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Limnol. Oceanogr., 35(4), 1990, 830-839
Q 1990, by the American Society of Limnology and Oceanography, Inc.
Type of suspended clay influences lake productivity
phytoplankton
community response to phosphorus
and
loading
Benjamin E. Cuker
Center for Marine and Environmental Studies, Hampton University, Hampton, Virginia 23668
Phumelele T. Gama and JoAnn A4. Burkholder
Botany Department, North Carolina State University, Raleigh 27695-76 12
Abstract
The effects on phytoplankton and limnetics of two different types of suspended sediments and
their interactions with P loading were tested in a small North Carolina Piedmont lake. Limnocorrals
were used in a complete, triplicated six-treatment, blocked design. Treatments were loaded with
P, kaolinitic clay (K), K+P, montmorillonitic clay (M), and M+P. A4 caused more turbidity and
stayed in suspension longer than K. Consequently, the light-dependent parameters, net community
productivity (NCP), chlorophyll concentration, and algal density were lowest in the M and highest
in the P treatment. Combined P and clay loading promoted clearing for both sediments and
mitigated their effects on algal densities and NCP. Flagellated algae and nonfilamentous cyanophytes
dominated the control community. The P treatment had blooms ofAnabaena. Without fertilization,
both clays resulted in sparse, flagellate-dominated communities. The M+P community, like that
of the P treatment, was dominated by Anabaena, but total algal densities were suppressed. In
contrast, the K-t-P community lacked Anabaena and was similar to the control in algal quantity
and composition.
Various mineral substances cause turbidity in lakes, including clays of distinct mineralogies from eroding soils, resuspended
bottom sediments, glacial flours, and calcite
precipitates. It is unlikely that such a heterogeneous array could exert uniform influences on lacustrine systems. Differential
composition of suspensions may be a source
of some of the conflict among investigators’
of the effects of mineral turbidity.
Some
studies show mineral turbidity to favor flagellated algae at the expense of filamentous
blue-greens
(Hergenrader
and Hammer
1971; Avnimelech et al. 1982; Cuker 1987),
yet other contrasting reports link blooms of
these filamentous blue-greens to loading with
suspended clays (Carriker and Taylor 1984;
Hart 1987). The effects of mineral turbidity
Acknowledgments
Much of this work was done under U.S. EPA grant
R8 133 15-O1-Oto B. Cuker while he and P. Gama were
at Shaw University.
We thank the following: The City of Raleigh which
owns the lake and provided use of facilities under the
direction of J. Connors; L. Gama and L. Hudson for
field and laboratory assistance; L. Shurtleff who trained
P. Gama in algal taxonomy; S. Weed of the North
Carolina State University Soils Department for determination of clay mineralogy; S. Mozley, R. Jordan, P.
Jumars, and three anonymous reviewers.
on grazing zooplankton are also controversial. Suspended sediments provide refuge
from visual predators and can interfere with
zooplankton feeding (McCabe and O’Brien
1983). Organic molecules adsorbed on the
surface of clay particles can provide nourishment to grazing zooplankton (Arruda et
al. 1983; Gliwicz 1986; Hart 1988). Distinguishing among suspension materials may
clarify some of these conflicting reports.
Laboratory studies, including early experimental turbidity work on zooplankton
(Robinson 19 5 7) and other research on clayP interactions
(Chiou and Boyd 1974),
linked the action of suspended clays with
their mineral composition. Using three different suspended sediments in laboratory
cultures, Soballe and Threlkeld ( 198 8) found
algal flocculation to depend on mineralogy
and concentrations of both sediment and
cells. Large tank experiments by Threlkeld
and Soballe (1988) definitively
ranked the
three minerals according to turbidity per unit
of mass (sihca > kaolin > bentonite), but
they were unable to trace unequivocally the
manifestation of these differences in the biota. Our study is the first to compare the
effects of different clays with an in situ experiment.
