THE EFFECT OF TURBIDITY CAUSED BY SEWAGE
POLLUTION ON THE PRODUCTIVITIES OF IRIDAEA
FLACCTDA, PHYLLOSPADIX TORREYI, AND LAMINARIA SETCHELLII
John Hainsworth
Hopkins Marine Station
Pacific Grove, Calif.
June 2, 1970
INTRODUCTION
Observation of the floral community surrounding an
intertidal sewage outfall shows that some species of benthic
algae and marine plants are able to exist in areas of high
sewage concentrations, while other species cannot tolerate
this altered environment. However, even the most resistant
species are small and stunted in the immediate vicinity
of the outfall. Since one of the most salient character¬
istics of sewage pollution is the increased turbidity
caused by the suspended particulate matter in the sewage,
it is logical to hypothesize that the increased turbidity
effects the viability of the marine flora by reducing the
amount of light received, therefore interfering with the
normal photosynthetic process. The specific absorbtion
spectrum of the sewage may also be a factor in determining
the different tolerances of different species, since each
has a unique pigment composition
An experiment was designed to answer two questions:
1) does turbidity effect the productivity of marine flora,
and 2) does turbidity have a different effect on different
species. In all species studied the productivity decreased
markedly at high turbidities. The productivities of the
three species studied also showed differences in their rates
of decline as the turbidity increased.
33.
METHODS AND MATERIALS
Two species of benthic algae (Iridaea flaccida and
Laminaria setchellii) and one marine plant (Phyllospadix
torreyi) were tested. These particular species were chosen
for two reasons: 1) Iridaea flaccida and Laminaria setchellii
are representative of the red and brown classes of benthic
algae, while the pigments in Phyllospadix torreyi are sim¬
ilar to those in the chlorophytes, and 2) all three species
are very common along the Monterey coast, where the study
was conducted. Samples were collected from an unpolluted
environment, cut into pieces weighing approximately 150 mg
(dry weight), and put separately into 300 ml BOD bottles.
(For both Iridaea flaccida and Laminaria setchellii, all
pieces used for one test were cut from a single leaf.) The
BOD bottles were filled with sea water which had a depressed
dissolved oxygen content achieved by bubbling with nit¬
rogen for 8-10 minutes. The bottles were then placed on
their sides in several two-gallon tubs containing water of
different turbidities, and incubated at 12° C. A light of
constant intensity was passed through a 3.5 cm. layer of
turbid water to the test bottles.
The bottles were submerged in water of three different
turbidities, and in sea water, which served as a control.
Sewage collected 1.2 meters from the Pacific Grove outfall
was used in the most turbid bath, the other two baths being
comprised of 1:2 and 1:4 dilutions of this sewage. Absorbtion
334
curves were determined for samples of sewage collected at
various times of the day, and although the absorbances at
specific wavelengths varied, the absorbtion spectra were
parallel for all samples tested. A typical absorbtion curve
for sewage is illustrated in Figure 1. Since the peak
absorbtion is at 430 millimicrons, this wavelength was
arbitrarily chosen as a measurement of turbidity. The
turbidity of each bath was measured daily and adjusted by
adding newly collected sewage. (The turbidity tended to
drop after standing for longer than 24 hours.) The sewage
was changed weekly to prevent gross changes in the ab¬
sorbtion spectrum due to bacterial growth or changes in
particle size.
Both Phyllospadix torreyi and Iridaea flaccida were
submerged in the baths for one hour. Laminaria setchellii
was incubated for two hours because of its slower rate
of photosynthesis. A single test included duplicate bot¬
tles at each turbidity (all with one type of test organism),
along with duplicate bottles in the control bath. When the
test period ended, the bottles were removed from the baths
and the dissolved oxygen in the water was measured using
the Winkler method as described by Strickland and Parsons
(1968). The dissolved oxygen in the water before the test
was subtracted from the dissolved oxygen in each of the bot-
tles, each piece of algae was dried and weighed, and the
net productivity (in ml oxygen/mg dry weight/hour) was
calculated using the formula:
LB - IP
net productivity - mg dry wt.
hrs.
where LB = dissolved oxygen in the light bottle, and IB
dissolved oxygen in the initial bottle.
