1. sports
2. fishing
3. sport fishing

July 1964
LIMNOLOGY
VOLUME
AND
NUMBER
OCEANOGRAPHY
EVIDENCE
FOR THE EUTROPHICATION
OF LAKE
PHYTOPLANKTON
RECORDS
ERIE
IX
3
FROM
Charles C. Davis
Department
of Biology,
Western
Reserve
University,
Cleveland,
Ohio
ABSTRACI’
The Division Avenue Filtration
Plant of the Cleveland Division of Water and Heat has
undertaken almost daily phytoplankton
counts of water samples from Lake Erie since 1919.
Data exist for 25 full years and for 7 additional
partial years between 1919 and 1963.
There has been a consistent increase in the average quantity of phytoplankton.
The vernal
and autumnal phytoplankton
maxima have consistently
become more intense and have
lasted longer. The periods of minimum phytoplankton
development
in winter and summer
have become shorter and less well marked, until the winter minimum failed to develop at
all in some of the latest years. Certain marked qualitative
changes also have occurred.
These effects are thought to have been caused by an increasingly
rapid eutrophication
of
the water in Lake Erie.
INTRODUCTIOK
in the lower Great Lakes, particularly
in
Most limnologists agree that many lakes Lake Erie and to a lesser degree in lower
Lake Michigan. Lake Erie lies in a highly
evolve slowly from oligotrophy
toward
industrialized
and heavily populated area.
eutrophy and that one of the important
effects of domestic wastes, whether un- Striking changes of the bottom fauna of
treated or fully treated in disposal plants,
western Lake Erie have occurred; these
is to speed up enormously the process of came to the attention of biologists espeeutrophication
through the addition
of cially in 1953 (Britt 1955a), when a stratification of water in the western basin relarge quantities of plant nutrient minerals.
Such man-made eutrophication
has been sulted in the destruction of the major porreviewed by Hasler ( 1947) ; since his re- tion of the theretofore heavy population of
view, much additional evidence has accu- nymphal Hexagenia ( Canadian soldiers ) .
partial
recovery
(Britt
mulated from various locations (for exam- A subsequent
195%) proved to be temporary
( Britt
ple, see Edmondson, Anderson, and Peter1963; Carr and Hiltunen
1963)) and at
son 1956 and Edmondson 1961 for condipresent the original benthic community
tions in Lake Washington).
dominated by Hexagenia has been reUntil recent years, the lakes that were
known to have become eutrophic because placed almost entirely by one dominated
and oligochaetes.
Simiof domestic drainage were relatively small, by chironomids
larly, there have been striking changes in
and it was widely held that the vast diluthe fish populations of the lake. These
tion factor characteristic
of the world’s
and other evidences of a rapid eutrophicalargest lakes would prevent any clear indication of eutrophication
in them. Re- tion of Lake Erie have been reviewed and
discussed by Beeton ( 1961, 1964).
cently, however, concern has arisen that
In his 1961 paper, Beeton was unable to
rapid eutrophication
may be taking place
275
276
CHARLES
point to any evidence of changes in the
plankton associated with eutrophication;
since then, however, additional information has been forthcoming.
An apparent
increase of abundance of the copepod
Diaptomus siciloides ( characteristically
an
inhabitant
of shallow, eutrophic waters )
has been discussed by Davis (lQ62, 1964)
and by Bradshaw ( 1964). In addition,
Bradshaw has reported, from Chandler’s
( 1940) 1938-39 western basin data and
from his own information
collected in
1948-49 and 1959, that there was a considerable increase in the total numbers of
Cladocera and Copepoda. To date, however, the only evidence suggesting that
phytoplankton
changes associated with
eutrophication
have occurred is that reported by Verduin
( 1964)) who stated
that in western Lake Erie the dominant
phytoplankters
before 1950 had been
Asterionellu formosa, Tabelluria f enestrata,
and Melosira ambiguu, whereas by 196O61 the dominants were Fragilaria capucina, Coscinodiscus radiutus, and Melosira
binderanu. Verduin associated this change
with increased fertility, caused principally
by an increased runoff of commercial fertilizers from farms in northwestern Ohio.
METHODS
The Division Avenue Filtration Plant of
the Cleveland Division of Water and Heat
has undertaken regular quantitative analyses of planktonic microorganisms (mostly
phytoplankters)
since 1919, usually for at
least 5 days a week, 52 weeks a year.