Soils of the southeastern U.S. are domi-
830
Algae, P, and two clays
nated by kaolinite, which is typically red
from a coating of iron oxide. Kaolinite is a
1 : 1 (Al : Si) clay of large particle size (0. l5 .O ,um). Other soils in this region are formed
from Triassic Basin sediments that are
mostly yellow-brown
montmorillonite,
which is a 2 : 1 (Si : Al : Si) small-particle
(0.01-2.0 pm) clay (Buckman and Brady
1969). Lakes in the region are often turbid
from either or both types of clay.
We compared influences of kaolinitic and
montmorillonitic
sediments on the pelagic
environment,
phytoplankton
community
structure and net community productivity
(NCP), emphasizing differential response to
P loading under the two types of clay. We
postulated that the smaller montmorillonite
particles would yield more turbidity per unit
of mass, scatter more light, and stay in suspension longer (Terwindt 1977). From this
assumption, we predicted lower NCP with
montmorillonite
than kaolinite. Kaolinite
favors flagellated and small cells at the expense of blue-green filaments (Cuker 1987).
We tested whether blue-greens in the presence of montmorillonite
would act similarly. We also tested whether community response to P fertilization was sensitive to the
type of suspended clay.
Materials and methods
Experimental design and manipulation The study extended from 1 June-l 8 August
19 8 7 in Durant Lake, a small impoundment
in the Piedmont of North Carolina (described by Cuker 1987). Limnocorrals were
used in a complete, triplicated,
six-treatment, blocked design. Treatments included
loading with P, kaolinite (K), K+P, montmorillonite (M), and M+P, as well as controls (C). Treatments were compared nonparametrically (Hollander and Wolfe 19 7 3).
Eighteen polyethylene limnocorrals (1 3-m3
cylinders, 2.25-m diam x 3 m deep, open
to the sediments) were installed on 1 June
by dropping the open bottoms through the
water column. Steel rings enclosed in sleeves
at the bottom of the limnocorrals
were
pressed into the sediment. Flotation came
from foam-filled collars. Fish longer than 2
cm were eliminated with a cast net. Minnow
traps set to capture the remaining juvenile
fish were only marginally
successful. To
831
standardize the fish effect, we added one
medium (9-12-cm SL) bluegill sunfish (Lepomis macrochirus) to each limnocorral on
15 July. Rotenone poisoning at the end of
the experiment confirmed the presence of
fish in all but two limnocorrals, one K and
one K+P replicate.
Loading rates were 3.3 mg P m-2 d-l and
100 g (dry mass) clay m-2 d-l. Batch additions were made 3 times a week (Monday,
Wednesday, and Friday) from 3 June to 18
August. Alone or in combination, clay slurries and phosporic acid were broadcast into
the limnocorrals. The upper 0.5 m of each
treatment (including the controls) was mixed
with a paddle. Clay came from construction
sites, kaolinite from within the lake’s watershed, and montmorillonite
from just
north of Durham, North Carolina.
To maintain realistic conditions, we used
natural soils of contrasting mineralogy rather than purified materials. Because natural
soils are mixtures, we use the labels kaolinite or montmorillonite
in this paper to denote the dominant mineral.
Sampling and analysis - Chemical and
physical analyses of the clay were performed
by the North Carolina Department of Agriculture and the North Carolina State University Soils Department. Light penetration
was determined with a 20-cm black-andwhite Secchi disk and a LiCor 1000 data
logger connected to a submersible spherical
PAR quantum sensor (LiCor 193SA). Separate wet and dry sensor calibrations were
used for the photometer readings. Secchi
depth was measured three times a week before the manipulations. The light meter was
used weekly on a day following manipulation. On 9 July, light was measured intensively over the first half of the solar day to
monitor changes in light penetration related
to solar angle. Temperature and oxygen were
measured at 0.5-m increments with a YSI
meter as part of the weekly estimate of NCP.
