Spatial and Temporal Variations of
Dissolved Organic Matter in the Arabian Sea:
Response to Monsoonal Forcing
Edward T. Peltzerč and Dennis A. HansellČ
1: Woods Hole Oceanographic Institution
2: Bermuda Biological Station for Research
ABSTRACT
As part of the US-JGOFS Arabian Sea Program, measurements of dissolved and
total organic carbon (DOC/TOC) were made on six cruises aboard the R/V
Thomas G. Thompson during 1995. In addition, samples for the
determination of dissolved and total organic nitrogen (DON/TON) were
collected during these same cruises, frozen and shipped back to Bermuda for
analysis. Preliminary results from the winter (northeast monsoon), spring
(inter-monsoon) and summer (southwest monsoon)seasons have revealed a dynamic
system where the mixed-layer concentration of organic matter varies in
response to physical and biological forcing. Surface concentrations of TOC
ranged from 60 to 100 ”MC and varied seasonally in a complex spatial
pattern. The lowest mixed-layer concentrations were observed during the
summer monsoon in areas of intense upwelling (characterized by low
mixed-layer temperatures and high nutrient concentrations) while the highest
concentrations (for oceanic sites) were observed at the oligotrophic stations
or (for coastal sites) near the coast during the winter monsoon.
ANALYTICAL
The measurement of organic carbon is a problem that has long challenged
marine organic geochemists. Shipboard measurements have proven especially
difficult owing to the motion sensitivity of the CO2 detector. Recent
advances in both the analyzer design and detector stability now make the
at-sea determination of DOC possible. The Li-Cor solid-state infra-red CO2
detector and a computer based data collection and peak integration program
allowed for instrument operation in varying sea-states without deterioration
of signal nor loss of analytical precision. A description of one of our
instruments, noting improvements over the Suzuki design, can be found in
Peltzer and Brewer (1993) and
Peltzer, et al. (1996).
This new hardware solved only some of the problems. Operating, quality
control and quality assurance protocols were also developed during the
US-JGOFS Equatorial Pacific Ocean cruises in 1992 to measure and correct for
the instrument blank; to determine instrument response factors on a regular
basis; and to eliminate routine inadvertent contamination. As a result of
these protocols, typical relative standard deviation of replicate analyses
was ± 2%. The instrument response factor was measured twice daily using
potassium hydrogen phthalate in seawater. Typically, the two response
factors differed by <2% and their mean was within ± 5% of the mean of
all calibrations. The instrument blank was measured frequently using
carbon-free distilled water, every 4-6 samples. Initially, the blank was
large (> 20 ”MC), but decreased rapidly. After several days of use,
the instrument blank "stabilized" between 10-15 ”MC. All TOC
concentrations reported are corrected for this instrument blank. Due to the
higher precision and stability of this instrument, seasonal and spatial
variations in mixed-layer TOC concentrations can now be observed.
Most of the samples collected were not filtered so in order to be consistent
with the older literature, it is more proper to call our measurements total
organic carbon (TOC) as opposed to dissolved organic carbon (DOC). However,
given the low surface particulate organic carbon (POC) concentrations [see
section below: TOC vs DOC], it is apparent that >
90-95% of the TOC is DOC at the surface. In the deep water, the fraction of
the TOC that is POC is even smaller.
STATION PROFILES 
In order to best demonstrate the seasonal variations in the dissolved organic
matter pool, the TOC profiles of the upper 200m for 8 stations from the
winter and summer monsoon cruises are shown.
For most stations, the summer time concentrations of TOC increased,
especially at the site in the northeast (Sta #7).
In addition to the expected changes in the mixed-layer concentrations,
equal or larger increases in the TOC was observed just below the mixed-layer.
In contrast to the open ocean stations, the
coastal station shows a decrease of TOC during the summer monsoon.
At these stations, the upwelling of cooler, nutrient rich, low TOC
waters is the dominant effect.
DEEP WATER PROFILE 
The deep-water (>2000m) profile from station 7
collected during the summer monsoon (Process 4) is shown. The shape of this
profile is reminiscent of the deep-water TOC profiles from most other major
ocean basins; however, the concentration of TOC in the Arabian Sea at 40
”MC is mid-way between the deep-water TOC concentrations found in the
Atlantic ocean (43-44 ”MC) and the Pacific (36 ”MC). In this profile
(and several others collected during the summer monsoon) there is the hint of
a slight TOC increase near the bottom suggesting that we may be seeing a flux
of organic material from the bottom.
TOC vs DOC 
In these profiles we have plotted both TOC (that is the organic carbon
measured in un-filtered samples) and DOC (from filtered samples). From DOC
analyses of deep-water (> 2000m) samples, it is know that there is a slight
(~3 ”MC) problem with contamination during the filtration process. To
correct for this 3 ”MC was subtracted from the DOC values reported
here. The particulate organic carbon (POC) concentrations were then calculated
by difference: POC = TOC - DOC. While this may not be as quantitatively
accurate or precise as the standard POC measurements, it does allow us to make
measurements while at-sea.
By comparison with the CTD transmissometer and fluorescence data, it is
apparent that this approach has some merit. Where the CTD data indicate that
the phytoplankton are most abundant there is a POC maximum. Likewise, the
shape of this distribution, agrees well with the calculated POC profiles.
SUMMARY
These preliminary results reveal some interesting features regarding the
distribution of DOM in the Arabian Sea and how these distributions respond to
monsoonal forcing:
- Mixed-layer TOC levels are generally high throughout the year relative to
other open ocean areas.
- Highest TOC levels are observed in the summer at the oligotrophic stations
and at the NE station. For coastal stations, the highest concentrations were
seen during the winter monsoon.
- Surprisingly, some of the greatest increases in concentration were
observed below the mixed-layer in the upper 200m.
- Lowest mixed-layer concentrations were observed at the coastal sites
during the summer monsoon as upwelling of nutrient rich but DOM depleted water
was the dominant process.
Acknowledgements
This research was supported by National Science Foundation grants
OCE-9310719 (to ETP) and OCE-9311012 (to DAH).
We thank program manager Don Rice for his guidance and support. We
gratefully acknowledge the captain, crew and technicians of the R/V
Thomas G. Thompson for their help and support. We also thank chief
scientists Mike Roman, John Marra, Dick Barber, Sharon Smith, Barney Balch
and Wil Gardner for cruise planning and leadership. We would also like to
thank Liz Caporelli, Nancy Hayward, Rachel Parsons and Tye Waterhouse for
technical support. Finally, we would like to thank Susan Kadar and Martin
Bowen for the excellent logistical support we received and their unwavering
determination in the face of often seemingly insurmountable obstacles and
inpenetrable barriers.
References
- Peltzer, E. T. and P. G. Brewer (1993). Some practical aspects of
measuring DOC: sampling artifacts and analytical problems with marine
samples. Marine Chemistry 41: 243-252.
(Abstract)
- Peltzer, E. T., B. Fry, P. H. Doering, J. H. McKenna, B. Norrman and U.
L. Zweifel (1996). A comparison of methods for the measurement of
dissolved organic carbon in natural waters. Marine Chemistry
54: 85-96. (Abstract)
This is the web version of a poster (OS11A-1) by Edward T. Peltzer, WHOI and
Dennis A. Hansell, BBSR. It was originally displayed at the
AGU / ASLO Ocean Sciences Meeting in San Diego, CA, 12-16 February 1996.
This page was created on 21 June 1996 and last updated on 19 April 1999
by Edward Peltzer, MBARI.
