NOTES
AND
DISTANf5E
l5Oh
25
I
50
I
5x3
COMMEAT
IN FEET
100
I
150
I
125
,
.
I.
.
I
i
t.
t
I
I
FIG. 2. Profile of beach at Will Rogers Beach State Park about two miles northwest
of Santa Monica,
California,
the site of repeated measluements of Table 1. Hased on measurements of Table 1 by observer
C uncorrected for earth curvature.
It is possible that other workers may find
the simple wooden rods useful for measuring profiles of beaches to determine seasonal
and other cyclic changes with respect to
waves, to relate slope to grain size of sand,
and for other purposes. For most such objectives the method appears to possess sufficient accuracy, particularly in view of the
THE STUDYOF IN-SITU
fact that the
irregularities
profile which
measurement
presence of cusps and other
produce local variations in
are greater than the error of
by the rods.
K.
0. EMERY
Unizjersity of Southern Cnlifornicl
I>os Angeles, Cnlifornicr
MARINE PHOTOSYNTHESIS
USING A LARGE PLASTIC BAG
Any precise study of the processes of phytoplankton growth and decay in the sea is
impossible unless the same body of water is
analysed over a protracted period of time.
This necessitates some form of containment
but the use of a tank has several disadvantages. Unless the tank is very deep the
lighting of the water mass as a whole is unrepresentative of that found in the euphotic
zone and the degree to which water in the
tank is heated will generally be inadmissibly
high. Finally no “balance” of dissolved oxygen or carbon dioxide can be readily computed using a shallow body of water with a
large surface area in contact with the atmosphere.
We wish to report the construction of a
piece of equipment the use of which has
overcome the above disadvantages and
which has enabled a study to be made of the
growth and decay of marine phytoplankton
under near “natural” conditions.
The equipment consisted of a thin plastic
container connected to the atmosphere by a
narrow neck, the body of the apparatus being some 6 ft beneath the surface of the sea
so that much of the sun’s heat was removed
before reaching the container. To lessen the
effects of wall growth and accidental contamination both the volume and the volume
to surface area ratio of any container should
be made as large as practicable. These facts
and considerations of ease of construction,
stirring and sampling suggested the use of a
spherical bag at least 20 ft in diameter. In
coastal waters it is doubtful whether increas-
94
NOTES
AND
ing the size of a bag by more than another
5-10 ft would have any advantage as for
much of the time the bottom would then be
beneath the euphotic zone.
The apparatus is shown in plan and to
scale by Figure 1. A custom-made bag made
of &gauge opalescent polyvinyl
chloride
sheet was used. The bag was exactly 20 ft in
diameter, with an l&in. diameter neck some
10 ft long and was supported by 61/2-in. mesh
netting of number 12 nylon seine twine.
About 80-85% of the incident light was
transmitted by this assembly. The netting
was fastened to steel rings 10 or 20 ft in
diameter, as shown, and the whole attached,
by a steel “cradle,” to a fiberglas buoyancy
bell. The latter could be flooded with water
or filled with air to counteract any increase
or decrease in weight of the system brought
about by salinity changes in the water surrounding the apparatus. (These changes
are very considerable in British Columbia
Coastal waters in the spring and a possible
weight increase of up to 4000 lb had to be
tolerated by the system). Weights were tied
to the bottom ring of the apparatus after it
was filled to ensure that no upthrust could
develop on the buoy. The level of the buoy,
and hence the bag, beneath the surface of
the sea, was kept constant by adding or
removing air as necessary (generally not
mere frequently than once a day).
The neck of the plastic bag was threaded
up through the buoyancy bell and tied to the
top. A fiberglas liner was then pushed down
the inside of the neck and projected a few
feet into the bag. This prevented the plastic
from closing when density gradients in the
water inside and outside the neck differed
and it also lessened wear-and-tear on the
plastic neck during sampling. At the end of
an experiment the bag could be replaced
without removing the whole apparatus. The
bottom lo-ft diameter ring was removed,
the old bag pulled through by SCUBA divers
and a new bag, suitably folded, threaded
through the neck of the buoy.
The whole equipment was anchored to
three one-ton anchor blocks by three nylon
ropes, about 150 ft long, fastened to halters
on the central 20-ft diameter ring. The bag
COMMENT
was thus at the center of an isosceles triangle,
with the anchors at the three points, and was
only able to move laterally within a circle of
lo-ft or less radius.
