A STUDY OF SELECTED
MECHANISMS
FOR THE
COEXISTENCE
OF DIAPTOMUS
SPP. IN
CLARKE LAKE, ONTARIO’
Gail A. Sandercock
Institute
of l?isheries,
University
of British
Columbia,
Vancouver
ABSTRACT
The following
mechanisms have been suggested to explain the coexistence of different
Three
Diaptomus
spccics : vertical segregation, seasonal separation, and size differences.
species of Diaptomus
(D. minutus, D. oregonends, and D. sanguineus, in o,rder of increasing
size) occur in Clarke Lake, Ontario.
D. mzhutus was vertically
separated from D. sanguinercs, and there was a distinct size difference between the two. D. minutus and L). oregonensis
showed different
seasonal maxima and were separated by size diffcrcnces.
D. sanguineus
and D. oregonensis were scparatcd vertically
and had different
seasonal maxima. A combination of two mechanisms allows cocxistcnce of each of the three Diuptomus species-pairs
in Clarke Lake.
INTRODUCTI.ON
Most studies of congeneric, coexisting
species have been concentrated on differences rather than similarities in the use of
a common environment (Hutchinson 1957a).
The basis for such an approach has been
stated by Hutchinson (1948, p. 239) : “closely
allied species living together practically always occupy slightly different niches or,
in other words, have different tolerances
and optima.” Mayr (1963) suggests that
the evolution of differences in overlapping
spccics is likely, and Hutchinson
( 1959)
has questioned how many diffcrenccs between two species are necessary to prevent
them from occupying the “same niche.”
Three mechanisms for the coexistence of
l The author wishes to thank Dr. R. R. Langford,
University
of Toronto, for making available plankton samples, oxygen and temperature
data ( 19601964); Dr. F. H. Rigler, Dr. R. R. Langford,
and
Mr. N. V. Martin
(Ontario
Dept. of Lands and
Forests) for use of equipment; Dr. T. G. Northcote,
University
of British Columbia, for helpful suggcstions and criticisms;
Dr. Northcote,
Dr. C. C.
Lindsey and Dr. N. J. Wilimovsky
for reviewing
the manuscript
when presented in partial fulfillment of the requirements
for the degree of M.Sc.
at the University
of British Columbia;
Dr. J, T.
McFadden,
University
of British
Columbia,
for
assistance with the statistical analysis and for criticizing both the original and revised manuscripts;
and acknowledge
financial support from Dr. F. II.
Rigler and Dr. R. R. Langford,
the University
of
British
Columbia,
and the National
Rcscarch
Council,
97
Calanoid copepods have been suggested: 1)
size differences implying food differences
(Fryer 1954; Hutchinson 1951)) 2) vertical
segregation and, 3) seasonal separation
(Cole 1961, 1963; Wells 1960; Pcnnak 1957;
Yeatman 1956; Davis 1954, 1961; Jahoda
1948; Hoff 1944; Langford 1938; Gurney
1931; Bigelow 1923). There has been no
single investigation
of the three mechanisms. Generally, if coexistence was observed without one particular mechanism
evident, one of the other two was assumed.
In this study, size measurements, vertical
distribution, and seasonal abundance were
investigated for three spccics of Diaptomus
(D. minutus Lilljeborg,
D. oregonensis
Lilljeborg, and D. sanguineus S. A. Forbes)
coexisting in Clarke Lake, Ontario.
D. minutus has been found with D. oregonensis by a number of workers (Davis
1961, 1962; Wells 1960; Jahoda 1948; Langford 1938). D. sanguineus is usually found
in ponds (Cole 1963; Wilson and Moore
1953; Wilson 1959), although Yeatman
( 1956) found it in Woods Reservoir, Tcnnessee.
Price ( 1951) reported only D. oregonensis
in Clarke Lake in 1949. Mrs. E. J. Jcrmolajev noted both D. minutus and D. sanguineus in collections from later years, so
the coexistence of Diaptomus spp, in Clarke
Lake seems to be recent. There have been
no obvious changes in the lake to account
for this.
