Comment
affected by changes in turbidity,
and the
Secchi disk can be a quick and easy way
to get meaningful
information.
W. T. Edmondson
Department of Zoology
University
of Washington
Seattle 98195
Limnol. Oceanogr.,
E(2),
@ 1980, by the American
1980,379-382
Society of Lnnnology
More complications
disk relationship
and Oceanography,
379
References
CHASAN, D. J. 1971. Seattle area wouldn’t
allow
death of its lake. Smithsonian 2(4): 6-13.
EDMONDSON, W. T. 1972. Nutrients
and phytoplankton in Lake Washington.
Am. Sot. Limnol. Oceanogr. Spec. Symp. 1: 172-193.
HAFFNER, G. D., AND J. H. EVANS. 1974. Relation
of light penetration
to particle distribution
in
vertically
mixed lacustrine
environments.
Br.
J. Phycol. 9: 261-267.
HUTCHINSON, G. E. 1957. A treatise on limnology,
v. 1. Wiley.
Inc
in the chlorophyll-Secchi
The comments
of Lorenzen
(1980),
Megard et al. (1980), and Edmondson
(1980) deal critically
with certain assumptions
underlying
the relationship
between Secchi disk transparency
and
algal chlorophyll.
Among other things,
Lorenzen and Megard et al. criticize the
use of the power function used by me and
others to describe this relationship
and
suggest that the attenuation
of light by
nonalgal substances adequately accounts
for deviations from simple light attenuation models.
These comments are especially important to me because I published a trophic
state index related to algal biomass (Carlson 1977). Although
algal biomass was
the basis for the index, I chose to base
the index values on Secchi disk transparency rather than on a direct measure of
algal biomass because I wanted the final
index values to be easily understood by
the lay public. It was fortuitous that the
normal range of transparency
values
could conveniently
be transformed
into
a scale of O-100.
As the philosophy
underlying
the index was that trophic
state variables
should be quantifiably
related, I chose
the Beer-Lambert
equation as the basis
for establishing
a theoretical
connection
between transparency
and other trophic
state variables. Although the equation is
a simplistic representation
of how a Secchi disk works (Hutchinson
1957; Tyler
1968), I felt that it was adequate to describe general relationships.
The equation predicted
that a linear
relationship
should exist between the inverse of Secchi disk transparency (l/SD)
and other biomass indicators. However,
when chlorophyll
and transparency data
for 14 lakes were plotted, the relationship
was not linear as predicted. I suggested
that this could be explained
if chlorophyll per cell increased as total algal biomass increased. There is evidence to support this hypothesis:
l-Chlorophyll
per
cell has been found to increase in certain
species as the mean light climate decreased (Steele 1962; Jgrgensen 1969).
This decrease could be produced in lakes
by the increased light attenuation
as algae increase in abundance. 2-The
relationship between Secchi disk transparency and total phosphorus
did fit the
simple inverse model. 3-The total phosphorus-chlorophyll
relationship
was also
nonlinear. 4-Chlorophyll
is nonlinearly
related to seston dry weight and to total
particle volume (as determined
with a
Coulter
Counter)
(Hallegraeff
1976).
Items 2,3, and 4 all suggest that the problem of nonlinearity
involves chlorophyll
rather than the other biomass variables.
The problem is that total phosphorus,
Comment
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I
.
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.
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.
.
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Chl=1
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.
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,
I
20 33
MEAN
20
CELL
30
VOLUME
40
(cm3
50
so
,
1
-40
CHLOROPHYLL
,
5:
61:
I
(mg
rn3)
/------u
250 260
Fig. 2. Plot of I’-intercepts
against mean chlorophyll concentration
for lakes listed in Table 1.
m3)
between total algal cell volFig. 1. Relationship
ume and chlorophyll
(1 concentration
in fi\.e Olinnesota lakes. Data from Shapiro and Pfankuch (unpubl.).
seston dry weight, total particle \-olume,
and Secchi disk t ransparency do not measure algal biomass exclusively.
Xny nonalgal Factor that is measured concurrentl>
with algae by these \,ariables
would
create the same nonlinearity
with chlorophyll.
The direct test would be to examine
the relationship
of chlorophyll
to a direct
measure of algal biomass such as cell \.olume. Laboratory studies might be in\-alid
because the hypothesis
depends on a
changing light climate. Using data supplied by J. Shapiro (Shapiro and Pfankuch unpubl.),
I plotted
chlorophyll
against total cell volume. The results,
based on five Minnesota
lakes (Fig. l),
strongly suggest that chlorophyll
is indeed a linear correlate of cell volume,
and the nonlinearity
is caused by some
other f&tor.
Lorenzen
(1980) and 1Iegard et al.
(1980) suggested that the nonlinearity
in
the plot of the inverse of transparency
against chlorophyll
was the result of laketo-lake \-ariations in the background
attenuation
coefficient,
k,,.. Megard et al.
suggested that if the data for each lake
were plotted separately-, the relationship
would be linear; I did this for the original
lakes as well as for some additional data
supplied hy J. Shapiro (Table 1). In all
the lakes the relationships
were either
linear or the data too scattered to fit any
curve well.
