DOMINANT
PLANKTONIC
ROTIFERS OF MAJOR
WATERWAYS
OF THE UNITED
STATES
Louis G. Williams1
US. Public IIealth
Service, R. A. Taft Sanitary
Engineering
Center, Cincinnati,
Ohio
ABSTRACT
Rotifers were the most numerous metazoans found in preserved plankton samples taken
routinely at 128 sampling stations on the major rivers and Great Lakes of the United States.
While rotifers of more than one genus often dominated a sample, the dominant form tended
stations with high population
densities had low species
to be one species. Generally,
diversity.
Turbulence,
silt, and some other cdaphic factors appeared to be more important
than industrial or domestic wastes, during winter and early spring, in reducing the number
of species of rotifers in the biota. During the summer and fall, differences in the abundances
of dominant species appeared to be the best criterion for indicating
differences
in water
quality or pollution.
METIIODS
INTRODUCTION
AND
MATERIALS
Type and preservation
Inadequate knowledge of the zooplankton
of rivers is a major impediment to a better
understanding of the trophic relationships
governing biological productivity
of the
of the United States
major wateiways
(‘Williams 1962). Beach ( 1960) reported
34 species in 24 genera of rotifers in plankton from the Ocqueoc river system of Michigan and, like Chandler ( 1937)) found that
rotifers in outlets of lakes and impounded
waters gradually dccreasc in numbers downstream. These studies indicated that planktonic rotifer fauna of lotic environments are
not qualitatively
distinct from those of the
lentic environment.
The common river
species have been reported, but no systematic attempt has been made to quantify
them for the major rivers on a nationwide
basis or to relate them to river environments.
This report is based on data taken from
semimonthly plankton samples at 88 of 128
stations of the National Water Quality Network of the US. Public Health Service
during the last two years of a five-year
study ending November 1962. Only those
stations were included that had been in
operation two or more years.
Acknowledgment
is given to Mr. Albert
Katko, who, during 1961 and 1962, did
much of the routine enumeration of the
rotifers.
of samples
In 1958-1959, an extensive study of fresh
and variously preserved plankton samples
from 19 widely scattered sampling stations
established that many of the delicate
protoeoa and a few of the more delicate
rotifers lost their identity when they died
or when preserved with formaldehyde. After
various preservatives and their combinations were tested, thimerosal (merthiolate)
0.007%, buffered with sodium borate and
containing 0.1% iodine (potassium iodide
and iodine) was found to be the least
destructive to plankton and was adopted as
the standard preservative. After the samples
arrived at the counting laboratory in Cincinnati, iodine was occasionally added to
them to enhance settling of dense plankton
blooms or to prolong preservation. During
the first 18 months oE this program, many
planktonic forms arrived in a more readily
identifiable condition when alive than when
preserved with 5% formaldehyde.
Major
alterations of samples occurred between the
time of collection and counting, preventing
the use of live samples for quantitative
mcasuremcnts. Although neither 5% formaldehyde nor thimerosal-iodine
preservative
proved to be completely bacteriostatic, the
latter arrested ccl1 division in phytoplankton
and zooplankton, and the samples were not
significantly degraded by bacterial action if
they were processed within a month. Since
samples were processed routinely within
l Present address:
FWPCA,
National
Water
@ality
Research Laboratory,
Duluth,
Minnesota
55812.
83
84
LOUIS
TABLE
1.