GAIL
hG.
1.
Contour
(m)
map
of
Clarke
A.
SANDERCOCK
Lake,
Ontario.
METHODS
Plankton samples and oxygen and temperature data collected at one-week intervals were available for the summers of
1960-1964. Samples were also obtained
during winter of 1963-1964, and three series
of samples were collected over 24 hr (diel
series) during summer 1963.
Temperature readings were taken with a
thermistor at approximately
l-m intervals
from the surface to the bottom. The oxygen
content of the water was usually determined
from four samples taken at 2-3-m intervals
using the standard Winkler method. A bottom contour map (Fig. 1) was drawn from
echo-soundings.
All plankton samples were taken in the
central basin of the lake with an unmodified
Juday plankton trap (Juday 1916; Welch
1948) fitted with No. 20 bolting silk. The
weekly midday samples ( 1960-1964) and
two diel series (17-18 June, 27-28 July
1963) were taken at depths of 0, 1, 3, 5, 7,
9, and 11 m. In the third diel series (20-21
August 1963), samples also were taken at
8 but not at 11 m. In the diel series, samples
generally were taken at 2hr intervals, ex-
ccpt at dawn and dusk, when the samples
were collected hourly.
Each sample was examined in a partitioned Petri dish. The adult Diuptomus
were first identified
to species (Wilson
1959) using a compound microscope. In
practice, the species were separated on the
basis of sex and size. Adults were sorted
to species working with low power of the
dissecting microscope. The numbers of juveniles and adults of each species were
recorded. Numbers of animals (x) at a
given depth were transformed to (x + 1) 4
in the analysis of variance. The number of
females with eggs and the number of eggs
per clutch were also listed, but the fluctuations over 24 hr were so great that no attempt was made to analyze the weekly
samples for seasonal change.
Size measurements of adults were made
using a dissecting microscope at 120~.
Wilson (1959) gives the following dimensions, including
urosome, for the three
species studied: D. minutus, 0.90-1.00 mm;
D. oregonensis, 1.25-1.40 mm; D. sanguineus, 1.00-2.10 mm. In this study, the measurement of length was made with the animal in lateral position, excluding the urosome. Males and females of each species
were measured from diel series samples in
May, June, July,, and August 1963, and from
January, February, and March 1964 samples.
Small sample sizes of the three species
at different depths precluded a separation
of sizes in relation to depth.
Ckzrke’ Lake description
Clarke Lake (13.0 hectares, maximum
depth 12 m) is located in Airy Township
at the east entrance to Algonquin
Park,
Ontario (45” 32’N lat, 78” 16W long). It
is part of the Madawaska watershed. The
lake’s single basin has a rather narrow littoral zone with abundant rooted vegetation.
Below the 3-m contour, the bottom consists
of highly organic mud. The lake water is
“tea-colored” and slightly acid. In lake cnrichmcnt studies involving fertilizer application in Algonquin Park, Clarke Lake was
used as a control. Summer plankton blooms
normally do not occur ( Langford 1951) .
COEXISTENCE
I
OF
8
1
-
CALANOID
COPEPODS
99
June
June
Juiy
FIG.
360
3.
July
Au<
Oxygen
August
‘:
OF
June
OF
isopleths
September
TIME
TIME
(ppm)
Oi
May
July
1961
0.
May
Lake
November
for Clarke
October
SAMPLING
I
SAMPLING
June
(19fS1964).
December
1
I
January
July
August
1962
February
I
March
1
COEXISTENCE
Temperature
OF
C4LANOID
and oxygen
The temperature isotherms (Fig. 2) are
based on .thermistor readings taken at approximately
l-m depth intervals.
Clarke
Lake is usually ice-covered in winter, and
experiences turnover twice a year (fall and
spring). It can be classified as a tempcratc
dimictic lake ( Hutchinson 1957b).