After several trial runs, the test was repeated ten
times with each species, and the first two runs for each
species were discarded.
RESULTS
Figures 2, 3, and 4 represent a compilation of eight
runs for each species tested. Duplicate bottles for each
test at each turbidity were averaged, so that the three final
graphs consist of eight points at each turbidity. Using
these eight points, the means and standard deviations were
calculated. The graphs show perecent of control product¬
ivity versus turbidity rather than net productivity versus
turbidity because the control productivity in a single
species varied from test to test since different plants
were used. This was especially true for Iridaea flaccida.
DISCUSSION
Figures 2, 3, and 4 clearly indicate that the product-
ivities of all three species studied are effected by tur
bidity. Although the standard deviations (especially in
the case of Laminaria setchellii) are quite large, it ap
pears that the three different species respond differently
340
to increasing turbidity. Although the curves for Laminaria
setchellii and Phyllospadix torreyi are quite similar, they
are both very different from the Iridaea flaccida curve.
While the productivities of both Laminaria setchellii and
Phyllospadix torreyi drop in the 1:2 and 1:4 dilutions of
the sewage, the Iridaea flaccida productivity remains ap
proximately constant.
The standard deviations which occur at some of the
points in the graphs of Iridaea flaccida and Phyllospadix
torreyi are misleadingly large, since one or two highly
deviant values had a greater effect on the standard dev¬
iations, because there were only eight values determining
the means. With many more tests using these two organisms
the standard deviations would probably become much smaller.
The values for Laminaria setchellii ranged quite widely
and showed no indication that additional tests would sig
nificantly lower the standard deviations. A number of
features probably contributed to the undesirability of
Laminaria setchellii as a test organism. Most important,
Laminaria setchellii contains a sticky sap, which oozes out
when the leaf is cut and then traps air bubbles when the
bottle is filled. Although the bottles were filled immed-
iately after the algae was cut, some air was probably still
trapped. In addition, the very slow productivity rate of
Laminaria setchellii (approximately eight times slower than
that of Phyllospadix torreyi) maximized the errors caused
by the trapped air bubbles. In spite of the wide variation
in the results, Laminaria setchellii shows a distinct de¬
crease in productivity when the turbidity increases.
The experimental results obtained for the three spe¬
cies studied correlate well with the locations of these
three types in the vicinity of the Pacific Grove outfall
at Point Pinos. None of the species is found within a
70meter radius of the outfall, where the turbidity at low
tide ranges from .800 to 1.40 (turbidity as absorbtion at
430 millimicrons). Iridaea flaccida exists at one loca¬
tion with a turbidity of approximately .500 (the exact
turbidity could not be measured since the rock was 7 meters
offshore), and fairly commonly in areas where the tur¬
bidity was .100-.200. Neither Phyllospadix torreyi nor
Laminaria setchellii appear except at low (.100-.200)
turbidities, and all of these sites are washed from one
side by clear water.
A comparison of the absorbtion curve of sewage with
the absorbtion curves of various plant pigments present
in marine flora provides possible explanations for the
detrimental effects of sewage turbidity on these organisms
in general, and also for its different effects on the three
species studied. All three types of chlorophylls, and
most carotenoids have absorbtion peaks at 430-460 milli-
microns (Franck, 1949), which is the range of maximum ab¬
sorbtion by the sewage. Since light at this wavelength is
being absorbed by the sewage, the light available for ab¬
sorbtion by the chlorophyll is reduced. This predicted
drop in productivity is well supported in Figures 2, 3,
and 4.