Population
densities
G. WILLIAMS
of rotifers
in the major waterways
of the United
States
A
Rotifers in semimonthly plankton samples,
counts/liter,
1961 and 1962
Basin, river, or lake,
and station
Colorado River Basin
Animas River, Cedar Hill, New Mexico
Colorado River, Yuma, Arizona
Parker Dam, Arizona
Boulder City, Nevada
Page, Arizona
Loma, Colorado
Southwest-Lower
Mississippi
River Basin
Arkansas River, Pcndleton Ferry, Arkansas
Ponca City, Oklahoma
Coolidge, Kansas
Mississippi River, New Orleans, Louisiana
Delta, Louisiana
West Memphis, Arkansas
Cape Girardeau, Missouri
Ouachita River, Bastrop, Louisiana ( 1962)
Red River, Alexandria,
Louisiana
Index, Arkansas
Dcnison, Texas
17.3
1.8
28.3
0.7
0
0.7
0.3
0.5
0.1
1.0
0
0
0.5
0.5
1.6
0.4
0
0
0.1
0.2
32.2
0
0
0
1.7
0.6
0.1
0
0
0
8.9
0
0.8
0
0
0.3
11.5
2.0
34.9’
1.5
12.7
43.8
5.1
1.2.
1.0
3.4
2.8
127.4
62.5
33.8
18.0
1.6
75.0
0.1
0.1
0.8
0.4
1.4
6.8
21.4
0.5
15.9
0.2
0.2
0.5
0.7
1.6
1.8
0
0.5
0
0.5
0.1
1.1
0.6
0
0.4
0
0
0
0
0
0
3.3
1.0
0.1
0
0.8
0
12.5
79.6
1.4
1.4
1.2
4.0
4.0
97.1
27.9
9.2
3%
14.4
1.8
G
1.3
0.1
fi
188.7
3.7 68.8
0
16.7
11
96.9
0.5
74.1
65.8
5.5
45.2 191.2
;
12
4
6
7
5
476’77
12:s
6.3
38.6
11.0
0’9
10
0:l
0.2
-
546
7’2
8:6
1.9
-
198
1’7
0:4
0
-
36
0’4
0:l
0
-
216
0’ 100.1
10.4
0.2
9.5
1.2
3.4
-
3
1
5.7
0.4
3.0
0.4
10.0
0.1
0.1
0
0
0
;
8
1
0
0
2’8
11
5416
3.0
0.2
0.7
0.7
05
2’3
012
l:o 11417
0
1.0
0
0
_
-
07
3818
4.2
0
_
_
04
310
0
0.1
_
0
42.05
30.8 188:5
0
5.8
0
0.2
-
::
220.1 329.0 136.9 134.9
5.7
41.7 38.2 29.0
50’67 179
47.04 65
11.15
119’5
50 6 29
82.1
0.5
2.4
5.9
23.1 714.4
3.3 77.8
24.2
1.1 345.1
101.4
1.2
13
159:2 104:3 175:3
4.0 105.1
161.3
29.7
3.9
18.2 324.6
0.9 168.2
13
3
0
3
4
4
6
2
3
3
3
7
7
Pacific Northwest
Basin
Columbia River, Clatskanie, Oregon
Bonneville,
Oregon
Pasco, Washington
Wenatchcc, Washington
Clearwater
River, Lcwiston,
Idaho ( 1962)
Snake River, Wawawai,
Washington
Payettc, Idaho ( 1962 )
Yakima River, Richland, Washington
( 1962 )
Missouri River Basin
Missouri River, St. Louis, Missouri
Kansas City, Kansas
St. Joseph, Missouri
Omaha, Nebraska
Yankton, S. Dakota
Bismarck, N. Dakota
Williston,
N. Dakota
N. Platte River, Henry, Nebraska (1962)
Platte River, Plattsmouth,
Nebraska ( 1962)
S. Platte River, Julesburg, Colorado ( 1962)
Big Sioux River, Sioux Falls, S. Dakota ( 1962)
Upper Mississippi River Basin
Illinois River, Peoria, Illinois
Mississippi River, E. St. Louis, Illinois
Burlington,
Iowa
Dubuque, Iowa
St. Paul, Minnesota
Red River, Grand Forks, N. Dakota (1961)
(TABLE
1 continued
9
8
9
2.2
23.3
32.4
on page 86)
47:0
49.3
0
0
13.3
5.0
PLANKTONIC
ROTIFERS OF UNITED
Jcm
STATES WATERWAYS
85
Among the metazoan plankters, cladocand copepods ranked next to the
rotifers in abundance, but they were USUally present in such small numbers that they
could not be adequately evaluated from a
l-liter sample. Insect larvae were present
only occasionally, and these forms, which
make up a major portion of net plankton,
were probably dcrivcd from the benthos.
