausc losses arc possible from
data, and the best fit was obtained for a export and grazing. Grazing by the ephydrid
flies that feed on these blue-green mats
second-degree polynomial (Y = a + bx +
(Brock ct al. 1969) was slight on these
cx2), where a and c were always ncgativc
board systems early in succession because
constants and b was positive. The negative
value of a indicates a lag time before the the relatively heavy flow of water kept
mat bccomcs established of about 3 days almost all parts of the mats at or above
for all boards and times cxccpt Bl in 1968. 40C. The cphydrid eggs and larvae do not
This new board did not have a source of survive well at temperatures above 40C
( Wicgert and Mitchell 1972). Only when
seeding above it and this plus the weathcring time for a new board caused a lapse of the mat approaches a steady state with porF
*
,' '\
THEHMAL
TADLE
3. Net production
and solar energy
EFFLUENT
conversion efficiencies
of calculations)
Bl
B2
B3
B4
(kcal m-2)
10-55
347
5-47
5-47
1,240
955
625
715
(kcal m-2 clay-l)
27.6
21.8
15.0
17.0
during
1968 (see text for explanation
Solar energy (kcal m-2)
Net production
Days*
221
SYSTEMS
Total
238,370
207,300
193,770
193,770
Available
78,662
68,410
63,940
63,940
Effi$;cyt
OO
1.58
1.40
0.98
1.12
* Bl-4
July, corrected clay 1; B2, B3, B4-18
July, clay 1.
t Net production/available
solar energy X 100.
tions above the water surface do cooler
spots dcvclop which can bc exploited by the
grazers.
Export
losses
Losses in the form of export of organic
matter, both particulate and dissolved, are
potentially
serious in lotic systems. Both
Brock ( 1967) and Stockncr ( 1968) reported no change in the dissolved organic
matter bctwccn upstream and downstream
flows. WC have had difficulty
obtaining
analyses of sufficient precision to allow calculation of the loss of dissolved organic
matter (DOM)
from our experimental
boards. Because more than 73,000 liters/day
flowed over the 10 m2 of board substrate,
an upstream-downstream
diffcrcncc in dissolved carbon of only 0.1 mg/liter
(0.1
ppm ) represents 0.9 g of carbon added or
lost per square meter per day. In both years
the DOM analysts showed far more than
this amount of variation, both bctwccn
replicate samples and between outflows.
The possibility
of significant
export of
DOM from these thermal communities
cannot bc ruled out on the basis of available data.
That considerable particulate
material
was washed downstream and lost was cvident, since large picccs of algae sometimes
passed through the outflow troughs. However, this occurred only following some disturbancc of the mat, such as a heavy rain.
A greater loss was at the continual passage
of small bits and filaments of algae in the
outflow water. We wcrc unable to monitor
this loss in 1968. During 1969, the particulate export was mcasurcd twice, once on
19 August, before the mat had grown to the
point whcrc any surface was cxposcd, and
again on 26 August after flits had begun to
cat sections of it.
Corrected for particulate material carried
by the source water, the particulate export
of ash-free organic matter from the outflow
of B4 on 19 August was 0.61 mg/min and
on 26 August, 10.53 mg/min. The minimal
export rate was assumed to have been constant over the first 11 days ( 12-23 August)
bcforc the mat cmergcd from the surface
an d could bc attacked by fly larvae. The
maximal rate was assumed to operate from
23 August-15 Scptcmbcr (23 days ) . Using
thcsc assumptions, we computed the total
kcal m-2 lost from the four boards during
the respective periods (total arca of all four
boards is 8 m2; 4.92 kcal/g ash-free dry wt
was used as the calorific equivalent of the
material exported) . Total export between
12 and 23 August was 5.9 kcal m-2, less than
6% of the net organic production remaining
on the boards on 18 August. However, as
much as 215 kcal could have been lost betwecn 23 August and 15 September, 31%
of the avcragc net accumulation of the mat
during this period. Overall, WC estimate
that 220.6 kcal were lost from the mat, or
28% of the avcragc net production
(793
kcal ln2) remaining on the boards after 34
days (22% of total net production minus
grazing and dissolved organic export). Although crude, these estimates indicate the
importance of particulate export from thcsc
flowing systems. Further, they suggest that
apart from temporary physical disturbances
of the mat, important factors in causing
particulate export are the activity of the
grazing insect larvae, changes that occur in
the algal mat as it approaches its maximum
standing crop value, or both.