The oxygen isopleths (Fig. 3) are based
on water samples taken every 2-5 m. Oxygen values of less than 2 ppm were found
at 10.5 m two to three weeks later in 1963
than in 1960 or 1962. Earlier oxygen depletion in 1960 and 1962 than in 1963 can
bc explained by the related temperature
data: heating of the upper 3 m to 16C
sccmcd to have been two to three weeks
later in 1961 and 1963 than in 1960 or 1962.
By the end of June in 1960 and 1962, a fairly
definite thermal stratification
was cstablished (which was not obvious in 1963)
restricting mixing in the lake, and the hypolimnion became dcoxygenated.
101
COPEI’ODS
BODY
LENGTH
(in
mm 1
)5
CLitLJQ
-kLcf
2 JXkJQ
J2LL-s
2
I
Q. minutus
El
0. oreaonensis
EU
Q sanquineus
rQ
-diLd
Pd
RESULTS
-0
Size measurements
At any one time that size measurements
were made, a unimodal frequency distribution was observed for each sex of all three
species.
Standard deviation and standard error of
the mean lengths were calculated. The
mean -t-2 SE and the mean +-2 SD are presentcd graphically (after Hubbs and Hubbs
1953) in Fig. 4. Comparison of the white
bars for the males and females of one species at one time shows that there is usually
considerable overlap in size, with one exception (July, D. sanguineus).
Thcrc is no size overlap between one
species and the next larger species in May,
July, or August 1963 or in winter 1964
samples. Of all specimens measured, 95%
fall within k2 SD of the mean-within
the
white bar, Thus, in cases of no overlap in
the diagram, 95% of the fem,ales of the
smaller species were definitely scparatcd
from 95% of the larger species on the basis
of metasome size alone.
Thcrc is a marked overlap of the females
L-.---L
---
--I
--__
1.
FIG
4. Vc?riation in size (length in mm, cxeluding urosome) of Diaptomus
min&us, D. oregoner&a, and D. sanguineus in May, June, July,
and August 1963, and in winter
1964. (small
peak = mean; length of bar showing species =
4 SE; total length of white bar = 4 SD; length of
underline = range. )
of D. oregonensis and the malts of D. sanguineus in June, so that if size of food consumed is directly related to body size in
Diaptomus
as suggested by Hutchinson
( 1951), then these two groups might be
competing for food.
Males and females of D. minutus were
larger in winter and May than in July or
August samples, as were those of D. oregonensis. The males of D. sanguineus were
slightly larger in July than in June, the
females being the same size in June and
July. Comita and Anderson ( 1959) and
Cole (1963) found adults of D. ashlandi
Marsh and D. siciloides Lilljeborg, respectively, to be larger in summer than in
winter.
102
GAIL
A.
SANDERCOCK
1. Results of three-way
analysis of uaria,nce u%h time, species, and depth as major effects for Diaptomus
min&us and D. oregonensis
in weekly samples (1960-l 964) and in three die2
series (1963)
TABLE
Source
of vari-
Sum of
squaws
df
ancc
___--
Mean
square
F
0.51
5.37
30.82
1.71
2.20
1.79
0.37
1.38*
14.51-t
83.3Ot
4.621’
5.9m5.t
4.84-f
Species
Time
Depth
TxD
TxS
SxD
Residual
Total
7;
181
Spccics
Time
Depth
T x D
TxS
SxD
Residual
Total
1
14
6
84
14
6
84
209
1961
29.42
145.07
440.64
213.42
134.83
51.97
168.36
lJ83.71
29.42
10.36
73.44
2.54
9.63
8.66
2.00
14.71-t
5.181
36.72t
1.27”
4.82t
4.33t
Species
Time
Depth
T x D
TXS
SxD
Residual
Total
1
15
6
90
15
6
90
223
1962
0.35
16.54
232.27
77.65
72.01
3.81
102.88
505.56
0.35
1.10
38.71
0.86
4.80
0.64
1.14
1.31”
0.96”
33.961
0.75””
4.21 t
0.56”
Species
Time
Depth
TxD
TxS
SxD
Residual
Total
1
16
6
96
16
6
96
237
Vertical
distribution
of Diaptomus
16.59-t
7.56-k
46.41t
1cw-b
A.“,
source
of variante
1.
