NOTES
AND
mitting the use of a decreased volume of
extractant.
YOUNG
SIK KIM
HARRY
ZEITLIN
Department of Chemist y,
and
Hawaii Institute of Geophp-ics,
University of Hawaii,
Honolulu, Hawaii
96822.
REFERENCES
BACHMANN, R. W., AND C. R. GOLDMAN.
1964.
The determination
of microgram
quantities
of molybdenum
in natural waters.
Limnol.
Oceanog., 9 : 143-146.
AN INSTRUMENT
FOR MEASURING
~IITCHELL, R. L., AND R. 0. SCOTT. 1947. Concentration methods in spectrographic
analysis.
J. Sot. Chem. Ind. ( London),
66: 330-336.
SANDELL, E. B. 1959. Coloumetric
determination of traces of metals, 3rd ed. Interscience, New York. 1032 p.
SILVEY, W. D., AND R. BRENNAN. 1962. Concentration
method
for the spectrochemical
determination
of seventeen elements in natural water.
Anal. Chem., 34: 784-786.
SUCAWARA, K., M. TANAKA, AND S. OKABE.
1959. Separation
and determination
of microgram quantities of molybdenum
in natural
waters.
Bull. Chem. Sot. Japan, 32: 221399
--i.
SUBTIDAL
Very little is known about the oxygen
consumption of the seabed-information
that is basic to our understanding of the
oxidation of organic matter and the regeneration of nutrients by biological processes
in the sea. Apart from several in situ intertidal (see Pamatmat 1966) and shallowwater subtidal (Stein and Denison 1966)
studies of oxygen uptake on the bottom,
subtidal investigation in deeper water have
been made by Kanwisher (1962) and Carey
( 1967) by measuring the oxygen uptake of
sediment cores on board ship or in the
laboratory.
The effect on the community’s metabolic
activity of raising the sediment from the
bottom is unknown. Both the decrease in
hydrostatic pressure and the delay in starting the experiment could conceivably lead
to erroneous estimates of oxygen consumption. One way to solve this problem is to
measure oxygen utilization
in situ and
compare results with those of the shipboard technique.
The authors express their gratitude to
Mr. J. Trimble for his electronics work, to
Skippers J. Gassert and G. Drewry for
their expert handling of the RV Onar,
1 Contribution
No. 452 from the Department
of
Oceanography,
University
of Washington,
Seattle.
This research was supported by National Science
Foundation
Grant GP-4902.
537
COMMENT
BENTHIC
METABOLISM
IN SITUP
FIG. 1. The tripod showing:
A-bell
jars, Bstirring motor, C--Distons,
D-television
camera. ,
E-panning
mechanism,
k-light,
and G-pressure-proof containers for electronic components.
and to Dr. K. Banse for reviewing
manuscript.
the
MODIFICATIONS
The in situ measurements were accomplished with an adapted stable instrument
platform originally designed for measuring
currents and sediment motion within 2 m
of the sea floor (Sternberg and Creager
1965), shown as modified in Fig. 1, above.
Mounted at the apex of an aluminum tripod are three watertight
cylinders containing the power-supply
regulator, the
538
NOTES
FIRST
STAGE
DIVING
REGULATOR
AND
COMMENT
---
FIG. 3. The bell jar tilted
upward to show:
A-the
openings with ball valves, B-the
oxygen
electrode, C-the
stirring motor, and D-the
stirring bar.
FIG. 2. Sketch of the hydraulic
system. The
first-stage diving regulator was adjusted to deliver
about 5 atm over the ambient hydrostatic
pressure. To push the bell jars down, solenoid valves
A and B were opened. When valves C, D, and
E were opened next (one at a time ), compressed
air forced water in the reservoir
through
the
needle valve.
To pull the bell jars out of the
sediment, valves F and G were opened; when C,
D, and E were subsequently
opened, the water
was pushed back into the reservoir.
receiver, part of the control system, and
the telemetry transmitter. The platform is
connected to the surface by an eightconductor electric cable and a winch cable.