222
RICIIARD
G. WIEGERT
AND
PETER
C. FRALEIGH
Comparisons with primary production
in other thermal ecosystems
Although our net production values do
not include losses from organic export, the
daily rate of carbon fixation on Bl was
similar to the most productive aquatic ccosys terns cited by Westlake ( 1963). The
maximum
value reported
by Stockncr
(1968) for accumulated growth was 0.5 g
org matter m-2 day-l or 2.5 kcal using our
calorific equivalent of 4.912 kcal/g ash-free
dry wt.
The net productivity
of Bl (25 kcal m-2
day-l, Table 3) as measured by daily accumulated net growth was an order of
magnitude greater than that of Stockner’s
Ohanapccosh
system as measured by
denudation.
However, his values for the
entire source spring (spring 3)) based on
photosynthetic
measurements, showed a
maximum net production of 2.2 g C m-2
day-l. If the carbon content of dry organic
matter is assumed to be about 50% (Westlake 1963)) this maximum rate represents
4.4 g org matter m-2 day-l. The mean
daily export rate during the growing season
was about 1.0 g rnd2 day-r leaving a net
accumulation of 3.4 g or 16.7 kcal m-2 day-l.
This is more than 7 times the maximum rate
obtained
from his denudation
studies.
Stockner ( personal communication ) attributes this difference to either seasonal variation (primary production was measured in
1966; the denudation studies were made in
1964-1965) or underestimation
of oxygen
liberation or oxygen diffusion rates in the
primary production methods.
Brock ( 1967) did not report the total
CO2 present in the water used for
his 14C-CO2 photosynthesis measurements.
Therefore, direct comparison of our results
with the Mushroom effluent stream is not
possible because his 14C-CO2 fixation data
cannot be expressed in absolute terms. Our
own general observations on the growth of
the Mushroom spring mat, together with
the recovery study of Brock and Brock
( 1969a), confirm the impression that net
productivity
is low. Lcnn ( 1966) reported
high production rates (7-12 g C rns2 day-l)
for the alkaline thermal algal mats at
lb
2’0
3’0
4’0
DAY
5-o
FIG. 3. Histogram of seasonal changes in total
pm) received by the
solar radiation (0.35-1.15
four experimental boards, 29 June-2 September
1968 (near Great Fountain Geyser, Yellowstone
National Park ) .
Drakesbad IIot Springs. Converting
his
values to calories in the same manner as we
did Stockncr’s, we obtained 69-119, kcal
m-2 day-l, at maximum more than 3 times
the highest rate of net production found
during this study.
Possibly the lowest net production recorded for a thermal spring was that of
Duke ( 1967), 231 kcal m-2 year-l. This was
caused in part by a high percentage of
the gross photosynthesis
being lost to
respiration (90-95%). In the Mimbres hot
springs that she studied, the regrowth of
the mat was slow, confirming her low cstimate of the net primary production.
Photosynthetic
efficiency
Summer light values in Ycllowstonc National Park arc very high. We have rccorded rates in excess of 1.6 cal cmd2 min-r,
more than 75% of the solar constant.
Aquatic autotrophs, unlike terrestrial plants,
should not have a problem getting rid of
excess heat under high light intensities.
Stockner
( 1968) found a linear relationship of light intensity to gross photosynthesis, with
no indication
of light
saturation
up to levels of 5,000 kcal m-2 day-l, approximately the average daily total radiation
TIIERMAL
TABLE
4. Mean value
Day
Bl
B2
B3
134
of mg Chl/g
*
*
*
223
SYSTEMS
erg matter for 5 samples, except
theses. Data from 1968 experiment
12
8
4.37G.58)
3.10(0.45)
3.18(0.43)
EFE’LUENT
3.18G.30)
3.01( 1.82)
4.11(0.73)
where
indicatecl;
21
14
34
27
5.02( 1.66)
-
4.73(0.85)
-
4.34(0.96) *
3.99(0.63)
5.07(0.88)
4.85( 1.99)
3.55(0.96)
3.12( 1.44)
3.05(0.81)
1 SD in paren-
* N=3.
delivcrcd
to the surface in Scrcndipity
Springs ( Fig. 3).
In 1968 solar radiation in the Ycllowstonc
area during August was below normal bccause of protracted cloudy weather.
In
Table 3 the total solar energy and the available solar energy are given for the periods
during which net primary production was
measured on each of the four boards. The
cfficicncy of net production (minus export
and grazing losses ) in terms of available
solar energy was in the range of values rcported by Stockner ( 1968).