NCP (pg O2 liter-’ h-l) was determined by
the difference between dawn and afternoon
oxygen profiles taken 1 d after manipulation. NCP estimates are conservative, not
being corrected for loss of oxygen to the
atmosphere.
Water samples were taken with a PVC
tube (3 m long, 5-cm diam) that delivered
832
Cuker et al.
discrete upper (EPI, O-l.5 m) and lower
(HYPO, 1.5-3.0 m) sections of the water
column (Cuker 1987). Total P (TP) samples
(collected on 9 June and 7 July) were digested with persulfate and autoclaved before spectrophotometry with citric acid-ammonium molybdate (Am. Public Health
Assoc. 1976). A pH meter (Haake-Buchler)
was used to test surface in situ waters and
to determine acid-titrated alkalinity in the
laboratory.
Weekly Chl a samples were drawn onto
replicate Gelman A-E glass-fiber filters and
then frozen. The filters were ground in glass
and extracted in 90% buffered acetone. Aftcr centrifugation, Chl a was determined on
a Turner 110 fluorometer with acid correction for pheopigments (Strickland and Parsons 1972). Aliquots of fresh, whole water
samples were measured for turbidity with
the fluorometer set up and calibrated as by
Cuker (1987).
Separate EPI and HYPO integrated phytoplankton samples were collected at the
middle (7 July) and end (11 August) of the
study. One-liter samples were preserved with
acid Lugol’s solution, settled for 2 weeks,
and concentrated by siphoning off the overlying water. Generally, 400 algal units per
sample were counted with a hemacytometer
at 400 x , with identifications
to species in
most cases.
Results
Light and temperature-Montmorillonite caused more turbidity per unit of mass
than kaolinite (Fig. 1). By the second week,
nephelometer turbidity units were -90 in
the M and M+P and 20 in the Kand K+P
treatments. As summer progressed, K and
K+P treatments began to clear, with Secchi
depths increasing from < 1 m to >2. Secchi
depth for the M and M+P treatments remained < 0.5 m, with some clearing in the
last few weeks. P fertilization
of the clay
treatments induced more rapid clearing, especially for M+P. Over the last 49 d, Secchi
depth for M+P was 24% greater (P < 0.00 1,
signed rank test) than for M, but in the K+P
treatment, Secchi depth was only 7% greater
(P < 0.009) than in the K treatment.
The comparative abilities of the two clays
to stay in suspension over the short term
was tested by taking Secchi depths just before and after clay additions on 8 July. Before adding clay, means were 150 cm (SE =
35) for K and 26 cm (SE = 5) for M. Adding
clay decreased them to 3 1 (SE = 2) and 18
cm (SE = 0), but both treatments returned
to their prior levels after 2 d. Thus, in the
2 d between manipulations
for this period
in July, turbidity dropped by 5 times for K
and only 0.3 times for M.
Transparency of the C and P treatments
increased in June and then declined in late
July due to algal turbidity. The decline -was
greater for P due to algal blooms in the second 5 weeks. Photometer results verified the
Secchi depth pattern, yielding the following
mean absorption coefficients for the second
5 weeks: C, 1.50; P, 1.75; K, 1.53; K+P,
1.42; M, 3.53; M+P, 3.14.
Distribution
of light (PAR) with depth
and solar angle was examined on 9 July (Fig.
2). During these prebloom conditions, illumination patterns were similar in both the
controls and the P treatment, with increasing penetration
as the solar angle approached zenith. The angular effect was diminished by scattering in the K and M
treatments. Intensive scattering in M created a narrow subsurface layer in which
photon fluxes measured exceeded those taken in the air above. This physically impossible result reveals stratification of scatterers
on a scale too small to resolve with the light
path in the sensor and introduces an error
of -6% in the measurements.