-
(Continued)
Sum of
squares
df
Mean
square
F
Winter 1963
185.11
185.11
218.74
21.87
67.38
11.23
149.12
2.49
55.00
5.56
27.63
4.61
106.02
1.77
809.00
104.58-I
12.36t
6.341’
1.41”
3.11-t
2.601
Residual
Total
1
10
6
60
10
6
60
153
Species
Time
Depth
TxD
TxS
SxD
Residual
Total
1
15
6
90
15
6
110
220
June 1963 diel
210.37
210.37
44.19
2.95
522.42
87.07
91.73
1.02
15.71
1.05
19.22
3.20
231.29
2.10
lJ34.93
100.18t
1.40”
41.46-k
0.49*
0.50”
1.52*
Species
Time
Depth
TxD
TxS
SxD
Residual
Total
1
14
6
84
14
7::
195
Species
Time
Depth
TxD
TxS
SxD
Residual
Total
1
14
6
84
14
6
84
209
Time
Depth
--
Summer 1963
24.39
24.39
177.88
11.12
409.36
68.23
280.22
2.92
130.03
8.13
42.75
7.13
141.56
1.47
1,206.19
-.-
Species
1960
0.51
64.43
184.94
122.96
26.43
10.75
26.85
436.87
1
12
6
72
12
TABLE
--
TxD
TxS
SxD
July
rhg
1963 diel
78.38
78.38
44.57
3.18
199.87
33.31
140.67
1.67
23.12
1.65
60.85
10.14
70.27
1.00
617.73
78.38-t
3.18.f
33.31-f
1.675:
1.65”
10.14t
1963 die2
29.67
29.67
87.44
6.25
168.05
28.01
1.37
114.83
3.59
OI26
10.21
1.71
33.99
0.40
447.78
74.1811.56*
70.03-t
3.43t
0.65’”
4.25-f
I
5.53t
4.85-t’
spp.
Weekly samples. The plankton data were
analyzed to test the null hypothesis that
there was no vertical separation of the three
Clarke Lake Diaptomus species. From a
preliminary
examination of the plankton
counts it was obvious that D. sanguineus
was distributed in the bottom 5 m 0.F the
lake (see Fig. 6) and was segregated vertically from the other two species; no fur-
* p>o.o5.
p p < 0.01.
$p<O.O5.
ther analysis was performed in the case of
D. sanguineus.
To test the null hypothesis that there was
no vertical separation of D. minutus from
D. oregonensis, a Model I three-way analysis of variance was used ( Snedecor 1956).
Time, depth, and species were considered
as major effects. In the analysis, each of
the effects was averaged or summed over
the other two f,actors. A square root transformation,
(x i- 1)4, was applied to the
COEXISTENCJJ
OF
CAI,ANOID
Depth
Depth(m)
103
COPEI?ODS
Depth
Cm)
(m)
1500V
d. Summel
f. 17-18 June
1963
1963
--r----l--e. Winter
19631964
3Y
z
b
X
0 100
+
g.
27-28
July
1963
h. 20-21 August
1963
c. Summe
1962
100
tit
L7
*--A
B. minutus
O----O D. oreqonensis
FIG, 5. The
formed to (x+
(1963).
depth distribution
l)“,
summed
over
of Dzizptomus minutus
time in the weekly
counts to stabilize the variance (x = actual
number of adults of a species at one depth).
The results ( Table 1) indicated that the
species-depth interaction was significant at
the 1% level in all years but 1962, and at
the 5% level in winter and fall 1963 samples, A significant species-depth interaction
means that the pattern of distribution with
depth differed for the two species. The
shapes of the curves (Fig. 5, a-e) differed
for the two species, yet were similar for
the different years within each species.