Few other studies of natural ecosystems
present data that permit comparison with
thcsc values. Botkin and Malone (1968)
summarize
the efficiencies
known
for
terrestrial systems, ranging from 0.03 for
desert shrubs to 5.1% for cultivated corn,
In a study of a temperate cold spring, Teal
(1957) found an cfficicncy of only-0.02%
(assuming I! of total reported radiation to
bc in the visible spectrum). Odum ( 1957)
found an efficiency of 0.15% for the primary
producers of Silver Springs. These values
are, however, based on total yearly net production. The values of Table 3, although
considerably higher than those of Teal and
Odum, reprcscnt measurements made during high productivity
and high light conditions. They would be somewhat lower
when calculated on an annual basis if, as
reported by Stockner ( 1968)) thcrc is a
direct relationship between light intensity
and productivity.
Despite the difference in the period of
the 1968 growth measurements between Bl
and B2-B4, the mean solar radiation per
day was similar on all four boards, and the
differences between boards in total net
production per day (Table 3) cannot bc
explained as a seasonal effect. Furthermore,
the similarity of the growth
ancl 1969 ( Fig. 2)) despite
diffcrcnce between the start
in the 2 years, suggests that
sonal effects on algal growth
found during summer months.
rates in 1968
the month’s
of succession
no great searate will bc
Chlorophyll:carotenoid
ratios
Chlorophyll
extractions in 1968 were
made on days 8, 12, 14, and 21 (B2, B3,
B4) and on days 21, 27, and 34 on Bl
( Table 4). A Friedman nonparamctric 2way RNOVA showed no differences betwccn boards in mg chlorophyll
a (665
nm)/g biomass ( p = 0.93). Combining the
data from all boards, a l-way ANOVA gave
a significant (p < 0.01) effect of successional
age ( Fig. 4). A decrease during the first
2 weeks was followed by an increase con-
II
'.'
2
I
1
1
4
6
6
1
I
I
I
I
I
1
I
I
I
I
I
1,
IO 12 14 16 16 20 22 24 26 26 30 32 34
DAY
FIG. 4. Mean values of mg Chl/g org biomass,
plotted as a function of successional age in days.
Vertical lines represent -L 1 SE of the average mean
( values in Table 4 ) .
224
RICIIARD
TABLE 5. Ratios
Day
Bl
B2
B3
B4
G. WIEGERT
AND
PETER
C. FRALEIGII
of optical density 480:665 nm. Values are means of 5 samples,
cated; 1 SD given in parentheses. Data from 1968 experiment
8
3.4OG.36)
3.42(0.24)
3.18(0.25)
12
*
*
*
2.99G.28)
3.56(0.38)
2.91(0.44)
14
21
where
27
2.30(0.04)
2.17(0.10)
1.70(0.19)
1.81(0.21)
2.41(0.19)
2.92(0.39)
2.76(0.38)
except
*
indi-
34
2.29(0.29)
-
1.82(0.14)
-
+ N=3.
tinuing through day 43. We assume the incrcnsc to bc a result of self-shading causing
a compensatory increase in the lower layers
of the mat (see Brock and Brock 1969b ).
This incrcasc is similar to that found by
Wilhm and Long (1969). Cooke (1967)
and Kormondy ( 1969) reported decrcascs
in chlorophyll
a concentration with time.
In the microccosystcms studied by Cooke
and the ponds studied by Kormondy, the
autotrophs arc planktonic forms, standing
crop is relatively low, and shading is much
less severe than in the blue-green algal mat
communities
characteristic
of thermal
springs. Significantly, the microcosm algal
mats studied by Wilhm and Long comprised blue-green algae and formed on the
bottom of the container, only gradually
ascending the sides; black paint prcvcntcd
light penetration from below.
Another measure of successional change
is the ratio bctwccn carotenoids and chlorophyll a. The USC of this ratio is attributable
to Margalcf
( 1963)) but his empirically
derivccl USCof the optical density at 430:665
nm is not gencrally applicable to the scparation of the two pigment groups bccausc
of a secondary absorption peak of chlorophyll a at 430 nm ( Kormondy 1969). A
better ratio for general USC is 480:665 nm;
variations in this ratio with time and with
board arc shown in Table 5. The same
statistical tests wcrc used as with the chlorophyll a data, with similar results. Boards
2-4 did not differ ( p = 0.43)) but the corn-bincd data showed a very highly significant
cffcct of successional age ( p < 0.001). In
Fig. 5 the mcans and standard errors arc
plotted. From day 8 thcrc was a constant
decrease in the ratio, with a variation generally much lower than in the case of chlorophyll measurcmcnts.