Afternoon thermal profiles always indicated stratification
of the lake and limnocorrals. Mixing of the entire water column
by convective
downwelling
of eveningcooled surface waters was, however, often
evident from profiles taken at dawn. M and
M+P treatments had the strongest afternoon stratifications
with significantly
warmer afternoon surface temperatures (P
< 0.00 1, Friedman test with time as blocks).
Mean afternoon surface temperatures (“C, n
= 7) were: C, 30.1 (SE = 0.8); P, 30.3 (SE
= 0.7); K, 30.2 (SE = 0.7): K+P, 30.2 (SE
= 0.7); M, 31.5 (SE = 0.9); M+P, 31.2 (SE
= 0.8).
pH, alkalinity, and TP-Mean surface pH
(measured on 6 and 8 July) ranged from 6.5
to 7.4, but there was no significant difference
Algae, P, and two clays
-I--.............,..,
20
833
1..
40
60
80
20
40
60
80
DAYS
Fig. 1. Mean Secchi disk depth recorded over the course of the experiment. Limnocorrals were installed on
day 0 (1 June). Secchi depths from the surrounding lake are indicated by dots in the control panel. Error bars
are +l SD, n = 3.
(P = 0.56, Friedman test) due to treatment.
The pH was consistently highest in the P
replicate that bloomed shortly after these
measurements (-pH 8). Alkalinity
deter-
8
9
10
11 12
minations from 21 July were not significantly different among treatments when all
six treatments were compared (P > 0.05 for
both EPI and HYPO, Kruskal-Wallis).
13
HOUR OF THE DAY
Fig. 2. Penetration of photosynthetically active radiation (PAR) from sunrise to solar noon on 9 July. Data
for K+P and M+P were essentially identical to the K and M treatments and are not displayed.
834
Cuker et al.
TOTAL PHOSPHORUS p g liter -1 (21 SE)
0
25
50
0
25
50 75
Fig. 3. Mean concentrations of TP from 9 June
(stippled area) and 7 July (entire bar). Error bars are
t-lSE,n=3.
Among just the clay treatments, mean alkalinities (as mg CaCO, liter-l) were significantly lower (rank sums, uncorrected for
multiple testing, P -=z0.05) for K (18-23)
than for M (26-30).
Both fertilization with P and the addition
of clay increased TP in the water column (P
= 0.002, for means of EPI and HYPO from
9 June and 7 July in one Friedman test, Fig.
3). All treatments, including unfertilized,
1501
3p
I
A HYPO
loo
accumulated TP over time. The clays alone
contributed substantial TP; K having 2 times
and A4 3 times the TP of the controls.
Chlorophyll and NCP-In
all treatments,
Chl a concentrations declined over the first
half and then recovered during the second
half of the study (Fig. 4). Initially, HYPO
values were twice those of the EPI in all
treatments. In the second 5 weeks, the M
and M+P treatments reversed this trend,
with Chl concentrating in the EPI. This reversal was also seen in the two P replicates
that bloomed, but not for either K or K-t P.
Analysis of the grand means taken over the
entire experiment revealed significant differences for the HYPO but not the EPI
(HYPO, P = 0.007; EPI, P = 0.21; Friedman test with time as blocks). Effects of
fertilization and clay loading were most evident in the last 5 weeks, with high levels
of Chl developing in the P treatment, but
in neither the K+ P nor the M+P treatments.
NCP fluctuations tracked changes in Chl,
with all treatments increasing after the midJuly minimum. NCP treatment means differed significantly (P < 0.0001, Friedman
test with time as blocks). In comparison to
C
0 EPI
0
20
40
60
0
20
40
60
DAYS
Fig. 4. Mean concentrations of Chl a over the course of the experiment. Day 0 is 1 June. Error bars are -t 1
SE, n = 3.