Random variations in k,,. , which is related to the Y-intercept
[k,,./ln(l,,/l,)],
could not account for the nonlinearity
of
my original
plot, as was suggested by
hlegard et al. and Lorenzen. A randomly
\-arying k,,. would be averaged in the
regression and the plot would remain linear. It is evident from Table 1 that the
slopes tend to decrease with increasing
mean chlorophyll
concentration
while
the Y-intercepts tend to increase (Fig. 2).
The nonlinearity
in my Secchi disk-chlorophyll plot appears to be largely the result of the Y-intercepts
co\.arying with
the mean lake chlorophyll
concentration.
This same relationship
between k,,. and
chlorophyll
is also e\-ident in the three
lakes cited by 1Llegard et al. In f&t, other
data supplied to me by Megard indicated
by a
that k,,. was related to chlorophyll
power function
(Carlson
1979). With
more data this cul7.e may pro\-e to saturate at higher chlorophyll
\-aloes, but the
evidence suggests that in some manner
k,,. covaries with chlorophyll.
If k,,. varied as a direct flmction of chlorophyll, then its effect would be in the
slope of the line rather than in the Y-intercept. The slope would be affected by
pigments other than chlorophyll
II or algal detritus, but the kr-intercept
would
not be affected. The Y-intercept is a measure of the attenuation
of materials not
covarying with chlorophyll.
The data base is certainly too small to
illustrate
the total variation
in k,,. between lakes. The addition of highly colored, low chlorophyll
lakes may eliminate any chlorophyll-k,,.
relationships,
but Fig. 2 suggests that a minimum
k,,.
Comment
Table 1. Linear regression relationships
parency (Y-axis) in several lakes.
between
381
chlorophyll
(X-axis) and in\ erse of Secchi disk trans-
Lnke
Minnetonka
Browns Bay
Carman Ba)
Upper Lake
Halsted Bay
10
10
10
37
9.10
6.09
14.68
31.64
Superior
20
0.79
0.0536
0.0688
0.1691
Schelske
4
0.15
0.0349
0.0340
0.8063
Powers et al. 1972
Waldo
0.0166
0.0289
0.0162
0.0225
0.3827
0.3057
0.4637
0.5001
0.3200
0.3321
0.1644
0.6370
\4egard
unpubl.
et al. 1972
Lake of the I\lea
1971
1972
13
13
45.50
56.35
0.0257
0.0154
0.4263
0.7553
0.3576
0.5653
Shapiro and Pfankuch
unpubl.
Calhoun
1971
1972
17
16
10.82
22.14
0.0075
0.0093
0.3816
0.4038
0.2289
0.4577
Shapiro and Pfankuch
unpubl.
Harriet
1971
1972
18
16
9.26
5.45
0.0168
0.0187
0.3894
0.2766
0.7093
0.4759
Shapiro and Pfankuch
unpubl.
0.0143
0.1556
0.8770
Carlson
Cottonwood
5
251
might exist for any giLTen mean chlorophyll concentration.
Another possibility
is that k,,. varies independently
of chlorophyll within a given lake, yet in comparisons of the mean k,,. between lakes a
relationship
exists because lake or inflowing stream characteristics
that affect
the growth of algae may also affect the
amount of nonalgal particles and color.
Areas near the mouths of streams ma)
have both high chlorophyll
\-alues and
high dissol\Ted fluorescence, a quality related to water color (Carlson et al. unpubl.). Shallow lakes ha\-e long been
thought to be more productive than deeper lakes, yet in shallow lakes there is also
a greater potential for wind-stirred
sediments and a higher k,,..
It seems evident that the relationship
between algal biomass and transparency
is not to be completely explained by simple equations. Certainly
more attention
should be given to the nature and effect
of nonalgal light attenuation.
If k,,. does
somehow covary with chlorophyll,
then
any use of these equations for predictive
purposes would be suspect. Estimates of
chlorophyll
from transparency
measurements can be made by using a constant
k,,. derived by the method described by
Megard, but this use of the Secchi disk
is of dubious importance.
Transparency
does not simply depend on the vertical
1975
attenuation
of light by chlorophyll,
nnd
Edmondson quite rightly points out that
scattering of light by particles is a irery
important f&or. The scatter found in my
graphs of l/SD versus chlorophyll
(as
seen in the r2 \ralues in Table 1) attest to
the f&t that these relationships
are quite
\-ariable. Th e Secchi disk would best be
relied on as a biomass indicator
only
when none better is available.
I ha\Te certainly used Secchi disk transparency as biomass indicator in ml’ trophic state index. Considering
all the potential error associated with the use of the
Secchi disk, I am amazed at the degree
to which the transparency
and chlorophyll indices do agree. Megard considered the trophic state index to be ambiguous because
lakes with
the same
transparencies
but different nonalgal attenuation coefficients
would be considered to have the same amount of algal
biomass and thus be assigned the same
trophic state value. This is not really a
problem if chlorophyll
were also meadeviation
of the
sured. A systematic
transparency
index from the chlorophyll
index would indicate that there were differences in k,,.. The index values based
on chlorophyll
should be gi\Ten greater
weight than transparency values.