Their samples have been obtained from
the raw Lake Erie water coming directly
through a conduit from an intake crib 5.9
km from shore and 13 m beneath the lake
surface. Through the courtesy of Plant
Superintendent Roy L. Simpson, the data
still in his files have been made available.
Permission to use the information has been
granted by Commissioner Thomas E. Stanton of the Cleveland Division of Water
and Heat. My sincere thanks are given for
the cooperation of these two men. Thanks
are also extended to Mr. John Wolk for
compiling some of the data used in Table 1.
C. DAVIS
Data exist for 25 full years and for 7
additional partial years, from 1919 through
1963. AS far as can be ascertained, the
methods of concentration and examination
have remained comparable throughout the
period, consisting of the sand-filtration
method
( Whipple,
Fair, and Whipple
1927), followed by examination of an aliquot in a Sedgwick-Rafter
cell. Results
have been expressed as phytoplankton
cells per milliliter of lake water.
The examinations of the samples were
undertaken to predict clogging rates in the
sand filters used at the filtration
plant.
Both living and dead cells were counted,
without distinction, but other studies (Davis 1955) have indicated that the fraction of
dead cells at this distance from the shore
of Lake Erie is relatively small for most
species. Because the same person could
not have made all of the counts over a
period of 44 years, there have probably
been variations in technique; the records
suggest that at times the samples were
concentrated by persons other than the
enumerator.
Some of the estimations of
phytoplankton
cell numbers ( particularly
the low values shown for- the years 1928
and 1929) may have come about through
On the other
variations in techniques.
hand, as will be shown, major changes
over the years have occurred gradually,
though with the expected up-and-down
fluctuations.
It does not seem likely that
changes of technique would occur gradually and consistently in one direction over
so many years. In spite of inevitable questions concerning their accuracy, it appears
worthwhile
to present some of the available data and to make certain gcneralizations concerning them.
Probably none of the analysts who made
the counts was a specialist in the taxonomy
of the phytoplankters involved. Some obvious taxonomic errors have been made,
as when ( occasionally) large numbers of
cells have been identified as Protococcus,
a nonplanktonic genus that may be either
terrestrial or attached to damp or submerged objects. In this instance, it would
bc: unwise to conjecture as to the identity
EUTROPIIICA’ITON
of the organism in question (Ch2or&?
Anacystis?
Chlamydomonas?).
Probably
Melmira and Mougeotia (and perhaps also
Ulothrix) were at times confused. In any
case, Mougeotia has tended in many instances in the present records to increase
and decrease in proportion to the changes
of the numbers of Melorira; also, other
more detailed studies of Lake Erie phytoplankton
( Davis 1954, 1962; Chandler
1940, 1942, 1944; Chandler and Weeks
1945; Burkholder
1960) have never reported Mougeotiu to be common. Aside
from Melosira, however, the phytoplankton genera that have usually been reported
as the predominant
forms are distinct
enough even at low magnifications
that
they probably have not been confused.
OF LAKE
277
ERIE
YEARS
FIG. 1. Avcragc
phytoplankton
cells per millilitcr for all years with complete records, 1920
to 1963 (2 weeks of records are lacking
for
1960 near the height of the autumnal phytoplankton maximum),
RESULTS
In the records for 13 years between 1919
and 1934, single counts exceeding 4,000
cells/ml occurred only in 2 years (4,072
cells/ml in November 1930 and 4,108/m1
in November 1932). After 1934, on the
other hand, counts of more than 4,000
cells/ml occurred in all of the 19 years for
which records are available except 1937.
Between 1934 and 1963, there was an irregular but persistent increase of records
higher than 4,000 cells/ml until the grcatest incidences were reached in 1961 (40
instances) and 1962 (42 instances). The
absolute maxima attained in single counts
fluctuated
considerably
but increased
fairly regularly until 1944; thereafter no
clear trend is shown in the records. Absolute maxima, however, are not thought to
be so significant as the number of high
records per year or the averages to be
discussed below. Maxima ranged from a
low of 421 cells/ml in 1928 to a high of
11,032 cells/ml in 1944. In contrast, in
oligotrophic Lake Baikal, the largest standing crop of phytoplankters
ever reported
was 415 cells/ml ( Koxhov 1963).
