PALEOLIMNOLOGY OF LAKE TEXCOCO, MEXICO. EVIDENCE FROM DIATOMS John P. Bradbury Limnological Research Center, Universit y of Minrlesota, Minneapolis 55455 ABSTRACT A 46-m core from the lacsutrine sediments beneath Mexico City was analyzed to establish a stratigraphic sequence of diatom assemblages for use in interpreting the climatic and limnologic history of ancient Lake Texcoco. Diatoms were found in nearly every 20-cm sample interval, and several major zones were established. Planktonic and benthonic-epiphytic assemblages alternate throughout the core, both of fresh- and brackish-water types. The alternations reflec t the fact that the coring site is marginal to the main basin of the la’ke, and limnologic conditions change as water levels rise and fall. A freshwater planktonic assemblage dominated by Stephanodiscus niagarne in decpcr parts of the core indicates that a large, cool, and possibly deep lak e exi sted about 100,000 years ago, either because of pluvial or because of tectonic factors. This is replaced (depth 35 to 30 m) by a freshw ater benthonic-epiphytic assemblage characterized by Denticula elegans and other marsh diatoms. The marshes were probably maintained by springs fr om the shore when the lake was reduced to saline pools in the center of the basin. As water levels rose again (core depths 30 to 5 m), brackish water flooded the marshes, and brackish benthonic diatoms (such as Anomoeoneis cost&a, Campylodiscus clypeus, and Nitzschia frustulum) replaced the earlier floras. These were periodical ly replaced by brackish planktonic diatoms (as Cycloteb la striatn and CycZoteZZu quillensis) when the lake was deeper, but the earlier frcshwatcr planktonic flora never recurred. The same brackish planktonic and benthonic diatoms tha t prevailed for several tens of thousands of years are found today confined to the brackish pools of Lake Texcoco that are remnants of the former larger lake. The long interval of fluchlating brackish floras probably rcprc- sents Wisconsin time. The last 10 ,000 years of the l ake’s history is marked by a return of the marsh flora, sllggcsting a climate drier than that of Wisconsin time. A marked climatic change, how- ever, is not necessary to explain this last c hange in the cliatom flora, and it seams likel y that the pluvial climate inferrecl for the southwcstcrn United States had less effect at the latitude of Mexico City ( 19” 30’) than farther north. INTRODUCTION This paleolimnolog ic study of Lake Tex- coca, Mexico, began in 19 68 when I was a postd octoral fellow at Yale University under the advi sement of E. S. Decvey and G. E. IIutchinson. It was partly supported by a grant from the American Philosoph- ical Society. The project was continued at the Limnological Research Center of the University of Minnesota where I was a National Science Foundation postdoctoral fellow under II. E. Wright. The cnthusi- asm and support of thcsc indi vidu als have greatly facilitated this work. In addition, I wish to acknowlcdgc the kind help of P. 13. Scars, of L. Zccvae rt, who provided 1 Contribution 92, Limnological Research Cen- tcr, University of Minnes ota. material for study, and of Prof. J. L. Lorcnzo and his colleagues at the Institut0 National dc Antropologia e Historia in Mexico City, who have share d with mc many o f their insights about the cnvi- ronm cntal history of the Basin of Mexico. GI3OLOGIC SETTING The general sequence of events that led to the formation of the Basin of Mexico h as been traced (Mooser ct al. 1956; Mooser 1963). The basin began as a gra- bcn in the Tertiary trans-Mexico volcanic belt, bounded on the cast and west by two fault zones and their associated volcanoes, Sierra de Las Cruces and the Sierra Nc- vada, the latter contai ning the famous volcanoes Popocatepctl and Iztaccihuatl. The faults and volcanoes have been active LIMNOLOGY AND OCEANOGRAPIIY 180 MARC11 1971, V. 16(2)
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PALEOLIMNOLOGY OF LAKE TEXCOCO, MEXICO.
EVIDENCE FROM DIATOMS
John P. Bradbury
Limnological Research Center, University of Minrlesota, Minneapolis 55455
ABSTRACT
A 46-m core from the lacsutrine sediments beneath Mexico City was analyzed to
establish a stratigraphic sequence of diatom assemblages for use in interpreting the climatic
and limnologic history of ancient Lake Texcoco.
Diatoms were found in nearly every 20-cm sample interval, and several major zones
were established. Planktonic and benthonic-epiphytic assemblages alternate throughout
the core, both of fresh- and brackish-water types.