The major ambiguity
inherent in the
index is not the possibility
of misinter-
382
Comment
pretation
of trophic state because of variations in k,,. , as suggested by Megard, but
that the index assumes that chlorophyll
increases exponentially
relative to algal
biomass. The evidence presented here
suggests that chlorophyll
is linearly related to biomass, and therefore the index
distorts changes in algal biomass. A lounit change in the index signifies a halving or doubling
of transparency,
but a
doubling
in chlorophyll
will only increase the index value by 7.
A total revision of the index would involve basing it on a direct measure of algal biomass such as total cell volumes or
chlorophyll.
A problem with revising the
index based on a lo-unit increase with
each doubling
is that the index will no
longer fit neatly between 0 and 100. The
range of biomass values found would
mean either that some lakes would have
negative values or that some would have
values of 150 or more.
A possible solution is to keep the same
scale and simply accept that each 7 units
rather than 10 units represents a doubling in algal biomass. The scale would
now be double: a lo-unit scale for transparency and a 7-unit scale for biomass.
The dual scale should be used with some
caution because it still implies 8 strong
relationship
between biomass and transparency. This comment as well as those
of Regard et al., Lorenzen, and Edmondson emphasize
that there is a great deal
of uncertaintyin that relationship
and
that these scales are coupled in only a
general way. Within my lake, the relationship between biomass and transparency will depend, at the \‘er\. least, on
the value of the background attenuation
coefficient. The scales cm be expected to
de\,iate significantly
at low chlorophyll
concentrations
when the effect of k,,. is
the greatest.
Lorenzen and _\legarcl
et al. are certainly correct that the background attenuation coefficient, k,,. , is an important faci II determining
tor
transparency.
However, their explanation that the nonlinearity
in the relationship
between 1/
SD and chlorophyll
is caused simply by
differences in k,,. in lakes cannot be substantiated. The nature of a k,, that covaries
with mean chlorophyll concentration makes
an enticing subject for future investigation. Equally interesting
is that the well
known power relationship
between chlorophyll
and total phosphorus
could be
caused by systematic differences in nonalgal phosphorus between lakes. If this
were true, we might expect that the relationship between chlorophyll
and total
phosphorus would be linear in individual
lakes.
Robert E. Carlson
Department of Biological
Kent State University
Kent, Ohio 44242
Science
References
CARLSON, R. E. 1975. Phosphorus
cycling in a
shallow eutrophic
lake in southwestern
\4innesota. Ph.D. thesis, Univ. Minnesota.
-.
1977. A trophic state index for lakes. Limnol. Oceanogr. 22: 361-369.
-.
1979. A review of the philosophy
and construction of trophic state indices, p. l-52. Zn T.
Maloney [ed.]. Lake and reservoir classification
systems. USEPA Ecol. Res. Ser. EPA-600/3-79074.
EDMONDSON, W. T. 1980. Secchi disk and chlorophyll. Limnol. Oceanogr. 25: 378-379.
HALLEGFUEFF,
G. X4. 1972. Pigment diversity,
biomass and species di\,ersity of phytoplankton
of three Dutch lakes. Ph.D. thesis, Univ. Amsterdam.
HVTCHINSON, G. E. 1957. A treatise on limnolog),,
v. 1. Wiley.
JBRGENSEN, E. G. 1969. The adaptation of plankton algae. 4. Light adaptation in different algal
species. Physiol. Plant. 22: 1307-1315.
LORENZEN, hf. W. 19&O. L-se of chloroph>.ll-Secchi disk relationsllips.
Limnol. Oceanogr. 25:
371-372.
\IEG.~RD, R. O., J. C. SETTLES, H. A. BOYER, AND
\V. S. COMBS, JR. 1980. Light, Secchi disks,
and trophic states. Limnol. Oceanogr. 25: 373,377.
POWERS, C. F., D. \I'. SCHPLTS. IL \Y. \I.\LcEG, R.
.\I. BRICE, ASD 11. D. SCHTZLDI‘. 1972. Algal
response\
to nutrient
addition\
in natural
waters. 2. Field experiments.
Xm. Sot. Limnol.
Oceanogr. Spec. Symp. 1: 141-1.54.
SCHELSKE, C. E., L. E. FELDT, Sl. .\. SO-TIAGO,
AND E. F. STOERMER. 1972. Nutrient enrichment and its effects on phytoplankton
production and species composition
in Lake Superior.
Proc. 15th Conf: Great Lakes Res. 1972: 149165.
STEELE, J. H. 1962. En\ ironmental control of photos)-nthesis in the sea. Limnol. Oceanogr. 7:
137-150.
TYLER, J. E. 1968. The Secchi disc. Limnol.
Oceanogr. 13: l-6.