835
Algae, P, and two clays
0
50
O2 j.q liter-’
h-’
(+ 1SE)
100
0
50
150
100
I24 HYPO
150
0
EPI
11 AUGUST
w
a
K
P
5
2
0
M M&P
K K&P
C
P
Fig. 6. Mean algal densities for all species combined from the end and middle of the experiment. Error
bars are f 1 SE, II = 3.
with time and sample depth as blocks). By
August, algal density in the P treatment was
50 100 150
0
0
50 100 150
twice that of the control. All the clay treatPAR PERCENT OF INCIDENT (+- 1SE)
ments had fewer algae than the control, with
being twice as effective as
Fig. 5. Grand means for NCP (0) and percent pen- montmorillonite
etration of surface light (stippled area) vs. depth for 11 kaolinite in reducing algal numbers. P ferJune, 9, 16, 23, 30 July, and 6 August. Error bars are tilization
increased algal densities for both
+l SE.
clays, but only in August for M+P. Assigning algal taxa to funtionally and taxonomthe controls the treatment grand means were: ically similar guilds aided analysis (Cuker
P, 173%;M+P,
116%; K+P, 107%; K, 78%; 1987; Figs. 7 and 8). To compare the relative contribution
of each guild within a
it4, 65%. Treatment determined the pattern
of productivity
with depth (Fig. 5). In the treatment, we transformed the actual values
P treatment, grand mean NCP peaked at 1 to proportions based on total algal counts
m and remained positive down to the bot- or total biovolume for that treatment. EPI
and HYPO estimates were combined to test
tom. In the other treatments, NCP followed
the depth profile of light. Consistent with
treatment effects on guild representation
the light conditions and Chl depth distri(Kruskal-Wallis
test).
butions, NCP in the M and M+P treatTreatment effects were significant (P <
ments was concentrated in the first 50 cm 0.05) for numbers of filamentous blue-greens
below the surface, with no production deep- and diatoms in both the July and August
er than 1.5 m. Patterns for the less turbid
samples (Figs. 7 and 8). Nonfilamentous
K and K+ P treatments were similar to those blue-greens, euglenoids plus flagellated
of the controls.
greens (Euglenophyta and Chlorophyta), and
Phytoplankton -The trend of increasing
Chrysophyta showed significant differences
Chl concentration in all treatments over the in July but not August. No significant treatsecond half of the experiment was supportment response was seen for unflagellated
ed by algal counts and biovolume estimates
greens or Cryptophyta in either month.
for July and August (Fig. 6). Treatment efThe control community in July was domfects on both total algal density and biovolinated by nonfilamentous
blue-greens
ume were significant (P = 0.003 for density;
(mostly Agmanellum sp.), flagellated greens
P= 0.004 for biovolume, Friedman test (Chlamydomonas spp.), and euglenoids
Cuker et al.
836
1
0
nfil
fil
CYA
n fig fig
CHL
CRY DIN CHR DIA
L--&Ln fil fil n flg fig
CYA
CHL
CRY DIN CHR DIA
Fig. 7. Mean densities of algal guilds from 7 July. CYA - Cyanophyta; nfil - nonfilamentous; fil- filamentous;
CHL-Chlorophyta
and Euglenophyta; nflg-nonflagellated; CRY-Cryptophyta;
DIN-F’yrrhophyta;
CHRChrysophyta; DIA-Bacillariophyta.
Error bars are k 1 SE, IZ= 3.
(Trachelomonas spp.). The green Kirchneriella lunaris, the chrysophytes Dinobryon
cylindricum and Dinobryon bavaricum, and
the dinoflagellate Peridinium pusilum were
also abundant. In August, Euglena gracilis,
Ankistrodesmus fusiformis, and Chrysosphaera paludosa were dominants.