The alternations reflect the fact that
the coring site is marginal to the main basin of the la’ke, and limnologic conditions change
as water levels rise and fall.
A freshwater planktonic assemblage dominated by Stephanodiscus niagarne in decpcr
parts of the core indicates that a large, cool, and possibly deep lake existed about 100,000
years ago, either because of pluvial or because of tectonic factors. This is replaced (depth
35 to 30 m) by a freshwater benthonic-epiphytic assemblage characterized by Denticula
elegans and other marsh diatoms.
The marshes were probably maintained by springs from
the shore when the lake was reduced to saline pools in the center of the basin.
As water levels rose again (core depths 30 to 5 m), brackish water flooded the marshes,
and brackish benthonic diatoms (such as Anomoeoneis cost&a, Campylodiscus clypeus,
and Nitzschia frustulum) replaced the earlier floras. These were periodically replaced
by brackish planktonic diatoms (as Cyclotebla striatn and CycZoteZZu quillensis) when the
lake was deeper, but the earlier frcshwatcr planktonic flora never recurred. The same
brackish planktonic and benthonic diatoms that prevailed for several tens of thousands of
years are found today confined to the brackish pools of Lake Texcoco that are remnants
of the former larger lake. The long interval of fluchlating brackish floras probably rcprc-
sents Wisconsin time.
The last 10,000 years of the lake’s history is marked by a return of the marsh flora,
sllggcsting a climate drier than that of Wisconsin time. A marked climatic change, how-
ever, is not necessary to explain this last change in the cliatom flora, and it seams likely
that the pluvial climate inferrecl for the southwcstcrn United States had less effect at the
latitude of Mexico City ( 19” 30’) than farther north.
INTRODUCTION
This paleolimnologic study of Lake Tex-
coca, Mexico, began in 1968 when I was
a postdoctoral fellow at Yale University
under the advisement of E. S. Decvey and
G. E. IIutchinson. It was partly supported
by a grant from the American Philosoph-
ical Society. The project was continued at
the Limnological Research Center of the
University of Minnesota where I was a
National Science Foundation postdoctoral
fellow under II. E. Wright. The cnthusi-
asm and support of thcsc individuals have
greatly facilitated this work. In addition,
I wish to acknowlcdgc the kind help of
P. 13. Scars, of L. Zccvaert, who provided
1 Contribution 92, Limnological Research Cen-
tcr, University of Minnesota.
material for study, and of Prof. J. L.
Lorcnzo and his colleagues at the Institut0
National dc Antropologia e Historia in
Mexico City, who have shared with mc
many of their insights about the cnvi-
ronmcntal history of the Basin of Mexico.
GI3OLOGIC SETTING
The general sequence of events that led
to the formation of the Basin of Mexico
h
as been traced (Mooser ct al. 1956;
Mooser 1963). The basin began as a gra-
bcn in the Tertiary trans-Mexico volcanic
belt, bounded on the cast and west by two
fault zones and their associated volcanoes,
Sierra de Las Cruces and the Sierra Nc-
vada, the latter containing the famous
volcanoes Popocatepctl and Iztaccihuatl.
The faults and volcanoes have been active
LIMNOLOGY AND OCEANOGRAPIIY
180
MARC11 1971, V. 16(2)
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PALEOLIMNOLOGY OF LAKE TEXCOCO
181
throughout the Tertiary, progressively cov-
ering the Crctaccous basement; par ticu-
larly massive extrusions of acidic lava and
probable sinking of the graben floor oc-
curred in the Miocene and Plioccnc, de-
fining a valley whose integrated river
system drained to the south (Figucroa et
al. 1968). The northern limits of the val-
ley arc defined by lower mountains pro-
duced by faulting and volcanism in the
region of Pachuca. The valley was closed
in the late Pliocene when basaltic volcanic
activity from centers located in the south-
ern part of the basin (the massive Chichi-
nautzin lavas and Sierra Ajusco) dammed
the valley. The basin thus formed was
rapidly scdimented with elastic and pyro-
elastic material to a thickness of 800 m
(Figueroa et al. 1968), and the regional
drainage converged to the lowest part,
where a lake has persisted until modern
times.