The contents of the bag were kept stirred
to uniformity to enable an accurate analytical balance of particulate and soluble material to be evaluated. The stirring was accomplished by an umbrella-like device, made of
nylon and stainless steel, which opened on
the up-stroke of a piston having a length of
2X+3 ft. An alternative arrangement with
two “umbrellas,” one at the top and one at
the bottom of the bag, connected by rigid
rod was also tried, the umbrellas being upside down so that they pushed water downwards. This latter system should, theoretically, give a greater upward velocity to
water in the bottom part of the bag and
hence better counteract the sinking of plant
cells but there may be little to choose between the two arrangements.
The stirring motor, suspended on the
buoyancy bell by a quadraped, was water
driven from a pipe from the shore. It consisted of a reciprocating water motor similar
to those used in water-powered washing machines. It was mounted over the neck of the
bag in a vertical position with its piston rod
projecting from the lower end.
In order to take samples the piston was
stopped at the top of its stroke, the stirring
line lowered into the bag and a meter block
fastened to the quadraped. Water samples
could then be taken from the center of the
bag using standard Nansen or Van Dorn
bottles. Light measurements were taken by
lowering a light meter into the water and
comparing the output with a “deck cell”
placed on the buoy. “Light” and “dark” bottles for photosynthesis measurements could
be fitted to the racks on the stirring assembly, as illustrated.
With extinction values
(base e) between about 0.3 and 1.0 it can be
shown that the rate of photosynthesis in a
bottle suspended 2 ft above the center point
of the bag should approximate to the mean
rate from a plant cell in random motion
throughout the entire water mass.
Fouling of the outside of the equipment,
with a consequent loss of light penetrating
NOTES
AND
95
COMMEY?
‘i
i
$
*
\\
/
‘\ \
/
\\
2-10’ dia. rings\-l
FIG. 1.
Cross-section
scale plan of plastic* bag container.
/
/
96
KOTKS .4X1) (‘o\lArE\‘l
into the bag, was restricted mainly to the
upper surfaces and on the nylon netting. The
growth, mainlv a hvdroid
in our experi,
merits, could be kept in check bv throwing
coarse crystals of copper sulphafe onto the
netting every few days.
IVe have used the above equipment suecessfull!7 at Departure Bay (near Nanaimo,
B. C. ) where it v7as anchored some 300 ft
from the end of a pier in water having a
tlepth of at least 11 m at low tide. Full
details of this work will be published later.
In our experiments the hag was filled with
\\Tater ( :32,000 U. S. gal) taken from a plastic
pipe from a depth of over 20 m and filtered
through a commercial diatomaceous earth
filtration unit which removed all plant and
animal life. The same water-line as that
ilsetl to power the stirring motor was used
for transporting water to the bag. The container MY~S“innoculatecl” with a few hundred
gallons of surface water, filtered through a
;3OO-pII!-lon net to remove most zooplankton.
SCREENI~II
PAIL FoH SIFTING
Rawson ( 195:3) described a screened pail
~~~hichhe used to process bottom samples.
AVo animal populations were found to clcvelop in the ensuing weeks. The tcmperature of the water in the bag kept to within
1°C of the temperature of the surrounding
sea and the assembly appeared to be “seaworthv” to a remarkable extent, although it
clearly should be llsed onlv in relativehsheltered waters.
We have reported the construction of this
equipment so as to point out its proven
feasibility. The apparatus is a most valuable
tool for the stud!7 of primary prodllctivit!
and could have many other applications.
such as a study of zooplankton growth and
grazing rates. We would be pleased to provide more detailed information to anyone
contemplating the construction of a similar
~lppU-~\tlls.
J. D. H. STRICKLASD ANJ
L.D.B. TERHUNE
Fisheries Resetwch Bocrrcl of Cnnncltr
Pacific Occcrnogrtrphic~ c*ro~q
Wnncrinzo, B. C.
HOTT~M-FAT_JNA
SAMI>T,E:S'
The follo\ving modified version of Rawson’s
pail has proved ver>T satisfactory during :3
years of bottom fauna work on the -Missisippi River. The metal paddles which were
mounted on the inside of Rawson’s pail have
been replaced by a stationary propeller beneath the screen bottom of the pail (Fig. 1).
Three screen windows increase the working
surface of the pail and also prevent it from
becoming too full of water. The propeller
circillates water rapidly in and out of tlicl
four screen snrfaces when the pail is s\virletl
in tlw
\wte1-.
CALWS 13. FREMLIN.