Fig. 1 shows the average phytoplankton
numbers per milliliter
for each year for
which records are available for the entire
year (2 weeks of records are lacking for
1960 at or near the peak of the fall phyto-
plankton maximum; had they been available, the average for that year might have
been somewhat higher). There were notabl e quantitative
differences in Lake Erie
from year to year; this is a regular feature
of the phytoplankton in any lake, probably
associated with varying meteorological and
cnvironmcntal
factors. Nevertheless, it is
clear that there has been a steady increase
in the average numbers since 1927. The
increase has been so steady and consistent
that it is obviously not associated with
changes of counting techniques. If WC accept the low values obtained for 1928 and
1929, the average number of cells varied
from 81/ml in 1929 to 2,423/m1 in 1962,
an increase of nearly 30 times. The mean
of the values for the first 13 complete
years (between 1920 and 1937) was 410
cells/ml, whereas the mean for the last 12
full years (between 1944 and 1963) was
1,254 cells/ml.
This was an increase of
slightly more than 3 times.
Yearlong phytoplankton
studies in Lake
Eric, such as those by Chandler (1940,
1942, 1944)) Chandler and Weeks ( 1945))
and Davis ( 1954, 1962)) have consistently
suggested that there are phytoplankton
maxima in the spring and the fall, separated by minima in winter and summer, a
classical pattern that occurs in a number
278
CIIARLES
of the larger temperate lakes, but not nccessarily in smaller lakes ( Pennak 1946).
The present records indicate that such a
typical pattern occurred in a number of
the earlier years (for example, see Fig, 2
for 1927)) and that, with considerable variations, the pattern continued throughout
most of the records. The maxima became
more marked and longer over the years,
whcrcas the minima became shorter and
less marked (Figs. 2-6). Although developing clearly in all other years, at least for
short periods, the winter minimum failed
to materialize at all in 1955-56, 1961-62,
and 1962-63, and it was only poorly shown
in 1959-60. The summer minimum also
tended to bc obscured in later years but
was always evident, except in some years
when there appeared to bc no great increase in fall as compared with summer
( for example, 1963).
The records of the Division Avenue Filtration Plant have been utilized twice prcviously in publications
dealing with the
phytoplankton
of Lake Erie. Inasmuch as
both references deal with periods for
which records are not now available, they
are referred to briefly here. Chandler
(1944) examined the records for 1941,
finding the vernal pulse to last from midMarch to late April and the fall maximum
to continue from mid-August to late December. Davis ( 1954) utilized the records
only to examine the termination
of the
phytoplankton
maximum in early June
1951 and to check a small maximum that
occurred during January of the same year.
It is of interest to compare the present
results for 1956-57 briefly with results for
the same year published previously in an
independent study by Davis ( 1962), inas-
+.
‘F’M’A
C. DAVIS
1935
3. The
dance for 1935.
record
of
phytoplankton
abun-
FIG. 4. The
dance for 1946.
record
of
phytoplankton
abun-
FIG. 5. The
dance for 1957.
record
of
phytoplankton
abun-
FIG.
640C
,
6000
1946
M’J’J
I 9 2J7
FIG.
2. The record oE phytoplankton
abundance for the year 1927. Typical vernal and autumnal maxima arc dcpictcd, separated by winter
and summer minima.
ETJTROPIIICATION
much as one of Davis’ stations lay only
about 1 km east of the intake crib used
by the Division Avenue Filtration
Plant.
Davis visited his stations only every 2
weeks, and hence more detail is shown by
the present records. Nevertheless, the data
dates correspond
fairly
for particular
closely, and differences can be accounted
for on the basis of varying collection and
counting techniques and a possible patchiDavis was
ness of the phytoplankton.
unable to visit his stations between midDecember and mid-March.
Because his
results for mid-March
were higher than
any others obtained during the entire year,
he concluded that he had returned at approximately the peak of the spring maxi@mm. This judgment was incorrect, as
indicated by the present records, which
show a somewhat obscure winter minimum in December-January,
followed by
an extensive pulse in February and early
March that is much higher than any value
reported as late as mid-March (Fig. 5).
The most abundant genera were ascertained for each month of each of the years.
Thcsc estimates of dominance are based
both on the cell counts reported in the
records and on an approximate knowledge
of relative sizes of cells. They are therefore somewhat subjective.