Electric power is supplied as 110-v d-c at
3 to 8 amp. The control system consists of
two parts: 1) a shipboard console that
transmits command signals by means of
tone generators and 2) the receiver housed
in the tripod consisting of tuned reed
distributing
relays.
relays and power
Transmission and reception of signals from
the different sensors are handled by FM/
FM telemetry.
This instrument system was adapted to
position three plastic bell jars in the sediment and telemeter the signals from electrodes to determine the decrease with time
of the dissolved oxygen in the enclosed
water. A shorter set of legs (total height,
1.75 m ) was substituted for the original
set to lower the center of gravity and prevent toppling when the platform is used
on a slope. Wide pads were provided to
prevent the tripod’s sinking too deeply in
soft mud. Between each pair of legs, a bell
jar was suspended from an arm attached
to a hydraulic piston. Above each bell jar
and to one side was mounted a 100-w projection bulb. In the center of the platform
was a panning device that rotated a television camera 360” in either direction for
observing the bell jars.
Compressed air was used to push the
bell jars into the sediment and pull them
out at the end of the experiment. At first,
compressed air was applied directly on
the pistons. The pistons, however, traveled too fast and disturbed the surface
layer, exposing reduced sediment and
causing a spuriously high oxygen consumption rate. The rate of descent of the
piston could not be sufficiently
retarded
by a needle valve. The hydraulic system
shown in Fig. 2 was subsequently found
NOTES
FIG. 4. A section of a strip chart showing
outputs from three electrodes and a platinum
tion check.
AND
COMMENT
539
successive recordings
(2.5-min duration each) of voltage
resistance thermometer.
Every third cycle was a calibra-
to be satisfactory,
moving the piston
smoothly at about 0.5 cm/set.
The bell jars (Fig. 3) were Plexiglas
cylinders (27-cm diam, 10 cm deep, 3.5
mm wall thickness) with opaque polyvinyl
chloride plastic tops. Each was fitted with
one-way relief ball valves on top, a leadsilver oxygen electrode (Mancy,
Okun,
and Reilley 1962)) a Teflon-covered magnetic stirring bar, and a motor rotating an
Alnico magnet to which the stirring bar
was magnetically coupled. The bell jars
were suspended in a horizontal attitude
so that they were pushed vertically into
the sediment. Proper balancing of the bell
jars was essential; a significant tilt caused
surface sediment to be pushed to one side
and the reduced subsurface sediment to
be exposed.
In the early developmental stage, a 5cm-wide flange and mechanical tripping
device were affixed to the bell jar to stop
the piston automatically
when the flange
touched the sediment surface. This arrangement was not satisfactory because the
sediment was either so soft that the device
did not trip soon enough or so hard that
the bell jar was not pushed all the way to
the flange. Hence, the lower half of each
bell jar was graduated with l-cm strips of
black and white tape that could be clearly
seen on the television screen, showing how
deep the jars penetrated the sediment. In
this manner, the volume of enclosed water
could be determined with an accuracy of
6 to 14% depending on the absolute volume. From 2 to 4 liters of water were
enclosed by the bell jars over an area of
570 cm2, depending on how deeply they
penetrated the sediment.
The electrodes had a positive temperature coefficient of about lO%/“C and were
also pressure sensitive. Increasing pressure decreased the electrode sensitivity.
Because pressure effects were unpredictable, it was necessary to calibrate each
electrode individually.
Actual pressure coefficients on various occasions ranged from
-0.1 to -0.2%/atm
when subjected to
hydrostatic pressure as high as 18.5 atm.
The signals from the three electrodes
and a platinum
resistance thermometer
were successively telemetered to the ship
where they were demodulated and presented on a strip-chart recorder (Fig. 4).
Although
the three bell jars were run
simultaneously, each electrode was monitored for only 2.5 min every 7.5 min. Experiments were run for periods of from 2
to more than 6 hr; the dissolved oxygen
concentration inside the bell jars decreased
by 20 to 50% during this time. Unless the
540
NOTES
AND
sediment had been disturbed, the rate of
oxygen consumption was constant with decreasing oxygen concentration.