Although the absorbtion spectra for red, green, and
brown algae are approximately the same, with absorbtion
peaks at 430 and 660 millimicrons, Haxo and Blinks (1950)
found that the action spectra of the red algae is greatly
different than the action spectra of either green or brown
algae. The action spectra of both green and brown algae
approximate the absorbtion spectra, and have peaks at 430
and 660 millimicrons. (Phyllospadix torreyi, which is a
marine flowering plant, contains the same pigments as does
a green alga.) However, in red algae, the water-soluble
phycobilin pigments are much more efficient at absorbing
light than are the chlorophylls; because of this the red
algae utilize light of 570 millimicrons most effectively,
while very little of the light with wavelengths of 430-450
millimicrons is absorbed. The absorbtion spectrum of sew¬
age shows that approximately 30% more light of wavelength
570 millimicrons passes through than does light of 430
millimicrons. Red algae can be predicted, because of this
fact, to be less effected by sewage turbidity than either
green or brown algae. This prediction is supported by
the results obtained in this investigation, using a single
species characteristic of each algal type. Iridaea flaccida
37
is able to tolerate turbidity (up to a certain level) much
better than either Phyllospadix torrevi or Laminaria set
chellii.
The ability to live in turbid water caused by sewage
pollution is one reason that Iridaea flaccida, and perhaps
red algae in general, can exist in water containing high
sewage concentrations. However, turbidity is not the only
parameter involved. Many other components of sewage, or
properties of the different algal species themselves, also
help to determine resistance to sewage pollution. Roy
(personal communication) has found that several species of
red algae in the vicinity of the Pacific Grove sewage out
fall contain greater quantities of pigments (both chloro¬
phylls and carotenoids) than do the same types growing
in unpolluted areas. This increased pigment concentration
could possibly be a response to a decreased amount of light
reaching the plant, due to sewage turbidity. Neither green
nor brown algae have demonstrated this property. Schreiber
(personal communication) has found that the productivity
of several types of algae is decreased by increased chlorine
concentrations in the water. Other pollutants found in
sewage may also effect productivity, and these could be
determinants of floral distribution in an outfall area.
SUMMARY
The productivities of all three species studied were
found to decrease with increasing turbidity; this decrease
34
was as great as 35% in turbidities approximating the tur-
bidity around the Pacific Grove sewage outfall. The ef¬
fects of this turbidity were also shown to differ for the
three species studied. Iridaea flaccida exhibits a greater
tolerance to sewage turbidity (except at very high values)
than do either Phyllospadix torrevi or Laminaria setchellii.
This finding is consistent with the prediction made for
the effects of sewage turbidity on red versus green or
brown algae using the action spectra of the different
types.
4
ACKNOWLEDGEMENTS
I wish to thank Dr. Ellsworth Wheeler for his assistance
both in my research and in the writing of this paper. This
research was supported in part by the NSF Undergraduate
Research Program Grant +GY7288.
REFERENCES CITED
Franck, J.
1949.
Photosynthesis in plants. Iowa State
College Press. 450 pp.
1950. Photosynthetic action
Haxo, F.L. and Blinks, L.R.
spectra of marine algae Journal of General Physiology,
33 (4): 389-421.
Strickland, J.D.H. and Parsons, T.R. 1968. A practical
handbook of seawater analysis. Fisheries Research Board
of Canada. 300 pp.
Figure 1.
Figure 2.
Figure 3.
Figure 4.
FIGURE CAPTIONS
Absorbance spectrum of sewage collected at
the mouth of the Pacific Grove outfall.
Change in productivity of Iridaea flaccida,
as a function of increasing turbidity.
Change in productivity of Phyllospadix torreyi,
as a function of increasing turbidity.
Change in productivity of Laminaria setchellii,
as a function of increasing turbidity.
(
O
JONVadosav
0
10

E
3
0.
120
100
6
40
078
IRIDAEA FLACCIDA
720
415
175
TURBIDITY
(Absorbance at 430 millimicrons
100
20
078
PHYLLOSPADIX TORREY
415
720
URBIDITY
(Absorbance at 430 millimicrons)
1175
6(
40
078
LAMINARIA SETCHELLI
720
1175
415
TURBIDITY
(Absorbance at 430 millimicrons)
52