LIMNOLOGIC SETTING
Today the Basin of Mexico is a plain
(elevation about 2,236 m) surrounded on
the east, south, and west by high moun-
tains (3,000-6,000 m). It was a closed
hydrographic system before being artifi-
cially drained in 1900, and precipitation in
the mountains and runoff from summer
rains drained into a chain of lakes that
nearly traversed it from north to south
( Fig. 1). After the rainy season ( May-
October) the lakes wcrc frequently joined
into a single sheet of water (clcvation 2,242
m), which during the dry winter months
was separated into a number of subbasins,
some artificially contained by dikes, The
principal ones and their elevation rclativc
to Lake Tcxcoco listed from north to south
arc [elevations from Zecvacrt (1952) and
Bonaparte et al. (ca. 1900) ] :
Zumpango
+6 m;
Xaltocan
+3 m;
San Cristobal
4-3 m;
Texcoco
0 m;
Mexico
+0.85 in;
Xochimilco
f3.5 m;
Chalco
d-3.5 m.
Lake Texcoco is the lowest in the scrics
and the most saline, both because of
evaporation and because thermal springs
flow into it (Mooser 1963). The Mexico
subbasin, artificially con taincd by Aztec
dikes to the west of Lake Texcoco, was
maintained by freshwater from Chalco,
Xochimilco, and numerous springs from
Chapultepcc,
southwest of Mexico City.
It drained into Lake Texcoco during times
of water surplus through an organized
system of canals and gates, In similar
fashion Lake Chalco was separated from
Lake Xochimilco. Because of the abun-
dance of frcshwatcr in the southern part
of the basin, Chalco, Xochimilco,
an d
Mexico were cxtcnsively used for chi-
nampa farming ( Dccvcy 1957).
The draining of the Basin of Mexico
and the growth of Mexico City have cre-
atcd some engineering problems.
The
early dikes, cspccially that of Nctzahua-
coyotl (ca. 1450 A.D. ), were built to pre-
vent seasonal floods of saline water from
cn tcring the highly productive chinampu
farms southwest of the capital. Flooding
continued in colonial times, and the need
for cffcctive sewage disposal for the city
that was rapidly grolwing onto the plain
of Lake Tcxcoco demanded that the lakes
bc systematically drained. The first cf-
forts began in the 17th century, and the
work was finally complctcd in 1945 with
the Tcquisquiac
tunnel, which has re-
duced the surface arca of Lake Texcoco
to a rainy-season arca of only 200 km2
( Mooscr 1963).
As the drainage of the lakes was cf-
fectcd, and more and more water was
pumped from aquifers of sands and silts
bcncath the lake plain, Mexico City began
to sink into the highly bcntonitic clays
that undcrlic it. This problem is cspccially
serious whcrc heavy buildings are con-
structcd in the metropolitan areas. Dr. L.
Zecvacrt has for many years conducted
stud& of the mechanical nature of the
lake sediments of the basin, principally
beneath Mexico City, to seek competent
strata for the placcmcnt of foundation
pilings. Hc has .taken numerous cores of
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182
JOIIN I’. BRADBURY
99OlOO’
ChaDultevec <-
3% Reforma - Hovre core
- maximum level
0 5 40
I
I
I
km
w m
mean level
.~**~****~* minimum level
FIG.
1. Index map showing lakes in the Basin of Mexico.
Adapted in part from J. L. Lorenzo
(in Mooser et al. 1956).
the scdimcnts with a simple Shelby tube
tcr content arc studied. Through this work
sampler that is forced into the lake sedi- Zccvacrt (1952, 1953) has compiled a dc-
mcnts by a mechanized coring rig.
In
tailed stratigraphy of the Basin of Mexico
resistant material a jar hammer is used.
to clcpths of about 70 m. Hc has been
Cores about lo-cm diam and 2 m long
most intcrestcd and cooperative with rc-
arc taken in this fashion and their sedi-
spect to ancillary scientific studies on his
mentology, mechanical propcrtics, and wa-
cores and has provided both Sears and
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Clisby ( 1955) and me ( Bradbury 1970b)
with samples.
LACUSTRINE STRATIGRAPIIY
The Plcistoccne stratigraphy in the Ba-
sin of Mexico was first studied in detail
by Bryan (1948). Hc worked with the
exposures of soils, tuffs, and alluvium on
the margins of the basin and divided them
into units of the following names, charac-
ters, and ages:
Nochc Buena-soils and alluvium, with
pottery shards;
Pre-Classic, Classic, and
Pus t-Classic.
Toltolzingo- dark brown soils, alluvium,
some eolian material.
Barrilaco -calichc-pedocal; Altithermal
4,500-7,500 BP.
Beccrra-alluvium-pedalfcr, Elephns,
Equus, Bison etc.; Cochranc-Mankato.