Highest numbers of both D. minutus and
D. oregonensis were in the upper 5 m. The
maximum for D. minutus was in the upper
2 m; in contrast, numbers of D. oregonensis
were low at the surface and increased to a
maximum at about 3 m. Numbers of both
species decreased gradually below the respective maxima and were low below 5 m.
and D. ore,~o~aensis with numbers, transsamples (1960-1964)
and three diel scrics
The depth-time
interaction was not significant in two of the four years nor in
winter samples, so that in three of five
cases the patterns of vertical distribution
with depth did not differ sufficiently from
time to time to bc detected by the sampling
method.
The vertical distributions are also shown
as histograms (Figs. 6 and 7). For each
species, the number at every depth sampled
was converted to a percentage of the total
number of adults of that species (obtained
The original
by summing over depth).
weekly counts were grouped on a four-week
basis from midmonth to midmonth. The percentages of the total numbers of the species
for the month at each depth were calculated
and graphed. The data for juveniles were
treated in the same way.
The significant species-depth interaction
104
GAIL
Adult
Weekly
A.
SANDERCOCK
Distribution
Samples
(1960
- 1964)
1960
1962
15 May-
15 Jun-
15 Jul-
15Aug-
act
15 May 15 Jun- 15 Jul-
15 Aug-
15 Jun
15 Jul
15 Aug
15 Sept
Nov
15 Jun
15 Sept
15 Jul
15Aug
Dee
mihtus
9rrr
D
0regZnensis
D
oreqZnensis
r
Q:
sanqulneus
(1
mgfi’neus
1963 -1964
1961
15 May-
15 Jun-
15 Jul-
15 Aug-
15 May-15 Jun-15 Jul-
15 Aug- Ott
15 Jun
15 Jul
15 Aug
15 Sept
15 Jun
15 Sept
15 Jul 15Aug
Nov
Jan
Feb
Dee
Mar
minutus
I
D-
Iii
orrgonensis
---
n.
oreqonensis
D
s;lnqi;neus
D
-*.
sanquineus
of adults of Diaptomus
FIG.
6. The vertical distribution
in weekly samples (1960-1964)
in Clarke Lake.
minutus,
Zl. oregonmsis,
and D. sanguineus
COEXISTENCE
OF
CALANOID
is reflected in the histograms, where the
highest pcrccntages of D. minutus occur
from the surface to 2 m and of D. oregonensis from 2 to 5 m. D. sanguineus was
more widely distributed in depth in May,
and the depth distributions of D. minutus
and D. oregonensis were more nearly uniform in fall and winter samples. Although
both D. minutus and D. oregonensis occurred within the upper 5 m, the pattern
of depth distribution
within that stratum
did differ. In carlicr studies of vertical distribution, Birge (1897) reported D.. oregonensis to be more abundant in the upper
strata; Langford ( 1938) found D. m.inutus
ctnd D. oregonensis adults above the 25-m
level.
During June, July, and August higher
percentages of juveniles occurred both in
the upper and lower few meters, with lower
percentages at mid-depths
(Fig. 7). In
early May and winter samples, the juveniles
were concentrated either in the upper levels
or in the lower few meters.
Diel samples. A three-way analysis of variance similar to that used in the weekly
samples was applied to the data from the
diel series. The analysis was performed to
test the null hypotheses that there was no
vertical separation of the species and that
thcrc were no changes in the depth distribution of the species with time. As in the
weekly samples, the first hypothesis was
rejected in the case of D. sanguineus (Fig.
8) by examination of the plankton counts.
For D. minutus and D. oregonensis, the
species-dcp th interaction was significant at
the 1% level (Table 1) in July and August
in two of the three diel series. The patterns
of depth distribution
of the two species
(Fig. 5, f-h) were thus significantly
different in July and August and were similar
to those found in the weekly samples resulting from averaging over four months
(Fig. 5, a-c).
The time-depth interaction (with numbers averaged over species in the three-way
analysis) was significant at the 5% level in
July and at the 1% level in August (Table
1) . To detcrminc whether the significance
of the time-depth interactions in July and
105
COPEPODS
__
____---
Juvenile
Weekly
15May-15Jun-15Jul15 Jun
15Jul
---
Distribution
15Aug
Samples
ISAug-
Ott
Jan
15 Sept
Nov
Dee
Mar
Feb
1961
1962
FIG. 7. The verlical
distribution
of juveniles in
weekly
samples (1960-1964)
in Clarke
Lake.
