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.

Arabian Sea 1995 Station profiles

Process 1Process 4LatitudeLongitudeTOC
Winter MonsoonSummer Monsoon°N°E”MC
Sta 02Sta 0222.561.2Profile-1
Sta 07Sta 0719.267.2Profile-2
Sta 11Sta 1115.468.8Profile-3
Sta 13Sta 1710.065.0Profile-4
Sta 17Sta 1314.565.0Profile-5
Sta 21Sta 2216.062.0Profile-6
Sta 26Sta 2717.259.8Profile-7
Sta 28Sta 2918.158.0Profile-8

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.

Process 1 -- Winter Monsoon

LatitudeLongitudeOrganic CTransmittanceFluorescence
°N°E”MC%fluor. units
14.565.0Sta 17 Cast-01aCast-01b
18.158.0Sta 28 Cast-10aCast-10b

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:

  1. Mixed-layer TOC levels are generally high throughout the year relative to other open ocean areas.
  2. 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.
  3. Surprisingly, some of the greatest increases in concentration were observed below the mixed-layer in the upper 200m.
  4. 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


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.