Morales-caliche-pcdocal; Wisconsin in-
tcrstadial.
Tacubaya-yellow-brown alluvium-pc-
dalfer; Tazcwell-Gary.
Tarango-acidic volcanic tuff, watcr-de-
posited; Plio/Pleistocenc?
Bryan (1948) assumed that these for-
mations or their cquivalcnts exist in the
lacustrine deposits in the center of the
basin, and Zeevaert (1952, 1953) corre-
lated Bryan’s strata with the alternating
layers of lacustrine clays, silts, and sands
undcrncath Mexico City. Lithology is used
as the basis for correlation of the coarser
units, whereas lake clays are thought to
bc contcmporanoous with soil formation
and roduccd alluviation. Deposits high in
calcium carbonate are considered cquiva-
lcnts of the calichcs on the basin margins,
Foreman (1955) more carefully analyzed
the sediments bcncath Mexico City, and,
although he did not USC: he formational
names ‘of Bryan and Zecvaert, he showed
their correlation to his findings. IIis stra-
tigraphy is generalized into scvcn zones,
but despite the complete lithologic dc-
scription, they do not have diagnostic
characteristics.
This seems to bc a result
oE high variability ,of the sediments and
the prodominancc of ash and wcathercd
as
h in them.
183
Bryan’s names applied to the stratigra-
phy beneath Mexico City are useful in
spcaking about the section.
Considering
the effects of erosion and the occurrence
of hiatuses in the marginal alluvium and
soils, as compared to the more oomplctc
dcpositional record in the central part of
this closed basin, temporal equivalents can
only be sporadic.
In addition, correlations
based on concepts such as pedalfers =
clays, pedocals = calichcs (which in scv-
cral cases
arc high concentrations of
ostracod carapaccs ) seem to be simplistic
representations of complicated and distinc-
tive cnvironmcnts. Mooser ct al. (1956)
pointed out that it is still adventurous to
identify the upper limit of the Tarango
formation bcncath Mexico City. For now,
despite the uscfulncss of Bryan’s forma-
tions, the lacustrine deposits of the Basin
of Mexico should bc characterized in their
own right and not equated with the mar-
ginal deposits.
FOSSIL STUDIES
Major advances in characterizing thcsc
deposits have come from fossil studies,
and foremost of these is the pollcn-strati-
graphic work of Sears (1952) and Sears
and Clisby ( 1955). Thcsc were preceded
by an exploratory study by Decvcy (1944).
Sears and Clisby studied two of Zcc-
vacrt’s cores to depths of more than 70 m.
They attempted palcoclimatic intcrpreta-
tion, but satisfactory pollen zonation is not
possible bccausc the percentage frcquen-
ties oE the major taxa are quite variable
throughout the core. In addition,
pollen
in the coarser scdimcnts is scarce and
poorly prcscrvcd.
Pollen abundance cor-
relates with clay zones and is used in
cnvironmcntal reconstruction.
Maxima in
the amount of oak, fir, and alder pollen
as opposed to pint pollen are considered
to represent warm-moist periods.
Foreman ( 1955)
noteld the prcscncc of
0s racods, sponge spiculcs, and diatoms;
he divides the latter into clongatc and
circular groups but dots not use any of
thcsc fossils for s ratigraphic zonation.
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-l’ALEOLIMNOLOGY Ol? LAKE TEXCOCO
185
t
. . . . .
. . . .
i
ashy
. .I..
sand 1
-....<..::::.-------
n
-
-
-
....,
-
-
-
xi--
-
--
-
.- .
. -.
.-.
-.-.
.-.
1:
,_I
-=-
- 7-z.
.-.
, .*. .7.’
.* a,*.:.
*::::
and 8 si
I
---
1-T
._. . fresh and 1
FIG. 3. Correlation of core P 366-2 with the
stratigraphic divisions of Zecvacrt (1952) and
Foreman (1955). Difference in depths results
from the marginal position of P 366-2 relative to
their sections.