PROCEDURE
The ship was held at one spot by threepoint anchoring.
The tripod was raised
and lowered several times at depth to
purge the bell jars of air bubbles. It was
then lowered to about 2 m above the
bottom. The stirring motors were switched
on, and the oxygen concentration and temperature of the water were continuously
monitored for about 30 min. When the
recordings indicated a constant oxygen
concentration
and temperature,
a water
sample was obtained by a Nansen bottle
and analyzed gasometrically
( Scholander
et al. 1955). This method of calibration
took both temperature and pressure effect on each oxygen electrode into consideration.
RESULTS
The tripod has been successfully lowered at 11 stations in Puget Sound, from
11 to 185 m deep. The sediment at these
stations ranges from soft mud to firm sand
and silty gravel with rocks and dead shells.
In January the rates measured at six stations were from 7.1 to 20.4 ml of 02 m-2
hr-l. The station consistently with lowest
rates of oxygen consumption is a firm,
clean, sandy bottom in 30 m of water.
Measurements there in March yielded an
average of 4.1 ml of 02 m-2 hr-l with a
standard deviation of 1.0. By comparison,
at a station 11 m deep and characterized
by sandy mud with debris, rates in March
averaged 23.2 ml of O2 m-2 hr-l with a
standard deviation of 3.9.
Comparisons between the in situ bell jar
and shipboard core techniques indicate
that the latter could underestimate
or
overestimate the rate of benthic oxygen
consumption, assuming the in situ measurements to be valid. Hydrostatic pressure up to 18.5 atm, approximating
the
pressure at the deepest area studied in
Puget Sound, had no evident effect on the
COMMENT
rate of oxygen uptake by sediment cores.
The divergent results by the shipboard
core techniques seem to arise from disturbance of the sediment by the coring device
itself or by the coring technique. Improvements are being made.
The results suggest the necessity for
making more in situ measurements of subtidal benthic metabolism as a means of
checking the accuracy of shipboard methods. Until the accuracy of the method is
demonstrated, the results of shipboard or
laboratory measurements must be considered tentative. Our search for the reasons
behind the discrepancy between our in
situ and shipboard techniques has revealed
many factors influencing
the measured
rate of benthic oxygen uptake.
MARIO
DOUGLAS
M.
PAMATMAT
FENTON
Department
of Oceanography,
University
of Washington,
Seattle
98105.
REFERENCES
CAREY, A. G., JR. 1967. Energetics of the benthos of Long Island Sound. I. Oxygen utilization of sediment.
Bull. Bingham Oceanog.
Collection,
19 : 136144.
KANWISHER,, J. W.
1962. Gas exchange of shallow marine sediments, p. 13-19.
In Proc.
Symp. Environmental
Chem. Marine
SediOccasional
Publ. 1, Grad. School
ments.
Oceanog., Univ. Rhode Island, Kingston.
MANCY, K. H., D. A. OKUN, AND C. N. REILLEY.
1962. A galvanic cell oxygen analyzer.
J.
Electroanal.
Chem., 4: 65-92.
The ecology
and
P~A-IWAT,
M.
M.
1966.
metabolism
of a benthic community
on an
intertidal
sandflat
(False
Bay, San Juan
Island,, Washington).
Ph.D. Thesis, Univ.
Wash., Seattle. 243 p.
SCHOLANDER, P. F., L. VAN DAM, C. L. CLAFF,
AND J. W. KANWISHER. 1955. Microgasometric determination
of dissolved oxygen and
nitrogen.
Biol. Bull., 109: 328-334.
STEIN, J. E., AND J. G. DENISON. 1966. In situ
benthal oxygen demand of cellulosic
fibers.
Third
Intern.
Conf. Water
Pollution
Res.,
Munich,
Germany.
Water Pollution
Control
Fed. 10 p.
STERNBERG, R. W., AND J. S. CREAGER. 19165.
An instrument
system to measure boundarylayer conditions
at the sea floor.
Marine
Geol., 3: 475-482.