The number of vertical
series taken during the
sampling period is given at the base of each
histogram.
August was due to one or both species, a
two-way analysis of variance ( considering
time and depth only) was used, with each
species treated separately. In each 24-hr
series, five daylight collections were treated
as replicates for each species and compared
with five vertical series taken in darkness
( which served as nighttime replicates ) .
The results (Tables 1 and 2) showed that
the depth-time interaction was never significant for D. minutus alone, indicating
that these samples failed to detect any
changes in depth distribution from day to
night. Only in July was the interaction
significant for D. oregonensis. Thus, only
D. oregonensis was shown to be giving rise
to the significance of the time-depth interaction in the three-way analysis of variance
in July.
The vertical distributions in the diel series
are shown in histograms ( Figs. 8 and 9).
The counts were grouped into three time
sequences : 1) noon to sunset; 2) sunset to
0400 hours; and 3) 0400 hours to noon.
106
GAIL
A.
SANDEIXCOCK
____.
Adult
Diel
Series
Q minutus
i
(1963)
P. oregonensis
m
ii
Jun
1
3
Q s_anquineus
III
i
Jun
rrr5
5
-
Distribution
Jun
6
Jul
M 5
Jul
rrr4
5
5
FIF4
5
6
I= Noon
-Sunset
Aug
a
ii
5
6
Jul
UL
5
5
Aw
AL~gL44L
a= Sunset
-0400
hours
a - 0400
5k
6
hours-Noon
FIG.
8. The vertical distribution
of adults of Diuptomus minutus, D. oregonends, and D. sanguineus
in three diel series ( 1963). The number of vertical series taken during the sampling period is given
at the base of each histogram.
The percentage of each species at each
depth in the group was then calculated and
graphed. Counts for juveniles were similarly grouped,
The significant depth-time interaction in
the two-way analysis of variance of D.
oregonensis in July was probably due to a
nighttime sinking of D. oregonensis, as seen
in the histograms where no changes in
depth distribution with time were seen in
ci thcr D. minutus or D. sanguineus (Fig. 8)
or in juveniles (Fig. 9).
Other workers (Wells 1960; Langford
1938) have reported upward movement of
D. oregonensis at night. Langford also described a bimodal distribution in D. minutzcs at night.
Seasonal changes in ahunclance of Diaptomus spp. The null hypotheses, that there
was no separation of the times of maximum
abundance of the three species or that there
were no changes in the relative abundance
of the species with time, were tested in the
three-way analysis of variance for D. minutus and D. oregonensis. The results (Table
1) showed that total numbers of both species changed significantly
in three of the
years. The species-time interaction
was
significant in all years, that is, the pattern
of change in numbers of D. minutus was
significantly different from that of D. oregonensis in all years. Thus, the null hypotheses were rejected.
For the weekly samples, the numbers,
transformed to (X + l)+, of adults of each
of the three species collected on a given
date were plotted against time (Fig. 10,
a-d). In 1961, 1962, and 1963, there were
initially high numbers of D. min&us adults,
but later D. oregonensis became the most
abundant species. In 1960 and 1963, the
high numbers of D. oregonensis were followed by increases in D. minutus. Numbers
of D. sanguineus were highest in early Nay
of both 1962 and 1963. A difference in time
of maximum a‘bundancc of D. oregonensis
from that of b’oth D. minutus and D. sanguineus is clearly shown.