Earlier studies of diatoms from the Plcis-
tocenc sediments of Lake Texcoco include
the taxonomic work of Ehrenbcrg (1869)
and Lozano ( 1917) : P. Congcr used din-
toms of the Bccerra formation to intcrprct
the environment of deposition of human
and mammoth remains in the study of
Tcpcxpan Man by dc Terra et al. ( 1949),
DIATOM STUIHES
The prcs’cnt work represents the first
attempt to produce a diatom stratigraphy
in the Basin of Mexico. The coring site
was at the intersection of Paseo dc la
Reforma and Callc Havre, about 4 km S
70” W from the center lof Mexico City
(the Zocalo) and about 2 km N 60” E
of Chapultcpcc IIill ( Fig. 1). The core
(P 366-2)) taken by Zcevacrt in May 1967
in connection with the construction of a
large hotel at this site, was similar to those
studied by Foreman and Scars and Clisby
in 1955 and to the many dcscribcd by
Zccvaert ( 1952, 1953). It was initially 50
m long, but after Zecvaert’s mechanical
analysis of some sections only the upper
35 m were well rcpresentcd, although
there were a few samples from 44 to 46
m available for study.
The coring site is shown in Fig. 2, a
reproduction of the 1550 Alonzo dc Santa
Cruz map of the Basin of Mexico (from
Linnc 1948). The spot cannot bc precisely
indicated, but it is probably very near the
canoeist shown hunting water birds with
a spear at the top ( west) of the map.
Chapultepcc Hill is behind and Ito the left
of the hunter.
The Alonzo de Santa Cruz map shows
the lacustrine cnvironmcnt of the Basin of
Mexico before any attempts were made
to drain the lakes and therefore reprcscnts
a more or less normal state ‘of affairs, al-
though the activities of man (dikes, canals,
hunting, fishing, and so forth) had clearly
modified the environment. What is im-
portant hcrc is to notice the abundance
of aquatic vegetation, the apparent shal-
lowness of the lakes, the placement of
Nctzahuacoyotl’s
dike separating Lake
Mexico from saline Lake Tcxcoco to the
cast, and the presence of springs or other
sources of water at Chapultcpcc Hill.
The coring site is clearly marginal rela-
tivc to the main basin of Lake Texcoco,
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186
JOHN I’. BRADBURY
and Zcevaert’s work shows that the forma-
tions beneath Mexico City dip basinward
and so are found at greater depths to the
northeast. The amount of clip is about 2
m/km.
SEDIMENTOLOGY AND CORRELATION
The strata found in the P 366-2 core can
be correlated with those of Zcevacrt and
Foreman only by lithology, bccausc the
depths arc not equivalent. The variability
of lithology makes this difficult, but the
scdimcntologic descriptions and analyses
of Zcevaert (unpublished but on file) in-
dicate that the upper units of his “Ta-
rango” formation arc reprcsentcd, as is the
“Tacubaya” formation. These sediments
are difficult to subdivide, and the likeli-
hood of lateral variation toward the ba-
sin margin further complicates correlation.
Noncthcless, they can be diffcrcntiatcd
from the overlying “Beccrra,” “Barrilaco,”
and “Toltolzingo” formations. The prob-
able correlation is shown in Fig. 3, but I
must stress that I do not feel thcsc names
are justly applied to the deposits beneath
Mexico City.
A detailed sedimcntologic study has not
been made of core P 366-2. The sediments
are generally similar to those of the cores
carefully described by Foreman ( 1955))
being predominantly fine sands, silts,
weathered ash and some clay, and occa-
sional unwcatl1ered ash. They are dia-
granlmatically characterized in Fig. 4 and
compared with Zcevaert’s (unpublished)
water-contcn t analyses. Fossils are abun-
dant; siliceous phytoliths, sponge spiculcs,
and diatoms are common, as are calcareous
ostracod carapaces.
Snails are less com-
mon but found in certain zones, especially
the coarser ones. Pollen is generally abun-
dant in the finer sediments, particularly
the clays and weathered ash, and occasion-
ally seeds and fish bones arc found. The
locations of high concentrations of ostracod
carapaccs and of fish fossils are indicated
in Fig. 4. The fish have been identified
by R. R. Miller and C. Barbour. In some
zones the sediments are penetrated by root
holes, suggesting emergent vegetation in
a shallow lake.
Diatom i?cology
Diatoms were examined at 20-~111 ntcr-
vals where possible throughout the core.
There wcrc 400 diatoms distributed in 12
to 60 taxa counted from each sample; 204
taxa were identified and their frequency
of occurrence calculated. Of those, 88 had
abundancies of at least 257, and their frc-
quencies are plotted in Fig. 4. The species
that had similar depth distributions wcrc
arranged in assemblage groups to facilitate
discussion of the limnological variation
against time;
these groups are identified
ecologically at the top of the figure. Each
spccics plotted is numbered consecutively
to help the reader locate its frequency sil-
houctte when it is mentioned in the text.