As shown in
Table 1, there has been a striking change
in the most abundant forms. In the spring
pulse (including
January and February
pulses that occurred during the later
years), Asterionella
dominated in all of
the records up to 1931; in 1932, dominance
by Asteriondu
was shared part of the
time by Synedra. In 1932, dominance was
shared by Asterioneh
and Melosira. Between 1933 and 1949, Atier~onetla was
dominant every year except 1937 (when
Melosira and Synedra were of greater importance), but the dominance was shared
by Melosirn in 1946 and 1947, and by Cyclotella in 1948. In the records from 1956
to 1963, Asteriondu
never dominated the
spring pulse, although at times it was common. Instead, Mekwira
was dominant
during this period every year except 1960
(when its importance was shared by Frag-
OF LAKE
279
ERIE
7600’
6000-
”
‘F’M’A’M’J’J’A’S’O’N’DI
Id2
FIG. 6. The
dance for 1962.
record
of
phytoplankton
abun-
ilaria and Tab&ha).
In 1958, Fragiluria
and Melosira dominated the spring pulse.
In carlier years, the autumn maximum
(and the large early winter populations,
when present), were dominated by Syne&n ( from 1920 to 1923). From 1927 to
1948, Melosiru always dominated, either as
the major or as one of the principal forms
( during 1927 and 1928 Synedru remained
important as a codominant with Melosira;
in 1929, Asterionella
was codominant ) .
Similarly, dominance was shared by Melosira and Synedrn in 1937, 1945, and 1946..
In 1947, the green alga Pediustrum was a
codominant with Melosira; similarly, Asterionella and Melosira were codominants in
1948. After 1948, Lh?eZosirawas among the
dominants in some years, but there was a
tendency away from this, so that Fragilaria, Peckstrum,
Anabaena, and others
became more important.
Hence, the main tendency in the spring
pulses has been a shift from dominance by
Asterionella to dominance by Melosira. A
corresponding shift in the autumnal pulses:
has been from Syneclra to Melosira, and
then from Melosira to Fragilariu. In addition, as shown in Table 1, during autumnal
pulses there has been a greater tendency
during later years (1947, 1949, 1957, 1958,.
1961, 1962) for an increased participation:
280
CHARLES
‘~ABLIS 1. Dominant
Spring
DAVIS
phytoplankters
during spring and autumn phytoplankton
(The dash signifies that there was no pulse)
pulse
Year
Astcrionella
Asterionclla
Asterionella
-Synedra, Asterionella
Aaterionella,
Melosira
Asterionella
Aslerionella
Asterionella
Metosira, Synedra
Asterionella
Asterionella
Asterionella
Asterionella,
Melosira
Melosira, Asterionella
Asterionella,
Cyclotella
Asterionella
2
M elosira
LMelosira
Fragilaria,
Melosira
Melosira
Fragilaria,
Tahellaria
Melosira
Melosira
M elosira
* Some
T From
C.
of the included
information
Chandler’s
(1944)
report
Autumn
1920
1921
1922
1923
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1941t
1944
1945
1946
1947
1948
1949
1955
1956
1957
1958
1959
1960
1961
1962
1963
of
has
the
pulse
Synedra
Synedra
Synedra
Synedra
Melosira, Synedra
Synedra, Melosira, Stephanodiscus
Asterionella,
Melosira
M elosira
M elosira
M elosira
Melosira
M elosira
M elosira
Melosira
Synedra, Melosira
M elosira
M elosira
Synedra, Melosira
hlelosira, Synedra
Melosira, Pediastrum
Melosi.ra, Asterionella
Synedra, Pediastrum
Nlelosira
Synedra, Melosira
Pediastrum, Fragilaria
Fragilaria,
Melosira, Anabaena
Fragilaria
M elosira
Fragilaria,
Melosira, Anabaena
Melosira, Anabaena, Oscillatoria
Fragilaria,
Synedra, Stephanodticus
been adapted
from
an
Filtration
Plant
records
on the part of the green and blue-green
algae, replacing in part the previous ,dominance by diatoms.
There has been a similar change of dominance during the winter phytoplankton
minimum, for Stephanodiscus and CycloteZ& were of greatest importance in most
years until 1944, whereas Melosira, Fmgila&z, and Asterionellu were most abundant
from 1945 to 1949. Since 1956, during a
period when little or no winter minimum
developed, hlelosiru and Frngilnria continued as the most frequent dominants, so
probably the phytoplankton
from 1945 to
1949 can be considered as indicating transitional features in the lake, leading to
the elimination of winter minima.
I)ominance
during
summer minima,
however, showed little indication of a con-
pulses, 1920-63”
undergraduate
for 1941.
project
written
by
Mr.