The comparison of year to year succession of species is readily made when the
“percentage species composition” is graphed
COEXISTENCE
OF
CALANOID
107
COPEPODS
2. Results of two-way analysis of uariante with time and depth as major effects fog
Diaptomus
minutus and D. oregoncnsis
comparing day and night samples of three diel series
TABLE
(1963)
Source
of variante
Time
Depth
TxD
Residual
Total
Time
Depth
TxD
Residual
Total
Time
Depth
TxD
Residual
Total
Time
Depth
TxD
Residual
Total
Time
Depth
TxD
Residual
Total
Time
Depth
TxD
Residual
Total
Sum of
squares
df
Mean
square
June, D. minutus
2.92
2.92
1
74.46
6
446.78
6
56
69
7.96
60.63
518.29
1.33
1.08
June, D. orcgonensis
0.12
0.12
1
6
5:
69
July,
1
6
5:
69
July,
1
6
27.22
2.30
22.85
52.49
3.44
34.25
4.19
27.12
69.00
3.44
5.71
0.70
0.48
1). oregoncnsis
0.95
0.95
154.02
127.70
333.78
25.67
8,52
2.28
D. minutus
1.45
1.45
6
34.95
0.95
21.83
59.18
Aug,
1
6
5:
69
Diel
Distribution
Series (1963)
2.70*
68.94t
1.23"
0.29"
11.07t
o-93*
7.17-l
11.90-l
1.72"
0.42"
11.26-t
3.74.t
t 6
5.83
0.16
0.39
0.37"
14.95$
0.41*
t 5
Au9
6
I= Noon -Sunset
a=0400
Aug,
1
5:
69
Juvenile
D. minutus
51.11
5:
69
4.54
0.38
0.41
F
n:= Sunset
hours
- 0400
hours
-Noon
l7IG. 9.
The vertical
distribution
of juveniles
in three diel series ( 1963 ) , The number of vertical
series taken during the sampling period is given
at the base of each histogram.
D. oregonensis
2.38
59.18
10.86
48.92
121.34
2.38
9.86
1.81
0.87
2.74"
11.33-t
2.08"
(Fig. 11, a-d), In each year D. oregonensis
increased from a low 20% value to 70-80%,
then returned to the 20-30% level. From
mid-June to the end of July 1960 and 1962,
from mid-July to the end of August 1961,
and from mid-July to the end of September
1963, over 50% of the adult Diaptomus sp.
were D. oregonensis. The increase of D.
oregonensis was approximately
a month
later in 1961 and 1963 than in 1960 or 1962.
In the 1964 winter samples and throughout
the rest of the sampling period when D.
oregonensis was not abundant, D. minutus
always was over 50% of the adult Diaptomus sp. The percentage of D. sanguineus
adults was higher (30-50%) in the June
and July 1962 and 1963 samples than at
other times.
The time of increase in D. oregonensis
percentage was earlier in 1960 and 1962
than in 1961 or 1963. The increase in temperature of the epilimnion also seemed to
108
GAIL
(I*xrOl.
aJ
a
pWJJO4SUeJ$)
S7lnpV
A,
SANDEnCOCK
40 SJaqUlnly
I
(I*xpO1.
paLUJO4SUeJl)
SllnpV
40 SJ3qLUnt-d
COEXISTENCE
OF
CALANOID
109
COPEPODS
1200
November
Eastern
daylight
Time
1600
2000
2400
December
(in hours)
400
coo
February
January
1203
March
50
Frc,. 11. The variation in pcrccntagc of adult Diu~Wry~z~s minutus,
in weekly samples ( 1960-1964)
and in three diel series ( 1963).
D. oregonensis,
Eastern
Time
May
ofJ;;ppling
$i;,YdaYs)
August
daylight
Time
and D. sanguineus
(in
hours)
September
50
October
November
December
Frc,. 12. The variation in percentage of adult and juvenile Diuptomus
weekly samples ( 1960-1964)
and in three diel series ( 1963 ) .
January
February
of the total
March
population
in
110
GAIL
A.
SANDERCOCK
be two to three weeks later in 1961 and
1963. Either the seasonal increase of D.
oregunensis may bc related to factors associated with lake temperature, or both increases may be related to some third factor and do not affect each other.
The number of adults of all three species
plus the number of juveniles was obtained
for each vertical series, and the percentage
of juveniles of this total Diaptomus population was obtained. The percentage of juveniles in the total population
remained
constant over much of the sampling period
(Fig. 12, a-d) with the exception of May
1961 and 1963, and fall and winter 19631964, when there were low percentages.