Bottom forms, which were more common in
samples from shallow streams, probably represented displaced organisms when found
Philoiha,
with many
in the plankton.
benthic spccics, was found frequently in
samples from shallolw streams. In all samples from the larger rivers, the most abundant forms were characteristically
planktonic.
crans
T
*
ONE LITER
MARK
Collection,
RUBBER STOPPE R
L POLYETHYLENE
DRAIN TUBE
RUBBER CONNECTOR
DETACHABLE
VIAL
IpIG. 1.
Schem:&ic drawing of spccinlly designed
graduated glass settling tube.
two weeks, long-term preservation was usually unnecessary.
!Study shotwed that the delicate protozoa
could not bc counted or identified in prcserved samples, and attempts to include
them in the routine plankton enumeration
were abandoned. Some protozoa with protective testaceous walls, such as CoduneZZa
cratera and Tintinnopsis
cylindrata, were
abundant in many samples and were well
preserved.
Because these species feed
largely on bacteria, their relative density
appears to bc an indication of the population of bacteria present.
concentration, and counting of
samples
Three liters of water were taken directly
from a river or lake or other continuously
flowing source; for example, from the intakes of water treatment plants. The samples were shipped to the counting laboratory in Cincinnati, Ohio, where they were
agitated and divided into three equal subsamples. The number of centric and pennate
diatoms and genera of other algae in one
portion were determined; in another, diatoms were identified to species (Williams
1964); in the third, metazoan invertebrates
wcrc identified to genus. The portions for
invertebrate
and diatom analyses were
settled in graduated glass settling tubes
( Fig. 1) . These tubes are 70 cm long and 5
cm diameter and taper near the bottom to
about 2 cm diameter. To concentrate and
clean the organisms for analysis, the sample
was thoroughly agitated, poured into the
settling tube, and allowed to settle for 18 hr
m more. After settling, the water in the
upper 50 cm was slowly drained from below
until it was at the level of the stopcock inserted in the side of the tube. The rubber
stopper and stopcock were then rcmovcd;
a small, pointed polyethylene tube was inserted in the opening and the remaining
supernatant water was drawn off by vacuum from the top of the water column to
86
LOUIS
G. WILLIAMS
TABLE
1 ( continued
)
Rotifers
in semimonthly
plankton samples,
counts/liter,
1961 and 1962
____.
Five most abundant genera of the network.
