ABSTRACT
A study was made of the populations of intermediate sized interstitial
organisms of Del Monte Beach, Monterey California. In the summer of 1969,
this beach was closed because of sewage pollution from the city's offshore
outfall. It was expected that populations of organisms constantly exposed to
this pollution might be affected. Samples were taken at every 100 meters on a
transect runningl000 meters north and 1500 meters south. Counts were substantially
lower to the north than to the south. This appears to be correlated to the
southerly flow of the sewage as reported by several other investigators. A
general enrichment of the environment is considered to account for the increase
in population.
INTRODUCTION
As part of a research project on the sewage pollution of Monterey Bay,
a survey was undertaken of the intermediate sized interstitial organisms of
Del Monte Beach, fronting the city of Monterey. During the summer of 1969, these
beaches were closed due to a high degree of bacterial contamination. It seemed
likely that an effect might be observable in populations of organisms constantly
exposed to this pollution. The organisms studied were intermediate in size
between the protozoa, and such macrofauna of the beach as sand crabs (Emerita sp.).
The animals surveyed were nematodes, nemerteans, flatworms, annelids, and
harpacticoid copepods. Others which occured regularly but not in surveyable
numbers included amphipods and mysids. While work had been done on the general
ecology of interstitial organisms, this appears to be the first study done to
correlate their ecology to possible pollution. General studies of their ecology
include those of Pennak (1951) and Swedmark (1964).
METHODS
From the location where the pipe from the Monterey sewage treatment plant
crosses the beach, running to the offshore outfall, a transect was established
with stations every hundred meters for 1000 meters north and for 1500 meters south.
The southern transect terminated at Monterey Municipal wharf f2. Sand samples
were taken at the same tidal height and stage of the tide once a week for three
weeks with a core 8cm deep and approximately 4cm in diameter to produce a sample
of 100 cc. The samples were stained and preserved with a mixture of rose bengal
and isopropyl alcohol. The organisms were counted by spreading loc sub-samples
in Syracuse dishes, adding water, and spinning so that a layer of sand grains was
produced over which the stained organisms (which were slightly bouyant) could be
easily seen. A count was also taken of the organisms caught when the preservative
fluid was filtered off.
24
RESULTS
The results of the counts are shown in Table 1. The tabulated values are
the means of the three weekly counts. At 100 south and 500 north, additional
samples were taken to refine the value of those stations which seemed extremely
variable. All counts are innumbers of organisms per l0cc. The third category
lumps nemerteans and flatworms together because of the low counts and the dif-
ficulty of differentiating preserved individuals of these two similar groups.
Graph 1 shows the total number of organisms of all categories per lOcc
plotted against distance. The line is the connection of the individual means
while the vertical lines are the standard deviations of the samples at each
station.
Graph 2 is the population of nematodes plotted against distance from the
outfall. As can be seen by comparison with the first curve, the nematodes
are the largest constituent in the total population, and the variation in the
total population is largely a function of the variation of the nematode population.
Graph 3 displays the distribution of annelids in the population, primarily
a Phyllodocid worm, possibly Eteone dilatae. As can be seen from the graph, the
population of this worm increases with distance north of the outfall. The populations
of the other organisms are not graphed because they did not appear to show any
definable trend over the length of the survey.
DISCUSSION
A general trend in distribution appears to be evident along the length of
this survey. The counts of the nematodes show a major change in the first 100
meters on either side of the outfall pipe. To the south the abundances are
consistently high with a variation that could be accounted for by the variation
in sampling. To the north, however, the population falls off sharly with increasing
distance from the outfall. The population of the annelid worm exhibits a reversal
of this trend, increasing with distance north. This would seem to indicate a
general ecological variance between the northern and southern half of the survey
There are several reasons for believing that the effect is related to
sewage pollution. The first is the relative homogeneity of the beach environment
along the length of the survey. The second is the abrupt change in the curve at
the area of the outfall. The third is the nature of the sewage field around the
Monterey outfall.
Several ecological factors important to the distribution of interstitial
organisms are relatively constant along the length of this beach, or else vary
in a manner inconsistent with the observed variation in the population.
Station
0 Pipe
N 100
200
300
400
500
600
700
800
900
1000
S 100
200
300
400
500
600
700
800
900
1000
1250
1500
Total
116.2
93.5
58.5
46.5
24.1
33.6
60.3
43.2
19.0
39.5
22.3
201.5
77.4
112.6
117.2
178.7
83.1
140.2
89.3
96.0
86.3
95.1
92.5
Nematodes
80.7
48.8
47.3
36.3
13.4
14.7
29.4
15.2
6.8
5.1
6.2
117.6
43.0
87.0
110.6
145.4
62.5
113.9
70.7
74.5
58.0
52.3
78.7
TABLE I
Annelids
6.7
5.4
4.7
4.7
4.0
11.7
9.7
9.1
30.0
12.1
3.8
4.6
3.3
2.6
3.6
3.6
5.0
3.6
5.4
5.0
1.6
1.7
Nemerteans
Fla.worms
5.2
3.8
2.5
3.7
3.2
3.8
9.5
9.9
1.7
2.3
8.7
2.6
6.3
3.3
6.2
5.2
12.0
8.4
7.7
14.3
11.5
12.0
Copepods
23.6
35.5
1.8
1.8
8.1
9.7
9.0
1.4
1.4
1.7
71.4
27.2
16.0
23.0
11.8
10.7
6.6
8.4
9.0
30.0
248
FIGURE CAPTIONS
Figure 1 - Map of Study area showing transect lines.