Kormondy (1969) rcportcd an increase
in the carotenoid: chlorophyll pigment ratio
with succession. Both Cooke (1967) and
Wilhm and Long ( 1969) observed an initial
increase followed by a decrease or lcvcling
off. But they both used the ratio 430:665
so that the relevance of their findings to
our study is unclear. The structural and
growth characteristics of the thermal algal
community explain this contrast with prcvious studies. The thermal springs in summer are rclativcly
devoid of green; the
predominance of orange-yellow carotenoid
pigments is a response to high light intcnsity (Castenholz 1967). In winter, or
under other conditions of low light, the
orange-yellow color changes to dark green.
On the boards, the amount of carotenoid
(as measured by OD 480 nm) was constant, but the amount of chlorophyll
increased with time. At first the algal mat
was a thin film cxposcd to high light in-
'.'2
4
6
6
1012
14 16 16 2022242628303234
DAY
FIG. 5. Ratio OD 480:665 nm as a function of
successional age in days. Values are from Table
5. Vertical
mean.
lines represent
+
1 SE of the average
1
TIIERMAL
IO
20
DAY
30
40
EFFLUENT
SYSTEMS
225
50
FIG. 6. Changes
in species-volume
diversity
of blue-green
algal mats with time.
Succession
on board 1 began on 5 July 1968 and for boards
2, 3, and 4 on 18 July 1968.
tensitics and thcreforc rclativcly
low in
chlorophyll a. As succession procecdcd, the
mat thickcncd, the arcas undcrncath were
exposed to lower light intensities,
the
chlorophyll
a increased relative to carotenoids, and the OD 480:665 nm dccrcased.
Species diversity
The use of the Shannon-Wicncr function
for the calc_ulation of an index of spccics
diversity ( H ) has been discussed by others
(Patten 1962; Piclou 1966; Monk et al.
1969) and used to examinc diversity in a
number of ecosystems. Although it has
some disadvantages for this purpose, including a relative insensitivity to the addition of rare spccics once the total spccics list
is fairly large, it dots intcgratc in a single
value the effects of change in number of
spccics and of distribution
of individuals
among the species.
Because of the difficulty of deciding what
constitutes an individual among blue-green
algae, we used the volume per square
mctcr of each spccics in calculating fl (see
Dickman 1968 ) . Computation of this value
using a continuous variable such as volume
in place of the discrete measure of an individual number invalidates statistical comparisons of diversity indicts using the IFPtest
of IIutcheson
( 1970), since neither the
degrees of freedom nor the variance arc
uniquely determined. Figure 6, shows its
Successional
changes in percentage
FIG. 7.
abundance
(by volume)
of the three dominant
species of blue-green
algae on the four expcrimental
boards.
Arrangement
from top down:
Board 1, 2, 3, 4. Dashed lines-Mastigocladus
laminosus;
dotted lines-Oscillatoria
princeps;
solid lines-Phormidium
ramosum.
change as a function of successional age in
days, The value increased rapidly during
the first 2 weeks on Bl and B2, decreased
during the third and fourth weeks, then
rose again. B3 and 134 did not reach peak
diversity until after 4 weeks.
Within the time span of the study WC can
say that diversity is generally higher the
further down the flow gradient one samples
and that trends in g on 131 and B2 wcrc
similar and diffcrcd from those of B3 and
134.
The majority of the volume of the blucgreen mat on each of the boards comprised
only three species ( Fig. 7) : MastigocZadus
226
RICHARD
G. WIEGERT
laminosus ( Cohn), Phormidium
ramosum
( Boyc Pet. ) and Oscillatoria
princeps
( Vauchcr ) . The first film of algae to
appear on the boards was a thin layer of
P. ramosum, never quantitatively
important
after its early colonization,
The relative
importance of Oscillatoria over Mastigoclacks increased from Bl through B4, but
the proportion of the total volume comprising these dominant species decrcascd from
Bl through B4.
Ten spccics of blue-green algae wcrc
recognized; of thcsc only LM. Zaminosus and
the species of Anabaena have hcterocysts.
This, together with the low levels of fixed
nitrogen in the source water, could explain
the early and continuing
dominance of
LMastigocladus. The thin film of Phormidium
at the start of succession was possibly due
to nutrients leaching from the boards.