The P treatment was dominated by the
filamentous blue-green Anabaena wiscon-
sinense. A nonfilamentous
blue-green,
Chroococcus minutus, codominated the EPI
for both months, while Chlamydomonas
(July) and Chrysosphaera paludosa (August)
were abundant in the HYPO. High variance
associated with the filamentous blue-greens
in the P treatment is traceable to irruptive,
out-of-phase blooming of A. wisconsinense
in two of the three replicates. The Ana-
6
5
4
3
2
g
-
1
0
-
a
2
1
0
n fil fil
CYA
n flg flq
CHL
CRY DIN CHR DIA
n fil fil
CYA
n flg flg
CHL
Fig. 8. As Fig. 7, but from 11 August.
CRY DIN CHR
DIA
837
Algae, P, and two clays
baena-dominated
replicate that bloomed
first (in July) succeeded to dinoflagellates
and nonfilamentous
blue-greens in August.
Composition
of the replicate that did not
bloom was similar to the others, but with
fewer Anabaena.
In the K treatment, nonfilamentous bluegreens (Chroococcus), dinoflagellates,
and
flagellated chrysophytes constituted most of
the sparse July community. By August, euglenoids plus flagellated and nonflagellated
greens replaced the dinoflagellates
and
chrysophytes. Oscillatoria angustissima, a
filamentous blue-green, codominated in the
HYPO during July. This alga was rare in
the control and P treatments, but it and congenerics were common constituents of all
the HYPO clay turbid trea’tments in July.
K+P supported a denser and structurally
different community than did K alone. During July, P. pusilum (EPI) and D. cylindricum (HYPO) were the dominants. Nonfila(Chroococcus,
mentous
blue-greens
Aphanocapsa elachista, and Gloeotheca rupestris) were dominant at both depths in
August.
The few surviving algae of the M treatment were dominated by dinoflagellates (P.
pusilum) in the EPI during both months.
Filamentous blue-greens (Oscillatoria spp.
and Spirulina) dominated the July HYPO,
giving way to nonfilamentous
blue-greens
(Chroococcus) and flagellated
greens
(Chlamydomonas) in August.
The M+P community was similar to the
M community
of July, but in August the
two treatments diverged; in the M+P treatment, Anabaena dominated the EPI and codominated with diatoms (Synedra rumpens) in the HYPO. This result is in marked
contrast to the K+P treatment, in which
there was almost no Anabaena. Although
Anabaena was of similar proportional importance in both P and M+P treatments,
its density was fourfold less with the clay.
Discussion
Because the two clays differed so markedly in their abilities to sustain turbidity, it
can be argued that the differences in their
effects on other limnological parameters are
simply a matter of scale. Montmorillonite
would alter light-driven processes to a great-
Table 1. Composition and properties of the two
types of clay.
Humics (% by volume)
Cation exchange capacity (100 cme3)
Base saturation (%)
PH
Total extractable P (ppm)
Total N (ppm)
K (pm-0
Montmorillonite (%)
Kaolinite (%)
Halloysite (%)
Vermiculite (%)
Gibbsite/goethite/hematite (%)
Kaolinite
Montmorillonite
0
13.9
90
6.6
3.2
18.5
51.9
0
75
0
15
10
0.1
4.7
40
5.0
1.4
9.4
55.2
50
25
25
0
0
er degree than kaolinite, but in the same
way. By extension of this argument, only
light attenuation needs to be considered in
predicting community
behavior of turbid
systems; the type of suspended other material would be of little consequence. In support of the scale argument, thermal stratification, TP concentrations
(Fig. 3), algal
densities (Fig. 6), and some species shifts,
such as that from Agmanellum to C. minutus, followed the turbidity gradient. The
scale argument, however, does not predict
the clay-P interaction, nor hold for NCP
under simultaneous clay and P loading, nor
explain the distribution of filamentous bluegreens.