Averages for 24 samples from
May to November
Basin, river, or lake,
and station
Western Great Lakes Basin
Niagara River, Lake Erie, Buffalo, New York
9
Detroit River, Detroit, Michigan
13
Lake Huron, Port IIuron, Michigan
11
Lake Michigan,
Milwaukee,
Wisconsin
9
Lake Michigan, Gary, Indiana
15
Lake Superior, Sault Ste. Marie, Michigan
6
Lake Superior, Duluth, Minnesota
5
Ohio River Basin
Allegheny
River, Pittsburgh,
Pennsylvania
( 1962 1 7
Kanawha River, Winfield
Dam, West Virginia
7
Little Miami River, Cincinnati,
Ohio
8
Ohio River, Cairo, Illinois
3
Evansville,
Indiana
13
Louisville,
Kentucky
( 1962 )
8
Cincinnati,
Ohio
10
Huntington,
West Virginia
9
E. Liverpool,
Ohio
Wabash River, New Harmony,
Indiana (1962)
;
106.3
22.1
14.1
63.7
45.3
12.0
8.5
10.9 104.2 100.4
1.2 25.5
6.2
0.7 33.1 12.5
1.2 82.0 45.0
0.9 38.6 22.4
0.3
4.2
8.0
0
15.2
3.0
29.0
23.8
21.2
11.8
104.3
84.2
84.0
68.1
74.9
79.0
14.1
4.2
1.2
0.2
0.6
3.8
7.0 119.6
16.4 43.3
17.9 126.0
29.8 125.0
-
Northeast Basin
Connecticut
River, Northficld,
Massachusetts
St. Lawrence River, Massena, New York
Hlldson River, Poughkeepsie,
New York
148.1
35.0
21.0
1.4 127.9
0.4 27.7
0.8 22.4
North Atlantic Basin
Delaware
River, Philadelphia,
Pennsylvania
Martins Creek, Pennsylvania
Schuylkill
River, Philadelphia,
Pennsylvania
Susquehanna River, Conowingo,
Maryland
Sayre, Pennsylvania
Potomac River, Great Falls, Maryland
Williamsport,
Maryland
Shenandoah River, Berryvillc,
Virginia
( 1962)
Western Gulf Basin
Rio Grande River, Brownsville,
Laredo, Texas
El Paso, Texas
Alamosa, Colorado ( 1962 )
Sabine River, Ruliff, Texas
10
;
11
6
10
8
12
z
8
10
4
Texas
;
7
Southeast Basin
Apalachicola
River, Chattahoochcc,
Florida
Escambia River, Century, Florida
Savannah River, Port Wcntworth,
Georgia
N. Augusta, South Carolina
Chattahoochec
River, Columbus, Georgia
Atlanta, Georgia
Tombigbee
River, Columbus, Mississippi
( 1962)
(TABLE
1 continued
13
7
1
2
9
2
6
6.1
1.2
2.5
25.6
6.6
0.3
1.3
5.9
3.4
1.3
1.2
26.6 248.7
3.5 37.0
2.5 50.2
1.5 135.1
1.9 67.3
0.7 19.6
0.3 19.8
zo.0
17.0
25.3
-
3.0
0.9
0.1
11.1
2.2
6.0
0.6
-
6.8
0.2
4.2
43.8
45.1
47.2
4.3
-
65.4
29.5
2.1
22.6
3.0
0
17.6 230.0
3.0 63.7
7.6 33.4
-
34.2
3.7
11.3
207.2
127.1
143.7
185.1
-
31.8
8.2
8.4
0.2
128.5 201.5
7.6
55.0
76.5
0.3
6.5
1.4
3.6
0
44.5
-
35.4
1.2
4.3
64.7
3.4
2.6
0.9
-
6.0
0.3
0.7
33.3
0.5
0.3
0.6
-
1.5
0.2
0.2
5.1
0.1
0
0.1
-
2.8 55.1
0.9
3.3
36.7 66.6
9.1 119.9
5.4
9.8
1.4
5.6
0.2
1.9
-
128.0
22.5
2.5,
10.0
16.5
26.6
0
1.2
0.3
48.7 115.4
0
23.2
0.3
0
3.9
0.5
22.7
0.1
0
0.1
79.7 288.2
18.5 37.3
0.1
1.7
5.4
9.8
225.1
27.4
1.3
3.0
76.3
2.3
20.2 323.6 140.5
0
2.1
1.3
0
0.2
0.3
0
0.7
0.5
15.4 15.4 61.4
0
0.6
0.2
_
-
34.2
on page 88 >
23.8 129.6 342.2
0.3
0.9
4.7
0
0.2
0.8
y.2 282'3
07 477'3
45
0
_
0:2
-
1:1
-
PLANKTONIC
ROTIFERS
OF UNITED
avoid disturbing the settled organisms. By
a series of steps, the organisms were carefully washed from the tapered portion of
the tube into the detachable vial. E’our
washings, with at least 20 min between
settling, were necessary to insure a concentrated sample without loss of organisms.