An alphabetical listing of all species found
is given in Table 1, with information about
their ecology and modern distribution.
Ideally, fossil diatoms that have similar
frequency distributions with depth consti-
tute fossil assemblages that reflect past
ecological associations. In practice the cf-
fects of reworking and transportation of
diatom frustules can obscure the internal
ecological coherence of fossil associations,
cspccially in shallow lakes, and it is not
always possible to interpret successfully
every elen1,cnt in a fossil assemblage. On
the whole, however, the species of the
diatom assemblage groups do reflect uni-
form ecology, within the rather broad lim-
its of diatom autecology. Some latitude in
ecological uniformity results from the jux-
taposition of similar but not identical dis-
tributions (in the interests of saving space).
Groups I-VI are dominantly freshwater
diatoms, the vast majority being bcnthonic
spccics preferring somewhat alkaline wa-
tcr and tolerant of small amounts of salt.
Group I has two species, Nitzschia tryhZi-
onella ( 1)
and N. hung&a (3), that
Cholnoky (1968) refers to as brackish-
water species, but IIustedt (1930) records
--- --
_- __~_____-
+
lb. 4.
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187
ALEOLIMNOLOGY OF LAKE l?EXCOCO
TABLE
1.
List of diatoms found in the Reforma-Ilavre (P 366-2) core
Ecological characteristics*
Occurrence in Mexico?
s
PII
IC
1 2 3 4 5 6
Achnanthes exigua Grun.
A. Jzauckiana Crun.
A. hungarica ( Grun. ) Grun,
A. lanceoluta ( Brelx ) Crun.
A. marginatula Grun.
A. minutissima Kutz.
Amphiprora alata Kutz.
Amphora acutiuscula Kutz.
A. coffaeiformis salina (W. Sm.) A. Cl.
A. macilenta Greg.
A. ovalis Kutz.
A. ovalis pedictdus Kutz.
A. veneta Kutz.
Anomoeoneis cost&a (Kutz.) IIust.
A. sphaerophora ( Ehr. ) Pfitx.
A. sphaerophora sculpta 0. Mull.
Caloneis hacillum ( Grun. ) Cl.
C. Zewisii influta (Schultzc) P&r.
C. Zimosa ( Kutz. ) Patr.
C. oregonica (Ehr. ) Patr.
C. pemqqxz (J. W. Bail.) Cl.
C. ventricosa subundulata ( Grun. ) Patr.
Cnmpglodiscus clyperu
Ehr.
C. noriczu Ehr.
Chaeloceras Ehr.
Cocconeis diminuta Pant.
C. placentzrlu Ehr.
C. thumensis A. Mayer
CycloteZZa comensis Grun.
C. kutxingiana Thwaites
C. meneghiniana Eaeuissima (v. Coor) IIust.
C. q uillensis Bailey
C. slriatu ( Kutz. ) Grun.
Cyclotella sp. cf. C. stylorum Brightwcll
Cymatopleura solea ( Breb. ) Wm . Sm.
CymbeZZa cistula (Hemp.) Grun.
C. cistula macuZata (Kutz. ) v. Heurck
C. helvetica Kutz.
C. mexicana (Ehr. ) Cl.
C. pusilla Grun.
C. ruttneri IIust.
C. triangulntum (I%. ) Cl.
C. turgida (Greg. ) Cl.
C. ventricosa Kutz.
Denticulu elegans Kutz.
F
8
OS
F 7.2-7.5 0,
F
8
02
B
F-B
F
F-B
B
F-B
F
F-B
B-M
8+
8.2
X X
8.0-8.2
8.0-8.5
x x x x
S-10 N&O:: x x
8.5
X
x x
X
A
X
B 8.6-8.7
B-M
A
X
F
8
F
8+
F 8
X X
F
8
F-B
>87
X X
B
B-M
(3.9-8.6 SS x
X X
F A
F G-9 OS
B A
B
7-8
F
8
X
X
F
8
X
* Ecological characteristics provided for the species plottecl in Fig. 4. S = salinity and F = freshwater, B = breck-
ish water, M = marine wntcr. pH = recorded pH and A = alkaline water, n = acidic.
IC = indicator choractertitics
and SS = stable salinity, AC = aerophil, W = warm water, E = cutrophic, hct = heterotroph, 0 = oligotrophic. This
information is largely from Cholnoky (1968), Patrick and Reimer (1966>, Hustedt (1930), and Bright (in prep.).