Graph 1 - Total abundance (Number/ l0cc) vs Distance from outfall
Graph 2 - Nematode abundance (Number/ l0cc) vs Distance
from outfall
Graph 3 - Annelid Abundance (Number/ 10cc) vs Distance
from outfall
Graph 4 - Semi-log Plot
Total Abundance (Grouped in 300 meter groups)
vs Distance from outfall
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The grain size of the sand is relatively constant along the portion of the
beach in question, with most sand grains having a diameter of O.2 — O.5 mm.
At 1500 meters south, however, there are large pebbles scattered in the sand.
The sand from the ends of the transectare indistinguishable in size, though
the sand form the southern end is darker. Temperature (as measured off shore
in another survey) varies only 0.2° C along the survey length. Salinity variations
offshore also do not appear to be significant
It seems reasonable to
assume that without any great variation in these factors in the offshore waters,
then a similar homogeneity can be expected in this regard along the length of
the beach. Other parameters of the interstitial environment such as dissolved
oxygen or the chemical composition of the water are very difficult to measure
in situ. Zinn (1968) mentions the development and testing of such a measuring
device. No such measurements were attempted in this study.
The problem of wave action is somewhat more difficult to assess, since there
is an appreciable difference in the intensity of wave action along the length of
the transect. At the extreme southern end the wave action is slight with the
waves being less than O.1 meters in height, while at the farthest north station,
waves of O.4 — 0.7 meters are present. As there are no major obstructions or
topographic anomalies along the beach, it seems likely that the height and
intensity of wave action increases in a continuous and smooth function up the
length of the beach. This is in contrast to the population distribution.
There is a large field of kelp on a bed of slate to the south of the outfall.
The influence of this on the interstitial environment ashore is not known.
If the changes of the beach population were independent of sewage pollution,
then it would be unlikely that any significant change in the population could be
relatable to the presence of the outfall. A semi-log plot of the counts grouped
in 300 meter groups in both directions from the pipe shows a definite change in
slope in the vicinity of the outfall. This method of presentation indicates more
clearly the major trend along the beach by reducing the random variations in
individual samples.
Studies of the Monterey outfall indicate that some of the sewage comes ashore
near the outfall. Trumbauer (1966) shows the highest coli-form counts within
200 meters of the outfall. His report also mentions that the area of highest
concentrations is quite variable, with peaks often occuring far to the south.
As has been noted before, coli-form organisms have certain limitations when used
in sewage tracking.
228.
NUMBER
1Occ
150
100
50
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1200
600
PIPE
600
NORTH
S.
A more informative study was that done by Stevenson (1964) of the water
circulation pattern off Del Monte Beach. In 10 surveys, a southward drift
was reported six times, a northward drift twice, and on shore drifts, normal to
the beach twice. He utilized drogues released seaward from the boil. He
concluded that the major factor in water movement in this area is wind. The
major wind component in May is from the north 30% ofthe time, northwest 33% and
northeast 5%. (Data from U. S. Navy Meteorological Atlas - North Pacific). This
wind pattern would give a southerly flow during the month of May. The finding
is supported by the dye tracking study and chemical analysis of Blencowe (personal
communication) and the diving observations of Baxter and Webster (personal
communication). These divers, participating in a benthic survey, report the
observation of what appears to be a visible sewage field moving to the south.
CONCLUSIONS
The result of the sewage pollution produced by the Monterey sewage treatment
plant is an enrichment of the populations of interstitial organisms to the
south of the outfall where a large amount of the sewage appears to be moving.
The sewage could be enriching the beach environment directly by providing
readily absorbable nutrients, it could be providing basic nutrients which would
promote the growth of bacteria, algae and fungi, or it could be providing a
constant supply of bacteria (live or dead) on which the organisms could be
feeding. Not enough is known about the viability of fecal bacteria in sea
water, the decay of complex organic molecules in seawater, or even about the
biology of these particular organisms to conclude which of the enrichment
mechanisms is most likely to be in operation.
ACKNOWLEDGEMENTS
Acknowledgement is gratefully made to Dr. E. H. Wheeler for his advice and
encouragement on this project and to Dr. Welton Lee and Dr. Donald Abbott for
assisting in the identification of some specimens. This study was supported
by grant 617280 of the National Science Foundation.
28.
REFERENCES
Pennak, R. W. 1951 Comparative ecology of the interstitial fauna of freshwater
and marine beaches. Annee biol. ser (3) 27, Ml9-180.
Stevenson, C. D. 1964 A study of the currents in southern Monterey Bay
M.S. thesis in oceanography. U.S. Naval Postgraduate School Monterey California
Swedmark, B. 1964 The interstitial fauna of marine sand. Biol. Rev. 39 1-12
Trumbauer, D. S. 1966 A coli-form bacteria survey of Monterey Bay off Del Monte
Beach. M. S. thesis in oceanography. U.S. Naval Postgraduate School
Monterey California
Zinn, D. J. 1968 A brief consideration of the current terminology and sampling
procedures used by investigators of marine interstitial fauna. Trans. Amer.
Microsc. Soc. 87 (2) 219-22:
254