Most samples for invertebrato
analysis
coiuld be counted without further washing,
but when ncccssary, the sediment was
washed with distilled water to remove unwanted colloidal turbidity and suspensoids.
Centrifugation, membrane filtration, and
the use of plankton nets were tested as
methods of concentrating
and washing
plankton from the samples, but were abandoned in favor of sedimentation. Studies of
samples containing 4 ml or more of silt per
liter contained few, if any, mctazoan invertebrates. The entire sediment from samplcs having less than 4 ml of silt was placed
in a special microslide, 70 mm long, 50 mm
wide, and 2 mm deep. Samples were analyzed as soon as possible after arrival to
avoid deterioration.
With aging, muddy
river samples became increasingly difficult
to count. The structural outlines of the organisms became less distinct, the organisms
became entangled with silt and debris, and
redispersing and washing the sediment was
cxtrcmcly difficult or impossible. About 5%
of the samples collected during the bloom
period of rotifers-May
to November-improved with different degrees of washing;
about 95% of the samples required no washing.
Highly silty conditions prevailed at many
of the stations during the November 1961 to
May 1962 period, and about 2% of the samples from this period were discarded bccause of excessive silt; data from this period
h avc been excluded in the report of the five
most abundant genera in Table 1. When the
sediment was not excessive, counting was
simplified by using the zooplanktcrs in the
entire sedimented sample. Random distribution of the sediment in the counting chambe:r was accomplished by thoroughly agitating and then quickly pouring the quantity
necessary to fill the counting chamber, The
vials containing the washed sediment and
STATES
87
T;VATEIW’AYS
rotifers were decanted so that the final
volume in each of the vials was equal to the
volume in the counting chamber. Only a
quarter of the chamber was counted when
counts reached 200 in one-quarter of the
chamber, and only a half was counted when
counts reached 100 in one-half of the chamber. Since random distribution in the chamber cannot be obtained for those invertcbrates larger than most of the rotifers, the
larger organisms were counted individually
by rapidly scanning the uncounted portion
of the chamber. The chambers were graduatcd for this partial counting.
A compound microscope with a 10 X
ocular and a 10 x objective was used, but
grcatcr magnification was used to identify
the smallest organisms and was essential to
make specific identification
of Keratellu,
Pompholyx, and Gnstropus. Some organisms
were difficult to identify because they were
contracted. l%spccially designed tally sheets
and codes to taxa and their densities wcrc
developed to expedite routine counting, rc:porting, and analysis of data (US. Public
IIcalth Service 1964).
DISCUSSION
Distribution
AND
RESULTS
of common genera and species
Pennak ( 1957, p. 229) states, “Any aquatic
biologist who has spent much time counting
quantitative
limnetic zooplankton samples
has been impressed with the fact that a
sample usually contains one species of copcpod, one cladoceran, and one rotifer which
are cxccptionally abundant and clearly dominant numerically over the other species in
those groups.” In this study, rotifers of
more than one genus often dominated the
same sample, but each dominant genus
strongly tended to have but one dominant
species. Table 1 gives the number of dominant genera found at the 88 sclccted stations during the two-year period.
For tho first two years of this study, the
total numbers of rotifers were reported and
no attempt was made to report their taxa.
But during this period, 19 widely-scattered
stations were selected and studied for rotifer
species composition,
In this study, 58
species, including
a few unknown ones,
.
88
LOUIS G. WILLIAMS
TABLE 1 (continued)
Rotifers in semimonthly plankton samples,
counts/liter,
1961 and 1962
Five most abundant genera of the network.