The successional picture given by Fig. 6
could be explained by the development of
more niches as distance from the source
increases, The development of tcmpcrature
and flow heterogeneity as the water moves
down the boards could be important factors,
as well as chemical heterogeneity secondarily related to temperature
and flow
changes. There is an initial rise in species
diversity on all boards, followed by a decline with time as 0. princeps becomes
dominant, particularly on B3 and B4.
Regulation
of primary productiona hypothesis
In constructing a hypothesis to explain
the factors controlling the primary production of the thermal algal mat, we must considcr two separate aspects of the autotrophic component:
What governs the
maximum standing crop and, by implication, the maximum rate of productivity that
can be realized in the steady state? What
governs the rate at which the successional
algal mat approaches the steady-state level
of standing crop?
The first question can be answered as
follows: In a very shallow effluent stream
the algal mat continues to grow until its
surface is raised above that of the water.
The algae are then simultaneously exposed
AND
PETER
C. FRALEIGII
to drying and to grazers and the nutrients
supplied by the flowing water are cut off;
the thickness of the mat dots not increase
further. If the flow is deep, or in quiet pools,
growth ceases when the mat reaches a thickncss of several ccntimcters, probably when
respiration losses from the dccpcr shaded
parts of such mats balance the net production added by the surface layers (see
Stockner 196S).
In support of these generalizations, WC
observed no case of a mat growing more
than a few millimeters above the surface of
a flow. In pools, deep streams, or where
periodic wetting is common (as around
gcyscrs ) the mats are ncvcr thicker than a
few centimeters.
The second question, concerning
the
factors governing the rate of net productivity during the successional approach to
a steady state, is more complex. We have
shown significant differences in the rate of
net production on the four boards. Each
board had the same dimensions, slope, and
water depth. There was therefore no rcason to suppose that either the daily rate or
the total solar insolation differed between
boards.
Total phosphate levels did not differ
between boards (Table 1) ; a downstream
dccrcase in the concentration of this nutrient could not, therefore, explain the diffcrcnces in productivity.
If competition
among the nitrogen-fixing
forms for Nz
were the major factor limiting productivity
[in summer 1969 an extremely high rate of
N-fixation was found on all four boards ( W.
Stewart, personal communication) ], then
WCshould see an increase downstream, since
the Na content of the water should increase
with exposure to an atmosphere composed
of 80% Nz ( 02, constituting only 20% of
the atmospheric air, increases with distance
from the source, even at night, when 02
demand is high).
From the source to the outflow of B4 a
dccreasc of 4-6C brings the temperature
below 40C-the
limit below which a blucgreen algal mat does not grow well. Howcvcr, when WC examined some of the other
outflows in Serendipity Meadow ranging
in temperature from 904X,
we were sur-
THERMAL
EFFLUENT
priscd to find poor development of blucgreen mats in many of the streams that wcrc
still 45C or higher. The distance that a
well-developed
mat extended downstream
seemed to be related rather to the volume
and depth of flow, i.c. to the length of time
that the water was exposed to the atmosphere, than to its initial
temperature.
Thus, we rcasoncd that the productivity and
steady-state standing crop of the algae
might bc determined by some change in
composition of the source water related to
the time of exposure to the atmosphcrc.
The source water loses free CO2 to the
atmosphere with a corresponding rise in
pH. The algae at any point in these flowing
systems are living in a constantly enriched
medium, but the level of enrichment decreases independently of any activity of the
algae themselves. We think this differing
enrichment by gaseous 002 may be primarily responsible for the inverse rclationship of productivity
of algae with distance
from the source.
In stagnant situations CO:! may bccomc
limiting at very low levels (see Fogg 1965;
King 1970). However, when other nutrients are abundant, bubbling algal cultures with 1% CO2 in air markedly increases
the growth rate ( Holton 1962)) and the
effect of this enrichment should continue
until the rate of entry into the cell of some
other essential nutrient bccomcs limiting.
Wright and Mills ( 1967) showed that
the free CO2 entering the Madison River
from a large thermal spring incrcascd the
primary production of that portion of the
river and concluded that fret CO2 added to
a system through oxidation
of organic
pollutants may bc important in causing eutrophication where other essential nutrients
are already in excess. Kuentzcl ( 1969) developcd a similar hypothesis from an extensive literature review. The conclusion
of Wright and Mills was clouded by their
inability to show clearly that the incrcascd
productivity
was related only to the COa
enrichment and not to the other inorganic
nutrients cntcring the river in the thermal
spring effluent. Our study does make this
separation, and our hypothesis, if proved
correct, supports their conclusions.
227
SYSTEbIS
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