John
Wolk.
sistent change, except for an increased frequency of the chlorophyte Pediustrum and
the cyanophyte Anabaena
Shifts of dominant species of phytoplankton comparable (though not identical) to these often have been interpreted
in other lakes as indications of rapid eutrophica tion ( Hasler 1947 ) .
DISCUSSJON
The earliest year-round studies of Lake
Erie plankton appear to have been those
of Vorce ( 1880u,b, 1881, 1882). He gathered his samples from the Cleveland water supply “by tying a muslin bag over
the house supply faucet . . . until the
pores . . . were clogged with the arrested
organisms . . .” (1881, p. 52). Although
such a method could give no quantitative
EUTROPIIICA’1’ION
picture, he reported that from February to
May the phytoplankton was dominated by
Stephanodiscus and Rhixosolenia, and that
Melosira and Tabellaria were the most
abundant from May to November, Rhixosole&a eriensis is a small and transparent
diatom and was not listed in any of the
records under consideration here. There
appears, however, to be considerable significance in its apparent abundance in
1880 (Vorce 1881)) and in the fact that
subsequent careful observations have indicated it to be less and less common over
the years. Thus, Burkholder ( 1960) found
it to be occasionally abundant in the central basin in 1929, as did Chandler (1940,
1942, 1944) and Chandler and Weeks
t-45)
in the western basin in 1939-42.
Davis (1954) listed it as a relatively rare
winter form in 1950-51, and he did not
find it at all in 1956-57 ( Davis 1962 ) .
Furthermore,
the genus is important in
some of the other, more oligotrophic Great
Lakes (Putnam and Olson 1961 for Lake
Superior; Daily 1938 for Lake Michigan).
It is unsafe to draw rigid conclusions
regarding other diffcrenccs between the
observations reported by Vorce and more
recent reports, but it should perhaps be
mentioned that there are no recent reports
either of Stephanodiscus as a dominating
member of the spring pulse or of TabelZaria as a dominant in the summer and
fall; the present results similarly support
the conclusion that a change has occurred,
for these diatom genera have never been
dominant in spring or fall. However, we
need more reports between 1880 and 1929
before definite conclusions can bc drawn.
Burkholder’s publication
( 1960) on the
phytoplankton of the central basin in 1929
covers only June to September. Although
his study was quantitative, the exact quantities are difficult to ascertain. Thcrc was
an autumnal phytoplankton
pulse in Scptember, with dominance by AsterioneZZa,
Fmgilaria, and Melosira. At the station
nearest Cleveland, about 100 phytoplankters/ml are indicated
( Burkholder
does
not state whether his units are cells or
colonies). In the present results, the only
OF LAKE
ERIE
281
real pulse in 1929 occurred in September,
with a maximum of around 600 cells/ml,
dominated by AsterioneZZa and Melosira.
Hence, the two studies agree rather
closely.
Metcalf ( 1942) examined the plankton
near Cleveland in the summer of 1938
(mainly in August and September, with
some results from June), but his methods
were nonquantitative.
No real pulses were
indicated among the few phytoplankters
he reported ( Asterionella, Anabaena, Ceratium, and Dinobryon.) .
As mentioned, Chandler (1944) briefly
described the now missing records for 1941
from the Division Avenue Filtration Plant
for the purpose of general comparison
with his own more detailed results from
the western basin. Dominants listed by
Chandler are given in Table 1.
The only other studies from the Cleveland area are those of Davis ( 1954, 1962)
for 1950-51 and 1956-57, rcspectivcly. The
1950-51 records of the filtration plant are
at present lacking, so that no comparisons
can be made. The year was characterized
by distinct vernal and autumnal maxima,
dominated in the autumn of 1950 by Melosira spp., Fragilaria crotonensis, and Pediastrum simplex, in the spring of 1951 by
CycZofeZZa and Melosira, and in the autumn of 1951 by Melosira spp., Pediastrum
simplex, and Fragilaria crotonensk
These
results fit reasonably well into the pattern
shown for the preceding and following
years.
In the 1956-57 study, the autumnal phytoplankton
of 1956 was dominated by
Microcystis
spp., and the early winter
plankton was characterized by an abundance of Stephanodiscus niagarae. Melosira spp. were most important during the
main portion of the vernal pulse of 1957,
and unidentified
phytoflagellates
dominated the summer minimum.