Avcragcs for 24 samples from May
to November
Basin, river, or lake,
and station
Tennessee River Basin
Tennessee River, Chattanooga,
Tennessee
Bridgeport,
Alabama
Pickwick
Landing, Tcnncssce ( 1962 )
Lenoir City, Tcnncssec ( 1962)
6
3
8
6
23.7
5.2
197.0
54.4
0.4
0.3
I
22.3
2.4
-
16.8
2.1
-
1.0
0.1
-
4.7
3.2
-
45.3
8.2
-
California
Basin
Klamath River, Kcno, Oregon
Truckec River, Farard, California
9
5
146.9
51.3
12.0
I
56.5
-
5.5
L
2.6
-
4.9
-
81.6
-
43.8
10.4
32.1
19.4
3.7
14.5
82.1
Average
( 1962)
counts per liter
were found among the dominants. An attempt was made to exclude washed-in or
dislocated metazoa in the plankton samples
by selecting the data so that only species
occurring two or more times and having
five or more individuals per liter were considcrcd as dominant. Keratella cochlea&
was by far the most widely distributed and
most abundant rotifer. Polyarthra vzdgaris
was the second most abundant and widely
distributed species. In this pilot study, the
five most widely distributed and dominant
genera in order of abundance wcro &Yatella, Polyarthra, Brachionus, Synchaeta,
and Trichocerca.
The kinds and numbers of genera of
rotifers were recorded routinely beginning
in 1959 and were reported to the Water
Quality Network after 1 October 1961.
From these many observations, it appears
that the one-species domination of rotifer
genera for any one sample is valid and provides the necessary information
for any
practical diversity study of the major rivers
of the United States for water quality
ecology.
Populations of a few of the more dominant species of rotifers found at selcctcd
stations
from May to November over a
three-year period, reported as the average
6.6
number of individuals per liter, arc as follows:
Peoria, Illinois, on the Illinois River,
Brachionus
calyciflorus,
615; Columbus,
Georgia, on the Chattahoochec River, Trichocerca Zongiseta, 512; and Chattahoochee,
Florida, on the Apalachicola River, POZZJarthra vulgaris, 262. KerateZZa cochlearis
averaged 181 per liter at stations on the
Ohio River, but the numbers fluctuated
with the phytoplankton
counts during the
rotifer season.
Some factors affecting population density
Most rotifers disappcarcd during winter
and during periods of high stream flow.
Rotifer populations were greatest from May
to November, Keratekla cochlearis occurred
at some stations at all seasons of the year,
but in much lower numbers during the
colder seasons. In the United States, rotifers
begin to multiply earlier in the southern
streams. Many species commonly found in
rivers were also found at some of the Great
Lakes stations, but some of the more common species of the Great Lakes, such as
KeZZicottia longispina and Filinia Zon&&n,
did not dominate typical river stations.
KerateZZa hiemalis became common in the
late fall at many cold water stations in the
north.
PLANKTONIC
TABLE
2.
Population
densities
ROTIFERS
OF UNITED
STATES
by rank orders of 12 of 88 sampling
and 1962
=
89
WATERWAYS
stations
May
to November
Rotifers
Algae
25
26
26
26
27
28
28
29
30
30
3I
32
26
27
25
14
12
3
24
17
10
24
1961
-
Twelve stations :
Ohio River, Cincinnati, Ohio
Ohio River, Evansville, Indiana
Ohio River, Louisville, Kentucky
Connecticut River, Northfield, Mass.
Niagara River, Lake Eric, Buffalo, N. Y.
Ohio River, Huntington, W. Va.
Chattahoochcc River, Columbus, Georgia
Mississippi River, Dubuque, Iowa
Rio Grande River, Brownsville, Texas
Mississippi River, St. Paul, Minn.