Although kaolinite contained twice the P
of montmorillonite
(Table I), because of its
shorter residence time it contributed less TP
to the water column (Fig. 3). Yet the greater
TP sustained by montmorillonite
was either
insufficient
to counter the comparatively
greater negative impact of montmorillonite’s turbidity on NCP, or the clay-bound P
was not readily available to the algae (sensu
Fitzgerald 1970). That additional P fertilization boosted NCP and algal growth in
both the K+P and M+P treatments indicates that neither of these clays caused productivity to be limited solely by light. Of
the two, montmorillonite
would have been
predicted to be less sensitive to P fertilization. Its higher turbidity suggests stronger
light limitation, and it is a better P sink than
kaolinite (Edzwald et al. 1976), yet the M+P
treatment was second only to the P treat-
838
Cuker et al.
ment in productivity
and by August had
twice the algal density of the M treatment
alone (Figs. 4 and 6).
Despite relatively high productivity
in
M+ P, algal concentrations remained low.
Sinking of algal-clay floes was a possible
source of mortality.
Zooplankton
grazing
also may have limited phytoplankton
populations. High densities of the cladoceran
Diaphanosoma were found in the M+P
treatment (Cuker unpubl. data). Further,
much of the NCP in the M+P treatment
can be attributed to periphyton on the enclosure walls (Burkholder and Cuker unpubl. data) and as such would not have contributed to planktonic populations.
In keeping with the scale argument, community structure was partitioned by depth
more so with montmorillonite
than with kaolinite (Figs. 7 and 8). This pattern was well
illustrated in the M+P treatment by Synedra, which developed a large population in
the cool, dark habitat of the HYPO. This
situation is analogous to winter dominance
of Synedra in the turbid Cahora Bassa Reservoir (Gliwicz 1986).
The scale argument is inconsistent, however, with the inability of the less turbid
K+P treatment to support the Anabaena
that thrived in the more turbid M+P treatment. Why is Anabaena able to grow with
montmorillonite
but not with kaolinite?
Perhaps the larger particle size of kaolinite
causes more rapid sinking of algal-clay floes;
sinking rates are proportional to particle size
squared (Chase 1979). This interpretation
is consistent with the brief residence time
of kaolinite inferred from short- and longterm Secchi depth studies. ‘But particle size
alone may be insufficient, however, to predict clay behavior. Soballe and Threlkeld
(198 8) found that although ground silica had
a larger average particle size than bentonite,
it was less likely to floc with Anabaena. In
that case, aspects of particle surface chemistry may be more important than particle
size.
Cuker (1987) showed that P fertilization
promoted clearing of kaolinite turbidity. In
our study, this effect was evident for both
clays, but primarily in the second half of
summer when temperature and productivity increased (Fig. 1). Although the effect
was proportionally
greater for montmorillonite, this clay caused such high turbidity
that the M+P treatment remained much
murkier than the K and K+P treatments.
Fertilization with P as a management practice to reduce mineral turbidity (Avnimelech and Menzel 1984) could work well for
kaolinite, but for montmorillonite
the clarification would be slow and likely accompanied by growth of Anabaena.
Unlike Threlkeld and Soballe (1988), we
were able to establish statistically significant
links between suspensions of different minerals and limnological
parameters beyond
just turbidity. Their use of a single pulse of
clay and their application of one of the clays
in a different season than the other two, as
well as the lack of replication in their study
all may have contributed to the discrepancies between those results and what they
predicted from the laboratory (Soballe and
Threlkeld 1988) and what we observed in
our field study.
Although we have shown that montmorillonite and kaolinite affected various limnological parameters differently, these effects may not be solely due to differences in
mineralogy. Suspended clays are coated by
layers of ions, organic molecules, and microbes (Loder and Liss 1985). The nature
of the coating could be more important than
the underlying mineral in controlling limnological interactions.
Further, we used
natural soils as turbidity sources, and minerals other than the dominant ones may have
contributed to the observed effects. Although
this heterogeneity constrains generalization
about the effects of montmorillonite
and kaolinite, this research demonstrates that suspended sediments of differing mineralogies
can produce distinct quantitative and qualitative effects.
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Submitted: 12 April 1989
Accepted: 4 January 1990
Revised: 27 February 1990