The late
summer and autumnal phytoplankton communities were dominated by a succession
of green algae, first Pediastrum duplex,
then OBcystis spp. and Coelustrum sp., and
finally Pediastrum simplex. It must be
emphasized that Davis’ methods of collec-
282
Cm4RLES C. DAVIS
tion and preservation
differed
greatly
from those used at the filtration plant and
that he reported his results as volumes of
the phytoplankters per liter of lake water
rather than as cells per milliliter.
Consequently, close comparisons are not feasible. The differing methods undoubtedly
contribute to the discrepancies between
his results for 1956-57 and those reported
for the same period from the filtration
plant. In addition, taxonomic differences
also are apparent, for example, in the lack
of recognition of the genera Microcystis
and O&y&is in the records of the filtration plant during 195&57, whereas Davis
found the genus Microcystis to be dominant in September 1956 and Oiicystis to
be the most important genus in August
1957.
The results surveyed in the present
paper show that 1) the phytoplankton has
consistently increased in quantity between
1920 and 1963, 2) the intensity and length
of the maxima have increased, 3) the minima have become shorter and less pronounced, and 4) the winter minimum has
failed to materialize in some of the later
definite qualitative
years. Furthermore,
changes have occurred. All of these results
appear to indicate an increased and rapid
eutrophication of the waters of Lake Erie,
as has been suggested by studies of the
bottom fauna, ichthyofauna,
and water
chemistry as summarized by Beeton (1961,
1964).
REFERENCES
1961. Environmental changes
BEETON, A. M.
Trans. Am. Fishcrics
Sot.,
in Lake Erie.
90: 153-159.
1964. The eutrophication
of the Great
-.
Lakes.
( Unpublished
manuscript. >
The crustacean zooBHAUSHAW, A. S.
1964.
plankton
picture : Lake Erie 1939-49-59;
Vcrhandl.
Intern. Ver.
Cayuga 1910-51-61.
Limnol., 15: 700-708.
BRITT, N. W.
1955a.
Stratification
in wcstcrn
Lake Erie in summer of 1953: effects on
Hexageniu
(Ephcmeroptcra)
population.
Ecology, 36 : 239-244.
-.
( Ephcmeroptera
)
1955b. E-Iexccg&n
recovery in western Lake Eric following
the
Ecology, 36 : 520-522.
I953 catastrophe.
Some changes in the bottom
-.
1963.
f auna of the island area of western Lake
Erie in the decade 1953-1963,
with special
reference
to the aquatic
insects.
( Abstr. )
Proc. Conf. Great Lakes Res. 6th: 268.
BUHKHOLDER, P. R.
1960.
A survey of the
microplankton
of Lake Eric.
In. Limnological survey of eastern and central Lake Erie,
1928-192,9.
U.S. Fish Wildlife
Serv. Spcc.
Sci. Rept., Fisheries, 334: 123-144.
CAM,
J. F., AND J. K. IIILTUNEN.
1963.
Changes in the bottom fauna of Lake Eric,
west of the islands,
1930-1961.
(Abstr.)
Proc. Conf. Great Lakes Res. 6th: 268.
GIANDLER,
D. C. 1940. Limnological
studies
of western Lake Erie. I. Plankton and certain physical chemical data of the Bass Islands region, from September,
1938, to Novembcr, 1939. Ohio J. Sci., 40: 291-336,.
1942. Limnological
studies of western
-.
Lake Erie. III. Phytoplankton
and physicalchemical data from November,
1939, to Novcmber 1940. Ohio J. Sci., 42: 24-44.
1944. Limnological
studies of wad-.
Lake Eric. IV. Relation of limnological
and
climatic factors to the phytoplankton
of 1941.
Trans. Am. Microscop.
Sot., 63: 203-236.
-,
ANI) 0. B. WEEKS. 1945. Limnological
studies of western Lake Erie. V. Relation of
limnological
and meteorological
conditions
to the production
of phytoplankton
in 1942.
Ecol. Monographs,
15 : 435-456.
study of
DAILY, W. A. 1938. A quantitative
the phytoplankton
of Lake Michigan
collected in the vicinity
of Evanston,
Illinois.
Butler Univ. Bot. Studies, 4: 65-83.
DAVIS, C. C. 1954. A preliminary
study of the
plankton
of the Cleveland
Harbor
area,
Ohio. II. The distribution
and quantity
of
the phytoplankton.
Ecol. Monographs,
24:
321-347.
A preliminary
study
of the
1955.
-*
plankton of the Cleveland Harbor area, Ohio.
IV,
Plankton
and industrial
pollution
in
Sewage
Ind.
Wastes,
Cleveland
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