Apalachicola River, Chattahoochee, Florida
Illinois River, Peoria, Illinois
All 88 stations:
Median ranking
Average ranking, lower 50 percent&
Average ranking, upper 50 percentile
-
Repeated observations
indicated
that
small rotifer populations at many stations
were probably a result of their intolerance
to turbulence and silt. In most streams,
dense lo,ads of silt arc associated with increased stream flow. Stations at which
highly silty conditions were associated with
extremely low rotifer populations
were:
Yuma, Arizona, and Loma, Colorado, on the
Colorado River; Coolidge, Kansas, on the
Atikansas River; all stations of the lower
Mississippi River; Wawawaii, Washington,
on the Snake River; St. Joseph, Missouri,
Omaha, Nebraska, and Williston,
North
Dakota, on the Missouri River; El Paso,
Texas, on the Rio Grandc River; and Cairo,
Illinois, on the Ohio River.
High densities of rotifers are generally
associated with water of high clarity and
littlle turbulence during the bloom period
of :rotifers. These stations usually have
average to high phytoplankton populations
(Table 2). Some of the rotifer-rich stations
are: Alexandria,
Lousisana, on the Red
River; Clatskanie and Bonneville, Oregon,
on the Columbia River; Yankton, South
Dakota, on the Missouri River; Peoria, Illinois, on the Illinois River, Burlington and
Dubuque, Iowa, and St. Paul, Minnesota, on
the upper Mississippi River; Evansville, Indi-
10
23
20
32
31
34
17
32
ana, Louisville, Kentucky, Cincinnati, Ohio,
and Huntington,
West Virginia,
on the
Ohio River; Northfield,
Massachusetts, on
the Connecticut River; Conowingo, Maryland, on the Susquehanna River; Brownsville, Texas, on the Rio Grande River; Chattahoochee, Florida, on the Apalachicola
River; and Columbus, Georgia, on the Chattahoochee River. Many of these stations
have high turbidities
from November to
May, when rotifers are practically absent,
and so the plankton data for this period
were excluded for the best ecological comparisons.
Because the magnitude of rotifer polpulations was usually associated with the density
of the phytoplankton
populations
(Table
2), exceptions are of ecological interest, In
the downstream stretches of some large
rivers, consumer populations tend to become more prominent where conditions toxic
to them do not exist and may actually
reduce the phytoplankton
populations and
their detritus (Williams 1964). This is important in stream management, because
heavy blooms of algae and their detritus
may be controlled by favoring conditions
for consumer populations, such as rotifers.
For instance, Cincinnati, Ohio, was noted
for high phytoplankton
populations while
90
LOUIS
C. WILLIAMS
Louisville and Evansville ( downstream ) had
lower phytoplankton densities and relatively
higher rotifer populations before Markland
Dam became operational below Cincinnati
in 1962.
Rotifer populations fluctuate seasonally,
and being small ( or absent) during winter,
yearly averages can be misleading.
For
comparative purposes, it is best to study
rotifer populations when populations are
normally high, Table 1 shows yearly averages far lower than averages for the six
months of high rotifer abundance, These
data show that the Colorado and Missouri
river basins are generally rotifer-poor, while
the upper Mississippi and Ohio river basins
are rotifer-rich.
But many of the rivers have
both rotifer-rich and rotifer-poor stations,
One shallow water station having high
species diversity and moderately high populations is Cedar Hill, New Mexico, on the
Animas River. At this station, benthic rotifers were usually common in the plankton
samples. Stations with both reasonably high
counts and high species diversity are Clatskanie, Oregon, and Wenatchee, Washington, on the Columbia River; Grand Forks,
North Dakota, on the Red River; Evansville,
Indiana, on the Ohio River; Conowingo,
Maryland, on the Susquehanna River; and
Chattahoochee, Florida, on the Apalachicola
River.
The three stations with the highest populations of both phytoplankton and rotifers
were: Peoria, Illinois, on the Illinois River;
St. Paul, Minnesota, on the Mississippi River;
and Brownsville, Texas, on the Rio Grande
River (Table 1). Comparison of phytoplankton and rotifer data indicates that stations with high phytoplankton
po,pulations
generally have high rotifer populations, and
the opposite relationship is also found, suggcsting a direct or indirect dependence of
rotifers on the phytoplankton for food.