Diel changes in abundance of Diaptomus
spp. If, in plankton sampling, certain depths
for collecting are chosen at random, then
the sum of the numbers of plankton collected at each depth should be representativc of the total population in the water
column, For a second series also collected
from randomly chosen sampling depths,
again the sum should be representative of
In this case, the sum of
the population.
numbers of plankton for depths sampled
should not change significantly
with time,
unless the population
changed in abundance.
With the sampling depths fixed, as in
this study, the sum of numbers over the
depths sampled would still be representative if the plankton distributions were random. The situation changes in the case of
sampling a clumped distribution
at fixed
depths, When aggregations of animals occur and shift from depth to depth, it is
possible that the fixed depths sampled could
miss the majority of the population. If however, the aggregations did not shift, then
the sum of animals collected at all depths
should not differ from a similar summation
made at another time, although these samples might not be representative of the total
population.
The null hypothesis that there were no
changes in relative numbers of the species
of D. minutus and D. oregonensis in time
over 24 hr was also tested in the three-way
analysis of variance of the data from the
three diel series. The results (Table 1)
showed that time was significant in only
one of the three diel series, and that the
species-time intcrraction was never significant. Thus, the results failed to disprove
the null hypothesis.
Greatest numbers of D. minutus and D.
oregonensis were found in the upper few
meters ( Figs. 6 and 8)) so the analysis
supports the idea that the populations sampled consist of aggregations that usually
do not change in their depth distributions
over 24 hr.
The numbers, transformed to (X +l)i, of
adults were plotted against time of day for
the diel series ( Fig. 10, e-g). The fluctuations in number were not statistically significant, and in the graph of the percentage
species composition (Fig. 11, e-g) little
change over 24 hr is evident. D. minutus
was always over 50% of the adult Diaptomus
sp, in the June series, with D. oregonensis
over 50% in July and August.
The percentage of juveniles in the total
population was constant in each of the diel
series ( Fig. 12, d-f).
DISCUSSION
This study sh.ows evidence for each of
the three suggested mechanisms of coexistence for species of Diaptomus.
The
seasonal maximum for D. minutus was in
spring, while that of D. oregonensis was in
summer; thus, the two species were separated seasonally. There was a distinct size
difference in the two species. If Fryer’s
( 1954) work has general application, the
consumption of food particles of different
size is a second possible mechanism of coexistence in the two species.
D. minutus occurred in the upper 5 m,
while D. sanguineus was found below the
6-m level, A definite size difference was
observed between these two species each
time measurements were made. Thus, the
two species were separated both vertically
and in size.
D. oregonensis was similar in depth
range to D. mircutus and so was also segregated vertically from D. sanguineus. The
seasonal maximum of D. sanguineus was
COEXISTENCE
OF
CALANOID
TABLE 3. Summary of mechanisms of coexistence
of three Diaptomus
species in Clarke Lake,
Ontario
-Mcchnnisms
of cocxistencc
Size
cliffcrence
Scasonnl
separation
Vertical
segregation
D. minutus
cand
D. oregonends
+
+
-
D. oregonend
an d
D. sanguineus
-
+
+
D. sanguineus
and
D. minutus
+
--
Species
pair
-
+
--
in spring, while that of D. oregonensis was
in summer; thus, the two species were separated seasonally as well as vertically.
Considerable
overlap in size between
the males of D. oregonensis and the females
of D. sanguineus in June indicated that differences in the size of food consumed cannot always be a means of coexistence in this
case. Similar observations of overlap are
given by Davis ( 1954) and Cole ( 1963).
Evidence for two of the three factors
permitting coexistence was found in each
of the three congeneric species pairs in
Clarke Lake, but different combinations of
factors were found in each case (Table 3).
It appears that the coexistence of Diaptomus species in Clarke Lake depends on the
additive effects of two factors or mechanisms. Additional factors not investigated
may also operate in combination with those
studied to permit coexistence.
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