Population densities were ranked for 88
stations from which there were two years of
*data from May to November 1961 and 1962
for both rotifers and algae (Table 2). The
average rank of the 12 stations with highest
rotifer density was 28 and the same 12
stations averaged 24 when ranked for algal
density, indicating that stations with high
rotifer abundance also have high algal abundance.
Some idea of the richness and variation of
the rotifer fauna may be obtained from
Table 1. Stations having 12 or more different genera of rotifers containing five or
more individuals
per liter from 48 semimonthly samples for two years are: Cedar
IIill, New Mexico oa the Animas River;
Wenatchee, Washington, Columbia River;
St. Paul, Minnesota,
Mississippi
River;
Grand Forks, North Dakota, Red River;
Detroit, Michigan,
Detroit River; Gary,
Indiana, Lake Michigan; Evansville, Indiana, Ohio River; Sayre, Pennsylvania, Susquchanna River; and Chattahoochee, Florida, Apalachicola River. This list contains
stations with enough algal abundance to
support a rich rotifer fauna. A number of
stations, such as Great Lakes stations at
Duluth, Minnesota and Sault Ste. Marie,
Michigan, did not have enough algae to
support a dense population of rotifers and
thus represent very clean stations. Several
of the other stations with low populations
of rotifers but with populations of phytoplankton sufficient to support much denser
po,pulations of rotifers may indicate conditions toxic to the rotifers.
Most of the stations are exclusively freshwater, but a marine influence is evidenced
in a few coastal stations where brackishwater diatoms occur and where rotifer po,pulations are far below average. High chlorides at some stations on the Arkansas and
Rio Grande rivers ( US. Public Health
Service 1964) completely destroy the rotifer
fauna and drastically reduce chloride-intolerant species of diatoms. Both phytoplankton and rotifer populations are much
reduced by industrial
chemicals in the
lower Kanawha River (U.S. Public Health
Service 1964 ) .
SUMMARY
1. Rotifers were the most numerous metazoans found in plankton samples from 128
stations on the major rivers and Great Lakes
of the United States. The ratio of rotifers
to other metazoans was approximately 30 to
1.
PLANKTONIC
ROTIFERS
OF UNITED
2. While more than one rotifer genus
often dominated in a sample, each dominant
genus tended to have only one dominant
species at any one time.
3.
Stations
with
a high
population
den-
sity of rotifers with low spccics diversity
were found widely.
4. There is no evidence to support the
indicator-organism
concept among the dominant rotifers present, regardless of what
species compose the population,
Rather,
the presence of dominants appeared to be
d&errnined by the principal cdaphic factors
and their abundance by the amount of available food.
5. Stations with high rotifer populations
were generally correspondingly
rich in
phytoplankton.
STATES
WATERWAYS
91
REFERENCES
BIZACII, N. W. 1960. A study of the planktonic
rotifers of the Ocqucoc River system, Presque
Isle County,
Michigan.
Ecol. Monographs,
30 : 339-357.
CIIANULER,
D. c.
1937. Fate of typical
lake
plankton in streams.
Ecol. Monographs,
7:
445-479.
PENNAK,
R. W.
1957. Spccics composition
of
limnetic
zooplankton
communities.
Limnol.
Oceanog., 2 : 222-232.
U.S. PUBLIC HEALTH
SERVICE.
1964.
National
Water Quality Network,
Annual compilation
of data from Oct. 1, 1961 to Sept. 30, 1962.
U.S. Public Health
Service Publ. No. 663
(1962 edition).
Washington,
D. C. 909 p.
WILLIAMS,
L. G. 1962. Plankton population
dynamics.
U.S. Public Health Service Publ.
No. 663, suppl. 2. Washington, D. C. 93 p.
-,
1964. Possible
relationships
between
plankton-diatom
species numbers and water
quality estimates.
Ecology, LE.5: 809-823.